In situ synthesis and nanoscale evolution of model supported metal catalysts: Ni on silica

The ability to disperse metal nanoparticles over a high surface area support is critical for catalyst preparation. For impregnation techniques, there are many fundamental questions about the spreading, diffusion, and coarsening behavior of metal intermediates during the drying and thermal treatments. We employ in situ environmental transmission electron microscopy to monitor the nanoscale processes taking place during the preparation of Ni on silica-supported metal model catalysts. Conventional salt impregnation techniques were employed, and the electron microscope was used to follow the spatial evolution of the material from salt impregnation through calcination and reduction. The best precursor dispersion was obtained with a nickel nitrate solution prepared in an ethanol solvent, followed by performing impregnation in an atmosphere saturated with ethanol vapor to avoid premature drying or deliquescence during mixing. In situ observations showed that no significant wetting or dispersion of the nickel precursor took place during calcination, and the final metal particles spatial distribution was controlled entirely by the initial spatial distribution of salt dispersion on the support in this case. Substantial diffusion of the nickel species took place when the precursor was directly reduced in hydrogen, resulting in high dispersion of Ni particles. The in situ approach should also yield valuable insight into the fundamental process taking place during the synthesis of other nanomaterials, especially for monitoring the structural changes taking place during thermal treatments.