Photo-Assisted Hydrogen Evolution in Aqueous Solution of Formic Acid with Silicon which is Supported with Noble Metals

Silicon is one of the most versatile semiconductors and also displays photocatalytic activity under visible light irradiation. We show that hydrogen evolution from 10% formic acid solution with silicon particles is enhanced by the support of noble metals on the surface of silicon. The most suitable loading metals and corresponding reagents, such as metal salts or metal nanoparticles, are examined for hydrogen evolution. Because carbon dioxide evolution is observed as a by-product and the amount of carbon dioxide is almost the same as evolved hydrogen through all reactions in the presence and absence of irradiation by visible light, we suggest that all evolved hydrogen is produced by the decomposition of formic acid. Furthermore, both the amounts of evolved H-2 and CO2 in the presence of visible light irradiation are larger than those without irradiation. Semiconductors absorb supra band gap photons to generate photoexcited electron (e(-)) and hole (h(+)) pairs in the conduction band (CB) and valence band (VB), respectively. Thus, the generated holes are consumed effectively in the oxidation of formic acid. The origin of evolved H-2 is also discussed from the consideration of SEM and XPS measurements of the catalysts through the photoreactions.