In Situ Investigations of Structural Changes in Cu/ZnO Catalysts

Abstract

Dynamic changes in the structure and catalytic activity of Cu/ZnO methanol synthesis catalysts have been investigated by a further developed in situ method, which combines X-ray diffraction (XRD), X-ray absorption fine structure spectroscopy (XAFS), and on-line catalytic measurements by mass spectrometry. The temperature-programmed reduction of copper was monitored both by in situ quick-EXAFS (QEXAFS) and on-line mass spectrometry. The results indicate that under typical mild reduction conditions very small copper particles (10–15 Å) are formed. Upon change in the reduction potential of the methanol synthesis gas, reversible changes of the Cu–Cu coordination number are observed by EXAFS. These structural changes are accompanied by changes in the catalytic activity and the highest activities were observed after exposure to the most reducing conditions. In this state the catalyst exhibited low Cu–Cu coordination numbers. These results support the model that reversible changes in the wetting of ZnO by Cu may occur upon changes in the reaction conditions. The results also show that such dynamical changes in Cu morphology may influence the catalytic properties. All the conditions used in the above studies are less severe than those observed to result in bulk alloy formation. However, additional XAFS measurements at higher temperatures have been performed and EXAFS spectra have been simulated to address the possibility for Cu–Zn alloy formation as suggested by recent results in literature. Only under severe reduction conditions was significant alloying of copper and zinc observed in EXAFS in addition to the morphological changes. Such changes have not been seen in the Cu/SiO2 system.