An experimental and theoretical study of the interaction of CH3OH and CH3SH 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 measured as a function of exposure and temperature. The adsorption is mainly dissociative, even though there is evidence for adsorbed molecular species at room temperature on the ZnO(0001) polar surface. As a whole, experimental results are in agreement with the usual model of dissociative 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. Calculations relative to the adsorption of CH3OH/CH3O- have also been carried out for comparison. The bonding between the CH3O-/CH3S- species and the Lewis-acid site is confirmed to play a leading role in determining the actual relative acidity scale on ZnO. As far as the molecular structure of the adsorbed species is concerned, the C-S bond is perpendicular to the surface, while the O atom of the CH3O- fragment is not on top of the Lewis-acid site and a non-negligible bonding interaction between the methyl group and the surface is found. These data agree very well with IR measurements which show, for the C-H stretching frequencies, significantly different red shifts for the adsorbed species with respect to the free molecules. Comparison with results recently published by us concerning the adsorption of H2O, HCN and H2S on ZnO(0001) indicate that, according to titration displacement reactions, the theoretical scale of the relative acidity is HCN > H2S > CH3SH > H2O > CH3OH.