CHAPTER 1
Advances in methanation catalysis
Hao Wang,a Yan Pei,a Minghua Qiao*a and Baoning Zong
DOI: 10.1039/9781788010634-00001
The hydrogenation of CO to CH4, the CO methanation reaction, has attracted considerable attention due to energy and environment concerns. The reaction is thermodynamically favourable, however, the catalyst should show appreciable activity and durability in this highly exothermic reaction. This chapter focuses on recent advances in methanation catalysis, addressing on the roles of the metals, supports, promoters, the reaction and deactivation mechanisms, and the reactor types, with the aim to provide a foundation for the rational design of CO methanation catalysts and processes that enables the production of synthetic natural gas (SNG) in a more economic and greener manner.
1 Introduction
Fossil fuels, such as petroleum, coal, and natural gas, are the major energy sources in industrial production and in our daily life. Among them, natural gas, which contains mainly CH4, is recognized as a clean energy carrier due to its high conversion efficiency, high calorific value, and environmental friendliness, as CH4 is completely combusted with smoke-free and slag-free characteristics. Alternatively, as illustrated in Fig. 1, CH4 is massively produced from coal and, more recently, biomass, that are firstly converted to synthesis gas (syngas, a mixture of CO and H2), followed by the methanation reaction (CO + 3H2 = CH4 + H2O, ?H298 K = -206.1 kJmol-1, AG =-141.8 kJmol-1). CH4 of this origin is also called SNG. The SNG can be readily transported and distributed by the existing natural gas pipeline grids, thus greatly lowering the utilization expenditure of coal and biomass. Another important application of the methanation reaction in the chemical industry is to remove trace amount of CO from the H2-rich gas for polymer electrolyte membrane fuel cells (PEMFCs) that have the advantages of high density and zero emission. The methanation reaction is also important in the purification of reformate for NH synthesis and in Fischer–Tropsch synthesis (FTS). It is worth mentioning that methane steam reforming (MSR) is the reverse reaction of methanation. A good catalyst for methanation is a good catalyst for MSR, as vice versa, and hence sharing similar deactivation mechanisms.
The methanation reaction was discovered by Sabatier and Senderens in as early as 1902, however, it came into the view of industry in the late 1970s during the oil crisis and gains renewed interests in recent years because of the huge demand and uneven distribution of the natural gas reserve. The main reactions involved in the methanation process are compiled in Table 1, and the corresponding equilibrium constants at different temperatures are plotted in Fig. 2. The CO methanation reaction is thermodynamically feasible, highly exothermic, favored at low temperatures, while limited at high temperatures. However, it is kinetically favored at high temperatures on most catalysts. Therefore, various catalysts, especially those based on the VIII group metals have been prepared and evaluated to achieve high activity and stability, among which Ni and Ru have been the most intensively studied. Promoters are also necessary to afford the catalysts with functions such as high sulfur resistance, and anti-sintering and anti-coking properties.
This chapter focuses on the transformation of syngas to SNG through the methanation reaction. In addition to the Introduction section and the Summary and Outlook section on the challenges and future opportunities in methanation catalysis, the second section discusses the roles of the catalyst components for CO methanation, including the metals, supports, and promoters. The third section presents the methanation mechanisms, mainly the direct dissociation mechanism and the hydrogen-assisted CO dissociation mechanism. The fourth section summarizes the catalyst deactivation mechanisms during CO methanation, including sulfur poisoning, carbon deposition, sintering, and Ni(CO)4 formation. The fifth section introduces the characteristics of the fixed-bed reactor, fluidized-bed reactor, and slurry-bed reactor in the CO methanation reaction.
2 Catalyst
2.1 Metal
Supported transition metal catalysts (Ni, Co, Fe, Ru, Rh, Pd, Os, Ir, and Pt) have been extensively studied in CO methanation. Fischer et al. ranked the methanation activities on the VIII group metals in 1925 with the order of Ru>Ir>Rh>Ni>Co>Os>Pt>Fe>Pd. In 1975, Vannice expressed the activity with respective to the number of surface metal atoms, and arrived at a different order of Ru>Fe>Ni>Co>Rh> Pd >Pt>Ir. The kinetic data were also obtained under well-defined experimental conditions, and the methanation reaction was described by a power-law equation with the form of [MATHEMATICAL EXPRESSION OMITTED]. Nevertheless, in light of these pioneering works, it is generally concluded that Ru is the most active metal for CO methanation. However, the use of Ni as the active metal for CO methanation is more preferred and has been heavily investigated, as it shows appreciably high activity and is much less costly than the noble metals.
2.2.1 Ni. The catalytic performance of the Ni-based catalysts in CO methanation depends on various parameters, including metal loading, preparation method, support, and promoter. The effect of Ni loading influences both its interaction with the support and its particle size and dispersion, thus affecting the catalytic behavior in CO methanation. Zhao et al.27 prepared the Ni-Al2O3 catalysts with the Ni loadings from 10 to 50 wt%. It is found that the catalytic activity in CO methanation is sensitive to the Ni particle size, and a maximum production rate of CH4 per unit mass of Ni was observed on Ni particles around 41.8 nm. Larger Ni particles have lower active surface area for CO conversion, while smaller Ni particles have more step sites that are easily covered by carbon, which causes fast catalyst deactivation. Hwang et al. developed the Ni-Al2O3 xerogel catalysts with the Ni loadings from 20 wt% to 60 wt%, and found that the CO conversion and the CH yield drastically increased with the Ni loading from 20 wt% to 40 wt%, and then became almost constant with the Ni loading above 40 wt%. Gao et al. prepared three Ni/a-Al2O3 catalysts with the Ni particle size in the ranges of 5–10, 10–20, and 20–35 nm, and found that the catalyst with the medium particle size showed the best catalytic performance and least carbon deposition. This result indicates that the dissociation and hydrogenation of CO may more easily occur on Ni particles in a proper size range giving rise to more terrace sites.
The preparation methods affect the chemical and physical properties of the catalysts, such as the Ni loading, surface area, particle size and distribution, microstructure, and electronic property. Therefore, they also have great influence on the catalytic activity. Various methods have been employed to prepare the methanation catalysts. The Ni-based catalysts are usually prepared by the impregnation, co-precipitation, sol-gel, and mechanical mixing methods. Other methods, such as deposition-precipitation, hydrothermal synthesis, hard-templating, dual templating, and solution combustion, have also been used to prepare the methanation catalysts. The effects of the support and the promoter will be discussed in Sections 2.2 and 2.3, respectively.
2.1.2 Co. A number of works have been carried out on the catalytic performance of Co3O4 in low-temperature CO methanation, FTS, low-temperature CO oxidation, and reduction of NOx. Co3O4 has a spinel structure with Co3+ in octahedral coordination geometry and Co2+ in tetrahedral coordination geometry. Theoretical and experimental works confirmed that the Co3+ cations in Co3O4 are the only sites favorable for CO adsorption. It is known that metallic Co is the active phase for FTS, while for low-temperature CO methanation, Zhu et al. believed that both Co2+ and Co3+ are active sites. However, Wang et al. suggested that CoO is the active phase for CO methanation. They found that the catalyst was active only when CoO was present. Without CoO, neither Co3O4 nor metallic Co could catalyze CO methanation. Co was also combined with Ni to prepare bimetallic catalysts to improve the activity and stability. Yu et al. prepared the SiC-supported bimetallic Ni–Co catalysts with different Ni/Co ratios. The interaction between Ni and Co and higher metal dispersion can enhance the adsorption and activation of CO, thus improving the methanation activity.
2.1.3 Fe. Fe is a well-known active component in catalysts for FTS. However, it was reported that the Fe-based catalysts are usually much less active and more prone to carbon deposition in CO methanation. Therefore, Fe is generally used as a second metal to Ni to prepare alloy or bimetallic catalysts. Density functional theory (DFT) calculations predicted that the Ni–Fe alloys are more active than the traditional Ni-based catalyst in CO methanation. On MgAl2O4 and Al2O3 supports, Kustov et al. found that the bimetallic catalysts with compositions of 25Fe75Ni and 50Fe50Ni showed much higher activities and in some cases also higher selectivities to CH4 than the monometallic Ni and Fe catalysts. Tian et al. studied the catalytic performance under the industrial total methanation conditions using a bimetallic Ni-Fe/?-Al2O3 catalyst. The results showed that the promotion by Fe effectively improved the activity of the Ni/[gamma-Al2O3 catalyst. Hwang et al. prepared the mesoporous Ni–M–Al2O3 (M = Fe, Co, Ce, and La) xerogel catalysts, among which the bimetallic Ni–Fe catalyst exhibited the best catalytic performance in terms of CO conversion and CH4 yield. The enhanced catalytic performance of the Ni–Fe catalyst was ascribed to the lowest CO dissociation energy and the largest H2 adsorption capacity arising from the formation of the Ni–Fe alloy and the improved reducibility.
2.1.4 Ru. The Ru-based catalysts are known to be more active and stable than the Ni-based catalysts and exhibit high activity even at low temperatures. Masini et al. found that on mass-selected Ru nanoparticles (NPs) deposited onto the planar SiO2, the TOF of CO methanation increased with the diameter of the Ru NPs in the range of 4-10 nm and pointed out the importance of the coordinatively unsaturated sites. Takenaka et al. concluded that the catalytic activities of the supported Ru catalysts strongly depended on the types of the supports, and the Ru/TiO2 catalyst was more suitable for CO methanation. Abdel-Mageed et al. studied on a set of Ru/TiO2 catalysts with similar Ru loading, Ru particle size, TiO2 crystal phase, but with very different surface areas. It was found that the catalytic activity strongly depended on the surface area of TiO2, but the selectivity was mainly determined by the Ru particle size. In situ infrared measurements further revealed that the CO adsorption strength changed significantly with the increase in the surface area, and the strong metal–support interaction led to partial overgrowth of the Ru NPs. Kinetic and in situ DRIFTS measurements showed that both the Ru/zeolite and Ru/Al2O3 catalysts were active and selective for CO methanation even at low temperature (190 °C). When using zeolite as the support, Ru showed significantly higher activity and selectivity, attributable to the weak Ru0–CO bonding arising from the interaction between the acidic support and Ru and the stabilization of very small metallic and oxidic Ru particles.
A great deal of efforts were devoted to selective CO methanation in the presence of H2O or CO2 in realistic reformates over the Ru-based catalysts. Panagiotopoulou et al. reported that the catalytic activity of Ru was not affected by H2O in the feed gas over the Ru/A2lO3 catalyst. Abdel-Mageed et al. investigated the impact of realistic high HO contents up to 30% on selective CO methanation in CO2-rich reformates on the supported Ru catalysts. They found that H2O did not change the selectivity on the Ru/zeolite catalyst, but decreased the activity, which was tentatively explained by the site-blocking effect due to H2O adsorption.
2.1.5 Other noble metals. For the Pd-based catalysts, the activity and selectivity in the hydrogenation of CO have been shown to depend on the types of the supports. Shen et al. studied the hydrogenation of CO over the Pd catalysts supported on Al2O3, SiO2, TiO2, and ZrO2. Interestingly, the Pd/Al2O3 catalyst mainly produced dimethyl ether, the Pd/SiO2 and Pd/ZrO2 catalysts favored the formation of methanol, while the Pd/TiO2 catalyst preferred the production of CH4. The higher CH4 yield on the Pd/TiO2 catalyst was ascribed to the presence of cationic Pd species stabilized by the support. For Pt, the monometallic catalysts are poor for CO methanation, since it promotes the undesired water-gas shift (WGS) reaction. However, Pt was reported as a good promoter for the Ni catalysts, which improved the activity in CO methanation. For the Rh-based catalysts, some works identified CH4 and other light hydrocarbons as the main products, while others observed considerable selectivity to C2+ oxygenates, such as acetaldehyde and ethanol.
2.2 Support
The supports are usually indispensable to the metal catalysts. The supports play an important role in the catalytic properties by affecting the metal dispersion, thermal stability, and electronic structure. Hence, the supports are one of the research focuses in CO methanation.
2.2.1 Al2O3. Al2O3 is the most commonly used support in CO methanation due to its diversified crystal forms and textural and chemical properties. In general, Al2O3 in different crystal forms can be synthesized through thermal dehydration of aluminum trihydroxide and aluminum oxyhydroxide at different temperatures. Because ?-Al2O3 has large surface area, well-developed pore structure, and wellcharacterized acidic property, the ?-Al2O3-supported Ni-based catalysts have been extensively studied in CO methanation. However, the Ni/?-Al2O3 catalyst always suffered from some drawbacks. ?-Al2O3 tends to undergo phase transformation at high temperatures, which often results in the collapse of the pore structure and catalyst sintering. The surface acidity of ?-Al2O3 tends to induce carbon deposition during CO methanation. Moreover, Ni is inclined to form the NiAl2O4 spinel phase with Al2O3, which is difficult to reduce even at high temperatures, thus lowering the catalytic activity.
Considering the drawbacks of ?-Al2O3, some studies turned to synthesize mesostructured ?-Al2O3 to improve its surface and textural properties. Cao et al. reported an approach to synthesize ordered mesoporous Ni/Al2O3 catalysts with high thermal stability using the evaporation-induced self-assembly (EISA) method. Ma et al. reported that the Ni/Al2O3 catalysts with a coral-reef morphology prepared by the co-precipitation method exhibited high activity and superior resistance to deactivation. Li et al. synthesized a macro-mesoporous Ni/Al2O3 catalyst by the sol–gel method in conjugation with supercritical drying treatment, which showed much higher activity and thermal stability in CO methanation. Liu et al. synthesized ordered mesoporous Ni–Zr–Al composite catalysts and used them in CO methanation. The Zr species effectively prevented the catalyst from structural collapse due to sintering and phase transition during the hydrothermal treatment.
Some researchers paid attention to the more stable, acid-free, and inert a-Al2O3 as the catalyst support. Gao et al.29 investigated the influence of the particle size of Ni on a-Al2O3 in CO methanation, and demonstrated that by properly controlling the Ni particle size, it is feasible to use the Ni/a-Al2O3 catalysts in CO methanation. However, the surface area of a-Al2O3 is too small to achieve a high metal dispersion. Therefore, Liu et al. developed a high-surface-area Ni/a-Al2O3 catalyst by the modified impregnation method, which exhibited better thermal stability and higher resistance to sintering than the low-surface-area a-Al2O3-supported counterpart.
2.2.2 SiO2 and mesoporous silica. SiO2 has attracted considerable attention over the past several years because of its excellent chemical resistance, controllable size, and high specific surface area and pore volume. SiO2 as the support for the Ni-based catalysts has been widely studied in CO methanation. For example, the Ni/SiO2 catalysts prepared by the sol-gel method exhibited excellent catalytic performance in CO methantion. The Si–Ni intermetallic compounds supported on SiO2 presented high activity for CO methanation and significantly higher resistance to sintering.
