Molecular approach to the mechanism of deposition - Precipitation of the Ni(II) phase on silica

A molecular mechanism explaining the chemical phenomena that occur during the preparation of silica-supported nickel catalysts by the method of deposition-precipitation is proposed. This mechanism takes into account the fact that either 1:1 nickel phyllosilicate or nickel hydroxide may be formed on silica during deposition-precipitation, depending on the silica surface area and the deposition-precipitation time, as shown in our previous paper (Burattin, P.; Che, M.; Louis, C. J. Phys. Chem. B 1997, 101, 7060). The mechanism is mainly based on the kinetic competition between two types of reactions: (i) Ni-O-Si heterocondensation/polymerization, which leads to the growth of 1:1 nickel phyllosilicate on a Ni(II) brucitic layer bonded to silica; (ii) Ni-OH-Ni olation/polymerization, which leads to the formation and the growth of nickel hydroxide on a Ni(LI) brucitic layer bonded to silica. The former type of reaction is faster than the latter but is limited by the concentration in solution of silicic acid arising from silica dissolution and by its diffusion rate. The changes in the QI-I curves, the yield of Ni(II) deposition-precipitation, and the nature of the supported Ni(II) phase during deposition-precipitation under different experimental conditions are also interpreted on the basis of the mechanism proposed.