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Abstract
UV light pre-treatment has been shown to boost catalytic oxygen activation by Pt/TiO2 particles. In this work, the key active sites generated during UV light pre-treatment of noble metals and metal oxide catalysts were studied to provide a deeper understanding of the generation of catalytic active sites following light pre-treatment.
As a model system, aqueous phase formic acid oxidation was used as the probe reaction. The enhancement effect of UV light pre-treatment on Pt/TiO2 was attributed to the generation of surface active oxygen species, comprising adsorbed oxygen, on Pt surfaces (PtOads) and active oxygen species (O-ads). Electrocatalytic assessment and DFT calculations indicated that UV light pre-treatment lowered the energy for oxygen activation. The UV light pre-treatment effect was observed for other noble metal deposits (Au and Pd) loaded on TiO2, proceeding via a process analogous to the Pt system although, in the instance of Au/TiO2, the effect was not as prevalent.
UV light pre-treatment was also able to boost the catalytic oxygen activation of Pt loaded on different metal oxide supports (TiO2, CeO2 and SiO2). The key active species were PtOads and O-ads under dark catalytic conditions while under photocatalytic conditions, photo-generated holes and electrons were believed to form reactive hydroxyl and superoxide radicals, respectively. Surprisingly, Pt/SiO2 showed the highest activity under dark catalytic conditions. The cuboctahedral shape of the Pt deposits on the SiO2 surface was believed to have been advantageous for activating adsorbed oxygen species.
Finally, catalytic oxygen activation on neat metal oxide-based catalysts was examined. The hydrogenation and subsequent UV light pre-treatment of neat SiO2 and TiO2-SiO2 generated two distinct defect types which provided a strong synergistic catalytic activity. Hydrogenation created oxygen vacancy sites capable of activating oxygen while UV light pre-treatment introduced silica-based non-bridging oxygen hole center sites with both sites working in tandem to accelerate formic acid oxidation. The findings were supported by XAS and DFT calculations.
Ultimately, understanding the role of noble metals and defects in metal oxide-supported catalysts for oxygen activation under UV light pre-treatment conditions opens an avenue for designing efficient and low-cost catalysts for energy storage and conversion.