Structure and Lewis acid properties of gallia–alumina catalysts

Abstract The surface structure of Ga2O3–Al2O3 catalysts was investigated by IR and ESR spectroscopy. As a result of modification by gallia, alumina hydroxyl cover changes significantly. The new band 3680–3688 cm−1 ascribed to bridging structure GaOHAl has been observed. By means of IR spectroscopy, adsorbed CO and ESR with anthraquinone as a probe molecule, two types of Lewis acid sites and their concentration were defined. These sites are coordinately unsaturated aluminium cation Alcus3+ and coordinately unsaturated gallium cation Gacus3+. In the case of Ga2O3–Al2O3 catalysts with a gallia content of 1–10 mol%, surface Lewis acid properties are in accordance with the assumption of solid solution formation. Only under significantly non-equilibrium conditions (large excess of dopant — 20 mol% Ga2O3–Al2O3, relatively low temperature of calcination −720 K, short time of interaction — 4 h) it becomes possible to stabilize highly dispersed state of gallium oxide, for which the characteristic feature is high concentration and local regulation of Gacus3+ on the surface. It was shown that on the alumina surface there are favourable (more than in the case of pure gallia) conditions for the Gacus3+ formation.