The small electron probe available in a scanning transmission electron microscope (STEM), when combined with X-ray energy dispersive spectroscopy (XEDS), enables the acquisition of 2D compositional maps with high spatial resolution and sensitivity. In addition to the spectroscopic information available, images sensitive to atomic number (so called Z-contrast images) can be acquired simultaneously using high-angle annular dark field imaging (HAADF). These capabilities make the STEM an ideal tool to study elemental distributions in catalysts containing 1-2 nm sized metal clusters within a support of low atomic number. The material examined in this work comprises of mesoporous silica in a honeycomb structure with channels of ˜3 nm diameter (designated MCM-41) in which nano-clusters of the bimetallic Pd6Ru6 have been introduced into the channels using metal-cluster carbonyls as precursors. Details of the specimen preparation are similar to that outlined by Shepard et al. for a Cu-Ru bimetallic catalyst.

Fig. 1 shows (a) a bright-field (BF) image, (b) a HAADF image and (c-f) X-ray elemental maps acquired on a VG 603 STEM, operated at 300 keV with probe size of ˜1.4 nm (FWTM) and probe current of ˜0.6 nA.