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...