Dynamic Self-Organization of Supported Pd/Au Catalysts during Vinyl Acetate Synthesis

Abstract

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.

Interaction of alkali acetates with silica supported PdAu

“…The Pd lattice expansion after LiOAc impregnation suggests the formation of a Pd/Li + mixed adlayer on the PdAu alloy particle. 16 First shell analysis of the EXAFS of the promoted sample indicates a Pd/ Li + distance of 2.14 Å, which agrees well with the Pd and Li + formed as a thin shell on the alloy particle core. The fit of the EXAFS oscillations improved significantly by including Pd/Li + contributions, while Au/Li + interactions were excluded as first shell analysis yielded nonrealistic parameters (i.e., unacceptable distances and partly negative coordination numbers).…”

“…33 The presence of Pd 1 Au 1 phases can be deduced from a 2 : 1 ratio between N Pd-Pd and N Pd-Au (the same applies for the coordination numbers of Au). 16 For the samples studied, the total coordination numbers for Pd (∼10) were lower than that of Au (∼12), which indicates that Au occupied with preference sites in the core of the particles, while Pd atoms were located with preference on the surface. With the addition of LiOAc N PdPd decreased from 7.9 to 7.1 indicating an enhanced surface segregation of Pd in presence of the promotor.…”

“…14,15 This change is induced by the strong interaction of KOAc with Pd. 14,16,17 The activity and selectivity of the structure sensitive VA synthesis reaction are determined by the relative arrangement of active Pd monomers suitably spaced by inert Au atoms on the solid surface. 18 Thus, the rate-enhancing effect of KOAc in VA synthesis is attributed to a geometric reordering in PdAu particles.…”

The PtAl− and PtAl2− anions: Theoretical and photoelectron spectroscopic characterization

“…The ground state of the neutral clusters has the calculated harmonic vibrational frequency of 397 cm −1 . The PtAl 2 − (C 2v , 2 B 1 ) cluster has the following valence electronic configuration: [core]1a 1 2 1b 2 2 1b 1 2 2a 1 2 1a 2 2 3a 1 2 2b 2 2 4a 1 2 2b 1 1 . Electron detachment from the doublet can result in the singlet and triplet final states.…”

“…[1][2][3][4][5][6][7][8]. Indeed, doping is being explored as one of the most promising means of tuning the catalytic activity of clusters and nanoparticles of Pt, Pd, and Au, although often in an empirical way.…”