Formation and thermal decomposition of rare-earth carbonates

The preparation of carbonate-containing rare-earth compounds and their thermal decomposition in 0%-17% CO₂/N₂ gas streams has been studied. Three types of rare-earth carbonate or hydroxycarbonate were produced by precipitation; with ammonium bicarbonate as the precipitant, La₂ (CO₃)₃,CeOCO₃ and Ln (OH)_x (CO₃)_y (Sm, Tb, and Yb) were obtained, whereas with Na₂CO₃, only the normal carbonate, Ln₂(CO₃)₃ (La, Sm, Tb, Er and Yb) was found. Temperature programmed decomposition studies revealed that the normal carbonate decomposed stepwise via a dioxocarbonate, Ln₂O₂CO₃, to the oxide. In contrast, the hydroxycarbonates decomposed directly to the oxide. The presence of CO₂ during heating had minimal effect on the decomposition of Ln₂(CO₃)₃ to La₂O₂CO₃ but raised significantly the decomposition temperature of Ln₂O₂CO₃ to the oxide. As CO₂ is a major product of the rare-earth oxide catalysed oxidative coupling of methane, these observations indicate that the state of catalyst carbonation will be dependent on the reaction temperature, overall catalyst selectivity and preparative method.