The chemistry of vinyl acetate synthesis from ethylene, acetic acid, and oxygen over palladium-gold catalysts has been investigated in a study which has concentrated on the use of cyclic voltammetry as a surface sensitive analytical technique. Three series of palladium-gold/graphite catalysts were prepared by slurry precipitation or by incipient wetness impregnation and by reduction with formaldehyde or hydrazine. These catalysts, which behaved similarly, contained metallic particles having bi-modal particle-size distributions and alloy compositions, as judged by X-ray diffraction, high resolution transmission electron microscopy, and energy dispersive X-ray analysis. Surface characterisation was achieved by cyclic voltammetry and X-ray photoelectron spectroscopy. In experiments intended to mimic the industrial LEAP process (in which potassium acetate is added to the reaction mixture) catalysts were treated with acetic acid or potassium acetate/acetic acid mixtures. Such treatments resulted in etching of the catalysts with consequent loss of palladium dissolution of the lost palladium as dimeric and trimeric palladium acetate complexes was confirmed by ultra-violet absorption spectroscopy. Such etching occurred at the surfaces of palladium-rich particles gold-rich particles were resistant to etching. Thus, the role of gold as a promoter is to stabilise the optimum alloy phase against loss of palladium. Reduction of palladium acetate complexes by ethylene led to plating of metallic palladium onto the surfaces of etched catalysts. Thus, a cycle of palladium dissolution and deposition occurred under reaction conditions leading to a redistribution of palladium between the catalyst particles. Palladium acetate forms complexes with acetic acid and water that are liquid at synthesis temperatures. Thus, even under gas-phase reaction conditions, a liquid-phase may be present in which palladium acetate complexes can facilitate the homogeneous catalysis of vinyl acetate synthesis. This investigation provides definitive evidence that the balance of heterogeneous and homogeneous synthesis is determined by the range of the palladium-gold alloy composition present at the surface of the active catalyst. Etching and dissolution would be avoided by suitably restricting this range, whereupon reaction would be confined to that achieved by heterogeneous catalysis.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:583976 |
| Date | January 2006 |
| Creators | Greenslade, Mark |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/56151/ |
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