Understanding the impact of small vanadia clusters and their coverage effects on undoped and Ni-doped ceria nanorod supports on propane oxidative dehydrogenation

Abstract

Using density functional theory calculations, the promotional effects of vanadia deposition over ceria-based nanorods on propane oxidative dehydrogenation (ODH) to propene were studied. Reliable catalyst models were modeled using thermodynamic and computational IR analyses. Electronic transfer between vanadia and the ceria surface kept a check on oxygen vacancy formation and ensured a lower degree of surface site heterogeneity. Ni²⁺ doping of the ceria support stabilized the vanadia clusters by strong interactions. Propane ODH occurred in two pathways (isopropyl radical and isopropoxide mediated), based on the type of the first C–H activation intermediate formed. The isopropyl radical-mediated mechanism favored propene formation via spontaneous C–H activation of the formed radical over the bridged oxygen of vanadia. The isopropoxide-mediated mechanism, with the second C–H activation occurring among the intra-row surface oxygen combinations, led to favorable propene formation, while those occurring between the inter-row surface oxygens led to the undesirable acetone formation and eventual direct oxidation of propane. Ni doping of the ceria support enhanced the first and second C–H activations kinetically, facilitated more sites for propene formation, and improved catalyst activity and propene selectivity. Increased vanadia coverage was shown to significantly improve propene selectivity via a heuristic prediction based on the mechanistic insights developed.