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

Paper SS1-MoM6
Vinyl Acetate Synthesis on Pd-based Catalysts: Structure Sensitivity and Active Site Identification

Monday, October 31, 2005, 10:00 am, Room 202

Session: Catalytic Chemistry of Hydrocarbons
Presenter: D. Kumar, Texas A&M University
Authors: D. Kumar, Texas A&M University
M.S. Chen, Texas A&M University
Y.F. Han, Texas A&M University
C.-W. Yi, Texas A&M University
K. Luo, Texas A&M University
D.W. Goodman, Texas A&M University
Correspondent: Click to Email
The synthesis of vinyl acetate (VA) is a very important industrial reaction and is produced by acetoxylation of ethylene over Pd-Au catalysts. Historically this highly selective reaction has been studied using conventional kinetic techniques; however, there is no consensus regarding a reaction mechanism. A combination of surface science techniques and kinetic measurements has been used in the current study to address the structure sensitivity and the active ensemble for the reaction. VA synthesis was carried out over Pd and Pd-Au high-surface-area and model planar catalysts and the reaction rates found to be: Pd(100) < 5 wt% Pd/SiO2(d@sub Pd@=4.0nm) < 1 wt% Pd/SiO2(d@sub Pd@=2.5nm). A particle size dependence of the reaction rates implies a degree of structure sensitivity. Furthermore, the addition of Au to Pd/SiO2 catalysts leads to a significant increase in the reaction rate and selectivity with the latter related directly to a change in the reaction order with respect to ethylene. Infrared reflection absorption spectroscopy (IRAS) of CO on Pd/Au(100) and Pd/Au(111) confirms that Pd is present as isolated monomers on a Au-rich surface. A pair of Pd monomers is the optimum active site for the adsorption of ethylene and acetate species, and leads to the formation of VA. The spacing between the two active Pd monomer pairs is crucial on Au(100) and Au(111), evident by the relative rates of VA synthesis on Pd/Au model catalysts, i.e. Pd/Au(111) < Pd/Au(100).