Inter-relationship between Pt oxidation states on TiO2 and the photocatalytic mineralisation of organic matters

Abstract

The inter-relationship between Pt oxidation state dynamics and the TiO2 photocatalysis of three groups of organic compounds (carboxylic acid, alcohol and phenolics) was investigated. Repetitive photocatalytic mineralisation of formic acid over the as-prepared Pt(II)/TiO2 produced free formate radicals which partially reduced Pt(II) to Pt(0, II)/TiO2. Further repetitive dark mineralisation of formic acid over this pretreated catalyst was as effective as the as-prepared sample under UV-A illumination. As for the photocatalytic mineralisation of methanol, the usually reported current doubling effect which would result in the reduction of Pt was not observed under the studied air-equilibrated condition and low organic concentration. However, the photocatalytic mineralisation of methanol was enhanced over formic acid-pretreated catalyst compared to the as-prepared and phenol-pretreated samples, highlighting the beneficial and detrimental effects of Pt(0) and Pt(IV), respectively. The enhancement was partially attributed to the favourable intimate dark catalytic interaction between Pt(0) and methanol. The photocatalytic mineralisation rates of phenol over as-prepared and formic acid-pretreated catalysts were found to be limited by the benzoquinone/hydroquinone short-circuit equilibrium. The photocatalytic mineralisation of trihydroxybenzene, the immediate product after the short-circuit equilibrium, over as-prepared Pt/TiO2 was faster relative to phenol and more so over the formic acid-pretreated sample. This corroborates the rate-limiting equilibrium while also demonstrates the beneficial effect of Pt(0). The enhancement effects on these phenolic compounds were in qualitative agreement with the observed dark catalytic mineralisation.