A ReaxFF Investigation of Hydride Formation in Palladium Nanoclusters via Monte Carlo and Molecular Dynamics Simulations

Palladium can readily dissociate and absorb hydrogen from the gas phase, making it applicable in hydrogen storage devices, separation membranes, and hydrogenation catalysts. To investigate hydrogen transport properties in Pd on the atomic scale, we derived a ReaxFF interaction potential for Pd/H from an extensive set of quantum data for both bulk and surface properties. Using this potential, we employed a recently developed hybrid grand canonical-Monte Carlo/molecular dynamics (GC-MC/MD) method to derive theoretical hydrogen absorption isotherms in Pd bulk crystals and nanoclusters for hydrogen pressures ranging from 10(-1) atm to 10(-14) atm, and at temperatures ranging from 300 to 500 K. Analysis of the equilibrated cluster structures reveals the contributing roles of surface, subsurface, and bulk regions during the size-dependent transition between the solid solution alpha phase and the hydride beta phase. Additionally, MD simulations of the dissociative adsorption of hydrogen from the gas phase were conducted to assess size-dependent kinetics of hydride formation in Pd clusters. Hydrogen diffusion coefficients, apparent diffusion barriers, and pre-exponential factors were derived from MD simulations of hydrogen diffusion in bulk Pd. Both the thermodynamic results of the GC-MC/MD method and the kinetic results of the MD simulations are in agreement with experimental values reported in the literature, thus validating the Pd/H interaction potential, and demonstrating the capability of the GC MC and MD methods for modeling the complex and dynamic phase behavior of hydrogen in Pd bulk and clusters.