Surface chemistry related with VAM synthesis on palladium and gold-palladium model catalysts.

摘要

The acetoxylation of ethylene using palladium-based catalyst is a commercial route for the synthesis of vinyl acetate monomer (VAM) by the following reaction, even though some controversy existed about the mechanism by which this reaction occurs. C2H4+CH3COOH+1/2 O2→C2H3OOCCH3 +H2O The addition of gold into the catalyst improves its activity and selectivity, but the role of Au has not been elucidated. Thus, a full understanding of the role of Au on the catalyst surface is essential to further improve its performance.;The gold-palladium surface alloy model catalysts on Pd(111) and Pd(100) were prepared. The AuPd alloys on Pd(111) and Pd(100) were titrated with CO. CO adsorption was dominated by the geometric effects. CO oxidation was also studied on the alloy surface, and the atomic oxygen was found to be the active species for CO oxidation.;The adsorption properties of ethylene, acetic acid and VAM were strongly affected by the presence of gold on the surface. At high gold coverages, the ethylene adopts a pi-bonded configuration and then transforms to the di-sigma bonded configuration at the gold coverage below 0.5 ML. The acetic acid forms eta 1 species and then forms eta2 species when the gold coverage is below 0.4 ML. The presence of oxygen stabilizes the eta 2 species to 410 K before it decomposes. Vinyl acetate on clean Pd surface adsorbs with vinyl group in a pi-bonded configuration; however, the presence of high coverage gold causes the VAM to adopt a di-sigma bonding configuration. The oxygen was found to have the same effect as gold on VAM adsorption. At low gold coverages (below thetaAu ∼80%), the VAM molecule will completely decompose on Au/Pd(100), while it is stabilized on Au/Pd(111).;Overall, the alloy formed on Pd(111) increases the VAM selectivity due to the stability of VAM on the surface; the alloy formed on Pd(100) surface shows high activity, the selectivity is low due to the complete decomposition of VAM when the thetaAu is below ∼80%, This also confirms the enhanced VAM selectivity for high gold coverage on Au(100).