High pressure CO2 photoreduction using Au/TiO2: Unravelling the effect of co-catalysts and of titania polymorphs

A series of Au/TiO2 based catalysts with low gold loading (0.1-0.5 wt%) were prepared by a modified deposition-precipitation method and their activity was tested for CO2 photoreduction in the liquid phase at high pressure (7 bar). Methanol, formic acid and formaldehyde were detected as major products in the liquid phase, whilst gas phase products, CO, CH4 and H2, were produced at different concentrations depending on the catalyst formulation. The deposition of small Au nanoparticles (3-5 nm) (NPs) onto TiO2 was found to quantitatively influence the product distribution and the productivities of various products. Characterization techniques, such as DRIFTS, XRD and TEM/HAADF, were used to correlate the photocatalyst features with the photoactivity obtained. Methanol productivities of 1360 and 1350 mmol kgcat-1 h-1 were attained for 0.2 wt% Au/TiO2 and 0.5 wt% Au/TiO2, respectively, which are impressive results with respect to the literature. However, the highest loaded sample revealed simultaneously the highest H2 and CH4 productivities. By contrast, lower activity was obtained using anatase and rutile pure phases, with respect to the mixed phase P25 commercial sample, confirming the key role of interfacial defects between borders of different crystalline phases.