An experimental and theoretical study of the interaction of CH3OH and CH3SH with ZnO

Abstract

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 Brønsted 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 ZnO(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.