Infrared Spectroscopic Study on the Surface Properties of γ-Gallium Oxide as Compared to Those of γ-Alumina

By hydrolysis of an ethanolic gallium nitrate solution, γ-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 γ-Al2O3 sample having a surface area of 290 m2 g-1 was also studied. On partially hydroxylated γ-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 γ-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 γ-Al2O3 (unsaturated Al3+ ions). Another significant difference between γ-Ga2O3 and γ-Al2O3 is the smaller thermal stability of pyridine and 2,6-dimethylpyridine Lewis adducts formed on the gallium oxide. The surface basicity of γ-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 γ-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 γ-Ga2O3 than on γ-Al2O3.