Selective Hydrogenation of Phenylacetylene on Bimetallic Cu-Pd and Cu-Pt Catalysts.

摘要

The development of selective catalysts has become a key concept in improving the efficiency of processes. Controlling the product distribution of a reaction can result in fewer by-products and reduce energy requirements for process equipment downstream. The selective hydrogenation of alkynes to alkenes is of major importance to industrial polymerization processes where alkyne/diene impurities can poison the polymerization catalyst and have an unwanted inhibiting effect on the growth of the polymer chain. In many circumstances, bimetallic catalysts have proved to have superior catalytic properties such as greater activity, selectivity or stability compared to their monometallic analogs. A study by the Sykes group (Chemistry, Tufts) in collaboration with our group has shown that in ultra-high vacuum (UHV), the addition of Pd minority species (0.01 ML) onto an otherwise inert Cu(111) single crystal surface can activate the Cu surface for selective hydrogenation reactions. This thesis work is an extension of the surface science study to the preparation of bimetallic catalysts at the nanoscale and their testing in hydrogenation reactions at ambient reaction conditions. The overall aim of this work was to develop single atom alloy Pd-Cu and Pt-Cu catalysts which are highly active and selective for the selective hydrogenation reaction of phenylacetylene to styrene.;The bimetallic catalysts were prepared by a colloidal synthesis of Cu nanoparticles immobilized on gamma-alumina support and the precious metals as a minority species were deposited by galvanic replacement. The prepared materials and synthesis technique were characterized with electron microscopy (TEM), UV-Vis spectroscopy, X-Ray diffraction (XRD), temperature programmed reduction (TPR), BET surface area measurements, chemisorption experiments and X-ray photoelectron spectroscopy (XPS). The resulting catalysts can be described as gamma-Al2O3 supported Cu nanoparticles with a narrow size distribution. The Pt/Pd species are alloyed and well-dispersed on the surface of the nanoparticles.;The bimetallic catalysts were tested for the liquid phase hydrogenation of phenylacetylene and were compared to their monometallic analogs for reactivity and selectivity. Both the Pt-Cu and Pd-Cu bimetallic catalysts display a significant improvement of the selectivity to styrene, especially at high conversions. Based on the rate data, activation energy measurements and the model study done in UHV, it is concluded that the reaction mechanism has been altered in the bimetallic samples. Pt and Pd serve as sites for molecular hydrogen dissociation to hydrogen atoms, which in turn spillover onto the Cu surface where the hydrogenation reaction occurs. In summary, this work has demonstrated how Pd-Cu and Pt-Cu bimetallic catalysts can serve as both active and highly selective hydrogenation catalysts, where the Pd and Pt entities promote the hydrogenation activity of a Cu surface which maintains high selectivity for phenylacetylene hydrogenation to styrene.