Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 20, Issue 13, Pages 3782-3792Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.201302599
Keywords
copper; heterogeneous catalysis; kinetics; methanol; X-ray absorption spectroscopy
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Funding
- Bayerisches Wissenschaftsministerium [NW-0810-0002]
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A Cu-based methanol synthesis catalyst was obtained from a phase pure Cu,Zn,Al hydrotalcite-like precursor, which was prepared by co-precipitation. This sample was intrinsically more active than a conventionally prepared Cu/ZnO/Al2O3 catalyst. Upon thermal decomposition in air, the [(Cu0.5Zn0.17Al0.33)(OH)(2)(CO3)(0.17)]mH(2)O precursor is transferred into a carbonate-modified, amorphous mixed oxide. The calcined catalyst can be described as well-dispersed CuO within ZnAl2O4 still containing stabilizing carbonate with a strong interaction of Cu2+ ions with the Zn-Al matrix. The reduction of this material was carefully analyzed by complementary temperature-programmed reduction (TPR) and near-edge X-ray absorption fine structure (NEXAFS) measurements. The results fully describe the reduction mechanism with a kinetic model that can be used to predict the oxidation state of Cu at given reduction conditions. The reaction proceeds in two steps through a kinetically stabilized Cu-I intermediate. With reduction, a nanostructured catalyst evolves with metallic Cu particles dispersed in a ZnAl2O4 spinel-like matrix. Due to the strong interaction of Cu and the oxide matrix, the small Cu particles (7nm) of this catalyst are partially embedded leading to lower absolute activity in comparison with a catalyst comprised of less-embedded particles. Interestingly, the exposed Cu surface area exhibits a superior intrinsic activity, which is related to a positive effect of the interface contact of Cu and its surroundings.
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