Effect of gold loading and TiO2 support composition on the activity of Au/TiO2 photocatalysts for H2 production from ethanol–water mixtures

Abstract

This paper systematically compares the activity of Au/TiO2 photocatalysts (Au loadings 0–10wt.%) for H2 production from ethanol–water mixtures under UV excitation. Degussa P25 TiO2 was used as the support phase. TEM analyses revealed that the average Au nanoparticle size at all loadings was 5±2nm, with the Au nanoparticles preferentially located at the interfacial sites between TiO2 crystallites. XRD, XRF, XPS, and UV–Vis measurements established that metallic Au was the only gold species on the surface of the photocatalysts. The Au/TiO2 photocatalysts showed an intense absorption maximum centred around 560–570nm due to the localised surface plasmon resonance (LSPR) of the supported gold nanoparticles. Photoluminescence measurements revealed that gold nanoparticles effectively suppress electron–hole pair recombination in TiO2, even at low Au loadings. All of the Au/TiO2 photocatalysts displayed high activity for H2 production from ethanol–water mixtures under UV irradiation, with the highest activities observed in the Au loading range 0.5–2wt.% (H2 production rate 31–34mmolg−1h−1). In order to deconvolute the role of the P25 TiO2 support in promoting H2 production, anatase and rutile nanoparticles were isolated from P25 TiO2 by selective chemical dissolution and then functionalised with gold nanoparticles (3wt.% loading, size 5±2nm). The H2 production activity of the resulting Au/anatase and Au/rutile photocatalysts was 22 and 10mmolg−1h−1, respectively, and substantially lower than the corresponding Au/P25 TiO2 photocatalyst (32mmolg−1h−1). The data provide strong evidence that synergistic electron transfer between the TiO2 polymorphs and supported Au nanoparticles is responsible for the high rates of H2 production observed in the Au/P25 TiO2 system. The interface between anatase and rutile crystallites, where gold nanoparticles preferentially deposit, is identified as a photocatalytic ‘hot spot’ for H2 production. High Au loadings reduce the efficiency of such ‘hot spots’.