Photocatalytic reactions under irradiation of visible light over gold nanoparticles supported on titanium(IV) oxide powder prepared by using a multi-step photodeposition method

Titanium(IV) oxide (TiO2) having both smaller and larger gold (Au) particles was successfully prepared by a multi-step (MS) photodeposition method. When 0.25 wt% Au loading per photodeposition was repeated four times, smaller and larger Au particles having average diameters of 1.4 and 13 nm, respectively, were fixed on TiO2, and the Au/TiO2 sample exhibited strong photoabsorption around 550 nm due to surface plasmon resonance (SPR) of the larger Au particles. Various Au/TiO2 samples were prepared by changing the Au loading per photodeposition and the number of photodepositions. Effects of the conditions in MS photodeposition and sample calcination on Au particle distribution and photoabsorption properties were investigated. These samples were used for hydrogen (H-2) formation from 2-propanol and mineralization of acetic acid in aqueous suspensions under irradiation of visible light. In the case of H-2 formation under deaerated conditions, the reaction rate of Au/TiO2 having both larger and smaller particles was 4 times higher than that of the Au/TiO2 sample without smaller Au particles, indicating that smaller Au particles acted effectively as a co-catalyst, that is, as reduction sites for H-2 evolution. On the other hand, in the case of mineralization of acetic acid under aerated conditions, carbon dioxide formation rates were independent of the presence of smaller Au particles, indicating that the smaller Au particles had little effect on the mineralization of acetic acid. To extend the possibility of Au/TiO2 for H-2 formation under irradiation of visible light, H-2 formation from ammonia (NH3) as biomass waste was examined under deaerated conditions; NH3 was decomposed to H-2 and nitrogen with a stoichiometric ratio of 3 : 1.