Front. Chem. Sci. Eng. 2011, 5(1): 2–10 DOI 10.1007/s11705-010-0528-3

REVIEW ARTICLE

Methanation of carbon dioxide: an overview Wei WANG, Jinlong GONG (✉) Key Laboratory for Green Chemical Technology (Ministry of Education), School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China

© Higher Education Press and Springer-Verlag Berlin Heidelberg 2011

Abstract Although being very challenging, utilization of carbon dioxide (CO2) originating from production processes and flue gases of CO2-intensive sectors has a great environmental and industrial potential due to improving the resource efficiency of industry as well as by contributing to the reduction of CO2 emissions. As a renewable and environmentally friendly source of carbon, catalytic approaches for CO2 fixation in the synthesis of chemicals offer the way to mitigate the increasing CO2 buildup. Among the catalytic reactions, methanation of CO2 is a particularly promising technique for producing energy carrier or chemical. This article focuses on recent developments in catalytic materials, novel reactors, and reaction mechanism for methanation of CO2. Keywords CO2 methanation, hydrogenation, catalysis, methane, environmental science

1

Introduction

Climate change is considered to be one of the greatest environmental threats of our times [1]. The atmospheric concentration of green house gases can roughly be divided into carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), fluorocarbons (CFs) and chlorofluorocarbons (CFCs) [2]. Carbon dioxide concentration in the atmosphere has constantly increased in the last century, where the current concentration of 3.86
10–4 is far from the preindustrial levels of 2.80
10–4 [3]. The drastic increase in CO2 concentration was deemed the main culprit for the increase in earth’s temperature. There has been increasing pressure from the public all over the world to curb CO2 emissions, and for industries to develop efficient carbon capture systems. For chemists, carbon dioxide turns out to be an attractive C1 building block in organic synthesis as it is a highly Received November 8, 2010; accepted December 1, 2010 E-mail: [email protected]

functional, abundant, renewable carbon source and an environmentally friendly chemical reagent [4–9]. The utilization, complementary to the storage of CO2, is indeed more attractive, especially if its conversion to useful bulk products is economical. Currently, the utilization of CO2 as chemical feedstock is limited to a few processes: synthesis of urea (for nitrogen fertilizers and plastics), salicylic acid (a pharmaceutical ingredient), and polycarbonates (for plastics) [10]. However, the actual use only corresponds to a few percentage of the potential CO2 suitable to be converted to chemicals. Therefore, the utilization of CO2 for the synthesis of fuels may result a convenient alternative to the production of methanol or syngas from conventional routes with low efficiency. Recycling of CO2 as carbon source for chemicals and fuels should be considered as a more sustainable use of the resources which can indeed lead to less consumption of carbon-based fossil resources without producing more CO2 from the whole system. Catalytic hydrogenation of carbon dioxide to methane, also called the Sabatier reaction, is an important catalytic process. CO2 þ 4H2 ↕ ↓CH4 þ 2H2 O ΔH298  K ¼ – 252:9  kJ=mol The methanation of carbon dioxide has a range of applications including the purification of synthesis gas for the production of ammonia and the production of syngas. The National Aeronautics and Space Administration (NASA) is remarkably interested in applications of this reaction in manned space colonization on Mars [11,12]. Bringing terrene hydrogen to Mars will make it possible to convert the Martian carbon dioxide atmosphere into methane and water for fuel and astronaut life-support systems [12]. The CO2 methanation is thermodynamically favorable (ΔG298 K = – 130.8 kJ/mol); however, the reduction of the fully oxidized carbon to methane is an eight-electron process with significant kinetic limitations, which thus requires a catalyst to achieve acceptable rates and selectivities. Extensive studies have been conducted on

Wei WANG et al. Methanation of carbon dioxide: an overview

metal-based catalytic systems on the hydrogenation of CO2 to methane. In this paper, we comprehensively discuss the recent progresses in the reactor innovation, optimization of the reaction conditions, reaction mechanism, and catalyst performance in CO2 methanation. We also place an emphasis on the nature of active sites and effects of promoter and support.

2

Metal-based heterogeneous catalysts

2.1

Ni-based catalysts

Hydrogenation of CO2 toward methane has been investigated using alternative suitable catalytic systems based on supported group VIII metals (e.g., Ru, Rh) on various oxide supports (TiO2, SiO2, Al2O3, CeO2, ZrO2). However, supported nickel catalysts remain the most widely studied materials. High surface area supports, usually oxides, have been used extensively for the preparation of metal catalysts. The nature of support plays a crucial role in the interaction between the nickel and the support, and thus determines catalytic performances toward activity and selectivity for the methanation of CO2 [13]. Because the support always has a significant influence on the dispersion of the active phase, preparation of highly dispersed supported metal catalysts has been the focus of research. Du et al. applied Ni/MCM-41 catalysts with different amount of Ni to CO2 methanation [14]. High selectivity (96.0%) and space-time yield (STY, 91.4 g$kg–1$h–1) were achieved on 3 wt-% Ni/MCM-41 at a space velocity of 5760 kg–1$h–1, superior to that of Ni/SiO2 catalysts and comparable to Ru/SiO2 catalysts [15–17]. The high selectivity was maintained at higher temperature (673 K) with increased space-time yield STY (633 g$kg–1$h–1). Reduction at 973 K produced stable catalysts yielding the best activity and selectivity, at which most Ni was reduced to highly dispersed Ni0 due to the surface anchoring effect [14]. Amorphous silica extracted from rice husk (thus commonly referred to as rice husk ash (RHA)) has a high specific surface area (125–132 m2/g), high melting point and porosity. Chang et al. reported that nickel catalysts supported on amorphous silica were active for hydrogenation of CO2 [18–20]. Table 1 shows the turnover

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numbers (TON) of methane (methane produced per nickel site per second) for hydrogenation of CO2 on highly dispersed nickel catalysts prepared by various methods. The authors noted that the hydrogenation activity of the nickel nanoparticles supported on amorphpus silica was better than those on silica gel [20]. Amorphous silica was also used as a raw material for preparing a series of silica-alumina composites as supports for nickel-based catalysts (Ni/RHA-Al2O3) synthesized via ion exchange method [21]. It should be noted that reduction of NiO from thermal decomposition of the layered nickel compound was particularly difficult. It is probably due to the presence of alumina trapped in the NiO particles resulting in an increase in activation energy of reduction [22]. Conversion of CO2 and the yield of CH4 are strongly dependent on the calcination and reduction temperatures. Hydrogenation activity decreases as content of alumina increases, indicating that acidic sites are not uniquely responsible for the reaction. Nickel nanoparticles supported on RHA-Al2O3 were also prepared by incipient wetness impregnation [13]. These catalysts with high surface area and mesopores structure are advantageous for chemical reactions. A strong interaction between metal and oxide (SIMO) was found for this system. Consequently, the nickel oxide nanocrystallites (e.g., NiO, NiAl2O4) are formed with high dispersion on the surface. At an optimized reaction temperature of 773 K, maximized yield (about 58%) and selectivity of CH4 (about 90%) were obtained. The performance of Ni/RHA-Al2O3 was better than that of Ni/SiO2-Al2O3 (Fig. 1). We certainly need to recall Raney nickel well-known as an active catalyst for hydrogenation, which appears to have high activity in methanation reaction [23]. Microscopically, each particle of Raney nickel is a three-dimensional mesh, with pores of irregular size and shape, of which the majority is created during the leaching process. Raney nickel is notable for being thermally and structurally stable as well as having a large BET surface area. Their unique physical properties attribute to the activation process and a relatively high catalytic activity. Ni-Al alloys were indeed active in the methanation of CO2 and the main products from the reaction were CH4 and CO [24]. The catalyst derived from the alloy exhibited that higher Ni content leads to higher specific activity and higher selectivity to methane (100%). This phenomenon can be explained with

Table 1 Comparison of activity of hydrogenation of CO2 on nickel catalysts [20] preparationa)

dispersion/%

reaction temperature/K

turnover number /(
103 s–1)

reference

4.3wt-% Ni/SiO2-RHA

IE

40.7

773

17.2

[20]

4.1wt-% Ni/SiO2-gel

IE

35.7

773

11.8

[20]

3.5wt-% Ni/SiO2-RHA

DP

47.6

773

16.2

[19]

I

39.0

550

5.0

[15]

catalyst

3.0 wt-% Ni/SiO2

a) IE: ion exchange; DP: deposition-precipitation; I: impregnation

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activity could be further improved by an redox pretreatment [29]. Additionally, Ocampo et al. studied the CO2 methanation over Ni-based Ce0.72Zr0.28O2 catalysts with Ni loadings ranging from 5 wt-% to 15 wt-% [30]. A catalyst with 10 wt-% Ni exhibited excellent catalytic activity and stability in the reaction during 150 h on stream, yielding a CO2 conversion and a CH4 selectivity of 75.9% and 99.1%, respectively. The high oxygen storage capacity of Ce0.72Zr0.28O2 and its ability to highly disperse nickel are the origin of the high performance. The incorporation of nickel cations into the Ce0.72Zr0.28O2 structure and the higher dispersion of NiO at its surface (Fig. 2) improve the redox properties of the material and thus restrict the metal sintering effect.

Fig. 1 Comparison of CO2 conversion and CH4 yield for CO2 hydrogenation over 15 wt-% Ni/RHA-Al2O3 and 15 wt-% Ni/SiO2-Al2O3 catalysts [13]

the better ability of Ni (compared to Al) to dissociate CO. Moreover, a series of mono- and bi-metallic Ni-based catalysts supported on alumina were tested for CO2 hydrogenation by a computational screening study [25]. The conversion of CO2 to methane was significantly increased over Ni-Fe alloy compared to pure nickel or iron catalyst, and the best catalyst had a Ni/Fe ratio higher than 1. ZrO2 is used as the support due to its acidic/basic features and CO2 adsorption abilities. Ni/ZrO2 catalysts with various amounts of tetragonal ZrO2 polymorph were prepared from amorphous Ni-Zr alloys [26]. The fraction of tetragonal ZrO2 increases with increasing nickel content, which subsequently influences the methanation activity. The tetragonal zirconia-supported nickel nanoparticles showed a higher turnover frequency (TOF, 5.43 s–1 at 473 K) and better CO2 adsorption than the monoclinic zirconia-supported nickel catalysts (TOF is 0.76 s–1 at 473 K). Ce-Zr binary oxides have been developed as automotive exhaust catalysts over the last decades. They contain lattic oxygen with high mobility for NOx reduction and can also activate H2O into H2 in the steam reforming process [27,28]. Perkas et al. developed Ni/meso-ZrO2 catalysts, in which the ZrO2 was modified with Ce and Sm cations [29]. The maximum porous volume and size were obtained with 30 mol% Ni loading, which coincidently exhibited higher catalytic activity for the methanation of CO2 (the TOF is 1.5 s–1 at 573 K). This could be ascribed to a synergistic effect of the increased surface area due to the mesopores structure in the support, which leads to the insertion of Ni particles into the pores, and the doping of the rare earth elements. The catalytic

Fig. 2 XRD patterns of Ni-Ce0.72Zr0.28O2 catalysts at various nickel loadings: (a) Ce0.72Zr0.28O2; (b) 5 wt-% Ni-Ce0.72Zr0.28O2; (c) 10 wt-% Ni-Ce0.72Zr0.28O2; (d) 15 wt-% Ni-Ce0.72Zr0.28O2 [30]

La2O3 has been used in Ni-based catalysts as an electronic modifier because it facilitates the dispersion of Ni and has the hexagonal crystalline structure with high K values that has an excellent capacity for charge trapping. A high, STY (1180 g$kg–1$h–1) of CH4 with both conversion and selectivity of 100% was obtained at a higher temperature (653 K) and gas hourly space velocity (GHSV, 11000 h–1) over 10 wt-% Ni/La2O3 [31]. However, under the same reaction conditions, the 10 wt-% Ni/γAl2O3 catalysts gave a low activity (the STY and conversion is 130 g$kg–1$h–1 and 6.9% respectively). The interaction between Ni and γ-Al2O3 was strong which resulted in difficult reduction of NiO to active Ni species. CO2 was activated on the La2O3 support (e.g., oxygen vacancies) and the adsorption or dissociation of hydrogen occurred on active sites originated from the interaction of Ni and La. Preparation method of catalyst has great effect on reactivity. For example, Guo et al. used glow discharge plasma to treat Ni-based catalysts, followed by calcination [32]. The treated Ni-La/γ-Al2O3 catalyst exhibited high activity at low temperature with CO2 conversion of 84.6% (57.4% for the conventional catalyst). Plasma treatment increased the surface area of the sample, favored the formation of small size crystals, and facilitated the high

Wei WANG et al. Methanation of carbon dioxide: an overview

dispersion and enrichment of the Ni species on the catalyst surface. 2.2

Noble metal catalysts

Ruthenium, although more expensive, is more active for CO2 methanation than nickel [33]. The problem of Nibased catalysts is its deactivation at low temperatures due to the interaction of the metal particles with carbon monoxide and formation of mobile nickel subcarbonyls [34]. Instead, ruthenium is stable at operating conditions. Ru/γ-Al2O3 was used as a probe catalyst to determine kinetic parameters of carbon dioxide methanation [35]. Apparent activation energy reaches a minimum (82.6 kJ/ mol) at ruthenium dispersion of 50%. Reaction order with respect to hydrogen decreased with the increase of H/Ru ratio, while reaction order with respect to CO2 changed slightly within examined dispersion range. Decrease in activation energies could be attributed to changes in heat of hydrogen adsorption and increase in amount of low coordination sites. Highly dispersed Ru nanoparticles were acquired on a TiO2 support prepared by a barrelsputtering method [36]. Remarkably, a 100% yield was achieved over this catalyst at 433 K, which was significantly higher compared to that prepared by conventional wet impregnation. Additionally, the methanation reaction over Ru/TiO2 prepared by barrel-sputtering can be proceeded at temperatures as low as 300 K with a reaction rate of 0.04 μmol$min–1$g–1. Effect of the support on the catalytic properties of Ru nanoparticles in CO2 hydrogenation has been studied by Kowalczyk et al. [37]. They found that the surface-based activities (e.g., TOF) of ruthenium are dependent on the Ru dispersion and the type of support for the metal. For high metal dispersion, the following order of TOFs (
103$s–1) for the reaction was obtained: Ru/Al2O3 (16.5) > Ru/ MgAl2O4 (8.8) > Ru/MgO (7.9) > Ru/C (2.5). It was suggested that the catalytic properties of very small ruthenium particles are strongly affected by metal-support interactions. In the case of Ru/C systems, the carbon moiety partially covers the metal surface, thus lowering the number of active sites (site blocking effect) [37]. Under steady-state conditions, the reaction rate determined for a highly loaded 15 wt-% Ru/Al2O3 sample was about 10 times higher than that obtained for the Ni-based system. Furthermore, Luo et al. have studied the effect of yttrium addition on the hydrogenation performance and surface properties of a Ru/sepiolite catalyst [38]. The presence of yttrium in Ru/sepiolite aids in increasing the catalytic activity and anti-poisoning capacity of the catalyst. The addition of yttrium increases the active surface area and the dispersion of ruthenium. Recently, a titania-nanotube-supported platinum (Pt/ Tnt) catalyst with a surface area of 187 m2/g has been prepared [39]. The catalyst contains scrolled multi-walled titania-nanotubing uniformly dispersed with Pt nanoparti-

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cles that are in the mixed-valence states as demonstrated by TPR and XPS. CO2-TPD results indicated that a large amount of CO2 adsorbed on the Pt/Tnt, which was ascribed to the synergetic effect of the tubular structure with high surface area and the mixed-valence Pt nanoparticles. The in situ FT-IR manifested that the Pt/Tnt was a highly active catalyst for the CO2 hydrogenation toward methane production at low temperature (Fig. 3).

Fig. 3 Variations of in situ FT-IR signal intensities with temperature raising (a) Pt/Tnt and (b) Pt/TiO2 [39]

Pd-Mg/SiO2 was investigated as a bifunctional catalyst for CO2 methanation motivated by the properties of Pd to dissociate molecular hydrogen and make available hydrogen atoms for the subsequent transfer and reaction with activated surface carbonate species formed by the reaction of CO2 on a Mg-containing oxide [40–44]. At 723 K, the Pd-Mg/SiO2 catalyst showed a high selectivity ( > 95%) to CH4 with CO2 conversion of 59%, whereas Pd supported on silica reduced CO2 primarily to CO, and Mg/SiO2 alone was inactive [45]. Replacing the Mg with Fe or Ni maintained the high activity for CO2 activation; however, the Fe-containing catalyst had little selectivity for methane. Since industrial feedstock typically contains a trace

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Front. Chem. Sci. Eng. 2011, 5(1): 2–10

amount of sulfur compounds, Szailer et al. have examined the effect of sulfur on methanation of CO2 [46]. A trace amount of H2S (e.g., 2.2
10–5) can promote the reaction on TiO2- and CeO2-supported metals (Ru, Rh, Pd), while on other supported catalysts (e.g., ZrO2-, MgO-) or when the H2S content was higher (1.16
10–4), the reaction was poisoned. IR, XPS, and TPD measurements demonstrated that the metal became apparently more positive when the catalysts were pretreated with H2S that diffused into the support. The promotion effect of H2S was explained by the formation of new active centers at the metal and support interface [46].

3

Reactor aspects

Most of the catalytic methanation of CO2 have been performed in fixed-bed reactors. However, applied electrochemistry can provide valuable cost efficient and environmentally friendly contributions to industrial process development with a minimum of waste production and toxic material [47–49]. There are many advantages for electrochemical processes, such as versatility, energy efficiency, and cost effectiveness. Electrochemical promotion of catalysis (EPOC) has been investigated for more than 70 catalytic systems using a variety of metal catalysts, solid electrolytes, and catalytic reactions [50]. CO2 hydrogenation using YSZ (Y2O3stabilized-ZrO2) solid electrolyte and Rh electrodes were studied in a single chamber reactor [51]. CO and CH4 were produced at temperatures of 346°C– 477°C, and the rate of formation of CH4 and CO was enhanced with positive potentials (electrophobic behavior) and negative potentials (electrophilic behavior), respectively. The maximum selectivity to CH4 was 35%. Moreover, a monolithic electropromoted reactor (MEPR) with up to 22 thin Rh/ YSZ/Pt or Cu/TiO2/YSZ/Au plate cells was used to investigate the hydrogenation of CO2 at atmospheric pressure and temperatures 220°C–380°C [52]. The Rh/ YSZ/Pt cells catalyzed CO and CH4 formation, and the open-circuit selectivity to CH4 wass less than 5%. Both positive and negative applied potentials significantly enhanced the total hydrogenation rate but the selectivity to CH4 remained below 12% [52]. The Cu/TiO2/YSZ/Au cells produced CO, CH4, and C2H4 with selectivities to CH4 and C2H4 up to 80% and 2%, respectively. The selective reduction of CO2 to CH4 started at 220°C with near 100% CH4 selectivity at open-circuit and under polarization conditions at temperatures 220°C–380°C. The results indicated the possibility of direct CO2 conversion in a MEPR via an electrochemical approach. The influence of different dopants in varying contents on the activity and selectivity of Ni-based methanation catalysts under proton exchange memebrane fuel cell (PEMFC) relevant conditions (hydrogen-rich gas reformate with low concentrations of CO and excess CO2) was

investigated [53]. The modification of Ni100Ox with 2.2 mol% of Re resulted in a catalyst highly inactive for both the methanation of CO and CO2 while the addition of Zr led to an enhanced CO and CO2 methanation activity. Further modification of Zr10Ni90Ox with small amounts of Re caused a drastically decreased reactivity toward the undesired hydrogenation of CO2 while that for CO was practically unchanged. Solo methanation experiments unambiguously revealed that this increase in selectivity was based on a loss of the intrinsic CO2 reactivity and not necessarily associated with any competition for active sites between the different kinds of CO2. As a part of the catalytically active Ni particles, Re changed the surface of the catalyst resulting in drastically altered catalytic properties while Zr or the Ni-ZrO2 interfacial region seemed to play a decisive role in the activation of the CO molecule.

4

Reaction mechanisms

Although the methanation of CO2 is a comparatively simple reaction, its mechanism appeared to be difficult to establish. There are differences of opinion on the nature of the intermediate compound involved in the ratedetermining step of the process and on the methane formation scheme. The reaction mechanisms proposed for CO2 methanation fall into two main categories. The first one involves the conversion of CO2 to CO prior to methanation, and the subsequent reaction follows the same mechanism as CO methanation [16,54–56]. The other one involves the direct hydrogenation of CO2 to methane without the formation of CO as intermediate [57,58]. Note that, even for CO methanation, there is still no consensus on the kinetics and mechanism. It has been proposed that the rate-limiting step is either the formation of the intermediate CHxO and its interaction with hydrogen or the formation of surface carbon in CO dissociation and its hydrogenation [59–62]. Steady-state transient measurements have been employed in the kinetic investigation on a Ru/TiO2 catalyst, couple with diffuse reflectance infrared spectroscopy and mass spectrometry [56]. The obtained information regarding surface intermediates and the gas phase components time evolution led to accurate identification of spectator species on the surface. A reaction mechanism (Scheme 1) was proposed including the formation step of the formate through a carbonate species. Reaction intermediates — carbon monoxide and formates — have been identified. The former was a key intermediate, and its hydrogenation led to methane formation. The latter was bound more strongly on the support, in equilibrium with the active formate species on the interface metal-support. The hydrogenation of the adsorbed carbon monoxide was presented in a lumped form involving six adsorbed hydrogens, which was obviously not an elementary

Wei WANG et al. Methanation of carbon dioxide: an overview

reaction, but the different hydrogenation steps cannot be distinguished by infrared spectroscopy. A pathway involving hydrogen carbonate was presented for the formation of the interfacial formate because DRIFTS experiments indicated that this species was formed on the support during the reaction and its transient response was consistent with the response of a carbon monoxide precursor [56].

CO2ads ↕ ↓COads þ Oads COads ↕ ↓Cads þ Oads 2COads ↕ ↓Cads þ CO2gas Cads þ Hads ↕ ↓CHads CHads þ Hads ↕ ↓CH2ads CH2ads þ 2Hads ↕ ↓CH4gas

S: the support, M: the metal, I: the metal-support interface

Scheme 1 The proposed reaction mechanism of CO2 methanation [56]

On the basis of isotope steady-state and non-steady-state kinetic investigation, the mechanism of CO2 methanation on copper and nickel catalysts has been examined [63]. Only CO and H2O were formed on the copper catalyst. Hydrogen was adsorbed in the dissociated form and CO2, which was strongly adsorbed, had a decelerating effect on the reaction rate. In contrast, CO2 on the nickel catalyst did not retard the reactions of CO and CH4 formation. A formate complex and hydrogen were involved in the ratelimiting step of methane formation on the nickel catalyst. The hydrogenation of carbon fragments was a fast process and the CH4 formation followed the consecutive scheme. Surface science approach from model catalysts is commonly helpful in understanding mechanistic aspects of reactions. Ni single crystals have been shown to be reasonable models of practical catalysts for methanation [64,65]. Peebles et al. studied the methanation and dissociation of CO2 on Ni(100) surface [16]. Activation energies of 88.7 and 72.8–82.4 kJ/mol were acquired for the formation of CH4 and CO, respectively. The activation energy and reaction rates for CH4 formation from CO2 were very close to the values for methanation of CO under identical reaction conditions. The results supported the mechanism that CO2 was converted to CO and then to carbon before hydrogenation. Using the ASED-MD (atom superposition and electron delocalization-molecular orbital) theory, Choe et al. investigated the CO2 methannation on a Ni(111) surface [66]. The elementary reaction steps are listed below.

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Step  1 Step  2 Step  3 Step  4 Step  5 Step  6

These elementary steps can be considered to consist of two mechanisms — carbon formation and carbon methanation. For the carbon formation mechanism, the activation energies were calculated to be 1.27 eV for CO2 dissociation, and 2.97 eV for the CO dissociation. For carbon methanation mechanism, the following activation energies were also calculated: 0.72 eV for methylidyne, 0.52 eV for methylene, and 0.50 eV for methane. Thus, the CO dissociation (Step 2) was the rate-determining of the process. In aiming to provide clear insights into the role of Pd and MgO, Kim et al. used computational and experimental methods investigating the reaction mechanism of CO2 methanation on Pd-MgO/SiO2 [67]. Density functional theory (DFT) calculations showed that MgO initiated the reaction by binding a CO2 molecule, forming a magnesium carbonate species on the surface. Pd species dissociates H2 molecule and supplies H atoms, essential for further hydrogenation of the oxygen species of the carbonate and residual C atom. The CO2-TPD results are consistent with DFT calculations that MgO initiates the reaction by binding CO2 molecules. A bifunctional mechanism was proposed that Pd similarly provides atomic hydrogen to Mg carbonates to form methane, and upon the desorption of the methane, the carbonate is reformed by gas phase CO2 (Scheme 2). Interestingly, it has been demonstrated that CO2 can react with H2 in the presence of Rh/γ-Al2O3 to produce methane at room temperature and atmospheric pressure with a high selectivity (99.9%–100%), even without photoexcitation [68]. The results showed that the recycling of CO2 to produce methane (chemicals), under friendly conditions (room temperature and atmospheric pressure), could be possible in the near future. The produced methane could be injected in petrochemical and/or chemical industries. Therefore, insight on the reaction mechanism is necessary to optimize the process and improve the performances. Jacquemin et al. looked into the reaction mechanism of CO2 methanation on the Rh/γ-Al2O3 catalyst to better understand this process [69]. Methane was the only hydrocarbon product which was observed in mass spectrometry analysis. The dissociative adsorption of CO2 into CO and oxygen on the surface of the catalyst has

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Front. Chem. Sci. Eng. 2011, 5(1): 2–10

Scheme 2 A potential bifunctional mechanism for Pd-Mg/SiO2 [45]

been evidenced by in situ DRIFT experiments (Fig. 4). The formation of CO(ads) was indeed confirmed by the presence of the bands corresponding to linear Rh-CO (2048 cm–1), Rh3+-CO (2123 cm–1) and gem-dicarbonyl Rh-(CO)2 (2024 and 2092 cm–1) [69]. CO2 adsorbed as gemdicarbonyl and CO associated with oxidized Rh were the most reactive species with hydrogen. Oxidation of rhodium was observed during the reaction which was correlated to the deactivation of the catalyst.

in the world’s population and improvement in living standards. Although various physical and chemical techniques have been proposed for the fixation of exhausted CO2, such as fixation in carbonates, geological or ocean storage, or afforestation, their immediate practical application has drawbacks in terms of economic factors, safety, efficiency, and reliability. The use of CO2 as a renewable and environmentally friendly source of carbon is a highly attractive approach. Various strategic considerations, technical approaches, and specific research directions have been presented in the methanation of CO2. The prevalent view of the active site is the synergy between the primary catalyst and support or promoter. Because the catalyst for CO2 methanation is sensitive to the structure of the catalyst, the preparation method, preparation conditions, and the component significantly influence its performance. An optimum catalyst should have a high specific surface area and ultrafine or nanostructured particles of the metal active sites. It is important to systematically characterize the nature of the active sites and interactions among active components, support, and promoter to be able to tailor the structure of the catalyst. The utilization of the new reactors and new catalysts for lower temperature synthesis would be a development trend. For practical CO2 utilization, however, better understanding of reaction mechanisms at atomic/molecular lever is necessary for rational design of high performance catalyst. As a green carbon source and renewable feedstock, the methanation of CO2 definitely has a promising future. Acknowledgements Financial support from the National Natural Science Foundation of China (Grant Nos. 21006068, 21050110425), Seed Foundation of Tianjin University (60303002), and the Program of Introducing Talents of Discipline to Universities (B06006) is gratefully acknowledged.

References

Fig. 4 DRIFT results after adsorption of CO2 and CO and after adsorption of CO2 and reaction with hydrogen [69]

5

Summaries and outlook

Carbon dioxide has been recognized as a major factor responsible for the greenhouse effect. The increasing amounts of CO2 in the atmosphere are causing climate changes on a global scale. Therefore, a reduction in CO2 emission into the atmosphere is an urgent necessity. However, the levels of CO2 are increasing with increasing

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