JOHNSON MATTHEY TECHNOLOGY REVIEWContents Volume 59, Issue 4, October 2015 JOHNSON MATTHEY TECHNOLOGY REVIEW Johnson Matthey’s international journal of research exploring science

JOHNSON MATTHEY TECHNOLOGY REVIEW

Johnson Matthey’s international journal of research exploring science and technology in industrial applications

www.technology.matthey.com

Volume 59, Issue 4, October 2015 Published by Johnson Matthey

ISSN 2056-5135

© Copyright 2015 Johnson Matthey

Johnson Matthey Technology Review is published by Johnson Matthey Plc.

All rights are reserved. Material from this publication may be reproduced for personal use only but may not be offered for re-sale or incorporated into, reproduced on, or stored in any website, electronic retrieval system, or in any other publication, whether in hard copy or electronic form, without the prior written permission of Johnson Matthey. Any such copy shall retain all copyrights and other proprietary notices, and any disclaimer contained thereon, and must acknowledge Johnson Matthey Technology Review and Johnson Matthey as the source.

No warranties, representations or undertakings of any kind are made in relation to any of the content of this publication including the accuracy, quality or fi tness for any purpose by any person or organisation.


Contents Volume 59, Issue 4, October 2015

JOHNSON MATTHEY TECHNOLOGY REVIEW

Johnson Matthey’s international journal of research exploring science and technology in industrial applications


291 Guest Editorial: Water Technologies at Johnson MattheyBy Nick Garner

293 “Heavy Metals in Water: Presence, Removal and Safety”A book review by Edward Rosenberg

298 “Particle-Stabilized Emulsions and Colloids: Formation and Applications”A book review by Cecilia Bernardini

303 Interplay between Silver and Gold Nanoparticles in Production of Hydrogen from MethanolBy Hany M. AbdelDayem

313 Carbon Formation in Steam Reforming and Effect of Potassium PromotionBy Mikael Carlsson

319 “Electrochemical Power Sources: Batteries, Fuel Cells, and Supercapacitors”A book review by Billy Wu

322 Selective Removal of Mercury from Gold Bearing StreamsBy James G. Stevens

331 In the Lab: Uranium Capture From High Sulfate and Nitrate Waste Streams with Modifi ed Silica Polyamine CompositesFeaturing Professor Edward Rosenberg

334 New Smopex® Ion Exchange Materials for the Removal of Selenium from Industrial Effl uent StreamsBy Carl Mac Namara, Javier Torroba and Adam Deacon

353 Johnson Matthey Highlights

www.technology.matthey.comJOHNSON MATTHEY TECHNOLOGY REVIEW

http://dx.doi.org/10.1595/205651315X688037 Johnson Matthey Technol. Rev., 2015, 59, (4), 291–292

291 © 2015 Johnson Matthey

In this issue the theme is water remediation. Johnson Matthey is working on a number of high technology purifi cation products for applications in the water industry. We are focusing our research and development efforts on creating technology to remove a range of low level toxic contaminants, such as mercury, from water.

Johnson Matthey is known for its expertise in adsorbent materials, such as Smopex®, with which readers of this journal may be familiar for their use in the recovery of precious metals from both waste and product streams (1, 2). In 2013 the company acquired further advanced ion exchange technology from Purity Systems Inc, forming the company’s Water Technologies business. This combination of technology fi ts well with Johnson Matthey’s core competences in advanced materials and catalysts. We place particular emphasis on some key challenges facing the mining and chemicals industries, where problem contaminants, increasing legislative requirements and focus on environmental and cost issues often mean current technologies are being stretched.

Providing Effective Solutions

The need for clean water is of major signifi cance across the world, with growing populations requiring access to improved quality water resources. Environmental legislation and regulation mean that there is increasing

need to clean up effl uents from industrial processes such as mining, agriculture and manufacturing. Pollutants including metals, non-metals and organic compounds may be present due to either man-made or natural processes.

An example of such a pollutant, selenium, is discussed in the article by Mac Namara et al. in the present issue of the Johnson Matthey Technology Review (3). In this article the performance and mechanisms of a new material based on the Smopex® range of ion exchange materials is described for Se remediation in effl uents from coal combustion plants and oil refi neries. A common co-contaminant is sulfur which poses signifi cant problems for previous generations of ion exchangers, although the technique of ion exchange offers attractive benefi ts over existing technologies (whether chemical or biological) which all have disadvantages in terms of high cost or high volumes of materials required. Strong-base functionalised materials were identifi ed by Johnson Matthey as being the most promising candidates for selective sorption of selenium ions and the article presents results and fundamental studies on these materials showing promising results in both fi xed bed and continuous stirred tank reactor trials.

Gold mining is another area which suffers from the presence of water soluble pollutants, in this case species of the heavy metal mercury which is frequently associated with gold in ore deposits. The health and

Guest Editorial

Water Technologies at Johnson Matthey

From mining to molecules – Johnson Matthey’s innovative processes and advanced scavenger technologies, built on and underpinned by continuous research and development, can help recover valuable metals and purify active pharmaceutical ingredients


http://dx.doi.org/10.1595/205651315X688037 Johnson Matthey Technol. Rev., 2015, 59, (4)

environmental implications of mercury are well-known, however it is a major challenge to remove the mercury from the gold processing circuit; technical diffi culties also exist since the most widely used method for extracting gold, employing cyanide as lixiviant, also extracts mercury and other metals along with the gold. It is therefore essential to identify a method that will remove only the mercury; any loss of gold during the process is deemed unacceptable. Johnson Matthey has now developed solid adsorbents which can achieve selective adsorption of mercury from gold cyanide bearing process streams and the technique is described in detail in this issue of the journal (4). Testing of the material in real process feeds is described and a pilot plant trial is now underway in Nevada, USA.

A Collaborative Approach

Johnson Matthey is always open to new collaborative efforts to solve problems for our customers. One such collaboration is with Professor Edward Rosenberg, University of Montana, USA. He develops advanced silica polyamine composite materials for metal ion separations and recovery from industrial and mining waste streams. Most recently these materials are being applied for uranium remediation with the University of the Witwatersrand in South Africa, and a forthcoming article in this journal is expected to present some further details on this project.

It is worth noting that many techniques based on

naturally occurring substances, bioremediation and even waste products are in use for removing heavy metals from water – but more technically advanced materials are required for heavy metal contamination arising from high technology industries in developed countries. For example, ion exchange is the go-to technology in the USA, where it constitutes a multi-billion dollar a year market. The technique of ion exchange shows great promise to help remediate wastewater streams around the world and provide safer, cleaner water for greater numbers of people than ever before.

NICK GARNERGroup Director, Corporate and Strategic

DevelopmentJohnson Matthey Plc, Orchard Road, Royston,

Hertfordshire, SG8 5HE, UKEmail: [email protected]

References

1. S. Phillips and P. Kauppinen, Platinum Metals Rev., 2010, 54, (1), 69

2. J. Frankham and P. Kauppinen, Platinum Metals Rev., 2010, 54, (3), 200

3. C. Mac Namara, J. Torroba and A. Deacon, Johnson Matthey Technol. Rev., 2015, 59, (4), 334

4. J. G. Stevens, Johnson Matthey Technol. Rev., 2015, 59, (4), 322




“Heavy Metals in Water: Presence, Removal and Safety”Edited by Sanjay K. Sharma (Jaipur Engineering College and Research Centre (JECRC), India), Royal Society of Chemistry, Cambridge, UK, 2015, 357 pages, ISBN: 978-1-84973-885-9, £175.00, €218.75, US$290.00

Reviewed by Edward Rosenberg Department of Chemistry and BiochemistryUniversity of Montana, Missoula, Montana 59812, USA

Email: [email protected]

Introduction

“Heavy Metals in Water: Presence, Removal and Safety” is published by the Royal Society of Chemistry and consists of 16 independent chapters. The chapters can be broadly divided into two groups: those covering the techniques and processes used to deal with heavy metal pollution and those discussing a particular pollutant or pollution problem. The chapters are divided approximately equally between these two topics. The techniques presented include the use of modern approaches such as photocatalysis and nanotechnology (Chapters 2, 4 and 9) but by and large the volume emphasises the use of naturally occurring substances, waste products and bioremediation for removing heavy metals from water (Chapters 3, 10, 11, 14 and 15). This is understandable in light of the fact that most of the contributors come from developing countries where the emphasis for remediation is on low-cost readily accessible technologies. As a consequence the volume does not deal with heavy metal contamination resulting from high technology industries such as nuclear power, computer manufacturing and related electronics

production where more technically advanced, but more expensive materials are employed in the industrially developed countries. The specifi c metal contamination problems presented are arsenic (Chapter 5), iron and manganese (Chapter 6), fl uoride (Chapter 13) and chromium (Chapter 16), with the remaining chapters dealing with techniques and general surveys of heavy metal contamination. Chapters 7 and 8 stand out as chapters that deal with Chinese government policies on toxic metal contamination and should be very useful for foreign entrepreneurs wanting to establish new businesses in metals related industries.

The editor of this volume, Sanjay K. Sharma, is currently Professor and Head of the Department of Chemistry at JECRC University, India. He has edited many volumes closely related to this one and was recently appointed editor for the series ‘Green Chemistry for Sustainability’.

Heavy Metals in Aquatic Media

It is beyond the scope of this review to give a detailed analysis of each chapter. A brief summary of each chapter will be provided with critical comments on the scientifi c contents, its relevance to the topic and where it complements or is redundant with the other chapters.

Chapter 1, ‘Contamination of Heavy Metals in Aquatic Media: Transport, Toxicity and Technologies for Remediation’ coauthored by the editor serves as a general introduction to the topic and deals with the



sources of heavy metal contamination, associated health risks and brief summaries of remediation methodologies, all of which are handled in more detail in later chapters. It concentrates on the removal of iron and manganese. This is strange as an entire chapter devoted to this subject is found later in the volume (Chapter 6). A useful summary of the health risks associated with heavy metals in water is provided in table form but several important contaminants are omitted. For example, uranium has become an important contaminant as a result of the development of nuclear energy and selenium remains a problem for the oil and coal industries. Both of these metals pose signifi cant health risks. There are a few notable misstatements in the chapter. For example the authors defi ne a heavy metal as having densities in the range of 3.5–7.9 g cm–3 while mercury has a density of 13.7 g cm–3 and many third row transition metals have densities of 19–22 g cm–3. Cadmium is defi ned as the most toxic heavy metal although the allowable release level of mercury is lower than that of cadmium.

Photocatalysis and Nanotechnology

Chapter 2, ‘Photocatalytic Processes for the Removal of Toxic Metal Ions’ describes the photocatalytic reduction of metals using titanium dioxide (TiO2) as the photocatalyst. The appeal of this method is that the electron holes created by the incident light can oxidise organic contaminants and the electrons released could be used to reduce metal ions in the same waste stream. In the absence of organic contaminants water needs to be oxidised. The authors do a good job of outlining the basic process and the relevant kinetic parameters, although one of the diagrams (taken from another source) is not adequately explained in the text. There is an appropriate discussion of the problems associated with scale-up of this technique followed by a case-by-case discussion of the reduction of specifi c metals on the bench scale. The tables in this chapter are basically redundant with those in Chapter 1 but it is interesting to note that the allowable release levels use the World Health Organization (WHO) values, which are different than those in Chapter 1.

Chapter 4, ‘Functionalized Magnetic Nanoparticles for Heavy Metals Removal from Aqueous Solutions’ addresses the timely and interesting topic of magnetic

nanoparticles as adsorption media for heavy metals. The chapter begins with a repetition of the same topics summarised in Chapters 1–3, sources and health effects of heavy metals, but does include selenium in the list. The chapter then goes on to discuss the synthesis of magnetic nanoparticles, focusing heavily on iron, and then goes on to explain the different materials and methods for coating the iron nanoparticles. This is a useful summary of the currently available technologies. The kinetic and isotherm models for the nanoparticles as applied to adsorption of heavy metals are also discussed. This is fairly standard for all adsorptive materials and the authors would have been better off giving more details on the synthesis of the iron-glutamic acid nanocages and on regeneration of magnetic nanoparticles in general. A minor point is that one of the equations given is not correct, it should read Fe2O3, not Fe2O4. The correct equation (Equation (i)) is given below.

Fe3O4 + 2H+ -Fe2O3 + Fe2+ + H2O (i)

Chapter 9, ‘Use of Nanotechnology against Heavy Metals Present in Water’, provides a brief overview of nano-adsorbents. This is a developing area of research that has not seen much use in the remediation industry. The author does a good job of putting this fi eld in perspective including a discussion of the environmental dangers of nanomaterials, a topic that is often sidestepped by other workers in this area.

Removing Heavy Metals From Water

Chapter 3, ‘Removal of Dissolved Metals by Bioremediation’, is perhaps the least useful and most superfi cial chapter in the book. The usual list of metals is followed by a cursory summary of the same remediation techniques outlined in Chapters 1 and 2. The actual subject of the chapter is bioremediation, which is summarised in three or four pages and consists of a laundry list of bacterial strains that absorb heavy metals with no conceptual or mechanistic insights.

Chapter 10, ‘Modifi ed and New Adsorbents for Removal of Heavy Metals from Wastewater’ presents a survey of industrial waste byproducts and modifi ed agricultural and biomaterials for heavy metal adsorption. Like the other chapters in this volume, the author starts out with the usual list of toxic metals,



their sources, health risks and methods of removal. The use of industrial and agricultural wastes is a good addition to this volume as is the discussion of modifi ed biopolymers. A very interesting magnetic core-shell particle modifi ed with a bio-hydrogel is reported but this fi gure is barely readable and should have been corrected prior to publication. An otherwise useful chapter is compromised by the statement that batch equilibrium studies can be used for designing industrial processes. This is a signifi cant misstatement as this type of study is only the beginning of the process, followed up by kinetic studies, adsorbent regeneration studies and evaluation of usable lifetime.

Chapter 11, ‘Natural Clays/Clay Minerals and Modifi ed Forms for Heavy Metals Removal’ presents a thorough and comprehensive survey of the use of clays and modifi ed clay. The fi rst few pages are devoted to allowable limits and then methods of treatment for activation of the clays. A comprehensive list of the applications of the various mineral clays to specifi c heavy metals is provided. The complex structures and structural modifi cations of the wide range of clay minerals available has prompted workers in the fi eld to develop new isotherm mathematical models for evaluating adsorption parameters. The chapter presents a list of the more recently developed isotherm models in addition to the more common Langmuir and Freundlich models and the clays to which they have been applied. Unfortunately, the chapter is already quite long and there was no critical evaluation of these models.

Chapter 14, ‘Use of Industrial and Agricultural Waste in Removal of Heavy Metals Present in Water’ describes a wide range of materials for this application (everything from banana peel to walnut dust), along with the methods used to modify them and their capacities for divalent metals and chromium. The authors discuss the methods in detail and some interesting images are included that describe the surface changes resulting from modifi cation of the surface of the waste product.

Chapter 15, ‘Biosorption of Metals – From the Basics to High Value Catalysts Production’ targets biosorption by living organisms. The fi rst six pages of the chapter, devoted to the sources and toxic effects of the metals arsenic, cadmium, chromium, copper, nickel, lead and zinc, are probably not necessary in light of the other chapters. The chapter goes on to briefl y describe the mechanisms of biosorption and the parameters

affecting effi ciency. The chapter would have benefi tted from more details on these topics rather than devoting half the chapter to information already covered elsewhere in the book.

Contamination Problems

Overall Chapter 5, ‘Arsenic Contamination: An Overview’ is an excellent chapter that summarises all aspects of the most pervasive water contaminant worldwide. The natural and anthropogenic sources of arsenic contamination are nicely described and the health risks and the different methods of arsenic removal are explained. In the conclusions section the reader is left with the impression that none of the currently available removal techniques are in widespread use. Although all the current methods have their disadvantages, processes using iron(III) chloride (FeCl3) precipitation, adsorption onto Fe particles (the ferrihydrite process) and composite materials are commercially available and are being used effectively. The most recent advances using ion exchange technologies are not covered at all and this is a glaring omission in an otherwise excellent review. The properties of elemental arsenic are listed, however, this has nothing to do with the topic of this chapter and should have been deleted.

Chapter 6, ‘Removal of Iron and Manganese from Water – Chemistry and Engineering Considerations’ deals with removal of these metals from ground and surface waters. The chapter begins with an excellent description of the aqueous redox chemistry of iron and manganese. These metals rank low on the toxicity index and are mainly a problem for the construction industry. Indeed, this chapter is a contribution from a civil engineering fi rm. The chapter goes on to describe the effective oxidation-fi ltration systems used to remove these metals in the construction industry. There are many other commercially available adsorbents for removal of these metals but none of these are discussed in this chapter and so it is a narrowly conceived contribution to the volume.

Chapter 13, ‘Fluorides in Different Types of Aquatic Systems and their Correlation with Metals and Metalloids’ deals with fl uoride contamination arising primarily from the use of fl uorine containing industrial chemicals. Sulfur tetrafl uoride (SF4) used for termite extermination in the USA is missing from the otherwise



fairly complete list. The chapter was hard to follow. The salt contents of an apparently random list of water sources are given in a table but are not discussed in the text. The authors attempt to correlate the presence of fl uoride with various cations and estimates of Al/F speciation as a function of pH are discussed in detail. The authors go on to explain the correlation between the presence of fl uoride with arsenic in ground waters but the discussion and the data presented are confusing and unconvincing. A useful but, perhaps over interpreted chapter overall.

Chapter 12, ‘Heavy Metals in Tannery Wastewater and Sludge: Environmental Concerns and Future Challenges’ and Chapter 16, ‘Chromium in Tannery Wastewater’ both deal with the problem of chromium in the wastewater and sludge associated with tanning leather. The chapters are complementary rather than redundant. Chapter 12 focuses more on the distribution of tanning sites worldwide and the demographics of risk from the toxic effects of Cr(VI). The legal discharge limits for Cr in various countries are included with some countries providing only Cr(III) limits, and estimates of the most at-risk populations are given. Chapter 16 contains much more detail on the chemistry of the tanning process and methods of treatment (Figure 1) including recovery and reuse of the chromium salts.

Taken together the two chapters provide an excellent overview of the challenges facing the tanning industry today.

Chinese Economy

Chapter 7, ‘Heavy Metal Pollution in Water Resources in China – Occurrences and Public Health Implications’ and Chapter 8, ‘Heavy Metals Distribution in Surface Water Samples of Taihu Lake, China’ are unique in this volume in that they both deal with the distribution of heavy metals in the environment. Chapter 7 deals with the sources of heavy metals across the Chinese economy while Chapter 8 focuses on the details of metal pollutants in one of China’s largest lakes (Figure 2). Both chapters contain an enormous amount of information that will be useful for planning future environmental clean-up in China and for foreign investors in the Chinese economy looking to get involved in balancing Chinese economic growth with sustainability.

Summary

The volume covers the fi eld of heavy metals in water very well for the most part. In general the chapters

CrVICrVI

CrVI

CrVI

CrIII CrIII

CrIII CrIII

Na+

Na+

The bacteria reduces Cr(VI) to Cr(III) and then the Cr(III) is retained in the zeolite by ion exchange

Formation of a biofi lm

Heat treatment – calcination

Catalysts to be applied in oxidation reactions of volatile organic compounds

M M

Fig. 1. New method of chromium removal from wastewater and catalytic reutilisation in volatile organic compounds oxidation (Reproduced by permission of Royal Society of Chemistry)



are well written and organised for facile retrieval of data. The references are recent and cite the most important journals in the fi eld. The one glaring omission is the cursory treatment of ion exchange. In the USA the use of ion exchange materials constitutes a

US$5 billion dollar a year market and represents the go-to technology for heavy metal removal from water. The volume would also have benefi tted from more careful editing. As mentioned several times in this review there is too much repetition of the sources, health effects and allowable release levels of the various heavy metal pollutants. This could have been covered in one introductory chapter (as it is) and omitted from the subsequent chapters. Overall however, this volume is a useful addition to the area of heavy metal pollution and remediation.

N

E

S

W

0 5 10 20Kilometers1:500,000

0–0.010.01–0.020.02–0.050.05–0.1

Pb, mg l–1

Fig. 2. Lead concentration distribution in Taihu Lake, China (Reproduced by permission of Royal Society of Chemistry)

The Reviewer

Edward Rosenberg received his doctorate at Cornell University, USA, and held post-doctoral fellowships at the University of London, UK, and the California Institute of Technology, USA. He is the author of 180 peer-reviewed publications, fi ve boo k chapters, eight patents and one book in the areas of environmental and organometallic chemistry. He has received awards for his research and student mentoring from the University of Montana and has had visiting faculty fellowships in Italy, Israel and South Africa.

“Heavy Metals in Water: Presence, Removal and Safety”




“Particle-Stabilized Emulsions and Colloids: Formation and Applications”Edited by To Ngai (The Chinese University of Hong Kong, China) and Stefan A. F. Bon (University of Warwick, UK), RSC Soft Matter Series, No. 3, Royal Society of Chemistry, Cambridge, UK, 2015, 337 pages, ISBN: 978-1-84973-881-1, £175.00, €218.75, US$290.00

Reviewed by Cecilia BernardiniJohnson Matthey Technology Centre,Blounts Court, Sonning Common, Reading, RG4 9NH, UK


“Particle-Stabilized Emulsions and Colloids: Formation and Applications”, edited by To Ngai and Stefan A. F. Bon, is the third book of the Royal Society of Chemistry (RSC) Soft Matter Series, published in 2015. Both editors have extensive expertise in polymer chemistry and its application to colloid science. Professor Ngai’s research interests focus on interparticle interactions at fl uid interfaces and using emulsions as templates for functional materials, whereas Professor Bon’s current research area is supracolloidal polymer chemistry, focusing on the design of assembled supracolloidal structures and the synthesis of their colloidal and macromolecular building blocks through a combination of polymer chemistry, colloid science, soft matter physics and chemical engineering.

This series, edited by Hans-Jürgen Butt, Ian W. Hamley, Howard A. Stone and Chi Wu, provides a review of recent developments in soft matter research. The scope of this volume is quite focused: the book

is devoted to the use of solid particles as a means to stabilise emulsions and more complex colloidal systems. The ambition of the book is to offer a comprehensive overview of not only the fundamental science behind Pickering emulsions and their stabilisation mechanism, but also of the current and future range of useful industrial applications, with the aim of fostering further development of these emerging technologies. The target audience is therefore the colloid science community at large, both in academia and in industry, rather than a general, non-specialised audience. Given the broad scope of the applications illustrated, only a selection of the most relevant chapters will be reviewed.

The Pickering Stabilisation Phenomenon

The fi rst chapter is written by Stefan Bon and it is a very short and basic introduction to the Pickering stabilisation phenomenon, with a brief historical perspective. The following chapter, authored by Bum Jun Park (Kyung Hee University, South Korea), Daeyeon Lee (University of Pennsylvania, USA) and Eric M. Furst (University of Delaware, USA), is a more extensive description of the physical-chemical interactions of particles adsorbed at fl uid-fl uid interfaces: from the wettability of a single particle, homogeneous or amphiphilic, to more complex topics, such as the interactions between pairs of



homogeneous and amphiphilic particles, with a focus on effects of geometrical anisotropy and non-spherical objects. Pair interactions are discussed not only from a theoretical viewpoint, but also by illustrating direct measurements done with optical laser tweezers and then related to bulk property measurements; further experiments reviewed include the effect of additives (salt and surfactant) and the evolution of interactions with time.

Polymer-brush Modifi ed Particles

Chapter 3 is nearly entirely dedicated to applications of polymer brush-modifi ed clay layers or gold nanoparticles (AuNPs) in Pickering emulsions. The chapter has been written by Hanying Zhao and Jia Tian (Nankai University, China). Brushes are generally sought after for their responsiveness to environmental conditions. First clay layers with block copolymer brushes are discussed: examples of preparations with poly(dimethylaminoethyl methacrylate) (PDMAEMA) and poly(methyl methacrylate) (PMMA) block copolymers are given. Then clay layers with homopolymer brushes are presented; fi nally examples of mixed homopolymer brushes and hydrophilic faces with hydrophobic polystyrene (PS) brushes on the edges are introduced.

Next the topic of AuNPs, functionalised with polymer brushes to stabilise emulsions, is highlighted. Interesting NP complexes with core-shell structures are made from AuNPs and iron oxide NPs with PS brushes. AuNP-stabilised emulsions are used as templates to fabricate hollow hybrid capsules.

Finally, the stabilisation of emulsions by Janus disks is summarised, with examples that include preparing amphiphilic Janus Laponite disks at the oil-water interface (Figure 1) and using metal-supporting Janus particles as interfacial catalysts.

Pickering Suspension, Mini-Emulsion and Emulsion Polymerisation

The terminology used in Chapter 4 by Stefan Bon is quite technical and confusion is likely for those who are unfamiliar with these topics. The opening paragraph explains the peculiarities of each system, followed by historical perspective and the more recent developments in suspension polymerisation, such as preparing deliberately armoured composite polymer particles. Examples of suspension polymerisations include using titanium dioxide, Laponite particles and iron oxide with different polymers and formulating inverse Pickering suspension polymerisation systems.

Pickering mini-emulsion polymerisation was fi rst reported in the literature by Landfester et al. in 2001 (2).

PS particle

Laponite disks

Solvent

Solvent

Solvent

Micelles

A

B

C

Fig. 1. Schematic outline for the synthesis of Janus Laponite disks: A a Laponite disk with hydrophobic PS brushes on one side and hydrophilic quaternised PDMAEMA on the other side; B a Laponite disk with PS brushes on one side; C a polymeric micelle with Laponite disks (Reprinted with permission from (1). Copyright (2013) American Chemical Society)



More examples of using Laponite disks as stabilisers are given. A successful synthesis needs particles that are able to adhere and cover the emulsion droplet, thus curvature is crucial and constraints on the size of particles are present: typically the particles need to be less than 200 nm in diameter, unless fl exibility and bending around the droplet are possible.

The fi nal part of the chapter concentrates on Pickering emulsion polymerisation: general references are suggested for further reading, and then a standard recipe for a free radical emulsion polymerisation is presented. The difference, in the case of Pickering emulsions, is the replacement of surfactants with solid, often nanosized particles: clay disks, amphiphilic polymer Janus particles, and silica NPs. An extensive historical overview of the fi eld is provided with several examples, followed by a mechanistic insight on the process, although work on the area is still limited. The outlook on technological application of armoured nanocomposite polymer latexes is promising, although still in its infancy: the latter developments in the synthesis techniques will allow more applied studies and a potential application has recently been reported by Wang et al. (3), where soft armoured latexes added to waterborne adhesives induce a marked increase in tack adhesion energy.

Bicontinuous Emulsions Stabilised by Colloidal Particles

Chapter 6 by Joe W. Tavacoli (Université Paris-Sud, France), Job H. J. Thijssen and Paul S. Clegg (University of Edinburgh, UK) describes how interfaces densely coated by particles can behave like an elastic sheet. Therefore unlike surfactant-stabilised systems, they do not necessarily form spherical droplets and their shape is dependent on the process history, allowing formation of liquid bicontinuous architectures (called bicontinuous interfacially jammed emulsion gels or bijels), which consist of two tortuously entwined percolated liquid phases that are separated and stabilised by solid particles. To adopt a bicontinuous morphology, two immiscible liquids must be induced through a critical quench, while the liquid-liquid interface is populated by neutrally wetted particles. Bicontinuous domains evolve when the system is quenched into the spinodal region of its phase diagram. This process was fi rst described by Stratford et al. (4) by computational methods and later experimentally obtained by Herzig et al. (5), using a water-lutidine system and Stoeber

silica particles. The characteristic size of bijels domains follows from the ratio of the particle diameter and the particle volume fraction used: this allows tuning of this size from the nanoscale to hundreds of microns. Other liquids successfully tested are nitromethane and ethanediol.

Apart from molecular liquids, polymers are also used to obtain bijels. Notably, there are some key differences with molecular liquids, thoroughly discussed in the chapter.

An essential element in the fabrication of bijels is the particles: colloidal silica particles have been used extensively, but more recently more exotic particles have been investigated as alternatives, (mainly with simulations) such as anisotropic or fi eld-responsive particles, magnetic particles and graphene oxide (GO) sheets.

Characterisation of bijel morphology is then introduced, followed by studies on the link between morphology and mechanical properties. Among the up-and-coming applications investigated so far, the use of three-dimensional (3D) bijels (Figure 2) to create porous materials is mentioned.

Materials with bicontinuous structures can be used in catalysis, sensors and gas storage, allowing simultaneous optimisation of active surface area and mass transport. Other potential applications include delivery (for example a dye and a bleach in haircare products or separate chemical reactants that form the desired product if released simultaneously at target, through a specifi c trigger) through bijel capsules and development of cross-fl ow microreactors. The challenges are the scaling up of fabrication, cost

Fig. 2. Silver monolith with continuous pores on two widely separated length scales (Reprinted with permission from (6). Copyright (2011) American Chemical Society)

100 μm

100 nm



of materials and use of benign materials instead of hazardous components. Finally, after dealing with liquid-liquid bicontinuous systems separated by a particle-coated, solid interface, the idea of creating a structure which has two co-continuous solid domains separated by a single liquid domain, called a bigel, is considered.

Hollow Spheres and Microcapsules Fabrication through Particle-stabilised Emulsions

Chapter 9, written by Simon Biggs (The University of Queensland, Australia) and Olivier Cayre (The University of Leeds, UK), explains the fabrication of colloidosomes, hollow core-shell microcapsules in which the capsule wall consists of close-packed colloidal particles that have been permanently locked to each other. Colloidosomes display a range of unique features: the shell thickness can be manipulated by choosing different-sized colloids; the porosity of the shell can be similarly adjusted; the system has inherent fl exibility due to the large range of particles that are used; and using particles solely as stabilisers of the emulsion yields enhanced stability.

The manufacture of colloidosomes include three main categories of methods that can lock the particles at the interface. The fi rst method involves using a sol-gel or a polymerisation reaction, either in the droplet or at the interface, leading to the formation of a shell that entraps the colloid particles permanently. A second method uses precipitation, by solvent extraction, of an existing polymer added to the dispersed phase. Another way to solidify the capsule surface is to perform a physical or chemical modifi cation of the particles, for instance fusing, by heating the particles above their glass transition temperature, or by using chemical cross-linkers to bind adjacent particles. Finally it is possible to adsorb one or more additional layers of polymer or polyelectrolyte onto the particle monolayer with two main advantages: adjacent particles are bridged on the surface of emulsion droplets and the permeability of the microcapsule shell so obtained is further controllable, and usually decreased, by this route. The main drawback consists of extra washing steps to remove the excess of stabilising particles and polymer or polyelectrolyte.

When the capsules have been synthesised, post-processing involves removing the oil phase to obtain a water-in-water microcapsule dispersion: often a gelling agent is added to the aqueous phase to impart

higher mechanical strength and more control over the release of encapsulated agents. The large variety of particles used allows for an easy introduction of functionality onto the microcapsule surface: magnetic particles can be used to direct the microcapsules into an area of the vessel or onto a specifi c delivery target. Other responsive materials have been used too. The main use of colloidosomes is for encapsulation and subsequent controlled release of drugs or substances: of more widespread interest for commercial applications are the approaches for encapsulation of highly volatile, low molecular weight molecules which are poorly soluble, or sensitive to their environment (perfumes), however none so far have succeeded in forming an impermeable shell. The variable pore size features have also been explored in order to encapsulate larger molecular weight materials or even NPs.

Conclusion

The book gives a very extensive coverage of particle-stabilised emulsions and it is a useful reference for industrial or academic researchers who are already familiar with the colloid science fi eld, but need to deepen their knowledge into this rather specifi c, although vast, branch of colloid science. The content is very technical and the style of presentation of the different topics is rather heterogeneous throughout the chapters: both aspects hinder somewhat the overall readability. Frequent overlapping of closely-related themes, developed to a different degree of extent, or across several chapters, can sometimes confuse the reader. As such, the volume is more suitable to a specialised audience rather than to a general one, with little or no previous knowledge of the areas covered by the book.

“Particle-Stabilized Emulsions and Colloids: Formation and Applications”



References1 J. Liu, G. Liu, M. Zhang, P. Sun and H. Zhao,

Macromolecules, 2013, 46, (15), 5974

2 F. Tiarks, K. Landfester and M. Antonietti, Langmuir, 2001, 17, (19), 5775

3 T. Wang, P. J. Colver, S. A. F. Bon and J. L. Keddie, Soft Matter, 2009, 5, (20), 3842

4 K. Stratford, R. Adhikari, I. Pagonabarraga, J.-C. Desplat and M. E. Cates, Science, 2005, 309, (5744), 2198

5 E. M. Herzig, K. A. White, A. B. Schofi eld, W. C. K. Poon and P. S. Clegg, Nature Mater., 2007, 6, (12), 966

6 M. N. Lee and A. Mohraz, J. Am. Chem. Soc., 2011, 133, (18), 6945

The Reviewer

Cecilia Bernardini studied Chemistry at Universitá degli Studi di Milano, Italy, and obtained a doctorate degree in colloid science in 2012 at Wageningen Universiteit, The Netherlands, under the supervision of Professor Martien Cohen-Stuart and Professor Frans Leermakers. Since July 2012 she works as a Coating Scientist at Johnson Matthey Technology Centre, Sonning Common, UK, on process chemistry research for automotive catalyst applications.




Interplay between Silver and Gold Nanoparticles in Production of Hydrogen from MethanolDeveloping a highly stable bimetallic catalyst for fuel cell applications

By Hany M. AbdelDayemChemistry Department, Faculty of Science, Ain Shams University, 11566 Abbassia, Cairo, Egypt

Email: [email protected]; [email protected]

Hydrogen production from methanol oxidation over silver-gold/zinc oxide (AgAu/ZnO) catalysts was investigated. Bimetallic catalysts produced higher hydrogen yield and lower carbon monoxide and water yields than Ag/ZnO catalyst without deactivation during 72 h on stream at 250ºC. In addition, the presence of Au in the bimetallic catalyst facilitated the preferential oxidation of CO to CO2. Structural analysis of bimetallic catalysts indicated that the strong interaction between Ag and Au particles in the nano-range (4.2 nm–7.2 nm) effi ciently enhanced the reducibility of non-selective silver oxide (Ag2O) species. Furthermore dispersion of metal particles in bimetallic AgAu/ZnO catalysts did not signifi cantly change after reaction; however, dispersion of Ag species in Ag/ZnO catalyst was remarkably decreased.

1. Introduction

Direct partial oxidation of methanol (POM) to hydrogen reduces the complexity of hydrogen-fuelled proton exchange membrane (PEM) fuel cells. Methanol can be easily oxidised into hydrogen at relatively low temperatures (<250ºC) (1, 2). In addition, it contains no carbon-carbon bond and it has a high H:C ratio

therefore reducing the risk of coke formation and catalyst fouling.

Bimetallic catalysts based on noble metals (gold, silver and platinum) and copper are known to be more active for hydrogen production from methanol than monometallic ones (3–18). Ag has the lowest price among noble metals, which makes it ideal for use as an industrial oxidation catalyst. The electrochemical and steam reforming activity of bimetallic catalysts based on Ag has been studied extensively (18–22). However, there are few studies in the literature dealing with direct POM to hydrogen on Ag-based catalysts (9, 23–25). Recently, Ag/ZnO catalyst was found to be active for POM to hydrogen; however, the catalyst produced high CO yield (~6%) and was rapidly deactivated (25). On the other hand, bimetallic combinations such as Au-Ag signifi cantly improved the activity and the stability of Ag catalyst in CO oxidation to CO2 at low temperature (26–32). Furthermore, much attention has been focused on using Ag-Au bimetallic catalysts for other important reactions such as oxidation of alcohols, dechlorination of organochlorides, hydrocarbon-selective catalytic reduction of nitrogen oxides (NOx), decomposition of organic pollutants, hydrogenation of esters and ethylene oxidation (33–40). The activity in a desired application is determined by the oxidation state of the reactive species, interaction between Ag and Au particles, the particle size, shape and location on the support controlled by the preparation process and the nature of the support (41, 42).

The main objective of this work was to develop a highly stable bimetallic catalyst based on noble metals (Ag and Au) with high performance in the oxidation



of methanol to hydrogen with low CO formation. The effect of adding Au on the physicochemical properties of Ag/ZnO was also studied.

2. Experimental2.1 Catalysts Preparation2.1.1 Preparation of Zinc Oxide Support

Nano-sized ZnO support was synthesised by direct precipitation (25). Analytical grade zinc nitrate (Zn(NO3)2) and ammonium carbonate ((NH4)2CO3) (Sigma-Aldrich, 99.5%) were fi rst dissolved in deionised water to form solutions of 1.5 mol l–1 and 2.25 mol l–1, respectively. The Zn(NO3)2 solution was slowly poured

into the (NH4)2CO3 solution with vigorous stirring and then the precipitate derived from the reaction was collected by fi ltration and rinsed three times with high-purity water and ethanol. The product was dried at 80ºC to form the ZnO precursor. Finally, the precursor was calcined at 550ºC for 2 h in a muffl e furnace to obtain nanoscale ZnO particles. The average crystal size of ZnO was ca. 35.2 nm, calculated from X-ray diffraction (XRD) by the Debye-Scherrer formula (25).

2.1.2 Preparation of Monometallic (Silver or Gold) Catalysts

The preparation of either Au/ZnO2 or Ag/ZnO2 catalyst with 5.0 wt% as theoretical loading was performed by deposition-precipitation (DP) with sodium carbonate (Na2CO3) at pH 8.5. Gold(III) chloride trihydrate (HAuCl4·3H2O) and silver nitrate (AgNO3), both from Sigma-Aldrich, were used as Au and Ag precursors. ZnO support was suspended in an aqueous solution of metal precursor, then the pH was controlled by the addition of 0.5 M Na2CO3. After DP, all samples were centrifuged, washed with water four times, centrifuged again and dried under vacuum for 2 h at 80ºC. After drying, the samples were stored at room temperature in desiccators under vacuum, away from light, in order to prevent any alteration (26, 43). Catalysts were calcined at 300ºC for 3 h.

2.1.3 Preparation of Bimetallic Catalysts

Preparation of bimetallic Ag1–yAuy (y = 0.1, 0.25 and 0.5, where y is the mass fraction of Au with respect to sum of weights of Au and Ag) catalysts supported on ZnO with 5.0 wt% as theoretical loading were also performed by DP with Na2CO3. The oxide support was suspended in an aqueous solution of HAuCl4·3H2O and AgNO3. The initial pH was ~3, which was then adjusted

to 8.5 by drop wise addition of 0.5 M Na2CO3 to promote metal hydroxide precipitation on zinc peroxide (ZnO2).The obtained samples were washed, dried, stored and calcined as described above.

2.1.4 Preparation of Silver and Gold Mechanical Mixture

The mechanical mixture (Ag0.5Au0.5Zn)mix catalyst was prepared by dispersing equal amounts of both 2.5 wt% Ag/ZnO and 2.5 wt% Au/ZnO powders (prepared by DP) in 200 ml n-pentane to give a total of 5 wt% metals as theoretical loading. The suspension was stirred vigorously for 20 min and then ultrasonically for 5 min. The n-pentane was evaporated at 40ºC and the obtained solid was dried at 100ºC overnight without further calcination.

2.2 Catalysts Characterisation

The Ag and Au content in these catalysts was determined by atomic absorption spectroscopy (AAS) on a Perkin Elmer model 3100. XRD measurements were performed on a Philips X’Pert multipurpose X-ray diffractometer (MPD) using Cu Kα1,2 radiation (λ = 1.5405 Å) for 2θ angles varying from 10º to 80º. Hydrogen temperature-programmed reduction (H2-TPR) was performed using a ChemBET 300 Quantachrome. 100 mg sample of the freshly calcined catalyst was subjected to a heat treatment (20ºC min–1 up to 1000ºC) in a gas fl ow (85 ml min–1) composed of a mixture of 5 vol% hydrogen and 95 vol% nitrogen. Prior to the TPR experiments, the samples were heated for 3 h under an inert atmosphere (nitrogen) at 200ºC. The surface areas (SBET) of the various samples were determined from the adsorption of nitrogen gas at liquid nitrogen temperature (–195.8ºC) using a NOVA3200e (Quantachrome Instruments, USA). Before the measurements, all samples were perfectly degassed at 150ºC and 10–4 Torr overnight. Transmission electron micrographs were obtained using a JEOL 1200 EX II transmission electron microscope (TEM) operated with an acceleration voltage of 50 kV. Nitrous oxide (N2O) pulse chemisorption was applied to determine the Ag degree of dispersion using ChemBET 3000 and the TPR-Win V. 1.50 software; further details can be found elsewhere (25). Energy dispersive X-ray analyses (EDX) were recorded using a Quanta FEG 250 microscope, equipped with EDX spectrometer (TexSEM Laboratories (TSL) EDAX, AMETEK, Inc, USA). Ultraviolet-visible (UV-Vis) refl ection spectra were recorded on a JASCO V-570 spectrophotometer.



2.3 Catalytic Test

Catalytic tests were performed at atmospheric pressure in a tubular quartz reactor with 6 mm internal diameter. The reaction was carried out at 250ºC and in a differential mode at conversion ca. 5% by varying the space velocity through changing the catalyst weight. The feed and product gas compositions were determined by online gas chromatography (GC), using a Bruker 450 GC equipped with three channels. The fi rst is for hydrogen analysis using a thermal conductivity detector (TCD). The gas separation was performed by HayeSep Q and 5 Å molecular sieves. Channel two is for analysing non-fl ammable gases (O2, N2, CO and CO2) using TCD and separation was accomplished by HayeSep Q and MolSieve 13X columns connected in series. The third channel is for analysing oxygenates (methanol, formic acid and formaldehyde) and separation was accomplished by HayeSep Q and Varian SelectTM columns. The catalyst was diluted with silicon dioxide (SiO2) to 10 wt% to prevent hot-spot formation in the bed. The catalyst activation was performed in situ by exposing the catalyst to 100 ml min–1 of 10% H2/N2 and increasing the temperature to 250ºC at 10ºC min–1. This temperature was maintained for 1 h. Subsequently, the furnace temperature was lowered to ~100ºC. The partial oxidation experiments were performed under a total fl ow rate of 220 ml min–1 with an O2/methanol molar ratio of 0.5, balanced with nitrogen and the weight hourly space velocity (WHSV) was from 8.8 × 104 ml h–1 g–1 to 13.2 × 104 ml h–1 g–1 (WHSV = fl ow rate of feed gas (ml h–1) per weight of catalyst (g)). The catalysts’ stability

was examined followed by the change of the rate of hydrogen production at a higher temperature i.e. 350ºC and at iso-conversion 5% by changing WHSV from 14.7 × 104 ml h–1 g–1 to 18.9 ×104 ml h–1 g–1. Furthermore, the external and internal mass transfer limitations of the catalytic system were tested. Catalytic tests were carried out using different particle sizes of AgZn and Ag0.5Au0.5Zn catalysts in the range 125 μm–1000 μm at the constant WHSV = 8.8 × 104 ml h–1 g–1 and 13.2 × 104 ml h–1 g–1, respectively. The results showed that catalyst of particle sizes 212 μm to 710 μm exhibited constant conversion without pressure drop at the constant WHSV. Thus the catalysts were ground and sieved to 355 μm−500 μm. In addition, the calculation of effectiveness factor (η) for AgZn sample with an average particle diameter of 356 μm showed that the values are close to 1.0; at iso-conversion 5%, T = 350ºC and WHSV = 8.8 × 104 ml h–1 g–1, indicating no internal diffusion limitation in the catalytic system. The activity of the catalyst was expressed in terms of experimental rate (Rateexpt = mmole of products per gram catalyst). The following formula (Equation (i)) was used to calculate the theoretical rate (Ratetheort) of hydrogen production over AgAuZn catalysts:

Ratetheort = Rateexpt of AgZn (1–y) + Rateexpt of AuZn (y) (i)

3. Results and Discussion

In the case of bimetallic samples, the actual Ag and Au fractions are also very close to the nominal values (Table I). Table I shows the measured values of the

Table I Characterisation of the Catalysts

Catalyst Nominal fraction Ag, %a Au,

%aActualfraction

SBET, m2 g–1b

Metal dispersionc, %

XRD crystal sizef

ZnO – – – – 38.8 –

AgZn – 4.6 0 – 36.3 42.0 (28.1)e 5.8

AuZn – 0 4.5 – 36.9 46.9 (32.0) 4.2

Ag0.9Au0.1Zn 0.1 4.3 0.4 0.09 36.4 41.5 (37.3) 6.1

Ag0.75Au0.25Zn 0.25 3.5 1.1 0.24 35.7 39.8 (38.2) 6.4

Ag0.5Au0.5Zn 0.5 2.3 2.2 0.49 37.0 38.3 (37.1) 7.2a Weight percentage from atomic absorption spectroscopyb From N2 adsorptionc Ag and/or Au e Value in parenthesis is metal dispersion after reactionf Calculated from XRD peak at 2θ ~38.1º



BET surface areas of catalysts. Interestingly, the BET surface area of the ZnO support was not signifi cantly altered after loading of Ag or Au or both. The results of N2O chemisorptions shows that the degree of dispersion of Ag in Au-containing catalysts (i.e. bimetallic samples) was less than in AgZn by ca. 4% (shown in Table I). This may suggest an interaction between Ag and Au particles in the bimetallic catalysts. The lower dispersion of Ag and Au particles on ZnO in bimetallic catalysts was clearly verifi ed by an increase of crystallite size of metals measured by XRD (Table I).

Figure 1 shows XRD patterns of AgZn, AuZn and Ag0.5Au0.5Zn catalysts. All of the diffraction peaks of ZnO could be indexed to the hexagonal phase reported in the Joint Committee on Powder Diffraction Standards (JCPDS) fi le (36-1451). For AgZn catalyst, the peaks characteristic of the cubic Ag2O phase were detected at 2θ = 38.08º and 44.29º (JCPDS fi le, 65-3289 and 42-0874). In the case of AuZn catalyst the peaks at 2θ = 38.17º and 44.38º are characteristic of the single pure metallic Au phase (JCPDS 04-0784). However, in the case of Ag0.5Au0.5Zn catalysts, Au-Ag alloy could not be distinguished from Ag2O and metallic Au based on the XRD patterns because the diffraction lines characteristic of Au and Ag are overlapped.

Based on both XRD and TEM results, both Ag particles and Au particles supported on ZnO in monometallic and bimetallic catalysts were in the nano-range 4.2 nm–7.2 nm (see Table I and Figure 2). The average size of the Ag particles in the bimetallic Ag0.5Au0.5Zn catalyst (Figure 2(b)) was larger than that in the

monometallic AgZn catalyst (Figure 2(a)). Furthermore, as shown from the TEM micrograph of (Ag0.5Au0.5Zn)mix, the contact between Ag and Au particles was lower than that between these particles in Ag0.5Au0.5Zn (Figure 2(c)). This suggests that Ag-Au ensembles may be formed due to the interaction between Ag and Au. On the other hand, the elemental ratio of Au:Ag by EDX for Ag0.5Au0.5Zn catalyst was 0.86 as shown in the EDX pattern (Figure 3(a)), i.e. the Ag-Au composite composition was approaching 1:1. However, the Au-Ag ratio for the same catalyst prepared by mechanical

Fig. 2. TEM micrographs of the catalysts: (a) AgZn; (b) and (c) Ag0.5Au0.5Zn prepared by DP method at different magnifi cations from ×100,000 to ×350,000 and (d) (Ag0.5Au0.5Zn)mix prepared by mechanical mixing

5 nm(a)

(b)

(c)(d)

10 nm

2.5 nm 2.5 nm

Au

AuAu

Ag

Ag

Ag

Ag

10 20 30 40 50 60 70 802, º

Inte

nsity

, arb

itrar

y un

itsAg0.5Au0.5Zn

AgZn

AuZn

37 39 41 43 45 472, º

Inte

nsity

, arb

itrar

y un

its

Ag0.5Au0.5Zn

AgZn

AuZn

Fig. 1. Wide-angle XRD patterns of the AgZn, AuZn and Ag0.5Au0.5Zn catalysts. Figure inset XRD patterns of the same catalysts in the range 37º to 47º. Peaks marked by the symbols “□”, “+” and “●” indicate the peaks assigned to ZnO, Ag2O and metallic Au, respectively



mixing was found to be 1.4, as estimated from the EDX pattern (Figure 4(a)). It was evident that the outer layers of the bimetallic mixture in this case is enriched in Au. From EDX mapping of the Ag0.5Au0.5Zn catalyst (Figures 3(b) and 3(c)), the contrast between Ag and Au was fairly clear in the homogeneous distribution occupying the same location on the ZnO top surface i.e. in good contact (44). However the EDX mapping of the mechanically mixed composition (Figures 4(b) and 4(c)) shows remarkably aggregated Ag nanoparticles on the top surface of ZnO indicating improper contact between the composites.

Figure 5 shows the H2-TPR patterns of AgZn, AuZn and Ag1–yAuy/ZnO (y = 0.25 and 0.5) catalysts. The TPR pattern of AgZn catalyst showed only a major peak at 192.5ºC and AuZn catalyst showed one main weak TPR peak at 132.4ºC. These peaks indicated the presence of Ag2O and gold oxide in the AgZn and AuZn catalysts (45, 46). The fact that the XRD analysis of AuZn catalyst did not reveal the presence of any Au oxide species (Figure 1), this may suggest that gold oxide crystallites are highly dispersed on the surface and/or Au crystallite sizes are smaller than 5 nm (47). Although AgZn and AuZn have approximately the same

0 1.3 2.6 3.9 5.2 6.5 7.8 9.1 10.4 11.7 13.0Energy, keV

70.2

62.4

54.6

46.8

39.0

31.2

23.4

15.6

7.8

Cou

nt, a

rbitr

ary

units

(a)

25% O K

7% Ag L

62% Zn K

6% Au L

Zn L

O K

Ag MAg L2

Ag LAg LAu M

Zn K

Zn KAu L

Au LAu L3

Au L

1 m

1 m

(b)

(c)

Fig. 3. Ag0.5Au0.5Zn: (a) EDX spectrum; (b) EDX mapping for Ag element: the region of mapping corresponds to (a), acquisition time 665.4 s; (c) EDX mapping for Au element; acquisition time 655.4 s

1 m

1 m0 1.3 2.6 3.9 5.2 6.5 7.8 9.1 10.4 11.7 13.0

Energy, keV

50

45

40

35

30

25

20

15

10

5

Cou

nt, a

rbitr

ary

units

(a)

16% O K

5% Ag L

72% Zn K

7% Au L

Zn L

O K

Ag M

Ag L2Ag L

Ag LAu M

Zn K

Zn KAu L

Au LAu L3

Au L

(b)

(c)

Fig. 4. (Ag0.5Au0.5Zn)mix: (a) EDX spectrum; (b) EDX mapping for Ag element: the region of mapping corresponds to (a), acquisition time 665.4 s; (c) EDX mapping for Au element; acquisition time 655.4 s



amount (wt%) of Ag and Au (Table I), yet the hydrogen consumption for AuZn was markedly lower than that for AgZn sample (Figure 5). This indicates that the majority of Au nanoparticles exist in a metallic state. On the other hand, the TPR features of Ag1–yAuy/ZnO catalysts did not signifi cantly change compared with AgZn. However, adding Au to AgZn catalyst promoted Ag2O reduction, namely the TPR peak characteristics of Ag2O reduction was shifted toward a lower temperature. As the Au content increased from 1.25 wt% to 2.5 wt% the main peak shifted from 179.8ºC to 163.2ºC.

The UV/Vis diffuse-refl ectance spectra of the monometallic Ag/ZnO and Au/ZnO catalysts as well as three bimetallic catalysts (Ag0.75Au0.25Zn, Ag0.5Au0.5Zn and (Ag0.5Au0.5Zn)mix) are compared in Figure 6. As evident from Figure 6 curve A for Ag/Zn and curve E for Au/Zn, the obtained spectra of the monometallic catalysts reduced at 300ºC show a broad absorption band due to the surface plasmon resonance (SPR) of Ag and Au nanoparticles at ca. 480 nm and 546 nm, respectively (26, 48). In addition, one plasmon band was observed for each bimetallic system and

0 50 100 150 200 250 300 350 400Temperature, ºC

Ther

mal

con

duct

ivity

sig

nal,

arbi

trary

uni

tsAg0.75Au0.25Zn

AgZn

AuZn

Ag0.5Au0.5Zn

700

600

500

400

300

200

100

0

Fig. 5. Temperature-programmed reduction profi les of monometallic AgZn and AuZn catalysts and bimetallic Ag1–yAuyZn (y = 0.25 and 0.5) catalysts

360 460 560 660 760 860 960 1060 1160Wavelength, nm

Refl

ect

ion,

arb

itrar

y un

its

60

50

40

30

20

10

0

Fig. 6. UV-visible spectra of diffuse refl ectance of the monometallic and bimetallic catalysts: A Ag/Zn; B Ag0.75Au0.25Zn; C Ag0.5Au0.5Zn; D (Ag0.5Au0.5Zn)mix; E Au/ZnO

0 400 800 1200 1600 2000Wavelength, nm

Refl

ect

ion,

arb

itrar

y un

its

ZnO

E

D

C B

A



the plasmon maximum was red-shifted from 480 nm to 540 nm with increasing Au content as shown in Figure 6 curves B and C, suggesting the formation of Au-Ag alloy (49). However the plasmon band characteristic of (Ag0.5Au0.5Zn)mix (Figure 6 curve D) was wider than that of Ag0.5Au0.5Zn prepared by DP. It seems that this peak can decompose to two surface plasmon peaks corresponding to the monometallic counterparts. Furthermore, Figure 6 inset shows a typical band at 400 nm which is characteristic of ZnO (50).

Hydrogen, CO, CO2 and H2O production rates at 250ºC and at ca. 5% methanol conversion over monometallic and bimetallic AgxAu1–xZn catalysts are presented

in Figure 7. It is clear that adding Au increased the selectivity of AgZn catalyst towards hydrogen and CO2. The optimal performance in methanol oxidation to hydrogen was achieved by Ag0.5Au0.5Zn catalyst. However, as shown in Figure 7, selectivity toward CO and H2O decreased with increasing Au content and reached a minimum in the case of Ag0.5Au0.5Zn catalyst. Furthermore, a complementary investigation to confi rm synergism between Ag and Au, including theoretical calculations, showed that the experimentally measured rates of hydrogen formation over AgAuZn catalysts are higher than the calculated ones (Figure 8). On the other hand, AgZn catalyst showed a decrease in the

AgZn Ag0.9Au0.1 Ag0.75Au0.25 Ag0.5Au0.5 AuZn

Hyd

roge

n pr

oduc

tion,

mm

ol g

cata

lyst

–1

250

200

150

100

50

Fig. 7. Hydrogen and byproducts (CO2, H2O and CO) production rates over monometallic catalysts and bimetallic Ag1–xAuxZn catalysts at 250ºC and iso-conversion 5%

H2 CO CO2 H2O4

3.5

3

2.5

2

1.5

1

0.5

0

Byproducts form

ation, mm

ol gcatalyst –1

Ag0.9Au0.1Zn Ag0.75Au0.25Zn Ag0.5Au0.5Zn (Ag0.5Au0.5 Zn)mix

Hyd

roge

n pr

oduc

tion,

mm

ol g

cata

lyst

–1

195

190

185

180

175

170

165

160

155

150

145

Fig. 8. Comparison between experimental and theoretical rates of hydrogen production over AgAuZn catalysts at 250ºC and iso-conversion 5%

Rate (theoretical) Rate (experimental)



activity during 20 h time-on-stream (TOS) (Figure 9). In contrast, a stable activity was observed after ca. 7 h TOS for Ag0.5Au0.5Zn catalyst up to 72 h (Figure 9 inset).

A signifi cant contribution of CO from methanol decomposition and/or reverse water gas shift (Equations (ii) and (iii)) was observed over Ag/ZnO catalyst; however, a decrease in CO formation over bimetallic AgAuZn catalysts was observed. This can be explained by the presence of Au particles, possibly by consuming oxygen with preferential oxidation of CO to CO2 (Equation (iv)) (26–32). This suggestion runs in good harmony with the observed increase of the rate of CO2 formation over AgAuZn catalysts especially Ag0.5Au0.5Zn catalyst. Sasirekha et al. (28) discussed the effect of promoting Ag catalyst with Au for the preferential oxidation of CO in a hydrogen-rich stream. It could be proposed that the formation of bimetallic alloy in Au-Ag/cerium(IV) oxide (CeO2) catalyst with Au/Ag ratio of 5:5, which showed a lower reduction temperature, is the reason for its excellent performance toward CO to CO2 reaction. Herein, the probable interaction between Au and Ag (XRD, TPR, EDX mapping, UV/Vis refl ectance and N2O chemisorptions) may be responsible for improving CO oxidation to CO2.

CH3OH → CO + 2H2 (ii)

CO2 + H2 → CO + H2O (iii)

CO + O2 → CO2 (iv)

Adjusting the valency of Ag species could lead to a variation in both hydrogen and CO selectivity. Ag on non-doped AgZn catalyst exists as Ag2O (as shown by XRD), namely it is in the Ag+ state. Ag+ is regarded as an inactive state for POM to hydrogen (24). On the other hand, it was reported that Ag+ is active in the conversion of methanol to CO. H2-TPR results showed that adding Au induced a change in the reduction profi le of Ag+ species and may result in the more active species Agn+ (n <1.0) on the AgAuZn catalysts surface. These Ag species may speed up the rate of hydrogen formation and decrease the rate of CO formation. Similarly, Yang et al. have reported that the reduction of CuO was enhanced by the presence of Au in Cu/ZnO (6). The enhanced reducibility of CuO has been explained in terms of the tendency of Au to decrease the strength of the Cu–O bond located in the vicinity of Cu. Therefore, it can be suggested that the Ag–O bond was weakened by the presence of Au which seems to be due to a certain degree of interaction between Au and Ag oxides in these catalysts.

In order to test the idea that the reduction of Ag2O is enhanced by the presence of Au, the (Ag0.5Au0.5Zn)mix catalyst was also synthesised by mechanical mixing to decrease contact between Ag and Au particles. Interestingly, the (Ag0.5Au0.5Zn)mix catalyst showed a lower hydrogen production rate than that of Ag0.5Au0.5Zn catalyst prepared by DP (Figure 8). In particular, as shown from the TEM micrograph and EDX mapping of

Hyd

roge

n pr

oduc

tion,

mm

ol g

cata

lyst

–1

400

350

300

250

200

150

100

50

0

Fig. 9. Time course of the methanol conversion over AgZn catalyst and bimetallic Ag1–yAuyZn (y = 0.25 and 0.5) catalysts at 350ºC and iso-conversion 5%

AgZn Ag0.75Au0.25Zn Ag0.5Au0.5Zn

0 2 4 6 8 10 12 14 16 18 20Time on stream, h

Hyd

roge

n pr

oduc

tion,

mm

ol g

cata

lyst

–1

300

250

200

150

100

50

00 20 40 60 80

Time on stream, h



the (Ag0.5Au0.5Zn)mix, the contact between Ag and Au particles was lower than that between these particles in Ag0.5Au0.5Zn (Figure 2(c)). These fi ndings may support the interpretation that the synergetic effects between Ag and Au were due to the strong interaction between Ag and Au nanoparticles (as show by UV-Vis). Furthermore, one cannot exclude that the synergetic effects between interacting Ag and Au in Ag0.5Au0.5Zn catalyst (prepared by DP) for production of hydrogen may also be due to the hydrogen spillover effect (24). Hydrogen adsorbed on Ag sites may be spilt over to neighbouring Au particles in high contact (51), which are known to have high affi nity for adsorbing hydrogen (52). The spillover hydrogen on Au particles may be desorbed as hydrogen through ZnO rather than being oxidised to water.

Even though adding Au to AgZn catalyst decreased metal dispersion however, dispersion of metal particles in bimetallic AgAu/ZnO catalysts did not signifi cantly change after reaction compared with AgZn catalyst (Table I). This can explain the observed higher stability of bimetallic Ag0.5Au0.5Zn catalyst with 72 h on stream ( Figure 9 inset) during POM reaction at 350ºC.

4. Conclusions

Bimetallic AgAuZn catalyst samples produced a lower amount of CO than AgZn catalyst which encourages the use of these catalysts in a hydrogen fuel cell to avoid any deactivation. The bimetallic catalysts containing 2.5 wt% Ag and 2.5 wt% Au exhibited the highest hydrogen production rate and had the lowest CO production rate. It was suggested that interaction between Ag and Au particles in the AgAuZn catalyst, detected from TPR and UV-Vis results, was responsible for enhancing the reducibility of Ag2O species in this catalyst. From TEM and EDX mapping investigations it was concluded that the contact between Ag and Au particles in AgAuZn catalyst prepared by DP is greater than that in a comparable catalyst prepared by mechanical mixing. The latter catalyst played an important role in the hydrogen spillover effect. The reaction pathway for POM over AgAuZn involved a preferential oxidation of CO to CO2 over Au sites.

Acknowledgement

The author would like to thank the supporters of the King Faisal University, Saudi Arabia, and the School of Catalysis at the Egyptian Petroleum Research Institute

who generously made resources available to undertake this study.

References 1. L. Mo, X. Zheng and C.-T. Yeh, ChemPhysChem,

2005, 6, (8), 1470

2. C.-T. Yeh, Y.-J. Chen and H.-S. Hung, ‘A Process of Producing Hydrogen under Low Temperature’, Taiwan Patent 226,308; 2005

3. T.-C. Ou, F.-W. Chang and L. S. Roselin, J. Mol. Catal. A: Chem., 2008, 293, (1–2), 8

4. F.-W. Chang, T.-C. Ou, L. S. Roselin, W.-S. Chen, S.-C. Lai and H.-M. Wu, J. Mol. Catal. A: Chem., 2009, 313, (1–2), 55

5. V. Dal Santo, A. Gallo, A. Naldoni, M. Guidotti and R. Psaro, Catal. Today, 2012, 197, (1), 190

6. H.-C. Yang, F.-W. Chang and L. S. Roselin, J. Mol. Catal. A: Chem., 2007, 276, (1–2), 184

7. Y.-C. Lin, K. L. Hohn and S. M. Stagg-Williams, Appl. Catal. A: Gen., 2007, 327, (2), 164

8. Y.-J. Huang, K. L. Ng and H.-Y. Huang, Int. J. Hydrogen Energy, 2011, 36, (23), 15203

9. E. Qayyum, V. A. Castillo, K. Warrington, M. A. Barakat and J. N. Kuhn, Catal. Commun., 2012, 28, 128

10. H. Zhu, Z. Guo, X. Zhang, K. Han, Y. Guo, F. Wang, Z. Wang and Y. Wei, Int. J. Hydrogen Energy, 2012, 37, (1), 873

11. C. Pojanavaraphan, A. Luengnaruemitchai and E. Gulari, Appl. Catal. A: Gen., 2013, 456, 135

12. C. Pojanavaraphan, A. Luengnaruemitchai and E. Gulari, Int. J. Hydrogen. Energy, 2013, 38, (3), 1348

13. S. Pongstabodee, S. Monyanon and A. Luengnaruemitchai, J. Ind. Eng. Chem., 2012, 18, (4), 1272

14. F.-W. Chang, L. S. Roselin and T.-C. Ou, Appl. Catal. A: Gen., 2008, 334, (1–2), 147

15. M.-L. Xu, X.-K. Yang, Y.-J. Zhang, S.-B. Xia, P. Dong and G.-T. Yang, Rare Metals, 2015, 34, (1), 12

16. R. Feng, M. Li and J. Liu, Rare Metals, 2012, 31, (5), 451

17. I. T. Schwartz, A. P. Jonke, M. Josowicz and J. Janata, Catal. Lett., 2013, 143, (7), 636

18. M. Rashid, T.-S. Jun, Y. Jung and Y. S. Kim, Sens. Actuators B: Chem., 2015, 208, 7

19. J. Cao, M. Guo, J. Wu, J. Xu, W. Wang and Z. Chen, J. Power Sources, 2015, 277, (1), 155

20. Y. Tong, J. Pu, H. Wang, S. Wang, C. Liu and Z. Wang, J. Electroanal. Chem., 2014, 728, 66



21. Y. Luo, Y. Xiao, G. Cai, Y. Zheng and K. Wei, Fuel, 2012, 93, 533

22. S. H. Israni and M. P. Harold, J. Membrane Sci., 2011, 369, (1–2), 375

23. L. Mo, X. Zheng and C.-T. Yeh, Chem. Commun., 2004, (12), 1426

24. L. Mo, A. H. Wan, X. Zheng and C.-T. Yeh, Catal. Today, 2009, 148, (1–2), 124

25. H. M. AbdelDayem, S. S. Al-Shihry and S. A. Hassan, Ind. Eng. Chem. Res., 2014, 53, (51), 19884

26. A. Sandoval, A. Aguilar, C. Louis, A. Traverse and R. Zanella, J. Catal., 2011, 281, (1), 40

27. Z. Qu, G. Ke, Y. Wang, M. Liu, T. Jiang and J. Gao, Appl. Surf. Sci., 2013, 277, 293

28. N. Sasirekha, P. Sangeetha and Y.-W. Chen, J. Phys. Chem. C, 2014, 118, (28), 15226

29. G. Nagy, T. Benkó, L. Borkó, T. Csay, A. Horváth, K. Frey and A. Beck, React. Kinet. Mech. Catal., 2015, 115, (1), 45

30. J. H. Byeon and J.-W. Kim, ACS Appl. Mater. Interfaces, 2014, 6, (5), 3105

31. Y. Iizuka, R. Inoue, T. Miura, N. Morita, N. Toshima, T. Honma and H. Oji, Appl. Catal. A: Gen., 2014, 483, 63

32. T. Déronzier, F. Morfi n, M. Lomello and J.-L. Rousset, J. Catal., 2014, 311, 221

33. W. Li, A. Wang, X. Liu and T. Zhang, Appl. Catal. A: Gen., 2012, 433–434, 146

34. X. Huang, X. Wang, M. Tan, X. Zou, W. Ding and X. Lu, Appl. Catal. A: Gen., 2013, 467, 407

35. X. Huang, X. Wang, X. Wang, X. Wang, M. Tan, W. Ding and X. Lu, J. Catal., 2013, 301, 217

36. L. V. Romashov, L. L. Khemchyan, E. G. Gordeev, I. O. Koshevoy, S. P. Tunik and V. P. Ananikov, Organometallics, 2014, 33, (21), 6003

37. P. M. More, D. L. Nguyen, P. Granger, C. Dujardin, M. K. Dongare and S. B. Umbarkar, Appl. Catal. B: Environ., 2015, 174–175, 145

38. D. X. Huy, H.-J. Lee, Y. B. Lee and W. S. Choi, J. Colloid Interface Sci., 2014, 425, 178

39. J. Zheng, H. Lin, Y.-n. Wang, X. Zheng, X. Duan and Y. Yuan, J. Catal., 2013, 297, 110

40. S. Rojluechai, S. Chavadej, J. W. Schwank and V. Meeyoo, Catal. Commun., 2007, 8, (1), 57

41. G. C. Bond, C. Louis and D. T. Thompson, “Catalysis by Gold”, Catalytic Science Series, Vol. 6, Imperial College Press, London, UK, 2006

42. T. Mallat and A. Baiker, Ann. Rev. Chem. Biomol. Eng., 2012, 3, 11

43. F. Zane, V. Trevisan, F. Pinna, M. Signoretto and F. Menegazzo, Appl. Catal. B: Environ., 2009, 89, (1–2), 303

44. J. Zhang, B. Lai, Z. Chen, S. Chu, G. Chu and R. Peng, Nanoscale Res. Lett., 2014, 9, 237

45. Y. Hu, W. Lü, D. Liu, J. Liu, L. Shi and Q. Sun, J. Nat. Gas Chem., 2009, 18, (4), 445

46. F. Yinga, S. Wang, C.-T. Au and S.-Y. Lai, Gold Bull., 2010, 43, (4), 241

47. C.-K. Chang, Y.-J. Chen and C.-t. Yeh, Appl. Catal. A: Gen., 1998, 174, (1–2), 13

48. A. N. Pestryakov, N. E. Bogdanchikova and A. Knop-Gericke, Catal. Today, 2004, 91–92, 49

49. S. Link and M. A. El-Sayed, J. Phys. Chem. B, 1999, 103, (40), 8410

50. T. Xu, L. Zhang, H. Cheng and Y. Zhu, Appl. Catal. B: Environ., 2011, 101, (3–4), 382

51. L.-T. Weng and B. Delmon, Appl. Catal. A: Gen., 1992, 81, (2), 141

52. C. Kartusch and J. A. van Bokhoven, Gold Bull., 2009, 42, (4), 343

The Author

Hany AbdelDayem holds a BSc degree in Chemistry and an MSc in catalysis from Ain Shams University, Egypt, as well as a PhD in Chemistry for research on chemical kinetics and catalysis from the Université Catholique de Louvain, Belgium. He subsequently held a Fulbright postdoctoral award at the University of Pittsburgh, USA, and a postdoctoral fellowship from the Agence Universitaire de la Francophonie (AUF) at the Université Catholique de Louvain. In 2001 he was appointed Assistant Professor in the Chemistry Department at Ain Shams University. He joined King Faisal University, Saudi Arabia, as Assistant Professor from 2005 up to 2014. In 2015 he is the appointing Associate Professor in physical chemistry at Ain Shams University. His main focus is the development of heterogeneous catalytic processes, including petrochemical and green energy. He is particularly interested in catalyst synthesis, ex- and in situ characterisation of catalysts and reaction kinetics.




Carbon Formation in Steam Reforming and Effect of Potassium PromotionPotassium dopants prevent carbon formation and aid catalyst recovery

By Mikael CarlssonJohnson Matthey Plc,PO Box 1, Belasis Avenue, Billingham TS23 1LB, UK


Introduction

When choosing a reformer catalyst, there are a number of important things to consider. Steam reforming of methane is an endothermic reversible reaction, whilst steam reforming of higher hydrocarbons is not reversible. The activity of the catalyst installed is critical in determining the reaction rate within the reformer. However, the steam reforming reaction is diffusion limited, so the geometric surface area of the installed catalyst is directly related to the catalyst activity. This article will show the mechanisms by which carbon can form on a catalyst and how a potassium dopant can prevent this and aid catalyst recovery following carbon formation (1).

Because the reaction is endothermic, the transfer of heat from the burners to the catalyst is just as important as the activity. Whilst within the reformer itself the primary heat transfer mechanism is radiation, within the tube it is convection and conduction. The hottest point inside the tube is the internal tube wall. The size and shape of the catalyst will impact on the tube-side laminar fi lm layer and therefore on the overall heat transfer coeffi cient as represented in Figure 1.

Due to the temperatures at which steam reformers operate, carbon is constantly being formed from the hydrocarbon feedstock, with the primary route being through cracking reactions. However, there are also carbon removal (or gasifi cation) reactions that simultaneously occur which remove the carbon laid down, meaning there is no net accumulation of carbon in a well-run plant. With a given catalyst loading in the reformer, the rate of gasifi cation is fi xed by the catalyst type and the process conditions. However, the rate of carbon laydown is a function of a number of conditions such as the catalyst activity, degree of sulfur poisoning and heat input to the tubes. The rate of laydown is therefore more likely to vary compared to the rate of gasifi cation. The selected catalyst should have appropriate activity or alkali promoters to ensure that the carbon removal rate is faster than the carbon formation rate, which would result in no net carbon laydown.

Finally, the catalyst should allow for the lowest possible pressure drop, as this will enable the highest possible plant throughput before compressor limits are reached. However the catalyst breakage characteristics are also important as all pelleted steam reforming catalysts will break due to the forces exerted on them when reformer tubes expand in operation and then contract during plant shutdowns, which will lead to an increase in pressure drop.

Carbon Formation

The three main reactions for carbon formation are hydrocarbon cracking (Equations (i) and (ii)), carbon



monoxide disproportionation (the Boudouard reaction) (Equation (iii)) and carbon monoxide reduction (Equation (iv)).

CH4 ⇌ C + 2H2 ΔH298 = +75 KJ mol–1 (i)

CnHm nC + (m/2)H2 (ii)

2CO ⇌ C + CO2 ΔH298 = –172 KJ mol–1 (iii)

CO + H2 ⇌ C + H2O ΔH298 = –131 KJ mol–1 (iv)

Cracking or decomposition of hydrocarbons is favoured at temperatures above approximately 620ºC (1148ºF) depending on the hydrocarbon species. The reaction with methane is reversible but with the heavier hydrocarbons they are not.

Both the carbon monoxide reduction and disproportionation reactions are more prevalent at lower temperatures but at those temperatures the concentrations of carbon monoxide would normally be low, depending on recycle rates, so the cracking reactions are normally the most important to consider. However, any combination of these reactions can lead to detrimental effects on catalyst activity and, if left untreated, eventually lead to permanent damage and carbon build-up.

There are three catalyst parameters that can be altered to prevent carbon formation. These are the activity, the inherent heat transfer coeffi cient and the catalyst alkali promoter content.

Increasing the catalytic activity can be achieved by using a higher surface area catalyst due to the diffusion limited nature of the reaction mentioned previously. This has a threefold effect; fi rstly, as there is more reforming reaction near the inlet of the tube, there is a lower process gas temperature due to the increased heat of reaction required. Secondly, the hydrocarbon content of the process gas is reduced. And fi nally as more hydrogen is produced carbon formation is suppressed.

By improving the heat transfer characteristics of the reforming catalyst, the rate of heat transfer within the tube can be increased. Intuitively this would appear to increase the process gas temperature thereby making the carbon forming potential worse. However since carbon is most likely formed on the inside tube wall which is the hottest part of the process, increasing the heat transfer characteristics of the catalyst reduces this temperature by transferring heat to the bulk of the catalyst. The additional heat transferred will in turn increase the reaction rate, which will also reduce the hydrocarbon content of the process gas making carbon formation less likely. The process gas temperature is

Flame heat release and radiation

Radiation from wall

Radiation from gas

Shadowing of tubes and circumferential temperature analysis

Radiation from coffi ns

Tube wall heat transfer tube stressCatalyst reaction

carbon formation

Gas radial temperature gradient

Pressure drop

Internal emissivity

Film heat transfer

Tube emissivity

Gas

Fig. 1. Heat transfer balance inside a steam reformer box from fl ames to reactant stream



also reduced. Overall, this has a similar effect to that seen by installing a highly active catalyst.

Another way of preventing the formation of carbon is to include a promoter in the catalyst to help increase the rate of carbon gasifi cation; one such promoter is potassium.

Potassium Promotion

It is well known that carbon formation on a surface, whether the support or catalyst, is affected by the acidity of that surface. Positively charged acidic sites on a surface will increase the rate of carbon formation, which is partly due to acidic sites catalysing the cracking reaction. Alpha alumina, which is a common catalytic support, contains acidic sites and adding Group 2 metals such as magnesium or calcium neutralises these making the surface less acidic.

For a supported nickel catalyst the steam ratio at which a catalyst would run without forming carbon can be decreased by approximately 16% compared to an undoped alumina through the addition of dopants such as calcium or magnesium. A way to further increase the surface basicity is to add a potassium-containing compound such as potash as a dopant, which will lead to an increased prevention of carbon formation. For alkalised calcium aluminate catalyst the steam ratio can be reduced by approximately 65% without forming carbon compared to an undoped alumina. The reason for this is due to both the acceleration of the carbon gasifi cation reaction and the suppression of carbon formation reactions.

In addition to increasing the surface basicity, the potassium will form hydroxide species in the presence

of steam and these will aid in any removal of carbon that is formed on the surface. As highlighted earlier, depending on the conditions, there are locations within the reformer where carbon will form on hot surfaces, for example, the inner tube wall. This is especially likely if heavier species slip further down the tube where the wall is hotter. That carbon will have to be removed at a faster rate than it is formed in order to prevent any build-up.

The history of potassium promoted catalysts goes back to 1975 when a trial was carried out on the No 1 Low Pressure Ammonia Plant in Billingham, UK, (2). During the trial it was shown that the promoted catalyst, where the potassium was incorporated in the support, was successful in the suppression of hot bands that had been seen for the previous charge of unpromoted catalyst. These hot bands associated with carbon formation appeared after only a few months of operation and it was thought at the time that they were due to a plant uprate. Alkali metals were known to inhibit the steam reforming reaction, but during the plant trial no such inhibition was seen due to the way in which the potassium was incorporated into the support. The effect was confi rmed by laboratory experimental testing. After nine months of operation the reformer was inspected and the tubes containing potassium promoted catalyst were running cooler with a more uniform temperature than adjacent tubes, which contained unpromoted catalyst. The material was discharged and when examined only a very limited potassium loss was detected.

The Johnson Matthey KATALCOJM catalyst range available today has been designed with different amounts of promoter for various operations. As can be seen in Table I the range spans from unpromoted

Table I Range of Johnson Matthey KATALCOJM Catalysts with Different Potassium Promotion

K2O, wt% Series Feedstock/carbon protection requirement

0 KATALCOJM 23-4 or 57 series Light feed/low C protection

Heavy feed/high C protection

1.5–2.5 KATALCOJM 25-4 series

4–5 KATALCOJM 47 series

6–7 KATALCOJM 46-3



KATALCOJM 23 and 57 series which are used for light feedstocks such as methane in combination with a low heat fl ux, up to KATALCOJM 46-3 which contains much higher levels of potassium for operations with heavy naphtha. The two catalysts series with intermediate levels of potassium promotion are for operations where the feed composition is heavier than methane but lighter than heavy naphtha, for example, liquid petroleum gas (LPG). In reality this summary is slightly oversimplifi ed as both the steam-to-carbon ratio and the overall heat fl ux also affect the amount of carbon protection required.

Potassium is incorporated into the catalyst in ceramic phase reservoirs with a precise stability to regulate the rate of release onto the surface. This leads to the right level of potassium and hydroxide species on the surface to ensure gasifi cation of carbon from all nickel sites throughout the catalyst’s lifetime.

The potassium-containing phases present in Johnson Matthey catalysts depends on the series but typically they are either a potassium-aluminosilicate or potassium-aluminate which is incorporated in the support. The use of a range of phases allows for the release of potassium at an appropriate rate under a range of process conditions and maintains high activity in terms of carbon removal. This also ensures that any adverse effect on the steam reforming activity is minimised.

Figure 2 shows an electron probe microanalysis (EPMA) of a potassium-promoted catalyst which clearly

shows areas which are rich in aluminium (Figure 2(a)) and potassium (Figure 2(b)). What can be seen is that where there is a high abundance of potassium there is also high aluminium content. This clearly indicates that there are areas of potassium-aluminates which act as potassium reservoirs for the catalyst.

Froment et al. examined different potassium loadings on a nickel catalyst and found that in conditions where methane cracking was taking place the presence of potassium seemed to have three effects on the carbon formation (3): (a) it reduced the fi nal level of carbon formed; (b) it reduced the rate of carbon formation; and (c) it apparently delayed the onset of carbon formation, which is speculated to be the result of decreasing the nucleation rate on the catalyst surface. Furthermore, the gasifi cation rate of fi lamentous carbon that had been deposited is also affected by the presence of potassium as shown in Figure 3. The rate of gasifi cation by steam as a function of the potassium content exhibits a maximum of around 1.6–2.0 wt% potassium oxide (K2O) for this catalyst system.

The presence of a potassium dopant will promote the adsorption of water which will in turn increase the carbon gasifi cation (4). The potassium will also affect the gasifi cation kinetics and increase the carbon monoxide production rate and, as steam adsorbs dissociatively, there could be an increase in oxygen on the surface as a result of the increased number of sites for water adsorption on an alkalised catalyst, which leads to an increase of the rate of gasifi cation.

Level

Average 3966

200 μm

6000525045003750300022501500

7500

(a) Level

Average

1000875750625500375250125

0129

200 μm

(b)

Fig. 2. EMPA images showing: (a) aluminium; (b) potassium distribution in a catalyst support highlighting areas of K-Al reservoirs



Carbon Formation Case Study

Figures 4–6 demonstrate the use of a potassium promoted catalyst to aid recovery after a carbon incident in a reformer on a European ammonia plant.

Figure 4 shows hot bands on the reformer tubes that appeared following a carbon incident due to LPG condensate trapped in a line being inadvertently fed into the reformer. This carbon led to an increase in pressure drop from 3.6 to 5.0 bar (52.2 to 72.5 psi) across the reformer.

Although carbon had been formed, the presence of potassium promoted catalyst limited the severity of this incident and a full shutdown was averted. As the plant needed to keep running the operators decided that it would be run at a higher steam-to-carbon ratio in an attempt to promote carbon gasifi cation. Over the following months the pressure drop was decreased to 4.7 bar (68.1 psi) and the extent of hot bands on the tubes decreased, which can be seen in Figure 5. The measurement of the tube wall temperature revealed a decrease of up to 30ºC (54ºF). This highlights the effect of carbon removal that is promoted by the potassium containing catalyst.

After two months of running at an increased steam-to-carbon ratio the plant tripped, providing an opportunity to steam the catalyst prior to restart. When the plant

0.16

0.14

0.12

0.1

0.08

0.06

0.04

0.02

00 1 2 3 4

Rat

e of

gas

ifi ca

tion

by s

team

, mol

C g

–1 c

at–1

h–1

PH2O = 3.0 barT = 550ºC

Alkali content, wt%

PCO = 0.1 barPH2 = 0.3 bar




Fig. 3. Carbon gasifi cation rate as a function of potassium loading. (Reprinted with permission from (2). Copyright (2002) American Chemical Society)

Fig. 4. Hot bands shown on tubes after a carbon forming incident

Hot bands

Fig. 5. Tube appearance two months after the carbon incident, illustrating some improvement

Hot bands – clearly reduced

Fig. 6. Tube appearance after plant shutdown and steaming showing conditions returning to normal

No hot bands visible



was restarted no hot bands were observed (as can be seen in Figure 6) and operation was back to normal with pressure drop at 3.8 bar (52.2 psi). This case study illustrates both how the KATALCOJM catalyst can slowly recover during normal operation and also the dramatic return to normal operating conditions after steaming.

Conclusion

There are a number of mechanisms by which carbon formation can occur on a nickel-based steam reforming catalyst, with the cracking of hydrocarbons most prevalent. Carbon deposition happens when the formation rate is greater than the removal rate which is a function of surface chemistry and the addition of promoters to reduce carbon formation. It is important that the potassium dopant is added to the catalyst in optimised phases with appropriate hydrothermal stability to give a controlled release rate. The

release and mobility of the potassium are required to keep tube walls free from carbon and also assist in recovery from plant upset conditions resulting in carbon formation.

References1 “Catalyst Handbook”, 2nd Edn., ed. M. V. Twigg,

Manson Publishing Ltd, London, UK, 1996

2 L. W. Lord, ICI Internal Report RD/CC430 , 1976

3 J.-W. Snoeck, G. F. Froment and M. Fowles, Ind. Eng. Chem. Res., 2002, 41, (15), 3548

4 R. A. Hadden, J. C. Howe and K. C. Waugh, ‘Hydrocarbon Steam Reforming Catalysts - Alkali Induced Resistance to Carbon Formation’, Catalyst Deactivation 1991, Illinois, USA, 24th–26th June, 1991, “Proceedings of the 5th International Symposium”, eds. C. H. Bartholomew and J. B. Butt, Studies in Surface Science and Catalysis, Vol. 68, Elsevier Science Publishing Co, New York, USA, 1991, pp. 177–184

The Author

Mikael Carlsson joined Synetix/ICI in 2002 after graduating from Napier University in Edinburgh, UK, with a MSc degree in Materials Technology. He also has a BSc in Chemical Engineering from Chalmers University of Technology, Sweden. Mikael has for the last nine years been developing catalysts for the steam reforming area and currently works as Reforming Technical Development Manager.




“Electrochemical Power Sources: Batteries, Fuel Cells, and Supercapacitors”By Vladimir S. Bagotsky, Alexander M. Skundin and Yury M. Volfkovich (A.N. Frumkin Institute of Physical Chemistry and Electrochemistry of the Russian Academy of Science, Russia), John Wiley & Sons Inc, New Jersey, USA, 2015, 372 pages, ISBN: 978-1-118-46023-6, £66.95, €81.99, US$99.95

Reviewed by Billy WuDyson School of Design Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK


“Electrochemical Power Sources: Batteries, Fuel Cells, and Supercapacitors” is a comprehensive textbook covering materials, applications and prospects of the aforementioned devices. The high level overview provided makes this book an excellent resource for readers new to electrochemical devices as it avoids going into excessive detail of each material, whilst providing an overall perspective and outlook. The book was edited by Alexander Skundin and Yury Volfkovich in honour of the late Vladimir Bagotsky, who is widely recognised for his scientifi c activities in electrochemistry and for his textbooks on this subject. Skundin and Volfkovich themselves are the chief scientists at the A.N. Frumkin Institute of Physical Chemistry and Electrochemistry of the Russian Academy of Sciences and are two of the leading experts in batteries and supercapacitors in Russia with over 200 peer reviewed articles between them.

The book covers three general topics: batteries, fuel cells and supercapacitors, with each section discussing fundamental operating principles, material considerations and technology prospects.

Application of Electrochemical Devices

Electrochemical devices are being employed in applications ranging from consumer electronics to electric vehicles due to their relatively high effi ciencies and environmental friendliness. However, major challenges include increasing the energy, power density and effi ciencies of these devices, especially when scaling up from a novel material or chemistry. An appreciation of the historic works, common challenges and potential future position of a technology is therefore extremely useful.

Batteries

The fi rst part of the book covers thermodynamic aspects in electrochemical devices with a focus on aqueous redox couples which have been historically considered for battery applications. The general operating principle of galvanic cells along with different electrode chemistries including zinc-based,



nickel-based and lead acid batteries are examined. The section follows with an explanation of the fi gures of merit relevant to batteries, and design considerations when translating the material chemistry into cell level systems, taking into account the accompanying separator and electrolyte. Applications of the technology, current limitations and technology prospects are then summarised.

The next section then leads onto non-aqueous systems which are currently the most prevalent in real world applications. Here a general introduction into different types of electrolytes is provided covering aprotic non-aqueous solutions, molten salts and solid electrolytes. Discussion of intercalation based lithium-ion electrodes then follows with their review concluding that existing materials will not be able to meet consumer demands for energy density and charging rates. Therefore, they believe a shift to alternative materials, i.e. silicon, tin or aluminium will be required, however cycle life then becomes the limiting factor with microstructural damage leading to poor lifetimes.

Their assessment of forthcoming battery technologies, from the perspective of increasing energy density, then concentrates on conversion based chemistries and thus a move away from the traditional intercalation based mechanisms. Lithium-sulfur, lithium-air and sodium-ion batteries are discussed with challenges identifi ed as again being lifetime issues associated with the microstructural changes occurring in the electrodes and the voltage hysteresis observed upon charge and discharge limiting device effi ciency.

The fi nal chapters of this section then briefl y cover solid state batteries and batteries with molten salt electrolytes which researchers have investigated in order to enable high device lifetimes, however other diffi culties such as electrolyte conductivities currently limit their mainstream adoption.

Fuel Cells

The second part deals with fuel cells, which has relevance in both transport and stationary power applications. Again, this starts with thermodynamic aspects of the device operation and basic defi nitions of components and concepts found in fuel cells. The subsequent sections then go into further explanation of the different types: proton exchange membrane fuel cell (PEMFC), direct methanol fuel cell (DMFC), molten carbonate fuel cell (MCFC), solid oxide fuel

cell (SOFC) and alkaline fuel cell (AFC). With PEMFC, which are the most relevant for automotive uses, a historical overview is given covering the development of the technology but it also touches on engineering challenges such as thermal management, cold start and water management. The progress and complications of each fuel cell technology then follows with a brief overview of phosphoric acid (PAFC), direct carbon fuel cell (DCFC), bacterial fuel cell (BFC) and redox fl ow battery (RFB). RFB in particular are receiving much attention in the large scale energy storage industry due to their potential cost savings and increased lifetime compared to batteries with the common all-vanadium and iron-chromium RFB discussed.

The section concludes with discussions about fuel cell applications and the historically installed fuel cell units for stationary appliances are summarised. Only PEMFC has received signifi cant industria l attention for automotive uses and the authors suggest the initial gains are in heavy goods vehicles. Their technology outlook underlines their interpretation of the forthcoming issues which are developing new catalysts for oxygen reduction, catalysts for the complete oxidation of ethanol and selective catalysts which do not promote the undesired side reactions.

Supercapacitors

The third section focuses on electrochemical double layer capacitors or supercapacitors, looking initially at carbon based systems which have the most industrial relevance. Here they present a detailed analysis of electrode optimisation for different kinds of carbon such as activated carbon, carbide derivatives, aerogels, nanotubes and graphene, with various electrolyte types in terms of pore size distribution and the effect of functional groups. The discussion then follows onto a range of electrolyte types: aqueous, non-aqueous and ionic liquids.

Observations suggest that eventually further growth in specifi c surface area of electrodes through reducing pore sizes will not result in improved supercapacitor energy density due to the infl uence of steric effects. Thus, there is an increased interest in pseudocapacitor electrode materials which includes metal oxides such as iridium(IV) oxide (IrO2), manganese(IV) oxide (MnO2) and ruthenium(IV) oxide (RuO2), conducting polymers and monomer redox systems. This text gives a comprehensive overview of these electrode materials, challenges and prospects, of which a current trend is



observed to be the creation of composite electrodes of metal oxides and conducting polymers to mitigate the conductivity issues of the metal oxides and achieve higher mass loadings for more practical devices.

The detailed discussion of carbon based supercapacitor electrodes and pseudocapacitor electrodes then leads onto asymmetric supercapacitors which combines the two together to give a higher energy density due to a wider operating voltage window. Refl ecting on all of this, the section closes with a comparison of commercially available supercapacitors and the prospects of the device.

Photoelectrochemical Devices

The closing chapter of the book covers electrochemical aspects of solar energy conversion. This gives a very brief review of the mechanisms of photoelectrochemical devices such as semiconductor solar batteries and dye-sensitised solar cells.

Conclusions

“Electrochemical Power Sources: Batteries, Fuel Cells, and Supercapacitors” is an excellent introductory

text to electrochemical energy devices which covers material considerations, historical developments of the technology and future prospects, spanning fundamental mechanisms to engineering challenges at a high level perspective. The supercapacitor section in particular goes into much more detail of the materials. This text would be most useful for students studying an introduction to electrochemistry course.

“Electrochemical Power Sources: Batteries, Fuel Cells, and Supercapacitors”

The Reviewer

Dr Billy Wu is a lecturer in the Dyson School of Design Engineering at Imperial College London where he works on electrochemical devices at a range of length scales from novel materials to engineering integration. He gained his PhD from Imperial College London in 2014 on PEMFCs, lithium-ion batteries and supercapacitors for automotive applications.




Selective Removal of Mercury from Gold Bearing StreamsExploring the use of solid adsorbents to avoid the undesirable loss of gold

By James G. StevensJohnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading, RG4 9NH, UK


Many gold ore bodies contain high levels of mercury which are co-extracted with the gold. This mercury then travels through the process circuit to pose health, environmental and technical issues. This article highlights a method to selectively remove the mercury whilst leaving the gold to be processed as normal. The removal of mercury from the circuit mitigates the need for retorting of the produced gold, reduces the potential environmental impact of any waste solutions and decreases any potential mercury exposure to plant workers.

The solid bound thiol species used have been shown by inductively coupled plasma optical emission spectrometry (ICP-OES) to reduce the mercury to undetectable levels whilst having no measurable effect on the gold concentration. The control of the cyanide concentration at the adsorption step has been shown to be key to ensuring that the mercury removal is achieved selectively. This in turn ensures that no precious metal value is lost in the mercury removal process. The process has been shown to be applicable to both batch and continuous operation which will allow the technology to be applied to a variety of flow rates and applications.

Introduction

In modern gold mining processes, typically it is necessary to extract gold from complex ores which comprise gold in addition to other metals, including mercury. A common technique for extracting gold from its ores is the cyanide process, wherein leaching of gold is achieved by the addition of cyanide at alkaline pH following the Elsner equation (Equation (i)) (1). Cyanide is a strong lixiviant for gold and so leaches the gold out of the ore into solution (2). The gold is typically present in the leaching solution as a gold cyanide complex. Silver can also be extracted from its ores using a similar cyanide leaching process.

4Au + 8CN– + O2 + 2H2O → 4[Au(CN)2]– + 4OH– (i)

A problem with this process is that cyanide is an equally strong lixiviant for many other metals, including mercury. Accordingly mercury, which is typically present in the ore along with gold or silver, is also leached into the solution. The mercury may be present in the leach solution as a variety of complex anions with the general formula [Hg(CN)2+x]x– (where x is 0, 1 or 2) depending on the ratio of mercury to cyanide ions. However, typically it is present as [Hg(CN)4]2–.The removal of mercury from mining waters is very

important, both on health and safety grounds and on environmental grounds. In particular, mercury volatilisation during extraction processes can be a threat to the health of plant workers and the presence of mercury in waste waters from mining is of significant environmental concern. Environmental legislation limits



the concentration of mercury permitted in waste waters to very low levels in many countries. Accordingly, effective removal of mercury from mining waters is of significant interest to the industry. However, it is important that mercury removal technologies do not remove significant quantities of the gold or silver being mined, to avoid undesirable loss of these products during processing.A range of different methods have been employed

for mercury removal in this field. Miller et al. reviewed different technologies for the removal of mercury, including precipitation with inorganic sulfides or sulfur-based organic compounds; adsorption with activated carbon or crumb rubber; solvent extraction by alkyl phosphorus esters or thiol extractants; ion exchange with isothiouronium groups or polystyrene-supported phosphinic acid; and electrochemical cementation all with varying degrees of selectivity. They deem further work on resins with thiol functionality necessary in order to achieve the desired selectivity (3).

Dithiocarbamates form stable mercury precipitates which have been used to selectively precipitate mercury from gold bearing solutions (4), this can be carried out more efficiently by using colloidal hydroxides to cause coagulation which can then be removed by dissolved air flotation (5). Hutchison and Atwood further reviewed mercury remediation methods highlighting dimethyldithiocarbamate as an effective precipitation reagent. However the long term stability has been questioned with suggestion that mercury leaches from the precipitate over time; this combined with its degradation into toxic byproducts means that it is only applicable under certain circumstances (6). Alkyl thiols such as 1,3-benzenediamidoethanethiol can also be used to precipitate mercury (7).

This paper explores the use of solid adsorbents and how they can be applied to the selective adsorption of mercury from gold cyanide bearing process streams such as those found within the gold mining circuit.

Experimental Procedures

Test solutions were made by dissolving the appropriate salts in deionised water adjusted to the correct pH using sodium hydroxide solution to prepare a stock solution. This stock solution was diluted by pipette to provide the required concentration solution.

Batch adsorption experiments were carried out by weighing 0.5 dry wt% of the adsorbent into a 60 ml glass tube, dry weight was determined using a

Sartorius infrared balance. 15 ml of the metal solution was then added and allowed to stir for the required time period on a Radleys 12 position stirrer. After stirring the solution was filtered using a 0.45 µm syringe filter, the solution was analysed by ICP-OES and compared against the initial solution. The percentage metal removed is calculated from the difference of the initial concentration to the final concentration divided by the initial concentration.

Kinetic data was obtained by running multiple tubes in parallel with each tube being filtered after an appropriate time. For the cyanide addition experiments an aliquot of cyanide solution was added to the appropriate tubes after an appropriate initial period, the tubes were then filtered as required.

Two samples of real mining process solutions were analysed by inductively coupled plasma mass spectrometry (ICP-MS). From this a model solution concentration of 4.0 ppm gold and 1.0 ppm mercury was set; this initial concentration was used unless stated otherwise.

The pH of the electrowinning (EW) pregnant and barren was pH 12.6 and 12.0 respectively; samples of heap leach were pH 10.0. The pKa of cyanide is reported as 9.2 (8); therefore, pH 9.2 is considered the minimum safe operating pH as below this the conversion to hydrogen cyanide occurs and loss of cyanide as HCN(g) causes both experimental and safety concerns. For these experiments a pH range of 10 to 13 was adopted.

Column experiments were carried out by loading the adsorbent material into a glass column, rinsing with pH adjusted deionised water and then pumping the test solution through the column using a peristaltic pump. The flow rate is controlled proportional to the bed volume (BV) with a standard flow rate of 6 BV h–1 being utilised. Outlet samples were collected and analysed by ICP-OES and compared against the inlet solution.

Materials and Reagents

Johnson Matthey produce a range of metal adsorbents for metal removal applications under the brands Smopex® (9), QuadraPure® and QuadraSil® which are based on polymer fibres, polymer beads and silica spheres respectively with an additional industrial silica based Functional Silica (FS series) range. Further details of the adsorbent materials are detailed in Table I. From internal knowledge and experience and preliminary screening several materials were identified to explore the selective adsorption of mercury. Initial screening showed both thiol and quaternary amine



materials to remove mercury; however quaternary amines strongly adsorbed gold therefore this selectivity study concentrated on the thiol based materials.

The following reagents were used as received: mercury cyanide (Sigma-Aldrich), potassium gold cyanide (Alfa Aesar), potassium cyanide (Sigma-Aldrich), sodium cyanide (Sigma-Aldrich), sodium cyanate (Sigma-Aldrich), sodium thiocyanate (Sigma-Aldrich), sodium sulfate (Alfa Aesar), sodium thiosulfate (Fisher Chemicals), sodium hydroxide (Fisher Chemicals). Deionised water was used from an Elga Purelab DV35 at 15 MΩ.

Results and DiscussionSpeciation

Metal cyanide coordination complexes are well known and Nakamoto discussed how the easily identifiable CN stretching band at 2200–2000 cm–1 can provide information on the structure of the complex, as coordination of cyanide to the metal centre causes the

CN band to be shifted to a higher frequency. This shift relates to both the coordination number and the metal oxidation state (10). Several mercury cyanide species are known to exist with varying coordination numbers which depends on the cyanide concentration of the solution. Gold cyanide does not vary its coordination but can exist as either gold(I) or gold(III); however, the Elsner equation (Equation (i)) predicts the gold to exist only as gold(I) in cyanidation process solutions.

Model solutions of gold and mercury cyanide were made and used to explore the likely species in a cyanidation circuit. Varying the cyanide ratio with mercury showed that the tetracoordinate [Hg(CN)4]2– is formed easily once the required ratio of cyanide is added (Figure 1). Above this ratio additional free cyanide is observed. Comparison of dissolved gold with known standards of potassium gold(I) cyanide (KAu(CN)2) and potassium gold(III) cyanide (KAu(CN)4) demonstrates that no gold(III) is present under standard conditions (Figure 2). This is in accordance with the Elsner equation. Model adsorption testing was carried

Table I Details of the Tested Materials

Name Support Material properties Functional group

FS1 Granular silica 250–710 µm, 90 Å pore size Alkyl thiol

Smopex®-111 Polymer fibre 300 × 50 µm (length × diameter) Benzyl thiol

Smopex®-112 Polymer fibre 300 × 50 µm (length × diameter) Alkyl thiol

2500 2400 2300 2200 2100 2000 1900Wavenumber, cm–1

Tran

smis

sion

, %

23422193

2160

21422079

Fig. 1. Speciation in a cyanidation circuit using model solutions of gold and mercury cyanide with varying ratios HgCl2:KCN. Formation of the 4 coordinate species is observed even at 1:3 ratios indicating that its formation is favoured. At higher ratios the free cyanide peak is observed at 2079 cm–1. Mercury concentration 0.1 M

1:0

1:1

1:2

1:3

1:4

1:10

KAu(CN)2

Au in KCN

KCN

KOCN

2500 2400 2300 2200 2100 2000 1900Wavenumber, cm–1

Fig. 2. Comparison of dissolved gold with known standards. The gold powder dissolved in KCN shows formation of KOCN from the oxidation under air of KCN. Gold concentration 0.1 M

2146

2079

21692146

2079

2169

Tran

smis

sion

, %



out using KAu(CN)2 and mercuric potassium cyanide (K2Hg(CN)4), made in situ (Equation (ii)):

2KCN + Hg(CN)2 → K2Hg(CN)4 (ii)

Model Adsorption

An initial comparison of the three materials (Figure 3) at pH 11 showed that Smopex®-111 gave low removal rates of mercury compared against the other materials. Both Smopex®-112 and FS1 gave excellent removal of mercury although in both cases some gold was removed. The lower performance of Smopex®-111 is probably due to the hydrophobic nature of the material. When it was tested at a higher pH or when using the sodium thiolate form of the material then the performance was

improved. Following this result, further testing focused on the FS1 and the Smopex®-112 which are both alkyl thiols in a more hydrophilic environment (Figure 4).

The amount of gold adsorbed by the materials generally increases with increasing pH whilst the adsorption of mercury is unaffected. This is likely due to the pKa of thiols occurring at around 12 to 13 (11); therefore, over the pH range 10 to 13 the thiols will become increasingly deprotonated changing the behaviour of their adsorption. Mercury forms a strong bond to sulfur and the adsorption is not affected by this change from free thiol to thiolate.

Various salts can be present in mining solutions depending on the source of the ore. These could potentially include sulfur containing salts such as thiosulfate or sulfate from oxidation of sulfide minerals, excess cyanide from the heap leach solution or products of cyanide decomposition including thiocyanate or cyanate. Cyanide is often used in the region of 100 to 500 ppm (12), this decreases during the process as the cyanide becomes bound to both the desired metal (gold) and undesired metals such as mercury, nickel and iron; it is also oxidised to the cyanate ion or reacts with sulfides to form thiocyanate. Infrared spectroscopy of the EW samples (Supplementary Information) shows no free cyanide but does show peaks indicative of cyanate and nickel cyanide. All of these species could potentially cause a change in the adsorption behaviour; particularly chelating species such as thiocyanide, cyanide and cyanate.

To explore the effect of these species, 100 ppm of each was added to the model solution and the adsorption was retested. Figure 5 shows that in all cases the addition of 100 ppm of the anions had

FS1 Smopex®-111 Smopex®-112

100

80

60

40

20

0

Met

al re

mov

ed, %

AuHg

Fig. 3. Comparison of the three materials for removing 4 ppm gold and 1 ppm mercury cyanide at pH 11. FS1 and Smopex®-112 show good mercury removal whilst Smopex®-111 had poor wetting and showed low mercury removal

Fig. 4. Effect of pH on removal of 4 ppm gold and 1 ppm mercury cyanides using: (a) FS1; (b) Smopex®-112. As the pH is increased the amount of gold adsorbed increases. This presumably relates to the pKa of the thiol functional group. The higher adsorption with Smopex®-112 at pH 10 has not been explained but may relate to additional hydroxyl functionality in the material

pH pH 10 11 12 13 10 11 12 13

AuHg

100

80

60

40

20

0

Met

al re

mov

ed, %

Met

al re

mov

ed, %

AuHg

100

80

60

40

20

0

(a) (b)



little effect on the adsorption of mercury when using FS1 with 99% removal being achieved in most cases and 97% removal with cyanide addition. For gold adsorption some significant differences were observed. The addition of sulfate, thiocyanate or thiosulfate had no effect; however both cyanate and cyanide caused a reduction in the removal of gold from 87% removal with no added anion to 65% and <1% removal with cyanate and cyanide respectively. Lewis and Shaw demonstrated that gold thiolate and gold cyanide are in equilibrium (as in Equation (iii)) (13). At low cyanide concentrations the gold forms an insoluble gold thiolate with the surface bound thiol and is removed from solution; at higher cyanide concentrations the equilibrium is shifted to the soluble gold cyanide and the gold stays in solution. Mercury more easily forms an insoluble complex with the thiol and the mercury cyanide equilibrium is not strong enough to maintain the mercury in solution.

Fig. 5. Effect on metal removal using FS1 when adding various sodium salts (100 ppm) to a solution of 4 ppm gold and 1 ppm mercury at pH 12. The commonly encountered ions show little change in the amount of gold removed from the system. Adding additional sodium cyanide meant that the gold removal was completely prevented whilst mercury removal was unaffected

100

80

60

40

20

0

Met

al re

mov

ed, %

None

Thiosu

lfate

Sulfate

Thiocy

anate

Cyanid

e

Cyana

te

AuHg

Adsorption Kinetics

Simple adsorption kinetics (Figure 6) show that mercury is rapidly adsorbed with both FS1 and Smopex®-112. From the initial mercury concentration of 1 ppm both materials reduced the solution concentration to 0.01 ppm

(instrument detection limit) within 5 minutes, less than 1 minute for Smopex®-112. Once adsorbed the mercury was tightly bound to the material and is not eluted under standard conditions, moderate pH or temperature.

Gold is adsorbed more slowly, particularly with Smopex®-112. This is not expected as Smopex® usually

Fig. 6. Adsorption kinetics of 4 ppm gold and 1 ppm mercury cyanides at pH 12 using: (a) FS1; (b) Smopex®-112. Both materials give rapid removal of the mercury with only 0.04 ppm mercury detectable after 1 minute and 0.01 ppm after 5 minutes. Gold adsorption is moderate with FS1 being reduced to 0.57 ppm (85% removed) after 15 minutes. Smopex®-112 had not yet reached equilibrium with gold after 1 h indicating gold adsorption is slow with that material

Time, min Time, min

Met

al c

once

ntra

tion,

ppm

Met

al c

once

ntra

tion,

ppm

5

4

3

2

1

0

5

4

3

2

1

0

0 20 40 60 0 20 40 60

AuHg

AuHg

(a) (b)

(iii)

NC Au CN[ ]– RS–

CN–RS Au CN[ ]– RS–

CN– RS Au SR[ ]–

CN–

RS Au



exhibits rapid kinetics due to its small particle size; therefore, the poor gold adsorption kinetics maybe indicative that the adsorption process is strongly equilibrium limited.

Further kinetic measurements were carried out whereby an aliquot of cyanide was added at 30 minutes, these are shown in Figure 7. This shows how upon cyanide addition the gold is rapidly desorbed from the resin. This provides further evidence for the gold thiolate ⇌ cyanide equilibrium and its effect on the gold adsorption. The addition of cyanide has little effect on the mercury which remains adsorbed onto the resin, confirming that the insoluble mercury thiolate is strongly favoured over the soluble mercury cyanide species.

Adsorption Isotherms

The mercury-only adsorption isotherms were measured for both FS1 and Smopex®-112 at an initial mercury concentration of 100 ppm (Figure 8); with mercury in the form of the K2Hg(CN)4 salt. The data were fitted to a Langmuir isotherm, shown in Equation (iv), where: qA = mg adsorbate per g adsorbent (mg g–1), b = adsorption parameter (l mg–1), Ce = equilibrium concentration (mg l–1) and Q0 = maximum capacity (mg g–1), which is deemed to be more suitable due to the expected chelation mechanism.

qA =b.Ce.Q

0

1+b.Ce (iv)

Both materials show a similar maximum capacity (Q0) from the Langmuir fitting at 28.8 mg g–1 and 31.2 mg g–1 for FS1 and Smopex®-112 respectively. However, the adsorption with FS1 is much more favourable with an adsorption parameter (b) of 5.1 l mg–1 compared to 0.50 l mg–1 for Smopex®-112.

Column Adsorption Trials

Due to the scale of liquid flow in a mining application the final process must run continuously; therefore columns are preferred. To test the feasibility column trials were run with a model solution containing 5 ppm gold and 5 ppm mercury at pH 10 with no added cyanide. Figure 9 shows that the outlet mercury concentration was below the instrument detection limit (0.01 ppm) during the entire test period; gold was initially adsorbed but then partially displaced as indicated by the outlet concentration going above the inlet concentration ([M]/[M]0>1). Figure 10 shows the effect of adding 100 ppm cyanide to the test solution. In this case no gold was adsorbed by the material whilst mercury adsorption was maintained; with the outlet mercury

Met

al c

once

ntra

tion,

ppm

4

3

2

1

0

Time, min0 20 40 60

AuHg

Fig. 7. Adsorption kinetics of 4 ppm gold and 1 ppm mercury cyanides at pH 12 using FS1. After 30 minutes an aliquot of sodium cyanide is added equivalent to 100 ppm. The full lines/symbols show the concentration for the standard adsorption; the dashed line/open symbols show the solution concentration with cyanide addition. This clearly shows that upon addition of cyanide the gold is rapidly desorbed from the resin whilst mercury remains adsorbed

Fig. 8. Graph showing adsorption isotherm of mercury as K2Hg(CN)4; initial mercury concentration 100 ppm, with the dashed lines showing the Langmuir isotherm fitting (anomalous data point with open symbol ignored for Langmuir fitting)

0 20 40 60 80 100Mercury equilibrium concentration, ppm

Mer

cury

ads

orbe

d, m

g g–1

35

30

25

20

15

10

5

0

FS1Smopex®-112



concentration being below the detection limit of the analysis (0.01 ppm). The low gold concentration of the first bed volume is due to the dilution effect of the column pre-rinse.

The material loaded in Figure 9 was subjected to washing with an increasing concentration of cyanide solution at pH 10, shown in Figure 11. Initially, when washed with dilute sodium hydroxide the gold concentration rapidly decreased as the load solution was rinsed from the packed bed (BV 1 to 6); when 100 ppm cyanide was added to the wash solution (BV 7 to 12) then the gold was rapidly eluted. The mercury was retained by the solid even when the

cyanide concentration was increased to 1000 ppm. This provides further evidence for the equilibrium nature of the gold-thiolate interaction which can easily be perturbed by increasing the free cyanide concentration. Meanwhile the mercury-thiolate interaction is considerably more stable.

Real Stream Adsorption

Due to the complexity of the real solution streams the batch adsorption was repeated using a real sample of heap leach pregnant solution containing 1.22 ppm gold and 0.31 ppm mercury at pH 10.0; both FS1 and Smopex®-112 were tested as adsorbents. The solution was tested both as received and with cyanide added at 100 ppm in order to assess whether the addition of cyanide would improve the selectivity of adsorption in a real feed. Gold adsorption was observed from the as received test solution with both materials (Figure 12), with 7% and 20% gold removal with FS1 and Smopex®-112 respectively. In any real world application non-selective adsorption would represent loss of product and is therefore unacceptable. When the cyanide was added to the received material the gold removal was reduced to less than 1% with Smopex®-112 and to undetectable levels with FS1. Both materials reduced the mercury concentration from 0.31 ppm to below the detection limit of the instrument (<10 ppb), this was regardless of whether cyanide was added to the solution. This represents the ideal result for the desired application whereby a minor modification or monitoring of the stream allows highly selective adsorption of mercury from a gold bearing stream. The process is currently being piloted in Nevada, USA (shown in Figure 13).

AuHg

0 3 6 9 12 15 18Bed volumes

1.21.00.80.60.40.2

0

[M]/[

M] 0

Fig. 9. Flow test of adsorption of 5 ppm gold and 5 ppm mercury by FS1 at a flow rate of 6 BV h–1. Mercury concentration was below the instrument detection limit (0.01 ppm) during the entire test period; gold was initially adsorbed but then partially displaced (outlet concentration above inlet concentration)

AuHg

0 3 6 9 12 15 18Bed volumes

1.21.00.80.60.40.2

0

[M]/[

M] 0

Fig. 10. Flow test of adsorption of 5 ppm gold, 5 ppm mercury and 100 ppm cyanide by FS1 at a flow rate of 6 BV h–1. Mercury concentration was below the instrument detection limit (0.01 ppm) during the entire test period; gold outlet concentration was at the inlet concentration immediately (ignoring any initial sample dilution effect)

0 6 12 18 24 30Bed volumes

Met

al c

once

ntra

tion,

ppm

5

4

3

2

1

0

Cyanide concentration, ppm

1000

800

600

400

200

0

Fig. 11. Elution of loaded resin from Figure 9. The material was washed with deionised water at pH 10 with increasing cyanide concentration. The initial gold concentration is a dilution effect of the load solution. As 100 ppm cyanide is added to the wash water the gold is rapidly eluted. No mercury is detected (<0.01 ppm) even at a cyanide concentration of 1000 ppm

AuHgCN



Conclusions

In this study infrared spectroscopy was used to show that [Hg(CN)4]2– and [Au(CN)2]– are the most likely species to occur within a mercury containing gold cyanide stream, as would be found in a gold mining circuit where cyanide is used as the lixiviant. These species were then used to conduct model adsorption tests in order to identify a method of selective mercury adsorption.

By adding free cyanide to the system (or ensuring that free cyanide exists in the solution) it was found that the selective adsorption of mercury in the presence of gold

could be achieved when using thiol based adsorbents. The adsorbents were effective regardless of whether the thiol was bound to a silica or polymer based support. The theory was then applied to a real sample from a mining circuit, with the real stream it was found that when the adsorbents were applied to the solution as received then an unacceptable level of gold adsorption was obtained in conjunction to the mercury adsorption. When cyanide was added to the received solution then gold adsorption was completely prevented whilst mercury adsorption was maintained, with the adsorbent reducing the mercury concentration to below the detection limit (0.01 ppm) of the analytical equipment.

AuHg

Fig. 12. Batch adsorption from real feed (heap leach pregnant solution) containing 1.22 ppm gold and 0.31 ppm mercury at pH 10.0 using: (a) FS1; (b) Smopex®-112. Both adsorbents show unacceptable levels of gold adsorption with the as received samples whilst adding 100 ppm of cyanide led to less than 1% of the gold being adsorbed with Smopex®-112 and undetectable gold adsorption with FS1

As received Plus 100 ppm As received Plus 100 ppm cyanide cyanide

100

80

60

40

20

0

Met

al re

mov

ed, %

Met

al re

mov

ed, %

AuHg

(a) (b)100

80

60

40

20

0

Fig. 13. 70 l pilot unit in Nevada, USA, operating at flow rates in the order of tens of litres per minute



The adsorbed mercury is strongly bound to the resin as an insoluble complex without leaching under mild conditions; therefore the material can be more easily handled or stored than many mercury complexes. Alternatively, it is envisaged that the material could be regenerated with strong concentrated acids (14).

The concept was also tested using a flow system which is likely to be the final application method. Here it was also found that the addition of cyanide to the solution led to the prevention of gold adsorption whilst maintaining a mercury outlet concentration below the detection limit of the equipment (0.01 ppm) during the test period. Additionally, the solution was run with no free cyanide and a dilute cyanide solution was then used to rinse any adsorbed gold from the material. This shows that alternative process solutions can be used depending on the individual process economics with both methods being based on the same scientific concept.

References 1 L. Elsner, J. Prakt. Chem., 1846, 37, (1), 441

2 J. S. MacArthur, R. W. Forrest and W. Forrest, ‘Improvements in Obtaining Gold and Silver From Ores and Other Compounds’, British Patent 14,174; 1888

3 J. D. Miller, E. Alfaro, M. Misra and J. Lorengo, ‘Mercury Control in the Cyanidation of Gold Ores’, in “Pollution Prevention for Process Engineers”, eds. P. E. Richardson, Frank Lanzetta Jr and B. J. Scheiner, Engineering Foundation, New York, USA, 1996, pp. 151–64

4 M. Misra and J. A. Lorengo, Board of Regents of the University and Community College System of Nevada, ‘Method of Removing Mercury From Solution’, US Patent 5,599,515; 1997

5 F. Tassell, J. Rubio, M. Misra and B. C. Jena, Min. Eng., 1997, 10, (8), 803

6 A. R. Hutchison and D. A. Atwood, J. Chem. Cryst., 2003, 33, (8), 631

7 M. M. Matlock, B. S. Howerton, M. A. Van Aelstyn, F. L. Nordstrom and D. A. Atwood, Environ. Sci. Technol., 2002, 36, (7), 1636

8 R. M. Smith and A. E. Martell, “Critical Stability Constants: Volume 4: Inorganic Complexes”, 3rd Edn., Springer Science+Business Media, New York, USA, 1976

9 S. Phillips and P. Kauppinen, Platinum Metals Rev., 2010, 54, (1), 69

10 K. Nakamoto, “Infrared and Raman Spectra of Inorganic and Coordination Compounds”, 3rd Edn., Wiley and Sons Inc, New York, USA, 1978

11 M. M. Kreevoy, E. T. Harper, R. E. Duvall, H. S. Wilgus III and L. T. Ditsch, J. Am. Chem. Soc., 1960, 82, (18), 4899

12 “Fact Sheet - Cyanide and its Use by the Minerals Industry”, Minerals Council of Australia, Australian Capital Territory, Australia, 2005

13 G. Lewis and C. F. Shaw III, Inorg. Chem., 1986, 25, (1), 58

14 A. Arencibia, J. Aguado and J. M. Arsuaga, Appl. Surf. Sci., 2010, 256, (17), 5453

The Author

James Stevens is a Senior Scientist at the Johnson Matthey Technology Centre, Sonning Common, UK. His work is focused on the development and application of solid adsorbents to the removal of metals from process water. Previously he gained his PhD at the University of Nottingham, UK, under the supervision of Martyn Poliakoff. His thesis focused on the hydrogenation of biorenewables in supercritical carbon dioxide.




In the Lab

Uranium Capture From High Sulfate and Nitrate Waste Streams with Modifi ed Silica Polyamine CompositesJohnson Matthey Technology Review features new laboratory research

Edward Rosenberg is a Professor of Chemistry at the University of Montana, USA. His research interests are in the areas of the applications of composite materials for metal ion removal, separation and concentration from aqueous systems.

About the Research

The objective of this research is to fi nd a solid phase adsorbent that is selective for uranyl cation (UO2

2+) in the types of waste streams found on the Navajo reservation in the southern part of the USA. The technology could potentially be used to remove this ion from ground and surface waters and to remove trace uranyl from drinking water supplies on the reservation.

There are several ion exchange materials on the market that report effective removal of uranium from water. Silica polyamine composites (SPC) are patented and commercialised ion exchange materials currently being developed for use in water remediation by Johnson Matthey Water Technologies division. SPC are made by the coating of amorphous silica gel particles with functionalised silane, which are further reacted with polymeric amines to provide the parent composites WP-1 and BP-1 (Figure 1).

The parent SPC can then be modifi ed to make them more specifi c to a given metal or group of metals (Figure 2).

Preliminary results on the removal of uranyl cation from solutions that mimic the contamination profi les of waste streams on the Navajo reservation have been obtained using a range of SPC materials. The most effective SPC is then compared with a polystyrene material

with related functional groups using equilibrium batch studies. The results of this exciting work are expected to be submitted for publication shortly.

Much work remains to be done before the group can go forward to applying this technology to remediation on the Navajo reservation. Breakthrough studies are currently underway as well as more direct comparisons with other materials. Most importantly cycle testing must be done with actual waste stream samples from the

About the Researcher

• Name: Edward Rosenberg

• Position: Professor

• Department: Chemistry and Biochemistry

• University: University of Montana

• Street: 32 Campus Drive

• City: Missoula, 59812

• Country: USA

• Email Address: [email protected]



NN

P NH

CuSelect (P = PAA) selective for Cu2+ over Fe3+ at low pH

Made by reaction with picolyl chloride or by hydramination of pyridine-2-carboxaldehyde

BPAP (P = PAA): selective for trivalent over divalent metals. Highly charged metals can be immobilised for anion capture

WP-2 (P = PEI), BP-2 P = PAA): pH dependent selectivity for divalent transition metals

P

HN CH2CO2H

Made by reaction with chloroacetic acid

WP-4 (P = PAA): selective for Fe3+, Ni2+ and Ga3+ over Al3+ at pH~2

P NH

OH

X

Made by Mannich Reaction with CH2O (X = H, Cl, SO3H)

P

HNP

OH

OOH

Made by Mannich Reaction CH2O + phosphorous acid

BPED (P = PAA), WPED (P = PEI) Very selective for Ni2+ over Co2+, Fe2+ and Zn2+

P

HN C

O

CH2 NCH2CH2NCH2CO2H

CH2CO2HHO2CH2C

Made by reaction with EDTA anhydride

BP-NTA (P = PAA), WP-NTA (P = PEI) Very selective for Ni2+ over Co2+, Fe2+ and Zn2+

P

HN

O

C

CH2 N CH2CO2HHO2CH2C

Made by reaction with nitrilo-acetic anhydride

Fig. 2. Ligand modifi ed SPC and their applications to date

N N N N N N

H2N H2N H2N

H2N

H

NH2

H2N H2N H2N H2N H2N

Poly(ethyleneimine) MW = 23 Kor

Poly(allylamine) MW = 11–15 K

+

CH3SiCl3:ClCH2CH2CH2SiCl37.5 1.0

+

Acid washed and humidifi ed amorphous silica gel

or

Si

NH2 H2NNH2

NH2

HN H2N HN H2N

O

O

O

O

O

O O

O OSi Si SiCH3 CH3

O O OSi Si Si

Si

OH

Si Si Si Si

OH OH OHO O O

O OOO

O OO

Si Si SiSi

WP-1

N

NH2

N N N N N

H2N

H

HN H2NHN

Si O SiCH3

O OO

CH3

Si O SiO

O

OSi Si SiO

OSi

BP-1

Fig. 1. Synthesis of the SPC materials



reservation to evaluate the usable lifetime of the solid phase adsorbents under real working conditions. The results obtained to date are certainly worth following up with pilot scale runs as the next milestone.

Acknowledgements

Ranalda Tsosie is a graduate student in chemistry and environmental studies at the University of Montana and her work on this project is acknowledged.

Selected Publications

E. Rosenberg, P. Miranda, Y. O. Wong, ‘Oxine Modifi ed Silican Polyamine Composites for the Separation of Gallium from Aluminium, Ferric from Nickel and Copper from Nickel, US Patent 8,343,446; 2012

Y. O. Wong, P. Miranda and E. Rosenberg, J. Appl. Polym.

Sci., 2010, 115, (5), 2855

M. A. Hughes, J. Wood and E. Rosenberg, Ind. Eng. Chem.

Res., 2008, 47, (17), 6765

M. A. Hughes and E. Rosenberg, Sep. Sci. Technol., 2007,

42, (2), 261

E. Rosenberg and R. J. Fisher, University of Montana,

‘Materials and Methods for the Separation of Copper

Ions and Ferric Iron in Liquid Solutions’, US Patent

7,008,601; 2006

E. Rosenberg and D. C. Pang, University of Montana,

‘System for Extracting Soluble Heavy Metals from

Liquid Solutions’, US Patent 5,997,748; 1999




New Smopex® Ion Exchange Materials for the Removal of Selenium from Industrial Effluent StreamsMaterial characterisation, modelling and process implementation

By Carl Mac Namara*, Javier Torroba# and Adam DeaconJohnson Matthey Plc, PO Box 1, Billingham, Cleveland, TS23 1LB, UK

Email: *[email protected]; #[email protected]

This article discusses new Smopex® ion exchange materials which have been developed by Johnson Matthey Water Technologies, and highlights their performance relative to other commercially available materials for the removal of selenocyanate, selenate and selenite ions from aqueous solutions. The ion exchange mechanisms by which these materials sorb these ions are also explained and modelled in order to highlight the additional benefits that these materials offer that non ion-exchange materials do not, such as the ability to achieve the full material exchange capacities at feed concentrations lower than 1 mg l–1 selenium. The unique characteristics of these fibrous type materials are also discussed, including fast sorption kinetics, facile regeneration and enhanced selectivity for selenium ions against competing sulfate ions. Finally, the performance of these materials in a continuous stirred tank reactor setup is demonstrated, showing that performance levels as high as in fixed bed processes can be achieved, due to the high selectivity and mass transfer kinetics of Smopex® materials.

1. Introduction

Selenium is a non-metal, trace element with crucial roles in animal and plant biology, although it becomes highly toxic at relatively low levels. The European Union suggests a maximum Se concentration of 10 mg l–1 in drinking water (1) while the guideline in USA was set at 50 mg l–1 (2). Selenium forms different water-soluble ions that can be found in aquatic environments from both natural and industrial origins. Fossil fuel and mining related activities, agriculture and glass manufacture constitute the most significant contributors to anthropogenic sources of selenium contamination (3).

Selenium remediation has been widely investigated during the past few decades, although today few technologies are being applied on a commercial scale (3). An efficient process must deal with the difficult task of removing selenium from large volumes at low concentrations (although above the regulated levels), the complex speciation of selenium, and the presence of high concentrations of sulfur. For example, wastewater from oil refineries can contain selenium in the order of few parts per million (ppm, mg l–1), typically as a mixture of selenocyanate (SeCN–) and selenate (SeO4

2–) ions (4). Sulfate (SO42–) would be found in the

same feeds at more than 10 times that concentration (1). Treated effluents from flue-gas-desulfurisation (from coal combustion power plants) can contain ppm levels of selenium, most commonly as selenate, but as much as 1 g l–1 of sulfur in the form of sulfate (5).



Current technologies include chemical and biological methods. Amongst the first group it is easy to find processes based on the reduction of selenium ions using reagents such as zero-valent iron (6, 7) or sodium sulfide (8). Although relatively cost-effective techniques, they are extremely dependent on pH and temperature conditions and require vast quantities of non-reusable reactants. Biological methods still demand rigorous control of pH, salinity and temperature, although they consume smaller amounts of additional chemicals (3, 9). Anaerobic tank reactors or packed bed systems can be designed to accommodate bioreduction of selenium ions by specific bacteria strains. Treatment of large volumes of slurry waste is necessary to separate the solid elemental selenium.

Physical separation methods have also been explored for selenium remediation, but only reverse osmosis and nanofiltration seem to decrease the concentration of selenium ions below the acceptable discharge levels. Both technologies present high operating costs and require efficient pretreatment of feeds to avoid fouling and degradation of membranes (10).

Ion exchange technology is widely used for many different municipal and industrial wastewater treatments. All kinds of natural and synthetic sorbents have been investigated as potential materials for selenium remediation at bench scale, but there is no reference to full scale processes being implemented (3). Materials from varied groups such as resins, carbon-based adsorbents and metal oxides have been found to show potential activity to adsorb Se ions from water media. Amongst them, only a few synthetic materials were reported as having the necessary selenium capacity to be considered potential solutions (3). Unfortunately, most of them are unable to achieve good levels of selectivity for the removal of selenium in the presence of sulfur or other contaminants, limiting the prospects of any potential applications. Advanced selective ion exchange technology could help in mitigating these disadvantages.

In this article we discuss our investigations of the fundamentals of selective sorption of selenium ions from aqueous solutions using strong-base functionalised materials. Exploratory tests highlighted the potential of Johnson Matthey’s synthetic scavengers for the removal of inorganic selenium species. Strong-base type materials were identified as the best candidates (11), although weak-base type materials sometimes also show high affinities for inorganic anions such as phosphate (12) and arsenate (13). Strong-base

functionalisation is generally based on fixed quaternary amines (ammonium cations), while weak-base type materials are based on alkyl and/or aromatic amines. The advantage of strong-base materials is that, thanks to having fixed positively-charged groups, they can act as anion exchangers in wider pH ranges than weak-base type materials. In this work we investigate in detail the performance of some materials within this class: the polymeric fibres Smopex®-103 and Smopex®-269, the silica-polymer composites WP11 and WP13, and two standard polystyrene resins from Dow Chemicals, AmberliteTM IRA 900 and DowexTM

1x2 100–200. All these materials have fixed positively-charged groups and mobile chloride ions as the exchangeable anions. In particular, Smopex®-103 bears trimethylammonium groups (similar to Dow’s resins) while the functionality of Smopex®-269 is based on the aromatic base benzyl-pyridinium (Figure 1).

Smopex® (Figure 2) is a unique type of material where the binding functionality lies on side chains grafted onto 0.3 mm long polymeric fibres (olefin or natural) (14), in contrast with standard spherical porous resin beads used in common adsorption or ion exchange processes. This structure grants an efficient, fast recovery of the target species with very high loading capacity. On the other hand, Johnson Matthey’s silica-polymer composites could seem, in principle, a very different kind of material (Figure 2) (15), formed by a silica particle core coated with a polymer bound to the silica chemically. However, the active functionality is homogenously distributed onto the surface of the coating polymer, allowing an excellent interaction with the liquid medium and thus enhancing properties such as kinetics and loading capacities, as in the case of Smopex®.

In the next sections we discuss experimental results and modelling work carried out in order to determine the principles governing the sorption of selenocyanate (SeCN–), selenite (SeO3

2–) and selenate (SeO42–) ions

onto the different strong-base sorbents investigated. A special attention is paid to the removal of selenium ions in the presence of the competing ion sulfate (SO4

2–).

Fig. 1. Chemical functionalisation of: (a) Smopex®-103; and (b) Smopex®-269

N+ Cl–Cl–

N+

(a) (b)



The effects caused by other potentially competing ions such as nitrate, phosphate or bicarbonate are not covered in this work.

2. Experimental

All chemicals were purchased from Alfa Aesar and scavenging materials were used as supplied by Johnson Matthey, Finland. Sodium selenite (Na2SeO3·5H2O), sodium selenate (Na2SeO4·10H2O) and potassium selenocyanate (KSeCN) were used as selenium sources for the model solutions, while sodium sulfate (Na2SO4 anhydrous) was used for sulfur containing feeds. Sodium chloride (NaCl, 99% min) solutions were used as eluent. Solutions were prepared at room temperature using demineralised water. First, an adequate amount of the corresponding salt was dissolved in water in a beaker and then diluted to the final volume in a volumetric flask. Further dilutions were carried out in order to prepare the very dilute solutions. pH was adjusted using small volumes of HCl 1 M or NaOH 1 M when necessary.

Please note that concentrations expressed in w/v terms in this document are based on elemental concentration, not on molecular concentration. For example, a KSeCN solution with Se concentration of 200 mg l–1 would actually contain 365 mg l–1 of KSeCN.

2.1 Analytical

Selenium and sulfur concentrations in the samples were determined by elemental analysis by ICP-MS (Perkin Elmer Elan 6100 DRC) or ICP-OES (Thermo Scientific iCAP 7600 Radial). All samples were acidified with 0.1 ml of 69% HNO3 prior to analysis.

Selenocyanate-containing samples were kept at basic pH (addition of 0.1 ml of 1 M NaOH) to prevent decomposition until just before the analysis. Chloride levels were analysed by ion chromatography. Dilution steps were included, when necessary, to adjust sample concentration to calibration window in the equipment. Results were compared to certified external standards.

2.2 Materials

The Smopex® materials are polypropylene fibres, having a trilobal shape and a typical length of 300 mm and diameter 50 mm (see Figure 2). The polypropylene fibres are cut to this length during the manufacturing stage, prior to functionalisation. The bulk density of the dry Smopex® fibres is approximately 275 g l–1. The dry content (mass of dry material per mass of the supplied material which includes moisture) of the supplied Smopex® is approximately 60%. The spherical silica polymer composite materials have a particle diameter range of 250–750 mm and bulk densities for the dry material of 500 to 600 g l–1. The silica polymer composite materials are supplied dry. Properties of the AmberliteTM IRA-900 and DowexTM

1x2 can be readily found online, however the dry contents of these materials as supplied were measured at 61% and 75%, respectively. Bulk densities of the dry material were measured at approximately 373 g l–1 and 468 g l–1, respectively. All of the materials were used in the experimental trials as supplied.

2.3 Adsorption Tests

Different procedures were followed to carry out tests, depending on the nature of the experiment. They are based on standardised procedures used by Johnson

(a) (b) (c)

(d)

20 mm

Fig. 2. (a) and (b) photographs of Smopex® fibres; (c) SEM of Smopex® fibres; (d) photograph of silica-polymer composite material



Matthey Water Technologies. In this investigation all tests were done at room temperature.

2.3.1 Determination of Batch Kinetic Profiles

These tests were carried out at neutral pH and 25ºC. Scavenger masses of 0.150 g (based on dry content) were put in contact with 15.0 ml of selenate, selenite or selenocyanate solutions with 1.0 mg l–1 of Se (0.013 mmol l–1). The mixtures were allowed to react for different lengths of time under gentle stirring (60 rpm), typically for 1, 2, 5, 10, 30, 60 and 120 minutes. Sorption tests with 30 seconds of contact time were also performed for Smopex® materials. Once the reaction time had passed, the solutions were filtered out from the suspensions and analysed by ICP-MS.

2.3.2 Determination of Adsorption Isotherms

The procedure requires using fixed concentration and volumes of solution and varying masses of scavenger material in parallel tubes in order to cover the expected range of loadings and equilibrium concentrations (Figure 3). These tests were carried out at room temperature and allowing enough reaction times to reach equilibrium conditions (generally overnight). The Se and S initial concentrations were kept fixed for a given isotherm trial but varied across different trials, and ranged from 10 to 500 mg l–1 (Se: 0.13 to 6.33 mmol l–1; S: 0.31 to 15.6 mmol l–1). The required masses of the materials were calculated based on an ‘expected maximum loading’, i.e. a mass range of material was added such that the equilibrium concentrations at the end of the trial were expected to range from 10% to 90% of the initial concentration value. Competitive

isotherms with sulfate were run using Se:S equimolar solutions with 150 mg l–1 of Se and 60 mg l–1 of S (1.9 mmol l–1).

2.3.3 Continuous Flow Column Tests

The general procedure is based on passing a feed of known Se or Se/S concentration through a column (Figure 4) containing a known amount of scavenging material, generally between 1 and 2 g (dry content). The fixed-beds in these experiments had volumes between 3 and 7 ml. Flow rates were kept constant at 36 ml h–1 while the lengths of the experiments were altered depending on the case. Initial Se concentration was adjusted to 1.0 g l–1 (12.7 mmol l–1) for Smopex®

fibres and Dow resins and to 0.50 g l–1 (6.3 mmol l–1) for silica-polymer composites. Lower initial concentrations were used for the latter materials in order to optimise the length of the experiment, as they have generally lower maximum materials concentration than Smopex® and Dow materials.

For competitive tests with sulfate, a Se:S mass ratio of 1:12 was used (1:30 molar ratio) with initial Se concentrations of 0.50 and 0.25 g l–1 (6.3 and 3.2 mmol l–1) and S levels of 6.0 and 3.0 g l–1 respectively (187 and 93.6 mmol l–1).

Desorption studies were carried out in a similar way, but passing NaCl solutions through the column at similar rates (36 ml h–1), containing in this case about 1.0 g of scavenger pre-loaded with selenium ions. Eluents with different chloride concentration were used, ranging from 0.06 mol l–1 (0.2 wt%) to 2.82 mol l–1 (10 wt%).

Fig. 3. Experimental apparatus (temperature controlled multi-vial carousel) for batch kinetics test and isotherms determination studies

Fig. 4. Experimental apparatus (column and fraction collector) for the continuous flow column tests



Samples were collected downstream at regular intervals using an automated fraction collector.

2.3.4 Continuous Stirring Tank Reactor

For this test a baffled cylindrical glass reactor,fabricated in house,was used (Figure 5) which allowed continuous flow (at 100 ml h–1) into the reactor of a SeCN– feed having a concentration 450 mg l–1 (5.7 mmol l–1). The reactor liquid level was controlled using a glass dip tube. Both the inlet tube at the bottom of the reactor and dip tube were fitted with glass frits to prevent Smopex® exiting the reactor. 30 g (dry content) of Smopex® 103 was vigorously mixed with the reactor solution for the duration of the test. The operating volume of the reactor was 0.64 l. Samples were manually collected downstream at regular intervals (every hour). The experimental results from a single continuous stirring tank reactor (CSTR) are presented in Section 4.4. The model was validated for the single CSTR system and deemed to be suitable for accurately predicting performance in the two CSTR system, therefore only the predicted performance for a two CSTR in-series system are shown with no experimental results to verify these predictions.

3. Modelling

The modelling of the ion exchange mechanisms discussed in this paper is an important task as it not only enables the rigorous scale-up of material behaviour from lab to plant scale, but also further informs material researchers with additional information which would

not usually be elucidated from simple charts and with which they can think up new interesting experiments. It must be emphasised that while modelling in this paper appears to be the representation of sorption mechanisms through mathematical equations, the bulk of the modelling exercise undertaken here is rather the understanding of the real sorption mechanisms taking place so that the correct mathematical expressions can then be applied. Special focus throughout the paper will thus be given to highlighting the mechanisms involved in the sorption processes and how they influence the choice of mathematical models.

Modelling the removal of ions from solution by ion exchange involves both mass transfer and equilibrium equations (16). Typical models used to represent sorption equilibrium are the Langmuir, Freundlich and mass action law models (17–18). The Langmuir and Freundlich models both consider a material as having adsorption sites with equal or different adsorption potentials for solution ions, and are often used to model the sorption isotherms of both adsorption and ion exchange systems as the fit to experimental data in both cases is often very good (19–21). Several authors (22–23) have already discussed the unsuitability of these models in accounting for all the mechanisms at play in ion exchange systems, leading to inaccurate and unreliable predictive models for scale-up and design of ion exchange processes. The effect of these mechanisms on the engineering of ion exchange processes is further discussed and featured in this study. The mass action law has been used to model the specific mechanisms of ion exchange systems, such as electroneutrality in the material, non-ideality of the solution and material phases as well as the effect of the ion released from the material during ion exchange (24). Equilibrium is represented in the mass action model by Equation (i):

Kqq

C

Ci jm i

Z

m jZ

s jZ

s iZ

i

j

j

i

j

i

i

j− =γ

γ

γ

γ

*

*

*

*

(i)

where i and j refer to two distinct ionic species in the system, Ki–j is the thermodynamic equilibrium constant, ɣsi and ɣsj are the activity coefficients in the liquid phase, ɣmi and ɣmj are the activity coefficients on the material, q*

i and q*j are the concentrations in the

material phase at equilibrium (eq g–1), C*i and C*

j are the concentrations in the liquid phase at equilibrium (eq l–1) and zi and zj are the absolute values of each species’ valency. The sum of the concentrations on the

Fig. 5. CSTR reactor, shown without Smopex® in the reactor for clarity



material phase is equal to the real exchange capacity (REC) on the material, as given by Equation (ii):

REC q q qn= + +…+1 2* * * (ii)

Where n is the number of ionic species in a given system, including the species initially present and then released from the material and with which ion exchange can occur. The REC is determined experimentally and is usually lower than the theoretical exchange capacity (TEC) of a material which is measured by chemical analysis of the material (23).

In order to estimate activity coefficients for a given system, the Bromley method (25) has been used for estimating the liquid phase coefficients and the Wilson method (26) for the material phase coefficients. Estimating the values of additional parameters in the Wilson model and the values of the equilibrium constants in Equation (i) is achieved by using nonlinear optimisation methods to minimise predicted and experimental equilibrium data (for example sorption isotherms) from binary systems. A full description on the use of these methods has been well described by other authors (24) and will not be repeated here.

The equilibrium models are then combined with mass transfer models to predict the dynamic sorption behaviour of adsorption and ion exchange processes. As most commercially available ion exchange resins are typically macroporous polymeric resin beads with chemical functionality located both within the pores of the resin and on the surface (27), bulk mass transfer models are combined with film diffusion and intra-particle diffusion models to predict the material concentration profiles as a function of time (23). Furthermore as commercial processes usually consist of fixed-beds through which fresh feed is continuously introduced, concentration profiles which vary along the axis of the column must be predicted by the model as well as axial dispersion effects, ultimately resulting in a large set of partial differential and non-linear equations which must be solved simultaneously.

The system under study here is somewhat simplified as first of all, the Smopex® materials have all of their functionality located on the surface of the material and thus only bulk and film diffusion mass transfer equations need be considered. Secondly, the Smopex® behaviour has specifically been studied in a CSTR setup, where perfect mixing was assumed and the dynamic behaviour can be represented by ordinary

differential equations. The film diffusion mass transfer equation is given by Equation (iii) (28):

dqdt r

k C Cif b si i

= −3ρ

( )*

(iii)

where r is the material particle radius (m), ρ the particle density (g l–1), kf the film mass transfer coefficient (m s–1), Cbi the bulk solution concentration of species i (eq l–1) and C*

si the concentration of species i (eq l–1) at the surface of the material, at equilibrium with the ion concentration in the material qi. C*

si is equivalent to C*i

in Equation (i). The bulk mass transfer in the CSTR is given by (Equation (iv) (28)):

dCdt

mVdqdt

Q C CV

b i f b fi i i= − +−( )

(iv)

where m is the mass of material in the system (g), V is the volume of liquid (l), Qf is the feed flow rate into the CSTR (l s-1) and Cfi is the concentration of species i in the feed (eq l–1).

In modelling the ion exchange sorption in the CSTR process (see Section 4.4), the following assumptions were made:• The only resistance to mass transfer is film

diffusion resistance• Ion exchange at the liquid/solid interface, i.e.

the material surface is instantaneous and the equilibrium between both phases can be represented by the mass action law

• The process occurs under isobaric and isothermal conditions

• Physical properties of the ion exchanger and liquid are constant

• As Smopex® materials are not spherical, the radius used in Equation (iii) corresponded to a sphere with equivalent surface area to a typical Smopex® particle (approximately 60 mm).

Equations (i)–(iv) were solved using the Athena visual studio (29) software to obtain the predicted liquid and material phase concentrations as a function of time of all ionic species involved in the ion exchange. Additional model parameters, such as the particle radius r and particle density ρ, were obtained from experimental measurements or material characterisation techniques, while the film mass transfer coefficient kf was estimated from the experimental bulk liquid concentration data.



4. Results and Discussion4.1 Material Selectivity

In Figures 6, 7 and 8, the sorption isotherm results for selenocyanate, selenate and sulfate ions, respectively, are plotted for the four functionalised materials. The points represent experimental measurements while the dashed lines are fitted lines with no physical significance but which assist in interpretation of the material sorption behaviour.

In Figure 6, Smopex®-269 is the most ‘selective’ material for selenocyanate, since it has the highest material concentration at low liquid concentrations, while the highest material concentration is achieved on Smopex®-103, at approximately 1.7 mmol per gram of dry Smopex®-103. Interestingly, this concentration is approximately equal to the measured nitrogen content on Smopex®-103 of 1.8 mmol g–1, i.e. the TEC, suggesting that nearly all of the functionality on the material is available for sorption. This is likely linked to design of the Smopex® material where all of the functionality is located on the surface of the polymeric fibres, in contrast to typical sorption materials where most of the functionality is located within the porous structure of the material. WP11 has the overall lowest material concentrations but appears to be more selective than WP13. The maximum material concentration of WP13 was not reached in these experiments but is likely to be similar to that of Smopex®-269 at approximately 1.2 mmol g–1.

It must be pointed out that there is one critical omission from these results: the equilibrium liquid

and material concentrations of chloride, which also change as selenocyanate concentrations change, due to ionic exchange of chloride and selenocyanate between the material and liquid solution. The sorption isotherms experimentally measured here are only valid for systems having equal levels of chloride initially present on the material. Thus a material having more or less functionality and hence differing initial concentrations of chloride on the material will lead to a different selenocyanate sorption isotherm. In the case of sorption by adsorption or chelation, there would be no exchanged ion from the material and so the isotherm itself would only depend on those ions initially in solution. Thus models such as the Langmuir or Freundlich isotherms are not applicable for modelling this system, even though they would probably fit the experimental data very well.

In Figure 7, Smopex®-103 is now the material with both the highest material concentration and selectivity for selenate. The maximum material concentration has approximately halved from 1.7 mmol g–1 for selenocyanate to approximately 0.85 mmol g–1 for selenate. This is because the selenate ion is divalent so two chloride ions must be exchanged in order to preserve electroneutrality in the material. The same decrease in capacity is observed for all four materials. This behaviour would not typically occur with materials where sorption occurs through chelation or adsorption, as there is no exchanged ion and thus valency is not important in determining material concentrations. The selectivity and maximum material concentrations of Smopex®-269 and WP13 are approximately equal for selenate. Contrasting Figures 6 and 7, it is clear that Smopex®-269 is no longer the material with highest

SeCN–

Smopex®-103Smopex®-269WP11WP13

0 0.5 1 1.5 2 2.5 3Equilibrium liquid concentration, mmol l–1

Equ

ilibr

ium

mat

eria

l co

ncen

tratio

n, m

mol

g–1

2.01.81.61.41.21.00.80.60.40.2

0

Fig. 6. Selenocyanate sorption isotherm. ‘Equilibrium material concentration’ is the concentration of the sorbed species on the material, expressed in mmol of species per gram of dry material. The initial SeCN– concentration was approximately 6 mmol l–1 and volume of solution per sample was 200 ml

SeO42–



Equ

ilibr

ium

mat

eria

l co

ncen

tratio

n, m

mol

g–1

1.00.90.80.70.60.50.40.30.20.1

0

Fig. 7. Selenate sorption isotherm. The initial SeO42–

concentration was approximately 3 mmol l–1 and volume of solution per sample was 200 ml



selectivity, as it was for selenocyanate. It is believed that the sorption sites of Smopex®-269 are hindered by bulkier molecular groups when compared to the other materials, which mitigates the sorption of the selenate ion, while the smaller and linear shaped selenocyanate ion is unaffected. Thus while Smopex®-269 still exhibits good recovery for divalent selenate ions, optimal use of its particular functionality is achieved when monovalent ions are recovered.

In Figure 8, WP13 has the highest selectivity but Smopex®-103 has again the highest overall maximum material concentration. The relative selectivity of both Smopex®-103 and Smopex®-269 for sulfate over selenocyanate has now decreased relative to the silica-polymer materials, with Smopex®-269 having the overall lowest selectivity for sulfate. Since sulfate recovery is generally undesired, this lower selectivity is a beneficial property of Smopex® materials, as selenium removal will be favoured in feeds containing both selenium and sulfate ions. Due to their design, the functionality of both Smopex® materials have a higher ionic character than those of the silica-polymer materials, which in turn leads to a stronger sorption interaction with the selenate, due to the selenate having a higher charge density on its oxygen atoms than sulfate. Note that the scale of the equilibrium liquid concentration axis is different in Figure 8 due to the lower molecular weight of sulfate over selenium and thus overall the selectivity of all materials for sulfate has decreased relative to selenium ions.

In Figure 9 the effect of equilibrium pH on the sorption perfomance of selenite on Smopex®-103 is depicted. The solution pH of each experimental trial was adjusted prior to the addition of material and then

measured at equilibrium, where they were found to have not significantly varied. The results found matched expectations and ion exchange theory. Sorption performances of selenocyanate and selenate were not affected by changes in pH within the range 3–10 (acidic conditions for selenocyanate were not investigated as the ion decomposes under those conditions). On the other hand, the performance for selenite was found to be highly influenced by the pH of the solution. The strong-base functionality on Smopex®-103 remains unaltered in a broad pH range, and neither the monovalent selenocyanate nor the divalent selenate ion change their protonation states in the pH range investigated (conjugated bases of very strong acids: pKa(HSeCN) < 1, pK1(H2SeO4) = –3.0, pK2(HSeO4

–) = 1.7). On the other hand, selenous acid is a considerably weaker acid than the other species (pK1(H2SeO3) = 2.6, pK2(HSeO3

–) = 8.1). Thus, the optimum performance for the adsorption of selenite ions takes place at pH 10, where almost all selenium is in the form of the fully deprotonated SeO3

2– (ca. 99% of the total selenium content). A selenite concentration on the material of 0.7 mmol g–1 was achieved at pH 10. Under neutral conditions, ca. 93% of the selenium present is HSeO3

– with the rest being in the form of SeO3

2– (ca. 7%). Thus, a significant decrease of the selectivity is observed at pH 7, as reflected in the shift to higher equilibrium liquid concentrations for the exchange isotherm, although maximum material concentration remains similar to that at pH 10. A major decline in performance is observed under acidic conditions, with a further shift to higher equilibrium liquid concentrations and a significant reduction in material concentrations. At pH 3, the major Se species is HSeO3

– (ca. 70%), with significant

SO42–Smopex®-103

Smopex®-269WP11WP13

0 3 6 9 12 15Equilibrium liquid concentration, mmol l–1

Equ

ilibr

ium

mat

eria

l co

ncen

tratio

n, m

mol

g–1

1.00.90.80.70.60.50.40.30.20.1

0

Fig. 8. Sulfate sorption isotherm. The initial SO42–

concentration was approximately 15 mmol l–1 and volume of solution per sample was 25 ml

SeO32–Smopex®-103, pH 11

Smopex®-103, pH 7Smopex®-103, pH 3


Equ

ilibr

ium

mat

eria

l co

ncen

tratio

n, m

mol

g–1

1.00.90.80.70.60.50.40.30.20.1

0

Fig. 9. Selenite sorption isotherm. The initial SeO32–

concentration was approximately 2.5 mmol l–1



presence of fully protonated, selenous acid (H2SeO3, ca. 30%) and nothing of the fully deprotonated SeO3

2–. In these conditions, a moderate loading of 0.3 mmol g–1 was measured, most likely originated by the uptake of available monovalent HSeO3

– ions.These results indicate that the Smopex®-103 material

presents a greater selectivity for divalent SeO32– than

for monovalent HSeO3–, and that it is incapable of

scavenging the neutral species H2SeO3, consistent with the ion exchange nature of the process. Different sorption mechanisms are required to efficiently remove selenous acid under acidic conditions, as demonstrated by Awual et al. by using silica based material functionalised with chromogenic Schiff bases (30, 31).

In Figures 10 and 11, the effect of initial selenium concentration in solution on the sorption isotherms is shown. Again, in the case of adsorption or chelation, a fixed isotherm would be expected with changing initial concentration. In the case of ion exchange, a varying isotherm can be observed depending on the relative ionic valency of the ions exchanged. In Figure 10, for the case of selenocyanate and chloride exchange, sorption isotherms at both initial selenium concentrations overlap, as the valency of each ionic species is one and hence the isotherm does not depend on initial concentration. In Figure 11, the sorption isotherm of the divalent selenate ion changes as a function of initial selenate concentration, with ‘apparent selectivity’ increasing with decreasing initial concentration. These effects are attributed to a phenomenon known as the Donnan potential (18). The material initially has a higher chloride anion concentration than the surrounding solution which results in the chloride ions

moving out of the material into solution by diffusion. However, since electroneutrality with the fixed cations in the material must be preserved, the chloride ions are pulled back into the material by an electric potential difference, the strength of which increases as the relative electric potential between material and solution increases (for example as solution dilution increases). For a given electric potential difference, the force with which it acts on an ion also increases with ionic charge. Divalent selenate ions are thus more strongly attracted by this electrical potential than the monovalent chloride ions and hence favoured by the material. As dilution increases and the electric potential force increases, this relative preference of the selenate over chloride ions increases further, resulting in the observed increase in ‘apparent selectivity’. The only suitable model for predicting this behaviour must thus include as parameters the selenium and chloride concentrations as well as the valency of the respective ions and activity coefficients to account for the non-ideal sorption behaviour due to the Donnan potential effects.

Figures 12 and 13 show the selenium sorption isotherms for solutions having both selenocyanate and sulfate initially present in solution, for Smopex®-269 and WP11 respectively. Only the results for these materials are shown as Smopex®-269 was observed (Figure 7) to have the highest overall selectivity for selenocyanate and lowest for sulfate, while WP11 had the lowest selectivity for selenocyanate. In Figure 12 the selenocyanate concentration on Smopex®-269 is very selective over sulfate, with sulfate desorbing from the material as liquid concentrations increase. A maximum 5:1 ratio of equilibrium concentrations for

SeCN–

Smopex®-103, 0.25 mmol l–1



Equ

ilibr

ium

mat

eria

l co

ncen

tratio

n, m

mol

g–1

2.01.81.61.41.21.00.80.60.40.2

0

Fig. 10. Selenocyanate sorption isotherm, two different initial solution concentrations. The initial SeCN– concentrations were approximately 0.25 and 6.0 mmol l–1




Equ

ilibr

ium

mat

eria

l co

ncen

tratio

n, m

mol

g–1

Fig. 11. Selenate sorption isotherm, two different initial solution concentrations. The initial SeO4

2– concentrations were approximately 0.1 and 3 mmol l–1

SeO42–

1.00.90.80.70.60.50.40.30.20.1

0



selenocyanate:sulfate is achieved. A maximum 3:1 ratio was achieved with Smopex®-103. By contrast, in Figure 13, there is no discernible difference in selectivity for selenocyanate versus sulfate on the WP11 material. The strong base Smopex® materials are thus well suited to selectively removing selenocyanate from solution in process streams containing both selenocyanate and sulfate.

Figures 14 and 15 show the selenium sorption isotherms in solutions having both selenate and sulfate initially present in equimolar concentrations, for Smopex®-103 and WP11. In this case, there is still a significant selectivity of selenate over sulfate on Smopex®-103 (an approximate ratio of 1.5:1), which is interesting considering the fundamental similarities between these ions. WP11 again exhibits no difference

in selectivity for both ions. Smopex®-269 achieved an approximate 2:1 ratio, greater than Smopex®-103, however the overall maximum material concentration is still greater for Smopex®-103. As mentioned previously, the higher ionic character of both Smopex® materials likely leads to a stronger interaction with the selenate.

4.2 Material Kinetics

In Figure 16 the sorption of selenate kinetics for the different materials is shown. For all materials, approximately 90% of the selenate concentration is transferred to the materials within ten minutes. For the two Smopex® materials, this material concentration is reached within one minute. Typical ion exchange materials would not exhibit such fast kinetics, as they are porous resins with most of the functionality located

SeCN–

SO42–

Smopex®-269, Equimolar SeCN– /SO42–

0 0.5 1 1.5 2Equilibrium liquid concentration, mmol l–1

Equ

ilibr

ium

mat

eria

l co

ncen

tratio

n, m

mol

g–1

1.61.41.21.00.80.60.40.2

0

Fig. 12. Competitive sorption isotherms for selenocyanate and sulfate on Smopex®-269, equimolar initial concentrations. The initial SeCN– and SO4

2– concentrations were approximately 1.9 mmol l–1

SeCN–

SO42–

WP11, Equimolar SeCN– /SO42–


Equ

ilibr

ium

mat

eria

l co

ncen

tratio

n, m

mol

g–1

Fig. 13. Competitive sorption isotherms for selenocyanate and sulfate on WP11, equimolar initial concentrations. The initial SeCN– and SO4

2– concentrations were approximately 1.9 mmol l–1

0.6

0.5

0.4

0.3

0.2

0.1

0

SeO42–

SO42–

Smopex®-103, Equimolar SeO42–/SO4

2–


Equ

ilibr

ium

mat

eria

l co

ncen

tratio

n, m

mol

g–1

0.6

0.5

0.4

0.3

0.2

0.1

0

Fig. 14. Competitive sorption isotherms for selenate and sulfate on Smopex®-103, equimolar initial concentrations. The initial SeO4

2– and SO42– concentrations were

approximately 1.9 mmol l–1

SeO42–

SO42–

WP11, Equimolar SeO42–/SO4

2–


Equ

ilibr

ium

mat

eria

l co

ncen

tratio

n, m

mol

g–1

Fig. 15. Competitive sorption isotherms for selenate and sulfate on WP11, equimolar initial concentrations. The initial SeO4

2– and SO42– concentrations were approximately

1.9 mmol l–1

0.6

0.5

0.4

0.3

0.2

0.1

0



within the material pores. The ions in solution must thus diffuse slowly through the resins in order to reach the functionality. In contrast, all of the functionality on these silica-polymer resins and the Smopex® materials is located on the particle surface and hence is much easier for the liquid ions to access. Only diffusion through the stagnant film surrounding the material particles needs to be considered in kinetic modelling of sorption for these materials. The Smopex® sorption kinetics are believed to be so fast relative to the silica-polymer materials, due to the rod shaped geometry of Smopex® fibres as well as their small particle size, resulting in a generally higher surface area per volume of material than the larger silica-polymer resins. Similar trends in the rates of sorption were observed for selenocyanate and selenite.

4.3 Continuous Flow Through Fixed Bed Columns

In Figure 17 the column outlet concentrations of selenocyanate, selenate and selenite are shown as a function of the mass of selenium fed to each fixed bed column of Smopex®-103. For all selenium species the column outlet concentration is 0 mmol l–1 then increases sharply to the feed selenium concentration value of approximately 12 mmol l–1. Data from fixed bed experiments is typically represented in this manner, however an alternative format has been adopted in this study for ease of performance comparison between different material types, and will be used in all subsequent column results figures.

In Figures 18 and 19 the performance of Smopex®, silica-polymer resins and two commercially available

polystyrene ion exchange resins, DowexTM 1x2 100–200 and AmberliteTM IRA 900 is plotted using the alternative format. The maximum concentrations achieved for the Smopex® and silica-polymer resins are also in agreement with the values seen in the sorption isotherms (Figure 6). An important factor in the design of fixed bed ion exchange systems is the material utilisation factor, defined here as the fraction of the maximum material concentration which has been reached at the point where a detectable concentration of the targeted ion for removal from solution (for example selenocyanate) appears in the

SeO42–


0 100 200 300 400 500 600Time, s

Liqu

id c

once

ntra

tion,

sca

led 1.0

0.90.80.70.60.50.40.30.20.1

0

Fig. 16. Kinetics of selenate sorption. Liquid concentrations are scaled relative to the initial liquid selenate concentrations. The initial SeO4

2– concentration was approximately 0.013 mmol l–1

0 1 2 3 4 5 6Selenium fed to column, mmol

Col

umn

outle

t con

cent

ratio

n,

mm

ol l–1

16

14

12

10

8

6

4

2

0

Fig. 17. Smopex®-103 column outlet concentration profile versus selenium fed to the column for selenocyanate, selenate and selenite feeds

Smopex®-103

SeCN–

SeO42–

SeO32–

Smopex®-103Smopex®-269WP11WP13AmberliteTM

DowexTM

SeCN–

0 0.2 0.4 0.6 0.8 1Material concentration, scaled

Col

umn

outle

t con

cent

ratio

n,

scal

ed

1.2

1.0

0.8

0.6

0.4

0.2

0

Fig. 18. Column material concentration versus column outlet concentration profile for a selenocyanate feed solution. The material concentrations are scaled relative to the maximum material concentrations. The column outlet concentrations are scaled relative to the feed concentrations. The feed SeCN– concentration was 12.66 mmol l–1 for the Smopex® and Dow materials and 0.66 mmol l–1 for the silica polymer composite materials



column outlet solution. For full-scale ion exchange processes, the drive is often to minimise column size while maintaining high (greater than 0.9) material utilisation factors, as column size dictates material (i.e. capital) costs. In Figure 18 the material utilisation factors are approximately 0.75, 0.85 and 0.5 for the Smopex®, silica-polymer and DowexTM resin, and AmberliteTM materials respectively. Also considering the maximum material concentrations shown in Figure 19, the Smopex®-103 is reaching the overall highest material concentrations before selenocyanate starts to appear in the column outlet. The WP11 material achieves the lowest material concentration. The AmberliteTM material presents the highest material concentration overall, but as mentioned above it also has the lowest utilisation factor. This reflects the low selectivity and slow sorption kinetics this material likely has for the selenocyanate species, resulting in a larger ‘mass transfer zone’ in the fixed bed, i.e. the length of the bed over which exchange of selenocyanate ions from liquid to the material occurs. As AmberliteTM is also a standard polystyrene type resin, most of the functionality is likely located within the particle and thus the sorption rate decreases significantly as the material concentration increases since more time is required for the selenium to diffuse into the particle to reach the available functionality.

The DowexTM material exhibits a similar maximum material concentration to Smopex®-269, with a higher utilisation factor. In the case of DowexTM this is believed to be predominantly due to its low particle size of 100 to 200 mm which increases material surface area in

contact with the solution and decreases the internal particle distance for porous diffusion, both likely to result in faster sorption kinetics and a smaller mass transfer zone. The silica-polymer materials exhibit similar utilisation factors to DowexTM while having a much larger average particle size of 500 mm, reaffirming the idea that having the functionality on the surface of the material and hence exposed to the solution is very beneficial to material performance in fixed beds. Larger particle sizes are also important for limiting the pressure drop across fixed beds in full-scale applications with high feed flow rates.

In Figures 20 and 21 the fixed bed performance charts are shown for a feed solution containing selenate ions. Again the Smopex® materials achieve the highest utilisation factors, approximately 0.9 for both materials, again with maximum values consistent with the sorption isotherms in Figure 7. In this case WP11 has the lowest material concentrations and utilisation factor. The AmberliteTM material achieves the highest material concentrations but again a low utilisation factor.

In Figures 22 and 23 the fixed bed performance for a feed containing both selenocyanate and sulfate, both present at a molar concentration ratio of 1 to 30, respectively is shown. This low selenocyanate:sulfate ratio was selected in order to study sorption performance under realistic conditions, similar to those in industrial wastewaters having a highly competitive environment for selenium removal. Considering this, low material concentrations of selenocyanate would be expected due to the competitive sorption

0 2 4 6 8Selenium fed to column, mmol

Fig. 19. Column material fed concentrations versus selenium fed to the columns, for a selenocyanate feed solution


DowexTM

Mat

eria

l con

cent

ratio

n, m

mol

g–1 3.0

2.5

2.0

1.5

1.0

0.5

0

SeCN–

SeO42–


DowexTM

Fig. 20. Column material concentration versus column outlet concentration profile for a selenate feed solution. The feed SeO4

2– concentration was 12.66 mmol l–1 for the Smopex® and Dow materials and 0.66 mmol l–1 for the silica polymer composite materials

Col

umn

outle

t con

cent

ratio

n,

scal

ed

1.2

1.0

0.8

0.6

0.4

0.2

00 0.2 0.4 0.6 0.8 1

Material concentration, scaled



of sulfate on the materials. The maximum material concentrations of selenocyanate on Smopex®-103 and Smopex®-269 are indeed lower than in the case of a feed with selenocyanate only, but for Smopex®-103 the maximum material concentration has only decreased to approximately 1.2 mmol g–1 from the maximum observed value of 1.8 mmol g–1 with only selenocyanate present in solution, while for Smopex®-269 the maximum material concentration is approximately 1.05 mmol g–1, down from the maximum of 1.2 mmol g–1. Furthermore, Smopex®-269 was shown in the sorption isotherms to have the highest relative selectivity for selenocyanate

over sulfate, while it is also the material with the lowest decrease in capacity, approximately 12%, when sulfate is introduced to the feed. The DowexTM, AmberliteTM and Smopex®-103 materials all show an approximate 30% decrease in maximum material concentration when sulfate is present in the feed. Overall, considering the absolute maximum material concentrations and utilisation factors, Smopex®-103 is the best material for selenocyanate removal in this competitive feed solution as it has the highest material concentration at the point when selenium appears in the column outlet. At even higher sulfate to selenocyanate ratios however, it is likely that the Smopex®-269 would perform better than Smopex®-103 due to its higher relative selectivity of selenocyanate over sulfate.

Another important factor to consider is the absolute values of the selenocyanate and sulfate concentrations in the feed, where these experiments were carried out with selenocyanate and sulfate feed concentrations of 6 and 200 mmol l–1, respectively. If the same ratio was maintained but absolute concentrations were decreased, a different fixed bed performance would be observed. This is again due to the nature of the ion exchange sorption mechanism, where the ‘apparent selectivity’ of the sulfate increases with decreasing initial concentration due to the Donnan potential effect. Conversely, at even higher absolute concentrations, higher selenocyanate sorption performance is to be expected due to lower ‘apparent selectivity’ of sulfate. To highlight the effect of this mechanism on fixed bed performance, a further experiment was carried out at the same selenocyanate to sulfate ratio but at absolute feed concentrations of 0.013 mmol l–1 and

0 1 2 3 4 5Selenium fed to column, mmol

Fig. 21. Column material concentrations versus selenium fed to the columns, for a selenate feed solution


DowexTM

Mat

eria

l con

cent

ratio

n, m

mol

g–1 SeO4

2–1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

SeCN–, (SO42–)


DowexTM

Fig. 22. Column material concentration versus column outlet concentration profile for a mixed selenocyanate and sulfate feed solution. The sulfate in brackets signifies that sulfate is present in the feed but is not plotted in the figure. The feed SeCN– and SO4

2– concentrations were 6.3 mmol l–1 and 187 mmol l–1 respectively, for the Smopex® and Dow materials and 3.2 mmol l–1 and 93.6 mmol l–1 respectively, for the silica polymer composite materials

Col

umn

outle

t con

cent

ratio

n,

scal

ed

1.2

1.0

0.8

0.6

0.4

0.2

00 0.2 0.4 0.6 0.8 1



Fig. 23. Column material concentrations versus selenium fed to the columns, for a mixed selenocyanate and sulfate feed solution


DowexTM

Mat

eria

l con

cent

ratio

n, m

mol

g–1

2.01.81.61.41.21.00.80.60.40.2

0

SeCN–, (SO42–)



0.37 mmol l–1, respectively (equivalent to 1 mg l–1 of selenium and 12 mg l–1 of sulfate). These results are shown in Figures 24 and 25. Due to the laborious nature of these tests (more than 14 weeks of continuous flow being required to reach maximum material concentrations on only 1 g of material in the column), only results for Smopex®-103 were obtained.

Contrasting Figures 25 and 23, maximum material concentration of selenocyanate on Smopex®-103 for the feed with selenocyanate to sulfate concentration ratio of 1:30 has decreased from approximately 1.2 mmol g–1 to 0.35 mmol g–1. This is due to the increased ‘apparent selectivity’ of the sulfate at these lower feed concentrations. During the experiment, once this maximum material concentration for selenocyanate was reached, the feed selenocyanate concentration was increased such that the selenocyanate to sulfate concentration ratio was decreased to 1:15. Maximum material concentration for selenocyanate subsequently increased to approximately 0.45 mmol g–1. The feed selenocyanate concentration was further increased to lower the concentration ratio to 1:2.5. Maximum material concentration subsequently increased to approximately 1.2 mmol g–1. Selenocyanate sorption versus sulfate has thus clearly been affected by the absolute feed concentrations of each ion in the feed.

At the higher absolute feed concentrations, the Smopex®-103 was the overall best choice of material for selenocyanate sorption, however at the lower absolute feed concentrations, the Smopex®-269 material is

likely to perform better than the Smopex®-103 due to its greater relative selectivity for selenocyanate over sulfate. Choosing the best material for selenocyanate sorption is thus a complex choice as it depends on many factors, hence the correct choice of model to predict material performance at differing absolute feed concentrations is critical.

Also shown in Figures 24 and 25 is the column performance for a feed containing only selenocyanate, but at the lower feed concentration of 0.013 mmol l–1, compared to 6 mmol l–1 used previously (Figure 22). A maximum material concentration of approximately 1.7 mmol g–1 was reached, very close to the observed maximum in the selenocyanate sorption isotherm (Figure 6).

Considering the frequently employed adsorption models, for example Langmuir or Freundlich, the reader might be surprised that the approximate total exchange capacity of the material was reached with such a low selenocyanate concentration in the feed. According to these sorption models, for a given feed concentration the corresponding material concentration, as given by the sorption isotherm, is also the maximum material concentration that should be attainable experimentally, since equilibrium between the material and solution is reached at these conditions and no further ions can be loaded without an increase in feed concentration. In the case of ion exchange however, the situation is different as there is also an ion from the material which is exchanged and released into solution. In the case of these particular experiments, the chloride released from

Smopex®-103, SeCN–, (SO42–)

Fig. 24. Column material concentration versus column outlet concentration profile for a mixed selenocyanate and sulfate feed solution at low concentrations (0.013 mmol l–1 SeCN– and 0.37 mmol l–1 SO4

2–). Results are also shown for a feed solution containing only selenocyanate at low concentration (0.013 mmol l–1)

Col

umn

outle

t con

cent

ratio

n,

scal

ed

1.2

1.0

0.8

0.6

0.4

0.2

00 0.2 0.4 0.6 0.8 1


SeCN–, (SO42–), 1:30

SeCN–, (SO42–), 1:15

SeCN–, (SO42–), 1:2.5

SeCN–, no SO42–


Fig. 25. Column material concentrations versus selenium fed to the columns, for a mixed selenocyanate and sulfate feed solution at low concentrations (0.013 mmol l–1 SeCN– and 0.37 mmol l–1 SO4

2–). Results are also shown for a feed solution containing only selenocyanate at low concentration

Mat

eria

l con

cent

ratio

n, m

mol

g–1

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

SeCN–, (SO42–), 1:30

SeCN–, (SO42–), 1:15

SeCN–, (SO42–), 1:2.5

SeCN–, no SO42–

Smopex®-103, SeCN–, (SO4

2–)



the material is also subsequently washed out of the fixed bed due to the feed flow through the fixed bed. Thus chloride concentrations inside the fixed bed are always decreasing and equilibrium is continuously pushed in favour of more chloride releasing into solution,which in turn requires selenocyanate to be exchanged into the material. In summary, provided there is no chloride in the feed itself then in a fixed bed the selenocyanate material concentration should always reach the highest maximum material concentration observed experimentally in sorption isotherms, regardless of the absolute feed selenocyanate concentration.

This phenomenon also highlights another point, that if only chloride ions were present in the feed, then regardless of their concentration, complete desorption of loaded selenocyanate from the material should be achievable, even though the selenocyanate ion has a much greater selectivity for the material than chloride ion. The results of fixed bed trials carried out to investigate this are shown in Figures 26 and 27.

Figures 26 and 27 both show that significant selenocyanate desorption is achievable regardless of the feed chloride concentration. While not all the experiments were continued until a maximum selenocyanate desorption was achieved, in all experiments the selenocyanate desorption trend is still increasing when flow to the fixed bed was stopped and heading towards an approximate desorption value of 1.6 mmol g–1 selenocyanate. It was previously mentioned that the total material capacity of Smopex®-103 is approximately 1.8 mmol g–1 of dry material,

however this is only valid when the material is initially concentrated with chloride ions. When concentrated with selenocyanate ions (having higher molecular weight than chloride ions) the mass of the material is subsequently increased which reduces the maximum material concentration per gram of dry material to 1.6 mmol g–1. This value is in agreement with the maximum achieved selenocyanate desorption for the experiment having feed chloride concentration of 2.82 mol l–1, indicating complete desorption of the selenocyanate from the material. Even for the experiment having fifty times less chloride in the feed, i.e. 0.06 mol l–1, a significant desorption of approximately 1.0 mmol g–1 selenocyanate was achieved after 30 bed volumes of flow through the fixed bed.

Considering adsorption theory, such a significant desorption of selenocyanate using a low concentration feed of a competitive ion such as chloride would not be expected. The material concentration of chloride ions on the material would quickly reach equilibrium with the low feed concentration, mitigating the exchange of further chloride ions into the material. In Figure 27, it can also be seen that the low concentration chloride feed is also the most effective at desorbing selenocyanate, per mass of chloride that has flowed through the fixed bed, with efficiency decreasing as feed chloride concentration increases. This is believed to be predominantly due to increases in the viscosity of the feed solution as chloride concentration increases, mitigating the diffusion rate of chloride ions to the surface of the Smopex® material, rather than any ion

0.06 mol l–1 Cl–

0.17 mol l–1 Cl–

0.28 mol l–1 Cl–

1.41 mol l–1 Cl–

2.82 mol l–1 Cl–

SeCN–

Fig. 26. Selenocyanate desorption versus material bed volumes of chloride solution flowed through the fixed bed, for varying concentrations of chloride in the feed solution. A bed volume is defined as the volume flowed through the column divided by the resin bed volume

Sel

eniu

m d

esor

bed,

mm

ol g

–1

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

00 10 20 30 40 50

Material bed volumes of flow, ml ml–1

0.06 mol l–1 Cl–

0.17 mol l–1 Cl–

0.28 mol l–1 Cl–

1.41 mol l–1 Cl–

2.82 mol l–1 Cl–

SeCN–

Fig. 27. Selenocyanate desorption versus mass of chloride flowed through the fixed bed, for varying concentrations of chloride in the feed solution

Sel

eniu

m d

esor

bed,

mm

ol g

–1

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

00 100 200 300 400 500

Mass of chloride flow through the column, mmol



exchange related phenomena. A 2.82 mol l–1 chloride solution has a viscosity of approximately 0.002 Pa s at 25ºC, compared to approximately 0.001 Pa s for water at 25ºC, a two-fold increase. Finally, while the low concentration chloride feed was the most efficient for selenocynate desorption, average selenocyanate concentrations in the outlet stream are also lowest for that feed. If a concentrated stream of selenocyanate is desired from the desorption process, then the higher chloride feed concentrations are required.

Selectivity of selenate over sulfate was also investigated in continuous flow experiments for Smopex®-103, the silica polymer composite WP13 and the Dow resins. Under the conditions tested, with a selenium:sulfur molar ratio of 1:30 and initial feed selenate concentration of 6.3 mol l–1, maximum material concentrations did not exceed 0.12 mmol g–1 (1 wt% Se) for any of the materials. Even though these results may seem very poor, a viable commercial process could be designed by implementing frequent regeneration cycles of the material once this low maximum concentration is reached (32).

4.4 Process Implementation of Smopex®

Smopex® materials have been shown to outperform all other tested materials in terms of kinetics, selectivity and maximum material concentrations. However due to the structure of Smopex®, where functionalised polymeric chains are attached to a relatively small

backbone polymer, a fixed bed of Smopex® can be easily compressed and at high feed flow rates (for example greater than 0.001 m s–1 superficial velocity) the pressure drop across a fixed bed can be significantly higher than if a silica or polystyrene ion exchange were used (often in excess of 5 bar per metre of fixed bed length). For such applications, an alternative process configuration is required. In this study, the CSTR has been explored as a viable process configuration for Smopex® materials. In contrast to fixed bed reactors, material utilisation factors in CSTR are generally lower, since concentration gradients between the bulk liquid and the material surface are lower than in fixed beds. Many CSTR connected in series are often required to achieve comparable material utilisation factors to fixed beds.

However, as will be shown from the trials carried out here, comparable material utilisation factors to fixed beds can be achieved using Smopex® and only two CSTR in series, due to the high selectivity and exchange kinetics of Smopex® materials.

In Figures 28 and 29 the experimentally measured and predicted reactor liquid and material concentrations are shown over the duration of the experiment using a single CSTR. The chloride liquid concentrations were only measured for the first 50 hours,in order to validate the predicted values. The experimental and model values are in agreement, likely due to the use of ion exchange models. Adsorption models would not have

SeCN– (expt.)SeCN– (pred.)Cl– (expt.)Cl– (pred.)

0 50 100 150 200 250Time, h

Rea

ctor

liqu

id c

once

ntra

tion,

m

mol

l–1

6

5

4

3

2

1

0

Fig. 28. Continuous stirred tank reactor liquid selenocyanate and chloride concentrations over time. Experimentally measured (expt.) and model predicted (pred.) values are shown. The volume of solution in the reactor was 0.64 l, the feed flow rate 100 ml h–1 and feed SeCN– concentration was 5.7 mmol l–1. The coefficient of determination (R2) value for the predicted versus experimental SeCN– concentration was calculated to be 0.97

SeCN–

SeCN– (pred.)Cl– (pred.)

0 50 100 150 200 250Time, h

Fig. 29. Continuous stirred tank reactor Smopex®-103 selenocyanate and chloride material concentrations over time. Only predicted values are shown as experimental material concentrations are estimated values based on the liquid solution concentrations

SeCN–

Mat

eria

l con

cent

ratio

n, m

mol

g–1 2.0

1.81.61.41.21.00.80.60.40.2

0



included the effects of the chloride ion in this system and thus would not give an accurate prediction.

Comparing these results to Figure 22, a much lower material utilisation of 0.25 has been reached, with a material concentration of only 0.5 mmol g–1 at the point in time when a significant concentration of selenocyanate (< 0.1 mmol l–1) is present in the reactor liquid solution (and hence the reactor outlet stream). As expected, the maximum material concentration of the Smopex®-103 (i.e. 1.8 mmol g–1) was eventually reached, as the exchanged chloride ions are again continuously flowed out of the reactor, driving the equilibrium towards further selenocyanate exchange.

Figures 30 and 31 show the predicted performance for the same system as Figures 28 and 29, but with the inclusion of a second reactor in series with the same solution volume and mass of Smopex® as the first reactor. In this system, the outlet concentration from the second reactor would exceed 0.1 mmol g–1 at approximately 125 hours. By this time, the material concentration of Smopex®-103 within the first reactor would reach approximately 1.8 mmol g–1. A material utilisation factor of approximately 1.0 could thus be reached for the Smopex® in the first reactor, with only a single additional reactor required. Even for commercial fixed bed systems, two fixed beds in series (i.e. lead-lag operation) are commonly required and utilised to achieve such high utilisation factors. Thus predicted performance in this CSTR system is comparable to fixed bed performance.

For the case of selenate or selenite, and competitive removal of these ions versus sulfate, the ion exchange models can similarly be employed to engineer process solutions for their removal, either in fixed beds or

CSTR process configurations, depending on the feed flow rates.

5. Conclusions

In this study it has been shown that Smopex® materials, i.e. functionalised polymeric chains attached to polypropylene fibre backbones, are very effective for the removal of selenocyanate, selenate and selenite ions from solution. Their particular structure has been shown to give very fast rates of ion exchange when compared to resin bead ion exchangers (DowexTM

1x2 100–200 and AmberliteTM IRA 900 were also tested in fixed bed trials), comparable with resin beads in the spherical diameter range of 100 to 200 mm. Furthermore, since all of the material functionality is readily exposed to solution and not contained within restricted pore spaces (as is the case for typical ion exchange resin products), the material utilisation factors achieved in fixed bed trials regularly exceeded 0.9. In competitive selenocyanate/sulfate feeds, the Smopex® materials were shown to outperform all other materials.

The sorption mechanisms by which these ions are removed from solution were also studied. It was shown that ion exchange clearly dominates the sorption process. Knowledge of this mechanism was shown to be very important in modelling and understanding how changes in initial ion concentrations and ionic species can affect the sorption performance. It was shown for example that, in contrast to typically employed models such as Langmuir, the relative selectivity of two ions such as selenocyanate and sulfate can change as a function of their initial concentrations and that sorption

0 50 100 150 200 250Time, h

Fig. 30. Continuous stirred tank reactor liquid selenocyanate and chloride concentrations over time, predicted values for two reactors in series (rct. 1 = Reactor 1; rct. 2 = Reactor 2)

SeCN–

Rea

ctor

liqu

id c

once

ntra

tion,

m

mol

l–1

6

5

4

3

2

1

0

SeCN– (rct. 1)SeCN– (rct. 2)Cl– (rct. 1)Cl– (rct. 2)

0 50 100 150 200 250Time, h

Fig. 31. Continuous stirred tank reactor liquid selenocyanate and chloride concentrations over time, predicted values for two reactors in series (rct. 1 = Reactor 1; rct. 2 = Reactor 2)

SeCN–

Mat

eria

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cent

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performance cannot be accurately predicted without considering the chloride ion exchanged from the material. It was also shown that maximum material concentrations (for example 1.8 mmol g–1 for the case of Smopex®-103 and selenocyanate) can be reached for feeds having selenium concentrations as low as 1 mg l–1 (0.013 mmol l–1), whereas such performance would rarely be seen when adsorption or chelation is the sorption mechanism.

The ion exchange models were then employed to predict the performance of Smopex® materials in one and two in-series continuous stirred tank reactors. The single reactor results were validated experimentally. It was shown that due to their excellent selectivity and kinetics, material utilisation factors approaching unity are achievable using only two reactors in series, i.e. full capacity of the material can be reached before outlet concentrations from the reactor system rise significantly. This level of performance is comparable to fixed bed performance, where typically two fixed beds in series (lead-lag operation) are employed in order to achieve such high material utilisation factors in commercial setups.

Smopex® materials are currently undergoing redevelopment so that their unique performance benefits are available without the pressure drop limitations attributed to their current morphology. Additionally, other reactor configurations are also under investigation to bring together the benefits of the Smopex® materials with the ease of use and performance efficiency of fixed bed setups.

Acknowledgements

The authors want to thank S. Colley, P. Kauppinen and J. Stevens for their support and useful comments and M. Lincoln for his assistance in some experiments. Special thanks also to D. Scott and J. Clarke for provision of crucial analytical services.

References 1. European Chemicals Agency (ECHA) A. Selenium –

Ecotoxicological Information: http://echa.europa.eu (Accessed on 1st October 2015)

2. “External Peer Review Draft Aquatic Life Ambient Water Quality Criterion for Selenium”, United States Environmental Protection Agency, USA, 2014, EPA-820-F-14-005

3. S. Santos, G. Ungureanu, R. Boaventura and C. Botelho, Sci. Total Environ., 2015, 521–522, 246

4. “Evaluation of Selenium Species in Flue Gas Desulfurization Waters”, Electric Power Research Institute, USA, 2009, Product ID: 1015586

5. H. L Reyes, S. García-Ruiz, B. G. Tonietto, J. M. Godoy, J. I. García Alonso and A. Sanz-Medel, J. Braz. Chem. Soc., 2009, 20, (10), 1878

6. L. Ling, B. Pan and W.-x. Zhang, Water Res., 2015, 71, 274

7. Y. H. Huang, P. K. Peddi, H. Zeng, C.-L. Tang and X. Teng, Water Sci. Technol., 2013, 67, (1), 16

8. E. Shah, P. Soni Hemant, 'An Improved Process for Removal of Heavy Metals', Indian Patent Appl. 11/MUM/2013, IPC: C10C003-02

9. T. M. Pickett, Y. Ma, J. Sonstegard, J. S. Kain, D.-C. Vuong and D. B. Fraser, General Electric Co, ‘Selenium Removal using Chemical Oxidation and Biological Reduction’, US Patent Appl. 2013/0,270,181

10. L. A. Richards, B. S. Richards and A. I. Schäfer, J. Membrane Sci., 2011, 369, (1–2), 188

11. S. W. Colley, P. M. Kauppinen, M. A. Lincoln and J. Torroba, Johnson Matthey Plc, ‘Selenium Removal’, World Appl. 2015/036,770

12. M. R. Awual, A. Jyo, S. A. El-Safty, M. Tamada and N. Seko, J. Hazard. Mater., 2011, 188, (1–3),164

13. M. R. Awual, S. Urata, A. Jyo, M. Tamada and A. Katakai, Water Res., 2008, 42, (3), 689

14. Johnson Matthey Advanced Ion Exchange: http://www.jmadvancedix.com/ (Accessed on 1st October 2015)

15. J. Allen, M. Berlin, M. Hughes, E. Johnston, V. Kailasam, E. Rosenberg, T. Sardot, J. Wood and C. Hart, Mater. Chem. Phys., 2011, 126, (3), 973

16. V. J. Inglezakis and S. G. Poulopoulos, “Adsorption, Ion Exchange and Catalysis: Design of Operations and Environmental Applications”, Elsevier BV, The Netherlands, 2006

17. “Perry’s Chemical Engineers’ Handbook”, 7th Edn., eds. R. H. Perry, D. W. Green and J. O. Maloney, The McGraw-Hill Companies, Inc, USA, 1999

Johnson Matthey’s Advanced Ion eXchange (AIX) business is committed to providing simple, robust metal removal solutions for API purification. To allow easier access to these products, the business is introducing their ThioSep kit. If you are interested in targeting Palladium removal during API and HPAPI manufacturing, we’d be keen to hear from you.

Contact our scavenging team on: [email protected] or +44(0)1763254640



18. F. Helfferich, “Ion Exchange”, Dover Publications Inc, New York, USA, 1995

19. A. A. Hekmatzadeh, A. Karimi-Jashni, N. Talebbeydokhti and B. Kløve, Desalination, 2013, 326, 125

20. T. R. Ferreira, C. B. Lopes, P. F. Lito, M. Otero, Z. Lin, J. Rocha, E. Pereira, C. M. Silva and A. Duarte, Chem. Eng. J., 2009, 147, (2–3), 173

21. I. C. Ostroski, M. A. S. D. Barros, E. A. Silva, J. H. Dantas, P. A. Arroyo and O. C. M. Lima, J. Hazard. Mater., 2009, 161, (2–3), 1404

22. I. C. Ostroski, C. E. Borba, E. A. Silva, P. A. Arroyo, R. Guirardello and M. A. S. D. Barros, J. Chem. Eng. Data, 2011, 56, (3), 375

23. Inamuddin and M. Luqman, “Ion Exchange Technology I: Theory and Materials”, Springer, The Netherlands, 2012

24. C. E. Borba, G. H. F. Santos and E. A. Silva, Chem. Eng. J., 2012, 189–190, 49

25. L. A. Bromley, AIChE J., 1973, 19, (2), 313

26. G. M. Wilson, J. Am. Chem. Soc., 1964, 86, (2), 127

27. J. C. Crittenden, R. R. Trussell, D. W. Hand, K. J. Howe and G. Tchobanoglous, “MWH’s Water Treatment: Principles and Design”, 3rd Edn., John Wiley & Sons, Inc, New Jersey, USA, 2012

28. R. B. Garcia-Reyes and J. R. Rangel-Mendez, Bioresource Technol., 2010, 101, (21), 8099

29. W. E. Stewart and M. Caracotsios, “Computer-Aided Modeling of Reactive Systems”, John Wiley & Sons, Inc, New Jersey, USA, 2008

30. M. R. Awual, M. M. Hasan, T. Ihara and T. Yaita, Micropor. Mesopor. Mater., 2014, 197, 331

31. M. R. Awual, T. Yaita, S. Suzuki and H. Shiwaku, J. Hazard. Mater., 2015, 291, 111

32. F. Mohammadi, P. Littlejohn, A. West and A. Hall, ‘Selen-IXTM: Selenium Removal from Mining Affected Runoff Using Ion Exchange Based Technology’, 7th International Symposium on Hydrometallurgy, Victoria, BC, Canada, 22nd–25th June, 2014

The Authors

Carl Mac Namara is a Process Engineer within the Johnson Matthey Water Technologies group, Chilton, UK. He obtained his MEng in Chemical Engineering from Cork Institute of Technology, Ireland, and an Engineering Doctorate from the University of Birmingham, UK. His doctorate and post-doctoral projects were based in Procter & Gamble’s Newcastle Innovation Centre, UK, where he carried out fundamental research on textile cleaning processes. His current role is focused on modelling and developing new water treatment technologies all the way from R&D through to commercial stages.

Javier Torroba is a Research Scientist at Johnson Matthey Technology Centre, Chilton, UK. He obtained his degree and doctorate in Chemistry from Universidad de La Rioja, Spain, and enjoyed post-doctoral stays at Universidad Complutense de Madrid, Spain, and University of York, UK, before joining Johnson Matthey. He developed his research work around the areas of Coordination Chemistry and Materials Science, in topics such as metal organic frameworks and organometallic liquid crystals. He now investigates the fundamentals of selectivity in sorption processes for water purification technologies.

Adam Deacon is a Research Technician at the Johnson Matthey Technology Centre. Before joining his current position, he obtained his degree in chemistry from Manchester Metropolitan University, UK. His interests and expertise include the synthesis of new sorbent materials and the underlying chelating properties of ligands. His work is now focused on the discovery and development of the next generation of products for water purification.

EMISSION CONTROL TECHNOLOGIES Increased NO2 Concentration in the Diesel Engine Exhaust for Improved Ag/Al2O3 Catalyst NH3-SCR ActivityW. Wang, J. M. Herreros, A. Tsolakis and A. P. E. York, Chem. Eng. J., 2015, 270, 582

The authors investigate the creation of higher NO2 concentration and its performance in the Ag/Al2O3 catalyst for the SCR route of eliminating NOx at low exhaust gas temperatures under real engine operation. The availability of NO2 concentration was increased for the SCR route with: (a) adding various NH3 and H2 mixtures upstream of the SCR catalyst and/or (b) using a Pt-based diesel oxidation catalyst (DOC) in front of the Ag/Al2O3-SCR catalyst. H2 improves the production of NO2 on the Ag/Al2O3 catalyst therefore the “Fast-SCR” like reaction is promoted by using the accessible NH3 primarily at low reaction temperature. The same effect was shown by the integration of the DOC in front of the Ag/Al2O3 as the NO2 availability was enhanced for the SCR process.

The Effect of Pt:Pd Ratio on Light-Duty Diesel Oxidation Catalyst Performance: An Experimental and Modelling StudyJ. Etheridge, T. Watling, A. Izzard and M. Paterson, SAE Int. J. Engines, 2015, 8, (3), 1283

This article represents a section of a two-part investigation on the effect of Pt:Pd ratio at a constant total Pt+Pd loading of 120 g ft–3 on the catalytic activity of a DOC for light-duty operations. In this study a one-dimensional model able to estimate the effect of Pt:Pd ratio on DOC activity was developed. This model was based on an earlier model and certain parameters are changed to take into consideration the variation in Pt:Pd ratio. A function to aid the interpolation to any Pt:Pd ratio was used to describe the difference in each kinetic parameter with ratio. The NEDC test data with optimised kinetic parameters was used to develop the model and to obtain the best fi t to measured data for each ratio. Good estimates of post-catalyst CO, THC and NO2 emissions over the NEDC across the entire range of Pt:Pd ratios were given by this model.

FINE CHEMICALS Functional Thin Film Coatings Incorporating Gold Nanoparticles in a Transparent Conducting Fluorine Doped Tin Oxide MatrixC. K. T. Chew, C. Salcianu, P. Bishop, C. J. Carmalt and I. P. Parkin, J. Mater. Chem. C, 2015, 3, (5), 1118

Gold nanoparticles (AuNPs) and F-doped SnO2 composites were combined by layering, making unique fi lms which display interesting optoelectronic properties such as high visible transparency and electrical conductivity. Aerosol assisted chemical vapour deposition (AACVD) was used to deposit both layers onto heated glass substrates. The authors produced and analysed four sets of fi lms: AuNPs, fl uorine-doped SnO2 (FTO), a layer of AuNPs on FTO and an FTO layer on AuNPs. Changing the precursor concentration could alter the sizes of the AuNPs. Layered Au:FTO composite fi lms were blue from the surface plasmon resonance of the AuNPs but demonstrate high transparency in the visible region and are electrically conducting. These are comparable to commercial FTO.

Au3+

Au(s)

C. K. T. Chew, C. Salcianu, P. Bishop, C. J. Carmalt and I. P. Parkin, J. Mater. Chem. C, 2015, 3, (5), 1118 (Reproduced by permission of The Royal Society of Chemistry)

A Convenient Palladium-Catalyzed Azaindole SynthesisR. De Gasparo, P. Lustenberger, C. Mathes, T. Schlama, G. E. Veitch and J. J. M. Le Paih, Synlett, 2015, 26, (2), 197

Azaindoles are diffi cult to access but of interest for promising pipeline drug candidates. A reaction cascade

Johnson Matthey HighlightsA selection of recent publications by Johnson Matthey R&D staff and collaborators





involving enamine formation followed by intramolecular Heck reaction was investigated as a possible route. Palladium based catalyst systems and reaction systems were screened. The fi rst generation XPhos catalyst was selective for a single regioisomer. Ultimately a one-pot synthesis was devised which provides direct access to azaindoles from amino-halopyridines and ketones.

NEW BUSINESS: FUEL CELLS Effect of Particle Size and Operating Conditions on Pt3Co PEMFC Cathode Catalyst DurabilityM. Gummalla, S. C. Ball, D. A. Condit, S. Rasouli, K. Yu, P. J. Ferreira, D. J. Myers and Z. Yang, Catalysts, 2015, 5, (2), 926

The stability and performance of Pt catalysts has been proven to depend on particle size. To fi nd out the effect of alloying and particle size in alloy catalysts, Pt3Co catalysts with approximately the same Pt:Co:carbon ratio and three different mean particle sizes (4.9 nm, 8.1 nm, and 14.8 nm) were prepared by heat treatment. A higher degree of ordering was found in the larger particles. Systematic tests were carried out. The cathode based on 4.9 nm catalyst exhibited the highest initial electrochemical surface area (ECA) and mass activity, while the cathode based on 8.1 nm catalyst showed better initial performance at high currents. Accelerated performance loss testing using electrochemical decay protocols showed similar trends to previous Pt studies with higher initial performance for smaller particles but higher durability for larger particles. Intermediate sized particles of ~8 nm provided the best balance of lifetime performance for Pt3Co catalysts.

Performance Measurements and Modelling of the ORR on Fuel Cell Electrocatalysts – the Modifi ed Double Trap ModelM. Markiewicz, C. Zalitis and A. Kucernak, Electrochim. Acta, 2015, doi: 10.1016/j.electacta.2015.04.066

Results for the ORR in perchloric acid for ultra-low loading Pt/C electrodes have been experimentally obtained for various ORR mechanisms which were accomplished as a function of temperature (280–330 K), oxygen partial pressure (0.01–1) and potential (0.3–1.0 V vs. RHE). The results confirm the reaction exponent for oxygen of 1 ± 0.1 through the potential range of 0.3–0.85 V vs. RHE and show that as the overpotential rises the surface becomes progressively blocked towards ORR. This was not taken into account in the double trap model therefore the present authors have created an alternative version to include the formation of OOHad intermediates. At higher overpotentials the OOHad intermediates block the electrode and lead to a reduction in electrocatalyst performance compared to a Tafel type approximation. Hydrogen peroxide can also be formed by these intermediates at high overpotentials and is poorly described by models.

Optimal ADF STEM Imaging Parameters for Tilt-Robust Image Quantifi cationK. E. MacArthur, A. J. D’Alfonso, D. Ozkaya, L. J. Allen and P. D. Nellist, Ultramicroscopy, 2015, 156, 1

ADF STEM can be used to obtain useful qualitative data about the atomic scale structure of materials including catalysts. The present study used the cross section approach, a statistical method of counting atoms with the advantage that it is not affected by image parameter errors. An fcc Pt nanocube was analysed in order to demonstrate that small detector angles are helpful in avoiding problems caused by inaccurate tilt due to rotation of the nanoparticle samples under the beam. Optimised experimental parameters were devised and the balance between thermal diffuse scattering and elastic scattering is explained.

NEW BUSINESS: WATER TECHNOLOGIES Structure and Properties of Highly Selective and Active Advanced Ion Exchange (AIX) MaterialsS. W. Colley, P. Kauppinen, J. Stevens and C. Mac Namara, Chim. Oggi, 2014, 32, (5), 72

There is a substantial loss of precious metal catalysts from active pharmaceutical ingredient (API) manufacturing processes into waste water streams. New composite materials for the recovery and purifi cation of precious and base metals from API production, platinum group metals refi ning, base metal mining and metal processing industries have been developed. These materials are created either by grafting active adsorption sites on the outer surface and large pores of silica or joining polymeric chains of active adsorption sites to non-porous polymer fi bres. The new AIX materials and conventional polystyrene resins are compared and the benefi ts are discussed.

Targeted Metal Purifi cation by ScavengingS. Phillips, Spec. Chem. Magazine, 2015, 35, (5), 12

Transition metal catalysts used in any API manufacturing process must be reduced to an approved impurity limit in the fi nal product. Highly potent APIs such as kinase inhibitors for cancer treatment pose a specifi c problem due to the low dosage required and the fact that they are synthesised using metal-catalysed aryl-aryl couplings or A-X couplings. Scavengers can remove metals and overcome problems such as the length of time required, meeting minimum contamination levels, avoiding product loss and solvent use. A large scale project to remove metal from APIs including kinase inhibitors was carried out using Johnson Matthey’s patented Sealed Flow Cartridge System. The main considerations were regulatory compliance (quality), cost and time. Implementation was tested from lab to plant scale and reduced the time taken to recover the



metal by a factor of 24, allowing the plant to reach its designed capacity.

PRECIOUS METAL PRODUCTS: ADVANCED GLASS TECHNOLOGIES A Combined Single Crystal Neutron/X-Ray Diffraction and Solid-State Nuclear Magnetic Resonance Study of the Hybrid Perovskites CH3NH3PbX3 (X = I, Br and Cl)T. Baikie, N. S. Barrow, Y. Fang, P. J. Keenan, P. R. Slater, R. O. Piltz, M. Gutmann, S. G. Mhaisalkar and T. J. White, J. Mater. Chem. A, 2015, 3, (17), 9298

Hybrid perovskites such as methylammonium lead halide perovskites, CH3NH3PbX3 (X = I, Br and Cl), are interesting as potential materials for photovoltaic devices. Variable temperature 1H and 13C magic angle spinning nuclear magnetic resonance (MAS-NMR) spectra were recorded for poly- and single crystalline samples of the perovskites. The CH3NH3

+ units were found to undergo dynamic reorientation due to tumbling of the organic component within the perovskite cage. Only the amine end of the CH3NH3

+ group was shown to interact with the inorganic network. Impedance spectroscopy showed that the conductivity changes signifi cantly at the phase transition temperature, with implications for the performance of the photovoltaic device at higher temperatures. The optical band-gaps of each perovskite were determined using UV-visible spectroscopy confi rming that they absorb strongly across the visible spectrum.

PROCESS TECHNOLOGIES Reactivity of Oxygen Carriers for Chemical-Looping Combustion in Packed Bed Reactors under Pressurized ConditionsH. P. Hamers, F. Gallucci, G. Williams, P. D. Cobden and M. van Sint Annaland, Energy Fuels, 2015, 29, (4), 2656

In order to effectively design, scale-up and optimise pressurised packed bed reactors for chemical-looping combustion (CLC) the infl uence of the pressure on the

reactivity of the oxygen carriers must be understood. The authors have measured the redox reactivity of CuO/Al2O3 and NiO/CaAl2O4 particles at high pressures in a pressurised high-temperature magnetic suspension balance. Pressure has an adverse effect on the reactivity and this effect is kinetically controlled. This may be caused by the decline in the number of oxygen vacancies at elevated pressures. The reactant gas fraction is an important parameter and may possibly be associated to the contest between various species for adsorption on the oxygen carrier surface. A kinetic model was proposed taking these effects into consideration. A particle model which acknowledges diffusion limitations and kinetics was used to study these results on packed bed CLC applications with bigger oxygen carrier particles. It was concluded that at high pressure the diffusion limitation decreases due to reduced reaction rates and a rise in diffusion fl uxes caused by Knudsen diffusion.

Continuous Catalytic Upgrading of Ethanol to n-Butanol and >C4 Products Over Cu/CeO2 Catalysts in Supercritical CO2 J. H. Earley, R. A. Bourne, M. J. Watson and M. Poliakoff, Green Chem., 2015, 17, (5), 3018

n-Butanol (BuOH) has advantages over EtOH as a biofuel as it can transported and used in a gasoline engine with little or no modifi cation, has a higher energy content, lower water miscibility and better gasoline compatibility. This paper uses a Cu-catalysed Guerbet reaction to investigate a more sustainable source of BuOH compared to the industrial OXO process, by upgrading EtOH. Six Cu catalysts on different supports were prepared and tested. Supercritical CO2 was the solvent and was used in a continuous fl ow reactor. The high surface area CeO2 support provided the best activity and gave over 30% yield and good selectivity. Increasing CO2 pressure was found to improve the performance in this reaction possibly due to its effect on the support’s redox cycle.


Platinum Group Metal Thin Films and Coatings Platinum Group Metal Thin Films and Coatings Johnson Matthey Noble Metals is developing pgm thin fi lms Johnson Matthey Noble Metals is developing pgm thin fi lms and coatings for medical and industrial markets.and coatings for medical and industrial markets.Please contact us if you have a requirement for high Please contact us if you have a requirement for high performance coatings and thin fi lms in your business, or if you performance coatings and thin fi lms in your business, or if you anticipate having coating related challenges in the future.anticipate having coating related challenges in the future.Contact Dr Shahram Amini at [email protected] for Contact Dr Shahram Amini at [email protected] for further details. further details.

500 nm

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JOHNSON MATTHEY TECHNOLOGY REVIEWContents Volume 59, Issue 4, October 2015 JOHNSON MATTHEY TECHNOLOGY REVIEW Johnson Matthey’s international journal of research exploring science