Atomistic Engineering of Catalyst Precursors: Dynamic Reordering of PdAu Nanoparticles during Vinyl Acetate Synthesis Enhanced by Potassium Acetate

The presence of potassium acetate (KOAc) on bimetallic PdAu catalysts increases the rate of reaction for vinyl acetate (VA) formation from ethene and acetic acid by a factor of 10 and the selectivity by 20%. The dynamic transitions of typical supported catalyst precursors with an atomic Pd/Au ratio of 2/1 were explored during synthesis in the presence and absence of KOAc. The dopant induces reordering of PdAu toward a Pd₁Au₁ phase, while Au-enriched Pd₄₀Au₆₀ bimetallic particles form primarily in the absence of KOAc. Pd-acetate species are generated via leaching of Pd from PdAu precursor particles during the reaction. These species are Pd₃(OAc)₆ and Pd₂(OAc)₄ in the absence of KOAc and K₂Pd₂(OAc)₆ in the presence of KOAc. Palladium in K₂Pd₂(OAc)₆ can be readily reduced by C₂H₄ to Pd⁰, while Pd₃(OAc)₆, which contains more stable, bridged acetate ligands remains stable. Reduced Pd either forms dispersed Pd⁰ or is incorporated into the metal particles. KOAc enhances rates and selectivity to VA by stabilizing, on the one hand, active Pd species at the bimetallic surface. On the other hand, KOAc enriches acetic acid close to the surface and forms Pd surface acetates, postulated to enhance the rate and the selectivity to VA by suppressing ethylene adsorption and oxidation.