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Cu‐Based Catalyst Resulting from a Cu,Zn,Al Hydrotalcite‐Like Compound: A Microstructural, Thermoanalytical, and In Situ XAS Study
Author(s) -
Kühl Stefanie,
Tarasov Andrey,
Zander Stefan,
Kasatkin Igor,
Behrens Malte
Publication year - 2014
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201302599
Subject(s) - catalysis , hydrotalcite , calcination , oxide , spinel , materials science , x ray absorption spectroscopy , xanes , mixed oxide , inorganic chemistry , heterogeneous catalysis , thermal decomposition , precipitation , metal , methanol , chemical engineering , chemistry , absorption spectroscopy , metallurgy , biochemistry , physics , organic chemistry , quantum mechanics , spectroscopy , meteorology , engineering
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/Al 2 O 3 catalyst. Upon thermal decomposition in air, the [(Cu 0.5 Zn 0.17 Al 0.33 )(OH) 2 (CO 3 ) 0.17 ] ⋅ m H 2 O precursor is transferred into a carbonate‐modified, amorphous mixed oxide. The calcined catalyst can be described as well‐dispersed “CuO” within ZnAl 2 O 4 still containing stabilizing carbonate with a strong interaction of Cu 2+ 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 ZnAl 2 O 4 spinel‐like matrix. Due to the strong interaction of Cu and the oxide matrix, the small Cu particles (7 nm) 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.