Characterization of Catalysts in Reactive Atmospheres by X-ray Absorption Spectroscopy

X-ray absorption spectroscopy (XAS) is a powerful method for probing the average local electronic and geometric structures of catalysts in the working state. Element-specific data can be obtained over a wide range of temperatures (room temperature to >1000 K), and pressures (from subambient to well more than 100 bar). Because the specimen can be investigated under such a wide range of conditions, XAS has become one of the most frequently applied techniques for structural characterization of working catalysts. Often there is no other way to obtain the structural information provided by this technique that allows one to develop specific structure-activity relationships in catalysis. XAS is applicable to a broad range of elemental concentrations (from tens of ppm to wt% levels), and therefore is used to characterize both high-surface-area supported catalysts and bulk catalysts (e.g., oxides). This review is focused on the application of XAS to the investigation of catalysts in the working state. The designs of the experimental reaction cells, which are essential for the XAS analysis of catalysts, are comprehensively reviewed. Specific examples illustrate the application of the technique to the investigation of catalysts at both steady-state and under dynamic conditions. The examples are chosen to demonstrate the wide variety of catalysts that can be investigated. Although XAS has been used for almost 40 years to characterize catalysts in the working state, methodology, equipment, and applications are still being advanced. Some recent major developments (as a result of improvements in optics of the X-ray beam lines, new synchrotron radiation sources, advanced detectors, and software) are therefore highlighted.