In situ/operando spectroscopic evidence on associative redox mechanism for periodic unsteady-state water-gas shift reaction on Au/CeO2 catalyst

Cyclic and repeatable CO2/H-2 formation under periodic CO (sic) H2O feeds, that is unsteady-state water-gas shift reaction (uss-WGS), was found to be catalyzed by a gold nanoparticles-loaded CeO2 (Au/CeO2) catalyst. Kinetics of the CO-reduction of Ce4+ to Ce3+ and Ce3+ reoxidation by H2O in combination with transient CO2/H-2 formation over Au/CeO2 were studied by operando ultraviolet-visible (UV-vis) and infrared (IR) spectroscopies at 175 degrees C. The Ce4+-OH species were reduced by CO to give Ce3+-square-Ce3+ (square: oxygen vacancy) and gas phase products (H-2, CO2). The Ce3+-square-Ce3+ species were oxidized by H2O to give H-2 and Ce4+-OH species. The reduction and reoxidation rates of Ce4+/Ce3+ redox couple were close to the rates of transient CO2/H-2 formation. The X-ray absorption spectroscopy results showed that the oxidation states of Au remained unchanged during the redox reactions. These results indicate that the uss-WGS is primary driven by the Ce4+/Ce3+ redox couple. Combined with the observation of adsorbed carboxylate intermediates as a precursor of H-2 and CO2, associative redox mechanism is proposed as the main pathway for the unsteady-state WGS reaction on Au/CeO2 at 175 degrees C.