Sonochemical preparation and properties of nanostructured palladium metallic clusters

Abstract

Nanoscale particles of palladium metallic clusters have been prepared at room temperature by sonochemical reduction of a 1:2 molar mixture of palladium acetate, Pd(O₂CCH₃)₂ , and myristyltrimethylammonium bromide, CH₃(CH₂)₁₂N(CH₃)₃Br (NR₄X), in tetrahydrofuran (THF) or methanol. Apart from its stabilizing effect, NR₄X acts as a reducing agent, probably due to the decomposition that occurs at the liquid-phase region immediately surrounding the collapsing cavity, and provides reducing radicals. Addition of 0.2 m ethanol–methanol in the THF process enhances the sonochemical reduction of Pd^II because of its highly volatile nature producing various reducing radicals inside the collapsing bubble. Analyses by UV–VIS spectroscopy indicated the initial formation of a Pd^II–NR₄X complex, which, in turn, reduced to Pd⁰ . Elemental analysis of the resulting solid (sonication residue) shows that the THF process yields NR₄X stabilized-palladium clusters, whereas the methanol process shows the formation of pure Pd agglomerates. X-Ray diffraction (XRD) and transmission electron microscopy (TEM) with selected area electron diffraction (SAED) techniques were carried out to ascertain the nature, size and morphology of the Pd clusters. TEM of NR₄X stabilized-Pd shows the presence of spherical particles of 10–20 nm in size. Selected area electron diffraction (SAED), along with TEM, reveals that the pure Pd consists of dense agglomerates, whereas NR₄X stabilized-Pd exists as thin crystallites. These Pd nanoclusters are catalytically active towards carbon–carbon coupling, or Heck reaction, in the absence of phosphine ligands, to a moderate extent of 30% conversion. Hydrogenation of cyclohexene to cyclohexane has also been studied using sonochemically generated Pd materials. The catalytic ability of these Pd materials was compared with the commercial Pd on carbon material.