SURFACE MOBILITY AND REACTIVITY OF OXYGEN SPECIES ON A COPPER-ZINC CATALYST IN METHANOL SYNTHESIS

A coprecipitated CuZnO/Al2O3 catalyst [Cu/(Cu + Zn) = 0.3] was prepared and characterized by XRD, XPS, N2O chemisorption, and its activity in methanol synthesis at 220°C, 1 atm. The size of the copper particles deduced from XRD line broadening agreed with that measured by N2O chemisorption (16-20 nm). Moreover, XPS showed that the composition of the surface, slightly enriched in zinc in the reduced catalyst, was similar to the composition of the bulk in the oxidized catalyst. Catalytic activity was similar to the activity already reported for this type of catalyst under similar conditions. Isotopic exchange of 18O2 and of C18O was performed on this catalyst to evaluate the surface mobility as well as the reactivity of the oxygen species on the support. Some experiments were also carried out on a Rh/ZnO catalyst. Cu16O was active in exchange with gaseous 18O2 but did not promote exchange with the oxygen of ZnO. By contrast, with Rh/ZnO, 18O2 could be exchanged with the 160 of ZnO at a significant rate. These results show that the surface oxygen species are mobile on ZnO but only in the direction ZnO to Cu: once they are bonded on the copper, oxygen atoms can no longer leave the metal particles. Oxygen transfer in both directions (metal to support and vice versa) being possible with rhodium, the rate of oxygen migration on ZnO could be measured by isotopic exchange of 18O2 with Rh/Zn16O. With C18O the most striking feature was the formation of large amounts of CO2, first C16O 18O and C16O2, then by C18O2. The formation of C16O, initially very fast, was interrupted as soon as the partial pressure of CO2 became significant. These results concerning isotopic exchange, either with 18O2 or with C18O, show that oxygen species (probably OH groups) can migrate from ZnO to the copper particles during methanol synthesis. This is discussed in the light of the different reaction mechanisms. ZnO can play a role in mechanisms with formate intermediates, especially in the synthesis with CO2/H2 mixtures. © 1990.