Imaging Single Atoms In Zeolites

Volkan Ortalan/UC Davis

HIDE AND SEEK The new imaging method pinpoints individual iridium atoms (red) in the pores of a zeolite crystal (Si and Al are light green, O is dark green).

The positions of individual metal atoms and nanoscale metal clusters anchored within the pores of zeolite crystals have, for the first time, been pinpointed with atomic-scale resolution by using an advanced electron microscopy method, according to researchers in California who conducted the study (Nat. Nanotechnol., DOI: 10.1038/nnano.2010.92). The imaging technique may provide a new way to study porous industrial catalysts and determine the mechanisms by which they change with use.

Zeolites are nanoporous crystalline aluminosilicates that are widely used as catalysts in petroleum refining. Researchers have typically depended upon X-ray methods and NMR spectroscopy to analyze the structures of zeolites and deduce how molecules interact with them during reactions. But those methods provide averaged structural values and only limited information about catalytically active metal atoms dispersed inside zeolite pores.

To zoom in on those atoms directly, a research team at the University of California, Davis, developed a way to apply aberration-corrected scanning transmission electron microscopy to fragile specimens such as zeolites, which would normally be damaged by the microscope's powerful electron beam. The team, which includes Volkan Ortalan, Bruce C. Gates, Alper Uzun, and Nigel D. Browning, reduced the beam's intensity during analysis to protect samples of a zeolite (called HY zeolite) loaded with iridium and then enhanced the resulting noisy data with image-processing methods to extract atomic-resolution results.

In this way, the group was able to identify the exact location of individual iridium atoms in the zeolite unit cell and to determine that the atoms reside some 70 Å below the crystal surface. In addition, by comparing the distribution of metal atoms in fresh catalysts with ones exposed to reactive gases, the team deduced a mechanism by which individual iridium atoms anchored at specific sites in the zeolite can dissociate from those sites, migrate, form clusters by bonding to other iridium atoms, and reanchor at other zeolite sites.

"This method provides direct physical evidence of the location of individual iridium atoms in a zeolitic structure," says Northwestern University chemical engineering professor Harold H. Kung. More interestingly, he adds, by tracking changes in the locations of these atoms with catalyst use, the method eliminates uncertainties about how closely freshly prepared zeolite samples resemble samples in operation.

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