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Three-dimensional structured carbon foam:
Synthesis and Applications
Tung Ngoc Pham
Department of Chemistry
Doctoral thesis
Umeå 2016
II
Tung Ngoc Pham, Umeå 2016
This work is protected by the Swedish Copyright Legislation (Act 1960:729)
ISBN: 978-91-7601-574-2
Cover picture: Tung Ngoc Pham
Electronic version available at http://umu.diva-portal.org/
Printed by: The service centre in KBC, Umeå University
Umeå, Sweden 2016
III
To my family
To my beloved wife and sons
IV
Abstract
Recently, due to the unique properties and structures such as large
geometric surface area, electrical conductivity and light weight, 3D
structured carbon materials have been attracting extensive attention from
scientists. Moreover, the materials, which can provide well-defined pathways
for reactants to easily access active sites, are extremely useful for energy
conversion as well as environmental and catalysis applications. To date,
many precursors have been used for fabrication of 3D structured carbon
materials including pitch, carbon nanotubes, graphene, and polymer foams.
This thesis, as shown in the thesis title, focus on two main aspects: the study
of the characteristics of melamine based carbon foam synthesized at
different conditions and their applications. In paper I, it was revealed that
through a simple, one-step pyrolysis process, flexible carbon foam
synthesized from melamine foam (Basotect, BASF) was obtained.
Additionally, through a pyrolysis-activation process, activated carbon foam
which possesses hydrophilic nature and high surface area was successfully
synthesized. The characteristics of carbon foam such as the
hydrophobic/hydrophilic nature, electrical conductivity, mechanical
properties and surface chemistry were studied. It was shown that carbon
foam could be successfully used as an absorbent in environmental
applications e.g. removing of spill oil from water (paper I) or as support for
heterogeneous catalysts, which in turn was used not only in gas phase
reactions (paper I and IV) but also in an aqueous phase reaction (paper II).
Importantly, when combined with a SpinChem rotating bed reactor (SRBR)
(paper II), the monolithic carbon foam/SRBR system brought more
advantages than using the foam alone. Additionally, the work in paper III
showed the potential of carbon foam in an energy conversion application as
anode electrode substrate in alkaline water electrolysis. In summary, the
versatility of the carbon foam has been proven through abovementioned
lab-scale studies and due to the simple, scalable and cost effective pyrolysis
and activation processes used for the production, it has potential to be used
in large-scale applications.
V
Table of Contents
List of publications .................................................................................. VII
Abbreviations ............................................................................................ IX
1. Introduction ........................................................................................... 1
2. Background ............................................................................................ 4
2.1. Melamine formaldehyde resin - Basotect foam .......................................4
2.2. Carbon materials .................................................................................5
2.2.1. Pyrolysis and activation process ....................................................5
2.2.2. Activated carbon .........................................................................6
2.2.3. Carbon foam ...............................................................................7
2.3. Water electrolysis ................................................................................8
2.4. The SpinChem rotating bed reactor (SRBR) ......................................... 10
3. Experimental section ............................................................................ 11
3.1. Carbon foam synthesis ...................................................................... 11
3.2. Catalyst preparation methods ............................................................. 11
3.2.1. Water-based methods ................................................................ 11
3.2.1.1 A traditional method ........................................................... 11
3.2.1.2. SRBR enhanced method ..................................................... 12
3.2.1.3. Incipient wetness impregnation method................................ 13
3.2.2. Organic solvent-based method .................................................... 14
3.3. Material characterization techniques .................................................... 15
3.3.1. X-ray photoelectron spectroscopy (XPS) ....................................... 15
3.3.2. Scanning electron microscopy (SEM) ............................................ 15
3.3.3. Transmission electron microscopy (TEM) ...................................... 15
3.3.4. Surface area determination......................................................... 16
3.3.5. X-ray diffraction (XRD) ............................................................... 16
VI
3.3.6. Thermogravimetric analysis (TGA) ............................................... 16
3.3.7. Raman spectroscopy .................................................................. 16
3.3.8. Electrical conductivity test .......................................................... 17
3.4. Oil/Solvent absorption capacity analysis ............................................... 17
3.5. Hydrogenation of acetone .................................................................. 17
3.6. Hydrogenation of D-xylose ................................................................. 17
3.7. Hydrochlorination of isopropanol ......................................................... 18
3.8. Electrochemical testing ...................................................................... 19
4. Results and Discussions ....................................................................... 21
4.1. Microstructure, surface chemistry and wetting behavior ......................... 21
4.2. Electrical conductivity of the carbon foam............................................. 24
4.3. Thermal stability of the carbon foam ................................................... 25
4.4. Applications ...................................................................................... 26
4.4.1. Solvent absorbent ..................................................................... 26
4.4.2. Catalytic application in gas phase reactions .................................. 27
4.4.2.1. Hydrogenation of acetone to isopropanol .............................. 27
4.4.2.2. Hydrochlorination of isopropanol to isopropyl chloride ............ 30
4.4.3. Catalytic application in a liquid phase reaction............................... 33
4.4.4. Carbon foam as electrodes in OER ............................................... 37
5. Conclusions .......................................................................................... 42
6. Acknowledgements............................................................................... 44
7. References ............................................................................................ 46
VII
List of publications
I. Industrially benign super-compressible piezoresistive carbon foams
with predefined wetting properties: from environmental to electrical
applications.
Tung Ngoc Pham, Ajaikumar Samikannu, Jarmo Kukkola, Anne-
Riikka Rautio, Olli Pitkänen, Aron Dombovari, Gabriela Simone
Lorite, Teemu Sipola, Geza Toth, Melinda Mohl, Jyri-Pekka Mikkola &
Krisztian Kordas
Scientific Reports 4, 6933 (2014), doi:10.1038/srep06933.
II. Catalytic Hydrogenation of D-xylose over Ru Decorated Carbon Foam
Catalyst in a SpinChem® Rotating Bed Reactor.
Tung Ngoc Pham, Ajaikumar Samikannu, Anne-Riikka Rautio,
Koppany L. Juhasz, Zoltan Konya, Johan Wärnå, Krisztian Kordas,
Jyri-Pekka Mikkola
Topic in Catalysis (2016) 59: 1165, doi:10.1007/s11244-016-0637-4.
III. Robust hierarchical 3D carbon foam electrode for efficient water
electrolysis.
Tung Ngoc Pham, Tiva Sharifi, Robin Sandström, William Siljebo,
Andrey Shchukarev, Krisztian Kordas, Thomas Wågberg and Jyri-
Pekka Mikkola
Manuscript, submitted.
IV. Gas phase synthesis of isopropyl chloride from isopropanol and HCl
over alumina and flexible 3-D carbon foam supported catalysts.
Natalia Bukhanko, Christopher Schwarz, Ajaikumar Samikannu,
Tung Ngoc Pham, William Siljebo, Johan Wärnå, Andrey
Shchukarev, Anne-Riikka Rautio, Krisztian Kordas and Jyri-Pekka
Mikkola
Manuscript.
Additional work (not included in the thesis)
Screening of active solid catalysts for esterification of tall oil fatty acids
with methanol.
Journal of cleaner production, accepted.
Jaroslav Kocik, Ajaikumar Samikannu, Hasna Bourajoini, Tung
Ngoc Pham, Jyri-Pekka Mikkola, Martin Hajek, Libor Čapek
VIII
Author contributions
Paper I. The author extensively contributed in planning, experimental work
and wrote the ‘chemistry part’ of the paper.
Paper II. The author planned the work, performed all experiments
(excluding the modelling part) and wrote the main part of the paper.
Paper III. The author extensively planned the work, performed most of
experiments and wrote the main part of the manuscript.
Paper IV. The author contributed in the preparation of the carbon foam
catalyst, measured B.E.T surface area and wrote some relevant parts in the
manuscript.
IX
Abbreviations
A800 Activated carbon foam synthesized at 800 °C
A800W A800 washed by water
B.E.T. Brunauer, Emmett and Teller
CCVD Catalytic chemical vapor deposition
CNTs Carbon nanotubes
CF Carbon foam
HER Hydrogen evolution reaction
OER Oxygen evolution reaction
P600 Pyrolyzed carbon foam synthesized at 600 °C
P700 Pyrolyzed carbon foam synthesized at 700 °C
P800 Pyrolyzed carbon foam synthesized at 800 °C
P800W P800 washed by water
P900 Pyrolyzed carbon foam synthesized at 900 °C
P900W P900 washed by water
LSV Linear sweep voltammetry
SEM Scanning electron microscopy
SRBR SpinChem rotating bed reactor
RHE Reversible hydrogen electrode
TEM Transmission electron microscopy
TGA Thermogravimetric analysis
XPS X-ray photoelectron spectroscopy
XRD X-ray diffraction
1
1. Introduction
The importance and potential of carbon-based materials simply cannot be
questioned, not only because of the recent premier scientific awards
(Nobel prizes for fullerenes and graphene in 1996 and 2010, respectively) but
also through the long history of carbon materials with many applications
throughout the human history [1,2]. Today, carbon-based materials and
their applications are under their fastest ever development and represent a
very important topic related to renewable energy conversion applications
such as: supercapacitor electrodes [3,4], heterogeneous catalysis [5,6] and
electrocatalysis [7,8]. Moreover, carbon-based materials can also be used in
other applications which are old but still very important such as: water
purification [9,10], gas storage [11,12] as well as traditional support materials
for metal catalysts [13,14] and as solid acid catalysts [15]. The extraordinary
versatility of carbon-based materials was brought by their diversity in forms
which range from the mundane activated carbon to the modern carbon
nanotubes and graphene. Recently, due to the large geometric surface area
and interconnected pores providing well-defined pathways for reactants to
easily access active sites [16,17], 3D structured carbon materials have
attracted much attention from scientists for their potential use in renewable
energy conversion applications such as hydrogen and electrical energy
storage [18,19] and flexible solar cell electrodes [20]. Generally, the 3D
structured carbon materials can be synthesized in several ways such as:
i) Blowing of carbon precursors e.g. pitch [21,22] which occurs at high
temperatures in inert atmosphere and under pressure. ii) Pyrolysis of
polymer foams like polyurethane [23] and phenolic foams [24], as well as
aerogels based on resorcinol formaldehyde resin [18]. iii) Materials
synthesized from carbon nanotubes [25], graphene [26], graphene oxide [27]
and their composites [28,29]. However, besides the abovementioned
fascinating properties, there are limitations that are specific for each type of
carbon foam. For example, foams based on carbon nanotubes and graphene
are difficult to synthesize in large scale, thus limiting their practical use and
commercialization, while 3D structured carbon materials synthesized from
pitches and polymers are often lacking good electrical conductivity,
mechanical integrity and flexibility.
2
In this study, flexible 3D structured carbon materials, hereafter called
carbon foams, were directly synthesized from commercially available
melamine foam (Basotect, BASF). Besides its natural flexibility, the carbon
foam possesses other valuable properties such as light weight (low density,
less than 8 mg/cm3), high thermal stability and tailorable properties which
can be easily achieved by applying suitable synthesis conditions. These
custom properties include surface area, hydrophobic/hydrophilic nature and
the electrical conductivity. An activation process can increase the surface
area of the carbon foam by almost two orders of magnitude and also change
the wetting behavior of the foam to hydrophilic (paper I [30]). In another
approach, carbon nanotubes were grown on the surface of carbon foam
which consequently resulted in an increased surface area while retaining the
hydrophobic nature of the foam (paper III). Moreover, carbon foam with
high electrical conductivity can easily be attained by using a proper pyrolysis
time and temperature. Due to these aforementioned properties, the carbon
foam has potential to be used in various applications including, but not limit
to, environmental, chemical and renewable energy conversion applications
[30,31] (paper I to IV).
Hydrophobic carbon foam can be used as absorbent for water purification
applications, e.g. removing spill oil from the surface of water, while the
activated form of the carbon foam with high surface area and hydrophilic
nature, resulting from the activation process, can be used as an excellent
porous substrate for metal catalysts. Nickel, ruthenium and zinc chloride
were successfully immobilized on the surface of carbon foam by oxygen
containing groups such as carboxylic and hydroxyl groups (paper I and II)
[30,31]. The use of catalyst on carbon foam was demonstrated in various
reactions which can be performed in gas phase as well as in liquid phase.
Moreover, due to their monolithic and flexible nature, the application of the
carbon foam can simplify the process and reduce costs related with powder
(packed bed, slurry) catalytic materials. Importantly, the monolithic carbon
foam was successfully used in a SpinChem rotating bed reactor (SRBR)
developed by SpinChem AB, Umeå, Sweden. The combination brought many
advantages such as providing isothermal conditions, improving the liquid-
solid and gas-liquid mass transfer to the solid phase of the catalyst active
sites, avoiding the mechanical grinding effects caused by the stirrer,
eliminating filtration steps and minimizing the loss of catalyst [31-33].
3
Renewable energy conversion e.g. the production of hydrogen from water is
another exciting application where carbon foam can be used as electrode
substrate in electrochemical cell setups. In order to be used as an electrode
in water electrolysis, the electrical conductivity of carbon foam can easily be
boosted through an appropriate pyrolysis process (Paper III). The as-
prepared carbon foam (CF) can be used directly as substrate for cobalt oxide
which later can give a decent electrocatalytic performance towards the
oxygen evolution reaction (OER). Moreover, the electrocatalytic
performance of the carbon foam can be improved by the growth of carbon
nanotubes (CNTs) on the surface without destroying the natural flexibility of
the foam. The hybrid cobalt oxide decorated CNTs/CF electrode showed
excellent electrocatalytic performance with a high catalytic activity in OER,
low overpotential (0.42 V vs RHE at 10 mA/cm2) and excellent stability
under testing conditions in 0.1 M KOH solution. The unique properties of
the three-dimensional structured carbon matrix such as large specific surface
area, good electrical conductivity, as well as an efficient electrolyte transport
into the whole electrode matrix concurrent with an ability to quickly dispose
oxygen bubbles into the electrolyte were well discussed in the paper III.
4
2. Background
2.1. Melamine formaldehyde resin – Basotect foam
Melamine formaldehyde resin, sometimes called melamine resin, is a
thermosetting resin formed by a 2-stage reaction of formaldehyde and
melamine: methylolation (Fig. 1a) and condensation (Fig. 1b and Fig. 1c).
At first, in the presence of catalyst (base or acid) and at elevated
temperatures, formaldehyde will react with melamine to form
an intermediate product, called methyolmelamine. In the condensation step
triggered by high temperature and low pH, there are two possible pathways
to form methylene crosslinks in the final product: i) Formation of
intermediate ether-linkage containing compounds followed by the
elimination of formaldehyde (Fig. 1b). ii) Direct reaction between
methyolmelamine and melamine (Fig. 1c) [34,35]. Due to the high density of
cross-linking, melamine resins offers extreme hardness, thermal stability,
excellent boiling resistance, abrasion resistance and flame retardant
characteristics, which allow them to be used in various applications such as
household goods, electrical applications and adhesive and coating products
[36].
Figure 1. Reactions to form melamine resin from melamine and
formaldehyde: methylolation (a), indirect condensation pathway (b) and direct
condensation pathway (c) [36].
5
Basotect is a trademark of melamine foam which is produced by BASF,
a German company. Based on their patent [37], the Basotect foam was
prepared by blending monomers, which are usually melamine and phenol
formaldehyde as well as melamine derivatives and other aldehydes. In
addition a blowing agent, a catalyst and an emulsifier are needed. According
to BASF, this open-cell melamine foam possesses a lot of unique properties
such as: flame resistance, heat resistance, good thermal and sound
insulation capacity and light weight. BASF has graded their melamine foams
to several different types based on application purposes such as Basotect G,
Basotect W, Basotect G+ and Basotect UF. In the studies performed,
Basotect G (Fig. 2) which is mainly used for noise insulation applications
and also Basotect W, similar foam but having a different color, were used as
precursor polymer foams to produce our carbon foams.
Figure 2. Monolithic melamine based polymer foam - Basotect G.
2.2. Carbon materials
2.2.1. Pyrolysis and activation process
Pyrolysis is the thermal decomposition of organic material at elevated
temperatures in the absence of oxygen. This endothermic process often has
been heavily used in industry to create valuable products from waste or less
valuable precursors such as: charcoal and activated carbon from wood or
wood waste product, syngas and biochar from biomass and carbon fiber
from polyacrylonitrile fiber [38-41]. During the pyrolysis process, organic
substances undergo chemical decomposition which leads to the formation of
carbon and also results in the release of small molecules as by-products.
Temperature, pressure, and heating programs e.g. ramping rate and heating
time are the most important parameters that control the pyrolysis reaction.
Depending on these factors and the chemical composition of the precursors,
the chemical composition of main and side products can be varied
[21,42,43].
6
For melamine formaldehyde resin, at pyrolysis temperature lower than
350 °C, mainly N-compounds such as isocyanic acid and ammonia were
found as degradation products. From 372 °C – 424 °C degradation products
are formaldehyde, methanol, amine, ammonia and sublimated melamine
[44]. From 435 °C the pyrolysis becomes faster with the deamination to form
tri-s-triazine structure, together with the formation of HCN and CH3CN
gases. Above 660 °C the resin undergoes extensive degradation and releases
HCN, CO and CO2 [45].
There are two major ways to perform the activation for carbon materials:
physical activation and chemical activation. In physical activation
experiments, the active oxygen in the activating agents, which could be
oxidizing gases e.g. air, O2, CO2, steam or their mixtures, was allowed to react
with carbon to form gaseous products e.g. CO (Fig. 3) and, consequently, led
to the development of carbon materials with microporous structures. On
another hand, upon chemical activation, activating agents such as KOH,
NaOH, H3PO4 or ZnCl2 act either as dehydrating agents, which enhance the
yield of carbon by inhibiting the formation of tar, or as oxidizing agents
under high temperature and inert atmosphere [46,47].
Figure 3. Possible reactions between CO2 and carbon in activation process
[48].
2.2.2. Activated carbon
Activated carbon can be prepared from carbonaceous materials such as
biomass [38,49,50], petroleum pitch [51,52] and synthetic organic polymers
[53,54] through carbonization-activation processes. Activated carbon has
been shown to possess a well-developed internal pore structure ranging from
micro to macroporous as well as a large number of functional groups such as
carboxyls, carbonyls, phenols, lactones and quinones formed during the
activation process [1,55]. Thus, due to the abundance, the inertness, and
high surface area which can be more than 1000 m2/g, activated carbon has
been used in numerous applications ranging from gas and water purification
[56,57] to more demanding applications, such as catalysis/electrocatalysis,
7
energy storage (supercapacitors and batteries), CO2 capture or H2 storage
[58-61].
2.2.3. Carbon foam
Carbon foam as defined by Scheffler [62] is: “..a porous carbon product
containing regularly shaped, predominantly concave, homogeneously
dispersed cells which interact to form a three-dimensional array throughout
a continuum material of carbon, predominantly in the non-graphitic state.
The final result is either an open or a closed-cell product.” The materials
possess a lot of unique characteristics e.g. low density, high thermal stability,
noise insulation, chemical inertness and tailorable electrical and thermal
conductivity as well as good mechanical properties [63].
A wide range of raw materials, extending from traditional materials such as
coal tar and pitch [64] to advanced materials e.g. aerogel polymer, graphene
and carbon nanotubes [18,25,26,29] has been used to synthesized carbon
foam (Fig. 4). The precursors used to synthesize carbon foam play an
important role in determining the graphitization ability of carbon foam
products. Thermoplastic raw materials such as coal tar and petroleum
pitches, which have the ability to form ordered stacks of graphene layers
under the high temperature of carbonization process, can yield graphitic
carbon foams [21,22]. On the other hand, due to their strong cross-linking
system which will prevent the movement of carbon layers to form oriented
structures during the carbonization process, thermoset raw materials like
polyurethane, phenolic and melamine foam will lead to the formation of
non-graphitic carbon foams [30,65].
Figure 4. Carbon foam from pitch [64] (a), melamine foam (b) and carbon
nanotubes [25] (c).
500 μm
a
40 μm
b
10 μm
c
8
2.3. Water electrolysis
The long history of water electrolysis started with the discovery by Nicholson
and Carlisle in the year 1800. In the next 100 years, water electrolysis
industry grew fast with more than 400 industrial electrolyzer units in
operation in 1903. Later in the 20th century the technique was superseded
by other techniques which can produce hydrogen at lower price such as
gasification of coal and petroleum coke, reforming of natural gas and
gasification and reforming of heavy oil [66-69]. Nevertheless, due to the
advantages such as very pure hydrogen product and environmental benefits,
especially when coupled with a renewable energy source [70,72], advanced
water electrolysis systems were still developed: solid polymer electrolyte
system (1966), solid oxide water electrolysis (1972) and advanced alkaline
system (1978) [73].
Figure 5. Alkaline water electrolysis scheme.
Usually, a normal alkaline water electrolysis unit includes: an anode,
a cathode, an electric supply (DC source) and an electrolyte (Fig. 5). When
a potential is applied, hydroxide ions move towards and donate electrons to
the anode which then flow to the positive terminal of the DC source.
The overall chemical reaction of the alkaline water electrolysis can be split
into two half reactions, oxidation at the anode where oxygen gas is formed
and reduction at the cathode where hydrogen gas is produced. In large scale
production, a membrane is used to have separated gaseous products
(hydrogen and oxygen) which are very explosive if mixed.
O2 H
2
OH-
+ -
Alkaline electrolytes
(anode)
(cathode)
9
The minimum necessary cell voltage for the start-up of electrolysis is 1.23 V
vs. reversible hydrogen electrode (RHE), at standard temperature (25 °C)
and pressure (1 atm) [73,74]. However, in reality, to start the reaction,
an extra energy is always required which means a voltage higher than 1.23 V
vs. RHE. Thus, this overpotential should be applied to activate the water
splitting reaction. The total cell voltage can be expressed as:
𝑉𝑐𝑒𝑙𝑙 = 𝑉0 + 𝑉𝑜ℎ𝑚 + 𝑉𝑎𝑐𝑡 + 𝑉𝑐𝑜𝑛 [75] where V0 is the lowest required voltage
for the reaction to happen. The ohmic overvoltage, Vohm, is due to total ohmic
resistance in the cell including external circuitry, electrolyte, electrodes, gas
bubbles etc. The activation overvoltage, Vact, is caused by the electrode
kinetics which strongly depends on the catalytic properties of the electrode
materials. Finally, the concentration overvoltage, Vcon, is affected by mass
transport processes the contribution of which, however, is very small
compared with other overvoltages, especially during an alkaline electrolysis.
The mechanism of the hydrogen evolution reaction (HER) is simple and
straight forward compared with the oxygen evolution reaction (OER) at the
anode which is more complex and also requires more energy [75,76]. Trasatti
and co-workers have proposed a three step reaction mechanisms where the
cycle begins with the adsorption of negatively charged groups (e.g. OH-) on
the surface of metal oxides and finishes by the recombination of two oxygen
adsorbates to form O2 [77] (Fig. 6a). Later, four step reaction mechanisms
proposed by Rossmeisl et.al. suggested an additional step with the presence
of oxyhydroxyl adsorbate groups [78] (Fig. 6b). The complexity of the water
oxidation reaction on metal oxide surfaces was shown through the diversity
in proposed reaction mechanisms which still haven’t been confirmed by
empirical studies [74].
10
Figure 6. Proposed OER mechanism for metal oxides surface: Trasatti’s
three-step mechanism (a) and Rossmeisl’s four-step mechanism (b).
2.4. The SpinChem rotating bed reactor
SpinChem rotating bed reactor (SRBR) is a new concept to improve the
mass transfer in heterogeneous catalytic reactions recently developed by
SpinChem AB, Umeå, Sweden. The structure of the SRBR is a hybrid of a
stirrer and a catalyst holder (Fig. 9) which could be considered as an
evolution version of a stirred contained solid reactor. Different sizes and
designs of SRBR based on the type of catalyst and the size of the reactor
vessel are commercially available. When rotating, the SRBR rapidly draws
the reaction solution from the bottom of the vessel and forces it through the
solid phase to the vessel. Consequently, better isothermal conditions as well
as faster liquid-solid and gas-liquid mass transfer to the catalytic sites of the
solid phase can be achieved. These advantages of the SRBR were proven by
empirical studies which also point out additional advantages of the SRBR
such as minimized production of fine particles caused by mechanical
grinding effects, elimination any filtration steps and avoiding loss of reagents
[31-33].
OH-
e- OH-
H2O + e-
OH-
e- OH-
H2O + O2 + e-
OH-
e- OH-
H2O + e-
½ O2
a b
11
3. Experimental section
3.1. Carbon foam synthesis
The system which was used to synthesize carbon foam includes: a horizontal
furnace, a quartz tube, gas connections and mass flow controllers.
Melamine-based polymer foam (BASF, Basotect G and sometimes
Basotect W) was cut into desired size and put into a quartz tube. Depending
on application different type of carbon foam can be synthesized as follows:
- Pyrolyzed carbon foams were produced by keeping the polymer foam
at desire temperature and under nitrogen flow (50 ml/min). These
carbon foams were denoted as “PXXX” where “P” means pyrolysis and
the following three digit number describes the pyrolysis temperature.
For example: P900 means the carbon foam was synthesized by a
pyrolysis process which was performed at 900 °C. The detail
procedures can be found at relevant works (paper I [30] and paper
III).
- Activated carbon foam was yielded through a pyrolysis process (1h,
800 °C, 50 mL/min of nitrogen) followed by an activation process at
800 °C under the flow of a gas mixture (2 % by volume of CO2 in N2
(50ml/min)) for 2 hours. The prepared carbon foam was denoted as
“A800” which means the carbon foam was synthesized by a pyrolysis-
activation process performed at 800 °C.
After the heat treatment process, the system was allowed to cool down to
room temperature under nitrogen atmosphere.
3.2. Catalyst preparation methods
3.2.1. Water-based methods
3.2.1.1. A traditional method (paper I [30])
Ni decorated carbon foams (Ni@A800) were synthesized by immersing
activated carbon foam (A800) in the solution of Ni(NO3)2 (400 ppm of Ni2+)
for 24 hours. To determine the amount of Ni adsorbed on the carbon foam
surface, the solution (before and after adsorption) was diluted 10 times in 3%
HNO3 and measured by using ICP-OES instrument (PerkinElmer OPTIMA
2000 DV). The as-obtained impregnated carbon foam was dried in air at 100
°C for 24 h and then reduced in H2 flow at 400 °C for 2 hours.
12
3.2.1.2. SRBR enhanced method (paper II [31])
In a typical process (Fig. 7), A800 which was selected for the application due
to its high surface area and the hydrophilicity of the foam, was inserted into
the SRBR. Then an “in-situ” catalyst preparation process where all steps
including catalyst decoration, washing and catalyst reduction was performed
within the SRBR. Generally, the A800-SRBR was immersed into a reactor
containing an aqueous solution of RuCl3 in distilled water and the absorption
process was continued for 24 hours at SRBR stirring speed of 400 rpm. Next,
the catalyst was washed several times by repeatedly changing the solution
inside the reactor with clean distilled water. During the whole washing
process, the SRBR stirring speed was maintained at around 400 rpm.
To reduce the ruthenium salt and produce the Ru catalyst, a solution
containing 150 ml deionized water was added into the reactor, followed by
drop-wise addition of 20 ml of 0.1g NaOH and 0.2 g NaBH4 in deionized
water (in 30 minutes) under stirring (approx. 200 rpm). After the reduction,
the catalyst was first washed several times with water (until the pH of rinse
water is neutral) followed by an additional washing with acetone. Finally, the
Ru@A800-SRBR was dried in vacuum oven overnight at 50 °C. In a
reference experiment, commercial ruthenium catalyst (5 wt. % ruthenium on
activated carbon, denoted as Ru@AC) containing the same amount of Ru as
our tailor-made Ru@A800 catalyst was used as slurry in batch reactor
method.
13
Figure 7. Overall process for hydrogenation of D-xylose over Ru@A800
catalyst inside a SRBR.
3.2.1.3. Incipient wetness impregnation method (paper IV)
The technique was used when the amount of catalyst on carbon foam was
defined at the beginning of the experiment. The catalyst precursor (ZnCl2)
was dissolved in an amount of distilled water which has the same volume as
the pore volume of the carbon foam, which was estimated based on the
density of the carbon foam (around 7 mg/cm3). Next, the solution was slowly
added into the carbon foam. During the process the carbon foam was
pressed and released several times to ensure a uniform dispersion of the
solution inside the foam. Finally, the water was evaporated from the carbon
foam by heating the foam in an oven at 100 °C overnight.
RuCl3 solution
Washing
Ru reduction
Insert A800 into SRBR
Cat
aly
st r
ecycl
ing
D-xylitol solution
H2
D-xylose solution
Washing and drying
Carbon foam (A800)
Ru@A800–SRBR catalyst
Reaction in
Parr reactor
Washing and drying
Catalyst decoration (Ru@A800-SRBR)
NaBH4 solution
Catalyst Ru@A800-SRBR
14
3.2.2. Organic solvent–based method (paper III)
Figure 8. Overall process scheme of utilizing carbon foam as electrode
material. CCVD: catalytic chemical vapor deposition.
As shown in Fig. 8, the polymer foam (melamine resin) was used as the
precursor upon synthesis of the carbon foam (A800 and P900).
Subsequently, cobalt was decorated onto the surface of the carbon foam by a
thiophene aided-impregnation method [79]. In a typical process, 20 mg of
cobalt(II) acetate tetrahydrate was dissolved in 5 ml of dimethyl formamide
(DMF) and sonicated for 3 minutes. The carbon foam sample (20 mg) which
can be either carbon foam (A800 or P900), was submerged into the mixture
together with 65 μl of thiophene. The mixture was then sonicated for
20 minutes, followed by a drying process at 125 °C under nitrogen flow.
Finally, the sample was annealed at 400 °C for 2 hrs in nitrogen atmosphere
to facilitate the formation of cobalt oxide (CoOx) to be used as the OER
catalyst.
Alternatively, the non-annealed cobalt acetate@P900 was stored and used as
the growth platform of CNTs. CNTs on carbon foam samples were produced
by means of the catalytic chemical vapor deposition (CCVD) technique.
Initially, the sample was placed on a quartz boat which was later inserted
into a horizontal quartz tube. The system was purged with the Varigon gas
(5% hydrogen in argon gas, 180 ml/min) for 20 minutes and then heated to
670 °C in around 20 min. When the desired temperature was reached,
acetylene was introduced into the system (~3.8 ml/min) for 30 minutes
while the Varigon gas flow was still pertained. Finally, the system was
allowed to cool down to room temperature under argon atmosphere (180
ml/min). The same method as described above was used to decorate cobalt
catalyst as well as to produce cobalt oxide on the CNTs/P900.
Impregnation
Thiophene A
nnea
ling
Thiophene
CCVD
Polymer foam
Carbon foam (CF: A800 and
P900)
Cobalt acetate@CF
CoOx@CF
Carbon nanotubes
(CNTs)/P900
Cobalt acetate@
CNTs/P900
CoOx@CNTs/P900
Pyro
lysis o
r
Pyro
lysis-
Activ
ation
Impregnation Annealing
15
3.3. Material characterization techniques
3.3.1. X-ray photoelectron spectroscopy (XPS)
XPS spectra are obtained by measuring the kinetic energy and number of
electrons that emit from the surface of a material under the radiation of
an X-ray beam. Through XPS spectra, the information such as elemental
compositions from the surface of the material can be obtained.
For measuring carbon foam, the XPS spectra were collected with a Kratos
Axis Ultra DLD electron spectrometer using monochromated Al Kα source
operated at 120 W. An analyzer pass energy of 160 eV for acquiring wide
spectra and a pass energy of 20 eV for individual photoelectron lines were
used. The surface potential was stabilized by the spectrometer charge
neutralization system.
3.3.2. Scanning electron microscopy (SEM)
SEM is used to reveal information about the sample including surface
morphology, and crystalline structure and orientation of materials.
In principal, SEM images are created based on the secondary electron which
is produced by scanning an incident beam over the surface of the solid
sample. Due to the interaction between electrons of sample atoms and
electrons from a highly focused beam, secondary electrons are emitted from
the sample and then collected by the electron detector. For carbon foam
samples, the voltage used for the electron beam was usually 4 kV. For low
conductive carbon foams such as P600 and P700, coating sample with a
conductive material i.e. gold was necessary to achieve good images,
conductive carbon foams like P800, A800 and P900, could be used in SEM
as-prepared. Throughout the study, the Zeiss Merlin FEG-SEM instrument
was the most frequently used instrument.
3.3.3. Transmission electron microscopy (TEM)
TEM images are created by the irradiation of ultra-thin sample with an
electron beam which will interact with sample as it travels through it.
A stronger interaction (scatter electron stronger) will be found at the place
where the sample is thicker or contains heavier elements. Thus TEM images
give the information of the morphology of the “bulk” sample. In the case of
carbon foam, due to the high thickness of the carbon skeleton which can be
up to 1 micron, it was almost impossible to acquire a good TEM image at the
center of the sample but an image taken at the edge was sometimes possible.
16
3.3.4. Surface area determination
Firstly, the carbon foam was passed through a degassing step where the
sample was heated at 120 °C in 2 h under nitrogen atmosphere. However, in
some cases a more intensive degassing program (e.g. 200 °C in 4 h) was
required to remove water or other adsorbed substances from the carbon
foam. In the next step, the specific surface area of sample was determined by
physical adsorption of nitrogen on the surface of the solid at the boiling point
of nitrogen (-196 °C). The Brunauer, Emmett and Teller (B.E.T.) method
was used to calculate for the surface area of the carbon foam materials.
The equipment used for the surface measurement was Tristar 3000 analyzer
(Micromeritics Instrument Corp.).
3.3.5. X-ray diffraction (XRD)
Crystal phase assessment of the carbon foam samples were performed with
XRD (Bruker D8 Discover, Cu Kα radiation source). The technique is
frequently used to determine the structural features of crystalline materials.
Therefore the technique was useless for amorphous carbon foam materials,
but it was expected to provide information about the catalyst decorated on
the surface of carbon foam. However, the use of XRD was quite limited for
carbon foam since samples with catalyst loading less than 5 wt. % didn’t
show any signals with XRD.
3.3.6. Thermogravimetric analysis (TGA)
TGA was conducted on a Mettler Toledo equipment (TGA/DSC 1LF/948)
operated at a heating rate of 5-10 °C/min up to 950 °C in air flow. The
measurement was used to quantify the metal loading on the carbon foam
(Paper III) by identification of the mass change in gradient temperature
under oxygen atmosphere. The metal (cobalt) loading was calculated based
on the residual weight of sample after finishing the measurement.
3.3.7. Raman spectroscopy
The technique is performed by radiation a laser as the light source on a
sample. Subsequently, sample molecules excited by photons from the light
source were reemitted light during the relaxation to lower energy levels.
Emitted light which have different wave length than the irradiation source
(inelastic scattering of the monochromatic light) provide information about
vibrational, rotational and other low frequency transitions in molecules. For
17
carbon foam samples, Raman spectroscopy was performed on a Renishaw
InVia Raman spectrometer using a laser excitation wavelength of 633 nm.
3.3.8. Electrical conductivity test (paper I & III)
The electrical conductivity between carbon samples can be compared by
performing current-voltage sweeps. The set-up of the current-voltage studies
can be varied based on the available facilities in the lab. In general, the
samples were cut to a size of ~ 12 x 5 x 2 mm with a surgical blade and placed
over copper electrodes (24 x 3 mm and the space between the two electrodes
was ~ 0.3 mm). To ensure an intimate contact with the electrodes, a weight
(around 1.5 g) was put on the top of the carbon foam. The copper electrodes
were connected to a Metrohm Autolab potentiostat (PGSTAT302N).
Current-voltage sweeps were performed from -1.0 V to 1.0 V at scan rate of
0.1 V/s in air and at room temperature. The resistance was estimated based
on the slope of resulting I-V curve with 95 % confidence interval.
3.4. Oil/Solvent absorption capacity analysis (paper I [30])
To identify the liquid absorption capacity, carbon foams pyrolyzed at 600 °C
(P600) were immersed into a range of solvents, including silicone oil,
benzene, turpentine oil, crude oil and isohexane, for 20 seconds. The
adsorption capacity, i.e. the change of mass in reference to the original mass
of the dry sponge, was calculated from gravimetric measurements. The final
results were averaged out from three tests.
3.5. Hydrogenation of acetone (paper I [30])
The hydrogenation reaction of acetone to isopropanol was carried out in
hydrogen atmosphere in a glass tube reactor (Fig. 16a). Around 0.2 g of the
sample (Ni@A800) was mounted in the center of the glass tube reactor
(inside diameter 1.3 cm and length 35 cm). Acetone was pumped into the
reactor with a feeding rate of 0.5 ml/min and mixed with hydrogen
(0.2 ml/min) at a specified temperature (150 and 250 °C). The vapor phase
product was condensed in a round-bottom flask at ~ -5 °C and the liquid was
collected for every 2 hours of reaction to perform gas chromatography (GC)
and GC coupled with mass spectroscopy (GC-MS) analysis.
3.6. Hydrogenation of D-xylose (paper II [31])
Hydrogenation of D-xylose was carried out in a high pressure autoclave
(PARR Inc. USA, 300 ml nominal volume) equipped with a mechanical
stirrer affixed to a SRBR (SpinChem AB) acting as catalyst holder and stirrer
18
(Fig. 9a). The SRBR ( 3.6 cm x 3.5 cm) consisted of a hollow cylinder
( 3.2 cm x 2.9 cm) equipped with rounded orifices at the sides (Fig. 9b) to
allow liquid flux across the carbon foam catalyst. For reference experiments,
instead of a SRBR, a stainless steel stirrer was used and a 7 μm sintered
metal filter was connected to the sampling line in order to avoid any loss of
catalyst.
Figure 9. Reaction setup in Parr reactor (inset: carbon foam catalyst
(Ru@A800) inside SRBR) (a) and SRBR pattern (b).
In a typical batch, 150 ml solution of 7.5 g of D-xylose in deionized water was
transferred to the reactor. In the next step, the reactor was purged several
times with nitrogen to remove any oxygen residing in the reactor.
Subsequently, the sugar solution was bubbled with nitrogen for 20 min,
followed by bubbling with hydrogen for 15 min before adjusting the pressure
to the pre-set value. The heater was then turned on and the stirring was
engaged (500 rpm). Further, in order to ensure operation in the kinetic
regime and to reduce any external mass-transfer resistances, various stirring
rates were tried to pinpoint the minimum required stirring speed of the
SRBR. Small samples of the reaction solution were periodically withdrawn
for HPLC analysis (around 0.5 ml each).
3.7. Hydrochlorination of isopropanol (i-PrOH)
The catalytic reactions were performed in a tailor-made pressurized fixed
bed reactor (inner diameter: 10 mm, length: 260 mm) constructed from
stainless steel tube with tantalum lining inside. Before every reaction, the
a b
19
catalysts were heated overnight at the reaction temperature under nitrogen
flow. The temperature was controlled by external heaters along the reactor
system. Two thermocouples were placed inside the reactor, well extending to
the catalyst bed (mcat = 0.25 g, lbed = 3 cm) and used to monitor the reaction
temperature. Glass beads were applied as spacers in the reactor.
Isopropyl alcohol was fed with an HPLC-pump and vaporized at 150 °C
before entering the reactor. The flow rates of HCl and i-PrOH were
controlled by means of PC-equipped mass-flow controller and calibrated
before each and every experiment. To remove traces of water and unreacted
acid, the mixture of products leaving the reactor was passed through the
vessel with a desiccant (calcium oxide powder mixed with small glass tubes
pieces for higher contact area heated to 100 - 105 °C). The reaction products
were collected in a cooling trap (T = -5 °C, water - ethylene glycol mixture)
and the samples for analysis were withdrawn every 15 min with a cooled
syringe (-5 °C). The condensed liquid-phase products were analysed by
means of Gas Chromatography (GC, Agilent 6890N). The device was
equipped with a flame ionized detector (FID) and HP-PLOT/U capillary
column (oven temperature 180 °C, isothermal regime). Identification of
unknown components in products mixture was performed by GC-MS
analysis with Agilent Technologies (Model No. 7820A) GS system equipped
by the same capillary column as a GC, coupled to Agilent Technologies
(Model No. 5975) MSD. All samples were mixed with methanol before
analysis. (For more details see paper IV)
3.8. Electrochemical testing (paper III)
Different electrode materials were cut to appropriate shape from the bulks of
the respective materials by a surgical blade. The sample size was around 5
mm x 5 mm x 1.7 mm. A copper wire was attached to the electrode material
using silver paste that was allowed to dry overnight. A platinum coil and
Ag/AgCl (1M KCl) were used as the counter and reference electrodes,
respectively, in a three-electrode electrochemical cell. Linear sweep
voltammetry (LSV) was performed at a scan rate of 2 mV/s. For the stability
test, chronoamperometric data were recorded at a constant potential of 1.7 V
vs RHE. After 5 hours, the old electrolyte inside the electrochemical cell was
replaced with a fresh electrolyte. The electrolyte used in all experiments was
the solution of 0.1 M KOH in water. The current density was normalized by
the geometrical area of the electrode. The measured potential vs. Ag/AgCl
20
were converted to a reversible hydrogen electrode (RHE) scale based on the
Nernst equation (ERHE = EAg/AgCl + 0.059 * pH + 0.222). The current density
was calculated by normalizing the anodic current with the projected area
(25 mm2). The overpotential (η) was calculated according to the formula: (η
(V) = ERHE – 1.23 V).
21
4. Results and Disscussions
4.1. Microstructure, surface chemistry and wetting behavior of
the carbon foam
After the pyrolysis process, carbon foam samples were not collapsed nor
turned into a rigid structure but instead a light and flexible foam was
achieved (Fig. 10a). Due to the high density of cross-linking bonds,
thermos-setting melamine resin is not melted during the pyrolysis process,
which helps to retain the structure of the foam (Fig.11). As reported in the
literature [26], the flexibility of the carbon foam could be resulting from the
high ratio of the length/cross section of the carbon filament. Depending on
the temperature and time of the pyrolysis process, the shrinkage of carbon
foam was increased from ~30 % to ~55 % by volume for P600 and P900,
respectively (Fig. 10b). Also, a higher temperature and longer time of the
pyrolysis process lead to a higher weight losses. The precursor polymer foam
lost around 70 wt. % of producing P600 but a weight loss more than 90 wt.
% was observed in the case of P900. At higher pyrolysis temperature,
nanoparticles and rod-like structures were gradually forming on the surface
of the carbon foam (Fig. 11) which later could be removed from the surface
by washing with water (Fig. 12a). Additionally, the XPS results of P800W
(after washing with water) showed that a large amount of sodium was
removed from the surface (Fig. 12b) which indicates that these observed
structures are actually sodium compounds migrating to the surface. At high
concentrations, the sodium compounds can dramatically alter the wetting
behavior of the carbon foam. For example, P800 showed the hydrophilic
nature at high sodium concentration (26 wt.%) on the surface but when a
major amount of sodium was removed by washing with water, the sample
turned to hydrophobic. Besides, the pyrolysis temperature didn’t show any
effect on the specific surface area of pyrolyzed carbon foams based on the
fact that the B.E.T. surface area showed similar results for all pyrolyzed
carbon foam (approx. 4 m2/g) [30].
22
Figure 10. Characteristics of carbon foam: the flexibility (a), the shrinkage (b)
and the wetting behavior in water (c). ‘W’ means after washing by water.
Figure 11. SEM images of melamine polymer foam (a), P600 (b), P700 (c)
and P800 (d). Insets: SEM images at low magnification.
The chemical composition of the surface of the carbon foam was revealed by
XPS measurements (Fig. 12b). The elemental analysis has shown the
presence of C, O, N and S at the surface of the precursor polymer foam which
is in line with BASF’s patent [37] according to which the melamine foam is
synthesized from melamine-formaldehyde copolymer including sodium
sulfonate as an anionic surfactant. At the lowest pyrolysis temperature
a b c P900
P800
P700
P600
Polymer
foam
P900W P700 P600
Polymer
foam A800W
2 μm
20 μm
2 μm
20 μm
2 μm
20 μm
1 μm
20 μm
d c
b a
23
(600 °C), all of the sulfur and most of the oxygen were removed from the
surface of the material, leading to the formation of carbon foam (P600)
which possessed high amount of carbon and nitrogen on the surface. When
increasing the pyrolysis temperature, the carbon content continued to
increase and less nitrogen was found on the surface (Fig. 12b). At the same
time, the amount of oxygen on the surface of carbon foam was also increased
which can be explained by the chemisorption of oxygen onto carbon radicals
when the carbon foam was exposed to air [80].
Figure 12. SEM images of P800W (after washing) (a) and elemental
compositions of different carbon foam detected by XPS (b).
On the other hand, as can be seen in Fig. 13a, the activation process also
didn’t destroy the structure but resulted in a dramatic increase in the specific
surface area of the foam (A800) to more than 300 m2/g. This was caused by
the forming of pores on the carbon foam surface (Fig 13b). Moreover, during
the course of the activation process, more oxygen containing groups such as
carboxylic, ester and hydroxyl formed on the surface of A800 [30] and
consequently turned the carbon foam to hydrophilic, a property that still
remained even after washing by water (Fig. 10c).
Polymer P600 P700 P800 P800W P900W A800W0
20
40
60
80
Com
posi
tion b
y X
PS
(w
t.%
)
C
O
N
Na
S
2 μm
a b
24
Figure 13. SEM image (a) and TEM image (b) of A800.
4.2. Electrical conductivity of the carbon foam
The nature of the electrical conductivity of carbon foam materials was
discussed well in the paper I [30]. Despite the linearity of the voltage-current
curves of the carbon foam, negative temperature coefficient of resistance was
observed for all carbon foam, which confirmed that the foams cannot be
considered as porous metallic conductors. Moreover, studies in this paper
also revealed that the multi-phonon hopping model could be considered as
the main charge transfer mechanism and that the pyrolysis temperature and
pyrolysis time play an important role in the electrical conductivity of the
resulting carbon foam. Higher electrical conductivity was always found for
samples treated at higher temperature and longer time (Fig. 14).
Figure 14. I-V curves of carbon foam samples at scan rate 0.1 V/s. Inset:
zoom-in view for P800 and A800 samples.
-1,0 -0,5 0,0 0,5 1,0-3
-2
-1
0
1
2
3
Cu
rren
t (m
A)
Voltage (V)
P800
A800
P900
-1,0 -0,5 0,0 0,5 1,0-0,020
-0,015
-0,010
-0,005
0,000
0,005
0,010
0,015
0,020
Curr
ent
(mA
)
Voltage (V)
P800
A800
500 nm
b
20 μm
a
25
There is a relation between the electrical conductivity of carbon foam and the
amount of carbon and nitrogen on the surface of samples. As the amount of
carbon on the surface of pyrolyzed sample increased, the electrical resistance
decreased (Fig. 12b and Fig. 14). It can be explained by the higher
sp2 π bonded carbon volume fraction formed at higher pyrolysis
temperature, leading to the increase in the electrical conductivity [81].
Moreover, it was shown earlier that doping with nitrogen has a significant
effect on the structure of the carbon material: The higher the nitrogen
content, the more defective structures of the carbon layers were found
[82,83]. At low amounts of nitrogen, quaternary nitrogen atoms which
promote n-type conductivity was preferably formed [84] and, consequently,
enhanced the electrical conductivity of the sample. However, it was reported
that at high nitrogen concentrations, the conductivity of nitrogen doped
carbon samples decreased [84,85]. This phenomenon is similar with our
observations where at around 8 wt. % of nitrogen, P900 showed much better
electrical conductivity than P800 and A800 samples which contained
~18 wt. % of nitrogen on the surface (Fig. 14). Moreover, at higher amount of
nitrogen (> 20 wt. %), P600 and P700 samples were acting as insulators at
small applied voltages (-1 V to + 1 V). The poor electrical conductivity of
samples having high amount of nitrogen can be explained by the formation
of an insulating phase e.g. amorphous CNx at high nitrogen concentration
[85].
4.3. Thermal stability of the carbon foam
It is obvious that the carbon foam is very stable under inert atmosphere e.g.
nitrogen. The structure of the carbon foam was still remained at elevated
pyrolysis temperature i.e. 900 °C and lengthy pyrolysis time (5 h). It was
reported that the presence of metals such as copper, nickel and iron didn’t
strongly affect the mechanical strength of the carbon fibers after a heat
treatment at low temperature (< 600 °C) in vacuum [86]. This statement is
in agreement with our observation where the structure of the carbon foam
was fully preserved after CCVD process (~15 wt. % cobalt oxide, 30 min at
670 °C and in Varigon gas (5% hydrogen in argon gas)). However, under
identical conditions, carbon foam was destroyed by the introduction of NH3
gas (~11 ml/min) into the system during the CCVD process. As shown on
Fig. 15a, in the presence of NH3 gas, pits were formed on the carbon body
and consequently destroyed the carbon body of the foam.
26
Figure 15. SEM image of a carbon foam sample after CCVD process taken
place in the presence of NH3 (a) and TGA measurements for bare P900 and
cobalt oxide decorated P900 (b).
Besides, in the presence of oxygen, the thermal stability of carbon foam with
and without cobalt oxide was revealed by means of TGA measurements
(Fig. 15b). For the bare carbon foam (P900), the initial weight loss could be
caused by adsorbed moisture. The decomposition of the P900 sample was
speed up at around 500 °C with the final weight loss in this temperature
range was ~80 wt. %. The final weight loss was found at around 850-900 °C
which can be characterized for the decomposition of sodium carbonate to
sodium oxide. Compared with P900, the decomposition of CoOx decorated
P900 (CoOx@P900) occurred at a lower temperature (around 400 °C) which
obviously showed the effect of cobalt oxide on the thermal stability of carbon
foam samples. The rapid degradation process of carbon foam can be
explained by the catalytic oxidation process where cobalt oxide particles act
as centers for oxygen dissociation. Oxygen atoms from the surface of cobalt
oxide particles can migrate and subsequently speed up the oxidation of
carbon at the metal-carbon interfaces, resulting in gaseous carbon products
[87].
4.4. Applications
4.4.1. Solvent absorbent
The hydrophilic nature of carbon foam seems to be governed by the amount
of oxygen on the surface. As can be seen from Fig. 10c, while all pyrolyzed
carbon foam showed the hydrophobic nature, only activated carbon foam
(A800) which has higher amount of oxygen than that of pyrolyzed sample
200 nm
a
-20
0
20
40
60
80
DTA_P900
DTA_CoOx@P900
TGA_P900
TGA_CoOx@P900
W/g
0 200 400 600 800 10000
20
40
60
80
100
Temperature (0C)
Wt.
%
b
27
(Fig. 12b) displayed a contrary characteristic. The multiplicity in the wetting
behavior of the carbon foam on water offers solutions for a wide range of
purification applications especially for water purification application.
Depending on the type of contaminants which can be soluble in water such
as heavy metal compounds or insoluble in water e.g. spill oil, hydrophilic or
hydrophobic foam can be selected. For example, due to the high
hydrophobicity, P600 can float on water surface and collect non-polar
contaminants (e.g. spill oil) from the water surface [30]. P600 sample was
also found to be able to absorb a large amounts of different nonpolar
solvents can reaching up to more than 100 times compared with the dry
weight of P600 itself (Table 1). Moreover, after being filled-up with
contaminants, the carbon foam can be easily recycled by squeezing the
oil/solvent out of it or by just burning out the organic liquid.
Table 1. Uptake of nonpolar organic liquids by P600. The quantity m/m0 denotes
the mass gain normalized to the starting weight of the dry carbon sponge.
Liquids (and its density) m/m0
Silicone oil (1.402 g/cm3) 106 ± 1
Benzene (0.879 g/cm3) 95 ± 3
Turpentine oil (0.856–0.867 g/ cm3) 101 ± 8
Crude oil (0.847–0.862 g/ cm3) 79 ± 2
Iso-hexane (0.653 g/ cm3) 77 ± 6
4.4.2. Catalytic application in gas phase reactions
4.4.2.1. Hydrogenation of acetone to isopropanol
The first application where the carbon foam was used comprised a system
where the foam was applied as a substrate for a nickel catalyst. Activated
carbon foam (A800) was selected due to its high surface area (more than
300 m2/g) and the high availability of polar functional groups on the surface.
A high surface area facilitates the accessibility of metal catalysts while the
presence of polar functional groups on the surface not only helps to
immobilize metal ions but also results in the hydrophilic nature that allows
the use of water as the solvent during the catalyst decoration process.
28
Figure 16. Reaction setup (a) and proposed reaction scheme for the
hydrogenation of acetone to isopropanol (b).
As depicted in Fig. 16a, there is a special aspect in the reaction setup which
eliminated the need of traditional supporting materials e.g. glass beads.
For the powder-type catalysts, the presence of additional supporting
materials as well as the special design for the tube reactor is crucial to keep
the catalyst in place and to avoid densification. Due to the flexible
monolith-type nature, the A800 supported-Ni catalyst (Ni@A800) can be
directly mounted in a normal (non-special design) tube reactor without the
need, at least in lab scale, of any additional means of immobilization to
retain the catalytic material. Apparently, further studies are required to
confirm for the advantages of the carbon foam in this case. However,
preliminarily results in this study indicated that the use of carbon foam as a
catalyst support in gas phase reaction could bring benefits such as
simplifying the process, and reducing the costs associated with packing and
recovering the catalytic materials.
Acetone Gases
Catalyst
Ni/Carbon foam
Heating
jacket
Sampling
line
Cooling bath
~ 5 °C a b
-
H2
29
Figure 17. SEM image (inset, scale bar is 1 μm) (a), TEM image (b) and
particle size distribution (c) of Ni@A800.
As shown in Fig. 17 a and Fig.17 b, respectively, large clusters as well as
smaller nanoparticles of nickel (around 10 wt. %) which mainly displayed
sizes of around 10 nm (Fig. 17c) were found on the surface of A800.
Although parameters such as reaction temperature, reactant flow rate and
catalyst amount have not been fully optimized, a high catalytic performance
(with the conversion of 86 % and selectivity of around 99 % for 2-propanol at
the reaction temperature of 150 °C) was easily obtained. Besides, a small
amount of side products such as methyl isobutyl ketone (MIBK) and
4-methyl-2-pentanol were also detected (Fig. 18). At higher reaction
temperatures (250 °C), even though a lower conversion was observed (44 %),
a higher amount of MIBK was also detected (selectivity 11%) (Fig. 18, inset).
It has been reported that basic pyridinic and quaternary nitrogen can act as
basic catalytic sites promoting the Knoevenagel condensation reaction [88].
Thus, owing to the similar nitrogen containing groups, it is possible that
carbon foam substrate also plays an important role in promoting aldol
condensation reaction (Fig. 16b) which has been reported for other
substrates [89-92].
a b c
30
Figure 18. A typical GC chromatogram of product taken after 2 hrs reaction at
150 °C (inset, at 250 °C).
4.4.2.2. Hydrochlorination of isopropanol to isopropyl chloride
In this study, ZnCl2 was decorated on activated carbon foam (ZnCl2@A800),
at different catalyst loadings (2, 5 and 10 wt. %) which in turn was used as
catalyst for gas-phase synthesis of isopropyl chloride from isopropanol and
HCl. As illustrated by the SEM images (Fig. 19), different morphology was
found on the surface of A800 at different levels of catalyst loading: film-like
clusters at 2 wt. %, a thin film at 5 wt. % and a semi-thin film plus thick
cluster at higher catalyst loading i.e. 10 wt. %. As shown in Fig. 19c, the
dispersion of the catalyst was not perfect and that there are places with less
catalyst found on the surface of the carbon foam. The presence of catalyst
was confirmed by XPS measurement where Zn and Cl elements were found
on the surface of A800 (Fig. 20).
Figure 19. SEM images of ZnCl2@A800 at: 2 wt. % (a), 5 wt. % (b) and
10 wt. % (c) catalyst loading.
2 4 6 8 10 12 14 16
0
1000
2000
3000
2 4 6 8 10
Sig
nal
(a.
u.)
Retention time (min)
at 250 0C
4-methyl-2-petanolMIB
K
Isopro
pan
ol
Ace
tone
Sig
nal
(a.
u.)
Retention time (min)
200 nm 200 nm 2 μm
a b c
31
Table 2. B.E.T. surface area, pore size and pore volume data for the catalysts used.
Catalyst Surface area (m2/g)
Average pore size (nm)
Average pore volume (cm3/g)
fresh spent fresh spent fresh spent
2 wt. % ZnCl2@A800
322 4.5
(269*) 14
40.9 (2.5*)
1.2 0.04
(0.17*) 5 wt. % ZnCl2@A800
172.0 4.5 2.6 11.8 0.1 0.01
10 wt. % ZnCl2@A800
4.0 2.76 68.0 62.9 0.06 0.04
* Sample was degassed at 200 °C for 4 hrs.
Figure 20. Survey XPS spectra of A800 (a) and 5 wt.% ZnCl2@A800 (b).
The effect of catalyst loading as well as the flow rate of HCl on the catalytic
activity of the catalyst was also studied. The flow rate of HCl plays an
important role in the yield of isopropyl chloride. As observed, the yield was
doubled when the flow rate was reduced by a half (Fig. 21a). Due to the
macroporous structure of the carbon foam, at high flow rate, the retention
time of reactants inside the ZnCl2@A800 catalyst was too short and resulted
in low yield. Lower flow rate resulted in longer retention time which,
consequently, increased the yield of isopropyl chloride. On the other hand,
the yield of isopropyl chloride was not proportional with the catalyst loading
(2, 5 and 10 wt. % of ZnCl2) where the optimum catalyst loading was found at
around 5 wt. % (Fig. 21b). Compared with 5 wt. % ZnCl2@A800, the lower
1000 800 600 400 200 00
10k
20k
30k
40k
50k
Na 1sO 1s
N 1s
C 1s
Inte
nsi
ty (
cps)
Binding energy (eV)
1000 800 600 400 200 0
0
10k
20k
30k
40k
50k
60k
Zn 2p3/2
Na 1s
Zn 2
p1/2
Cl
2p
Zn 3
s
Zn 3
p
O 1sN 1s
C 1s
Inte
nsi
ty (
cps)
Binding energy (eV)
a b
32
catalytic activity of 2 wt. % ZnCl2@A800 could be assigned to a smaller
amount of catalyst, while for the 10 wt. % ZnCl2@A800 catalyst it was rather
an effect of the smaller surface area (Table 2). At the same time, the flow
rate of HCl as well as the catalyst loading showed minor effects in terms of
the selectivity to isopropyl chloride (Fig. 21a and Fig. 21b). As shown in
Fig. 21c, the developed ZnCl2@A800 catalyst gave rise to comparable
selectivity but lower yield of isopropyl chloride than that of ZnCl2 decorated
on commercial aluminum oxide (ZnCl2@Al2O3). It is noteworthy to know
that the major part of the catalytic activity of ZnCl2@Al2O3 was contributed
by the substrate (Al2O3) while activated carbon showed low catalytic activity
toward the hydrochlorination reaction [93,94].
Figure 21. Effect of the flow rate of HCl (a) and catalyst loading (b) on
catalytic activities (average yield and selectivity of isopropyl chloride) of
ZnCl2@A800 and (c) catalytic activities of different catalysts. No. I: particle
size < 250 μm.
Due to the fact that the carbon foam was not the material of interest in this
specific study (paper IV), no specific stability tests were performed for the
ZnCl2/carbon foam catalyst. On the other hand, after 3 h in stream, there
Yield Selectivity
0
20
40
60
80
100
120
Y
ield
/Sel
ecti
vit
y (
%)
2 wt. % ZnCl2
5 wt. % ZnCl2
10 wt. % ZnCl2
Yield Selectivity
0
20
40
60
80
100
120
Yie
ld/S
elec
tivit
y (
%)
5 wt. % ZnCl2@A800
20 ml/min HCl
10 ml/min HCl
Yield Selectivity
0
20
40
60
80
100
120
140
160
Yie
ld/S
elec
tivit
y (
%)
Bare Al2O
3 (No. I)
5 wt. % ZnCl2@Al
2O
3 (No. I)
5 wt. % ZnCl2@A800
a b
c
33
occurred a dramatic loss of the surface area in case of 2 wt. % and 5 wt. %
ZnCl2@A800 catalysts. The reduction in the surface area could have
occurred due to blockage of the pores by water and also maybe by other
chemicals strongly trapped inside the small pores of the foam. The presence
of trapped substances was confirmed by the increasing of the surface area of
2 wt. % ZnCl2@A800 ‘spent’ catalyst treated at higher temperatures and
longer time periods (200 °C, 4 h) to 269 m2/g which is much higher
comparing with just only 4.5 m2/g at standard degassing conditions (120 °C,
2 h) (Table 2).
4.4.3. Catalytic application in a liquid phase reaction
In the study [31], a monolithic-type catalyst (Ru@A800) with ruthenium
decorated on the surface of activated carbon foam (A800) was immobilized
in a SpinChem rotating bed reactor (SRBR). The new system, called
Ru@A800-SRBR in this study, was used in the hydrogenation reaction of
D-xylose to D-xylitol (Fig. 22a and Fig. 22b). As depicted in the SEM image
(Fig. 22c), a combination of nanoparticles as well as large clusters of
ruthenium was found on the A800 surface. The presence of ruthenium was
also confirmed through XPS results where ruthenium was found in both
metallic and higher oxidation states (Fig. 21d) most likely formed upon
oxidation of ruthenium metal when the catalyst was exposed to air after the
reduction process.
34
Figure 22. Hydrogenation reaction route of D-xylose (a), a typical HPLC
chromatogram of product and by products (b), SEM image (c) and Ru 3d XPS
spectra of Ru@A800 (d).
The influence of temperature, hydrogen pressure and stirring speed on the
reaction rate was studied. It was shown that when the hydrogen pressure
exceeded 40 bar, higher pressures only resulted in slightly higher reaction
rates (Fig. 23a) which is in agreement with previous studies [95,96].
Similarly, the SRBR stirring speeds (300, 500 and 700 rpm) also gave rise to
a minor effect on the reaction rate (Fig. 23c). Typically, higher stirring speed
results in faster external mass-transfer and consequently higher reaction
rate. In this case, however, such an effect was possibly counteracted by the
observation that at higher stirring rates, more gas bubbles (H2) were formed
in the reaction media. After some time, the bubbles could accumulate inside
the SRBR or/and the pores of carbon foam, thereby inhibiting the
accessibility of the liquid reactant to the active sites on the surface of carbon
foam and consequently reducing the reaction rate. Finally, the reaction rate
was strongly affected by reaction temperature, especially when a lower
hydrogen pressures was applied (Fig. 23b).
294 292 290 288 286 284 282 280 2780
1000
2000
3000
4000
5000
6000
C1s RuO2
Ruo
RuO3
Inte
nsi
ty (
cps)
Binding energy (eV)
0 10 20 30 40 50
0
20000
40000
60000
80000
24,7
896
29,6
424
36,8
928
D-arabinitol
D-xylitol
D-xylose
nR
IU
Time (min)
b a
1
c d
35
Figure 23. The effect of hydrogen pressure (a) reaction temperature (b) and
SRBR stirring speed (c) on the conversion of D-xylose.
Ru@A800-SRBR demonstrated a very good catalytic performance with high
conversion of D-xylose (ca. 99 wt. %) and high D-xylitol selectivity (ca. 98 %)
which is on par with the commercial slurry catalyst (Ru@AC) in terms of
both conversion and selectivity toward D-xylitol (Fig. 24a). Moreover, the
use of Ru@A800-SRBR allowed for an “in-situ” strategy where the
monolithic-type Ru@A800 catalyst was kept inside the SRBR through all
steps from catalyst preparation to catalyst testing. The “in-situ” approach not
only renders the whole process simpler and less time consuming than slurry
batch reactor methods but also eliminates the use of a filtration step during
the catalyst recovery process which consequently avoids the loss of catalyst
material. Practically, a normal filtration step for Ru@AC catalyst can cause
the loss of up to 8 wt. % after recuperation of catalysts from the filter [97],
which will dramatically reduce the catalytic performance just after a few
catalyst recycling cycles. Ru@A800-SRBR showed that catalyst recycling can
be achieved with only a minor loss in catalytic activity [31] even after many
0 50 100 150 200 2500
20
40
60
80
100
Conver
sion (
%)
Time (min)
At constant temperature (110 oC)
40 bar
50 bar
60 bar
0 50 100 150 200 2500
20
40
60
80
100
Conver
sion (
%)
Time (min)
At constant pressure (40 bar)
100 oC
110 oC
120 oC
0 50 100 150 200 2500
20
40
60
80
100
Conver
sion (
%)
Time (min)
At constant presssure (50 bar)
and temperature (110 oC)
300 rpm
500 rpm
700 rpm
a b
c
36
recycling cycles (Fig. 24b). This result confirmed that the catalyst
loss/deactivation (if any) in Ru@A800-SRBR is very minor.
Figure 24. The catalytic performance of the developed Ru@A800 (with and
without SRBR) and slurry commercial catalyst Ru@AC (at 100 °C, 50 bar) (a)
and catalytic deactivation study of Ru@A800-SRBR (at 110 °C, 50 bar) (b).
The catalytic activity of Ru@A800 when used as ‘slurry’ (after being cut into
small pieces) without SRBR was lower compared with that of
Ru@A800-SRBR (Fig. 24a). This could be due to slower mass transfer inside
the carbon foam pieces which consequently decrease the overall reaction
rate. When operating in ‘slurry’ mode, there was evidence suggesting that Ru
was peeled off from the surface of the carbon foam. As shown in SEM image
(Fig. 25) and XPS results (Table 3), lesser amounts of ruthenium was found
on the surface after the reaction, which could be due to the attrition of
Ru@A800 catalyst floating freely inside the reaction medium. When the
Ru@A800 catalyst was maintained inside the SRBR, the attrition of catalyst
didn’t occur and thus the Ru concentration was maintained (Table 3).
Table 3. Ruthenium concentration on the surface of Ru@A800 catalyst by XPS.
Unused Used * Used**
Ru (%.at) 5.9 5.4 2.3
*. After 15 recycling rounds with SRBR.
**. After running only one time without SRBR
0 50 100 150 200 2500,0
0,2
0,4
0,6
0,8
1,0
Conver
sion (
x100%
)
Time (min)
Ru@AC-without SRBR
Ru@A800-SRBR
Ru@A800- without SRBR
0 50 100 150 200 2500,0
0,2
0,4
0,6
0,8
1,0
5th cycle
12th cycle
Conver
sion (
x100%
)
Time (min)
a b
37
Figure 25. SEM images of Ru@A800 catalyst; ’fresh’ (a), after 15 recycling
cycles with SRBR (b), and after one run without SRBR (c).
4.4.4. Carbon foam as electrodes in OER
Due to the high surface area, the richness in oxygen containing groups on the
surface, which could contribute useful for electrochemical performance [98],
and the utility for previous applications (paper I and II [30,31]), activated
carbon foam (A800) was also being considered as anode electrode material
for the OER. However, the low electrical conductivity of A800, as discussed
in section 4.2, could possibly hamper the electrochemical performance when
used as electrode material. Therefore, due to the high electrical conductivity,
pyrolyzed carbon foam (P900) was also selected for this application even
though the low surface area and the hydrophobic nature of P900 could
inhibit the electrocatalytic activity of the electrode.
Figure 26. SEM image of CoOx@A800 (a) and CoOx@P900 (b). Scale bars,
200 nm (inset 2 μm).
In order to find out which of the two materials would be better for the
application, cobalt oxide (CoOx) was decorated on both A800 and P900
(CoOx@A800 and CoOx@P900, respectively) and subsequently the
electrocatalytic activities of all samples (together with “blank” reference
(A800 and P900)) were evaluated in alkaline electrolyte (0.1 M KOH). As
shown in Fig. 26, even at similar catalyst loadings (around 17 and 16 wt. %
1 μm
a b
1 μm 1 μm
c
a b
38
for CoOx@A800 and CoOx@P900, respectively), different morphology of
cobalt oxide was found: cobalt oxide nanoparticles on the CoOx@A800
surface versus a semi-thin film of cobalt oxide on the surface of CoOx@P900.
The difference in morphology could be the result of the difference in surface
chemistry as well as surface area between A800 and P900. As shown in
Fig. 27a, the bare P900 electrode has shown much better electrocatalytic
activity compared with that of the bare A800 electrode, which gives the first
hint to an answer for the question. After cobalt oxide was decorated on
P900, a better electrocatalytic performance (higher anodic current and lower
over potential) was achieved by CoOx@P900 electrode compared with bare
P900 electrode. In contrast, no enhancement in electrocatalytic performance
was found after doping A800 by cobalt oxide (Fig. 27a). Thus, based on the
electrocatalytic performances, it is obvious that A800 is completely
unsuitable to be used as electrode material while P900 is a promising
candidate for the application.
Figure 27. Polarization curves for OER on different carbon foam based
electrodes (scan rate of 2 mV/s) (a), Chronoamperometric responses of
CoOx@CNTs/P900 at a constant potential of 1.7 V vs. RHE (b) and 100
consecutive polarization scans obtained with CoOx@CNTs/P900 electrode (scan
rate 5mV/s) (c).
0 4000 8000 12000 160004
8
12
16
20
24
Cu
rren
t d
ensi
ty (
mA
/cm
2)
Time (s)
1st round
2nd
round
1,2 1,4 1,6 1,8 2,00
10
20
30
40
50
Curr
ent
den
sity
(m
A/c
m2)
V vs RHE (V)
CoOx@CNTs/P900
CoOx@P900
CNTs/P900
P900
CoOx@A800
A800
1,2 1,4 1,6 1,80
10
20
30
40
Cu
rren
t d
ensi
ty (
mA
/cm
2)
V vs. RHE (V)
100th cycle
50th cycle
1st cycle
a b
c
39
The electrocatalytic performance of P900, which was restricted by the low
surface area (around 4 m2/g), was enhanced by the growing of CNTs on the
surface. As can be seen in Fig. 28a CNTs were not homogeneously
distributed on the surface of P900, but the B.E.T. surface area of the hybrid
material (CNTs/P900) was increased from 4 m2/g to around 120 m2/g.
Other characteristics of P900 such as the flexibility and the electrical
conductivity were maintained in the CNTs/P900 sample (paper III). Aided
by a capping agent (thiophene) [79], a narrow size distribution of CoOx
nanoparticles in the range of 5 – 10 nm (Fig. 28b) was successfully achieved
on the surface of CNTs/P900. The presence of cobalt on the surface of
CNTs/P900 was confirmed by Raman (paper III), HRTEM (Fig. 28c) and
XPS results (Fig. 29) which, due to the effect of the annealing process taking
place in inert (nitrogen) atmosphere, showed not only cobalt oxide (CoO and
Co3O4) but also metallic cobalt on the surface of the CoOx@CNTs/P900
electrode. The observation is also in agreement with reported results in the
literature [99-101] where metal oxides could be reduced on the surface of
carbon materials in inert atmosphere and at high temperature. However,
both the surface metallic cobalt and cobalt (II) oxide could be oxidized to
Co3O4 by newborn oxygen atoms formed during the OER process (Fig. 29).
Figure 28. Structure and composition of P900 decorated with CoOx
nanoparticles: Low (a) and high (b) magnification SEM images and HRTEM
image (c). Scale bars, 10 μm (a), 200 nm (b) and 10 nm (c).
As expected, comparing with the CoOx@P900 electrode, much higher
electrocatalytic performance of CoOx@CNTs/P900 was observed, which
could be explained by: the higher surface area and the introduction of CoOx
nanoparticles on the surface. The higher surface area of CoOx@CNTs/P900
brought by CNTs growing directly on the electrode surface which was clearly
observed by comparing the electrocatalytic activity between “blank”
CNTs/P900 and “blank” P900 electrodes (Fig. 2a). Additionally, CNTs bring
b a
0.21 nm CoO (200) 0.23 nm
Co3O4 (222)
c
40
along not only higher surface area but also better electron transport occurs at
the interface between the CNTs and the substrate [102] which also
contributed for the better performance of CoOx@CNTs/P900 electrode. The
introduction of cobalt oxide nanoparticles on CNTs (Fig. 28b) brought along
better electrocatalytic performance compared to the cobalt oxide film found
on the CoOx@P900 electrode (Fig. 26b), at the same catalyst loading. It was
reported that metal oxide catalyst films do not perform well upon electrolysis
of water due to their low catalytic activity (low surface area) and the platelet
morphology (increased hydrophobicity) that hampers the ability of bubbles
to detach from a surface [103]. Consequently, the CoOx@CNTs/P900
electrode showed an excellent electrocatalytic performance with high anodic
current density and low overpotential (0.42 V at 10 mA/cm2) (Fig. 27a)
which is comparable and even better compared with the values presented in
previous studies (Table 4). It is noteworthy that while iR correction which
can lead to a huge difference in the recorded overpotential was applied in
some of these cited studies, no iR correction was used in our study. In a
specific study [104] a 0.07 V reduction in the overpotential value at
10 mA/cm2 was obtained after iR compensating.
Figure 29. XPS Co 2p spectra of CoOx@CNTs/P900 electrode; ‘fresh’ sample
(a) and ‘spent’ sample (after 100 cycles) (b). *: satellite peaks; Co0: metallic
cobalt.
As shown in Fig. 27b, during the stability test which was performed at
constant potential (1.7 V vs RHE) in a 0.1 M KOH solution, the current
density of the CoOx@CNTs/P900 electrode showed a minor downward trend
during the first 5 hrs, where the degradation of the electrolytes [105] could
be the main reason. This was revealed by the fact that after the replacement
810 805 800 795 790 785 780 7752000
3000
4000
5000
6000
* *
Co 2p1/2
Co 2p3/2
Inte
nsi
ty (
cps)
Binding energy (eV)
810 805 800 795 790 785 780 7751500
2000
2500
3000
Co0
**
Co 2p1/2
Co 2p3/2
Inte
nsi
ty (
cps)
Binding energy (eV)
a b
41
of the electrolyte in the electrochemical cell by a fresh electrolyte after the 1st
round, the electrocatalytic performance of the electrode was recovered and
the chronoamperometric curve of the 2nd round shows a very similar trend as
the 1st round.
Table 4. OER activities of some electrocatalysts in 0.1 M KOH at a current density of
10 mA/cm2.
Materials Overpotential (mV) References
CoOx@CNTs/P900 420 This work
Co3O4@NCNTs/CP 470 [102]
Co3O4/mMWCNT 390 [105]
Fe-Co3O4 486 [106]
Au/mCo3O4 440 [107]
20 wt% Ir/C 380 [108]
20 wt% Ru/C 390 [108]
Mn oxide 540 [108]
Mn3O4/CoSe2 450 [109]
Beside the unique characteristics earlier discussed, the good electrocatalytic
properties of CoOx@CNTs/P900 were also brought by a well-balanced
morphology of the 3D electrode. The macroporous carbon foam with large
pores (ranging from 50 – 100 μm) (Fig. 28a) allows an efficient diffusion of
gas bubbles throughout the whole electrode, similar as observed for nickel
foam [110]. It has been reported that attachment of gas bubbles on an
electrode surface will block the active catalyst sites and hamper ionic
transport which results in lower electrocatalytic performance, as usually
observed in case of a 2D electrode [102,110]. The ability to quickly dissipate
the oxygen bubbles formed during OER of CoOx@CNTs/P900 electrode was
strongly confirmed by the experiment data where even after 100 cycles, there
was no decrease in electrocatalytic performance (Fig. 27c).
42
5. Conclusions
Herein, flexible carbon foam was directly synthesized from commercially
available polymer foam (Basotect) by a simple pyrolysis or
pyrolysis/activation process. Besides unique characteristics such as:
flexibility, low density, large geometric surface area, it have been proven that
other important properties such as surface area,
hydrophobicity/hydrophilicity and electrical conductivity of the carbon foam
can be easily modified by changing of the pyrolysis conditions such as
duration and temperature or the introduction of an additional activation
step. Moreover, CNTs were successfully grown on the surface of carbon foam
introducing new valuable characteristics e.g. high surface area but retain
other properties such as flexibility and hydrophobicity. Due to these
aforementioned characteristics, the monolithic carbon foam was utilized in
various applications ranging from environmental application to catalytic and
electrochemical applications.
Different types of carbon foam which was aimed for specific applications
were easily synthesized. For instance, the hydrophobic pyrolyzed carbon
foam was selected for environmental application e.g. removing spill oil from
water whereas activated carbon foam having high surface area and
hydrophilic nature was shown to work perfect as substrate for metal catalysts
which in turn can be used for gas phase as well as liquid phase reactions.
Moreover, due to the flexibility and the hierarchical 3D structure, carbon
foam as catalyst substrate could bring valuable advantages such as:
simplifying the process, improving the catalytic performance, reducing costs
associated with packing and recovering the catalyst materials, especially in
combination with a SRBR.
On the other hand, it was revealed that the bare carbon foam is very stable in
inert atmosphere (N2) whereas a lower thermal stability of the metal
decorated carbon foam could be observed. Moreover, the thermal
decomposition of metal decorated carbon foam was faster with the
introduction of NH3 into the system. Besides, the homogeneity of catalyst
and CNTs decorating on the surface of carbon foam is another issue that
should be considered when working with carbon foam. In general, these facts
do not represent a weakness of the carbon foam but provide valuable
43
information when planning catalyst decorating strategies as well as other
related processes in any future carbon foam studies.
To summarize, the results of the study indicate that starting from relatively
cheap precursor polymer foam, a range of different multifunctional materials
could be easily synthesized. The application of our developed carbon foam is
not only in abovementioned studies but also in other potential applications
such as other electrical applications and mechanical applications which
haven’t yet been proven by actual studies. Moreover, the technologies used
to synthesize of carbon foam such as pyrolysis and activation are well
developed and available in large scale. Thus, we have faith in the potential of
3D carbon foam material that the material is ready to be synthesized and
taken further to larger scale.
44
6. Acknowledgements
The first person who I would like to express my gratitude is my supervisor,
Jyri-Pekka Mikkola, without him none of this would have been possible.
Thank you for your guidance and for giving me the possibility to developed
my own ideas throughout the years of my Ph.D. studies.
I would like to thank my co-supervisors, Krisztian Kordas and William
Siljebo for their technical advices and scientific discussions. A big thank you
to William, who spent a lot of time to help me with this thesis.
I would like to thank Knut Irgum for introducing me as a Ph.D. candidate
with my supervisor and your support since my master study.
I would like to extend my gratitude to my colleagues, former and present:
Hasna, Ajai, Natalia, Santosh, Anjana, Mikhail, Dilip, Rakesh and Lakhya for
being kind and friendly. For Hasna, who shared with me the funniest as well
as the most awkward moments of the Ph.D. life, I wish all the best for you
and your family and good luck for your Ph.D. study.
I would like to send my heartfelt thanks to the Vietnamese community in
Ålidhem, Umeå, especially to Nguyễn Văn Liêm, Hoàng Tuấn Tình, Đinh
Ngọc Phước, Võ Đức Duy, Võ Phạm Long, Nguyễn Anh Minh, Lê Đình
Tĩnh and many others for being friendly and supportive. My homesickness
could be much worse without them.
Cuốn luận văn này, như đã viết ở trang đầu: “dành cho gia đình, cho vợ tôi
và các con”. Tôi cảm thấy thật khó khi viết lời cảm ơn cho gia đình mình,
bởi vì tôi mang ơn họ quá nhiều và vài dòng cảm ơn là quá ngắn để bộc lộ
hết tình cảm của tôi dành cho gia đình. Tôi không biết nói cảm ơn như thế
nào với mẹ tôi, người luôn lo lắng, quan tâm và động viên tôi, với cha tôi,
mặc dù đã quên hầu hết mọi thứ nhưng vẫn nhớ động viên tôi hoàn thành
luận văn đúng thời hạn. Tôi cũng không biết nói lời cảm ơn như thế nào với
vợ tôi (Nguyễn Trần Thanh Loan), em đã phải chịu cảnh xa chồng và phải
nuôi con một mình trong suốt thời gian tôi đi du học, hoặc với bố mẹ vợ tôi
vì đã luôn giúp đỡ và động viên gia đình nhỏ của tôi...Tôi cũng xin gởi lời
cảm ơn chân thành và sâu sắc nhất đến: chị cả (Phạm Thị Thu Thủy) và anh
rể (Đặng Quang Hướng) vì đã tận tình gánh vác việc chăm sóc ba mẹ già để
cho 2 em có thể yên tâm học và làm việc tại Thụy Điển; và chị ba của tôi
45
(Phạm Kim Chung) vì đã khuyến khích và giúp đỡ rất nhiều về mặt vật chất
cũng như tinh thần trong thời gian đầu khi tôi đi du học.
SpinChem AB is acknowledged for supplying the precursor polymer foam
and the SpinChem rotating bed reactor for this research.
The Artificial Leaf project and Bio4Energy programme are acknowledged for
the financial support.
"After I got my Ph.D., my mother took great relish in introducing me as,
'This is my son, he's a doctor but not the kind that helps people.'"
— Randy Pausch, The Last Lecture
46
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