Survey of Heterogeneous Catalysts for the CO 2 Reduction to CO via Reverse Water Gas Shift

Abstract

Conversion of CO2 to CO using H2, known as the reverse water gas shift (RWGS), is one of the promising ways to utilize CO2 as a renewable feedstock. This chapter reviews the performance of different heterogeneous catalysts reported so far for RWGS reaction. CO or syngas is already being used as a feedstock for the production of many chemicals and fuels. Different categories of catalysts such as supported noble metal catalysts (Au, Pd, Pt, Rh, and Ru), supported non-noble metal catalysts (Cu and Ni), bimetallic catalysts, oxide systems such as perovskites, BaFe–hexaaluminate catalysts, iron-based oxides, and cobalt-based oxides and transition metal carbides have been studied for RWGS reaction. Supported metal catalysts with dual functionalities from the support and the metal sites that can activate CO2 and dissociate H2 in a synergistic manner are extensively studied for the RWGS reaction. A range of supports such as Al2O3, TiO2, CeO2, SiO2, ZnO, MoOx, ZrO2, MgO, zeolites, and Fe2O3; transition metal carbide (TMC), SBA-15, and a metal–organic framework (MOF) were used to stabilize different metal nanoparticles and to achieve high metal dispersion and optimum particle size so as to enhance their catalytic performance with maximum CO selectivity. The structural and electronic characteristics of the metal and support, particle size, as well as the dispersion of the active metal component, metal loading, metal–support interaction, and nature of active species formed at the metal–support interfacial sites are all major factors that influence the catalytic activity and long-term stability for RWGS reaction. The supported noble metal catalysts, particularly Pt, Rh, and Ru, are resistant to coking and corrosion. They also display high activity toward H2 dissociation, CO2 conversions close to equilibrium, and avoid the formation of the main side product CH4 above 500°C. However, the high cost and limited availability of noble metals necessitate the development of cheaper and more abundant non-noble metal catalysts, particularly 3d transition elements such as Cu and Ni. However, Cu nanoparticles have poor thermal stability and have strong tendency to sinter at reaction temperatures above 300°C, whereas Ni-based catalysts give relatively low CO selectivity because of the favorable methanation reaction during the hydrogenation of CO2. Efforts have been made to improve the activity, stability, and CO selectivity over Cu- and Ni-based catalysts, such as by dispersing the active metal components on different kinds of supports, alloying Cu or Ni species with other metals (Ru, Zn, Fe, etc.), using promoters such as Cs and K, controlling the metal particle size, metal loading, the oxidation state of metal, and the metal–support interaction. Transition metal carbides (TMCs) with noble metal-like properties are also reported in the literature, which are active at low temperatures with relatively lower CO selectivity. They can also act as a promising support for dispersing active metal species for RWGS. Metal oxides are attractive for RWGS reaction because of their low cost, resistance to sintering when compared to noble metals, and metal-supported catalysts. Among the metal oxide catalysts, perovskite (ABO3) compounds have been effective and emerge as one of the potential systems for RWGS reaction because of their structural flexibility, thermal stability, high oxygen mobility and stability under nonstoichiometric conditions, and harsh reaction conditions. Two different mechanisms have been postulated for RWGS reaction in the literature, namely, dissociative (also known as regenerative or redox mechanism) and associative. In the redox (dissociative) mechanism, the reactants are oxidized or reduced separately on the catalyst surface, whereas in the associative mechanism, the reaction occurs via a surface intermediate such as bicarbonate species from adsorbed CO2, followed by its conversion to formate species in the presence of hydrogen.