Hydrothermal synthesis attempts of dawsonite-type hydroxymetalocarbonate precursor compounds for catalytic Ho, Sm, and La oxides

Chemical interactions in mixed, aqueous solutions of NH4HCO3 and M(NO3)3·9H2O, where M stands for Ho, Sm, or La, were facilitated under various hydrothermal treatment conditions (pH 8–12 and temperature = 75–135 °C). The solution chemistry established did not make available necessary concentrations of soluble HCO3− and MO(OH)2− species for the formation of dawsonite-type ammonium hydroxymetalocarbonates, NH4M(CO3)(OH)2, but, alternatively, high concentrations of soluble CO32−, and M(H2O)n3+ or M(H2O)n−1(OH)2+ facilitating, respectively, precipitation of corresponding hydrated carbonate, M2(CO3)2·2H2O, or carbonate hydroxide, MCO3(OH). X-ray powder diffractometry, infrared spectroscopy, and thermal analyses proved alternative formation of Ho2(CO3)3·2H2O or LaCO3(OH) under the whole set of hydrothermal treatment conditions probed, and Sm2(CO3)3·2H2O at pH < 10 or SmCO3(OH) at pH ≥ 10, thus implying dependence of the composition of the product carbonate compound on the hydrolysability of the initial M(H2O)n3+ species and, hence, the metal ionic size (La > Sm > Ho). Calcination of the various hydrothermal treatment products at ≥600 °C resulted in the thermal genesis of the corresponding sesqui-oxides (M2O3). Bulk and surface characterization studies of the product oxides, employing N2 sorptiometry and scanning electron microscopy, in addition to the above analytical techniques, revealed overall strong crystallinity, large average crystallite size, and well-defined particle morphology. They revealed, moreover, surfaces, though of limited accessibilities (≤13 m2/g), exposing OH groups of various coordination symmetries and, hence, acid–base properties, thus furnishing promising surface catalytic attributes.