In situ EXAFS study of the nucleation and crystal growth of Ni particles on SiO₂ support

Ni/SiO₂ catalysts were prepared by wetness impregnation method (INi) and by a new two-step method (ENien_2.5/600+ INi_2.5). The two-step method consists of formation of chemical glue, "nickel nuclei", having strong metal support interaction with SiO₂ and preparation of the "nickel reservoir" which has weak interaction with support. The nucleation and growth behaviors of such Ni/SiO₂ catalysts in the preparation stage were continuously monitored by in situ EXAFS spectroscopy. The in situ EXAFS spectra of INi catalyst began to change after 320°C and bulk NiO like structure was formed at 500°C heat treatment. The Ni/SiO₂ prepared with the two-step method had larger coordination number of Ni with silica support in the initial stage of in situ monitoring than that of INi catalyst, which showed the coexistence of Ni nuclei and Ni reservoir. The ENien_2.5/600+INi_2.5 had smaller particles of Ni compared with INi after high temperature calcination. The formation of small Ni particles could be attributed to the strong ion-support interaction of nickel nuclei with nickel reservoir in ENien_2.5/600+ INi_2.5 catalyst. Slight particle growth occurred after 370°C calcination in Ni/SiO₂ prepared by two-step method.