City University of Hong Kong
Vinyl acetate monomer (VAM) is formed on supported palladium and palladium-gold alloy catalysts from ethylene, acetic acid and oxygen, were 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.
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Event: |
MEEM Seminar 0809_002 |
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Date: |
14 July 2008 (Monday) |
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Time: |
10:30 am - 12:00 noon (Tea reception at 10:00 am) |
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Venue: |
Room B6605 (FSE Conference Room) |
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Speaker: |
Professor Wilfred T. Tysoe |
Wilfred Tysoe is a Distinguished Professor of Chemistry at the University of Wisconsin-Milwaukee. He received his B.Sc. in Chemical Physics at the University of Manchester, England after which he spent a year teaching Chemistry and Physics in Ghana, Africa. He then obtained an M.Sc degree at the University of Sydney, Australia following which he held the position of Senior Scientist at the Plessey Company in England. He then went on to receive his Ph.D. degree in 1982 in Physical Chemistry at the University of Cambridge, England and worked as a post-doctoral researcher at the University of California, Berkeley, before moving to his present position at the University of Wisconsin-Milwaukee in 1984. His research interests focus on chemical properties at surfaces with particular emphasis on understanding catalytic reactions both in ultrahigh vacuum and under realistic conditions and in the area of tribology to understand the interplay between lubricant chemistry and the resulting frictional and wear properties of reactively formed tribological films. He is an Editor in Chief of Tribology Letters, which he co-founded about ten years ago. He is a member of the American Chemical Society, the American Vacuum Society and the Society for Tribologists and Lubrication Engineers (STLE), for which he serves on the publications committee and is a co-contributor to the "Cutting Edge" column in the STLE magazine, Tribology and Lubrication Technology. He has published over 200 papers in scientific journals and presented over 200 talks.
Enquiry: MEEM General Office (Tel: 2788 8420 Email: mego@cityu.edu.hk )