Infrared study of the adsorption of ammonia, pyridine and hydrogen chloride on barium sulphate

Abstract

Infrared spectra are reported of ammonia, pyridine and hydrogen chloride adsorbed on barium sulphate in the presence and absence of surface hydroxyl groups and adsorbed water. Surface hydroxyl groups do not exhibit Brønsted acidity but form hydrogen bonds with adsorbed acceptor molecules. Exposed Ba²⁺ cations constitute Lewis-acidic adsorption sites. Surface sulphate anions act as hydrogen-bond acceptor sites in the presence of hydrogen-bond donor molecules. The dissociative adsorption of water on barium sulphate generates hydroxyl groups at Ba²⁺ sites but does not lead to the concomitant formation of HSO₄^– anions. The latter may be generated by the dissociative adsorption of hydrogen chloride and exhibit Brønsted acidity in the presence of adsorbed pyridine.