Acid-base properties of Mg-Ni-Al mixed oxides using LDH as precursors
Résumé
The thermal behavior of a series of Mg/Ni/Al hydrotalcites with (Mg + Ni)/Al = 2 and different Mg/Ni ratios has been studied (by means of X-ray diffraction (XRD), thermogravimetry and X-ray photoelectron spectroscopy (XPS), as well as the acid-base properties of the mixed oxides resulting from their calcination at 723 K (by means of adsorption microcalorimetry). Up to 500 K, only the physisorbed and interlayer water is released, causing a small loss of crystallinity. The decomposition of carbonate anions begins above 500 K, but this loss does not destroy the LDH structure which remains up to temperatures between 600 and 650 K. Above, the structure collapses and dehydroxylation occurs, leading to the formation of mixed oxides with large surface areas. At 723 K, a temperature usually chosen for the catalytic reactions, about 30% of the initial carbonates are still detected and poison a part of the strongest basic sites. The decarbonation is practically completed at 923 K. When the stability of the carbonates increases with the nickel content, the stability of the hydroxides increases, on the contrary, with the magnesium content. The NH3 adsorption data indicate an increase of the concentration and strength of acid sites with the nickel content. The compound SO2 seems to be more efficient than CO2 for the analysis of the basicity, because of its higher acidity and the presence of remaining carbonates in the mixed oxides. The highest concentration of basic sites is observed for the Mg/Ni/Al sample with low magnesium and high nickel contents. This gives evidence of the synergy effect of these metals favoring the basic properties and the role of MgO as promoter. Above 723 K, the concentration of the basic sites of mixed oxides decreases, which can be related to a more complete dehydroxylation. © 2001 Elsevier Science B.V. All rights reserved.