Effect of precipitate aging on the microstructural characteristics of Cu/ZnO catalysts for methanol steam reforming

Microstructural characteristics of the copper phase in Cu/ZnO catalysts for methanol steam reforming (MSR) were investigated as a function of aging of the precipitated hydroxycarbonates during catalyst preparation. The bulk structure of active catalysts under MSR reaction conditions was determined by in situ X-ray diffraction (XRD) and in situ X-ray absorption spectroscopy (XAS) combined with on-line mass spectrometry. Reduction kinetics and phase compositions obtained from XAS data analysis were compared with conventional TPR and TG/MS results. With increasing aging time of the precipitate, the onset of reduction of the CuO/ZnO precursor shifted from 462 to 444 K, whereas a decrease in crystallite size from 110 Å (0 min) to 70 Å (120 min) was detected. A strong increase in catalytic activity was observed for Cu/ZnO catalysts obtained from precipitates aged for more than 30 min. The microstrain in the copper particles as detected by XRD and XAS was determined as an additional bulk structural parameter correlating with the increase in catalytic activity. Moreover, continuous precipitate aging led to a decreasing amount of Zn in the copper clusters of the Cu/ZnO catalysts. A schematic model of the structural characteristics of Cu/ZnO catalysts as a function of precipitate aging is proposed. This model emphasizes the defect-rich state of the homogeneous microstructure of Cu/ZnO catalysts and its implication for the catalytic activity in the steam reforming of methanol.