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

Abstract

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.