High-Resolution Scanning Electron Microscopy and Microanalysis of Supported Metal Catalysts
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The use of a high-brightness field emission gun and novel secondary electron detection systems makes it possible to acquire nanometer-resolution surface images of bulk materials, even at low electron beam voltages. The advantages of low-voltage SEM include enhanced surface sensitivity, reduced sample charging on non-conducting materials, and significantly reduced electron range and interaction volume.
High-resolution images formed by collecting the
backscattered electron signal can give information about the size and spatial distribution of metal nanoparticles in supported catalysts. Low-voltage XEDS can provide compositional information of bulk samples with enhanced surface sensitivity and significantly improved spatial resolution. High-resolution SEM techniques enhance our ability to detect and, subsequently, analyze the composition of nanoparticles in supported metal catalysts. Applications of high-resolution SEM imaging and microanalysis techniques to the study of industrial supported catalysts are discussed. INTRODUCTION
The development of a commercially viable catalyst requires understanding the factors that enhance selectivity, increase activity, and improve lifetime. Most industrial catalysts are highly complex mixtures of different phases including active components, promoters, stabilizers, supports, etc. The understanding of the structure-performance relationship is crucial to successful development of industrial catalysts. The size and spatial distribution and the surface structure of metal or alloy particles are the most important parameters in determining the performance of supported metal catalysts. The morphology of the supports and the particle-support interactions,
however, can profoundly affect the distribution and the nature of highly dispersed metal or alloy particles. A complete structural characterization of both the metal or alloy particles and the supports is essential for a full understanding of the catalyst's performance. Traditionally, transmission electron microscopy or scanning transmission electron microscopy (TEM/STEM) has to be used to visualize highly dispersed metallic nanoparticles. A major limitation of TEM/STEM techniques is, however, the stringent requirement of samples that can be examined: useful information can be extracted from only very thin areas of a sample. It is very difficult, if not impossible, to use TEM/STEM techniques to extract information about the surface properties of bulk catalyst samples such as powders, cylinders, or beads that are most frequently used in industrial catalytic reactions. With the use of a high-brightness field emission gun and novel electron detection systems in field emission scanning electron microscopes (FE-SEM), it is now possible to examine surface features of bulk samples on a nanometer scale [1]. Highly dispersed metal nanoparticles as well as detailed surface topography of catalyst supports can now be examined inFE-SEM instruments [23]. Secondary electron (SE) and backscattered electron (BE) signals can be used independently or i
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