Synthesis and characterization of spinel-type gallia-alumina solid solutions

By precipitation with ammonia of ethanolic solutions containing the appropriate proportions of gallium and aluminium nitrate, following by calcination of the resulting gels at 773 K, mixed Ga2O3/Al2O3 oxides having GaAl ratios of 9:1, 4:1, 1:1, 1:4 and 1:9 were obtained. Powder X-ray diffraction showed that these mixed metal oxides form a series of solid solutions having the spinel-type structure; also shown by gamma-AL(2)O(3) and gamma-Ga2O3. The specific surface area (determined by nitrogen adsorption at 77 K) was found to range from 160 m(2) g(-1) for the mixed oxide having Ga:Al = 9:1 up to 370 m(2) g(-1) for that having Ga:Al = 1:9. High resolution MAS NMR showed that Ga3+ and Al3+ ions occur at both tetrahedral and octahedral sites in the spinel-type structure of the mixed metal oxides, although there is a preferential occupation of tetrahedral sites by Ga3+ ions. A proportion of penta-coordinated Al3+ ions was also found. IR spectra of carbon monoxide adsorbed at 77 K showed that the mixed metal oxides have a considerable Lewis acidity, related mainly to tetrahedrally coordinated metal ions exposed at crystal surfaces. The characteristic infrared absorption band of coordinated (adsorbed) CO appears in the range 2205-2190 cm(-1), and its peak wavenumber is nearly independent of GaAl ratio in the mixed gallia-alumina oxides.