Experimental methods in chemical engineering:X‐ray absorption spectroscopy—XAS,XANES,EXAFS

Abstract

Although X-ray absorption spectroscopy (XAS) was conceived in the early 20th century, it took 60 years after the advent of synchrotrons for researchers to exploit its tremendous potential. Counterintuitively, researchers are now developing bench type polychromatic X-ray sources that are less brilliant to measure catalyst stability and work with toxic substances. XAS measures the absorption spectra of electrons that X-rays eject from the tightly bound core electrons to the continuum. The spectrum from 10 to 150 eV (kinetic energy of the photoelectrons) above the chemical potential-binding energy of core electrons-identifies oxidation state and band occupancy (X-ray absorption near edge structure, XANES), while higher energies in the spectrum relate to local atomic structure like coordination number and distance, Debye-Waller factor, and inner potential correction (extended X-ray absorption fine structure, EXAFS). Combining XAS with complementary spectroscopic techniques like Raman, Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) elucidates the nature of the chemical bonds at the catalyst surface to better understand reaction mechanisms and intermediates. Because synchrotrons continue to be the light source of choice for most researchers, the number of articles Web of Science indexes per year has grown from 1000 in 1991 to 1700 in 2020. Material scientists and physical chemists publish an order of magnitude articles more than chemical engineers. Based on a bibliometric analysis, the research comprises five clusters centred around: electronic and optical properties, oxidation and hydrogenation catalysis, complementary analytical techniques like FTIR, nanoparticles and electrocatalysis, and iron, metals, and complexes.

In Situ Kinetic Observations on Crystal Nucleation and Growth

“…224 Meanwhile, different from diffraction methods (sensitive to long-range ordered structure), the EXAFS method is element specific and sensitive to the oxidation state and local (fine) structure with short-range order. 225,226 Hence, EXAFS is suitable for the characterization of liquid solutions, disordered materials, amorphous and crystalline samples, and is critical for probing the chemical environments at the initial crystal nucleation stages in various material systems.…”

“…For the latest progress on EXAFS spectroscopy, one can follow the excellent review written by Guerrero-Peŕez and co-workers. 225 EXAFS characterizes the oscillatory variations of the X-ray absorption in ∼30−1000 eV. By measuring the constructive and destructive interference of the emitted electrons as a function of energy, the information on the number, type, as well as distances of the neighboring atoms could be obtained from the spectrum.…”

Reaction mechanism – explored with the unified reaction valley approach

“…A promising way to move into this direction are so-called in situ / operando techniques 18 including in situ electron paramagnetic resonance spectroscopy (EPR), 19 sum frequency generation vibrational spectroscopy, 20 Raman spectroscopy, 21 absorption near edge structure (XANES) spectroscopy and K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, 22,23 X-ray powder diffraction (XRPD) 24–26 or synchrotron and neutron scattering-based techniques, 27 complemented with computational efforts, 7 just to name a few. These techniques collect data about the chemical reaction as it proceeds.…”

Coupling metal stable isotope compositions and X-ray absorption spectroscopy to study metal pathways in soil–plant systems: a mini review

“…This technique is almost exclusive to synchrotron facilities due to the required photon flux and spectral resolution, 24 although bench-top XAS systems have been developed for samples with high metal concentrations. 25 …”