The effect of preparation methods on the thermal and chemical reducibility of Cu in Cu–Al oxides

Abstract

The influence of the preparation methods on the physicochemical properties of Cu and Al oxides was studied. Four different preparation methods were used to synthesize the oxides (catalysts): co-precipitation by the hydrotalcite precursor route (CuAl-HT-c), the solid-state reaction route (CuAl-s), the physical mixing of the oxides (CuAl-o), and individual oxide precipitation followed by physical mixing (CuAl-p). Chemical analysis, X-ray diffraction and textural analysis results of the precursor CuAl-HT-c showed that the respective preparation method is efficient to obtain the hydrotalcite phase with high purity as well as generating a mixed oxide with the greatest surface area among all of the catalysts in this study. The efficacy of this method should be highlighted as it is very difficult to obtain high purity hydrotalcites with copper and aluminum. Furthermore, through the evolution of the FWHM with temperature, this method provided a structurally and texturally more stable mixed oxide. The thermal reducibility of the CuAl-HT-c catalyst was also much higher than that observed in the Cu and Al oxides prepared by other methods. Under an He flow, the CuAl-HT-c catalyst showed a significant reduction of CuO starting at 440 °C, with the formation of Cu₂O and metallic Cu, confirmed by XRD and XANES, both in situ. When investigating the effect of the pretreatment gas for this catalyst, H₂ reduction occurs faster than with CO, and with less Cu⁺ formation. Thus, Cu and Al mixed oxides obtained from a precursor with the hydrotalcite structure showed advantages over other methods, presenting unique textural and reducibility properties.