Development of the ReaxFF reactive force field for mechanistic studies of catalytic selective oxidation processes on BiMoO_x

Abstract

We have developed a new reactive force field, ReaxFF, for use in molecular dynamics (MD) simulations to investigate the structures and reactive dynamics of complex metal oxide catalysts. The parameters in ReaxFF are derived directly from QM and have been validated to provide reasonable accuracy for a wide variety of reactions. We report the use of ReaxFF to study the activation and conversion of propene to acrolein by various metal oxide surfaces. Using high-remperature MD-simulations on metal oxides slabs exposed to a propene gas phase we find that (1) Propene is not activated by MoO_3 but it is activated by amorphous Bi_2O_3 to form allyl which does not get oxidized by the surface; (2) Propene is activated by Bi_2Mo_3O_(12) to form an allyl-radical and the hydrogen gets abstracted by a Mo=O bond, which is bridged via an O to a Bi-site; (3) Propene is activated over V_2O_5 to form an allyl, which is then selectively oxidized on the surface to form acrolein. The propene reations on V_2O_5 occur at lower temperatures than on Bi_2O_3 or Bi_2Mo_3O_(12). The results are all consistent with experimental observations, encouraging us that such investigations will enhance our mechanistic understanding of catalytic hydrocarbon oxidation sufficiently to suggest modifications for improving efficiency and/or selectivity.