One of the main stumbling blocks in developing rational design strategies for heterogeneous catalysis is that the complexity of the catalysts impairs efforts to characterize their active sites. We show how to identify the crucial atomic structure motif for the industrial Cu/ZnO/Al(2)O(3) methanol synthesis catalyst by using a combination of experimental evidence from bulk, surface-sensitive, and imaging methods collected on real high-performance catalytic systems in combination with density functional theory calculations. The active site consists of Cu steps decorated with Zn atoms, all stabilized by a series of well-defined bulk defects and surface species that need to be present jointly for the system to work.

Getting under the surface of aging: The bulk structure of copper nanoparticles can be tailored during the preparation by exploiting known structure–activity relationships. Ageing of freshly precipitated precursors leads to characteristic phase transitions and enables control of microstrain and particle size (see picture) properties which influence catalytic activity.

Microstructural characteristics of the copper phase in Cu/ZnO catalysts for methanol steam reforming (MSR) were investigated as a function of ageing 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 to conventional TPR and TG/MS results. With increasing ageing time of the precipitate the onset of reduction of the CuO/ZnO precursor is shifted from 462 K to 444 K, while a decrease in crystallite size from 110 Å (0 min) to 70 Å (120 min) is 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 that correlates with the increase in catalytic activity. 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.

A technique of contact angle measurement was applied to the nano-scale oxide-supported metal particles. For Cu supported on ZnO and ZrO2 the angles were found to increase and the work of adhesion to decrease with increasing particle size. Such a trend is interpreted as an effect of negative contact line tension of 2.1 x 10(-9) J m(-1) and 1.0 x 10(-9) J m(-1) in the Cu/ZnO and Cu/ZrO2 system, correspondingly. For the small-sized Cu particles the apparent work of adhesion on ZnO support is higher than that on ZrO2.

Die Autoren bedanken sich bei der ZEIT-Stiftung (Nanotechnologie für eine zukünftige Automobiltechnik) und der Deutschen Forschungsgemeinschaft, DFG (SP "Brückenschläge in der heterogenen Katalyse) für die finanzielle Unterstützung. KeywordsIn situ, heterogeneous catalysis, structure-activity relationships, X-ray diffraction, NMR spectroscopy Abstract Ziel eines rationalen Designs von Cu/ZnO-Katalysatoren ist es, die Volumenstruktur der Kupfernanoteilchen durch Ausnutzen bekannter Struktur-Funktionsbeziehungen während der Präparation durch geeignete Verfahren zielgerichtet einzustellen. Altern der frisch gefällten Präkursoren führt zu charakteristischen Phasenumwandlungen und erlaubt es, Mikroverspannung und Größe der Kupfernanoteilchen zu beeinflussen.