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Insights into Photocatalytic Oxidation by Bare and Platinised TiO2: The Impact of Adding Hydroxyl Functional Groups to Butanedioic Acid and Propanol
Wenny Irawaty, 2013
The University of New South Wales
Abstract
The effect of molecular structure on photocatalytic oxidation was evaluated by comparing the performance of butanedioic acid, propanol and their hydroxylated derivates in an aqueous TiO2 suspension. The influence of Pt deposits had on the photooxidation of these compounds was also studied.
The photoactivity was found to depend on the nature of the parent substance and its level of hydroxylation. The presence of additional hydroxyl functional groups on butanedioic acid accelerated the diacid disappearance rate but decreased the overall mineralisation rate. Accelerated diacid disappearance was attributed to increased parent organic adsorption via enhanced bonding between the extra OH groups and the TiO2 surface. The slower mineralisation rates were ascribed to the additional hydroxyl groups altering or enhancing intermediate formation to produce more recalcitrant intermediates (e.g. ethanoic acid). In contrast, no consistent relationship was found between the additional hydroxyl group number and mineralisation rate for the alcohols. Hydroxyl addition to propanol was proposed to not significantly alter its adsorption mode on TiO2 leading to similar disappearance rates for the alcohols. Differences in mineralisation rates were observed and thought to be dominated by differences in degradation rates of the generated intermediates.
Pt/TiO2 displayed a higher photoactivity than bare TiO2 for both organic groups. Similar intermediates were formed irrespective of the Pt presence with its function as an electron sink primarily responsible for the enhancement. The Pt deposits also displayed a thermal (dark) catalytic effect for oxidising formic acid. Its dark catalysis was augmented by pre-irradiating the Pt/TiO2 with UV light. Pre-irradiation was suspected to enhance electron accumulation in the Pt deposits, in turn facilitating oxygen dissociation which then enhanced formic acid oxidation. The pre- irradiation effect decayed with time but could be readily recovered by re- irradiating the Pt/TiO2. This dark catalytic effect may further contribute to the overall Pt/TiO2 photoactivity as formic acid is a commonly generated intermediate from photocatalysis.
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