Coverage effects on the palladium-catalyzed synthesis of vinyl acetate: comparison between theory and experiment.

The high adsorbate coverages that form on the surfaces of many heterogeneous catalysts under steady-state conditions can significantly lower the activation energies for reactions that involve the coupling of two adsorbed intermediates while increasing those which result in adsorbate bond-breaking reactions. The influence of the surface coverage on the kinetics of metal-catalyzed reactions is often ignored in theoretical and even in some ultrahigh vacuum experimental studies. Herein, first principle density functional theoretical calculations are combined with experimental surface titration studies carried out over well-defined Pd(111) surfaces to explicitly examine the influence of coverage on the acetoxylation of ethylene to form vinyl acetate over Pd. The activation energies calculated for elementary steps in the Samanos and Moiseev pathways for vinyl acetate synthesis carried out on acetate-saturated palladium surfaces reveal that the reaction proceeds via the Samanos mechanism which is consistent with experimental results carried out on acetate-saturated Pd(111) surfaces. The rate-limiting step involves a beta-hydride elimination from the adsorbed acetoxyethyl intermediate, which proceeds with an apparent calculated activation barrier of 53 kJ/mol which is in very good agreement with the experimental barrier of 55 +/- 4 kJ/mol determined from kinetic measurements.