Synthesis and characterization of cobalt nanoparticles, and development of well defined metal on support model catalyst systems
Nanotechnology is a term used when working with components in the range 0.1-100 nanometer (nm). One nanometer corresponds to 1×10-9 m. Nanomaterials attract huge attention due to their often unexpected physical and chemical properties relative to the same materials when composed of larger components. This makes also nanomaterials commercially important and scientifically very interesting. For example, in the oil industry “metal-on-support” catalysts are frequently used for converting natural gas (via synthesis gas) to valuable oil products. Fischer-Tropsch (FT) synthesis is one such process, and this process is controlled by a “metal-on-support” catalyst fabricated by nanometer sized metal particles deposited onto a porous support. The metal component of the FT-catalyst is often cobalt (Co).
In order to optimize the industrial process to give high reaction rates and no undesired by-products we need to understand how the small catalytic particles must be designed with respect to size, shape and atomic arrangement. Figure 1 shows some possible shapes of such metal nanoparticles.
Figure 1 Some possible shapes of cobalt nanoparticles.
In cobalt metal and in cobalt nanoparticles, the cobalt atoms can be arranged in different ways giving rise to slightly different atomic arrangements, or polytypes. The two most common polytypes are face centered cubic (fcc) and hexagonal closed packed (hcp), see Figure 2.
For large particles the hcp type is stable below 420 oC, whereas the fcc-type exists at higher temperatures. However, when the cobalt particle size become smaller than approximately 20 nm, the fcc-type will also be the favoured polytype below 420 oC, which in turn may affect the catalytic properties in a cobalt based ”metal-on-support” catalyst.
Figure 2 Structural representation of fcc Co presented in a pseudo hexagonal cell (left panel) and hcp Co (right panel).
Several efforts have been done to correlate cobalt particle size, shape, atomic arrangement and controlled addition of an extra element to the cobalt particle with respect to catalyst performance. Most studies are hampered by not being carried out in a systematic manner, varying just one or a minimum materials parameters. Also the preparation route may be a factor affecting the atomic arrangement and composition of the particles, in particular when more elements are added to cobalt. In this project we target on developing synthetic routes that yield particles with narrow size distribution, well-defined atomic arrangement and chemical composition. These carefully designed particles will in a next step be applied for preparation of well defined model “metal-on-support” catalysts. Finally, systematic studies will be undertaken to search for significant correlations between particle properties and catalyst performance.