Photocatalytic hydrogen production at titania-supported Pt nanoclusters that are derived from surface-anchored molecular precursors

Abstract

Degussa P-25 TiO2 bearing surface-anchored Pt(dcbpy)Cl2 [dcbpy = 4,4′-dicarboxylic acid-2,2′-bipyridine] prepared with systematically varied surface coverage produced Pt0 nanoparticles under bandgap illumination in the presence of methanol hole scavengers. Energy-dispersive X-ray spectroscopy confirmed the presence of elemental platinum in the newly formed nanoparticles during scanning transmission electron microscopy (STEM) experiments. According to the statistical analysis of numerous STEM images, the Pt0 nanoclusters were distributed in a segregated manner throughout the titania surface, ranging in size from 1 to 3 nm in diameter. The final achieved nanoparticle size and net hydrogen production were determined as a function of the Pt(dcbpy)Cl2 surface coverage as well as other systematically varied experimental parameters. The hybrid Pt/TiO2 nanomaterials obtained upon complete decomposition of the Pt(dcbpy)Cl2 precursor displayed higher photocatalytic activity (300 μmol/h) for hydrogen evolution in aqueous suspensions when compared with platinized TiO2 derived from H2PtCl6 precursors (130 μmol/h), as ascertained through gas chromatographic analysis of the photoreactor headspace under identical experimental conditions. The conclusion that H2 was evolved from Pt0 sites rather than from molecular Pt(dcbpy)Cl2 entities was independently supported by Hg and CO poisoning experiments. The formation of small Pt nanoparticles (1.5 nm in diameter) prevail at low surface coverage of Pt(dcbpy)Cl2 on TiO2 (0.5 to 2% by mass) that exhibit enhanced turnover frequencies with respect to all other materials investigated, including those produced from the in situ photochemical reduction of H2PtCl6. PtII precursor absorption in the ultraviolet region appeared to be partially responsible for attenuation of the H2 evolution rate at higher Pt(dcbpy)Cl2 surface coverage. The nanoparticle size and hydrogen evolution characteristics of the surface-anchored materials generated through photodeposition were directly compared with those derived from chemical reduction using NaBH4. Finally, Degussa P-25 thin films deposited on FTO substrates enabled electrochemically induced (−1.0 V vs Ag/AgCl, pH 7.0, phosphate buffer) electron trapping (TiO2(e–)) throughout the titania. After removal of the applied bias and the anaerobic introduction of Pt(dcbpy)Cl2, the accumulated electrons reduce this molecular species to Pt0 nanoparticles on the titania electrode surface, as confirmed by TEM measurements, with the concomitant production of H2 gas. The combined experiments illustrate that TiO2(e–) generated with bandgap excitation or via electrochemical bias affords the reduction of Pt(dcbpy)Cl2 to Pt0 nanoparticles that in turn are responsible for heterogeneous hydrogen gas evolution.