Time-Resolved X-Ray Absorption Spectroscopy (XAS)

X-ray absorption spectroscopy (XAS) plays a crucial role in the characterization of catalysts as it can in principle characterize any chemical element under well-defined conditions (i.e., in the liquid, gas, and solid phase) as well as under reaction conditions (i.e., at elevated temperatures and pressures, in the so-called in situ or operando mode). The past decades have seen an increase in XAS capabilities in part due to the higher brilliance of X-ray sources at synchrotrons, which in combination with more powerful detectors and optics allows to conduct time-resolved measurements, such as sub-second X-ray absorption near-edge spectroscopy (XANES). Such measurements allow investigation of solid catalysts at different stages of existence, i.e., their birth, life, and death. Furthermore, by the combination of XANES with microscopic capabilities by use of Fresnel zone plates to focus, X-rays allow for 2D and 3D imaging of a catalyst material as a function of reaction time, while the further development of lab-based X-ray source has made it possible to bring the X-ray experiment into both academic and industrial labs, comparable to what we currently do for measuring, e.g., X-ray diffraction (XRD) and Raman spectroscopy. Finally, as no single analytical technique can offer the ultimate answer to a scientific question, often (time-resolved) XAS is combined with optical, diffraction, and/or scattering methods, thereby allowing to distinguish between local and structural (bulk) properties of catalyst materials. The above-described developments will be illustrated by using a selection of showcases. The chapter concludes with some general observations as well as with an outlook.