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/Pd0.8Au0.2/SiO2 had a 5.6-fold higher formation rate (89.0% selectivity) than Pd0.8Au0.2/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.

High-Entropy Intermetallics Serve Ultrastable Single-Atom Pt for Propane Dehydrogenation.

Propane dehydrogenation has been a promising propylene production process that can compensate for the increasing global demand for propylene. However, Pt-based catalysts with high stability at ≥600 °C have barely been reported because the catalysts typically result in short catalyst life owing to side reactions and coke formation. Herein, we report a new class of heterogeneous catalysts using high-entropy intermetallics (HEIs). Pt-Pt ensembles, which cause side reactions, are entirely diluted by the component inert metals in PtGe-type HEIs. The resultant HEI (PtCoCu) (GeGaSn)/Ca-SiO2 exhibited an outstandingly high catalytic stability, even at 600 °C (kd-1 = τ = 4146 h = 173 d), and almost no deactivation of the catalyst was observed for 2 months for the first time. Detailed experimental studies and theoretical calculations demonstrated that the combination of the site-isolation and entropy effects upon multi-metallization of PtGe drastically enhanced the desorption of propylene and the thermal stability, eventually suppressing the side reactions even at high reaction temperatures.