Influence of metal dispersion on selectivity and kinetics of phenylacetylene hydrogenation catalyzed by supported palladium

Several Pd metal dispersions on vitreous supports and on charcoal have been prepared and used in the half-hydrogenation of phenylacetylene. The metal particle size distributions have been determined both by small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS). The degree of dispersion was dependent on the nature of the support; in the most favorable cases the Pd load was dispersed in metallic particles smaller than 30 Å in diameter. These catalysts hydrogenate phenylacetylene under mild conditions in heptane suspension. The kinetic results indicate that both half-hydrogenation rate and selectivity were largely affected by the degree of dispersion of Pd: a considerable reduction of catalytic effectiveness and selectivity has been observed as the averaged particle diameter was increased. Results have been interpreted and discussed in terms of hydrogen availability on the reacting catalytic center and of the metallic phase features