Adsorbed CO2‑Mediated CO2 Photoconversion Cycle into Solar Fuel at the O Vacancy Site of Zirconium Oxide

ZrO2 photoreduces 13CO2 under ultraviolet–visible light to 13C-product(s) negligibly affected by adventitious carbon. The dual-site reaction pathway was theoretically clarified from CO2 to CO using monoclinic ZrO2, followed by multiple hydrogenation steps to methane over Ni nanoparticles. The oxygen vacancy (VO ••) played essential roles, including the M-shaped CO2 adsorption over the VO •• site and the direct occupation of the VO •• site by the dissociated O and/or hydroxy group from the hydroxycarbonyl species favorably on the ZrO2(111) surface. The rate-limiting step was for the regeneration of the VO •• site with an activation energy (E act) of 2.6 eV, but the water desorption energy was greatly compensated by the CO2 adsorption energy at the VO •• site, in contrast to the first-row transition-metal oxides. The COH and/or CO species transfer from ZrO2 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.