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 COads 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.