Infrared studies of adsorbed species on rutile at the solid—vapour and solid—liquid interfaces

Abstract

Infrared spectra of rutile confirm the existence of several types of surface hydroxyl group with different reactivities towards adsorbate molecules. One type of hydroxyl group is unreactive towards adsorbates and exists at sub-surface lattice sites. Water, amines and carbonyl compounds are adsorbed in part on rutile through coordinative interactions with exposed Ti 4+ ions which act as Lewis acidic surface sites. Oxide ions on rutile exhibit basic or nucleophilic character and chemisorption reactions often involve the oxidation of adsorbate molecules. The formation of oxidation products on the surface was promoted by the pretreatment of rutile in hydrogen at high temperature. In contrast, reduction in hydrogen decreased the Lewis acidity of the rutile surface. Reaction of rutile with silicon tetrachloride and water led to a modified surface which exhibited Brønsted acidity in the presence of adsorbed pyridine. Infrared spectra of rutile immersed in solutions with liquid hydrocarbon as the predominant component suggest that the general characteristics of the surface are unchanged by the transfer from the solid/gas to the solid/liquid interface.