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 °C, p = 3.5 bar) using plasmonic Au/CeO_2−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 CeO_2−x particle size, Au particle size and loading, and the concentration of oxygen vacancies. Upon illumination, all Au/CeO_2−x catalysts showed a CO production of up to 2.6 ± 0.2 mmol CO per g_Au per h (104 ± 8 μmol CO per g_cat 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/CeO_2−x. A photothermal contribution was found from the exponential relationship between the CO production and the solar irradiance. In the dark, all Au/CeO_2−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 CeO_2−x and Au was found, which is in line with an associative formate-mediated reaction mechanism occurring at the metal–support interface. Tuning the V_O content through thermal treatments yielded decreased photocatalytic activity for oxidised samples, identifying them as pre-catalysts. The stability of the Au/CeO_2−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.