Photodecomposition of Adsorbed 2-Chloroethyl Ethyl Sulfide on TiO₂:  Involvement of Lattice Oxygen

Abstract

The photodecomposition of chemisorbed 2-chloroethyl ethyl sulfide (2-CEES) on the TiO2 surface was investigated using several experimental methods involving both a rutile TiO2(110) surface and powdered P-25 Degussa TiO2. It is found that photooxidation of 2-CEES occurs in the absence of gas-phase oxygen. The oxygen for this process is supplied by the TiO2 lattice. For UV irradiation in the range 3.0−5.0 eV, two simultaneous photodecomposition pathways are measured: one fast process with a measured cross section of 7.4 × 10-19 cm2 and one slow process with a measured cross section of 5.4 × 10-20 cm2. These two photodecomposition pathways probably involve different modes of binding of the 2-CEES molecule to sites on the TiO2(110) surface. Aldehydic adsorbed products are observed to be produced by UV irradiation and are measured by surface-sensitive IR spectroscopy. In addition, adsorbed carboxylates and carbonates are also produced. GC−MS studies of the gas-phase photodecomposition products produced in the absence of O2 also indicate the production of similar oxygenated products upon UV irradiation of 2-CEES over TiO2 powder as well as many products characteristic of free radical initiated processes on the TiO2 surface. We postulate that free radical species, produced from the 2-CEES molecules by electron or hole attack, are able, in a sequence of reactions, to extract lattice oxygen from the TiO2 photocatalyst surface.