CO₂ methanation reaction pathways over unpromoted and NaNO₃-promoted Ru/Al₂O₃ catalysts

Catalytic CO₂ sorbents, materials that adsorb and pre-concentrate CO₂ on the catalyst surface prior to subsequent conversion, are becoming important materials in CO₂ capture and utilization. In this work, a prototypical CO₂ methanation catalyst - Ru/Al₂O₃ - and a related catalytic sorbent - NaNO₃/Ru/Al₂O₃ - are used for CO₂ methanation in flowing hydrogen in a fixed bed reactor at temperatures ranging from 220 to 280 °C. Activation energies for the NaNO₃/Ru/Al₂O₃ material are slightly higher than unpromoted Ru/Al₂O₃ catalysts, and the reaction orders vary more significantly. In situ IR spectroscopy and steady-state isotopic kinetic analysis (SSITKA) using in situ IR/MS spectroscopy show that bicarbonate and linear carbonyl species are the likely reaction intermediates over unpromoted Ru/Al₂O₃, while bidentate carbonate, formate and linear carbonyl species are among likely reaction intermediates over NaNO₃/Ru/Al₂O₃. Rate laws consistent with the obtained experimental data are proposed after kinetic modeling of multiple plausible reaction pathways. Results suggest that the pathway over the NaNO₃/Ru/Al₂O₃ catalyst likely has an additional kinetically relevant irreversible step in the CO₂ methanation reaction pathway.