Reactions involving electron transfer at semiconductor surfaces. Part 12.—Nature and origins of photoactivity on oxides of 3d transition metals for elimination reactions of secondary alcohols

Abstract

Activity for conversions of the vapours of propan-2-ol and butan-2-ol into products corresponding formally to elimination of H₂, H₂O or a (C_α—C_β) bond from the parent alcohol were compared for oxides of the 3d transition metals under thermal- and photo-activation. Use of a gas-chromatographic, continuous reactant-flow technique with f.i.d. detection favoured the detection of products corresponding to a large turnover per surface site (t.a.p.s.). Under these conditions photoenhancement of the (– H₂) and (C_α—C_β) products at significant levels was detected only over ZnO and TiO₂ and in the presence of gaseous oxygen. Such photocatalytic activity, and the contrasting absence of photoactivity continuing to high turnover over oxides featuring cations with partially filled 3d levels, is attributed to predominance and persistence of an O^–-type character and reactivity only for holes photogenerated in the diamagnetic ZnO and TiO₂ samples. Photoassisted reaction of alcohol with several oxides at t.a.p.s. ≃ 1, i.e. with the first monolayer of those oxides, could be observed using a more sensitive mass-spectrometric technique. This also revealed incorporation of oxygen-18 into acetone produced from propan-2-ol under these conditions, and the origins of this exchange at low t.a.p.s. are considered. Poisoning experiments employing a gas-chromatographic pulsed reactant technique at low t.a.p.s. provide evidence that photoassisted activity on ZnO involves photoinitiated one-electron transfer processes.