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 degrees C induces structural transformation of two gallia-based propane dehydrogenation (PDH) catalysts, viz. nanocrystalline gamma/beta-Ga2O3 and amorphous Ga2O3 (GaOx) supported on silica (gamma-Ga2O3/SiO2 and Ga/SiO2, respectively) and how it affects their activity, propene selectivity and stability with time on stream (TOS). Ga/SiO2-H-2 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 gamma-Ga2O3/SiO2-H-2 decline with TOS but to a lesser extent than in calcined gamma-Ga2O3/SiO2. In addition, gamma-Ga2O3/SiO2-H-2 cokes less than gamma-Ga2O3/SiO2. Ga K-edge X-ray absorption spectroscopy suggests an increased disorder of the nanocrystalline gamma/beta-Ga2O3 phases in gamma-Ga2O3/SiO2-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 gamma-Ga2O3/SiO2-H-2 compared to gamma-Ga2O3/SiO2 but lower than in Ga/SiO2 and Ga/SiO2-H-2. Dissociation of H-2 on the Ga-O linkages in gamma-Ga2O3/SiO2-H-2 yields high-frequency Ga-H bands that are observed in Ga/SiO2 and Ga/SiO2-H-2 but not detected in gamma-Ga2O3/SiO2. We attribute the increased amount of Ga-IV sites in gamma-Ga2O3/SiO2-H-2 mostly to an increased disorder in gamma/beta-Ga2O3. X-ray photoelectron spectroscopy detects the formation of Ga+ and Ga-0 species in both Ga/SiO2-H-2 and gamma-Ga2O3/SiO2-H-2. Therefore, it is likely that a minor amount of Ga-IV sites also forms through the interaction of Ga+ (such as Ga2O) and/or Ga-0 with silanol groups of SiO2.ISSN:2044-4753ISSN:2044-476