Photocatalytic oxidation of ethanol on micrometer- and nanometer-sized semiconductor particles

The photocatalytic oxidation reaction of ethanol on small nanometer-sized and large micrometer-sized ZnS particles as well as on nanometer-sized SnO2 under stationary illumination has been investigated in order to study the particle size effect in this size regime. It has been. demonstrated that the reaction mechanism of alcohol oxidation on micrometer- and nanometer-sized particles is quite different. Ethanol is selectively oxidized on mum-ZnS to acetaldehyde without side products by a two-hole process. This reaction required two absorbed photons in one particle which are transferred in a very short time interval (55 ns) from the mum-ZnS particle to the ethanol molecule forming directly acetaldehyde. In the case of nm-ZnS, long-lived alpha-hydroxyethyl radicals are formed via a one-hole process due to the low generation rate of charge carriers (about 18 e(-)/h(+) s(-1)) in one nm-ZnS particle. These radicals undergo secondary reactions, i.e. dimerization to 2,3-butanediol and disproportionation to acetaldehyde in the electrolyte. The electrochemical rate constant k(et) for the slowest partial reaction occurring on the illuminated nm- and mum-ZnS particles has been calculated. One obtains similar rate constants, namely k(et) = 9.8 x 10(-9) and 4.3 x 10(-9) cm s(-1) for mum-ZnS and nm-ZnS, respectively. In the case of illuminated nm-SnO2 particles, only the oxidation product acetaldehyde was observed. Here, the initially formed et-hydroxyethyl radical via a one-hole process is further oxidized to the corresponding aldehyde by electron injection from the radical into the conduction band of the same SnO2 particle by avoiding the formation of 2,3-butanediol. (C) 2002 Elsevier Science B.V. All rights reserved.