Visible light accelerated CO2 hydrogenation to CO over a Cu/ZnO/Al2O3 catalyst: Teaching an old dog new tricks

The Cu-ZnO-Al2O3 is a proven catalyst for thermally-driven RWGS reaction. In this work, its ability to function as photo-catalyst in the light-assisted RWGS reaction was analyzed. By utilizing the synergy of visible light and thermal energy, CO formation was accelerated almost 6-times compared to reaction in dark. Wavelength-dependent and kinetic analysis confirmed that blue photons are most efficient for accelerating the CO rate. Differences in the reaction mechanism under dark and illuminated conditions are postulated, based on the drop in apparent activation energy from 83 to 44 kJ/mol and strongly changing reaction orders of H2 during kinetic analysis. CO formation through the redox reaction mechanism occurs both under dark and illuminated conditions, yet represents only 10 % of the overall CO yield. Transient in-situ DRIFTS data showed the decomposition rate of the most abundant surface species (carbonates and formates) is not accelerated by light despite higher catalytic activity, indicating that these abundant surface species are minimally involved in CO formation under illumination. The RWGS reaction under light-assisted conditions proceeds via several contributory routes, with the carboxylate mechanism as the main pathway. Light-induced hydrogen dissociation, increased H coverage and its reaction with surface hydroxyls accelerates water formation and desorption after carboxylate decomposition, resulting in accelerated CO rates.