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

A series of hydrotalcite-like (htl) compounds of file general composition (Cu,Zn)(1-x)Al-x(OH)(2)-(CO3)(x/2)center dot 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 degrees 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 oil 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)(ZnAl2O4)(y)" of homogeneous microstructure. Crystallization occurs upon carbonate decomposition at temperatures higher than 500 degrees C. Despite the small size of the Cu nanoparticles (around 7 nmm) formed upon reduction, the accessible Cu surface area is below 5 m(2)g(-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.