Infrared spectroscopic studies on the surface chemistry of high-surface-area gallia polymorphs

Polymorphs alpha, beta, and gamma of Ga2O3 having hexagonal (corundum-type), monoclinic and cubic (spinel-type) structure, respectively, were prepared in a high-surface-area form, and characterized by powder X-ray diffraction. Nitrogen adsorption at 77 K showed these gallia samples to have specific surface areas of 77 (alpha-Ga2O3), 40 (beta-Ga2O3) and 120 m(2) g(-1) (gamma-Ga2O3). Fourier transform infrared spectroscopy of adsorbed carbon monoxide (at 77 K) and pyridine (at room temperature) showed that the three gallia polymorphs have a very similar surface Lewis acidity, regardless of their different crystal structures. This Lewis acidity was assigned, mainly, to coordinatively unsaturated tetrahedral Ga3+ ions situated on the surface of the small crystallites which constitute the different metal oxide varieties. Ga3+ (...) CO adducts formed after CO adsorption gave (in all cases) a characteristic C-O stretching band at 2195-2200 cm(-1), while Lewis-type adducts formed with adsorbed pyridine were characterized by IR absorption bands at 1610-1612 and 1446-1450 cm(-1). The three (partially hydroxylated) gallia polymorphs showed also a very weak Bronsted acidity, which they manifested by forming hydrogen-bonded adducts with both CO and pyridine; however no protonation of adsorbed pyridine occurred.