In Situ Formation and Dynamic Evolution of Metastable Boron Species on BCN for Propane Oxidative Dehydrogenation

Boron-based materials have demonstrated their ability to catalyze propane conversion to olefin via oxidative dehydrogenation (ODH) of propane, and their surface restructuring during the reaction has been observed in ex situ nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS) studies, while understanding of the influence of specific chemical environments at a molecular level is not established, which hinders the extensive mechanistic understanding vital for catalyst design and optimization. Herein, we sampled the chemical space accessible for boron oxides, including boron-rich and oxygen-rich states, supported on the BCN surface under the ODH conditions. Dynamic restructuring of boron oxides was observed, and in situ formation and coexistence of multiple metastable boron species were captured, implicating higher surface activity of BCN compared to h-BN. Electronic structure analysis of metastable species combined with free energy calculations reveals the strong electrophilicity of the B atom in the –B═O site and stronger nucleophilicity of the O atoms in the –BO2 ring when compared with >BOOB<, which can promote the activation of dioxygen, water, and propane. The ensemble-averaged thermodynamic phase diagram was then mapped to illustrate the phase space of the BOx/BCN surface in the space of temperature vs dioxygen pressure and speciation population vs chemical potential of oxygen, which was confirmed by experimental XPS and NMR characterizations of fresh and spent BOx/BCN catalysts. These findings demonstrate the broad accessible chemical space of boron-based catalyst surfaces during their restructuring under ODH conditions and provide a molecular level of understanding on the surface dynamic evolution of BOx/BCN catalyst, which contribute to the design of supported boron-based ODH catalysts with high activity.