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

Abstract

Pd-Pt nanoclusters are obtained by the focusing of an Nd:YAG laser onto rods of alloys. The aggregates, which are produced by plasma cooling via short helium bursts synchronized with the laser pulses, are collected on amorphous carbon or silicon substrates, in a IIFIV chamber. Transmission electron microscopy (TEM) experiments show that the diameters of the clusters range between 1,5 and 4.5 nm, and analytical microscopy indicates that they have the sanie 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 Pd₆₅ Pt₃₅ alloy. The catalytic activity of these clusters in the hydrogenation of 1,3-butadiene to butenes and butane is measured in static mode, with mass spectrometry detection. The reactivity of the bimetallic clusters is explained by the atomic local order and low-coordination sites considered as “hot sites”.