Radiotracer Studies of Adsorption on Methanol Synthesis Catalysts

[摘要] The adsorption of carbon dioxide and carbon monoxide on a Cu/ZnO/Al2O3 methanol synthesis catalyst and the ZnO and Al2O3 components of this catalyst, at ambient temperature, has been investigated using a [14-C]radiotracer technique. Before adsorption of the various gases, the catalysts were activated in a stream of 6% hydrogen in argon at 463K. The relative strengths of adsorption of carbon dioxide and carbon monoxide, on the various catalysts, have been investigated by evacuation and molecular exchange experiments. Exchange between one of the [14-C]labelled carbon oxides pre-adsorbed on each of the catalysts and the other non labelled carbon oxide in the gas phase, together with adsorption measurements for each of the [14-C]carbon oxides after admission of the other non labelled gas, was used to examine the degree of competitive adsorption between the two adsorbates. The effects, both separate and combined, of oxygen (from nitrous oxide) adsorbed on the copper surface and of hydrogen on the adsorption of carbon dioxide and carbon monoxide on Cu/ZnO/Al2O3, have also been examined using both the radiotracer method and a microreactor system coupled to a mass spectrometer. It has been shown that, carbon dioxide adsorbs both weakly and relatively strongly on each of the components of the catalyst. On the copper component, carbon dioxide dissociates at high energy sites to give adsorbed carbon monoxide and surface oxygen. This adsorbed carbon monoxide is different to that from the species obtained by adsorption of gaseous carbon monoxide in that it is strongly adsorbed and perhaps multiply bonded to the surface. Further slow adsorption of carbon dioxide takes place with oxygen atoms, formed by the dissociative adsorption of carbon dioxide, as well as with surface oxygen remaining after the reduction process to form a carbonate species. Similarly, a slow adsorption of carbon dioxide occurs on the support, forming a more strongly adsorbed species. A limited amount of surface oxygen on the copper component, does not affect the amount of carbon dioxide adsorbed on the catalyst, although its strength of adsorption is increased. In sharp contrast, when the copper component is fully oxidised, sites for the adsorption of carbon dioxide are initially blocked, adsorption of this gas on fully oxidised copper being a very slow process. Although hydrogen has little effect on the adsorption of carbon dioxide on reduced Cu/ZnO/Al2O3, complex formation takes place between carbon dioxide and hydrogen adsorbed on partially oxidised Cu/ZnO/Al2O3. Hydrogen adsorbs at Cu+-Cu sites to form hydroxyl groups which react with carbon dioxide possibly to form surface formate species. In sharp contrast with carbon dioxide, carbon monoxide adsorbs, almost exclusively, on the copper component of the catalyst, where it is adsorbed relatively weakly. Although.

[发布机构] University:University of Glasgow

[关键词] Physical chemistry, Organic chemistry