Abstract
AbstractBy 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 Ga:Al 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 γ‐Al2O3 and γ‐Ga2O3. The specific surface area (determined by nitrogen adsorption at 77 K) was found to range from 160 m2 g−1 for the mixed oxide having Ga:Al = 9:1 up to 370 m2 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 Ga:Al ratio in the mixed gallia‐alumina oxides.
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