The effects of ageing a bimetallic catalyst under industrial conditions: a study of fresh and used Pd-Au-K/silica vinyl acetate synthesis catalysts

The effects of ageing under industrial conditions commercial Pd-Au-K/SiO2 vinyl acetate (VA) synthesis catalysts was studied by XRD, HREM/EDX, XPS, DRIFTS and temperature-programmed oxidation (TPO). The fresh catalysts contain palladium in two forms: Pd-Au alloy particles 4-5 nm in diameter and a very highly dispersed Pd metal component. As a consequence, the alloy particles are far more An rich than predicted on the basis of the metal loadings (60% An versus 18% Au). Usage resulted in both pronounced sintering of the Pd-Au alloy particles and the appearance of a I'd acetate component, but without significant change in composition of the alloy particles. Thus, the migration of palladium acetate species does not contribute significantly to sintering. DRIFT spectra revealed the presence of palladium acetate, acetic acid and silyl ester species on the aged sample, in addition to the KOAc also observed on the fresh samples. A further band observed at 1730 cm(-1) on the aged sample is assigned to a monodentate "ester-like" palladium acetate species, previously postulated as a key reaction intermediate in vinyl acetate monomer (VAM) formation. (C) 2003 Elsevier B.V. All rights reserved.