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 degrees 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.