AVS 45th International Symposium
    Surface Science Division Monday Sessions
       Session SS2-MoA

Paper SS2-MoA8
The Catalytic Chemistry of Small Hydrocarbons on Palladium: Cyclization, and Hydrogenation

Monday, November 2, 1998, 4:20 pm, Room 309

Session: Surface Chemistry on Model Catalysts
Presenter: W.T. Tysoe, University of Wisconsin, Milwaukee
Authors: W.T. Tysoe, University of Wisconsin, Milwaukee
H. Molero, University of Wisconsin, Milwaukee
M. Kaltchev, University of Wisconsin, Milwaukee
Correspondent: Click to Email
The reaction pathways for acetylene trimerization catalyzed by palladium is investigated using a range of surface-sensitive techniques. It is found that benzene is formed from acetylene via the intervention of a metallocyclic C@sub4@subH@sub4@sub intermediate. This further reacts with a third acetylene to form benzene. However, the catalytic reaction proceeds in the presence of a carbonaceous layer. which consists of vinylidene species. At high pressures, acetylene is formed by reaction between acetylene adsorbed on the vinylidene-covered palladium surface and adsorbed vinylidene itself to form a C@sub4@subH@sub4@sub intermediate. It is shown that the hydrocarbon part of this layer can be removed by reaction in high pressures (several Torr) of hydrogen. The activation energy for this process on molybdenum surfaces is about 6 kcal/mol. Correspondingly, the rate of acetylene cyclotrimerization is found to be accelerated by the addition of hydrogen to the reaction mixture. The implication of these phenomena on alkene and alkyne hydrogenation reactions catalyzed by transition metals will also addressed and a model for hydrogenation reactions under high-pressure conditions is proposed that yield kinetic parameters in good agreement with those measured experimentally.