Aluminum oxide and systems based on it: Properties and applications

Abstract

The metastable forms of aluminum oxide that exist in the range of 300–800°C are characterized; differences in the microstructures of homogeneous γ-, η-, and χ-Al2O3 are demonstrated; and the acid-base properties of the above modifications are compared. The catalytic properties of aluminum oxide in ethanol dehydration and propionitrile ammonolysis were studied. It was found that an increased surface concentration of Lewis acid sites, including strong acid sites (ν(CO) = 2237 cm−1), is required for preparing an effective catalyst for the dehydration of ethanol, whereas the rate of propionitrile conversion increased proportionally to the surface concentration of Bronsted acid sites. γ-Aluminum oxide was used to prepare catalysts for carbon monoxide oxidation. It was found that the supporting of Pd on γ-Al2O3 did not change the support structure. Palladium on the surface of γ-Al2O3-550 (Tcalcin = 550°C, SBET = 300 m2/g) occurred as single particles (2–3 nm) and aggregates (∼100 nm). The single particles were almost completely covered with a layer of aluminum oxide to form core-shell structures. According to XPS data, they were in atypical states (BE(Pd 3d5/2) = 336.0 and 338.0 eV), which were not reduced by hydrogen in the range of 15–450°C and were resistant to the action of the reaction mixture. Palladium on the surface of γ-Al2O3-800 (SBET = 160 m2/g) was in the states Pd0 and PdO, which are typical of Pd/Al2O3, and the proportions of these states can change under the action of the reaction mixture. An increase in the Tcalcin of the Pd/Al2O3(800)-450 catalyst from 450 to 800 → 1000 → 1200°C led to the agglomeration of palladium particles and to an increase in the temperature of 50% CO conversion from 145 to 152 → 169 → 189°C, respectively. α-Aluminum oxide was used in the preparation of an effective Mn-Bi-O/α-Al2O3 supported catalyst for the synthesis of nitrous oxide by the oxidation of ammonia with oxygen: the NH3 conversion was 95–97% at 84.4% N2O selectivity.