NUCLEATION AND PARTICLE GROWTH-PROCESSES INVOLVED IN THE PREPARATION OF SILICA-SUPPORTED NICKEL MATERIALS BY A 2-STEP PROCEDURE

A new method of preparation of silica-supported nickel materials makes possible the control of the particle size of metallic nickel in the range 20-60 Angstrom and in particular the obtention of decreasing particle size for increasing metal content. The preparation involves a two-step procedure: nickel in strong interaction with the support is deposited first, then nickel in weak interaction. After temperature-programmed reduction up to 700 degrees C, metal nickel particles are obtained whose size depends on the respective amounts of nickel deposited at each step of preparation. The average particle size is always smaller and the size distribution narrower than those of samples prepared by classical impregnation. This preparation method involves an easy to carry out procedure: (i) nickel in strong interaction is obtained by impregnation of nickel nitrate followed by water-washing or by ion-exchange followed by calcination; (ii) nickel in weak interaction is obtained by impregnation. It is shown that a mechanism of particle growth on the nickel nuclei is responsible for the changes in particle size. It takes place after impregnation during the drying step at 90 degrees C via condensation reactions between the two types of nickel. Nickel in strong interaction, identified as phyllosilicates or as grafted nickel depending on the preparation conditions, acts as nucleation sites for the particle growth arising from nickel in weak interaction. During drying in air at 90 degrees C, impregnated nickel nitrate transforms into a basic nitrate whose OH groups can react either with those in phyllosilicates or with H2O in the coordination sphere of grafted nickel.