REACTIVITY OF CO ON A TIO2(110) DEFECTIVE SURFACE STUDIED BY ELECTRON-STIMULATED DESORPTION

CO adsorption on a defective TiO2(110) surface at 150 K incorporating about 1.7 X 10(14) cm(-2) vacancies produced by Ar+ sputtering has bee n studied by electron stimulated desorption, ESD, and with Auger elect ron spectroscopy, AES, as a complementary technique. When the surface was exposed to CO up to 5 X 10(5) L 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 ES D technique showed dramatic changes in ion yield and energy. The chara cteristic 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 defectiv e surface changed to a triple peak structure with maxima at 3.4, 4.3 a nd 7.4 eV. The first is identified with the most probable kinetic ener gy 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 TiO2(1 10) surface, respectively. The growth of the 7.4 eV peak upon adsorpti on is a consequence of the filling of the bridging O-ligand initial va cancies by dissociated O from the CO molecule. The ion yield curves sh ow, in addition to the onset at 35 eV electron energy of the clean sur face, a threshold at about 290 eV electron energy corresponding to exc itation of the C Is core level. The total ion yield also shows a decre ase upon CO adsorption. The Of ion yield curves are found to be relate d to the secondary electron yield. The chemisorption of CO is discusse d in terms of a model where the CO is molecularly adsorbed on the Ti f ive-fold coordination sites with a part being dissociated and leading to O adsorbed on bridging O-ligand vacancies.