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Direct Imaging of Metal Atoms and Clusters on Real Supports

Nellist PD1, Pennycook SJ1

Science 274, 413 (1996)

Full Article (PDF 1.2 MB)

For the first time, metal atoms and small clusters have been imaged on the surface of a real automotive catalyst. Using Z-contrast microscopy in ORNL’s atomic resolution scanning transmission electron microscope, single atoms and monolayer rafts of platinum and rhodium have been directly imaged on a g-Al2O3 support. Monolayer rafts can be simply distinguished from three-dimensional metallic clusters. The outer atoms of nanometer-sized three-dimensional clusters are clearly imaged, allowing their exposed crystal facets to be directly determined. By simultaneously imaging the atomic structure of the support, it is even possible to determine individual adatom adsorption sites. This ability to detect the absolute state of aggregation of a catalyst offers a new approach to overcoming long-standing barriers in catalyst science. Atomic mechanisms of cluster formation, cluster coarsening behavior, and degradation phenomena can all be elucidated. In the future, detailed atomic scale characterization, in conjunction with standard chemical tests of activity and selectivity, could identify the key reaction sites involved, leading to the first detailed scientific understanding of actual reaction pathways in automotive catalysts.

This advance in catalyst microcharacterization is made possible through Z-contrast microscopy using ORNL’s unique 300-kV scanning transmission electron microscope. With a probe diameter of only 1.26 Å, this instrument gives sufficient scattering to render a single heavy atom visible on a typical light support such as g-Al2O3. Figure 1 compares bright field images (as available on conventional microscopes and dominated by the support) with Z-contrast images taken simultaneously (dominated by the metal). In the Z-contrast image, single platinum atoms and monolayer rafts are visible, shown filtered and enlarged in Fig. 2, with possible sites on the support suggested by their orientation and spacing.

Investigating real catalyst systems is crucial to understanding the scientific issues involved. The support plays a pivotal role, from changing the band structure of the cluster and affecting catalytic activity, through determining adatom mobility and affecting formation mechanisms, to reacting with the metal leading to degradation. It has not been previously possible to study these complex atomic mechanisms on the rough, insulating surface of a real automotive catalyst. Z-contrast microscopy now offers a new opportunity to study these crucial phenomena at a fundamental level.

 

Ultradispersed Pt on g-alumina

 
 
Figure 1. Stem bright field (BF) and Z-contrast (ADF) images taken simultaneously on the HB603U STEM.
 
 

Pt trimers and dimers

  Figure 2. Higher magnification Z-contrast image showing single Pt atoms and suggested sites.
   
 
  1. Solid State Division, Oak Ridge National Laboratory

 Oak Ridge National Laboratory