Phase transition and lattice expansion during hydrogen loading of nanometer sized palladium clusters

In situ X-ray diffraction (XRD) measurements for Pd-clusters (3.8 and 6.0 nm) are performed during hydrogen loading and unloading. The lattice parameter increases as a function of the hydrogen partial pressure. The expansion is smaller than that of bulk palladium and is shown to be cluster-size dependent. An (α-α') phase transition was observed for the large clusters but small clusters do not show this transition. XRD analysis of the as-prepared clusters show that the 3.8-nm sized clusters predominantly have an icosahedral structure, while the 6.0-nm sized clusters have a cubic structure. The effect of size and structure of the cluster on the lattice expansion and on the phase transition will be discussed