Absence of spillover of hydrogen adsorbed on small palladium clusters anchored to graphene vacancies

Experimental evidence exists for the enhancement of the hydrogen storage capacity of porous carbons when these materials are doped with metal nanoparticles. One of the most studied dopants is palladium. Dissociation of the hydrogen molecules and spillover of the H atoms towards the carbon substrate has been advocated as the reason for the enhancement of the storage capacity. We have investigated this mechanism by performing ab initio density functional molecular dynamics (AIMD) simulations of the deposition of molecular hydrogen on Pd6 clusters anchored on graphene vacancies. The clusters are initially near-saturated with atomic and molecular hydrogen. This condition would facilitate the occurrence of spillover, since our energy calculations based on density functional theory indicate that migration of preadsorbed H atoms towards the graphene substrate becomes exothermic on Pd clusters with high hydrogen coverages. However, AIMD simulations show that the H atoms prefer to intercalate and absorb within the Pd cluster rather than migrate to the carbon substrate. These results reveal that high activation barriers exist preventing the spillover of hydrogen from the anchored Pd clusters to the carbon substrate.The authors acknowledge financial support by the Gobierno Vasco-UPV/EHU Project No. IT1246-19, the Spanish Ministerio de Ciencia e Innovación [Grants No. PID2019-107396 GB-I00/AEI/10.13039/501100011033 and PID2019-104924RB-I00], Junta de Castilla y León [Grant VA021G18], and University of Valladolid (Grupo de Física de Nanoestructuras). A. G. acknowledges a predoctoral fellowship from Junta de Castilla y León.Peer reviewe