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

Abstract

A new series of CuCoZnAl layered double hydroxides (LDHs) with various Cu/Co atomic ratios were synthesized by a coprecipitation method at constant pH ≈ 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·mH2O]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 °C), loss of structural water and CO32− (250–400 °C) and the loss of some strongly held CO32− anions (above 500 °C). 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 °C. 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.