Metal Particle Size Effects on the Photocatalytic Hydrogen Ion Reduction

The main characteristics of noble metals on semiconductor photocatalysts within the context of hydrogen ions reduction are particles' size, electronic structure, and dispersion. In this work, we have systematically studied Au, Pd, and Pt particles on TiO2 with mean diameters of 5.2-5.8, ca. 2, and 1.2-1.5 nm, respectively, at different coverages. Ethanol photo-reforming with water was used as the example from which extraction of reaction rates per mass, per particle, and per atom has been analyzed. Irrespective of the metal nature, a narrow range for maximum catalytic performance was observed when the rate is normalized per unit mass or unit mole. Starting from a very low metal content (0.25 wt % for each metal), the H-2 production rates decreased with increasing number of Pd, Pt, or Au particles. However, the highest rate per particle is that of gold at any metal coverage. This rate exceeded by 2 orders of magnitude that of Pt and by 1 order of magnitude that of Pd. These results indicate that, unlike the case of thermal catalytic reaction, large particles perform better than small particles. Extraction of reaction rates from this study and from previous studies on Ni and Ag deposited on TiO2 indicated a direct relationship with the work function of the metals and a volcano-shape relationship with their d-band center position.