Photodeposition of platinum nanoparticles on wel-defined tungsten oxide : controlling oxidation state, particle size and geometrical distribution

Abstract

In this thesis, structure-directed photodeposition of the cocatalyst platinum (Pt) on monoclinic tungsten oxide (WO3) nanoplates is described, both considering fundamental aspects, as well as usefulness for applications in photocatalytic propane oxidation. Before such studies are described, the concepts of crystal facet engineering of WO3 nanoplates and photodeposition of platinum on commercial WO3 are explained. For the crystal facet engineering studies, WO3 with plate-like morphology was synthesized through a hydrothermal synthesis method. A solution containing sodium tungstate, hydrochloric acid and citric acid was treated in a sealed autoclave at elevated temperatures. It is demonstrated that in the synthesis procedure the pH should be sufficiently low and the temperature sufficiently high to obtain monoclinic WO3 nanoplates. In photodeposition studies of Pt on commercial WO3, the influence of the sacrificial agent methanol was investigated in detail. When methanol is used, all platinum in the solution is deposited in a metallic state on the WO3 surface, in the form of large clusters of particles. In the absence of methanol, not all platinum can be deposited on the WO3. Platinum is then found on the surface as highly dispersed nanoparticles in an oxidized state. The results make clear that the dispersion and oxidation state of platinum photodeposited on tungsten oxide can be carefully controlled by the potential usage of methanol or, if needed, hydrogenation. When structure-directed photodeposition studies of Pt on WO3 nanoplates are performed, Pt particles are found on the edges/subordinate facets of the plate-like WO3. This is likely due to sorption effects rather than light-induced charge separation. It is concluded that the anionic Pt precursor [PtCl6]2- must have adsorbed on the positive sites of the WO3 nanoplates before being reduced. The presence of Pt on WO3 nanoplates has negative consequences on the photocatalytic activity of the samples in propane oxidation, especially when it is present as PtII rather than Pt0. The results demonstrate that structure-directed photodeposition of Pt on WO3 nanoplates does not have a beneficial effect on the activity in photocatalytic propane oxidation, in contrast to popular believe that structure-directed photodeposition should result in enhanced photocatalytic activities.