Hydrogenation of Toluene over #gamma#-Al_2O_3-Supported Pt,Pd,and Pd-Pt Model Catalysts Obtained by Laser Vaporization of Bulk Metals

摘要

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 (y-A1203). The bimetallic clusters have a perfectly well-defined stoichiometry, as demonstrated by energy-dispersive X-ray analysis experiments. Transmission electron mi-croscopy 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 condi-tions. The activation energies are 44 ± 6 kJ/mol. The pressure de-pendence on toluene varies from 0 at 333 K to 0.7 at 500 K whereas this dependence on H2 increases from approximately 05 at 333 K up to 2.2 at 500 K. The results obtained with the pure metals cor-roborate those reported in the literature for catalysts prepared by 1681 chemical methods. The activities of the bimetallic catalysts do notshow 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.