This study concerns the alkali promotion of Cu/ZnO/Al<SUB>2</SUB>O<SUB>3</SUB> catalysts which are used for the low temperature water-gas shift reaction. Preliminary high pressure microreactor studies revealed that, on addition of the promoter, an enhanced selectivity to the desired products was achieved rather than for by-product methanol formation. Diffuse Reflectance Infra-red Fourier Transform Spectroscopy (DRIFTS) has been employed during adsorption and subsequent temperature programmed desorption (TPD) and temperature programmed reaction (TPR) experiments in order to determine the mode of action of the promoter. The catalysts were prepared <I>via</I> a layered hydrotalcite-type precursor by co-precipitating the mixed metal nitrates with ammonium bicarbonate, thus generating the catalyst alkali free. Promotion was achieved by impregnation with alkali metal salts. Detailed carbon monoxide adsorption experiments have been used to explore the various parameters in the pre-treatment procedure and their effect on the resultant carbon monoxide stretching band. These studies have also allowed the presence of stepped copper planes to be identified following reduction of the sample, along with partially oxidised copper and a small amount of copper oxide. Catalysts promoted with potassium to different levels (0.02 to 1 weight %) were studied using carbon monoxide as a probe molecule, where the main observation was a sharp attenuation in the CO stretching frequency intensity and a site blocking mechanism of the promoter was invoked in explanation. No shift in the CO stretching frequency was observed. Another, more subtle, effect was identified for the support, bicarbonate formation on alumina was hindered while another surface species with a higher desorption temperature dominated. This was suggested as being a potassium-associated carbonate. Similarly, the effect on the support was seen in analogous carbon dioxide adsorption studies. In addition, the formation of CO following the exposure of the catalysts to CO<SUB>2</SUB> was found to increase on promotion, and, again, a potassium-associated species such as a carboxylate was invoked to explain this.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:652646 |
| Date | January 1998 |
| Creators | Howieson, Kathryn E. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/14113 |
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