Hydrogenation of Toluene over γ-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 of bulk metals or alloys have been deposited on a high surface area alumina (γ-Al2O3). The bimetallic clusters have a perfectly well-defined stoichiometry, as demonstrated by energy-dispersive X-ray analysis experiments. Transmission electron microscopy shows that pure or bimetallic clusters have 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 activation 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 H2 increases from approximately 0.5 at 333 K up to 2.2 at 500 K. The results obtained with the pure metals corroborate those reported in the literature for catalysts prepared by chemical methods. The activities of the bimetallic catalysts do not show any synergy between Pd and Pt but rather a perfect additivity of their individual catalytic properties. Pt has been shown to have no electronic influence on the reactivity of Pd atoms. The nonlinear variation of the turnover frequency as a function of the Pd surface concentration is interpreted on the basis of an active site composed of a pair of atoms.