Structure, Texture, and Acid-Base Properties of Alkaline Earth Oxides, Rare Earth Oxides, and Binary Oxide Systems

Abstract

The effect of synthesis conditions on the formation of the phase composition, dispersity, pore structure, and acid-base properties of alkaline earth oxides, rare earth oxides, and the Mg-M-O (M = Y, La, or Ce) and Y(La)-M-O (M = Ca, Sr, or Ba) binary systems was studied. It was found that the nature of the system was responsible for the character of phase transformations: the Mg-M-O samples were a mixture of either MgO with Y2O3 or MgO with a solid solution based on rare earth oxides (LaMg)2O3 or (CeMg)O2); the Y(La)-M-O samples (M = Ca, Sr, or Ba) contained the M2Y2O5, MY2O4, and MLa2O4 compounds, which differ in chemical stability, in addition to La2O3 and Y2O3 phases. According to XPS data, the M/Mg atomic ratios were much higher than the bulk values; this is indicative of an enrichment of the surface of samples in the second component. An increase in the concentration of M2O3 from 5 to 25 mol % resulted in a decrease in the S sp of the Mg-M-O samples from 220 ± 10 to 110 ± 10 m2/g; the S sp of samples calcined at 750°C was lower by a factor of ∼1.5–2. The S sp of the Y(La)-M-O samples was higher than the S sp of individual La2O3 and Y2O3. The samples were characterized by a biporous texture. The concentrations and strength distributions of surface OH groups, Lewis acid sites, and Lewis base sites depend on the nature and concentration of rare earth elements in the binary samples. The activity of the Mg-M-O samples in the oxidative dehydrogenation reaction of propionitrile correlated with the acid-base surface sites. Among the Ru/Y(La)-M-O catalysts for ammonia synthesis, Ru/Y-Ba-O was the most active; this catalyst provided a higher yield of NH3 at 250–300°C, as compared with catalysts prepared with the use of other supports (Sibunit, KVU-1, and C/MgO).