Credit: © 2008 ACS

Heterogeneous catalysts are used in a wide range of industrial processes. One example is the Fischer–Tropsch reaction, where cobalt nanoparticles supported on porous alumina are highly active in the production of fuel from various sources. However, studying the catalysts' properties often relies on average measurements of bulk samples, rather than looking at individual particles or their active sites.

Now, Ilke Arslan and colleagues1 based at the University of Cambridge, UK, and in Trondheim, Norway have used a scanning transmission electron microscope (STEM) to look directly at how cobalt oxide particles fit into an alumina support. Although the active catalysts contain cobalt metal, the corresponding oxide was examined in the STEM because it is more stable and the overall morphology and distribution of the cobalt particles would be expected to remain the same in the reduced form. Over one hundred STEM images were combined to create a 3D picture of a catalyst particle. With these pictures, the team were able to investigate why adding nickel to the support improves the activity of the catalyst.

It was found that the cobalt oxide particles coat the nickel–aluminate pore walls, rather than filling the pores as they do in the pure-alumina support. Arslan and co-workers suggest that adding nickel creates imperfections that allow the cobalt oxide to precipitate on the pore walls. Crucially, the porosity of the nickel-containing areas measured by tomography is higher than the bulk value evaluated by traditional methods. This indicates that the technique can directly measure the active parts of the system, an advantage over other methods that take an average that includes areas not involved in the catalysis.