Catalysis by metal colloids : Trajectories for atom assembling in Pd and Pt colloids

The methods for the synthesis of giant clusters and the pathways of their formation from palladium and platinum complexes are discussed. The approaches to the study of these compounds, based on the concepts of both cluster chemistry and physics of solids, are discussed. Giant clusters contain a positively charged, close-packed nucleus of metal atoms, whose number can be several hundreds and have discrete values, "magic numbers," for example, Pt-55, Pt-309, and Pd-561. The metal nucleus is stabilized by the coordinated ligands, for example, PPh3, phen, dipy, O2-, and Cl-, and the outer-sphere anions like OAc- and PF6-. The structures of giant clusters are determined by high-resolution electron microscopy, electron diffraction, tunneling microscopy, small-angle X-ray scattering, EXAFS, and other techniques combined with elemental microanalysis. The polynuclear low-valence complexes [Pd(4)phen(oAc)(2)(H)(x)](n), [Pt(4)phen(OAc)(3)](n), and [Pt(8)phen(3)(OAc)(4)(OH)(4)(H2O)(6)](m) (n, m similar to 100), which are the precursors of giant clusters, are isolated and characterized using the same techniques. Their pathways of transformation to giant clusters are studied using temperature-programmed EXAFS combined with thermal analysis (DTA-TG). Giant clusters exhibit high catalytic activity in the oxidation of alkenes, CO, and alcohols, the oxidative acetoxylation of alkenes and alkylarenes, the reduction of nitriles and nitroarenes, acetal formation, and the oxidative carbonylation of phenol.