TiO2 photocatalysis: Impact of the platinum loading method on reductive and oxidative half-reactions

Environmental pollution and shortage of energy resources are considered as the most serious threats faced by mankind. Heterogeneous photocatalysis has become one of the most frequently investigated technologies, thanks to its dual ability to convert solar energy into chemical energy and to perform environmental remediation. However, the low quantum efficiencies achieved so far with TiO2 photocatalysts represent a big challenge that needs to be overcome before their potential can be fully realized. Among other possibilities, the loading of noble metals (e.g. platinum) is a proven strategy to enhance the TiO2 photoactivity. However, the method by which Pt/ TiO2 is prepared appears to play a crucial role in tuning the photocatalytic activity. In this work, platinum-loaded anatase TiO2 photocatalysts were prepared using two alternative methods: photodeposition by reduction of PtCl(62-)and physical mixing of TiO2 with Pt nanoparticles synthesized by laser ablation. The effect of the Pt deposition method was evaluated for the photoreforming reaction of two organic substrates: naphthalene, and methanol. To explain the different activities, a full physicochemical characterization was performed for these samples. Moreover, the charge carrier dynamics in pristine and platinized TiO2 were investigated by transient absorption spectroscopy (TAS), and by the electron paramagnetic resonance (EPR) technique, respectively. The photodeposition of Pt caused a significant decrease in the charge carrier recombination rates, which in turn led to an increased rate of the photocatalytic reactions. This effect was mainly attributed to the strong metal-semiconductor interaction resulting from the photodeposition process, aided by the preferential deposition on crystalline facets with stronger reducing properties.