The interpretation of HREM images of supported metal catalysts using image simulation:: profile view images

The inherent problems of image simulation when applied to the study of supported metal catalysts are analysed and discussed. The paper focuses on the consideration of profile view images with reference to the fine details of the contrasts both in the metal particles and in the outer support layers. As a general conclusion we prove that complex contrasts which very often appear in the images can be interpreted on the grounds of the structural features of the catalysts and on the recording conditions in the microscope. This conclusion is supported by simulation of several experimental images showing excellent fitting with the simulated ones. One key feature to face for a successful interpretation of metal/support discrete interfaces is the availability of a methodology to construct the complex supercells which are required as input data for the multislice simulation programs. The paper includes a description of the approach followed for this aim in our lab, allowing to model size, shape, faceting, and relative orientation of metal and support particles. Some other definite aspects addressed specifically in this contribution are: (a) influence of support thickness, (b) influence of the metal particle position on the support, (c) effect of the metal particle size on its visibility and resolution, (d) assessment to the determination of reliable metal particle size by direct measurement from the images, and (e) influence of the crystal tilts in the imaging process. The influence of such variables in the image contrasts are analysed. (C) 1998 Elsevier Science B.V. All rights reserved.