Microcalorimetry, TPR and XPS studies of acid–base properties of NiCuMgAl mixed oxides using LDHs as precursors

NiCuMgAl layered double hydroxides (LDHs) with hydrotalcite-like structure containing different proportions of Ni2+, Cu2+, Mg2+ and Al3+ cations have been prepared. Thermogravimetry and X-ray diffraction data indicate that the transformation of LDH into mixed oxides is effective after calcination at 450 °C. The acid–base properties of these mixed oxides have been investigated using adsorption microcalorimetry and X-ray photoelectron spectroscopy with SO2 (for basicity) and NH3 (for acidity) as probe molecules. The basicity depends principally on the Ni/Cu ratio and increases with the Ni proportion. The strength (strong and medium) of the basic sites is particularly homogeneous. On the surface of the oxides, they are both Brönsted (∼60%) and Lewis sites (∼40%). The acidity (only Lewis type) is rather weak and dependant, as the basicity, of the Ni/Cu ratio. The observed heterogeneity of these sites can be related to the heterogeneity of the interactions between the nickel atoms and the other elements.