Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases p. 2777 - 2802 (1981)
Update date:2022-08-16
Topics:
Cunningham, Joseph
Hodnett, Benjamin K.
Kinetics of conversion of propan-2-ol and butan-2-ol to the corresponding ketones by photoassisted dehydrogenation, (-H2)*, to lower aldehydes by a photoassisted Cα-Cβ bond cleavage process, (α-β)*, and to trace quantities of alkene by photoassisted dehydration (-H2O)* have been investigated at oxygen partial pressures in the range 0-700 Torr and at alcohol partial pressures of 0-60 Torr, utilising dynamic flow photoreactors and gas-chromatographic analysis.Qualitatively similar results were obtained either with a 'continuous reactant flow plus continuous u.v. illumination' procedure allied to intermittent sampling, or with a 'pulsed reactant and analysis' procedure.Both procedures yielded dependence upon alcohol pressure indicative of two parallel photoassisted pathways to (-H2)* product, one being Langmuir-Hinshelwood (LH*) and the other Eley-Rideal (ER*) in character.A decline in activity of the metal oxide surfaces as photoassisted conversion of alcohol increased was observed with both procedures and affected the LH* process for (-H2)* more strongly.It is proposed that this LH* process involved alcohol chemisorption and hole localisation at coordinatively unsaturated O2-cus ions.The ER* process is envisaged to have involved encounter of alcohol (from the gas phase or from a reversibly adsorbed weakly bound state) with a much more numerous type of surface location capable of being photoactivated by hole capture.The proposed mechanisms account for observed dependences of the (-H2)* process upon square root of the incident light intensity, upon alcohol pressure and upon oxygen pressure.
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