Atomistic Engineering of Catalyst Precursors: Dynamic Reordering of PdAu Nanoparticles during Vinyl Acetate Synthesis Enhanced by Potassium Acetate
2015-10-02T00:00:00Z (GMT) by
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 Pd<sub>1</sub>Au<sub>1</sub> phase, while Au-enriched Pd<sub>40</sub>Au<sub>60</sub> 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 Pd<sub>3</sub>(OAc)<sub>6</sub> and Pd<sub>2</sub>(OAc)<sub>4</sub> in the absence of KOAc and K<sub>2</sub>Pd<sub>2</sub>(OAc)<sub>6</sub> in the presence of KOAc. Palladium in K<sub>2</sub>Pd<sub>2</sub>(OAc)<sub>6</sub> can be readily reduced by C<sub>2</sub>H<sub>4</sub> to Pd<sup>0</sup>, while Pd<sub>3</sub>(OAc)<sub>6</sub>, which contains more stable, bridged acetate ligands remains stable. Reduced Pd either forms dispersed Pd<sup>0</sup> 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.