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 TbO_1·5.

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 PrO_1·5 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.