Multinuclear NMR characterization of microemulsions: Preparation of monodisperse colloidal metal boride particles

One major goal of heterogeneous catalysis by metals is the preparation of small and uniform particles. This can be achieved partially by depositing them on a support, which in turn decreases their ability for sintering. Nevertheless, these particles show, in most cases, a rather broad distribution in size and/or a bimodal distribution. A new method is proposed for the preparation of very small (20–70 Å) monodisperse particles of Ni2B, Co2B and NiCoB, and their formation is examined from a mechanistic point of view. These catalysts are used in room-temperature liquid-phase hydrogenation and they are synthesized by reduction with NaBH4 of Ni (II) and Co (II) ions solubilized in one of the microemulsions composed of cetyltrimethylammonium bromide (CTAB), hexan-1-ol and water. The solubilization sites and the interaction of Co (II) and Ni (II) ions with CTAB or hexan-1-ol molecules were studied by 13C-NMR and by UV-VIS absorption spectroscopy. It is concluded that the metallic ions are basically solvated in the inner water cores and one or more hexan-1-ol molecules are included in their first salvation shells. The size of the inner water core is measured by an indirect method (19F-NMR), based on the distribution of 6-fluorohexan-1-ol between the interface and the hexan-1-ol organic medium. The size of metal boride particles depends upon the micellar composition (especially on the water concentration) and on the metal ion concentration. Larger particles are obtained at higher water content, while the curve of particle size as a function of Co (II) or Ni (II) concentration showed a clear-cut minimum. This latter can be explained, provided a critical number of Co (II) or Ni (II) ions is assumed for the formation of nuclei.