Effect of gold loading and TiO_2 support composition on the activity of Au/TiO_2 photocatalysts for H_2 production from ethanol–water mixtures

摘要

This paper systematically compares the activity of Au/TiO_2 photocatalysts (Au loadings 0–10 wt.%) for H_2 production from ethanol–water mixtures under UV excitation. Degussa P25 TiO_2 was used as the support phase. TEM analyses revealed that the average Au nanoparticle size at all loadings was 5 ± 2 nm, with the Au nanoparticles preferentially located at the interfacial sites between TiO_2 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/TiO_2 photocatalysts showed an intense absorption maximum centred around 560–570 nm 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 TiO_2, even at low Au loadings. All of the Au/TiO_2 photocatalysts displayed high activity for H_2 production from ethanol–water mixtures under UV irradiation, with the highest activities observed in the Au loading range 0.5–2 wt.% (H_2 production rate 31–34 mmol g~(-1) h~(-1)). In order to econvolute the role of the P25 TiO_2 support in promoting H_2 production, anatase and rutile nanoparticles were isolated from P25 TiO_2 by selective chemical dissolution and then functionalised with gold nanoparticles (3 wt.% loading, size 5 ± 2 nm). The H_2 production activity of the resulting Au/anatase and Au/rutile photocatalysts was 22 and 10 mmol g~(-1) h~(-1), respectively, and substantially lower than the corresponding Au/P25 TiO_2 photocatalyst (32 mmol g~(-1) h~(-1)). The data provide strong evidence that synergistic electron transfer between the TiO_2 polymorphs and supported Au nanoparticles is responsible for the high rates of H_2 production observed in the Au/P25 TiO_2 system. The interface between anatase and rutile crystallites, where gold nanoparticles preferentially deposit, is identified as a photocatalytic ‘hot spot’ for H_2 production. High Au loadings reduce the efficiency of such ‘hot spots’.