Moderate and high temperature doping of cobaltic oxide catalyst with lithium oxide

Abstract

Pure and lithium-doped cobaltic oxide (Co 3O 4) samples were prepared by thermal decomposition of pure basic cobalt carbonate and of mixtures of Co 3O 4 with different proportions of lithium hydroxide. The thermal dissociation of pure and doped solids was carried out in air at temperatures in the range 500–1100 °C. The effects of lithium doping on the surface area of Co 3O 4 and on its catalytic activity in CO oxidation at 125–220 °C were investigated. The influence of doping on the thermal reduction of Co 3O 4 to CoO was also studied using X-ray diffraction and differential thermal analysis (DTA) techniques. Incorporation of lithium ions into the Co 3O 4 lattice at temperatures equal to or greater than 700 °C decreased drastically its catalytic activity and its specific surface area. By contrast, doping at 500 °C, although it increased substantially the surface area of the doped oxide, decreased its catalytic activity. X-ray diffraction showed that the thermal treatment of pure basic cobalt carbonate in air at 500–700 °C effected its complete conversion into Co 3O 4 which underwent a partial reduction at 900 °C yielding CoO. At 1100 °C, Co 3O 4 decomposed entirely, producing cobaltous oxide. The presence of lithium ions in Co 3O 4 (3%–6% Li + (mol Co 3O 4 -1) effectively retarded its reduction into CoO from 900 to 1050 °C. A prolonged heating of doped oxide at 1100 °C affected only its partial decomposition into CoO. The retardation of cobaltous oxide formation was also confirmed by DTA. The incorporation of lithium ions into Co 3O 4 stabilized its lattice against the thermal reduction to the cobaltous oxide phase. New diffraction lines appeared in all doped oxide samples heated in air at 900–1100 °C. These lines may be due to the formation of a new lithium oxide-cobalt oxide compound. The possible mechanisms of lithium doping are discussed.