Interstitial Carbon Dopant in Palladium–Gold Alloy Boosting the Catalytic Performance in Vinyl Acetate Monomer Synthesis

Vinyl acetate monomer (VAM), an important chemical intermediate in industry, is produced by the well-established commercial process of acetoxylation of ethylene with Pd-Au/SiO₂ and a KOAc promoter. No paper has since decades defined the true effects of Au and KOAc, despite numerous attempts to clarify them. The role of subsurface carbon as a catalyst booster for enhanced catalytic performance in VAM synthesis was found by us for the first time. X-ray diffraction and X-ray absorption fine structure studies revealed that carbon atoms spontaneously doped into the Pd-Au alloy lattice while maintaining the alloy's size, metallic state, and alloy composition. Additionally, during the process, the KOAc addition dramatically raised the equilibrium carbide fraction. Because of the high carbide fraction, KOAc/Pd_0.8Au_0.2/SiO₂ had a 5.6-fold higher formation rate (89.0% selectivity) than Pd_0.8Au_0.2/SiO₂ (69.2% selectivity) due to high carbide fraction. Surprisingly, kinetic and theoretical analyses showed that the coupling of acetate and ethylene, which is a rate-determining step, is effectively promoted by the synergistic contributions of Au (electronic/geometric effects) and interstitial carbon (electronic effect). Additionally, the synergy inhibits ethylene dehydrogenation, which ultimately slows the formation of CO₂. The contentious debates about the roles of Au and KOAc in the acetoxylation of ethylene have been resolved thanks to experimental and theoretical insights into the roles of Pd-Au formation, Au/Pd ratio, and interstitial carbon atoms. These insights also open the door for the logical design of catalysts with desirable catalytic performance.