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Abstract

Microstructural characteristics of copper zinc oxide catalysts for methanol steam reforming (MSR) were investigated as a function of ageing of the precipitated hydroxycarbonates during catalyst preparation. Microstructural changes of the active catalysts were determined by in situ X-ray diffraction (XRD) and in situ X-ray absorption spectroscopy (XAS) combined with on-line mass spectrometry, nuclear magnetic spin resonance (NMR) and transmission electron microscopy (TEM). The strong increase in catalytic activity observed for Cu/ZnO catalysts obtained from precipitates aged for more than 15 min cannot be explained alone by the higher Cu surface area of the catalysts, as determined by N2O decomposition. The microstrain in the copper particles as detected by XRD, NMR, and XAS was determined as an additional bulk structural parameter that correlates with the increase in catalytic activity. Additionally, TEM investigations show that the less active Cu/ZnO catalyst (non-aged precipitate) comprises a heterogeneous microstructure of large and isolated Cu and ZnO particles, whereas the more active catalyst (120 min aged precipitate) shows small and intimately mixed Cu and ZnO particles (homogenous microstructure). Moreover, continuous precipitate ageing leads 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 ageing is proposed. The 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. The present work reports, that microstructural characteristics of a “real catalyst” can be controlled by suitable preparation conditions. The preparation of Cu/ZnO catalysts by precipitation of mixed metal hydroxycarbonates is a multi-step process, including ageing, washing, drying, calcination and reduction. Ageing of freshly precipitated precursors leads to characteristic phase transformations and enables designing a desired catalyst performance by modifying the precipitation conditions instead of varying the chemical composition. All materials obtained from the intermediate preparation procedures (i.e. calcination and reduction) exhibit a “chemical memory” on their treatment during the first stages of preparation (i.e. ageing) and none of the subsequent treatment procedures appears to erase that memory and level out the characteristic structural differences.