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Atomistic Engineering of Catalyst Precursors: Dynamic Reordering of PdAu Nanoparticles during Vinyl Acetate Synthesis Enhanced by Potassium Acetate
journal contribution
posted on 2015-10-02, 00:00 authored by Elisabeth K. Hanrieder, Andreas Jentys, Johannes A. LercherThe 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.