Reactivity of CO on a TiO 2(110) defective surface studied by electron stimulated desorption

Abstract

CO adsorption on a defective TiO 2(110) surface at 150 K incorporating about 1.7 × 10 14cm −2 vacancies produced by Ar + sputtering has been studied by electron stimulated desorption, ESD, and with Auger electron spectroscopy, AES, as a complementary technique. When the surface was exposed to CO up to 5 × 10 5L the AES spectra revealed a small C signal. From the O(KLL) Ti(LMM) ratio increase a CO coverage of 0.06 ± 0.01 ML was determined. In spite of this small amount of CO the ESD technique showed dramatic changes in ion yield and energy. The characteristic dominant peak at 4.0 eV and the small structure at about 7.0 eV of the ion kinetic energy distribution curve of the clean defective surface changed to a triple peak structure with maxima at 3.4, 4.3 and 7.4 eV. The first is identified with the most probable kinetic energy of O + ions ejected from non-dissociated CO molecules. The 4.3 and 7.4 eV maxima correspond to the most probable kinetic energy of O + ions desorbed from O in-plane and from the bridging O-ligand of the TiO 2(110) surface, respectively. The growth of the 7.4 eV peak upon adsorption is a consequence of the filling of the bridging O-ligand initial vacancies by dissociated O from the CO molecule. The ion yield curves show, in addition to the onset at 35 eV electron energy of the clean surface, a threshold at about 290 eV electron energy corresponding to excitation of the C 1s core level. The total ion yield also shows a decrease upon CO adsorption. The O + ion yield curves are found to be related to the secondary electron yield. The chemisorption of CO is discussed in terms of a model where the CO is molecularly adsorbed on the Ti five-fold coordination sites with a part being dissociated and leading to O adsorbed on bridging O-ligand vacancies.