Scanning Electron Microscopy (SEM)

Abstract

Scanning electron microscopy (SEM) is perhaps the first technique a catalyst researcher should use to start characterizing a catalyst. In this chapter, we present an overview of instrumental considerations when studying heterogeneous catalysts via SEM, methods to achieve high resolution for imaging and elemental analysis, and applications and case studies highlighting the use of SEM for the study of catalysts. The SEM images show the morphology of the sample and provide elemental composition and an indication of the uniformity of the sample as well as information on pore structure and particle size at the micron scale. The ability to see the material over this length scale is critical to early identification of issues with catalyst preparation, such as poor distribution of the active components. Using a high-resolution SEM, or when possible, a STEM detector within the SEM, it is possible to achieve resolutions rivaling a TEM. Then it becomes possible to see individual pores, the size and morphology of nanoparticles, and their composition. SEM is also ideally suited to see exposed surface facets in nanoparticles. Sample preparation for SEM is so much easier than a TEM, since the sample can be simply sprinkled on an SEM stub. The disadvantage is that only the surface of the sample is visible. But one can probe deeper by generating cross sections of extrudates. Such cross sections can help in identifying the location of various components, such as, for example, when preparing eggshell morphologies where the active phase is present only in the near-surface region of an extrudate.