Hexagonal Boron Nitride Surfaces Under Condition of Oxidative Dehydrogenation of Propane—Off-Stoichiometric Restructuring and Metastable Active Species

Hexagonal boron nitride (h-BN), a 2D material usually considered chemically inert, was found surprisingly active towards oxidative dehydrogenation of propane (ODHP). Previous experimental studies evidenced the in-situ formed partially oxidized boron layer to be catalytic, however the surface structure or the active site remains elusive.Here, we performed grand canonical global optimizations at DFT level to explore the off-stoichiometric restructuring of the face and edges of h-BN, under realistic reaction conditions. The surface is found to populate a grand canonical ensemble of states of different stoichiometries, rearranging in timescale of picoseconds. Ab initio phase diagrams are constructed, and surface-gas phase radical ODHP mechanism is proposed based on the “hot” surface states (only accessible at temperature of catalysis) in collaboration with experimental kinetics studies. The grand canonical ensemble representation provides rich structural insights into the surface oxyfunctionalization of hBN and helps resolving experimental edge-selective ¹¹B solid state NMR spectra, as well as B 1s signal residual in operando X-ray Raman spectroscopy. In addition, the sliding of the h-BN sheets could solely lead to an ensemble of sliding configurations, and the metastable configurations exhibit lower barriers for the key activation steps of ODHP, which is likely the reason why hBN is more active towards ODHP than more rigid metal borides. The dataset generated during the grand canonical sampling can be utilized to train ML models for low-cost prediction of surface free energy, Bader charges, and NMR spectra.
This collection of works demonstrates the necessity and benefits of adopting an ensemble-level view of restructuring thermal catalysts in identifying strictures, reactivity, and spectroscopy simulations.