Lanthanide oxides: preparation and ageing

Abstract

La₂O₃, CeO₂, Sm₂O₃, Dy₂O₃, and Yb₂O₃ were obtained by calcining in air phases prepared by precipitation with ammonia from solutions of the corresponding nitrates. During the calcination to oxides, under the experimental conditions used, the nitrate ions are removed to an extent which could not be detected by temperature programmed desorption–mass spectrometry or i.r. spectroscopy, contradictory to the established literature. For comparative purposes, Yb₂O₃ has also been prepared by thermal decomposition of oxalate. The Brunauer–Emmett–Teller surface area was always lower for the oxide obtained from oxalate. This was so even after thermal treatment at 1 173 K. Independent of the preparation method, the heaviest 4f oxides undergo partial conversion to bulk carbonate hydroxides when exposed to air. According to our results and those reported in the literature, lanthanide oxides, when used as catalysts, are often activated at temperatures lower than those necessary for the complete elimination of CO₂ and H₂O.