Adsorbed CO<sub>2</sub>-Mediated CO<sub>2</sub> Photoconversion Cycle into Solar Fuel at the O Vacancy Site of Zirconium Oxide
Keisuke Hara, Misa Nozaki, Rumiko Hirayama, Rento Ishii, Kaori Niki, Yasuo Izumi
Abstract
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.