Materials Research Bulletin, Vol.43, No.1, 16-29, 2008
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)center dot 9H(2)O, where M stands for Ho, Sm, or La, were facilitated under various hydrothermal treatment conditions (pH 8-12 and temperature = 75-135 degrees 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)(n)(3+) or M(H2O)(n-1)(OH)(2+) facilitating, respectively, precipitation of corresponding hydrated carbonate, M-2(CO3)(2)center dot 2H(2)O, or carbonate hydroxide, MCO3(OH). X-ray powder diffractometry, infrared spectroscopy, and thermal analyses proved alternative formation of Ho-2(CO3)(3)center dot 2H(2)O or LaCO3(OH) under the whole set of hydrothermal treatment conditions probed, and Sm-2(CO3)(3)center dot 2H(2)O 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)(n)(3+) species and, hence, the metal ionic size (La > Sm > Ho). Calcination of the various hydrothermal treatment products at >= 600 degrees C resulted in the thermal genesis of the corresponding sesquioxides (M2O3). Bulk and surface characterization studies of the product oxides, employing N-2 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 m(2)/g), exposing OH groups of various coordination symmetries and, hence, acid-base properties, thus furnishing promising surface catalytic attributes. (c) 2007 Elsevier Ltd. All rights reserved.