Abstract

X-ray photoelectron spectroscopy (XPS) is used to characterize the surface region of a commercial Cu/ZnO/Al₂O₃ (33/66/1 wt %) catalyst. A systematic study of the effects of oxidation, reduction, and the steam reformation of methanol on the oxidative state of the Cu component is presented. The Zn XPS features show no changes due to the various treatments. Peak fitting procedures were developed to quantitate the Cu oxidation states on the basis of the XPS Cu 2P_3/2 main and satellite features. After oxidation in pure O₂ at 300°C, all Cu exists as Cu⁺². The Cu/Zn ratio changes from 0.28 to 0.37 as a result of this oxidation, in comparison to the ratio in the catalyst as-received. The reduction studies involved different H₂/N₂ mixtures (15 to 100% H₂) and temperatures (250 to 300°C). The catalyst always contains Cu⁺¹ (7.0 ± 5.0%) and Cu⁰ (93.0 ± 5.0%) after reduction. The Cu/Zn ratio decreases from approximately 0.37 in the oxidized catalyst to 0.13 after reduction. After methanol-steam reformation with a 50/50 vol % mixture, the Cu 2P_3/2 and Auger features are indicative of complete reduction of all Cu in the catalyst to a reduced Cu⁰ state not seen previously. Changes in the Cu/Zn ratio of the surface are interpreted in terms of changes in surface morphology of the Cu species.