Energetics and dynamics of hydrogen adsorption, desorption and migration on a carbon-supported palladiumcluster

Abstract

The functionality of metal-doped carbon materials in catalytic and hydrogen storage applications is governed by the characteristics of their interaction with hydrogen. The dynamics of hydrogen chemisorption, its migration and desorption are investigated by first-principles molecular dynamics simulations of a model Pd₄ cluster supported on a coronene molecule. Longer time scale events are accelerated using the metadynamics technique. This article is a comprehensive report of the energetics and dynamics of (i) dissociative chemisorption of molecular hydrogen on a carbon-supported Pd cluster, (ii) transport of atomic hydrogen on the cluster, towards the carbon support and (iii) the associative desorption of atomic hydrogens in a molecular form. It is found that the dissociative chemisorption of hydrogen on the cluster involves a negligible energy barrier and takes place readily at room temperature. The migration of atomic hydrogen from the tip of the tetrahedral Pd₄ cluster to its sides, on the other hand, involves an energy barrier of ∼6 KJ/mole. This energy barrier, however, is reduced when the cluster is partially saturated with pre-existing hydrogens adsorbed on its sides. The energy barrier for the subsequent migration of atomic hydrogen, from the sides of the cluster towards the carbon support, is however not affected significantly by the presence of pre-existing hydrogens. Overall, the transport of atomic hydrogen from the tip of the cluster towards the carbon support is energetically favourable and only involves low energy barriers. The associative desorption of two hydrogen atoms from the cluster to form molecular hydrogen is an endothermic process and also involves a small energy barrier. This work sheds light onto the mechanism of the hydrogen interaction with Pd-doped-carbon-supported catalytic materials and, more generally, onto the mechanism of hydrogen spillover on such materials.