AN EXPERIMENTAL AND THEORETICAL-STUDY OF THE INTERACTION OF CH3OH ANDCH3SH WITH ZNO

The adsorption of CH3SH on ZnO surfaces has been studied by means of X -ray photoelectron and IR spectroscopies. The CH3SH uptake was measure d as a function of exposure and temperature. The adsorption is mainly dissociative, even though there is evidence for adsorbed molecular spe cies at room temperature on the ZnO(0001) polar surface. As a whole, e xperimental results are in agreement with the usual model of dissociat ive adsorption of Bronsted acids on oxides. Decomposition of adsorbed thiolate species to sulfide occurs to a significant extent above 523 K . The interaction of CH3SH/CH3S- with the Lewis-acid site available on the Zn0(0001) surface has also been theoretically studied by coupling the molecular cluster approach to density functional theory. Calculat ions relative to the adsorption of CH3OH/CH3O- have also been carried out for comparison. The bonding between the CH3O-/CH3S- species and th e Lewis-acid site is confirmed to play a leading role in determining t he actual relative acidity scale on ZnO. As far as the molecular struc ture of the adsorbed species is concerned, the C-S bond is perpendicul ar to the surface, while the O atom of the CH3O- fragment is not on to p of the Lewis-acid site and a non-negligible bonding interaction betw een the methyl group and the surface is found. These data agree very w ell with IR measurements which show, for the C-H stretching frequencie s, significantly different red shifts for the adsorbed species with re spect to the free molecules. Comparison with results recently publishe d by us concerning the adsorption of H2O, HCN and H2S on ZnO(0001) ind icate that, according to titration displacement reactions, the theoret ical scale of the relative acidity is HCN > H2S > CH3SH > H2O > CH3OH.