Acido-basicity of Mg−Ni−Al Mixed Oxides from LDH Precursors: A FTIR and XPS Study

The acido-basicity of Mg−Ni−Al mixed oxides generated from the calcination of layered double hydroxides (with the atomic ratios Mg/Ni variable and Mg + Ni/Al equal to 2) has been investigated by adsorption experiments, using FTIR with pyrrole and pyridine as probe molecules and XPS with ammonia and sulfur dioxide. The spectroscopic data were compared with microcalorimetric data and with the results of catalytic tests (cyclohexanol conversion). The acidity, comparable to that of alumina, is mainly of Lewis type. It increases with the nickel content. The FTIR spectra of chemisorbed pyrrole indicate the presence of strong, medium and weak basic sites. The bulk basicity of trimetallic oxides is stronger than that of the bimetallic ones. The effective basicity of the bimetallic Mg−Al oxide is minimized by the presence of numerous remaining carbonate ions. The XPS analyses show that the surface basicity increases with the Mg content, as indicated by the reaction test. Moreover it appears that the majority of the basic surface sites are hydroxyl OH- groups.