Surface chemistry and pore structure of β-Ga2O3

Mesoporous beta-Ga2O3 was obtained by calcination at 1073 K of the gamma-Ga-O-3 polymorph previously prepared by adding aqueous ammonia to an ethanolic solution of gallium nitrate. Nitrogen adsorption-desorption at 77 K showed the beta-Ga2O3 sample to have a BET surface area of 40 m(2) g(-1) and a bimodal distribution of pore radii, with maxima at 2.1 and 14 nm. The surface chemistry of this material was studied by FTIR spectroscopy of carbon monoxide adsorbed at liquid nitrogen temperature. Lewis acidity was monitored by the corresponding C-O stretching mode of CO interacting with coordinatively unsaturated Ga3+ ions; main IR absorption bands of Ga3+...CO adducts were found at 2218 and 2195 cm(-1), revealing two types of surface-exposed metal cations. The surface OH groups (which were present in partially hydroxylated beta-Ga2O3) gave main O-H stretching bands at 3650 and 3695 cm(-1). These hydroxy groups were found to have a Bronsted acid character, as revealed by formation of hydrogen-bonded adducts with adsorbed carbon monoxide. (C) 2002 Elsevier Science B.V. All rights reserved.