Bulk and surface transformations of Ga₂O₃ nanoparticle catalysts for propane dehydrogenation induced by a H₂ treatment

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.ChemE/Inorganic Systems Engineerin