In situ investigations of structural changes in Cu/ZnO catalysts

Dynamic changes in the structure and catalytic activity of Cu/ZnO methanol synthesis catalysts have been investigated by a further developed in situ m ethod, which combines X-ray diffraction (XRD), X-ray absorption fine struct ure spectroscopy (XAFS), and on-line catalytic measurements by mass spectro metry. The temperature-programmed reduction of copper was monitored both by in situ quick-EXAFS (QEXAFS) and on-line mass spectrometry. The results in dicate that under typical mild reduction conditions very small copper parti cles (10-15 Angstrom) are formed. Upon change in the reduction potential of the methanol synthesis gas, reversible changes of the Cu-Cu coordination n umber are observed by EXAFS. These structural changes are accompanied by ch anges in the catalytic activity and the highest activities were observed af ter exposure to the most reducing conditions. In this state the catalyst ex hibited low Cu-Cu coordination numbers. These results support the model tha t 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 temp eratures have been performed and EXAFS spectra have been simulated to addre ss the possibility for Cu-Zn alloy formation as suggested by recent results in literature. Only under severe reduction conditions was significant allo ying of copper and zinc observed in EXAFS in addition to the morphological changes. Such changes have not been seen in the Cu/SiO2 system. (C) 2000 Ac ademic Press.