Understanding the CO2 methanation mechanism on nanostructured bimetallic Ru/NiO–CeO2 catalysts

• Oxygen vacancies are created on Ni 2+ –CeO 2 interface where CO 2 is evenly chemisorbed and dissociated. • CO 2 exchanges oxygen with the catalysts and carbon intermediates are simultaneously reduced to CH 4 . • The most active catalyst has the best balance between CO 2 chemisorption and hydrogenation of reaction intermediates. • Ru 0 remains stable under reaction conditions. The CO 2 methanation reaction mechanism has been studied by isotopic pulse experiments followed by rapid scan DRIFTS-MS and NAP-XPS using synchrotron radiation for mono- and bimetallic Ru/NiO–CeO 2 catalysts. CeO 2 and NiO–CeO 2 (10 % wt. Ni) nanoparticles (NP; 7–8 nm) were prepared by reversed microemulsion, and Ru (1.6 % wt.) was impregnated afterwards. The reaction mechanism starts with the reduction of the catalysts by H 2 , H 2 O release and generation of a high population of oxygen vacancies on the catalysts surface where CO 2 is evenly chemisorbed and dissociated. The exchange of oxygen atoms between the chemisorbed CO 2 molecules and the catalysts occurs simultaneously to the hydrogenation of the carbon intermediates to yield CH 4 . The most active catalyst (Ru/NiO–CeO 2 (NP)) has the best balance between CO 2 chemisorption and further hydrogenation of the reaction intermediates. NAP-XPS experiments confirm the redox participation of the Ni 2+ –CeO 2 interface in the chemisorption and dissociation of CO 2 , and the importance of the Ru 0 stability under reaction conditions. Ru/NiO–CeO 2 (NP) combines both features, while Ru 0 is not stable and is partially oxidised under reaction conditions on Ru/CeO 2 and on a counterpart Ru/NiO–CeO 2 catalyst prepared without particle size control. Rapid scan DRIFTS-MS confirms the efficient CO 2 chemisorption and fast hydrogenation of the reaction intermediates observed (mainly carbonyls and bicarbonates) on Ru/NiO–CeO 2 (NP), while the counterpart catalyst prepared without size control is not able to chemisorb CO 2 so efficiently and the Ni-free catalyst is less active for CO 2 methanation because the surface carbon species created upon CO 2 chemisorption are more difficult to be hydrogenated.