EXAFS - example 1
 

(a) Schematic illustration of Pt atoms deposited on nitrogen-doped carbon dots (Pt–NCDs) and (b) Fourier transformed Pt L₃-edge EXAFS spectra of Pt–NCDs hybridized with TiO₂ film, and of PtO₂, PtCl₂ and Pt foil as references for comparison (measured at beamline B18, Diamond Light Source).

The absence of a scattering peak in the region 2-3.5 Å, corresponding to a Pt-Pt bond, indicates that only isolated Pt atoms are bound to the NCD support, and the single peak at 1.6 Å reveals that they are coordinated to light atoms (4-5 carbon atoms) on the support. 

This example shows that EXAFS data can be used to distinguish between single-atom catalysts and small clusters or nanoparticles bound to the support. Pt single-atoms are important as catalysts for photocatalytic hydrogen production. 

Adapted from Hui Luo, Ying Liu, Stoichko D. Dimitrov, Ludmilla Steier, Shaohui Guo, Xuanhua Li, Jingyu Feng, Fei Xie, Yuanxing Fang, Andrei Sapelkin, Xinchen Wang and Maria-Magdalena Titirici, Pt single-atoms supported on nitrogen-doped carbon dots for highly efficient photocatalytic hydrogen generation, J. Mater. Chem. A, 2020, 8, 14690.
DOI: 10.1039/d0ta04431h
 

EXAFS - example 2
 

(left) Preparation of 17 atoms Pt clusters deposited on γ-Al₂O₃ (Pt₁₇/γ-Al₂O₃)

(right) Fourier transform of Pt L₃-edge EXAFS spectra of [Pt₁₇(CO)₁₂(PPh₃)₈]Cl_n, Pt₁₇(CO)₁₂(PPh₃)₈/γ-Al₂O₃, and Pt₁₇/γ-Al₂O₃ together with EXAFS data of Pt foil and PtO₂. In the spectrum of Pt₁₇/γ-Al₂O₃ the peak at 1.7 Å is attributed to Pt–C or Pt–O bonds at the Pt₁₇/γ-Al₂O₃ interface.

EXAFS data shows that in the Pt₁₇/γ-Al₂O₃ system the supported Pt₁₇ is not present in the form of oxide but has a framework structure like a metal cluster. Moreover, it was shown that supported Pt₁₇ clusters are covered by CO molecules at normal temperature. CO molecules adsorbed on fine Pt₁₇ supported clusters generally has a longer C–O bond compared to larger Pt supported nanoparticles, promoting the oxidation reaction and possibly contributing to the high catalytic activity of the Pt₁₇/γ-Al₂O₃ system for carbon monoxide and propylene oxidation in comparison to γ-Al₂O₃-supported larger Pt nanoparticles (PtNP/γ-Al₂O₃)prepared by conventional impregnation.

Adapted from Yuichi Negishi, Nobuyuki Shimizu, Kanako Funai, Ryo Kaneko, Kosuke Wakamatsu, Atsuya Harasawa, Sakiat Hossain, Manfred E. Schuster, Dogan Ozkaya, Wataru Kurashige, Tokuhisa Kawawaki, Seiji Yamazoe and Shuhei Nagaoka, γ-Alumina-supported Pt₁₇ cluster: controlled loading, geometrical structure, and size-specific catalytic activity for carbon monoxide and propylene oxidation, Nanoscale Adv., 2020, 2, 669-678. https://doi.org/10.1039/C9NA00579J

XANES - example 1

Sulfur K-edge XANES spectra of substances with sulphur atoms in different oxidation states, attached to different ligands and in different coordination geometries. Obviously, different coordination environments cause characteristic features in XANES spectra, which can serve as ´spectral fingerprints´, and the sulfur speciation in an unknown sample can be determined by comparison with reference XANES spectra. Moreover, XANES spectra can be fit to linear combinations of reference spectra, from which the relative concentrations of the different species can directly be obtained. 

Adapted from Gøril Jahrsengene, Hannah C. Wells, Stein Rorvik, Arne Petter Ratvik, Richard G. Haverkamp & Ann Mari Svensson, A XANES Study of Sulfur Speciation and Reactivity in Cokes for Anodes Used in Aluminum Production, METALL MATER TRANS B 49, pages1434–1443(2018) https://doi.org/10.1007/s11663-018-1215-x

XANES - example 2

Linear combination fitting (LCF) of Cl K-edge XANES spectra of Ce glass-ceramics with nominal (a) 0.9 wt% Cl and (b) 1.7 wt% Cl; (c) comparison of the weighted contributions of reference compounds fitted to the Cl K-edge XANES data of Ce-free and Ce-incorporated glass-ceramics (samples were fabricated with CeO₂, which model a PuO₂ surrogate). XANES spectroscopy was used to investigate the potential application of an albite glass-zirconolite ceramic material for immobilisation of chloride contaminated plutonium oxide residues. XANES data confirm partitioning of Cl to the glass phase with exsolution of crystalline NaCl above the chlorine solubility limit. LCF results indicate an association of Cl with Na and Ca modifier cations, with environments characteristic of the aluminosilicate chloride minerals eudialyte, sodalite, chlorellestadite and afghanite. This study demonstrates the compatibility of the glass-ceramic wasteform toward Cl solubility at the expected incorporation rate (below the determined solubility limit) and provides confidence that upstream heat treatment or blending of waste feed are not required.

Adapted from Stephanie M. Thornber, Lucy M. Mottram, Amber R. Mason, Paul Thompson, Martin C. Stennett and Neil C. Hyatt, Solubility, speciation and local environment of chlorine in zirconolite glass–ceramics for the immobilisation of plutonium residues, RSC Adv., 2020, 10, 32497. DOI: 10.1039/d0ra04938g   

XANES - example 3

Vanadium K-edge XANES spectra of (a) electrochemically lithiationed V₂O₃(SO₄)₂ electrodes at various states of discharge and charge and (b) chemically lithiated V₂O₃(SO₄)₂ samples compared to the reference materials V₂O₅, VOSO₄·3H₂O and V₂O₃; (c) vanadium K-edge Energy (at half-height) as a function of oxidation state. In this case, XANES spectroscopy was used to investigate Li⁺ insertion into V₂O₃(SO₄)₂ (promising material for high energy density lithium-ion batteries) via electrochemical and chemical routes. The results show that 2.0 Li⁺ ions can be incorporated electrochemically, resulting in Li₂V₂O₃(SO₄)₂ attributed to the V⁴⁺/V⁵⁺ redox couple. For the chemically lithiated materials up to 4.0 Li⁺ ions can be inserted into V₂O₃(SO₄)₂, reducing V⁵⁺ to V³⁺.

Reproduced from Stephanie F. Linnell, Julia L. Payne, David M. Pickup, Alan V. Chadwick, A. Robert Armstong and John T. S. Irvine, Lithiation of V₂O₃(SO₄)₂ – a flexible insertion host, J. Mater. Chem. A, 2020, 8, 19502. DOI: 10.1039/d0ta06608g