Ab initio dynamical simulations based on the density functional formalism have been performed for molecular hydrogen impinging on a Pd6 cluster anchored to a vacancy defect in graphene. Under the conditions assumed in the simulations, most H2 molecules rebound after colliding with the Pd6 cluster, but a number of molecules stay adsorbed on its surface. Depending on whether the substrate is initially at 0 or 300 K, either one-third or one-half of those adsorbed molecules later on dissociate on the cluster, leading to two chemisorbed H atoms. For both substrate temperatures, dissociation of H2 triggers a transition from the original octahedral structure of the anchored Pd6 to an incomplete pentagonal bypiramid structure, which is essentially produced by a severe elongation of the distance between two particular neighbor Pd atoms. Interestingly, no such structural changes were previously observed for Pd6 adsorbed on pristine graphene. Although this new result comes for a specific reaction—this one occurring, for instance, in the anode of hydrogen fuel cells—we anticipate that the observation of a structural change, which means that the cluster structure is not immune to the reaction taking place on its surface, can be relevant for many catalytic processes occurring on the surface of small metal particles.
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