Thermodynamic aspects of the interaction of hydrogen with Pd clusters

Clusters are agglomerates of a few to some hundred atoms. A large fraction of the atoms find themselves on a surface site. The geometrical structure of a cluster, in contrast to the structure of crystalline bulk material, cannot be described with a repeated, space filling unit cell. Clusters appear often in regular geometrical polyhedrons, e.g. cuboctahedron or icosahedron. The interaction between hydrogen and palladium clusters of different sizes was investigated. The sorption properties of the clusters are compared with the well-known properties of Pd bulk material. The Pd clusters reversibly absorb and desorb hydrogen. The miscibility gap (plateau in the pressure concentration isotherm) narrows with decreasing cluster size and therefore, an increased hydrogen solubility was found for Pd clusters as compared to bulk Pd. The hysteresis between the absorption and the desorption pressure decreases with decreasing cluster size. (C) 2000 Elsevier Science B.V. All rights reserved.