Operando Spectroscopy: the Knowledge Bridge to Assessing Structure-Performance Relationships in...

www.advmat.dewww.MaterialsViews.com

EARCH N

Operando Spectroscopy: the Knowledge Bridge to Assessing Structure–Performance Relationships in Catalyst Nanoparticles

Progress in nanomaterials and catalysis stands on three pillars: 1) synthesis of nanomaterials, including the preparation of hierarchically dispersed nano-particles; 2) theoretical studies of materials that enable experimental results to be understood; and 3) advanced , in situ characterization during operation (operando methodology). This Research News brings a perspective on how these three pillars are blending for research in materials science, with par-ticular emphasis on catalysis.

1. Introduction

The chemical industry is one of the largest industries; catal-ysis lies at the heart of the petroleum-refi ning industry, and is the gateway to the future of petroleum-free chemical and fuel industries in the search of a sustainable future. Advances in catalysis will reduce our use of feedstock and energy and maxi-mize recycling: there are many exciting challenges and oppor-tunities for the development of new catalytic technologies and for the improvement of existing ones. The shift from petroleum supply to natural gas and renewables demands research for technologies that will permit conversion of light hydrocarbons, shale and renewables into chemicals and fuels in an economic and effi cient manner. There is indeed the need to increase the research effort aimed at the discovery and development of novel catalytic processes and systems.

Catalysts must exhibit a number of properties; mainly activity and selectivity , but also durability and regenerability . A high catalytic activity permits less-severe operating conditions (optimizing energy use); a high selectivity optimizes feedstock use, minimizing environmentally harmful emissions. Eventu-ally, catalysts deactivate and they must be either regenerated or replaced. Thus, high stability and ease of regeneration are important properties.

Materials research is increasingly emphasizing nano-technology. As the size reduces, the surface area increases and becomes more and more important. Surface chemistry is an integral part of nanomaterials research, and, in particular,

Prof. M. A. Bañares Catalytic Spectroscopy Laboratory Instituto de Catalisis y Petroleoquímica CSIC Marie Curie 2; E-28049-Madrid, Spain E-mail: miguel.banares@csic.es

DOI: 10.1002/adma.201101803

of catalysis. The development of new or improved catalysts is complex, involving extensive and expensive testing and evalu-ations. Knowledge derived from scientifi c studies provides a basis for conceiving new catalysts and catalytic reactions, and for interpreting the results of experimental observations. In situ spectroscopy signifi -cantly advances catalysis research, for it provides fundamental information about catalytic structure and surface species under catalytic conditions.

Catalysis is a science that cannot be understood without spectros-copy . Spectroscopy is the enabling tool for the knowledge-based design of catalysts. Operando spectroscopy combines in situ spectroscopic characterization of a catalytic material during a reaction with the simultaneous measurement of the catalytic activity/selectivity in a kinetically relevant in situ cell fi tted to a reaction-product analysis system. Software and hardware progress in the last few years have made computational model-ling the perfect match for understanding in situ and operando studies.

2. The Catalyst Is It, and its Circumstance

The Spanish social philosopher, Don José Ortega y Gasset said “ man is he, and his circumstance ”. This concept is transferable to catalysis: “ The catalyst is it, and its circumstance ”. Operando meth-odology brings catalysis research into its real circumstance . Just like man, catalysts are affected by their environment. Catalysts undergo physical and chemical changes under reaction condi-tions. We need to know the interaction between catalysts and reactants, intermediates and products to understand the catalytic act. Research strategies must focus on developing methods to observe the catalytic reaction steps in situ, or at least the catalytic sites. Catalyst characterization should be preferably carried out in situ during catalytic operation, and, more preferably, with simul-taneous activity/selectivity determination (i.e., operando meth-odology). Operando methodologies have delivered substantial progress in recent years. Reviewing all developments is not the objective of this Research News; but it is interesting to highlight some recent conceptual developments that increase the quan-tity and quality of information, mainly a combination of several operando techniques, and imaging of profi les during reaction. Probably the most-remarkable progress is the combination of several operando measurements in a single experiment, which

m 5293wileyonlinelibrary.com

EARCH N

bination of several optical spectroscopies, or their combination with resonance or synchrotron-based spectroscopies. Comple-mentary simultaneous information is valid not only to develop better insight, but also as a tool for an effi cient real-time process control (e.g., for the controlled regeneration of coked catalyst, the combined operando insight minimizes the thermal increase during coke burning from 75 to 25 ° C). [ 2 ] Another important development operando real-time mapping and imaging during reaction. Complementary real-time determination of chemical profi les along a catalyst bed during nitrogen-storage reduction on Pt–BaO/Al 2 O 3 catalysts by Raman spectroscopy and diffuse-refl ection IR Fourier transform spectroscopy (DRIFTS) dem-onstrates a different evolution of nitrates and nitrites, which is relevant to understanding the reaction mechanism and profi les along the catalyst bed. [ 3 ] Imaging is also valuable to investigate and monitor processes in microreactors, such as hydrogena-tion of cyclohexene along the channels of a microreactor, [ 4 ] but it is also good to investigate reactants mixing, as in the case of the reaction of ethanol and acetic acid to make ethyl acetate. [ 5 ] Electrocatalysis and operando investigation is another area of progress; Bell and coworkers reported the presence of Au–OOH species on gold electrodes by Raman spectroscopy at the oxygen release-potential values, [ 6 ] delivering experimental evidence of theoretical predictions. [ 7 ]

2.1. From in situ to Operando

Spectroscopic techniques are powerful, because they pro-vide fundamental information about catalyst structures in the working state. Most techniques have been reviewed else-where. [ 8–14 ] In particular, there have been recent compilations in Chem. Soc. Rev . (2010) vol. 39, issue 12 and in Adv. Catal ., (2006) vol. 50, (2007) vol. 51, and (2009) vol. 52. Raman spec-troscopy will essentially illustrate this Research News because it is one of the most-powerful tools used for characterizing working catalysts; it normally works in refl ectance mode and with visible radiation (it works from UV up to 1500 nm excita-tion lines) and may work in very-broad pressure and tempera-ture ranges (with the appropriate excitation wavelength). In addition, catalytic reactors are easily accessible to spectroscopy using quartz fi ber optics. It is also possible to study reactions in the liquid phase [ 15 ] or under supercritical conditions. [ 16 ]

Knowing the structure under in situ conditions is only part of the story. Ortega y Gasset followed up his comment on cir-cumstance: “It is wrong to tell that we are determined by our circumstances. On the contrary, circumstances are the dilemma on which we must take a decision. It is our character who determines our decision”. The same circumstances will have different effects on different individuals, or on different cata-lysts. The catalyst’s character (acidic, basic, redox, bifunctional) will show up in different performances. We need to know the catalytic activity too. By determining both structure and activity simultaneously, operando studies provide molecular-level infor-mation about the states of the catalyst and surface-reaction intermediates during reactions. For a correct assessment, we need structural and kinetic pieces of information that are rel-evant to the catalytic act.

“In situ” spectroscopy is a well-established methodology, while operando spectroscopic methodology was proposed in the literature at the beginning of this century. [ 10,11 , 17 ] “Operando” is Latin for “working”, and “in situ” for “on site”, the latter means that there is no temporal discrimination. Advanced in situ studies have uncovered detailed information during variable-programmed (pressure, temperature, reactant concen-tration, amongst others) experiments (e.g., during temperature-programmed processes [temperature-programmed reduction (TPR) in situ, temperature-programmed oxidation (TPO) in situ, or temperature-programmed surface reaction (TPSR) in situ] or modulation studies). Because of the dramatic effect of the reaction environment on the catalyst structure at work, it is necessary to combine in situ studies during genuine cata-lytic operation with simultaneous catalytic-activity/selectivity measurements. Such a combination of experiments is the key concept of operando methodology. With a single word, “oper-ando” underlines the simultaneous evaluation of both the cata-lyst active-site structure and the catalytic activity/selectivity in a structurally and kinetically relevant manner. Operando is about structure and about activity, and each has its requirements.

2.2. Kinetic Relevance

The fi rst requirement of operando methodology is that catalytic-activity data in an operando reactor cell should agree with those in conventional reactors. These requirements have been at the core of the operando concept since it was fi rst proposed, [ 11 , 17 ] so that activity/selectivity data are consistent [ 18 ] and it is possible to perform Arrhenius analyses. [ 19 ] In situ cell design is typically not the best for delivering true catalytic-reaction data since sig-nifi cant temperature and diffusional gradients may dominate; also, the design is not always the most accurate from a reactor point of view. [ 10 , 20 ] A thorough revision on the kinetic require-ments of operando cells can be found elsewhere. [ 21 ]

2.3. Structural Relevance: “There is no Life in Earth!”

The second requirement of operando methodology is that the structure determined by the spectroscopic operando study has to be relevant to the catalytic process: we have to wisely select the catalyst features and the corresponding characterization techniques. An essential hurdle is that catalysis is a surface phe-nomenon (be it an internal or external surface), and most spec-troscopic techniques acquire signals from both the bulk and the surface. It is a major challenge to obtain surface-relevant infor-mation of real catalysts.

In many catalyst particles, the surface-to-volume ratio is low, and this is a problem in characterizing the surface. If an alien civ-ilization were to study Earth, and perform global analyses of our planet, they may get the wrong feedback. Earth is ca. 12 700 km diameter, and life occurs in only the very-few outermost kilo-meters. If our world were analyzed, they would get valuable information on the composition, with a metallic core and dif-ferent kind of oxides and perovskites, or an interesting magnetic fi eld, amongst other things. Signals from the outermost layer, where life exists, can hardly compete with the overwhelming

GmbH & Co. KGaA, Weinheim Adv. Mater. 2011, 23, 5293–5301

EARCH N

signals from the bulk. Their conclusion would be clear: “There is no life in that planet”. We know this is wrong: we know there is life on this planet. Understanding catalysis is not that dif-ferent from interplanetary exploration. Catalytic life occurs at the surface, not in the bulk , even if it is affected by the bulk. Reac-tants and intermediates will not interact with the bulk structure, but with the exposed layer. The surface structure and surface reactivity are determined by the bulk, but it is wrong to directly connect the bulk structure with the surface reactivity.

We know that the surfaces of catalyst particles are not like the bulk. The surface is a dramatic change to the bulk structure: it is the end of a 3D order; in summary, it is a “catastrophe” for bulk regularity; most particles may actually have a core–shell structure. The structure that covers the surface is not only deter-mined by the bulk material but by the catalytic reaction condi-tions: reaction feed, temperature, and pressure, amongst others.

2.4. Surface Species, Nanoparticles and Microparticles

Catalysts possess multiple functional properties, such as acid-base, redox and electron transfer and transport, which result in different kinds of chemisorption and transformations of the reactants, such as O-insertion, H-abstraction, isomerization, etc. The determination of the structure–performance relation-ship requires the right assessment of the structure (i.e., that which is relevant to the performance). We may see a collection of scenarios and we will focus on oxide catalysts to highlight this aspect. Characterizing the surface layer is a major challenge due to the overwhelming signal from the bulk. Oxide catalysts may be used as pure bulk oxide catalysts, supported bulk oxide particles or as molecularly dispersed oxide species ( Figure 1 A).

Molecularly dispersed oxide catalysts (typically known as “supported oxides”) are particularly convenient since their active

Figure 1 . A) Bulk oxides, nanoscaled bulk oxides, molecularly dispersed oB) Transmission electron microscopy (TEM) and high-resolution TEM (Hoctahedra (e, f). Reproduced with permission. [ 22 ] Copyright 2010 American

Reactant

Product

Bulk metal oxide

Nanoscaled-bulk metal

O||V – O-H

/ \O O| |

SUPPORT SUPPORT

/ | \O O O| | |

Molecularly dispered oxides Support-stablized nanosbulk metal oxides

sites are essentially dispersed on the oxide support surface, which leaves them directly exposed to reactants. Raman spec-troscopy is particularly appropriate to study supported oxides, since most oxides employed as support materials are not Raman active (gamma-Al 2 O 3 ), possess only very-weak Raman bands (SiO 2 ) or do not exhibit intense Raman bands above 700 cm − 1 (e.g., ZrO 2 , TiO 2 , Nb 2 O 5 , zeolites, etc.). Thus the interference with most informative Raman modes is minimized or non-existent for supported oxide catalysts. [ 10 ]

Bulk oxides are at the other end of site exposure. Bulk oxides possess a 3D character, but only the outermost layer is exposed to reactants. The signal from the bulk oxide, rather than that from outermost surface, dominates the spectroscopic signal. In addition, the reaction environment may simultaneously modify both the bulk lattice and the surface layer of the catalyst, like in the V–P–O system. [ 23 ] The catalytic performance of surface vanadium and phosphorus oxide species on titania is similar to that of bulk V–P–O during butane oxidation. [ 24 ] Several inves-tigations have revealed that the surface composition of many mixed metal oxide catalytic systems is very different from the bulk-lattice composition. [ 25,26 ] In some cases, a monolayer of one of the metal oxide components decorates the surfaces of bulk mixed metal oxides. [ 27 ] These fi ndings have enormous implications for catalysis science, since mixed metal oxide cata-lysts are essentially supported oxides . However, in these systems, the signal from the support (bulk phase) will overlap with that of the supported phase (outermost layers). Thus, we need a new strategy: maximizing the surface-to-volume ratio .

The surface-to-volume ratio is rather low for most bulk mixed metal oxide systems because of the rather-high primary-particle size. As we go to the nanoscale, decreasing particle sizes lead to increasingly higher surface-to-volume ratios. Moving from the microscale to the nanoscale will facilitate characterization of the outermost layers in the presence of the bulk phase. In addition, a

5295bH & Co. KGaA, Weinheim wileyonlinelibrary.com

xides and support-stabilized, nanoscale oxides (source: M. A. Bañares); R-TEM) images of ceria nanorods (a, b), nanocubes (c, d), and nano-

Chemical Society.

Product

EARCH N

lattice, and, very often, these defects possess the relevant catalytic properties: there are a signifi cant number of cases, but a repre-sentative one may be the preparation of ceria nanocrystals. Dif-ferent surface planes of ceria nanocrystals exhibit different surface defects (Figure 1 B), which Wu et al. characterized using Raman spectroscopy and probed the surface reactivity for CO preferential oxidation in the presence of H 2 . [ 22 ] This approach allowed Over-bury et al. to study high-surface-area ceria with controlled surface terminations; in their work, they could, for the fi rst time, detect the Raman bands of O vacancy sites on reduced ceria surfaces and investigate the formation of superoxide and peroxide species, determining that superoxides are more reactive towards CO. [ 22 ] These data stand on advanced experimental synthesis and in situ studies, but they also feed back from theoretical calculations, which indicate that different crystal planes of ceria exhibit different properties, such as the surface-formation energy and the interac-tion with surface molecules. [ 28 ] The integration of all of these com-plementary approaches brings a solid insight on the structure–performance relationships in ceria-based catalysts that would not have been possible with larger particles, in which the signals obtained from polycrystalline ceria would average all of these effects.

Catalysis is among the earliest applications of nanotech-nology, and actually, as we go into nanoscaled materials, we approach catalytic reality. We have seen that the use of nano-particles is a valuable approach for operando studies. As we go into nanoscaled materials, we also face additional problems. The handling of nanoparticles is not easy, and they also pose a health risk. Ideally, we should combine the properties of nano-particles with the handling and safety advantages of micropar-ticles, (i.e., there is a need to immobilize nanoparticles ). There are several approaches, but these tend to be tedious and complex. One method that we have traditionally used is to disperse oxides on a support, adding up to twice the dispersion-limit loading (typically called “monolayer” loading) by impregnation. At such loading, the supported phase cannot be totally dispersed on the oxide support as a molecularly dispersed phase. They have to aggregate into 3D aggregates, forming bulk mixed oxide phases; due to the rather low loading, the segregated phase cannot be large, and it is typically limited to a few nanometers. Thus, we have been able to obtain nanocrystals on alumina and other supports by simple impregnation. These support-stabilized nanoparticles exhibit a catalytic performance equivalent to their conventional bulk counterparts for propane ammoxidation to acrylonitrile on SbVO 4 and for propane oxidation to acrylic acid on rutile Mo–V–O phases. [ 29 ] A simpler approach would be to use a solvent-free and experimentally simple deposition of nanoparticles on microparticles. Fernández and coworkers in Madrid have reported a simple method to immobilize nanopar-ticles that needs no solvent or other methodology that would modify the nanoparticles and their properties. [ 30 ] This is a low-energy, dry nanodispersion method [ 30 ] that consists of shaking a mixture of nanoparticles and microparticles in a tubular mixer at room temperature ( Figure 2 A), resulting in a hierarchical dispersion of nanoparticles decorating the microparticle sub-strate (Figure 2 B). Such a method is very convenient since it stabilizes nanoparticles on larger particles that act as a support. Additionally, the interaction between the support and supported

nanoparticles can be used to tune or create magnetic, piezo-electric and catalytic properties (Figure 2 C), depending on the temperature treatment for the nanodispersed material; [ 31 ] due to the small particle sizes, it is possible to spectroscopically characterize changes in structure (Figure 2 D). We feel that this is a powerful method of preparing hierarchically dispersed nanoparticles on microparticles, which is environmentally friendly and easy to scale up. A key challenge in catalysis is to be able to translate investigation on model systems to large-scale operation; this won’t be necessary if we deal with the same samples in both scenarios.

3. Operando Highlights

3.1. Stories of Birth and Life

Understanding a catalyst is understanding three stories: a story of birth, a story of life, and a story of death . I will briefl y highlight examples where the simultaneous determination of struc-ture and activity is critical for understanding such stories. Such a combination of experiments is the core of operando methodology.

3.1.1. Birth of a Metal Catalyst derived from an Organometallic Cluster

This experience goes back to my early postdoc times: I studied metal catalysts for hydrogenation reactions, but the precursor was not a common one, it was an organometallic cluster in which the ligands were clusters too. Controlled decomposition of these clusters of clusters in helium, hydrogen or in a hydro-genation-reaction feed resulted in a high-surface-area metal system, such as pure cobalt or bimetallic Co–Zn, amongst others. [ 32 ] The main ligand was a tricobaltnanocarbonyl cluster. The activation of these clusters of clusters was monitored by temperature-programmed DRIFTS-mass-spectrometry (MS) studies. [ 32 ] During the activation in inert conditions, hydrogen or in the hydrogenation reaction feed, the carbonyl ligands pro-gressively desorb as CO gas, which was confi rmed by on-line mass spectrometry. The progressive release of CO results in a progressive shift in the C–O IR vibration from the characteris-tics of a carbonyl ligand to the characteristics of CO chemisorbed on a metal surface. The same trend occurs when the cluster of clusters is heated in a reaction mixture (butadiene hydrogena-tion); in addition, the IR bands of the adsorbed hydrocarbons grow stronger and, simultaneously, on-line mass spectrometry indicates that the butadiene becomes hydrogenated. These operando DRIFTS-MS studies illustrate the transformation of an organometallic compound into a metal surface; furthermore, they illustrate the transition from an inert organometallic com-pound to a catalytic active system: the birth of a catalyst.

3.1.2. Birth and Life of an Oxide Catalyst: Support-Stabilized Nanoscale ~SbVO 4

A rutile-SbVO 4 catalyst for propane ammoxidation is the per-former for this story of birth and life. It Illustrates the advan-tages of operando spectroscopy and of support-stabilized,

GmbH & Co. KGaA, Weinheim Adv. Mater. 2011, 23, 5293–5301

EARCH N

Figure 2 . A) Schematic of the attachment of nanoparticles to the outer surface of an individual ZnO microparticle obtained by dry nanodispersion method: Co 3 O 4 /ZnO mixtures before (a) and after (b) the dry nanodispersion process. B) Representative image of cobalt oxide nanoparticles dispersed on alumina microparticles [ 30 ] C) Raman spectra illustrating incipient and extensive interaction of oxide nanoparticles with the support. D) Catalytic performance of Co 3 O 4 /ZnO catalytic systems for the synthesis of glycerol carbonate at 145 ° C during 4 h. Figure 2A, 2C and 2D reproduced with permission. [ 31 d ] Copyright 2010, Elsevier.

nanoscale bulk mixed oxides along with the increasingly impor-tant feedback from computational modelling. The rutile-SbVO 4 phase appears intimately related to the propane-ammoxidation reactivity: its formation is more extensive in aged catalysts. [ 26 ] Preliminary reports in the literature have already provided valuable insight. Based on IR spectra, Centi and Perathoner reported that a change in the reactivity had to be due to the for-mation of an ~SbVO 4 phase by the reaction of vanadium and antimony oxides that had not reacted during calcination. [ 33 ] The fi rst paper using the term operando reported an investigation of support-stabilized, nanoscale SbVO 4 -based catalysts for pro-pane ammoxidation. [ 11 ] To do so, a total coverage of V + Sb of one monolayer was deposited on an alumina support. The fresh, calcined catalyst exhibited no SbVO 4 crystalline phase, which forms during reaction, in line with reports in the literature of unsupported bulk SbVO 4 An operando Raman-gas-chroma-tography (GC) study showed that the ammoxidation-activity performance runs parallel to the build up of the rutile-SbVO 4 phase ( Figure 3 A). [ 11 ] This is a nanocrystalline phase, probably less than 4 nm long, since it barely generates X-ray diffraction

patterns. [ 34 ] These nanocrystals possess a very-high surface-to-volume ratio that minimizes the bulk-to-surface signal ratio. Most importantly, the performance of the monolayer of SbVO 4 on alumina was equivalent to that of a bulk SbVO 4 catalyst for propane ammoxidation. [ 34 ] Thus , the proof of concept was estab-lished : support-stabilized nano-SbVO 4 performs like conven-tional bulk SbVO 4 . Raman spectra during the reaction show how the ammoxidation-reaction environment is the driving force for rearranging the surface amorphous antimony oxide species and dispersed vanadium oxide species into rutile-SbVO 4 nanocrys-tals. [ 11 ] The story of birth goes now into the story of life. Since the bulk signal was minimized in the nanocrystalline SbVO 4 , it was possible to assess the presence of the dispersed vanadium oxide species interacting with the nano-SbVO 4 , along with that of segregated Sb 2 O 3 or Sb 2 O 4 interacting with the nano-SbVO 4 during the reaction. Dispersed vanadium oxide species could not be observed in the large bulk SbVO 4 particles. During cat-alytic operation, the interaction of the segregated vanadium and of the segregated antimony oxides with SbVO 4 are closely related. [ 35 ] Raman spectra show that dispersed V 5 + oxide species

5297bH & Co. KGaA, Weinheim wileyonlinelibrary.com

EARCH N

Figure 3 . A) Raman spectra and activity data during propane ammoxidation in the operando fi xed-bed reactor of alumina; supported Sb–V–O mixed oxides at monolayer coverage; 25% O 2 + 9.8% C 3 H 8 + 8.6% NH 3 in He. Reproduced with permission. [ 42 ] Copyright 2005, Elsevier. and simultaneous illustrations of structural chances occurring during reaction. B) Small-area-electron-diffraction (SAED) patterns of SbVO 4 crystals oriented along the [101] basic rutile direction and prepared at different oxygen pressures, indicated in the upper-left part of each diagram. C) Scheme showing the domain of existence of ~SbVO 4 as a function of composition and partial oxygen pressure at 800 ° C. The three described sample series are depicted in magenta, green and dark-red lines and their expressions and reaction mechanisms at the left of the scheme in the corresponding colours. The general expression for the whole domain of existence that consists of the cyan stripes region of the scheme is formulated at the bottom of the fi gure. Figure 3B and 3C reproduced with permission. [36] Copyright 2005, Elsevier.

migrate into the SbVO 4 lattice as reduced vanadium ions during the ammoxidation reaction (Figure 3 A).Concomitantly, Sb 5 + ions leave the SbVO 4 lattice to go into the segregated Sb 2 O 4 , which is a mixed-valence oxide constituted of Sb 3 + and Sb 5 + ions. Reoxi-dation of the catalyst oxidizes the vanadium ions to V 5 + , which are not stabilized in the rutile-SbVO 4 lattice [ 36 ] and redisperse on the alumina support. Concomitantly, Sb 5 + ions return from the

segregated antimony oxide phase to the rutile-SbVO 4 lattice. [ 35 ] This work demonstrated that the redox cycle of vanadium spe-cies on mixed Sb–V–O catalysts for the ammoxidation reaction is coupled to a migration cycle of Sb 5 + ions. The high mobility of the Sb 5 + ions had already been calculated by computational chemistry, which underlines that there is very little barrier for the migration of Sb 5 + ions in the rutile-SbVO 4 phase. [ 37 ] The rutile

mbH & Co. KGaA, Weinheim Adv. Mater. 2011, 23, 5293–5301

EARCH N

vanadium antimonates possess cation vacancies that strongly depend on moderately reducing or oxidizing conditions, [ 38 ] which is the turning point determining its reactivity. [ 35 ] Recently, Landa-Cánovas et al. [ 36 ] provided a detailed insight on the struc-tural reactivity of the rutile-SbVO 4 phase by transmission elec-tron microscopy (TEM) and small-area electron diffraction (SAED) (Figure 3 B). Raman spectroscopy shows the presence of surface alkoxides, that are not detected in the absence of sur-face vanadium oxide species. The activity becomes increasingly higher as the Raman bands of SbVO 4 grow stronger. [ 39 ] The sur-face vanadium oxide species are critical for propane activation: this has been confi rmed by a study of substituted SbVO 4 . [ 40 ] An important lesson from these results is that a combined study of the system by different complementary techniques brings a more-solid and reliable insight into structure–activity relation-ships at a molecular scale. What is the role of the rutile-SbVO 4 phase? Electron microscopy underlines the great structural reac-tivity (Figure 3 C), [ 36 ] which was confi rmed by computational chemistry, indicating a very-low energy barrier for migration of antimony ions. [ 37 ] This accounts for the redox reactivity. In addi-tion, density-functional-theory (DFT) calculations demonstrate that vanadium ions surrounded by antimony ions via bridging

Figure 4 . A) Initial cartoon describing the vanadia-ceria interface in 200(reproduced with permission from, [ 45 ] copyright 2009 Wiley. C) Operando Rof activity data from operando reactor before and after deactivation. FigureChemical Society.

oxygens are the preferred sites for ammonia activation, which is necessary for nitrogen insertion in the hydrocarbon, no other site is preferred for ammonia chemisorption. [ 41 ] This is also a nice example of how spectroscopy is the bridge that connects experimental and theoretical knowledge.

3.2. A Story of Death

The interaction between the supported oxide and the support determines its reactivity; on occasions, the support and sup-ported oxide undergo a solid-state reaction. Martínez-Huerta et al. studied the nature of the active sites for the ethane oxida-tive-dehydrogenation reaction on ceria-supported vanadia. [ 19 , 43,44 ] Extended-X-ray-absorption-fi ne-structure (EXAFS) analyses showed that the vanadia-ceria interaction is particularly strong, being the fi rst co-ordination sphere of vanadium, remarkably similar to that in cerium vanadate, leading to a dramatic sta-bilization of V 5 + ions and to the reduction of cerium ions at the interface with ceria, which is backed up by in situ electron-paramagnetic-resonance (EPR) spectroscopy; the V 5 + –O–Ce 3 + bond is thus characteristic of the ceria-supported vanadium oxide catalyst ( Figure 4 A). [ 43 ] Raman spectroscopy studies

5299mbH & Co. KGaA, Weinheim wileyonlinelibrary.com

4 (source: Bañares). B) DFT + U description of the vanadia-ceria interface aman spectra during ethane oxidative dehydrogenation. D) Arrhenius plot

EARCH N

reducing conditions, which in turn promotes the formation of CeVO 4 by 200 ° C. [ 44 ] A qualitative cartoon of the vanadia-ceria interface presented in 2004 by Martínez-Huerta is presented in Figure 4 A, illustrating the progressive interaction between the ceria support and the surface vanadium oxide species. In 2009, the model of the vanadia-ceria interface [ 19 , 43,44 ] was confi rmed by a DFT study (Figure 4 B). [ 45 ] Such an interaction stabilized Ce 3 + at the vanadia-ceria interface, [ 43,44 ] and DFT calculations bring the rationale for such a stabilization. [ 45 ] This system con-stitutes a nice example of the synergetic interaction between operando and theoretical modelling approaches.

Having set the interface scenario, Martinez-Huerta et al. [ 19 ] reported the transformation of surface vanadium oxide spe-cies into CeVO 4 during ethane oxidative dehydrogenation at temperatures lower that those in air and higher than those in a reducing environment. [ 19 ] This is due to the redox cycle, which partially reduces the ceria sites, promoting a solid-state reaction to irreversibly form CeVO 4 , which deactivates the cata-lyst. [ 19 ] Operando Raman spectra also showed that the forma-tion of CeVO 4 does not correlate directly with deactivation; [ 19 ] such an observation could only be made using real-time operando Raman-GC analyses . The catalyst is deactivated above 500 ° C, but the formation of CeVO 4 is evident at 460 ° C (Figure 4 C). The Raman bands of CeVO 4 become sharper at temperatures above 500 ° C. [ 19 ] This trend is consistent with a decrease in the exposure of the active sites, rather than a change in the struc-ture of the active phase. The Arrhenius plots measured using the operando cell show that the apparent activation energy does not change signifi cantly as the catalyst ages (Figure 4 D) (i.e., as the structure changes from surface vanadia on ceria to a material in which the surface vanadia has reacted with the ceria to form CeVO 4 ). The Arrhenius plots underline a decrease in the number of active sites. It is concluded that the V 5 + –O–Ce 3 + bonds present in both the fresh and aged catalysts are directly related to the active sites, regardless of whether they belong to the ceria-supported vanadia or the CeVO 4 phase. The redox cycle is related to the cerium ions at the interface with the vanadia. Deactivation is due to development of better-defi ned CeVO 4 domains, which decreases the number of exposed sites.

4. Conclusions

Progress in nanomaterials and catalysis stands on three pillars: 1) the synthesis of nanomaterials, including the preparation of hierarchically dispersed nanoparticles; 2) theoretical studies of materials that enable experimental results to be understood; and 3) advanced, in situ characterization during operation (operando methodology). These three pillars blend synergis-tically in catalysis science. In situ and operando studies sig-nifi cantly advance catalysis science by providing fundamental information about catalytic structure and surface species under controlled environments; this signifi cantly assists in the estab-lishment of fundamental molecular structure–activity/selec-tivity relationships for catalytic systems, which, in turn, can be used to control the catalytic process and design new catalysts. It is envisaged that the combination of operando studies on nano-scale particles (properly supported) and theoretical chemistry is

the approach that will become the common powerful approach to understand catalysis in the years to come.

Special care has to be taken in the design of operando cells; since it was fi rst proposed, operando requires that the reactor cell satisfi es the requirements of an in situ cell and those of the catalytic reactor used for the target reaction. Such an approach will generate solid understanding of structure–activity relation-ships on a molecular scale. The combination of spectroscopic techniques with imaging is becoming powerful to understand not only the state of the catalytic sites, but also its context and its evolution. Operando methodology brings catalysis research into its real circumstance ; operando imaging brings an even-broader vista on the circumstances of catalyst life. Signifi cant progress is anticipated in the coming years towards increasing the knowledge of catalytic science and engineering, thus fos-tering its progress.

Acknowledgements The Spanish Ministry of Science and Innovation (project CTQ2008-04261-PPQ and EULANEST/MICINN PIM2010EEU-00138) is gratefully acknowledged.

Received: May 13, 2011 Published online: September 23, 2011

[ 1 ] a) S. J. Tinnemans , J. G. Mesu , K. Kervinen , T. Visser , T. A. Nijhuis , A. M. Beale , D. E. Keller , A. M. J. van der Eerden , B. M. Weckhuysen , Catal. Today 2006 , 113 , 3 ; b) U. Bentrup , Chem. Soc. Rev. 2010 , 39 , 4718 .

[ 2 ] C. S. M. Bennici , B. M. Vogelaar , T. A. Nijhuis , B. M. Weckhuysen , Angew. Chem. Int. Ed. 2007 , 46 , 5412 .

[ 3 ] E. Rödel , A. Urakawa , S. Kureti , A. Baiker , Phys. Chem. Chem. Phys. 2008 , 10 , 6190 .

[ 4 ] A. Urakawa , F. Trachsel , P. R. von Rohr , A. Baiker , Analyst 2008 , 133 , 1352 .

[ 5 ] P. D. I. Fletcher , S. J. Haswell , X. Zhang , Electrophoresis 2003 , 24 , 3239 .

[ 6 ] B. S. Yeo , S. L. Klaus , P. N. Ross , R. A. Mathies , A. T. Bell , ChemPhys-Chem 2010 , 11 , 1854 .

[ 7 ] a) J. Rossmeisl , A. Logadottir , J. K. Nørskov , Chem. Phys. 2005 , 319 , 178 ; b) Y. H. Fang , Z. P. Liu , J. Phys. Chem. C 2009 , 113 , 9765 .

[ 8 ] I. E. Wachs , Surf. Sci. 2003 , 544 , 1 . [ 9 ] a) B. M. Weckhuysen , In situ Spectroscopy of Catalysts , American Sci-

entifi c Publishers , Valencia , CA, USA, 2004 ; b) B. M. Weckhuysen , Chem. Commun. 2002 , 97 ; c) B. M. Weckhuysen , Phys. Chem. Chem. Phys. 2003 , 5 , 4351 .

[ 10 ] M. A. Bañares , I. E. Wachs , J. Raman Spectrosc. 2002 , 33 , 359 . [ 11 ] M. O. Guerrero-Pérez , M. A. Bañares , Chem. Commun. 2002 , 1292 . [ 12 ] S. Kuba , H. Knözinger , J. Raman Spectrosc. 2002 , 33 , 325 . [ 13 ] G. Mestl , J. Mol. Catal. A 2000 , 158 , 45 . [ 14 ] M. O. Guerrero-Pérez , M. A. Bañares , Catal. Today 2004 , 96 , 265 . [ 15 ] a) D. E. Pivonka , J. R. Empfi eld , Appl. Spectrosc. 2004 , 58 , 41 ;

b) R. Schmink , J. L. Holcomb , N. E. Leadbeater , Chem. Eur. J. 2008 , 14 , 9943 ; c) V. Calvino Casilda , E. Pérez-Mayoral , M. A. Bañares , E. Lozano Diz , Chem. Eng. J. 2010 , 161 , 371 ; d) V. Calvino-Casilda , M. A. Bañares , E. Lozano Diz , Catal. Today 2010 , 155 , 279 .

[ 16 ] J. D. Grunwaldt , R. Wandeler , A. Baiker , Catal. Rev. 2003 , 45 , 1 . [ 17 ] M. A. Bañares , M. O. Guerrero-Pérez , J. L. G. Fierro , G. G. Cortez ,

J. Mater. Chem. 2002 , 12 , 3337 .

mbH & Co. KGaA, Weinheim Adv. Mater. 2011, 23, 5293–5301

EARCH N

[ 18 ] S. J. Khatib , R. Guil-López , M. A. Peña , J. L. G. Fierro , M. A. Bañares , Catal. Today 2006 , 118 , 353 .

[ 19 ] M. V. Martínez-Huerta , G. Deo , J. L. G. Fierro , M. A. Bañares , J. Phys. Chem. C 2008 , 112 , 11441 .

[ 20 ] M. A. Bañares , I. E. Wachs , Adv. Catal. 2009 , 52 , 43 . [ 21 ] a) F. C. Meunier , A. Goguet , S. Shekhtman , D. Rooney , H. Daly ,

Appl. Catal. A 2008 , 340 , 196 ; b) F. Meunier , Chem. Soc. Rev. 2010 , 39 , 4602 .

[ 22 ] Z. Wu , M. Li , J. Howe , H. M. Meyer III , S. H. Overbury , Langmuir 2010 , 26 , 16595 .

[ 23 ] a) G. J. Hutchings , A. Desmartinchomel , R. Olier , J. C. Volta , Nature 1994 , 368 , 41 ; b) V. V. Guliants , J. B. Benziger , S. Sundaresan , N. Yao , I. E. Wachs , Catal. Lett. 1995 , 32 , 379 .

[ 24 ] G. Deo , I. E. Wachs , J. Catal. 1994 , 146 , 335 . [ 25 ] V. V. Guliants , J. B. Benziger , S. Sundaresan , N. Yao , I. E. Wachs ,

Catal. Lett. 1995 , 32 , 379 . [ 26 ] a) A. Pantazidis , A. Burrows , C. J. Kiely , C. Mirodatos , J. Catal. 1998 ,

177 , 325 ; b) H. W. Zanthoff , W. Grünert , S. Buchholz , M. Heber , L. Stievano , F. E. Wagner , G. U. Wolf , J. Mol. Catal. A 2000 , 162 , 435 ; c) L. E. Briand , O. P. Tkachenko , M. Guraya , I. E. Wachs , W. Grünert , Surf. Interface Anal. 2004 , 36 , 238 ; d) L. E. Briand , O. P. Tkachenko , M. Guraya , X. T. Gao , I. E. Wachs , W. Grünert , J. Phys. Chem. B 2004 , 108 , 4823 .

[ 27 ] a) L. J. Burcham , L. E. Briand , I. E. Wachs , Langmuir 2001 , 17 , 6164 ; b) L. E. Briand , A. Hirt , I. E. Wachs , J. Catal. 2001 , 202 , 268 ; c) L. E. Briand , J. M. Jehng , L. Cornaglia , A. M. Hirt , I. E. Wachs , Catal. Today 2003 , 78 , 257 .

[ 28 ] a) M. Baudin, M. Wójcik , K. Hermansson , Surf. Sci. 2000 , 468 , 51 ; b) M. Nolan , S. Grigoleit , D. C. Sayle , S. C. Parker , G. W. Watson , Surf. Sci. 2005 , 576 , 217 ; c) M. Nolan , J. E. Fearon , G. W. Watson , Solid State Ionics 2006 , 177 , 3069 ; d) M. Nolan , G. W. Watson , J. Phys. Chem. B 2006 , 110 , 16600 .

[ 29 ] a) M. O. Guerrero-Pérez , J. L. G. Fierro , M. A. Vicente , M. A. Bañares , J. Catal. 2002 , 206 , 339 ; b) R. López-Medina , J. L. G. Fierro , M. O. Guerrero-Pérez , M. A. Bañares , Appl. Catal. A 2010 , 375 , 55 .

[ 30 ] J. F. Fernández , I. Lorite , F. Rubio-Marcos , J. J. Romero , M. A. García , A. Quesada , M. S. Martín-González , J. L. Costa-Krämer , ( Consejo

Superior de Investigaciones Científi cas, CSIC ), WO2010010220-A1; ES2332079-A1, 2010 .

[ 31 ] a) A. Serrano , E. Pinel , A. Quesada , I. Lorite , M. Plaza , L. Pérez , F. Jiménez-Villacorta , J. De La Venta , M. S. Martín-González , J. L. Costa-Krämer , J. F. Fernandez , J. Llopis , M. García , Phys. Rev. B: Condens. Matter 2009 , 79 , 144405 ; b) M. S. Martín-González , J. F. Fernández , F. Rubio-Marcos , I. Lorite , J. L. Costa-Krämer , A. Quesada , M. A. Bañares , J. L. G. Fierro , J. Appl. Phys. 2008 , 103 , 083905 ; c) F. Rubio-Marcos , M. A. Bañares , J. J. Romero , J. F. Fernandez , J. Raman Spectrosc. 2011 , 42 , 639 ; d) F. Rubio Marcos , V. Calvino-Casilda , M. A. Bañares , J. F. Fernandez , J. Catal. 2010 , 275 , 288 .

[ 32 ] a) M. A. Bañares , L. Dauphin , V. Calvo-Pérez , T. P. Fehlner , E. E. Wolf , J. Catal. 1995 , 12 , 396 ; b) M. A. Bañares , L. Dauphin , X. Lei , W. Cen , M. Shang , E. E. Wolf , T. P. Fehlner , Chem. Mater. 1995 , 7 , 553 .

[ 33 ] G. Centi , S. Perathoner , Appl. Catal. A 1995 , 124 , 317 . [ 34 ] M. O. Guerrero-Pérez , J. L. G. Fierro , M. A. Vicente , M. A. Bañares ,

J. Catal. 2002 , 206 , 339 . [ 35 ] M. O. Guerrero-Pérez , M. A. Bañares , J. Phys. Chem. C 2008 , 111 , 1315 . [ 36 ] A. R. Landa-Cánovas , F. J. García-García , S. Hansen , Catal. Today

2010 , 158 , 156 . [ 37 ] B. Irigoyen , A. Juan , N. Castellani , J. Catal. 2000 , 190 , 14 . [ 38 ] G. Xiong , V. S. Sullivan , P. C. Stair , G. W. Zajac , S. S. Trail , J. A. Kaduk ,

J. T. Golaband , J. F. Brazdil , J. Catal. 2005 , 230 , 317 . [ 39 ] M. O. Guerrero-Pérez , M. A. Bañares , Catal. Today 2004 , 96 , 265 . [ 40 ] A. Wickman , A. Andersson , Appl. Catal. A 2011 , 391 , 110 . [ 41 ] E. Rojas , M. Calatayud , M. O. Guerrero-Pérez , M. A. Bañares , Catal.

Today 2010 , 158 , 178 . [ 42 ] M. A. Bañares , Catal. Today 2005 , 100 , 71 . [ 43 ] M. V. Martínez-Huerta , J. M. Coronado , M. Fernández-García ,

A. Iglesias-Juez , G. Deo , J. L. G. Fierro , M. A. Bañares , J. Catal. 2004 , 225 , 240 .

[ 44 ] M. V. Martínez-Huerta , G. Deo , J. L. G. Fierro , M. A. Bañares , J. Phys. Chem. C 2007 , 111 , 18708 .

[ 45 ] M. Baron , H. Abbott , O. Bondarchuk , D. Stacchiola , A. Uhl , S. Shaikhutdinov , H.-J. Freund , C. Popa , M. V. Ganduglia-Pirovano , J. Sauer , Angew. Chem. Int. Ed. 2009 , 48 , 8006 .

5301mbH & Co. KGaA, Weinheim wileyonlinelibrary.com

Operando Spectroscopy: the Knowledge Bridge to Assessing Structure-Performance Relationships in...

Documents

BANKSIDE, BOROUGH & LONDON BRIDGE

CROWN & BRIDGE - WHW Plastics

courses-job market bridge

SpaciMS: spatial and temporal operando resolution of reactions within catalytic monoliths

Highway Bridge Structures

Partnership Bidding at Bridge

Analyzing the Influence of H 3 PO 4 as Catalyst Poison in High Temperature PEM Fuel Cells Using in-operando X-ray Absorption Spectroscopy

asean catalyst - CIMB

Mabey Bridge

Obligacje na rynku Catalyst

Role of catalyst pellet activity distribution in catalyst poisoning

Earthquake vehicle bridge

Catalyst characterization (Materials Characterization

Bridge Bearings

The Bridge Kingdom - VISIONVOX

Bridge summary

IMTApprenticeship Program - Catalyst Connection

A Bridge Between - Oapen

Steam Reforming - Catalyst Loading

Bridge to Higher Mathematics