AVS 52nd International Symposium
    Surface Science Monday Sessions
       Session SS1-MoM

Paper SS1-MoM5
The Reaction Pathway for the Synthesis of Vinyl Acetate on Pd(111)

Monday, October 31, 2005, 9:40 am, Room 202

Session: Catalytic Chemistry of Hydrocarbons
Presenter: W.T. Tysoe, UW-Milwaukee
Authors: W.T. Tysoe, UW-Milwaukee
F. Calaza, UW-Milwaukee
D. Stacchiola, UW-Milwaukee
Correspondent: Click to Email
The palladium-catalyzed synthesis of vinyl acetate from acetic acid, ethylene and oxygen is studied on Pd(111) single crystal surfaces. The rate of reaction between gas-phase ethylene and adsorbed acetate species is followed using infrared spectroscopy by measuring the time dependence of the intense OCO acetate mode at 1414 cm@super -1@. The acetate removal kinetics are modeled by assuming that ethylene adsorption is blocked by acetate species. Temperature-programmed reaction confirms that vinyl acetate is formed in the reaction. The reaction pathway is explored by comparing the reaction rates of d-@sub 4@-, 1,1-d@sub 2@-, 1,2-d@sub 2@ and normal ethylene with surface acetates. d@sub 4@-ethylene reacts about six times more slowly than C@sub 2@H@sub 4@ indicating that hydrogen is involved in the rate-limiting step. The reaction rates are different for 1,1-C@sub 2@H@sub 2@D@sub 2@ and 1,2-C@sub 2@H@sub 2@D@sub 2@. This indicates that vinyl acetate formation occurs via the insertion ethylene into the surface acetate species to form an acetoxyethyl-palladium intermediate, which then reacts to form vinyl acetate via a @beta@-hydride elimination reaction, rather than by vinyl formation and addition to the acetate species to form vinyl acetate. The different reactivities of the isotopomers is rationalized by making the reasonable assumption that ethylene adsorption is blocked by the acetoxyethyl-palladium intermediate. When reaction is carried out using 1,1-C@sub 2@H@sub 2@D@sub 2@, the intermediate can be formed with exclusively deuterium atoms in the @beta@-position, slowing its rate of decomposition, blocking ethylene adsorption, and slowing the reaction rate. Reaction with 1,2-C@sub 2@H@sub 2@D@sub 2@ results in one hydrogen being in the @beta@-position, allowing it to decompose more rapidly. Final conformation of this pathway comes from detecting the acetoxyethyl-palladium intermediate using infrared spectroscopy when reaction is carried out using C@sub 2@D@sub 4@.