Karl Franzens University Graz
Graz University of Technology
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Karl Franzens University Graz | Graz University of Technology | |
Probing Reaction Pathways on Model Catalyst Surfaces: Vinyl Acetate Synthesis on Palladium and Gold/Palladium Alloys Vinyl acetate monomer (VAM) is formed on supported palladium and palladium-gold alloy catalysts from ethylene, acetic acid and oxygen, where the reaction selectivity on the alloy is significantly higher than that on pure palladium. Two reaction pathways have been proposed. The first due to Samanos suggests that reaction occurs between ethylene and surface acetate species to form an acetoxyethyl-palladium intermediate, which decomposes via a ?-hydride elimination reaction to produce vinyl acetate. A second pathway, proposed by Moiseev, is initiated by vinyl formation, which inserts into a surface acetate to form VAM. The pathway for the synthesis of vinyl acetate is explored on a Pd(111) single crystal in ultrahigh vacuum, where it is shown that vinyl acetate is formed via the Samanos pathway. The activation energy of the reaction, calculated by density functional theory (DFT), agrees very well with the experimentally measured value for an acetate-saturated surface. This emphasizes the importance of repulsive lateral interactions on the crowded surface, which favor reaction pathways that are initiated by bond-forming reactions, as in the Samanos pathway. This implies that reactions that are initiated by bond-breaking reactions should be more favored on less crowded surfaces and this is confirmed by DFT calculations. Decreasing the acetate coverage and reducing crowding is accomplished by diluting the palladium sites with gold by forming a Au/Pd(111) alloy. In this case, it is shown that VAM is now formed via the Moiseev pathway. These results illustrate that surface science experiments on well-characterized samples can be used not only to understand relatively complex reaction pathways, but also to identify underlying principles. It is anticipated that the effect of crowding on the reaction is a general phenomenon in heterogeneous catalysts, and ways in which this can be exploited to lead to enantioselective surfaces is also discussed. |