XANES Simulations Research Articles

Understanding of Anion Effect on Iron Redox Behavior and Oxygen Reduction Activity of Fe-N-C Electrocatalysts

Fe-N-C catalysts are the most promising platinum group metal (PGM) free electrocatalysts for oxygen reduction reaction (ORR). Significant progress has been made in understanding the active site structure and activity and improving the ORR performance. It is generally accepted that the Fe-N-C sites are associated with FeNx centers embedded in a defective nitrogen-doped graphene. As reported in literature, both pyrrolic N and pyridinic N can support Fe single site; however, it is difficult to distinguish between the two. Furthermore, experimental characterization, such as in-situ X-ray absorption spectroscopy (XAS) and Mössbaruer, has elucidated the redox behavior of Fe2+ to Fe3+ during the reaction. Interestingly, in different acidic environments, the ORR activity exhibits differently. For instance, in comparison between H2SO4 and HClO4 solutions at the same pH value, the Fe redox happens at lower potential in H2SO4; however, the right shift of half-wave potential (E1/2) of ORR in H2SO4, compared with HClO4, indicates higher ORR activity. This indicates that the electrolyte systems have a significant impact on ORR activity. To further understand the active site structure and their ORR activity, density functional theory (DFT) calculations utilizing explicit solvation effect are carried out. XANES simulation using FDMNES suggests that as prepared catalysts could consist of Fe single atom sites supported by both pyrrolic and pyridinic N atoms; however, the pyrrolic species are not as stable as pyridinic species in solution. As presented by DFT simulation, the anions in solutions can interact with FeN4 site differently. SO4 2- binds strongly with the FeN4 center, while HSO4- and ClO4- interact rather weakly. Furthermore, the redox potential and limiting potential are evaluated via mechanistic study. By focusing on more stable Fe center supported by pyridinic N, the results suggest that the transformation of Fe2+ to Fe3+ happens at 0.52 V in H2SO4, which is lower than that in HClO4, 0.69V. More interestingly, the limiting potential (activity) of ORR in H2SO4 is higher than that in HClO4 (0.94 vs 0.68 V). DFT simulation indicated that the anions in electrolyte can impact the Fe redox behavior and ORR performance, which agrees well with the experimental investigation. This work also provides significant guidance to further improve ORR activity of Fe-N-C catalysts.

Recent Advances on Computational Modeling of Supported Single-Atom and Cluster Catalysts: Characterization, Catalyst–Support Interaction, and Active Site Heterogeneity

To satisfy the need for catalyst materials with high activity, selectivity, and stability for energy conversion, material design and discovery guided by theoretical insights are a necessity. In the past decades, the rise in theoretical investigations into the properties of catalyst materials, reaction mechanisms, and catalyst design principles has shed light on the catalysis field. Quantitative structure–activity relationships have been developed through incorporating spectroscopic simulations, electronic structure calculations, and reaction mechanistic studies. In this review, we report the state-of-the-art computational approaches to catalyst materials characterization for supported single-atom and cluster catalysts utilizing spectroscopic simulations, i.e., XANES simulation, and material properties investigation via electronic-structure calculations. Furthermore, approaches regarding reaction mechanisms, focusing on active site heterogeneity, are also discussed.

Insight into the cation migration and surface structural evolution of spinel LiNi0.5Mn1.5O4 cathode material for lithium-ion batteries

High-voltage spinel LiNi0.5Mn1.5O4 (LNMO) is considered to be a promising cathode material for electric vehicles (EV) and hybrid electric vehicles (HEVs). Nevertheless, the structural transformation under a deep discharge voltage is still ambiguity. Herein, we report detailed studies of structural change of LNMO cathode under a wide voltage range of 5.0–1.5 V cycling by using synchrotron-based X-ray absorption spectroscopy, aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). The fundamental findings demonstrate that LNMO would undergo a complex phase transformation from cubic to tetragonal during deep discharge to a voltage lower than 3 V. It gives rise to a further reduction of Mn4+ to Mn3+ and this serious structural distortion leads to the formation of Li2Mn2O4-type tetragonal phase with space group of I41/amd, which is also confirmed by XANES simulations and density functional theory (DFT) calculations. Meanwhile, migration of Ni ions toward the surface from bulk lattice, leading to the rock-salt NiO phase formation with a space group of Fm-3 m. Finally, these observed evolutions on the surface are associated to the fast capacity fading for LNMO under such a wide voltage range. This work provides insight into understanding the capacity degradation mechanism of spinel cathodes and would inspire us to further design high-performance cathode materials for advanced lithium-ion batteries.

Complementary Operando Spectroscopy identification of in-situ generated metastable charge-asymmetry Cu2-CuN3 clusters for CO2 reduction to ethanol

Copper-based materials can reliably convert carbon dioxide into multi-carbon products but they suffer from poor activity and product selectivity. The atomic structure-activity relationship of electrocatalysts for the selectivity is controversial due to the lacking of systemic multiple dimensions for operando condition study. Herein, we synthesized high-performance CO2RR catalyst comprising of CuO clusters supported on N-doped carbon nanosheets, which exhibited high C2+ products Faradaic efficiency of 73% including decent ethanol selectivity of 51% with a partial current density of 14.4 mA/cm−2 at −1.1 V vs. RHE. We evidenced catalyst restructuring and tracked the variation of the active states under reaction conditions, presenting the atomic structure-activity relationship of this catalyst. Operando XAS, XANES simulations and Quasi-in-situ XPS analyses identified a reversible potential-dependent transformation from dispersed CuO clusters to Cu2-CuN3 clusters which are the optimal sites. This cluster can’t exist without the applied potential. The N-doping dispersed the reduced Cun clusters uniformly and maintained excellent stability and high activity with adjusting the charge distribution between the Cu atoms and N-doped carbon interface. By combining Operando FTIR and DFT calculations, it was recognized that the Cu2-CuN3 clusters displayed charge-asymmetric sites which were intensified by CH3* adsorbing, beneficial to the formation of the high-efficiency asymmetric ethanol.

Investigating cubane formation and effect of co-crystallization agents in oxo-bridged Co complexes using X-ray absorption spectroscopy

• Cubane core in Co-oxo complexes studied using XAS to probe its spectroscopic nature. • XANES shows presence of octahedrally coordinated Co 3+ ions in cubane structure. • EXAFS confirm octahedral Co with 6 O/N, 3 Co-Co backscattering forming cubane core. • XANES simulation indicate interaction with co-crystallizing salts NaNO 3 and NaClO 4 . • Formation of defect dicubane with Co 2+ /Co 3+ ions predicted by XANES simulation. The formation of cubane core and effect of co-crystallization agent have been studied in the oxo-bridged Co complexes using X-ray absorption spectroscopy. XAFS at Co K-edge have been measured in the oxo-bridged Co complexes [Co 4 O 4 (O 2 CCH 3 ) 4 (C 5 H 5 N) 4 ]( 1 ), 1 .0.5NaNO 3 ·8H 2 O( 2 ), 1 .NaClO 4 ·3·5H 2 O( 3 ) and [Co 4 O 4 (O 2 CCH 3 ) 4 (NH 3 ) 4 ]( 4 ). Complexes 2 and 3 actually contain the same complex 1 co-crystalized by NaNO 3 and NaClO 4 moieties. While 1 and 4 are in powder form, 2 and 3 are in crystalline form. Different XANES features have been correlated to the coordination around the absorbing Co atom in all the complexes. EXAFS analysis has been performed to determine coordination environment around the Co atoms to confirm the formation of the oxo-cubane structure in these complexes. For each of the Co centers present at the four different positions of the cubane core in complexes 2 and 3, XANES spectra have been theoretically simulated and simultaneously p-DOS calculated to examine the effect of co-crystallizing moieties interactions with the individual Co centres present in these two complexes. After preliminary indication by EXAFS analysis, XANES simulations were also performed to examine the presence of defect dicubane structure in 4. The study can be helpful in the detection of these structures during in-situ/operando studies of these catalytically active complexes.

Identification of barium-site substitution of BiFeO3–Bi0.5K0.5TiO3 multiferroic ceramics: X-ray absorption near edge spectroscopy

In this work, the effects of barium substitution on the local structure, dielectric and magnetic properties of the polycrystalline ceramics 0.6BiFeO3–0.4(Bi0.5K0.5)TiO3 (0.6BFO–0.4BKT) system was investigated. A solid-state reaction technique was used to synthesize the materials with barium (Ba) doping of 1, 3, 5, 7, and 10 mol%. XRD analysis reveals the coexistence between tetragonal and rhombohedral phases of single-phase perovskite in pure 0.6BFO–0.4BKT and the rhombohedral reach phase was found with increasing Ba content. XANES simulations indicate that the majority of Ba atoms occupy A-site in BKT lattice of Ba-doped 0.6BFO-0.4BKT, the oxidation state of Fe, Ti, and Ba ions are +3, +4 and+2, respectively. At 5 mol% of Ba doping content, the dielectric measurement shows the morphotropic phase boundary (MPB) and the maximum value of ferromagnetic characteristic were observed, indicating an optimum composition, properties and production conditions.

Exploring structure and reactivity of Cu sites in functionalized UiO-67 MOFs

The exceptional thermal stability of the UiO-67/68-family MOFs makes them ideal candidates to explore post-synthetic functionalization routes aiming to the heterogenization of homogeneous catalysts. We previously demonstrated that a small fraction of the linkers in the UiO-67 MOF can be replaced by bipyridine-dicarboxylate (bpydc) moieties exhibiting metal-chelating ability and enabling the grafting of Pt(II) and Pt(IV) ions in the MOF framework [Chem. Mater., 27 (2015) 1042]. Herein, we investigate a novel Cu-functionalized UiO-67 MOF obtained by a simple synthesis method, consisting in contacting the bpydc-containing UiO-67-bpy MOF with a CuCl2 dihydrate precursor. By combining in situ and operando XAS and FTIR spectroscopies, we assessed the successful incorporation of well-defined Cu complexes in the UiO-67 framework and explored local coordination geometry, redox properties and reactivity of the dominant Cu species formed in different conditions relevant to potential future applications in catalysis. EXAFS fits and XANES simulations, based on DFT-optimized geometries, yielded detailed structural and electronic information on the major Cu-species formed. Data analysis revealed three-coordinated Cu(I) complexes with the bpydc linker of the Cu-UiO-67 MOFs and a Cl− ligand, formed after thermal treatment at 523K in inert gas flow able to efficiently and reversibly form Cu(I)-mono-carbonyl adducts with CO adsorbate.

Delta-mu XANES reveals the electronic structure of the adsorption of propene on gold nanoparticles

Experimental delta-mu spectra for the H oxidated and the propene co-fed Au nanoparticles were reproduced with XANES simulations with FEFF9. The results support that the propene has pi back-bonding to the Au with binding at defects or steps or in very small nanoparticles. Cationic, or even better, the neutrally charged Au nanoparticles, reproduce the experimental results of the propene co-fed catalyst found in the literature.

Kβ Detected High‐Resolution XANES of FeII and FeIII Models of the 2‐His‐1‐Carboxylate Motif: Analysis of the Carboxylate Binding Mode

AbstractProteins sharing the same “2‐His‐1‐carboxylate” structural motif have little amino acid sequence similarity and are able to perform many different reactions. Many factors have been cited to explain their different specificity and turnover rates, like protein environment, coordinated ligand geometry, electronic structure of the active site, etc. In this paper, we present a combined approach applying high‐resolution XANES spectroscopy and theory simulations to different model complexes that mimic the binding modes of the amino acids to the metal site. Experiments were performed on three compounds showing three metal sites: ferrous hexacoordinate, ferric pentacoordinate and ferrous pentacoordinate. The first two compounds bear an N,N,O‐tridentate 3,3‐bis(1‐alkylimidazol‐2‐yl)propionate ligand that features a monodentate carboxylate group. These complexes mimic the activity of extradiol dioxygenases but also exhibit intradiol cleavage activity. The third compound features a bidentate terphenylcarboxylate ligand and a sterically hindered bidentate N,N‐donor, thus providing a good structural mimic of the ternary enzyme‐tetrahydrobiopterin‐substrate complex in pterin‐dependent phenylalanine hydroxylase, which also contains a bidentate carboxylate. Modeling of high‐resolution XANES on well‐defined model complexes of different geometry can aid in protein structure elucidation. XANES gives the oxidation state and coordination number of the metal in the non‐crystallized protein at natural pH. The accuracy of the results is limited by the core‐hole and experimental broadenings. We found that high‐resolution XANES experiments give increased resolution at the pre‐edges, but limited improvement at the main edge. These high‐resolution pre‐edges can be accurately simulated by using crystal field multiplet theory (CFM). We show that by combining modelling and XANES simulations with FEFF8, detailed structural and chemical information can be obtained. We found that a short O2–metal distance for the carboxylate oxygen atom not bound to the metal causes a higher white line in FeII, which is similar to the results obtained for the pterin‐dependent hydroxylase, tyrosine hydroxylase (TYH). Full‐potential FDMNES simulations for each sample confirm the accuracy of the main results with muffin‐tin approximation (FEFF8).

X-Ray absorption spectroscopy quantitative analysis of biomimetic copper(ii) complexes with tridentate nitrogen ligands mimicking the tris(imidazole) array of protein centres

In this study copper(ii) complexes with the tridentate nitrogen ligand bis[2-(1-methylbenzimidazol-2-yl)ethyl]amine (2-BB) are considered as model compounds for the Cu-tris(imidazole) array found in several copper proteins. 2-BB chelates copper(ii) forming two six-membered rings and the complexes contain methanol, nitrite, azide and water as ancillary ligands; both the coordination numbers and stereochemistries differ in these complexes. Their key structural features were investigated by using full multiple-scattering theoretical analysis of the copper K-edge X-ray absorption spectrum with the MXAN code. We showed that using cluster sizes large enough to include all atoms of the ligand, the analysis of the XANES region can give both a structural model of the metal centre and map the structure of the 2-BB complexes. Complex [Cu(2-BB)(N(3))](+) provided a critical test through the comparison of the XANES simulation results with crystallographic data, thus permitting the extension of the method to the complex [Cu(2-BB)(H(2)O)(n)](+) (n = 1 or 2), for which crystallographic data are not available but is expected to bear a five-coordinated Cu(3N)(2O) core (n = 2). The structural data of [Cu(2-BB)(MeOH)(ClO(4))](+) and [Cu(2-BB)(NO(2))](+), both with a Cu(3N)(2O) core but with a different stereochemistry, were used as the starting parameters for two independent simulations of the XANES region of the [Cu(2-BB)(H(2)O)(2)](+) cation. The two structural models generated by simulation converge towards a structure for the aqua-cation with a lower coordination number. New calculations, where four-coordinated Cu(3N)(O) cores were considered as the starting structures, validated that the structure of the aqua-complex in the powder state has a copper(ii) centre with a four-coordinated Cu(3N)(O) core and a molecular formula [Cu(2-BB)(H(2)O)](ClO(4)).(H(2)O). A water solvation molecule, presumed to be disordered from the simulations with the two Cu(3N)(2O) cores, is present. The successful treatment of this Cu-2-BB complex system allows the extension of the method to other biomimetic compounds when a structural characterization is lacking.

Thermal detoxification and bloating of chromium(VI) with bentonite

This study stabilizes and bloats Cr(VI)-sorbed bentonite by heating at high temperature. Cr leaching decreases with increasing temperatures. Heating the sample at 1100 °C results in a non-detectable Cr concentration in the leachate, equivalent to a Cr leaching percent less than 0.001% (i.e., Cr TCLP concentration <0.018 mg of Cr L −1 of leachate). Morphology observed with a scanning electron microscopy indicates the occurrence of sintering of the sample heated at 1100 °C. The heated samples also show the occurrence of a vesicant process at 1100 °C. X-ray absorption spectroscopy results indicate that heating at 500 °C for 4 h can convert approximately 87% Cr(VI) into Cr(III) that is negligibly toxic; Cr 2O 3 was detected to be the most abundant Cr species. After heating at higher temperatures, namely 900–1100 °C, almost all doped Cr(VI) is reduced to Cr(III) as inferred from the height of the pre-edge peak of XANES spectra and/or from XANES simulation.

XAS study of the residual copper after desorption from rice husk ash

Rice husk ash is a good sorbent to remove copper ions from industrial wastewater. To recover the sorbed copper, two reagents (nitric acid and sodium thiosulfate) were used to desorb copper from the rice husk ash. XAS was used to investigate speciation of the residual copper in the rice husk ash after desorption. XANES simulation indicates that the copper species distributions in all pre-desorbed and post-desorbed solid samples are 40–45% CuSO 4 + 41–52% CuO + 7–16% Cu(OH) 2. On the other hand, in the liquid phase collected from desorption by Na 2S 2O 3, Cu 2O accounts for 48% of the total copper. Formation of Cu 2O is attributed to the chemical reduction of Cu(II) with Na 2S 2O 3 that is a reductant.

The manganese complex of oxygenic photosynthesis: conversion of five-coordinated Mn(III) to six-coordinated Mn(IV) in the S2-S3 transition is implied by XANES simulations

Photosynthetic water oxidation proceeds at a tetra-manganese complex bound to a cofactor-protein complex denoted as Photosystem II (PSII). To understand this process of fundamental importance for life on Earth, the crucial changes of the active-site occurring upon the individual transitions in the catalytic S-state cycle have been investigated by x-ray absorption spectroscopy at the Mn K-edge. For the S2-S3 transition, the changes in position and shape of the x-ray edge have been interpreted to be either associated with oxidation of Mn(III) to Mn(IV) or to formation of a ligand radical without manganese oxidation. Here we simulate XANES spectra of simple Mn-complexes using modern multiple-scattering code. Features of the XANES spectra are assigned to molecular orbitals by means of comparative semi-empirical calculations. It is concluded that the changes in position and shape of the edge spectra observed for the S2-S3 transition are explainable by the transformation of five-coordinated Mn(III) in a square-pyramidal geometry to six-coordinated Mn(IV).

Simulation of XANES spectra for protein-bound metal centers: analysis of linear dichroism data

For protein-bound transition metals, the structure of the metal site (geometric arrangement of the nuclei) and the electronic structure (oxidations state and orbital occupancy) can be investigated by x-ray absorption spectroscopy. While the extended x-ray absorption fine-structure (EXAFS) is theoretically well understood and the advantages of EXAFS spectroscopy on protein-bound metal centers have been repeatedly demonstrated, quantitative analysis of the near-edge structure XANES is more demanding. However, by now XANES simulations, for which a number of codes are available, may be useful to understand geometric and electronic structure effects.For a simplified model of the manganese complex in photosystem II we simulate polarization-dependent XANES spectra using the FEFF 8.2 code [Ankudinov et al, Phys. Rev. B 12 (1998)], which includes self-consistent muffin-tin potentials and polarization-dependent full multiple-scattering calculations. We derive a scheme to analyze the polarization dependence of XANES data that facilitates a treatment of partially uni-directionally oriented samples. Reasonably good agreement between simulations and experiments is obtained for the chosen model of the photosystem II manganese complex and its orientation with respect to the protein-carrying membrane.