Radiotracer studies of chemisorption on copper-based catalysts. Part 1.—The adsorption of carbon monoxide and carbon dioxide on copper/zinc oxide/alumina and related catalysts

Abstract

The adsorption of ¹⁴CO and ¹⁴CO₂ on a hydrogen-reduced Cu/ZnO/Al₂O₃, a Cu/Al₂O₃ and a ZnO catalyst at 293 K is reported. With each adsorbate evidence for both a strongly and a weakly absorbed state has been detected on the copper component of the catalyst. Both a relatively rapid and a very slow adsorption of CO₂ on each of the adsorbates have been detected. Adsorption of CO in the presence of preadsorbed CO₂ and vice versa shows that the presence of one gas does not completely prevent the adsorption of the other. The adsorptive capacity of the Cu/ZnO/Al₂O₃ for CO is approximately twice that for CO₂, although a larger fraction of the latter is strongly adsorbed and will displace some preadsorbed CO from the copper surface. Evidence for the dissociative adsorption of some of the CO₂ to CO_ads and surface oxygen, as well as the formation of surface carbonate by reaction of CO₂ with catalyst oxygen, has been obtained. It is proposed that this latter process is responsible for the very slow CO₂ adsorption process. Whilst CO₂ is adsorbed on each of the components of the Cu/ZnO/Al₂O₃ catalyst, adsorption of CO is severely limited on the zinc oxide component and does not occur on the alumina. The results are interpreted in terms of a model in which at least three types of adsorption site exist on the copper surface. The role of residual oxygen on the surface, following hydrogen reduction, in the adsorption of CO₂ is also discussed.