Diffusion of Hydrides in Palladium Nanoclusters. A Ring-Polymer Molecular Dynamics Study of Quantum Finite Size Effects.

Abstract

The diffusion kinetics of hydrogen in bulk palladium and in Pd nanoclusters containing up to 512 atoms has been theoretically investigated at 3% loading using ring-polymer molecular dynamics simulations. The electronic ground-state energy surfaces are modeled using an explicit many-body potential fitted to reproduce the properties of bulk palladium and palladium hydrides. The diffusion constant, calculated by integration of the velocity autocorrelation function, shows Arrhenius behavior with inverse temperature. In addition, both the prefactor and activation energy are found to exhibit approximately linear variations with inverse cluster radius for sizes exceeding 128 Pd atoms. Vibrational delocalization generally enhances diffusion, this effect being stronger in clusters than in bulk. An inherent structure analysis from the positions of the centroids was used to characterize the diffusion mechanisms. Quantum effects lead to not only a higher coordination of hydrogen atoms both in bulk (fcc) palladium and in clusters but also favor further softening of the outer layers.