Credit: © 2008 AAS

Metal nanoparticles dispersed on a surface act as catalysts for the synthesis of many industrial chemicals and fuels. For such catalysts, the relationship between structure and properties is delicately poised, and so far most fundamental insights have come from experiments on model catalysts, such as single-crystal surfaces in controlled environments. However, real catalysts are inherently more complex, and their size, shape and surface structure can also change during a reaction. Andreas Stierle and colleagues at the Max Planck Institute for Metals Research in Stuttgart and the CEA Institute for Nanoscience and Cryogenics in Grenoble have now shown that changes to rhodium nanoparticles during the course of a reaction can be followed directly by using high-resolution X-ray diffraction (Science 321, 1654–1658; 2008).

Stierle and co-workers examined the rhodium nanoparticles, which were supported on a magnesium oxide surface, during oxidation and reduction reactions. To obtain atomic-scale information on the average shape and size of the nanoparticles, they recorded reciprocal-space maps at elevated temperatures and under various gas atmospheres. From the analysis, the particles were found to have a truncated pyramidal shape. The figure shows fitted diffraction maps for the (110) plane of the clean rhodium nanoparticles (top) and the oxidized nanoparticles (bottom), and the difference between the two (middle). The ridges pointing towards the bottom corners correspond to (111) facets at the side of the nanoparticles, whereas the ridge running down the centre corresponds to a (001) facet at the top of the nanoparticle. The signal intensity (top and bottom) and the difference in intensities (middle) are represented by the spectrum of colours, running from blue (low) to red (high).

The researchers found that the addition of oxygen led to flatter nanoparticles: this can be seen in the difference map — the intensity is increased along the (001) ridge and decreased along the (111) ridge. However, this change could be reversed by exposure to carbon monoxide. Complementary information about the shape of the nanoparticles was also obtained with transmission electron microscopy. Stierle and colleagues showed that this reversible change of shape was driven by the formation of an oxygen–rhodium–oxygen surface oxide trilayer at the facets of the nanoparticle.