Development of a ReaxFF potential for Pd∕O and application to palladium oxide formation

Oxide formation on palladium surfaces impacts the activity and selectivity of Pd-based catalysts, which are widely employed under oxygen rich operating conditions. To investigate oxidation processes over Pd catalysts at time and length scales inaccessible to quantum based computational methods, we have developed a Pd∕O interaction potential for the ReaxFF reactive force field. The parameters of the ReaxFF potential were fit against an extensive set of quantum data for both bulk and surface properties. Using the resulting potential, we conducted molecular dynamics simulations of oxide formation on Pd(111), Pd(110), and Pd(100) surfaces. The results demonstrate good agreement with previous experimental observations; oxygen diffusion from the surface to the subsurface occurs faster on the Pd(110) surface than on the Pd(111) and Pd(100) surfaces under comparable conditions at high temperatures and pressures. Additionally, we developed a ReaxFF-based hybrid grand canonical Monte Carlo∕molecular dynamics (GC-MC∕MD) approach to assess the thermodynamic stability of oxide formations. This method is used to derive a theoretical phase diagram for the oxidation of Pd935 clusters in temperatures ranging from 300 K to 1300 K and oxygen pressures ranging from 10(-14) atm to 1 atm. We observe good agreement between experiment and ReaxFF, which validates the Pd∕O interaction potential and demonstrates the feasibility of the hybrid GC-MC∕MD method for deriving theoretical phase diagrams. This GC-MC∕MD method is novel to ReaxFF, and is well suited to studies of supported-metal-oxide catalysts, where the extent of oxidation in metal clusters can significantly influence catalytic activity, selectivity, and stability.