It is known that vinyl acetate monomer synthesis over bimetallic Pd 1 Au 1 catalysts is highly structurally sensitive and that these structures are dynamically formed and disintegrated in reactant gas atmosphere. Here we show via a combination of bulk and surface sensitive methods that independent of their nominal composition, Pd/Au bimetallic particles undergo marked reconstruction and phase partition during vinyl acetate synthesis. While temperature-induced reorganization of SiO 2 -supported Pd x Au y particles leads to all three thermodynamically metastable phases, i.e., Pd 3 Au, Pd 1 Au 1 , PdAu 3 , with mainly Au-enriched surface compositions, the reactive atmosphere induces selectively the formation of the Pd 1 Au 1 phase and separated Pd particles. The Pd in the Pd 1 Au 1 has higher specific activity and selectivity than the sole Pd nanoparticles, exemplifying the importance of changing its electronic nature. As the Pd 1 Au 1 phase dominates the catalytic activity, novel tailored catalysts could be produced, limiting the presence of dispersed Pd.
SiO 2 -supported Pd−Au catalysts with Pd/Au molar ratios varying from 0.8 to 7.0 were used as catalysts for vinyl acetate synthesis under industrial conditions. Continued operation of the bimetallic catalysts at 150 °C led to the formation of the Pd 1 Au 1 phase in the particles, with the remaining Pd atoms forming Pd nanoparticles by leaching of Pd as acetate. The presence of these phases was monitored by X-ray absorption spectroscopy (XAS) of the used catalysts. Temperature-resolved in situ XRD of the reduced samples in an inert atmosphere confirmed the phase separation into a Pd-rich phase and a Au-rich phase above 160 °C. CO adsorption and XRD of the catalysts used at 180 °C showed that phase separation also took place during vinyl acetate synthesis. The pronounced temperature dependence of the morphology and surface composition of the bimetallic Pd−Au catalysts determines the selectivity; the activity; and, in particular, the stability during vinyl acetate synthesis.
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