Radiotracer studies of chemisorption on copper-based catalysts. Part 2.—Adsorption of carbon monoxide, carbon dioxide and dihydrogen on partially and fully oxidised copper–zinc oxide–alumina catalysts

Abstract

The adsorption of ¹⁴CO and ¹⁴CO₂ at 293 K, on the surface of a copper–zinc oxide–alumina catalyst, which had been partially (30%) or completely oxidised by reaction with N₂O at 353 K, is reported. The reactive adsorption technique for H₂ at 293 K on a fully oxidised Cu–ZnO–Al₂O₃ catalyst and of CO₂ at 503 K on a hydrogen-reduced Cu–ZnO–Al₂O₃ catalyst is also reported. Adsorption of hydrogen on a 100% oxidised surface occurs at 293 K to give a surface stoichiometry of 2 Cu_s : 1 H_ads : 1 H_ads : 1O_ads, although no water is desorbed below 373 K. At low coverages of surface oxygen, the amount of CO₂ adsorbed and retained as a strongly bound species on the copper component is increased compared with the corresponding values on a freshly reduced surface. Adsorption of CO₂ at 293 K on oxidised surfaces in the presence of H₂ results in a further enhancement in both the amounts adsorbed and retained on the copper component. Little adsorption of CO₂ occurs on fully oxidised surfaces. Admission of CO to partially or fully oxidised surfaces, both in the presence and absence of hydrogen, results in the formation of CO₂, which is subsequently adsorbed on the surface. Whilst the presence of H₂ reduces the total amount of CO adsorbed, the amount of strongly adsorbed CO is significantly increased. The results are interpreted in terms of the formation of OH_ads by reaction of hydrogen with preadsorbed oxygen and formation of a surface formate species by interaction of either CO₂ or CO with the OH_ads.