By hydrolysis of an ethanolic gallium nitrate solution, gamma-Ga2O3 was prepared as a single-phase polymorph having a specific surface area of 160 m2 g(-1). Surface acidity and basicity of this material was studied by IR spectroscopy, using pyridine, 2,6-dimethylpyridine, acetonitrile, and carbon dioxide as spectroscopic probe molecules. For comparison, a gamma-Al2O3 sample having a surface area of 290 m2 g(-1) was also studied. On partially hydroxylated gamma-Ga2O3, the main O-H stretching bands were found at 3693 (sharp) and at 3660-3630 cm(-1) (broad), and the material proved (by adsorbed dimethylpyridine) to have a weak Brønsted acidity. Surface Lewis acidity of gamma-Ga2O3 was revealed (mainly) by adsorbed pyridine, which gave the characteristic IR absorption bands of Lewis-type adducts at 1612, 1579, 1488, and 1449 cm(-1) (values noted under an equilibrium pressure of 1 Torr at room temperature); the corresponding Lewis acid centers (coordinatively unsaturated Ga3+ ions) were found to be weaker, although more abundant, than those present on the surface of gamma-Al2O3 (unsaturated Al3+ ions). Another significant difference between gamma-Ga2O3 and gamma-Al2O3 is the smaller thermal stability of pyridine and 2,6-dimethylpyridine Lewis adducts formed on the gallium oxide. The surface basicity of gamma-Ga2O3 was studied by using carbon dioxide and deuterated acetonitrile as IR probe molecules. Adsorbed CO2 gave carbonate and hydrogen-carbonate surface species similar to those formed by gamma-Al2O3. Adsorbed acetonitrile gave rise to acetamide species, which revealed the basic character of surface O2- ions. These acetamide species were found to be more abundant on gamma-Ga2O3 than on gamma-Al2O3.

g-Ga 2 O 3 and a-Ga 2 O 3 samples were prepared as single-phase materials having a high surface area. 71 Ga NMR showed the simultaneous presence of tetracoordinated (Ga IV ) and hexacoordinated (Ga VI ) in g-Ga 2 O 3 , whereas in a-Ga 2 O 3 most of the gallium present was found to be as Ga VI . However, the apparent rate of methanol conversion into dimethyl ether (at 473 K) was nearly the same for both samples, when due account was taken of their respective surface area. The unexpected high activity of a-Ga 2 O 3 was explained from IR spectroscopic results obtained by using acidity probes like CO and pyridine. These spectroscopic probe molecules showed, for both samples, the presence of strong Lewis acid sites related to coordinatively unsaturated (cus) Ga IV ions which, for the case of a-Ga 2 O 3 , strongly suggests surface reconstruction. By contrast, CO 2 adsorption revealed a higher basicity for the a-polymorph, as compared to g-Ga 2 O 3 . This basicity can be directly correlated to a larger amount of surface Ga VI ions which results in a higher degree of ionicity.

No abstract