Elucidation of the Reaction Mechanism for the Palladium-Catalyzed Synthesis of Vinyl AcetateWe gratefully acknowledge support of this work by the US Department of Energy, Division of Chemical Sciences, Office of Basic Energy Sciences (Grant No. DE-FG02-92ER14289).

The first, suggested by Samanos et al. , involves the coupling of ethylene directly with chemisorbed acetate. The resulting acetoxyethyl–palladium intermediate then undergoes b-hydride elimination to form vinyl acetate. Alternatively, ethylene could first dehydrogenate to form a vinyl–palladium intermediate, which then couples with an acetate species adsorbed on the surface to form the VAM directly. This latter mechanism was proposed by Moiseev et al. 4] as well as by Nakamura and Yasui. We have recently shown that ethylene in the gas phase reacts with a model-catalyst surface of h-acetate moieties adsorbed on a Pd(111) surface precovered with a (252) oxygen overlayer to form the VAM, thus implying that the acetate species is the precursor to formation of the VAM. The structures of the adsorbed h-acetate and ethylene species were determined separately on a Pd(111) surface by using low-energy electron diffraction (LEED) and reflectionabsorption IR spectroscopy (RAIRS), which showed that both species adsorb on the bridge site. Herein, we discuss the investigation into the nature of the reaction pathway by using isotopically labeled reactants, which allowed the mechanism for the formation of the VAM to be unequivocally identified. The RAIRS experiments were performed in an ultrahigh vacuum apparatus; the experimental procedure has been described in detail elsewhere. Kinetic measurements were typically carried out by the collection of spectra over 100 scans, which is a relatively short procedure that took 25 seconds because of the large intensity of the asymmetric vibrational mode of the acetate moiety (O-C-O) at 1414 cm . Some experiments were carried out with a larger number of scans to yield spectra with better signal-tonoise ratios and, therefore, allow additional surface species to be identified. A flux of ethylene impinged onto the sample from a collimated dosing source to obtain an enhanced flux at the Pd(111) single-crystal surface while the background pressure was minimized. C2H4 (Matheson, Research Grade), acetic acid (Aldrich, 99.99+ %), C2D4 (CIL, 98% D), CD2CH2 (CDN, 99% D), CHDCHD (CDN, cis/ trans mixture, 99% D), and O2 (CIL, 95% O2) were transferred into glass bottles, which were attached to the gas-handling line for introduction into the vacuum chamber. It has been shown previously that acetate species adsorbed on a Pd(111)–O(252) surface undergoes reaction with ethylene in the gas phase to yield the VAM. The experimental titration curves obtained with acetate species can be explained by using a procedure described previously, in which it was assumed that adsorbed ethylene underwent reaction with the acetate species and that ethylene adsorption was blocked by the acetate species. Acetate species adsorbed on clean Pd(111) surfaces (namely, in the absence of the (252) oxygen overlayer) are also removed by reaction with ethylene and also can be analyzed by using the same procedure used to reproduce the kinetic data for the oxygen-covered Pd(111) surface; furthermore, these acetate species yield similar rate constants to those found for reactions on the oxygen-covered surface, and therefore the presence of coadsorbed oxygen does not substantially affect the reaction kinetics. A series of IR spectra for the reaction of the acetate species on a clean Pd(111) surface was collected just after the acetate moieties had been completely removed using C2H4, C2H4 (on an O-covered surface), and C2D4 (Figure 1). The spectrum of CO on the Pd(111) surface is shown for comparison (Figure 1a). The CO exposure was selected to