α-Ga2O3, β-Ga2O3, and I -Ga2O3 as well as the silica-supported catalysts I -Ga2O3/SiO2, β-Ga2O3/SiO2, and Ga(NO3)3-derived Ga/SiO2 were prepared, characterized, and evaluated for propane dehydrogenation (PDH) at 550 °C. The coordination environment and acidity of surface sites in stand-alone and SiO2-supported Ga2O3 catalysts were studied using FTIR, 15N dynamic nuclear polarization surface-enhanced NMR spectroscopy (15N DNP SENS), and DFT modeling of the adsorbed pyridine probe molecule. The spectroscopic data suggest that the Lewis acidic surface Ga sites in I -Ga2O3 and β-Ga2O3 (the latter obtained from colloidal nanocrystals of I -Ga2O3 via thermal treatment at 750 °C) are similar, except that β-Ga2O3 contains a larger relative fraction of weak Ga3+ Lewis acid sites. In contrast, α-Ga2O3 features mostly strong Lewis acid sites. This difference in surface sites parallels their difference in catalytic activities: I.e., weak Lewis acid surface sites are more abundant in β-Ga2O3 relative to α-Ga2O3 and I -Ga2O3 and the increased relative abundance of weak Lewis acidity correlates with a higher initial catalytic activity in PDH, 0.41 > 0.28 > 0.14 mmol C3H6 m-2 (Ga2O3) h-1 at 550 °C, for respectively β-, α-, and I -Ga2O3 with initial propene selectivities of 86, 83, and 88%. Dispersion of I -Ga2O3 or β-Ga2O3 on a silica support introduces strong as well as abundant weak Brønsted acidity to the catalysts, lowering the PDH selectivity. The I -Ga2O3/SiO2 catalyst was slightly more active than β-Ga2O3/SiO2 in PDH (Ga normalized activity) with initial propene formation rates of 11 and 9 mol C3H6 mol Ga-1 h-1 (sel = 76 and 73%, respectively). However, these catalysts deactivated by ca. 55% within 100 min time on stream (TOS) due to coking. In contrast, Ga/SiO2, with mostly tetracoordinated surface Ga sites and abundant, strong Brønsted acid sites, gave a lower activity and selectivity in PDH (3.5 mol C3H6 mol Ga-1 h-1 and 49%, respectively) but showed no deactivation with TOS. DFT calculations using a fully dehydroxylated oxygen-deficient model β-Ga2O3 surface show that tetra- A nd pentacoordinated Ga Lewis acid sites bind pyridine more strongly than tricoordinated Ga sites and a higher relative fraction of strong Lewis acid sites correlates with increased coking. Overall, our results indicate that weakly Lewis acidic, tricoordinated Ga3+ sites are likely driving the superior PDH activity of β-Ga2O3.@en

Three γ/β-Ga2O3 nanoparticle catalysts that differ in the relative ratio of γ-Ga2O3 to β-Ga2O3 were prepared to evaluate the effect of H2 treatment (500 °C, 2 h) on the coordination environment of bulk and surface Ga sites, Lewis acidity and catalytic activity in propane dehydrogenation (PDH). Independent of the H2 treatment, the initial PDH activity of the γ/β-Ga2O3 catalysts increases with the fraction of the β-Ga2O3 phase. This is explained by the presence of weak Lewis acid sites (LAS) in β-Ga2O3 while such sites are absent in γ-Ga2O3. Treatment with H2 increases the catalytic activity of all three γ/β-Ga2O3 catalysts but for different reasons. For catalysts with higher fractions of β-Ga2O3, H2 treatment increases further the relative abundance of weak LAS, likely by generating coordinatively unsaturated Ga sites (such as tricoordinated Ga sites nearby oxygen vacancies). In contrast, H2 treatment of a catalyst containing a predominant fraction of γ-Ga2O3 phase induces disorder in the sub-surface structure of the nanoparticle, that is, it forms gallium and oxygen vacancies in the bulk and favors migration of gallium, and likely also of oxygen, to the surface. This induces a surface reconstruction that notably increases the fraction of strong LAS (and proportionally decreases the fraction of medium LAS), while creating no weak LAS in γ-Ga2O3-H2. Therefore, the increase in the catalytic activity of H2-treated γ-Ga2O3 is explained by the higher density of surface Ga sites in γ-Ga2O3-H2 relative to calcined γ-Ga2O3. H2-treated catalysts that contain a higher relative amount of weak LAS also feature a higher relative abundance of gallium hydride species associated with a low frequency FTIR band at ca. 1931–1939 cm−1, that is, weak LAS likely give weakly-bound hydrides in β-Ga2O3. Our results highlight that weak LAS in unsupported Ga2O3 catalysts are more active in PDH than mild or strong LAS.@en