The Hermans group reported hexagonal boron nitride (hBN) and several metal borides to exhibit the unexpectedly excellent catalytic performance towards oxidative dehydrogenation (ODH) of propane to propene. While it is clear that boron plays the crucial role in this catalysis, structures of the catalytic surface under reaction conditions, and the resultant catalytic mechanism remain elusive. We theoretically characterize the surface of partially oxidized hBN under reaction conditions. Using global optimization, we find the structures of the global and thermally-accessible local optima of hBN surface with different access B and O coverages. Intriguingly, some stoichiometries have a variety of competing thermally-accessible structures in conditions of ODH, whereas other stoichimetries have very prevalent global minima, and spread minimally over their structural ensembles. As a result, the phase diagram of partially oxidized hBN constructed based on just the global minima is very different from the ensemble-based phase diagram. In particular, on the ensemble-average phase diagram, at a certain partial pressure of O2 and chemical potential of B, several stoichiometries have comparable stabilities and meet at a point, potentially indicating the sweet spot for reactivity in ODH. In addition, molecular dynamics simulation show the highly fluxional behavior of the BO<sub>x</sub>-layer, dynamically exposing the possibly reactive -B=O units at reaction temperatures. Our results reveal the need to take into account configurational entropy, structural diversity, and fluxionality of catalytic interfaces in realistic modeling.

Dynamic phase diagram for the catalytic surface of hexagonal boron nitride in conditions of oxidative dehydrogenation of propane

2019 Spring National Meeting

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