Godfrey C. Chinchen

The activity and state of the copper surface in methanol synthesis catalysts

Abstract The measurement of copper metal surface areas by monitoring nitrous oxide chemisorption is a well established technique. A frontal chromatographic version of this technique has been developed which is very suitable for in situ measurements and this has enabled the apparent copper areas of various catalysts to be measured after exposures to methanol synthesis gases of different compositions at typical industrial conditions in microreactors commonly used for assessing the methanol synthesis activity of such catalysts. Using such techniques, it has been shown that, first, there is a linear relationship between the methanol synthesis activity of copper/zinc oxide/alumina catalysts and their total copper surface area. Second, that copper supported on other materials has approximately the same turnover number as copper/zinc oxide/alumina catalysts. Third, that under industrial conditions the copper surface of the catalyst is partially oxidised, to an extent which depends on the composition of the synthesis gas, particularly the CO 2 /CO ratio.

Mechanism of methanol synthesis from CO2/CO/H2 mixtures over copper/zinc oxide/alumina catalysts: use of14C-labelled reactants

Abstract The addition of 14 CO or 14 CO 2 tracers to CO 2 /CO/H 2 reactant mixtures for methanol synthesis over a commercial copper/zinc oxide/alumina catalyst was used to determine the origin of the carbon in the product. For PCO 2 /PCO ratios from 0.02 to 1 the fraction of methanol made from carbon dioxide rises from ∼0.7 to ∼1, so carbon dioxide is the major reactant under industrial conditions. There is no carbon-containing surface intermediate common to methanol synthesis and the water-gas shift reaction.

Promotion of methanol synthesis and the water-gas shift reactions by adsorbed oxygen on supported copper catalysts

The surfaces of the copper metal crystallites of working Cu/ZuO/Al2O3 and other copper catalysts are partially oxidised in reaction mixtures for methanol synthesis and the water-gas shift reaction. Work with unsupported polycrystalline copper has confirmed earlier results that copper metal is the active phase in supported copper catalysts. The coverage of adsorbed oxygen, O(a), up to half-monolayer, was determined by reaction with N2O and it was found to be controlled by the overall reaction CO2(g)= CO(g)+ O(a). The free energy of formation of O(a) was calculated to be –240 kJ mol–1 at 513 K. The induction period found in methanol synthesis from CO–CO2–H2 mixtures is consistent with the calculated rate of formation of O(a). The role of O(a) in the methanol synthesis and water-gas shift reactions is both as promoter and reaction intermediate. The dissociative chemisorption of hydrogen on copper is promoted by O(a) but this is not necessary for the reactions. Experiments with unsupported polycrystalline copper have shown that O(a) both increases the extent of physisorption of CO2 and creates new chemisorbed states of CO2, with desorption energies of 109, 113 and 125 kJ mol–1. O(a) is also essential for the dissociative chemisorption of water on copper. A regenerative mechanism for the water-gas shift reaction on copper [involving the formation and reaction of O(a)] has been established by observation of the separate stages. The adsorbed formate intermediate, pivotal in methanol synthesis from carbon dioxide, is irrelevant to the water-gas shift reaction.

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

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.

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

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 Cus : 1 Hads : 1 Hads : 1Oads, 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 OHads by reaction of hydrogen with preadsorbed oxygen and formation of a surface formate species by interaction of either CO2 or CO with the OHads.