CHARACTERIZATION OF NI/SIO2 CATALYSTS DURING IMPREGNATION AND FURTHER THERMAL-ACTIVATION TREATMENT LEADING TO METAL PARTICLES

Nickel nitrate impregnated onto silica, identified as such after drying at 25-degrees-C, is gradually transformed into basic nickel nitrate, Ni(NO3)2.2Ni(OH)2, and then into nickel phyllosilicate (1:1 nickel phyllosilicate) upon drying at 90-degrees-C. At this temperature, the amount of phyllosilicates which depends on the drying time, can reach 20% of the overall Ni. The temperature programmed reduction (TPR) profiles of impregnated nickel exhibit three peaks of hydrogen consumption at 300, 380, and about 500-degrees-C, attributed to the decomposition of nickel nitrate or basic nitrate into NiO, the reduction of NiO, and the reduction of 1:1 nickel phyllosilicates, respectively. The last peak corresponds to the reduction of 20% of the overall Ni content regardless of the Ni loading and the conditions of drying at 25 or 90-degrees-C. This fixed percentage proves that phyllosilicate may be still formed at the beginning of the TPR. After TPR up to 700-degrees-C, the nickel is totally reduced and the metal nickel particles have a constant average size of about 65 angstrom while the concentration of particles per surface unit increases with the Ni loading. Before complete reduction, the nickel phyllosilicates are probably located at the interface between silica and the remaining nickel, for which they act as anchoring sites. During TPR, after nitrate decomposition, the NiO particles located on the phyllosilicates are reduced into Ni0 at 400-degrees-C without any migration. Between 400 and 700-degrees-C, the increase in metal particle size is due not only to the reduction of the phyllosilicates but also to Ni0 migration induced by thermal effect and probably also by the weakening of the anchoring strength because of the reduction of phyllosilicates. Calcination prior to TPR leads to the formation of larger metal particles and to their aggregation, owing to the partial decomposition of phyllosilicates and leading to the formation of larger oxide particles during calcination.