Hydrogenation of toluene over gamma-Al2O3-supported Pt, Pd, and Pd-Pt model catalysts obtained by laser vaporization of bulk metals

Pure Pd, Pt, and bimetallic Pd-Pt clusters produced by laser vaporization o f bulk metals or alloys have been deposited on a high surface area alumina (gamma -Al2O3). The bimetallic clusters have a perfectly well-defined stoic hiometry, as demonstrated by energy-dispersive X-ray analysis experiments. Transmission electron microscopy shows that pure or bimetallic clusters hav e similar size distributions. The catalytic properties of Pd, Pt, Pd17Pt83, and Pd65Pt35 supported clusters have been investigated in the vapor-phase toluene hydrogenation over a wide range of reaction conditions. The activat ion energies are 44 +/- 6 kJ/mol. The pressure dependence on toluene varies from 0 at 333 K to 0.7 at 500 K whereas this dependence on H-2 increases f rom approximately 0.5 at 333 K up to 2.2 at 500 K. The results obtained wit h the pure metals corroborate those reported in the literature for catalyst s prepared by chemical methods. The activities of the bimetallic catalysts do not show any synergy between Pd and Pt but rather a perfect additivity o f their individual catalytic properties. Pt has been shown to have no elect ronic influence on the reactivity of Pd atoms. The nonlinear variation of t he turnover frequency as a function of the Pd surface concentration is inte rpreted on the basis of an active site composed Of a pair of atoms. (C) 200 1 Academic Press.