From Palladium Clusters in Solutions to Supported Nanocatalysts for Hydrocarbon Synthesis

Abstract

Literature data on palladium clusters obtained by the interaction of Pd(II) compounds in solutions with hydrogen are summarized and analyzed. The conclusion is drawn on the influence of ligands and reduction conditions on the polynuclearity of clusters and on the autocatalytic nature of their formation. Palladium clusters become active in the selective hydrogenation of alkynes and dienes only after their treatment with hydrogen containing traces of oxygen or hydrogen peroxide. Clusters containing phosphide ligands are active in the homogeneous hydrogenation of unsaturated hydrocarbons with a high selectivity being of a thermodynamic nature. Most of the distinctive features of nanocluster synthesis in solution are also observed in the synthesis of metal-containing nanoparticles on the support surface from precursors. Catalysts for the liquid-phase selective hydrogenation of phenylacetylene on the Pd supported on coal with a nanoparticle size of 2.8 ± 0.2 nm are obtained. This size is comparable with the size of clusters in solution. The mechanism for the reduction of palladium acetate on carbon is also autocatalytic and is similar to the same process in solutions. A kinetic study of the selective hydrogenation of phenylacetylene to styrene in the presence of Pd supported on coal shows that it is impossible to adequately describe the experimental data in the framework of the Langmuir, Hinshelwood, or Eley–Rideal mechanisms. It is proposed to use this mechanism in the framework of cluster (polynuclear) active sites on the surface of Pd particles capable of adsorbing more than one substrate molecule or its fragments on the same site. This approach makes it possible to describe the mechanisms of other reactions occurring in the presence of nanoscale heterogeneous supported catalysts: deoxygenation of fatty acids obtained from renewable raw materials to paraffins on supported palladium catalysts and olefins on supported nickel sulfide catalysts.