Phase-Pure Cu,Zn,Al Hydrotalcite-like Materials as Precursors for Copper rich Cu/ZnO/Al2O3 Catalysts

摘要

A series of hydrotalcite-like (htl) compounds of the general composition (Cu,Zn)_(1-x)Al_x(OH)_(2-) (CO3)_(x/2)·m H2O was prepared with a fixed Cu:Zn ratio of 70:30. Phase pure samples could be obtained for 0.3 < x < 0.4. The htl precursors thermally decompose in multiple steps. After dehydration and dehydroxylation amorphous materials were obtained at 330 °C. Phase segregation during this mild calcination was only observed for samples with a Zn: Al ratio deviating strongly from 1:2. A mechanism for this low-temperature segregation process basing on the preformation of the ZnAl2O4 phase within the amorphous material is proposed. Samples with Zn:Al ratios near 1:2 form an amorphous carbonate-modified mixed oxide "(CuO)_x(ZnM2O4)_y" of homogeneous microstruc-ture. Crystallization occurs upon carbonate decomposition at temperatures higher than 500 °C. Despite the small size of the Cu nanoparticles (around 7 nm) formed upon reduction, the accessible Cu surface area is below 5 m~2g~(_1). This can be explained by the unfavorable microstructure of the resulting Cu/ZnAl2O4 catalyst: The Cu particles are to a large extent embedded in a compact oxide matrix. The applicability as Cu/ZnO/Al2O3 catalysts and the role of htl precursor phases in course of industrial catalyst preparation are discussed.