Oxidation of terbium oxide. Part I. Chemisorption of oxygen on terbium oxide

Abstract

The type C modification of terbium sesquioxide Tb2O3 chemisorbs oxygen rapidly at –196°. The adsorption of oxygen was studied by volume and weight measurements on both 99·9 and 99·99% purity terbium oxide, and both hydrogen and vacuum reduction techniques were used to regenerate TbO1·5.The fraction of the surface area covered by chemisorbed oxygen amounted to the equivalent of 0·8 of a physically adsorbed monolayer on the 99·99% pure preparation and 0·5 of a monolayer on the 99·9% pure preparation. Substitution of vacuum reduction for diffusion-purified hydrogen reduction on the same sample produced no significant difference in the chemisorption value.Much of the chemisorbed oxygen could be reversibly desorbed by evacuation at –196° and considerably more by raising the temperature to 60°, but the equivalent of about 0·1 of a physically adsorbed monolayer resisted even prolonged pumping either at –196° or 60°. This behaviour differs from that observed previously with UO2 and type C PrO1·5 in which systems little, if any, of the chemisorbed O2 could be desorbed, either at low or high temperatures. Partial dissociation of molecules on the surface is invoked to explain why oxygen is both reversibly and irreversibly chemisorbed.