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

•OH− or e− encaged in mayenites promotes the activation of CO2 on the supports.•The catalytic activity of Ru/C12A7:OH− is drastically higher than that of Ru/C12A7:e− and Ru/γ-Al2O3.•*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).