Hydrogen and Pd-clusters

Isotherms of Pd-H clusters with different sizes differ from bulk isotherms: they show a largely enhanced solubility in the low-concentration regime. The isotherms resemble those of bulk above the critical point, they show no flat plateau region, but a slope. However, the existence of a hysteresis gives evidence for a phase transition even in small Pd-H clusters. Structural studies of 6.0 nm clusters show a transition between two cubic phases. 3.8 nm Pd-H clusters always show an icosahedral structure in the low and high concentration regime. For an intermediate size of 5.0 nm Pd-H clusters, the lattice structure changes during H-absorption, from cubic to, most probably, icosahedral. The slope in the cluster isotherm's two-phase region is here attributed to stress emerging between the surfactant shell and the cluster during hydrogen-loading. (C) 2003 Elsevier B.V. All rights reserved.