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 materials by exchange, impregnation, and deposition-precipitation methods may give rise to supported phases of talc-like, serpentine-like structure (phyllosilicates) and/or nickel hydroxide. The pH of the impregnation solution and the specific surface area of the support strongly influence the nature of the supported phase. Supports with low specific surface areas favor the formation of a nickel hydroxide phase whatever the preparation method, while supports with high specific surface areas allow the growth of phyllosilicate to occur. The role of the Si/Ni ratio of soluble species close to the surface to produce polymerization (nickel hydroxide) or copolymerization (nickel phyllosilicates) reactions is discussed. The identification of the supported phase by FTIR spectroscopy is made by comparison with reference bull; compounds of various degrees of crystallinity. Crystalline phyllosilicates are easily identified by their delta(OH) mode, while ill-organized phyllosilicates which cannot be identified by this mode are distinguished by the position of the stretching nu(SiO) and nu(OH) vibrations. The position of the SiO band of the supported phase is obtained by subtracting the 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 phases, 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 FTIR 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 phyllosilicates of talc- and serpentine-like structure from hydroxide-like phases.