Deciphering the role of chemisorbed CO in CO2 methanation: kinetic and mechanistic investigation over monometallic (Ru) and bimetallic (Ru–Ni) catalysts†

Abstract

Supported metal catalysts have made prominent contributions to CO₂ mitigation through conversion into useful chemicals. However, intermediates and mechanisms involved in this process remain ambiguous. Herein, we present the kinetics, mechanistic route and impact of chemisorbed CO in CO₂ methanation on Ru/γ-Al₂O₃ and Ru–Ni/γ-Al₂O₃ catalysts. Both the catalysts show minimal variation in adsorbed species on changing the duration of reduction, as confirmed through in situ IR spectroscopy. A notable observation is that the adsorbed CO exhibits a red shift at a longer reduction time and a more reactive nature on the Ru/γ-Al₂O₃ surface. Conversely, stable bridged CO mode is detected on Ru–Ni/γ-Al₂O₃ under similar conditions, leading to catalyst poisoning in all instances. This indicates that pre-reduction duration does not have much effect on the surface but interference of CO has more effect at lower concentrations of reactant gases. In situ XRD analysis reveals limited changes in the metallic or mixed oxide species during these conditions. Reaction kinetic analysis showed that Ru–Ni/γ-Al₂O₃ has better rate performance at higher concentrations of CO₂, whereas Ru/γ-Al₂O₃ exhibits better rate performance at lower concentrations. The activation energy was found to be 74.07 kJ per mole for Ru/γ-Al₂O₃ and 89.38 kJ per mole for Ru–Ni/γ-Al₂O₃. The turnover frequency (TOF) is directly proportional to the rate of formation of methane.