Reactions involving electron transfer at semiconductor surfaces. Part 9.—Oxygen labelling study of photo-oxidation of C3 and C4 alcohols over rutile

Abstract

The secondary alcohols, propan-2-ol and butan-2-ol, and also the tertiary alcohol 2-methyl-propan-2-ol are shown to undergo selective photo-oxidation over rutile surfaces, when exposed to u.v.-illumination in the presence of gaseous oxygen. Gaseous oxygen labelled with oxygen-18 experienced a rapid photoinitiated depletion from the gas phase over these (alcohol +¹⁸O₂)/TiO^*₂ interfaces but very little oxygen-18 label was incorporated into the ketone products of selective photo-oxidation over pure TiO₂. Photo-oxidation of an oxygen-18 labelled sample of the tertiary alcohol over preoxidised rutile in the presence of ¹⁶O₂ resulted in almost total loss of the oxygen-18 label from the acetone product of photo-oxidation. Over prereduced rutile surfaces, however, photo-oxidation was much slower and the acetone photoproduct contained the same percentage of oxygen-18 as the parent alcohol. The mechanisms developed to account for these results assign an important role to hole-trapping at coordinatively unsaturated O^2– sites on the rutile surface and envisage large differences in ease of detachment of such lattice oxygen species from preoxidised and prereduced surfaces of pure TiO₂.