Preparation and Study of Cu−Al Mixed Oxides via Hydrotalcite-like Precursors

Abstract

Thermogravimetric analysis (TGA), X-ray diffraction (XRD), BET areas, FT-IR spectroscopy, scanning electron microcopy (SEM) and temperature programmed reduction (TPR) techniques were performed to characterize copper−aluminum mixed-oxide samples with Cu/Al ratios between 0.5 and 3.0. The thermal stability, crystallinity, and purity of the materials obtained depended on the Cu/Al atomic ratio. The FT-IR and TG detected carbonate (mainly) and nitrate as counteranions interacting in the interlayer region. Loosely bound carbonate and nitrate anions and one strongly bound type of carbonate were found. The evolution of phases during calcination was studied using dynamic XRD experiments. The copper hydrotalcite phase is only stable at calcination temperatures lower than 500 K. All the samples showed well-dispersed CuO and/or CuAl2O4 phases. A pure copper aluminate of high surface area (>150 m2/g) can be obtained. The rate of formation of copper aluminate depends inversely on the amount of copper in the sample. The TPR experiments showed that all carbonate decomposed at much lower temperatures during the reduction process compared with the decomposition process in a vacuum. However, the consumption of hydrogen was always very close to that required for the stoichiometric reduction of Cu2+ to Cu. The degree of reduction of the samples calcined at higher temperatures decreases both with the calcination temperature and the copper content.