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

Abstract

This work explores how H₂ pretreatment at 550 °C induces structural transformation of two gallia-based propane dehydrogenation (PDH) catalysts, viz. nanocrystalline γ/β-Ga₂O₃ and amorphous Ga₂O₃ (GaO_x) supported on silica (γ-Ga₂O₃/SiO₂ and Ga/SiO₂, respectively) and how it affects their activity, propene selectivity and stability with time on stream (TOS). Ga/SiO₂–H₂ shows poor activity and propene selectivity, no coking and no deactivation with TOS, similar to Ga/SiO₂. In contrast, the high initial activity and propene selectivity of γ-Ga₂O₃/SiO₂–H₂ decline with TOS but to a lesser extent than in calcined γ-Ga₂O₃/SiO₂. In addition, γ-Ga₂O₃/SiO₂–H₂ cokes less than γ-Ga₂O₃/SiO₂. Ga K-edge X-ray absorption spectroscopy suggests an increased disorder of the nanocrystalline γ/β-Ga₂O₃ phases in γ-Ga₂O₃/SiO₂–H₂ 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-Ga₂O₃/SiO₂–H₂ compared to γ-Ga₂O₃/SiO₂ but lower than in Ga/SiO₂ and Ga/SiO₂–H₂. Dissociation of H₂ on the Ga–O linkages in γ-Ga₂O₃/SiO₂–H₂ yields high-frequency Ga–H bands that are observed in Ga/SiO₂ and Ga/SiO₂–H₂ but not detected in γ-Ga₂O₃/SiO₂. We attribute the increased amount of Ga_IV sites in γ-Ga₂O₃/SiO₂–H₂ mostly to an increased disorder in γ/β-Ga₂O₃. X-ray photoelectron spectroscopy detects the formation of Ga⁺ and Ga⁰ species in both Ga/SiO₂–H₂ and γ-Ga₂O₃/SiO₂–H₂. Therefore, it is likely that a minor amount of Ga_IV sites also forms through the interaction of Ga⁺ (such as Ga₂O) and/or Ga⁰ with silanol groups of SiO₂.