Unexpected similarities between the surface chemistry of cubic and hexagonal gallia polymorphs

gamma-Ga2O3 and alpha-Ga2O3 samples were prepared as single-phase materials having a high surface area. Ga-71 NMR showed the simultaneous presence of tetracoordinated (Ga-IV) and hexacoordinated (Ga-VI) in gamma-Ga2O3, whereas in alpha-Ga2O3 most of the gallium present was found to be as GaVI. 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 alpha-Ga2O3 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 alpha-Ga2O3, strongly suggests surface reconstruction. By contrast, CO2 adsorption revealed a higher basicity for the alpha-polymorph, as compared to alpha-Ga2O3. This basicity can be directly correlated to a larger amount of surface Ga-VI ions which results in a higher degree of ionicity.