Hydrogen induced melting of Palladium clusters

Abstract

To investigate the effect of adsorbates on struc- tural transitions in small palladium aggregates, we have per- formed molecular dynamics simulations of cluster "melting" in presence of atomic hydrogen. Intriguing questions are how the exposure to hydrogen modifies the metal-metal interac- tion, and whether hydrogen could lower the onset of melting. Various structural and caloric methods were used to cap- ture signatures of melting. We present results for the bare icosahedral Pd55 cluster and also Pd55 exposed to different amounts of hydrogen. Our results indicate that the melting transition is significantly lowered as hydrogen adsorbs on the cluster, and that the decrease in melting temperature depends sensitively on the hydrogen loading. pre-melting and an almost size independent heat of trans- formation per atoms for Pd13 ,P d 55 and Pd147. In connection with reactions involving clusters, an especially interesting question is the influence of adsorbates and a carrier gas at- mosphere on the thermodynamical properties of metal clus- ters. In this work we study this issue by molecular dynamics simulations of palladium clusters melting in presence of hy- drogen. We will present results for the bare icosahedral Pd55 cluster, as well as Pd55 exposed to various amounts of hy- drogen. To study the temperature dependent dynamics of palla- dium clusters, we need an accurate description of interatomic interactions. The Hamiltonian describing the system should be capable of correctly selecting between different minima in the potential energy surface, and also account for the re- hybridization due to adsorbed hydrogen. We have used the Many-Body-Alloy (MBA) Hamiltonian (10), which is based on the second-moment approximation for the electronic den- sity of states, and a pairwise repulsive Born-Mayer interac- tion. The parametrized binding energy for each atom is given by