Hydrogen storage in Pd clusters

Hydrogen storage in a Pd cluster is studied theoretically using a self-consistent density-functional scheme with a norm-conserving pseudopotential. We mainly focus on the cuboctahedral Pd₁₃H_n clusters with n = 1, 6, 8, and 14. Two stable sites for hydrogen adsorption are found; one is slightly inside the square face of the cluster and the other is outside of the triangular face. Electronic states relevant to hydrogen adsorption are well described by the cluster-centre electronic states (CCSs); this term refers to the states produced by the centre cluster (hydrogen atoms and the centre Pd atom) interacting with peripheral Pd atoms. When the hydrogen atoms are adsorbed inside the tetrahedral face, the cluster is stabilized by expanding the Pd-Pd distance by 3.7%, which is close to the lattice expansion in the β-phase of bulk PdH. As the face-centred-cubic bulk can be fully `clusterized' by these clusters, we estimate the entropy change of the bulk upon transition as the change in configuration entropy between these clusters.