ZrO 2 photoreduces 13 CO 2 under ultraviolet−visible light to 13 C-product(s) negligibly affected by adventitious carbon. The dual-site reaction pathway was theoretically clarified from CO 2 to CO using monoclinic ZrO 2 , followed by multiple hydrogenation steps to methane over Ni nanoparticles. The oxygen vacancy (V O•• ) played essential roles, including the M-shaped CO 2 adsorption over the V O•• site and the direct occupation of the V O •• site by the dissociated O and/or hydroxy group from the hydroxycarbonyl species favorably on the ZrO 2 (111) surface. The rate-limiting step was for the regeneration of the V O•• site with an activation energy (E act ) of 2.6 eV, but the water desorption energy was greatly compensated by the CO 2 adsorption energy at the V O•• site, in contrast to the first-row transition-metal oxides. The COH and/or CO species transfer from ZrO 2 to Ni in a concerted mechanism was energetically favorable, and the apparent E act value from hydroxycarbonyl species to methane was reduced to 0.67 eV.

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