Dynamic Phase Separation in Supported Pd–Au Catalysts

Abstract

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.

Catalysis in Flow: Why Leaching Matters

“…However, selective leaching may also occur during a catalytic reaction, which can alter the productivity and selectivity of the process. More recently, XAS has also been successfully employed to study the leaching of Pd from SiO 2 -supported bimetallic Pd/Au catalysts during the synthesis of vinyl acetate from ethylene, carbon monoxide and acetic acid [16]. Under industrially relevant conditions, exposure of the catalyst to acetic acid led to leaching of Pd from the alloy as palladium(II) acetate, leading In an earlier paper, we have found that a sample of 5% Pd/Al 2 O 3 undergoes leaching when it was exposed to a heated solution of aqueous ethanol [17].…”

Effect of Pd Coordination and Isolation on the Catalytic Reduction of O2 to H2O2 over PdAu Bimetallic Nanoparticles

“…PdAu alloys can provide catalytic rates and selectivities that exceed those of monometallic Pd or Au catalysts; however, ascertaining the reasons for these differences remains a challenge. The formation of PdAu alloys modifies both the electronic and geometric structure of Pd, which has been demonstrated by investigations that involve X-ray photoelectron spectroscopy (XPS), − X-ray absorption spectroscopy (XAS), ,− and Fourier transform infrared spectroscopy (FTIR) of adsorbed CO. ,,,,,,,− These two effects are entangled and complicate the interpretation of data obtained by these methods, because the atomic composition and arrangement of PdAu alloy surfaces affect the electronic structure. Pd and Au are miscible and form a substitutional, exothermic alloy (−2 > Δ H mix > −9 kJ mol –1 ) for all compositions. − As such, Monte Carlo simulations of PdAu surfaces predict the formation of significant numbers of isolated Pd monomers when the mole fraction of Au greatly exceeds that of Pd. − Most results indicate that Pd atoms gain d-electrons as the Au contents in PdAu alloys increase.…”

Interstitial Carbon Dopant in Palladium–Gold Alloy Boosting the Catalytic Performance in Vinyl Acetate Monomer Synthesis

“…The major technical challenge of the Pd–Au system is the preparation of homogeneously distributed Pd–Au alloy nanoparticles with high-phase purity. − Employing nonuniform Pd–Au alloy nanoparticles has complicated this system and hampered the elucidation of the roles of Au and KOAc additives in the commercial catalyst. Indeed, Pd–Au alloys, such as Pd/Au(111) − and Pd/Au(100), − have been primarily used as model surfaces despite their distinct structures from the commercial catalyst (KOAc/Pd–Au/SiO 2 , Au/Pd = 1/4) .…”