Combined experimental and modellistic approach to unravel the pH dependence of photocatalytic hydrogen production

Hydrogen production via formic acid reforming on Pt-loaded TiO2 was characterized by marked pH dependence with higher rates at acidic pH, a maximum around pH 4, and a sharp decrease at pH 5–6. We observed a similar behavior with methanol reforming. With formic acid, hydrogen and CO2 production rates were within the limits of experimental errors, while with methanol, CO2 evolution was delayed because of the formation of partially oxidized C1 intermediates. We observed the same trends photodepositing Pt from hexachloroplatinic acid in a preceding irradiation experiment at fixed pH, thereby demonstrating that Pt photodeposition was not responsible for the observed pH-dependence. We modelled the photocatalytic process to account for pH dependence by analyzing the rate constant for the oxidation of formic acid promoted by valence band holes. Our study demonstrates that the electrostatic interaction between substrate and TiO2 surface is the cause of the pH dependence of hydrogen production rate.