Credit: © 2010 NPG

The crystalline microporous materials known as zeolites are intensively studied and widely used as catalysts, sorption agents and for separating molecules. They are also used as supports for metal catalysts, giving the added advantage of size or shape selectivity through their pore structure. A wide range of techniques has been used to understand the structure of these supported catalysts, to gain mechanistic insights crucial to improving their performance. Among these techniques, transmission electron microscopy (TEM) methods are useful, but the zeolite samples can be damaged by the electron beam.

Now, Volkan Ortalan and colleagues from the University of California Davis have used a low-dose form of TEM, combined with image-processing techniques, to directly image single iridium atoms within a zeolite's pores. After obtaining aberration-corrected high-angle annular dark-field scanning TEM images that minimized the effect of the beam on the zeolite, the team used real-space averaging after Fourier filtering. The averaging technique preserved the zeolite structure in the images, removing the iridium atoms. Superimposing the original images therefore revealed the metal positions.

One of the advantages of the technique is that its nanometre-scale depth of focus makes it possible to analyse different depths within a sample. Ortalan and colleagues took advantage of this to look at regions where there are few or no mesopores. They also examined catalysts after exposing them to flowing hydrogen for 30 minutes, simulating a reactive environment. The iridium atoms form clusters, identifiable as Ir4 and Ir6.