Abstract
During the last decades, X-ray absorption spectroscopy (XAS) has become an indispensable method for probing the structure and composition of heterogeneous catalysts, revealing the nature of the active sites and establishing links between structural motifs in a catalyst, local electronic structure, and catalytic properties. Here we discuss the fundamental principles of the XAS method and describe the progress in the instrumentation and data analysis approaches undertaken for deciphering X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra. Recent usages of XAS in the field of heterogeneous catalysis, with emphasis on examples concerning electrocatalysis, will be presented. The latter is a rapidly developing field with immense industrial applications but also unique challenges in terms of the experimental characterization restrictions and advanced modeling approaches required. This review will highlight the new insight that can be gained with XAS on complex real-world electrocatalysts including their working mechanisms and the dynamic processes taking place in the course of a chemical reaction. More specifically, we will discuss applications of in situ and operando XAS to probe the catalyst’s interactions with the environment (support, electrolyte, ligands, adsorbates, reaction products, and intermediates) and its structural, chemical, and electronic transformations as it adapts to the reaction conditions.
Highlights
While more work is still needed to determine the applicability of this approach, this study provides an interesting example of how extended X-ray absorption fine structure (EXAFS) analysis can provide insight into the thermodynamic characteristics of the material
The oxygen evolution reaction is at the heart and is often the limiting step of promising enabling technologies such as the electrochemical water splitting for hydrogen production, rechargeable metal-air batteries and reversible fuel cells that can resurge the fuel through electrocatalysis.[549,550]
A previous study of ZnCo2O4 catalysts yielded somewhat conflicting results.[330]. While in this case X-ray absorption near edge structure (XANES) and EXAFS analysis confirmed similar well-ordered normal spinel structure, and no changes in the Co and Zn oxidation states were observed under oxygen evolution reaction (OER) conditions, higher OER activity was observed for ZnCo2O4 than for Co3O4.330 The enhanced activity in the bimetallic catalyst was partially attributed to the leaching of the Zn species, which resulted in a structure with a high number of vacancies, facilitating the formation of the active CoOOH phase at the catalyst surface.[330]
Summary
The last decades have been marked by drastic developments in experimental in situ and operando characterization techniques as well as great progress in theory Such advanced new methods have allowed us to gain deeper insight into the complex processes that take place in thermal- and electrocatalysts while at work.[1−9] The concept of catalysts being dynamic systems that actively transform and respond to the reaction conditions, rather than just a static arrangement of atoms, is almost commonplace now. Fundamental understanding of the working principles of existing catalysts is still lacking, which hinders the development of novel catalytic materials.[14] Another reason for the emphasis on electrocatalysis in this Review includes the additional challenges in the XAS studies under an electrochemical environment.
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