Reaction Mechanism of the Multiple-Electron Oxygen Reduction Reaction on the Surfaces of Gold and Platinum Nanoparticles Loaded on Titanium(IV) Oxide

Au and Pt nanoparticles with varying mean particle size and comparable loading amounts were loaded on the surface of TiO2 (Au/TiO2 and Pt/TiO2, respectively). The photocatalytic activities of Au/TiO2 and Pt/TiO2 for the oxygen reduction reaction (ORR) in an aerated aqueous solution containing 4% ethanol were compared under ultraviolet-light irradiation at 298 K. The initial rate of H2O2 generation (or H2O2 formation rate) in the Au/TiO2 system is much greater than that in the Pt/TiO2 system regardless of the metal particle size. To clarify the origin for the striking difference in the activity, the photocatalytic ORR on the model slabs (M28/(TiO2)32 and M50/(TiO2)96, M = Au and Pt) was simulated by density functional theory (DFT) calculations taking the solvation effect into consideration. The DFT calculations clearly show that regardless of the cluster size, H2O2 formation more easily occurs structurally and energetically for the Au/TiO2 system, whereas H2O is generated with the O-O bond cleavage in the Pt/TiO2 system.