Credit: © 2008 NPG

Catalysts composed of nanoparticles dispersed across a solid support lie at the heart of the chemical industry. Despite their importance, however, acquiring a molecular-level understanding of the chemical and physical processes taking place during a catalytic reaction is still a significant challenge. Now, Bert Weckhuysen, Frank de Groot and colleagues1 at Utrecht University, Delft University of Technology and the Lawrence Berkeley National Laboratory have shown that a highly focused, monochromatic beam of X-rays can be used to obtain chemical maps of working catalysts at a spatial resolution of 15 nm.

The technique — termed scanning transmission X-ray microscopy — uses low-energy (soft) X-rays that are scanned across a sample while the absorption of the beam by the sample is measured. By repeating this process at different X-ray energies, absorbance can be plotted as a function of X-ray energy. This information can then be used to image and distinguish between specific chemical species. Soft X-rays are, however, easily absorbed by matter, restricting the thickness of the sample, and the distance and pressure the X-rays can travel through. To overcome this challenge, the researchers constructed a nanoreactor, which allows them to carry out measurements at atmospheric pressure and temperatures up to 500 °C.

The team have demonstrated their method by monitoring in situ changes to an iron-based catalyst for the Fisher-Tropsch synthesis — a reaction in which a mixture of CO and H2 is converted into hydrocarbons.