Structure and reactivity of Pd-Pt clusters produced by laser vaporization of bulk alloys

Pd-Pt nanoclusters are obtained by the focusing of an Nd:YAG laser onto rod s of alloys. The aggregates, which are produced by plasma cooling via short helium bursts synchronized with the laser pulses, are collected on amorpho us carbon or silicon substrates, in a UHV chamber. Transmission electron mi croscopy (TEM) experiments show that the diameters of the clusters range be tween 1.5 and 4.5 nm, and analytical microscopy indicates that they have th e same composition as the vaporized rods. Low-energy ion scattering (LEIS) also shows that the surface of the obtained clusters is Pd enriched: the Pd concentration in the first atomic layer is found to be equal to 38% for a Pd17Pt83 rod composition and 87% for the Pd-65 Pt-35 ahoy. The catalytic ac tivity of these clusters in the hydrogenation of 1,3-butadiene to butenes a nd butane is measured in static mode, with mass spectrometry detection. The reactivity of the bimetallic clusters is explained by the atomic local ord er and low-coordination sites considered as "hot sites".