ADSORPTION AND ELECTRON-STIMULATED DESORPTION OF NH3/TIO2(110)

Adsorption and electron stimulated processes of NH3 on rutile TiO2 (110) have been studied by means of x-ray photoemission spectroscopy (XPS) and low-energy ion scattering (LEIS). Defects, mostly surface oxygen vacancies, can be produced easily on TiO2 surfaces by thermal treatment or by rare gas ion sputtering. We have used three differently prepared TiO2 surfaces to study the influence of the substrate defect structure on the interaction with NH3: a stoichiometric surface, a thermally treated (slightly oxygen deficient), and a sputtered (highly oxygen-deficient) surface. The adsorption behavior of NH3 on all these surfaces is quite similar. NH3 adsorbs molecularly, with a saturation coverage of approximately 0.16-0.19 monolayers, the latter for a highly oxygen-deficient surface. The electron stimulated desorption (ESD) behavior is quite different: In the limit of a stoichiometric surface, electron irradiation induces,the desorption of NH3 molecules, with a desorption cross section of approximately 1 X 10(-16) cm2. When the highly oxygen-deficient TiO2 surface is used as substrate, besides desorption, electron stimulated dissociation of NH3 takes place, with atomic nitrogen being the final product of the dissociation process. This atomic nitrogen is adsorbed in subsurface sites, as confirmed by angle-resolved XPS. Under certain conditions, an electron stimulated formation of surface nitride is observed. These measurements provide direct evidence for the role of surface defects in electron stimulated reaction pathways.