Fabrication, Characterisation and Photocatalytic Properties of Au/TiO2, Ag/TiO2, Pd/TiO2 and Pd-Ag/TiO2 Photocatalysts

Abstract

The development of a sustainable hydrogen economy for electricity generation and transportation hinges on the discovery of simple, low cost and efficient technologies for H2 generation, which this project aimed to address. In this work, a series of Au/TiO2, Ag/TiO2, Pd/TiO2 and Pd-Ag/TiO2 photocatalysts were prepared at different loadings (0.5-8.0 wt.%) and their photocatalytic activities for H2 production from ethanol-water mixtures systematically evaluated under UV irradiation. Degussa P25 TiO2, comprising of anatase (85%) and rutile (15%), was used as the support phase for the preparation of all the photocatalysts. Transmission electron microscopy (TEM) showed that all of the photocatalysts comprised metal nanoparticles of size 1-5 nm dispersed uniformly over the TiO2 support, with the size of the nanoparticles being independent of the metal loading. X-ray photoelectron spectroscopy (XPS) confirmed that all of the metals were present in metallic form (i.e. Au0, Ag0, Pd0 or Pd0-Ag0), with the actual metal loading being proportional to the nominal metal loading. The phase composition and crystallite size of the TiO2 support particles were unaffected by metal deposition. Photoluminescence (PL) measurements demonstrated that metal co-catalyst deposition effectively suppressed electron-hole pair recombination in TiO2, which in turn had a positive impact on the photocatalytic activity of P25 TiO2 for H2 production. Photocatalytic tests were carried out on all samples in ethanol-water mixtures (80:20 vol. ratio) under UV fluxes comparable to those present in sunlight. The H2 production rate of the photocatalysts depended on the metal co-catalyst and the metal loading. In general, the activity of the samples decreased in the following order Pd/TiO2 > Au/TiO2 ≈ Pd-Ag/TiO2 ≫ Ag/TiO2 > P25 TiO2. The optimum metal loading for the monometallic photocatalysts were 1 wt.% for Pd/P25 TiO2 (49.4 mmol gcatalyst-1 h-1), 2 wt.% for Au/P25 TiO2 (25.7 mmol gcatalyst-1 h-1) and 2 wt.% for Ag/P25 TiO2 (3.9 mmol gcatalyst-1 h-1), which were all higher than P25 TiO2 (1.0 mmol gcatalyst-1 h-1). The very high and stable activity of Pd and Au co-catalysts, compared to Ag, is explained in terms of the work functions of the various metals (Φ = 5.2 eV, 5.4 eV and 4.7 eV for Pd, Au and Ag, respectively) with the high work function metals more readily forming efficient Schottky junctions and accepting electrons photoexcited in the conduction band of TiO2, and in turn transferring them to H+ or H2O to form H2. Protons were generated by oxidation of ethanol and water on the TiO2 surface by photogenerated holes in the valence band of TiO2. Results presented suggest that Pd/P25 TiO2 and Au/P25 TiO2 are promising candidates for first generation semiconductor photocatalysts for H2 production from water and biofuels. This research supports the development of a sustainable H2 economy.