The influence of size, metal loading and oxygen vacancies on the catalytic performance of Au/CeO2-x in the sunlight-powered reverse water gas shift reaction

This study reports the conversion of CO2 and H2 to CO and H2O at low temperature and low pressure (up to 203 degrees C, p = 3.5 bar) using plasmonic Au/CeO2-x photocatalysts, with mildly concentrated sunlight as the sole energy source (up to 9 kW m-2). Systematic catalytic studies were carried out by varying the CeO2-x particle size, Au particle size and loading, and the concentration of oxygen vacancies. Upon illumination, all Au/CeO2-x catalysts showed a CO production of up to 2.6 +/- 0.2 mmol CO per gAu per h (104 +/- 8 mu mol CO per gcat per h), while the supports without Au did not show any activity. We determined that both photothermal and non-thermal effects contribute to the light-driven reverse water-gas shift reaction catalysed by plasmonic Au/CeO2-x. A photothermal contribution was found from the exponential relationship between the CO production and the solar irradiance. In the dark, all Au/CeO2-x photocatalysts and supports without Au produced CH4 instead of CO with >= 97% selectivity, indicating a significant non-thermal contribution in light. A linear dependence of catalytic activity on the accessible interface area between CeO2-x and Au was found, which is in line with an associative formate-mediated reaction mechanism occurring at the metal-support interface. Tuning the VO content through thermal treatments yielded decreased photocatalytic activity for oxidised samples, identifying them as pre-catalysts. The stability of the Au/CeO2-x photocatalysts was evaluated, demonstrating that the catalytic performance was affected by adsorption of H2O as a reaction product, which could be fully restored upon heating in vacuo.