EFFECT OF THE PREPARATION METHOD ON THE THERMAL-STABILITY OF SILICA-SUPPORTED NICKEL-OXIDE AS STUDIED BY EXAFS AND TPR TECHNIQUES

EXAFS analysis and Temperature-Programmed Reduction experiments (TPR) have been carried out on calcined Ni/SiO2 materials prepared by incipient wetness impregnation at different pH values, ion exchange with ammonia and ethylenediamine solutions, and deposition-precipitation. The interaction of the Ni(II) ions with silica is shown to be dependent on the first steps of catalyst preparation, i.e., deposition, washing, and drying. Silicates with a layer structure are formed in samples prepared by deposition-precipitation and ion exchange from ammoniacal solutions. Silicate formation inhibits the NiO crystallization up to 500°C. The silicate decomposition is favored when the thermal treatment is performed under reduced pressure. The reducibility of the nickel oxide phase resulting from silicate decomposition is strongly hindered in comparison with that of unsupported nickel oxide. The silicate formation is avoided using a strong chelating ligand such as ethylenediamine during ion exchange. With ethylenediamine, isolated Ni(II) species stable to 500°C are obtained. © 1992.