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

Abstract

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.

Microstructure and Optical Properties of CoxZn1–xGa2O4 (0 ≤ x ≤ 1) Nanoparticles

“…The main doublet peak at δ ≈ 100 ppm is assigned to Ga 3+ ions in the octahedral sites (Ga VI–1 ), and the two other relatively weak peaks at δ ≈ 180 and 9 ppm are due to Ga 3+ ions in the tetrahedral (Ga IV ) and second octahedral (Ga VI–2 ) sites, respectively. The assignment is reasonable when compared with those previous reported 38,39. Because 71 Ga is a quadrupolar nucleus, its lineshape is influenced by both first‐ and second‐order quadrupolar interactions, and the doublet peak (Ga VI–1 ) can be due to the second‐order splitting of the central transition band by nuclear‐quadrupolar interactions.…”

Ga/HZSM-5 Catalysed Acetic Acid Ketonisation for Upgrading of Biomass Pyrolysis Vapours

“…The Lewis:Brønsted ratio exhibited a small increase for 10Ga/HZSM-5, in accordance with literature reports [56,64,65]. Corresponding DRIFTS for pyridine on m-Ga 2 O 3 revealed two weak bands at 1452 cm −1 and 1614 cm −1 indicative of pure Lewis acid character as previously reported [66,67]. Acid strength was subsequently probed by temperature-programmed reaction spectroscopy (TPRS) of propylamine.…”

The structure of low-index surfaces of β-Ga2O3

“…The only theoretical studies of b-Ga 2 O 3 surfaces of which we are aware are those of Kohl et al [31] and Gonzalez et al [32] (discussed below) which employed semi-empirical cluster treatments to examine the adsorption of CH 4 and H, respectively, on (1 0 0) surfaces. In the present work, ab initio calculations will be done to study the structure and charge distribution of low-index surface planes of b-Ga 2 O 3 since, as is already known [33], the Ga 2 O 3 surface structure can have a pronounced effect on reactivity. Attention is focused on the (1 0 0), (0 1 0), (0 0 1) and ð1 0 1Þ faces since, as noted above, these appear to be among the most technologically important.…”