ReaxFF Monte Carlo reactive dynamics: Application to resolving the partial occupations of the M1 phase of the MoVNbTeO catalyst

Recently high quality structures have become available for the M1 and M2 phases of the MoVNbTeO multi-metal oxide (MMO) catalysts capable of selectively activating propane to form acrylonitrile and other important monomers for modern polymer materials. However the details of the chemical mechanisms controlling selectivity and activity have remained elusive because important sites in these structures are occupied by mixtures of Mo and V atoms, obscuring the actual distributions of the metals and oxides at the active sites. We have developed the ReaxFF-MC-RD computational approach to resolve such partial occupations of crystallographic sites, which we apply here to determine the atomistic arrangements in the M1 phase of the MoVNbTeO system. We find ordering of four distinct crystallographic sites with partial occupations in the z direction. There is a strong energetic preference for Mo–O–Mo and V–O–V interlayer bonding between adjacent M1 sites and also between adjacent M7 sites, leading to Vdouble bond; length as m-dashO Vdouble bond; length as m-dashO and Mdouble bond; length as m-dashO Mdouble bond; length as m-dashO chains perpendicular to the layers and bordering the heptagonal channels. On the other hand, for M3 sites in adjacent sites Mo–O–V bonding is preferred, while there is no observed energetic preference for different ordering between adjacent M2 sites. We consider that these ordered structures lead to local environments that strongly affect the reactivity of V and Mo atoms in the M1 catalyst, demonstrating the need to resolve the partial occupations from X-ray analysis into atomistic structures with whole atoms. The ReaxFF-MC-RD approach provides a means for accomplishing this resolution. We expect that such studies will provide additional insights into the chemical reaction steps on MMO catalysts that should be useful in designing more selective and more active systems.