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Detection and Imaging of Supported Catalyst Particles

Published online by Cambridge University Press:  25 February 2011

Michael M. J. Treacy
Michael M. J. Treacy*
Affiliation:
Exxon Research and Engineering Company Route 22 East, Clinton Township, Annandale, New Jersey 08801
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Abstract

In this short review we contrast and compare the relative merits of three imaging techniques in the CTEM and STEM, for detecting subnanometer size catalyst clusters. It is shown that incoherent dark field signals, such as those obtained by hollow-cone illumination in the CTEM, and the annular detector in the STEM, are significantly more sensitive to subnanometer size clusters than the bright field signal. Sensitivity to clusters of high atomic number increases as the illumination (or detector) semi-angle increases, although at the expense of decreased signal-to-noise. The STEM annular detector signal offers advantages over the CTEM hollow-cone signal in this respect since it has a higher electron collection efficiency. A further advantage of the STEM is that image contrasts may be enhanced by electronically combining the annular detector signal with the axial energy loss signal. This ratio method, or Z contrast technique, has been used with success to detect single Pt atoms on thin low atomic number supports.

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