Acid-base properties of alumina supported M2O3 (M = B, Ga, In) catalysts

The acid-base properties of γ-Al<inf>2</inf>O<inf>3</inf> and alumina-supported B<inf>2</inf>O<inf>3</inf>, Ga<inf>2</inf>O<inf>3</inf> and In<inf>2</inf>O<inf>3</inf> have been determined by microcalorimetry of ammonia and sulfur dioxide adsorption. From the adsorption of NH<inf>3</inf>, it was found that the addition of B<inf>2</inf>O<inf>3</inf> on alumina leads to an increase of the number of acid sites, while Ga<inf>2</inf>O<inf>3</inf> and In<inf>2</inf>O<inf>3</inf> additives caused a decrease in the acidity of alumina. Using SO<inf>2</inf> as a probe molecule to study the basicity, the number of surface basic sites on alumina was found to be strongly decreased by the addition of boron oxide, while it was only slightly affected by the addition of gallium oxide and decreased by the addition of indium oxide. The differential heats of adsorption are discussed as a function of the coverage by the probe molecules. The electronic properties of the oxides are examined in order to explain the acid-base properties of the supported oxides.