Atomic-scale changes of silica-supported catalysts with nanocrystalline or amorphous gallia phases: implications of hydrogen pretreatment on their selectivity for propane dehydrogenation

This work explores how H(2) pretreatment at 550 °C induces structural transformation of two gallia-based propane dehydrogenation (PDH) catalysts, viz. nanocrystalline γ/β-Ga(2)O(3) and amorphous Ga(2)O(3) (GaO(x)) supported on silica (γ-Ga(2)O(3)/SiO(2) and Ga/SiO(2), respectively) and how it affects their activity, propene selectivity and stability with time on stream (TOS). Ga/SiO(2)–H(2) shows poor activity and propene selectivity, no coking and no deactivation with TOS, similar to Ga/SiO(2). In contrast, the high initial activity and propene selectivity of γ-Ga(2)O(3)/SiO(2)–H(2) decline with TOS but to a lesser extent than in calcined γ-Ga(2)O(3)/SiO(2). In addition, γ-Ga(2)O(3)/SiO(2)–H(2) cokes less than γ-Ga(2)O(3)/SiO(2). Ga K-edge X-ray absorption spectroscopy suggests an increased disorder of the nanocrystalline γ/β-Ga(2)O(3) phases in γ-Ga(2)O(3)/SiO(2)–H(2) and the emergence of additional tetrahedral Ga sites (Ga(IV)). Such Ga(IV) sites are strong Lewis acid sites (LAS) according to studies using adsorbed pyridine and CO probe molecules, i.e., the abundance of strong LAS is higher in γ-Ga(2)O(3)/SiO(2)–H(2) compared to γ-Ga(2)O(3)/SiO(2) but lower than in Ga/SiO(2) and Ga/SiO(2)–H(2). Dissociation of H(2) on the Ga–O linkages in γ-Ga(2)O(3)/SiO(2)–H(2) yields high-frequency Ga–H bands that are observed in Ga/SiO(2) and Ga/SiO(2)–H(2) but not detected in γ-Ga(2)O(3)/SiO(2). We attribute the increased amount of Ga(IV) sites in γ-Ga(2)O(3)/SiO(2)–H(2) mostly to an increased disorder in γ/β-Ga(2)O(3). X-ray photoelectron spectroscopy detects the formation of Ga(+) and Ga(0) species in both Ga/SiO(2)–H(2) and γ-Ga(2)O(3)/SiO(2)–H(2). Therefore, it is likely that a minor amount of Ga(IV) sites also forms through the interaction of Ga(+) (such as Ga(2)O) and/or Ga(0) with silanol groups of SiO(2).