The effect of cobalt on the structural properties and reducibility of CuCoZnAl layered double hydroxides and their thermally derived mixed oxides

A new series of CuCoZnAl layered double hydroxides (LDHs) with various Cu/Co atomic ratios were synthesized by a coprecipitation method at constant pH approximate to 9. The effects of Co on the structural properties and reducibility of both as-synthesized and calcined materials were investigated thoroughly by employing powder X-ray diffraction (XRD), ultraviolet-visible diffuse-reflectance spectroscopy (UV-Vis DRS), thermogravimetry-differential thermal analysis (TG-DTA), and temperature programmed reduction (TPR) methods. Characterization of as-synthesized materials using these techniques revealed the formation of well-crystallized hydrotalcite (HT)-like LDHs with a general formula[Cu1-(x+ y + z)CoxZnyAlz(OH)(2)](z+)[(CO32-)(z/2). mH(2)O](z-) as a major phase. Thermal analyses of materials indicated three stages of endothermic weight loss processes due to the loss of interlayer water and some loosely bound CO32- (100-250 degreesC), loss of structural water and CO32- (250-400 degreesC) and the loss of some strongly held CO32- anions (above 500 degreesC). The structural and redox properties of mixed oxides obtained upon thermal activation were investigated by calcining the LDH precursors at 300, 450, 700 and 900 degreesC. The Cu-rich sample generated a mixture of CuO, ZnO and CuAl2O4 spinel, while Co-based spinel phases were observed at all temperatures in the case of the Co-rich sample. Materials containing both Cu and Co showed a mixture of CuO, Co-based spinel and CuAl2O4 spinel. The interesting observation of the present study was that the reducibility of Cu2+ species of the CuO was enhanced with increasing Co content. On the other hand, the reducibility of Co species decreased with increasing Cu content.