PREPARATION AND CHARACTERIZATION OF WELL-DISPERSED AND STABLE CO SIO2CATALYSTS USING THE AMMONIA METHOD/

The potential of the ammonia method, initially proposed for preparing stable and well-dispersed Ni/SiO2, has been explored for the preparati on of new Co/SiO2 catalysts. This method consists of adding ammonia to a solution of cobalt II nitrate to obtain ammine ions and of reacting the solution with silica support before filtration, washing and dryin g. When superparamagnetism occurs (no remanence, cobalt particle size less than 16-20 nm), it is shown that magnetic measurements are well a dapted to the characterization of Co/SiO2 catalysts (surface average d iameters calculated from magnetic techniques are in good agreement wit h those obtained from transmission electron microscopy). Magnetic meth ods also give the amount of metallic cobalt in the catalyst. The diffe rent parameters of precursor preparation (order of introduction of rea ctants, time of contact between silica and the solution, degree of mix ing, number of washings of the filtrate, concentration of reactants, s urface area of the silica support) have been varied. Increasing mixing and the silica surface area from 50 to 200 m(2)g(-1) leads to a simul taneous increase of Co loading and dispersion. By comparing the extent of reduction versus reduction temperature curves, it is suggested tha t the cobalt phase in precursors with cobalt loading less than ca. 8 w t% consists of a two-dimensional phyllosilicate. Reduction under flowi ng hydrogen at 923 K leads to an almost complete reduction with surfac e average diameters as small as D-s =3.8 nm associated with a narrow d istribution and a cobalt loading of 3.2 wt%. Highly loaded catalysts ( 45.6 wt%) can also be obtained by the ammonia method with D-s=20 nm. F ive typical preparation recipes are given, leading, after reduction at 923 K, to Co/SiO2 catalysts with surface average diameters spanning t he range of 3.8-16 nm and cobalt loadings between 3.2-45.6 wt%. (C) 19 98 Elsevier Science B.V.