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.