Volumetric and infrared studies of the adsorption of gaseous benzene, toluene and chlorobenzene by zinc oxide

Abstract

The adsorption isotherms of gaseous benzene, toluene and chlorobenzene have been measured at 25 °C for ZnO samples covered with different numbers of surface hydroxyl groups. With increasing surface hydroxyl content of the ZnO sample, the amount of reversibly adsorbed molecules increased, while the amount of irreversibly adsorbed molecules decreased. A strong interaction between the surface zinc atoms and the π-electrons of the adsorbate molecules was responsible for an irreversible adsorption taking place on the dehydroxylated surface. Toluene was found to be chemisorbed dissociatively to produce a hydroxyl group on the dehydroxylated surface sites, indicating the basic nature of the ZnO surface. The interaction of free hydroxyl groups on the surface with the π-electrons of the aromatic ring contributed to the reversible adsorption on the fully hydroxylated surface. The shift of the OH absorption band in the i.r. spectra increased in the order chlorobenzene < benzene < toluene, in accordance with the order of increasing electron density of the aromatic ring. It is likely that these molecules are adsorbed with the plane of the aromatic ring parallel to the solid surface, irrespective of the degree of surface hydroxylation of the ZnO samples.