CO2 methanation over Ru/12CaO∙7Al2O3 catalysts: Effect of encaged anions on catalytic mechanism.

• OH− or e− encaged in mayenites promotes the activation of CO 2 on the supports. • The catalytic activity of Ru/C12A7:OH− is drastically higher than that of Ru/C12A7:e− and Ru/ γ -Al 2 O 3. • *CO adsorbed on Ru is the vital intermediate for methane formation and carbon deposition. • Electrons in mayenite electrides are transferred to Ru metal, thus accelerating the dissociation of *CO into *C (carbon deposition). Mayenites (C12A7) encaged with OH− or e− have been synthesized and applied as supports for ruthenium catalyst in CO 2 methanation. The catalytic activity of Ru/C12A7:OH− (TOF at 225 °C = 6.04 × 10-2 s-1, E a =70.9 kJ mol-1) is drastically higher than that of Ru/C12A7:e− (TOF at 225 °C = 2.41 × 10-2 s-1, E a =94.4 kJ mol-1) and Ru/ γ -Al 2 O 3 (TOF at 225 °C = 0.23 × 10-2 s-1, E a =105.8 kJ mol-1). It has been proposed that the OH− or e− encaged in mayenite promotes the formation of carbonates and bicarbonates from CO 2 on the supports. It is supposed that at the interface between Ru and mayenite support the surface carbonates and bicarbonates are hydrogenated into formates, which are further dissociated into *CO adsorbed on Ru. The formed *CO is the intermediate for methane formation and carbon deposition. The different anions (OH− or e−) encaged in the framework of mayenites have been found to influence drastically the ionization potential of Ru. Electrons in mayenite electrides are transferred to Ru metal, thereby enhancing the back donation from Ru to the antibonding orbitals of *CO intermediate. As a consequence, the dissociation of *CO into *C and subsequent into deposition of carbons are accelerated, which is responsible for the inferior catalytic performance of Ru/C12A7:e−. Based on the experimental results, we propose an effective technique for tailoring the catalytic performance of metallic sites supported on mayenites via changing encaged anions. [ABSTRACT FROM AUTHOR]