1. UNIVERSITY OF CALIFORNIA
Los Angeles
Methanation of Carbon Dioxide
A thesis submitted in partial satisfaction
of the requirements for the degree Master of Science
in Chemical Engineering
by
Daniel Jacob Goodman
2013
2.
3. ii
ABSTRACT OF THE THESIS
Methanation of Carbon Dioxide
by
Daniel Jacob Goodman
Master of Science in Chemical Engineering
University of California, Los Angeles, 2013
Professor Selim M. Senkan, Chair
The emission of greenhouse gases into the atmosphere has been linked to global warming.
Carbon dioxide’s (CO2) one of the most abundant greenhouse gases. Natural gas, mainly
methane, is the cleanest fossil fuel for electricity production helping meet the United States ever
growing energy needs. The methanation of CO2 has the potential to address both of these
problems if a catalyst can be developed that meets the activity, economic and environmental
requirements to industrialize the process. Producing methane using carbon dioxide as a reactant
would have the dual effect of keeping CO2 from entering earth’s atmosphere by consuming it to
produce natural gas which in turn can produce electricity to meet growing power needs. This
thesis aims to introduce the current methanation of carbon dioxide research, provide guidance
into what needs to be considered before entering this field and how one might go about it.
4. iii
The thesis of Daniel Jacob Goodman is approved.
Yunfeng Lu
Yi Tang
Selim M. Senkan, Committee Chair
University of California, Los Angeles
2013
5. iv
DEDICATION
I would like to dedicate this thesis and the work to produce it to the following people, for without
them it would not have been completed. Thanks to my advisor, Dr. Selim Senkan, for his
patience, belief in my abilities and guidance in completing this degree. As well as my family and
friends for their support throughout my time in graduate school and wishing the best for me even
if it took longer than intended and was not what I initially set out to do. Lastly, a great thanks to
my Girls for their love, understanding, and help getting back on track toward finishing this
degree.
6. v
TABLE OF CONTENTS
Abstract of the Thesis……………………………………………………………………….…….ii
Committee Page…………………………………………………………..………………………iii
Dedication Page…………………………………………………………..………………………iv
Introduction………………………………………………………………………………………..1
Thermodynamics of the Methanation of Carbon Dioxide………………………………………...3
Methanation Catalyst.....…………………………………………………………………………..8
Experimental………..……………………………………………………………………………17
Mechanism……………………………………………………………………………………….21
Conclusions………………………………………………………………………………………26
References………………………………………………………………………………………..27
7. 1
INTRODUCTION
The United States emits 6 billion tons of carbon dioxide (CO2) per year into earth’s atmosphere.
Greenhouse gases are so named because of their ability to absorb and emit infrared radiation.
Water vapor and CO2 are the most common greenhouse gases in earth’s atmosphere. Recent
studies indicate a high probability of a link between anthropogenic greenhouse gas emissions and
observed effects on global warming, precipitation patterns, ocean acidification, and weather
patterns [1].
Petroleum use for transportation accounts for about 1/3 of the total annual U.S. emissions of
CO2. Fossil fuel use for electricity generation accounts for more than another 1/3. For electricity
generation coal emits twice as much CO2 as natural gas which emits 20-40 times what nuclear or
renewable methods do [1]. Nature gas is the cleanest of the fossil fuels, based on greenhouse gas
emissions. 86% of the natural gas consumed in the U.S. is produced domestically and most of the
remainder from Canada. This domestic production removes dependence on foreign sources and
the international market price fluctuations that come with them. Steady prices in natural gas
would allow for the construction of cleaner natural gas combined cycle plants, possibly with
carbon capture and storage (CCS) technology, to replace existing coal plants helping reduce
emitted CO2 [1].
There are 3 main strategies for reducing CO2 emission: reduce the amount of CO2 produced,
storage of CO2, and usage of CO2. Hydrogenation of CO2 is an attractive C1 building block for
making organic chemicals, materials, and carbohydrates (i.e. foods) if considering reducing
emissions by usage of CO2. CO2 as a chemical feedstock in current industrial processes is
8. 2
limited: synthesis of urea and its derivatives, salicylic acid, and carbonates. This limitation is due
to the thermodynamic stability of CO2, which requires high energy substances to transform it
into other chemicals [2]. The hydrogenation of CO2 into more useful fuels or chemicals uses
hydrogen as the required high energy material for transformation. The products of CO2
hydrogenation currently being researched include carbon monoxide, methane, methanol, ethanol
and higher alcohol, hydrocarbons, dimethyl ether, formic acid, formates and formamides. Some
of these products can be fuels in internal combustion engines, raw materials and intermediates in
many chemical industries, easily liquefied allowing for easy storage and transportation, and in
general are more desirable than CO2. Effecting the scale up of CO2 hydrogenation to industrial
levels include lack of satisfactory catalysts (with desirable cost, activity, selectivity, stability,
recovery, reuse, and handling), efficient and economic reactor design and the availability of
hydrogen which has issues with its production, storage and transportation [2].
Methane is the main component of natural gas [3]. If a natural gas plant with carbon capture and
storage technology were utilized for producing electricity using methane/natural gas produced
from CO2 all 3 strategies for reducing CO2 emissions would be implemented. This is why there
is great interest in producing methane from CO2. Catalytic hydrogenation of CO2 to methane,
CO2(g) + 4 H2(g) CH4(g) + 2 H2O(g), also called the Sabatier reaction (named after chemist Paul
Sabatier in 1902 who observed the reaction over a Nickel catalyst) [4] is a topic most recently
reviewed by Wang et al [5], who also reviewed the hydrogenation of carbon dioxide into other
products [2].
9. 3
This paper is meant as an introduction into the research being conducted on CO2 methanation:
The thermodynamics of the CO2 methanation reaction; types of catalysts being explored, what
factors need to be considered when choosing which to use (i.e. type of metal(s), support, metal
loading, and preparation method), and a table of CO2 catalysts investigated in the literature with
their relevant conditions and results; The Sabatier reaction mechanism and how to design a
reactor system to test your catalyst; Characterization techniques to use to better understand and
evaluate your catalyst’s performance; and some conclusions and recommendations developed
from reviewing the current state of methanation technology.
THERMODYNAMICS OF THE METHANATION OF CARBON DIOXIDE
Thermodynamic equilibrium calculations of chemical systems can give answers to important
questions, such as the type of thermodynamically stable reaction products produced along with
their selectivity and yield, if a reaction proceeds endothermically or exothermically, the impact
of reaction parameters like temperature, pressure and reactant ratios [6]. Comparing calculations
with experimental results allows for the identification of kinetic hindrances, i.e.
thermodynamically allowed but somehow suppressed chemical reactions, providing guidance in
catalyst development and process control of methanation.
CO2, and its Carbon-Oxygen double bonds, is a stable molecule. Both terms of the Gibbs free
energy disadvantage the conversion of CO2 into other products. At atmospheric pressure and 298
K, the ΔH is about 293 kJ/mol CO2 for the dissociation of CO2 into CO and O2. The addition of
the higher Gibbs free energy co-reactant H2 in the reversible and exothermic methanation
reaction makes the conversion of CO2 thermodynamically easier with a ΔHo
of -167 kJ/mol [7].
10. 4
Gao et al. conducted systematic thermodynamic analysis of carbon oxide (CO and/or CO2)
methanation using the total Gibbs free energy minimization method which is without any
hindrances caused by kinetics, transport phenomena, or hydrodynamics. Table 1 lists possible
reactions involved in the methanation of carbon oxides. Their calculations were based on
gaseous compounds containing H2, O2, N2, CO, CO2, CH4, H2O, C2H4, and solid carbon. Note
that all the reactions may simultaneously happen and since 3 of the reactions produce CO2 it is
difficult to completely convert it. The van’t Hoff equation was used to calculate equilibrium
constants (K), which are plotted versus temperature in Figure 1. It can be seen that the
exothermic CO2 methanation (R2) is suppressed as temperature increases and that it plays an
important role in the methanation reaction system because of its high equilibrium constant in the
200-500 o
C range [6].
Table 1. Possible reactions involved in the methanation of carbon oxides [6]
11. 5
Figure 1. The calculated K values of the reactions involved in methanation. [6]
Figure 2 shows the typical product fraction for CO2 methanation determined by Gibbs
minimization at equilibrium for a stoichiometric 4:1 H2:CO2 feed at 1 atm. At low temperatures
CH4 and H2O are the main products. As the temperature is increased above 450 o
C, the formation
of CO by-product by the reverse water-gas shift reaction (R5) occurs and then dominates above
550 o
C. Increasing the temperature is unfavorable since CO2 methanation is strongly exothermic,
and operating below 300 o
C is necessary for high CH4 yield at 1 atm.
12. 6
Figure 2. Product fraction of CO2 methanation at equilibrium. [6]
Gao et al. also investigated the effect of pressure and temperature, Figure 3, and different H2/CO2
ratios, Figure 4, on CO2 methanation, i.e. CO2 conversion, CH4 selectivity and yield. From
Figure 3 it can be seen that CO2 conversion is inversely proportional to temperature and directly
proportional to pressure. The switch to a direct proportionality with termperature at 1 atm
beyond 600 o
C is attributed to the reverse water gas-shift reaction dominating the system and
consuming CO2. The authors note that CO2 is more difficult to methanize than CO at the same
temperature and pressure even though CH4 selectivity is better in CO2 methanation. As seen in
Figure 4, higher H2/CO2 ratios lead to higher CO2 conversion and CH4 selectivity.
13. 7
Figure 3. Effects of pressure and temperature on CO2 methanation: (a) CO2 conversion, (b)
CH4 selectivity, and (c) CH4 yield. [6]
Figure 4. Effect of different H2/CO2 ratios on CO2 methanation: (a) CO2 conversion, (b) CH4
selectivity, (c) CH4 yield, and (d) carbon yield. [6]
14. 8
As shown by the Gibbs minimization method the methanation of CO2 is thermodynamically
favorable and low temperature, high pressure and proper H2:CO2 ratio (equal to or more than 4)
are required to optimize the process [6].
METHANATION CATALYST
The reduction of the fully oxidized carbon, CO2, to methane is an eight-electron process with
significant kinetic limitations, which requires a catalyst to achieve acceptable rates and
selectivities for potential industrial use [8]. For a catalyst to be of use in industry it needs to meet
certain cost, activity, selectivity, stability, recovery, reuse, and handling requirements.
Metal
The catalysts being investigated for CO2 methanation are generally made up of Group VIII, IX,
X and XI transition metals. Nickel and Ruthenium based catalysts produce almost exclusively
methane, while less reactive metal constituents Pd, Pt, Rh, Mo, Re and Au catalyze
simultaneously CH4, CH3OH and CO (by reverse water-gas shift reaction). Cu and Ag catalyze
mainly CH3OH [9]. Nickel based catalysts are the most common studied because of their high
activity and low price, but sintering at reaction conditions diminishes their industrial viability
[10]. Ruthenium has been shown to be the most active metal for methanation, but its high cost
makes it less attractive as an industrial catalyst [11].
Multi-metallic catalysts that seek to utilize the functionality of different metals in a concerted
manner are also of interest. Park et al. investigated a Pd-Mg on Silica catalyst based on
Palladium’s ability to dissociate molecular hydrogen, which can then react with surface species
15. 9
formed by the reaction of CO2 on a Mg-containing oxide [8]. While Pd/SiO2 is active for CO2
reduction to CO and Mg/SiO2 (without Pd) is relatively inactive, when combined they attained
selectivities to methane greater than 95% with 59% CO2 conversion for this system.
The metal loading of a catalyst can affect dispersion, pore volume and pore size and blockage of
micropores caused by the agglomeration of species leading to a reduced surface area. Although
the number of active sites increases with increasing metal loading, the dispersion of metal will
decrease with increasing loading [12]. As metal loading is increased a significant decrease of the
BET surface area is usually observed, usually due to formation of large oxide aggregates, which
limits the access to the internal surface of the catalyst [13].
Support
The type of support used for heterogeneous catalysts is another important factor to consider. The
interaction between the metal and support plays a very active role on catalyst performance and
are termed “metal-support effects” [12]. The support plays an important role on the active site
dispersion, activity and stability [14]. Typical supports include Silica (SiO2) [8], Aluminum
Oxide (Al2O3) [15], Lanthanum Oxide (La2O3) [16], and composite supports (ZrO3-AlO3) [17].
Chang et al. used an amorphous silica rice husk ash-alumina support (RHA-Al2O3) with Nickel
for a catalyst because they found the support to exhibit high selectivity for CO2 hydrogenation
[12].
Zhang et al. used Mobile Composition Matter (MCM) based mesoporous silica nanoparticle
supports for Nickel based catalysts. MCM supports exhibit high specific surface area, regular
pore structures and adjustable heteroatom contents. When nickel was incorporated into MCM-41
16. 10
supports they were found to be novel, thermally stable and highly active for CO2 methanation, on
par with the best Ru/SiO2 catalysts [14].
Table 2 lists CO2 catalysts investigated in the literature with relevant conditions and results.
Another summary of the results of methanation of CO2 on various catalysts can be found
elsewhere [40, 9].
Table 2. CO2 catalysts investigated in the literature with their relevant conditions and results.
Catalyst Prep.a
Temp, o
C Feed Ratio,
H2:CO2
CO2 Conversionb
,
%
CH4 Selectivityb
,
%
CH4
Yieldb
, %
Ref.
4.29% Ni/RHA-Al2O3 IE 500 4:1 34 56 19 18
4.09% Ni/ SiO2-gel IE 500 4:1 25 45 11 18
15% Ni/RHA-Al2O3 IWI 500 4:1 63 90 58 12
15% Ni/ SiO2-Al2O3 IWI 600 4:1 63 29 19 12
NiFeAl-(NH4)2CO3 CP 220 4:1 58.5 99.5 58.2 15
NiFeAl-Na2CO3 CP 220 4:1 55.7 99.5 55.4 15
NiFeAl-NH4OH CP 220 4:1 54.5 99.4 54.2 15
NiFeAl-NaOH CP 220 4:1 49.1 99.6 48.9 15
69.1% Ni5Al-R CP 400 4:1 92.4 99 -- 47
LaNi4Al AM 400 4:1 91.5 95 -- 48
25% Ni/Al2O3 CP 235 9:1 99 99.7 -- 49
15% Ni/Al2O3 I 250 2:1 14.5 97 -- 50
15% Ni-5%Mo/Al2O3 I 250 2:1 17.2 97 -- 50
15% Ni/Al2O3 I 300 4:1 45 99 -- 51
15% Ni-2%CeO2/Al2O3 I 300 4:1 71 99 -- 51
40.4% Ni/Al2O3 SG 220 4:1 61.5 99.2 -- 52
10% Ni/Al2O3 I 400 4:1 5 99 -- 53
9.8% Ni/Al2O3 I 516 11:1 98 100 -- 36
10% Ni/Al2O3 I 350 3.5:1 69 92 -- 58
15% Ni/Al2O3 I 350 3.5:1 71 100 -- 58
20% Ni/Al2O3 I 350 3.5:1 76 100 -- 58
23. 17
EXPERIMENTAL
Preparation Method
An important consideration when designing a catalyst is its preparation method. How the metal
is combined with the support can affect the metal dispersion, crystal structure, and allowable
metal loading.
Incipient wetness impregnation is the simplest preparation method, allowing for easier control of
loading and no liquid waste generation, but is not the best for attaining good dispersion of metal
on support [12].
Catalysts prepared by ion exchange yield finely dispersed and evenly distributed metal
crystallites, but are limited to catalysts with low loading [18].
For high metal loadings a precipitation-deposition technique should be used in place of
impregnation of a support which is not practical [19]. The precipitation agent selected has been
shown to greatly influence the catalyst performance and should be carefully considered and
investigated when using this preparation method [15].
The method of reverse microemulsion can be utilized to achieve a highly dispersed catalyst when
intimate mixing of the heterospecies is required and phase separation needs to be minimized [8].
Based on the expansive array of metals, supports, loading and preparation methods being
considered currently a high throughput combinatorial catalysis approach to discovery and
optimization of CO2 catalysts seems ideal to expiating the advancement of this technology to
industrial standards.
24. 18
Reactor
Catalyst evaluations can be performed using the Senkan lab computer controlled array channel
microreactor system described previously [20] in which up to 80 catalysts can be screened in
parallel. In the array microreactors, reactant gases flow over the flat surfaces of catalyst powder
which are individually isolated within reactor channels; the flow regime is similar to that of a
monolithic reactor [21]. All experiments should be performed under atmospheric pressure and at
a gas hourly space velocity (GHSV) of 20,000 h-1
, to represent differential reactor conditions.
GHSV can be calculated using the gas flow rate and the volume of the catalyst used in each
channel. Prior to activity measurements, all catalysts should be reduced in hydrogen at
temperatures above those used for the reaction (i.e. 500 °C). Initial screening experiments should
be performed within the temperature range 25-450°C, using a feed gas, supplied by pre-filled
high-pressure bottles of H2, CO2 and inert carrier gas (i.e. Ar, He, N2), with a composition of
carbon dioxide and hydrogen at a ratio of CO2:H2 = 1:4 and balance inert carrier [12]. Reactor
effluent gases will be analyzed by withdrawing the products using a heated capillary sampling
probe followed by on-line gas chromatography (Varian CP-4900 Micro GC with thermal
conductivity detector, Porapak Q (10m) and Molecular sieve 13X (10m) columns) and mass
spectrometry (MS, SRS RGA-200, Sunnyvale, CA).
Should the array channel microreactor be unavailable for these experiments a micro fixed-bed
reactor (4-6 mm i.d. quartz tubing) can be constructed [22]. The feed gas control and effluent gas
analysis systems described above can be incorporated into or reproduced for this new reactor,
while a tubular furnace can be used to control the reactor temperature.
25. 19
Marwood et al. note that the methanation reaction rate is inhibited by water [23]. A water trap
can be placed at the entrance to the reactor to control the partial pressure of water in the reactor.
A water trap for maintaining a constant water partial pressure would consist of 20 mL glass U-
tube filled with 1mm glass beads, which is filled with a predetermined volume of water (i.e. 0.5
mL H2O to maintain 8.1 mbar partial pressure) before each experiment and kept at 4°C [24].
If catalyst deactivation caused by excess temperature generated by the exothermic nature of the
Sabatier reaction needs to be avoided the use of a microchannel reactor can be considered. These
reactors allow for precise temperature control along the length of the reactor [3].
Characterization
Characterization of a catalyst is essential when trying to better understand and evaluate your
catalyst’s properties and performance. Below is a list of techniques being used and how they can
help you understand the microscopic and macroscopic properties of your catalyst and provide
guidance on how to move forward in your research.
Inductively coupled plasma-atomic emission spectrometer + Elemental analyzer (Elemental
analysis) can be used for determining metallic impurities. Contaminants can affect catalyst
activity and the chemical reaction [12].
Temperature programmed reduction (TPR) can be used to characterize the effect of metal
loading on the fraction and dispersion of catalyst species, and the reduction behavior of catalyst
[12].
26. 20
Temperature programmed desorption (TPD) can be used to determine the metal dispersion by
identifying the amount of surface metal versus total metal atoms in the catalyst and thereby the
metal surface sites surface area [25].
X-ray photoelectron spectroscopy (XPS) can be used for understanding the role of the metals
interactions with active components of the catalyst support [12].
X-ray diffraction (XRD) can be used to determine the crystal structure of the components of the
catalyst and how metal loading affects dispersion and crystal growth (size, structure,
crystallinity, crystal form) [12]. XRD techniques combined with TPR can be used to confirm
metal-support interactions [22].
Scanning election micrographs (SEM) can be used to study the surface structure of the catalyst
and support and how calcination and other catalyst preparation techniques affect it [12].
A transmission electron microscope (TEM) can be used to determine the catalyst morphology, its
particle size distribution, and average particle size. Elemental distribution can be determined by
energy dispersive X-ray spectroscopy (EDS) with a system coupled to the TEM [8].
The specific surface area (BET), pore volume, and pore size of a catalyst can be analyzed by a
Nitrogen adsorption-desorption analyzer. This information can help determine how varied metal
loading can affect dispersion, pore volume and pore size and blockage of micropores caused by
the agglomeration of species leading to a reduced surface area [12].
Thermogravimetric analysis can be conducted to determine the loss in weight taking place in the
decomposition process [25].
27. 21
MECHANISM
Insight into the mechanism of a chemical reaction is necessary to optimize the reaction process
and improve performance. Much work has gone into establishing the mechanism of CO2
methanation, but to date no consensus on the kinetics and mechanism exists in the field. The split
in opinion has to do with the nature of the intermediate compound involved in the rate-
determining step and the process of methane formation. There are two main categories for the
proposed mechanism of CO2 methanation. The first and more agreed upon path involves
converting CO2 to CO which is converted to CH4 following the same mechanism as CO
methanation, which itself does not have a consensus mechanism either. The other proposed
mechanism directly hydrogenates CO2 to CH4 without the formation of a CO intermediate.
Mechanism 1: CO intermediate
The CO intermediate pathway for CO2 methanation involves the conversion of the gaseous CO2
into CO, which is subsequently hydrogenated to methane, CH4, following the CO methanation
mechanism. While CO as the direct intermediate for methanation is consistent for this pathway
the procedure for how CO is formed from gaseous CO2 proposed can differ. There is also
disagreement on the CO methanation mechanism and its intermediates. The varied CO
intermediate mechanisms proposed are discussed below.
Using transient measurement techniques to measure gas phase components time evolution and
diffuse-reflectance infra-red (DRIFT) spectroscopy to indentify species on the surface of a 2%
Ru/TiO2 catalyst Marwood et al. propose the mechanism reproduced here in Figure 5. Gaseous
CO2 reacts with a surface hydroxyl group to a form surface-bound hydrogen carbonate species,
HCO3
-
, which is reduced with metal adsorbed hydrogen, HM, to an interfacial formate species,
28. 22
HCOO-
. The formate diffuses in an equilibrium reaction between the interface and the support,
where it can be identified by DRIFT spectroscopy, but transient experiments indicate the support
formate is not active in the methanation reaction. The interfacial formate decomposes to
adsorbed CO species while regenerating the surface hydroxyl group. Infrared spectroscopy was
unable distinguish the steps of adsorbed CO hydrogenation and they are thus presented in a non-
elementary lumped form involving 6 metal adsorbed hydrogens [23].
Figure 5. CO2 methanation reaction mechanism proposed by Marwood et al. Reproduced from
[23].
Beuls et al. recently conducted methanation experiments at low temperatures on a Rh/γ-Al2O3
catalyst. DRIFT results showed identical adsorbed species for CO2 and CO methanation, but no
formate was detected in either case indicating CO2 and CO methanation followed the same
CH4 + H2O
S: the support; M: the metal; I: the metal-support interface
+ 6 HM
(CO)M
(OH-
)S
CO2
(HCO3
-
)S
2HM
H2O
(HCOO-
)I (HCOO-
)S
29. 23
mechanism, but one different from the formate pathway Marwood proposed. XPS showed the
catalyst being oxidized following exposure to CO2, which is believed to be caused by the
adsorbed oxygen species generated by the dissociation of gaseous CO2 , CO2(g) CO(a) and O(a),
which also generates the CO intermediate for methanation [26].
Atom Superposition and Electron Delocalization-Molecular Orbital (ASED-MO) theory was
applied to a Ni (111) surface by Choe et al. for a surface science approach to determining a
reaction mechanism where CO2 is converted to CO and then to surface C before hydrogenation.
The elementary reaction steps proposed are reproduced in Figure 6 and consist of two
mechanisms: carbon formation (steps 1-3) and carbon methanation (steps 4-6) [27]. The
dissociation of CO2 to form the CO intermediate, step 1, occurred with an intermediate complex
composed of atomic oxygen and π-bonded CO and a calculated activation energy of 1.27 eV
[28].
Figure 6. CO2 methanation reaction mechanism proposed by Choe et al. Reproduced from [27].
Steps 2-6 concern the CO methanation mechanism. There are two pathways in CO dissociation.
Dissociation of CO into C and O, step 2, is rate-determining based on a calculated activation
CO2ads COads + Oads Step 1
COads Cads + Oads Step 2
2COads Cads + CO2gas Step 3
Cads + Hads CHads Step 4
CHads + Hads CH2ads Step 5
CH2ads + 2Hads CH4gas Step 6
30. 24
energy of 2.97 eV, highest of any of the elementary steps. This CO dissociation pathway was
originally proposed by Fitzharris et al. [29], and has the support of thermodynamics [6]. The
alternative adsorbed carbon formation path is the CO disproportion into surface carbon and
gaseous CO2, step 3, proposed by Martin [30].
There is support for a CO methanation mechanism that does not require the formation of
adsorbed carbon, known as the associative pathway. Employing DRIFTS spectroscopy
techniques over a 5% Ru/TiO2 catalyst Panagiotopoulou et al. argue that CO2 methanation occurs
via intermediate formation of adsorbed CO species produced via the reverse water-gas shift
(RWGS) reaction at the metal-support interface where the (TiO2)Ru-CO species interacts with
adsorbed hydrogen atoms to form a partially oxygenated carbonyl species and, eventually,
methane in the gas phase without surface carbon being involved in the hydrogenation. Transient
mass spectroscopy techniques indicated that the interaction of the reaction mixture with the
catalyst surface does not result in oxidation of Ru crystallites. This indicates that the dissociation
of CO2 into CO and O, as proposed by Beuls on Rh/γ-Al2O3, does not take place in this Ru/TiO2
system and thus the associative pathway dominates in CO2 methanation [31].
Mechanism 2: Direct Hydrogenation (No CO intermediate)
The direct hydrogenation of CO2 to CH4 without the formation of a CO intermediate is the other
proposed mechanism for CO2 methanation. By means of conventional ultrahigh vacuum surface
science techniques, combined with density functional theory calculations and corroborated by
high pressure reactivity tests on Ni (110) Vesselli et al. support the direct hydrogenation
mechanism. They noted that at low temperature, CO2 is negatively charged and is chemically
bonded mainly via the carbon atom. The molecule receives electronic charge from the metal,
31. 25
bends, and binds to the surface with the carbon atom in a “V” configuration: the resulting energy
barrier (0.43 eV) for its hydrogenation is relatively small and, most importantly, smaller than that
for dissociation into CO + O and that for CO2 desorption, both of which were found to have an
activation barrier of 0.60 eV. When adsorbed H approaches CO2, the H-CO2 complex flips and
binds to the surface with the two oxygen atoms and H binds to the carbon atom, thus yielding a
formate intermediate. The presence of hydrogen prevents the formation of CO [32].
Sharma et al. concluded that CO2 methanation does not take place through a CO intermediate.
For CO2 methanation their Ce0.95Ru0.05O2 catalyst yields 55% CO2 conversion with 99%
selectivity for CH4. Though when performing temperature-programmed reaction (TPR) of CO
with H2, on the catalyst essentially no activity for CO methanation is seen. Since this catalyst
methanates CO2 but not CO a mechanism with a CO intermediate is rejected. Additionally IR
adsorption in the region between 2700 and 3100 cm-1
where formate bands are expected were not
seen, so this catalyst methanates CO2 without formate or CO intermediates [33].
Another factor to consider when investigating CO2 methanation mechanisms is the impurities in
the feed gas. As noted thermodynamically the reverse water-gas shift reaction tends to dominate
over carbon oxide methanation as temperature is increased. This can cause CO2 in the feed to be
converted to CO. This is significant because Schlid et al. observed that while CO2/H2 feed
mixtures rapidly form surface formate and gas phase CH4 during catalytic hydrogenation on
Pd/ZrO2 catalysts, the main reaction product is methanol, CH3OH, when a CO/H2 feed is used
[34]. The presence of CO in the feed mixture, due to CO2 conversion by RWGS or feed
impurities, can alter what product is formed by the catalyst.
32. 26
From this summary of proposed mechanisms for just a few of the CO2 methanation catalysts the
difficulty of settling on a consensus mechanism is evident. A change in any or all of the possible
variables (i.e. metal(s), support, feed composition, approach (experimental vs. theoretical),
reaction conditions, preparation method) can lead to compelling evidence for a different CO2
methanation mechanism, or lead to a catalyst that yields a different product. Even this variability
doesn’t always hold true. Schlid et al. observed that two differently prepared Ni/ZrO2 catalysts
exhibit an almost identical catalytic behavior; both are good CO2 methanation catalysts and
reveal about equal activation energies, indicating that the reaction mechanism is probably the
same and that the active sites are similar on both catalysts. This result is remarkable in the light
of the vastly different structural properties of both systems. The performance of the catalysts for
CO2 methanation does not appear to be significantly influenced by the chemical and structural
properties of the zirconia matrix [35].
CONCLUSIONS
The high throughput combinatorial catalysis approach to discovery and optimization of catalysts
used in the Senkan lab, with the help of the computer controlled 80 array channel array
microreactor system, is well suited for investigating catalysts for the methanation of CO2 and
advancing them to performance levels desired by industry for scale up. This is based on the
expansive array of metals, supports, loading and preparation methods being considered currently
for the methanation of CO2. The factors to consider for designing, testing and characterizing a
methanation catalyst have been introduced in this thesis with the hope that it will expedite the
determination of an acceptable industrial catalyst and allow the pursuit of a solution to an issue
of environmental, political and economic significance.
33. 27
REFERENCES
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