FTIR IDENTIFICATION OF THE SUPPORTED PHASES PRODUCED IN THE PREPARATION OF SILICA-SUPPORTED NICKEL-CATALYSTS

Depending on the experimental conditions, the preparation of Ni/SiO2 m aterials by exchange, impregnation, and deposition-precipitation metho ds may give rise to supported phases of talc-like, serpentine-like str ucture (phyllosilicates) and/or nickel hydroxide. The pH of the impreg nation solution and the specific surface area of the support strongly influence the nature of the supported phase. Supports with low specifi c surface areas favor the formation of a nickel hydroxide phase whatev er the preparation method, while supports with high specific surface a reas allow the growth of phyllosilicate to occur. The role of the Si/N i ratio of soluble species close to the surface to produce polymerizat ion (nickel hydroxide) or copolymerization (nickel phyllosilicates) re actions is discussed. The identification of the supported phase by FTI R spectroscopy is made by comparison with reference bull; compounds of various degrees of crystallinity. Crystalline phyllosilicates are eas ily identified by their delta(OH) mode, while ill-organized phyllosili cates which cannot be identified by this mode are distinguished by the position of the stretching nu(SiO) and nu(OH) vibrations. The positio n of the SiO band of the supported phase is obtained by subtracting th e spectrum of the silica support conditioned in the same way as the Ni /SiO2 sample. In the case of a mixture of phyllosicate and hydroxide p hases, the position of the delta(OH) mode is used to discriminate the hydroxide phase since, for this phase only, this mode is sensitive to hydrogen bonding with interlayer water molecules. The advantage of FTI R spectroscopy as compared to other techniques, such as UV-visible and EXAFS spectroscopies, is to permit the identification of a mixture of ill-organized supported phases and to discriminate nickel phyllosilic ates of talc- and serpentine-like structure from hydroxide-like phases .