X-ray Absorption Spectroscopy Experiments Research Articles

X-ray Absorption under Operating Conditions for Solid-Oxide Fuel Cells Electrocatalysts: The Case of LSCF/YSZ

We describe a novel electrochemical cell for X-ray absorption spectroscopy (XAS) experiments during electrical polarization suitable for high-temperature materials such as those used in solid oxide fuel cells. A half-cell LSCF/YSZ was then investigated under cathodic and anodic conditions (850 °C and applied electrical bias ranging from +1 V to −1 V in air). The in situ XAS measurements allowed us to follow the LSCF degradation into simple oxides. The rapid deterioration of LSCF is ascribed to the formation of excess of oxygen vacancies leading to the collapse of the mixed perovskite structure.

Exploring reaction dynamics in lithium-sulfur batteries by time-resolved operando sulfur K-edge X-ray absorption spectroscopy.

A time-resolved operando X-ray absorption spectroscopy experiment was successfully designed to explore the sulfur reaction dynamics in lithium-sulfur (Li-S) batteries. It presents direct real-time experimental evidence for the distinct sulfur dynamics at different discharge rates, deepening the understanding of reaction mechanisms in Li-S batteries.

Iron Speciation of Natural and Anthropogenic Dust by Spectroscopic and Chemical Methods

In this work, we have characterized the iron local structure in samples of two different types of atmospheric dust using X-ray absorption spectroscopy and selective leaching experiments. Specifically, we have investigated samples of long-range transported Saharan dust and freshly emitted steel plant fumes with the aim of individuating possible fingerprints of iron in the two cases. Findings include (1) prevalence of octahedral coordinated Fe 3 + for all samples; (2) presence of 6-fold coordinated Fe 3 + , aluminosilicates and iron oxy(hydr)oxides in Saharan dust and (3) of Fe-bearing spinel-like structures in the industrial fumes; (4) general predominance of the residual insoluble fraction with a notable difference: 69% for Saharan dust and 93% for steel production emissions, associated with aluminosilicates and non-reducible iron oxy(hydr)oxides, and Fe spinels, respectively. The remarkable differences between the X-ray absorption spectroscopy (XAS) spectra and leaching test results for the two sample types suggest the possibility to exploit the present approach in more complex cases. To this aim, two additional case studies of mixed aerosol samples are presented and discussed.

Development of a high temperature diamond anvil cell for x ray absorption experiments under extreme conditions

X-ray absorption spectroscopy (XAS) is presently a powerful and established tool to investigate solid and liquid matter at high pressure and high temperature (HP-HT). HP-HT XAS experiments rely on high pressure technology whose continuous development has extended the achievable range up to 100 GPa and more. In high pressure devices, high temperature conditions are typically obtained by using internal and external resistive heaters or by laser heating. We have recently developed a novel design for an internally heated diamond anvil cell (DAC) allowing XAS measurements under controlled high temperature conditions (tested up to about 1300 K). The sample in the new device can be rapidly heated or cooled (seconds or less) so the cell is suitable for studying melting/crystallization dynamics when coupled with a time-resolved XAS setup (second and sub-second ranges). Here we describe the internally heated DAC device which has been realized and tested in experiments on pure selenium at the energy dispersive ODE beamline of Synchrotron SOLEIL. We also present results obtained in XAS experiments of elemental Se using a large volume Paris-Edinburgh press, as an example of the relevance of structural studies of matter under extreme conditions.

Ultralow-temperature device dedicated to soft X-ray magnetic circular dichroism experiments.

A new ultralow-temperature setup dedicated to soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) experiments is described. Two experiments, performed on the DEIMOS beamline (SOLEIL synchrotron), demonstrate the outstanding performance of this new platform in terms of the lowest achievable temperature under X-ray irradiation (T = 220 mK), the precision in controlling the temperature during measurements as well as the speed of the cooling-down and warming-up procedures. Moreover, owing to the new design of the setup, the eddy-current power is strongly reduced, allowing fast scanning of the magnetic field in XMCD experiments; these performances lead to a powerful device for X-ray spectroscopies on synchrotron-radiation beamlines facilities.

Crude oils as ore fluids: An experimental in-situ XAS study of gold partitioning between brine and organic fluid from 25 to 250 °C

Organic matter can be associated with mineralization in hydrothermal ore deposits. One hypothesis is that this organic matter represents remnants of organic fluids (crude oils) that were competing with aqueous fluids for metal transport and contributed to metal endowment. We investigated the transport of gold (Au) in model oil compounds (S-free n-dodecane, CH3(CH2)10CH3, DD; and S-bearing 1-dodecanethiol, CH3(CH2)10CH2SH; DDT) from 25 °C to 250 °C using in-situ synchrotron X-ray absorption spectroscopy (XAS) experiments to determine the speciation and the structural properties of gold complexes in the aqueous- and oil-based fluids. For most experiments, DD or DDT were in contact with Au-bearing acidified water, or acidified water plus 10wt% NaCl (pH25°C = 1.85 in both cases). Gold rapidly partitioned from the aqueous phase into DD and DDT. Below 125 °C, Au(III)Cl is dominant in the DD and the adjacent water with a refined coordination number (CN) of chloride of 4.0(3) and an AuCl bond length of 2.28 Å, consistent with the tetrachloroaurate complex (AuCl4−) being stable in both the aqueous and organic phases. In contrast, Au(III) is rapidly reduced in the presence of DDT and an Au(I) complex dominates in both water and adjacent DDT with a CN of sulfur ∼2.0, suggesting a [RS-Au-SR]− (RS = DDT with deprotonated thiol group) complex with AuS bond lengths ranging from 2.29(1)Å to 2.31(3)Å. In an open system of DDT in contact with water, of which the water and DDT were analyzed separately, AuCl4− was dominant in the water phase, and Au(RS)2− dominant in DDT, possibly due to different equilibration kinetics in the beaker and glassy carbon tube.Since sulfur and organothiol compounds are ubiquitous and abundant components in natural oils, this study demonstrates the potential of natural oils to scavenge and enrich gold from co-existing gold-bearing brines. In particular, Au(I) organothiol complexes may contribute to transport in low-temperature (<125 °C) ore fluids such as those in basinal environments – in both hydrothermal fluids and oils. At temperatures ≥125 °C, gold was reduced to metallic gold in all experiments, suggesting that organo-stabilized nanoparticles may be the major form of gold to be scavenged, concentrated or transported in crude oils at these conditions. The results imply that brine-oil interactions may enrich Au in oils, and that oils may be an effective ore fluid in sedimentary environments.

Determining Cu–Speciation in the Cu–CHA Zeolite Catalyst: The Potential of Multivariate Curve Resolution Analysis of In Situ XAS Data

The Cu–CHA zeolite today represents an attractive platform to design catalysts for deNOx applications by NH3-assisted selective catalytic reduction (NH3–SCR) and for the low-temperature selective oxidation of methane to methanol (MTM). Accessing a quantitative understanding of Cu–speciation in this material is a key step to unveil structure–performance relationships for these high-impact processes. Herein, we select Cu–CHA as a case study to demonstrate the potential of chemometric approaches, such as multivariate curve resolution (MCR) applied in combination with principal component analysis (PCA). We employ these methods to assist the interpretation of X-ray absorption spectroscopy (XAS) experiments in the near-edge (XANES) region, determining the spectroscopic signatures and concentration profiles of the pure Cu–species formed. We pinpoint the composition impact on the material reducibility and highlight Cu–speciation–productivity relationships for the MTM process. Furthermore, we report novel insights on the formation of O2-derived species in Cu–CHA, obtained from MCR analysis of high energy resolution fluorescence detected (HERFD) XANES data collected during thermal treatment of Cu–CHA in both He and O2 gas flow. Multivariate analysis, in combination with the superior energy resolution adopted, allows us to identify an additional Cu(II) species. This component, different from the previously characterized Z[Cu(II)OH] moiety, is only formed at significant concentrations in O2 and it is envisaged to play an important role in the MTM conversion.

Metal-Organic Frameworks Mediate Cu Coordination for Selective CO2 Electroreduction.

The electrochemical carbon dioxide reduction reaction (CO2RR) produces diverse chemical species. Cu clusters with a judiciously controlled surface coordination number (CN) provide active sites that simultaneously optimize selectivity, activity, and efficiency for CO2RR. Here we report a strategy involving metal-organic framework (MOF)-regulated Cu cluster formation that shifts CO2 electroreduction toward multiple-carbon product generation. Specifically, we promoted undercoordinated sites during the formation of Cu clusters by controlling the structure of the Cu dimer, the precursor for Cu clusters. We distorted the symmetric paddle-wheel Cu dimer secondary building block of HKUST-1 to an asymmetric motif by separating adjacent benzene tricarboxylate moieties using thermal treatment. By varying materials processing conditions, we modulated the asymmetric local atomic structure, oxidation state and bonding strain of Cu dimers. Using electron paramagnetic resonance (EPR) and in situ X-ray absorption spectroscopy (XAS) experiments, we observed the formation of Cu clusters with low CN from distorted Cu dimers in HKUST-1 during CO2 electroreduction. These exhibited 45% C2H4 faradaic efficiency (FE), a record for MOF-derived Cu cluster catalysts. A structure-activity relationship was established wherein the tuning of the Cu-Cu CN in Cu clusters determines the CO2RR selectivity.

Revisiting the Dependence of Cu K-Edge X-ray Absorption Spectra on Oxidation State and Coordination Environment.

X-ray absorption spectroscopy (XAS) at the Cu K-edge is an important tool for probing the properties of copper centers in transition-metal chemistry and catalysis. However, the interpretation of experimental XAS spectra requires a detailed understanding of the dependence of spectroscopic features on the local geometric and electronic structure, which can be established by theoretical X-ray spectroscopy. Here, we present a systematic computational study of the Cu K-edge XAS spectra of selected Cu complexes based on time-dependent density-functional theory in combination with a molecular orbital analysis of the relevant transitions. For a series of Cu ammine model complexes as well as a comprehensive test set of 12 Cu(I) and 5 Cu(II) complexes, we revisit the dependence of the pre-edge region in Cu K-edge XAS spectra on oxidation state and coordination geometry. While our calculations confirm earlier experimental assignments, we can also reveal additional signatures of the ligand orbitals and identify the underlying orbital interactions. The comprehensive picture revealed by this study will provide a reliable basis for the interpretation of in situ Cu K-edge XAS spectra of catalytic intermediates.

An intermetallic Pd2Ga nanoparticle catalyst for the single-step conversion of CO-rich synthesis gas to dimethyl ether

Well-defined Pd/Ga-nanoparticles were prepared and used as a precursor for the methanol active component in a bifunctional syngas-to-dimethyl ether catalyst. In situ X-ray absorption spectroscopy experiments were employed both to unravel the initial formation of the active catalyst phase in reductive H2 atmosphere and to further monitor changes of the nanoparticles under conditions of dimethyl ether synthesis at a pressure up to 20 bar (250 °C). The catalytic studies were conducted using simulated biomass-derived, CO-rich syngas in a continuous-flow reactor, with the bifunctional catalyst offering the two types of active sites, i.e. for methanol synthesis (Pd/Ga nanoparticles) and its subsequent dehydration (γ-Al2O3), in close proximity. As compared to the conventional Cu/Zn-based reference catalyst prepared via a similar procedure, the Pd/Ga-based catalyst showed a promising activity together with a notable stability with time on stream and a high temperature tolerance (up to 300 °C). A kinetic model which considers the individual reactions involved in direct DME synthesis based on power law equations was used to fit the experimental data, and the apparent activation energies were compared to the Cu/Zn-based catalyst.

Non-Destructive Chemical State Mapping Using Laboratory XANES and EXAFS

X-ray absorption spectroscopy (XAS) is a popular technique for determining chemical states, such as oxidation states by XANES (x-ray absorption near edge structure) or interatomic distances and coordination numbers by EXAFS (extended x-ray absorption fine structure). These techniques are widely utilized by electrochemical researchers in synchrotron facilities worldwide, addressing a range of studies: for example, XANES can help to probe the oxidation states of battery electrode materials at various stages in their operational lifetime, EXAFS can elucidate the dynamics of conversion reactions in operando, thus capturing transients that may otherwise be lost in ex situ studies, and XAS experiments encompassing both XANES and EXAFS have shown unique promise for examining the behavior of sulfur in electrodes for Li-S batteries. The popularity of these techniques in tandem with the 3D microstructural information from X-ray tomography (XCT) and the compositional information available through other types of spectroscopy (e.g., XRF, EDS, and Raman) has grown substantially over the past two decades, as more synchrotron beamlines have become available for these experiments and the true power of the techniques have been demonstrated. In spite of the excitement generated by the various XAS experiments documented in the literature and the growing number of synchrotron facilities worldwide, access to suitable instrumentation remains challenging. This is due largely to the general popularity of synchrotron radiation for studying a variety of materials across a range of disciplines, leading to high levels of competition for limited amounts of available beam time. Efforts have been made in the past to migrate the XAS techniques to laboratory instrumentation, but such efforts have been met with many challenges. Often the exposure times are very long, resulting in multi-day measurements for a single experiment, or the energy resolution is too crude to provide sufficient sensitivity for precise quantification. Thus, a need remains for expanded access to XAS techniques within the electrochemical energy storage and conversion community, which has, so far, not been adequately addressed. Here, we present a novel, compact, laboratory-based XAS system operating in the 4.5-12 keV energy range with a powerful range of capabilities. This instrument provides sub-eV energy resolution over a fairly large energy range, short data collection times (seconds to minutes), and high spatial resolutions (~10 micrometers) for small spot analysis or mapping. The performance of the system is achieved in part with a patented high brightness x-ray source, a novel system design, and the use of the latest x-ray crystal analyzers and spatially resolved detector. Through the unique capabilities of this laboratory instrumentation, many experiments traditionally reserved for the synchrotron can now be accomplished in the laboratory. In this presentation, we will briefly review the spectrum of studies where XAS can provide insight into battery electrode materials and proceed to demonstrate how this laboratory XAS setup can address both routine and more specialized needs for electrochemical characterization. We will also discuss the relationship of this technique to other laboratory approaches, providing some perspectives on both the present and future of non-destructive characterization.

Time-resolved copper speciation during selective catalytic reduction of NO on Cu-SSZ-13

Practical catalysts often operate under dynamic conditions of temperature variations and sudden changes of feed composition that call for understanding of operation and catalyst structure under analogous experimental conditions. For instance, the copper-exchanged small-pore SSZ-13 catalyst used currently in the selective catalytic reduction of harmful nitrogen oxides from the exhaust gas of diesel-fuelled vehicles operates under recurrent ammonia dosage. Here, we report the design of unsteady state experiments that mimic such a dynamic environment to obtain key mechanistic information on this reaction. Through the combination of time-resolved X-ray absorption spectroscopy and transient experimentation, we were able to capture an ammonia inhibition effect on the rate-limiting copper re-oxidation at low temperature. The practical relevance of this observation was demonstrated by optimization of the ammonia dosage on a catalyst washcoat on cordierite honeycomb, resulting in lower ammonia consumption and an increase in nitrogen oxide conversion at low temperature. Selective catalytic reduction is employed at the exhaust of diesel vehicles to abate nitrogen oxide emissions. Now, guided by time-resolved X-ray absorption spectroscopy and transient experiments using Cu-SSZ-13 as the catalyst, the authors unravel important features of the reaction mechanism that allow the performance of the catalyst to be improved.

Iron oxidation dynamics vs. temperature of synthetic potassic-ferro-richterite: a XANES investigation.

We investigated the oxidation behaviour of a synthetic potassic-ferro-richterite up to 750 °C by using simultaneous X-ray absorption spectroscopy and X-ray diffraction experiments with synchrotron radiation. From the X-ray diffraction results, we observed an abrupt decrease of cell dimensions at ∼335 °C accompanied by an anomalous increase in the monoclinic cell angle β. From the analysis of the XANES spectra at the iron K-edge, we observed that the structural shrinkage is due to the iron oxidation process, coupled to hydrogen loss, occurring at ∼315 °C, slightly before the cell contraction. Combining these results with previous studies performed on similar samples by single-crystal structure refinement, Mössbauer, high temperature-Fourier transform IR and Raman spectroscopies, we show that the temperature evolution in Fe-amphiboles is a multi-step process. Following the iron oxidation driven by temperature, the structural dynamics in this double-chain silicate is ruled by local strains in the ribbon of iron-filled octahedra, mainly due to the contraction of the M(1) site.

X-ray Absorption Spectroscopy Combined with Time-Dependent Density Functional Theory Elucidates Differential Substitution Pathways of Au(I) and Au(III) with Zinc Fingers.

A combination of two elements' (Au, Zn) X-ray absorption spectroscopy (XAS) and time-dependent density functional theory (TD-DFT) allowed the elucidation of differential substitution pathways of Au(I) and Au(III) compounds reacting with biologically relevant zinc fingers (ZnFs). Gold L3-edge XAS probed the interaction of gold and the C-terminal Cys2HisCys finger of the HIV-1 nucleocapsid protein NCp7, and the Cys2His2 human transcription factor Sp1. The use of model compounds helped assign oxidation states and the identity of the gold-bound ligands. The computational studies accurately reproduced the experimental XAS spectra and allowed the proposition of structural models for the interaction products at early time points. The direct electrophilic attack on the ZnF by the highly thiophilic Au(I) resulted in a linear P-Au-Cys coordination sphere after zinc ejection whereas for the Sp1, loss of PEt3 results in linear Cys-Au-Cys or Cys-Au-His arrangements. Reactions with Au(III) compounds, on the other hand, showed multiple binding modes. Prompt reaction between [AuCl(dien)]2+ and [Au(dien)(DMAP)]3+ with Sp1 showed a partially reduced Au center and a final linear His-Au-His coordination. Differently, in the presence of NCp7, [AuCl(dien)]2+ readily reduces to Au(I) and changes from square-planar to linear geometry with Cys-Au-His coordination, while [Au(dien)(DMAP)]3+ initially maintains its Au(III) oxidation state and square-planar geometry and the same first coordination sphere. The latter is the first observation of a "noncovalent" interaction of a Au(III) complex with a zinc finger and confirms early hypotheses that stabilization of Au(III) occurs with N-donor ligands. Modification of the zinc coordination sphere, suggesting full or partial zinc ejection, is observed in all cases, and for [Au(dien)(DMAP)]3+ this represents a novel mechanism for nucleocapsid inactivation. The combination of XAS and TD-DFT presents the first direct experimental observation that not only compound reactivity, but also ZnF core specificity, can be modulated on the basis of the coordination sphere of Au(III) compounds.

Amorphous MoS3 as the sulfur-equivalent cathode material for room-temperature Li–S and Na–S batteries

Many problems associated with Li-S and Na-S batteries essentially root in the generation of their soluble polysulfide intermediates. While conventional wisdom mainly focuses on trapping polysulfides at the cathode using various functional materials, few strategies are available at present to fully resolve or circumvent this long-standing issue. In this study, we propose the concept of sulfur-equivalent cathode materials, and demonstrate the great potential of amorphous MoS3 as such a material for room-temperature Li-S and Na-S batteries. In Li-S batteries, MoS3 exhibits sulfur-like behavior with large reversible specific capacity, excellent cycle life, and the possibility to achieve high areal capacity. Most remarkably, it is also fully cyclable in the carbonate electrolyte under a relatively high temperature of 55 °C. MoS3 can also be used as the cathode material of even more challenging Na-S batteries to enable decent capacity and good cycle life. Operando X-ray absorption spectroscopy (XAS) experiments are carried out to track the structural evolution of MoS3 It largely preserves its chain-like structure during repetitive battery cycling without generating any free polysulfide intermediates.

Soft X-ray Absorption Spectroscopy of Liquids and Solutions.

X-ray absorption spectroscopy (XAS) is an electronic absorption technique for which the initial state is a deeply buried core level. The photon energies corresponding to such transitions are governed primarily by the binding energies of the initial state. Because the binding energies of core electrons vary significantly among atomic species, this makes XAS an element-selective spectroscopy. Proper interpretation of XA spectra can provide detailed information on the local chemical and geometric environment of the target atom. The introduction of liquid microjet and flow cell technologies into XAS experiments has enabled the general study of liquid samples. Liquids studied to date include water, alcohols, and solutions with relevance to biology and energy technology. This Review summarizes the experimental techniques employed in XAS studies of liquid samples and computational methods used for interpretation of the resulting spectra and summarizes salient experiments and results obtained in the XAS investigations of liquids.

Mn K-edge X-ray absorption studies of mononuclear Mn(III)-hydroxo complexes.

Mn K-edge X-ray absorption spectroscopy experiments were performed on the solid- and solution-phase samples of [MnII(dpaqR)](OTf) (R=H, Me) and [MnIII(OH)(dpaqR)](OTf). The extended X-ray absorption fine structure (EXAFS) data show distinct differences between the MnII and MnIII-OH complexes, with fits providing metric parameters in excellent agreement with values from X-ray crystallography and density functional theory (DFT) computations. Evaluation of the EXAFS data for solid-phase [MnIII(OH)(dpaq)](OTf) resolved a short Mn-OH bond distance of 1.79Å; however, the short trans-amide nitrogen bond of the supporting ligand precluded the resolution of the Mn-OH bond distance in the corresponding solution-phase sample and for both [MnIII(OH)(dpaqMe)](OTf) samples. The edge energy also increases by approximately 2eV from the MnII to the MnIII-OH complexes. Experimental pre-edge analysis shows the MnII complexes to have pre-edge areas comparable to the MnIII-OH complexes, despite the presence of the relatively short Mn-OH distance. Time-dependent density functional theory (TD-DFT) computations illustrate that Mn 3d-4p mixing, a primary contributor to pre-edge intensities, decreases by~0.3% from the MnII to MnIII-OH complexes, which accounts for the very similar pre-edge areas. Collectively, this work shows that combined EXAFS and XANES analysis has great potential for identification of reactive MnIII-OH intermediates, such as those proposed in enzyme active sites.

Preparation and characterization of Ce1−xPrxO2 supports and their catalytic activities

In this work, the addition of praseodymium (Pr) into ceria as a mixed oxide support in a form of Ce1−xPrxO2 (x = 0.01, 0.025, 0.050, 0.075 and 0.10) was prepared using a co-precipitation method. The structural and textural properties of the synthesized supports were characterized by X-ray diffraction (XRD), N2 adsorption–desorption, Raman spectroscopy, H2-temperature programmed reduction (H2-TPR) and H2-chemisorption. Upon addition of Pr, XRD patterns and Raman spectra indicated an enlargement of ceria unit cell and the characteristics Raman broad peak at 570 cm−1 which was attributed to the existence of oxygen vacancies in the ceria lattice. This indicated that some Ce4+ ions in ceria were replaced by larger Pr3+ cations. To evidence the incorporation of Pr3+ cations into ceria lattice, X-ray absorption near edge structure (XANES) was employed. The results showed that the oxidation states of Ce in mixed oxide supports were slightly lower than 4+ while those of Pr were still the same as a precursor salt. Therefore, the incorporation of Pr3+ into ceria lattice would lead to strain and unbalanced charge and result in oxygen vacancies. The reducibility of Ce1−xPrxO2 mixed oxide supports was investigated by H2-TPR and temperature-resolved X-ray absorption spectroscopy experiment under reduction conditions. XANES spectra of Ce L3 edges showed a lower surface reduction temperature (Ce4+ to Ce3+) of Ce0.925Pr0.075O2 than that of CeO2 which agreed with H2-TPR results. H2-chemisorption indicated that Pr promoted the dispersion of the metal catalyst on the mixed oxide support and increased the adsorption site for CO. For WGS reaction, 1% Pd/mixed oxide support had higher WGS activity than 1% Pd/ceria. The increase of WGS activity was due to the increase of Pd dispersion on the support and the existence of oxygen vacancies produced from incorporation of Pr into the ceria lattice.

Investigating the Role of Fe in Transition-Metal (Oxy)Hydroxide Electrocatalysts for the Oxygen Evolution Reaction

The efficiency of water electrolysis for hydrogen production is limited in part by the slow kinetics of the oxygen evolution reaction (OER).1,2 Nickel-iron and cobalt-iron (oxy)hydroxides have been shown to be the most active, low-cost electrocatalysts for the OER in alkaline conditions.3–5 Although it is evident that Fe is essential for high activity,6,7 its role in the catalytic active site is still largely debated.8,9 We investigate the role of Fe in NiOOH by comparing the effects of Ti, Mn, La, and Ce incorporation on the OER activity and electrochemical behavior of NiOOH in Fe-free 1 M KOH.10 We show that other metal cations in NiOOH can increase its reduction potential in a similar way to Fe, but without the same orders of magnitude increase in OER activity. This suggests that Ni is not likely the active site as its electronic properties are not correlated to activity. We also evaluate the OER activity and Tafel behavior of Fe3+ impurities on different noble metal substrates. We find that the activity varies by substrate suggesting that the local atomic and electronic structure of [FeO6] units play an important role in catalysis of the OER as it can be tuned by substrate interactions. Finally, through in situ and in operando X-ray absorption spectroscopy experiments, we find that the local structure around Fe atoms in Co(Fe)OOH changes during OER while that of Co stays the same. Given that the OER activity on a per Fe basis is independent of [Fe] in Co(Fe)OOH, the potential dependent structural changes further indicate that Fe is essential to the catalytic active site for the OER on transition-metal (oxy)hydroxides. References (1) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Chem. Rev. 2010, 110(11), 6446. (2) Ursua, A.; Gandia, L. M.; Sanchis, P. Proc. IEEE 2012, 100(2), 410. (3) Trotochaud, L.; Ranney, J. K.; Williams, K. N.; Boettcher, S. W. J. Am. Chem. Soc. 2012, 134(41), 17253. (4) Burke, M. S.; Zou, S.; Enman, L. J.; Kellon, J. E.; Gabor, C. A.; Pledger, E.; Boettcher, S. W. J. Phys. Chem. Lett. 2015, 6(18), 3737. (5) Burke, M. S.; Enman, L. J.; Batchellor, A. S.; Zou, S.; Boettcher, S. W. Chem. Mater. 2015, 27(22), 7549. (6) Corrigan, D. A. J. Electrochem. Soc. 1987, 134(2), 377. (7) Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. J. Am. Chem. Soc. 2014, 136(18), 6744. (8) Friebel, D.; Louie, M. W.; Bajdich, M.; Sanwald, K. E.; Cai, Y.; Wise, A. M.; Cheng, M.-J.; Sokaras, D.; Weng, T.-C.; Alonso-Mori, R.; Davis, R. C.; Bargar, J. R.; Nørskov, J. K.; Nilsson, A.; Bell, A. T. J. Am. Chem. Soc. 2015, 137(3), 1305. (9) Chen, J. Y. C.; Dang, L.; Liang, H.; Bi, W.; Gerken, J. B.; Jin, S.; Alp, E. E.; Stahl, S. S. J. Am. Chem. Soc. 2015, 137(48), 15090. (10) Enman, L. J.; Burke, M. S.; Batchellor, A. S.; Boettcher, S. W. ACS Catal. 2016, 6 (4), 2416.

Revisiting the hydrothermal geochemistry of europium(II/III) in light of new in-situ XAS spectroscopy results

Knowledge of the mobility of Rare Earth Elements (REE) in crustal fluids is important for understanding both the formation of REE deposits and their use as geochemical tracers. This is particularly true for europium (Eu), which exists in both Eu(II) and Eu(III) states in natural fluids, making it a sensitive probe of hydrothermal redox processes. Currently, our understanding of Eu complexation in hydrothermal fluids relies mainly on extrapolations from room-temperature data, as there are no high-temperature experimental data available for Eu(II) species. In this study we conducted in-situ synchrotron X-ray absorption spectroscopy (XAS) experiments to investigate the complexing of Eu(II) and Eu(III) in chloride- and bromide-rich acidic solutions and Eu(III) sulphate complexes, at 35–400°C, 600bar. For Eu(III) chloride complexes, the total number of ligands decreased and the ratio of H2O:Cl ligands in the first coordination shell decreased with increasing temperature. For the Eu(II) system, our data confirm Sverjensky's (1984, EPSL 67, 70–78) theoretical predictions that Eu(II) species become increasingly stable (relative to Eu(III)) at elevated temperatures. However the new data indicate that complexation between Eu(II) and chloride is much weaker than Haas's (1995, GCA 59, 4329–4350) theoretical estimates. The thermodynamic properties of Eu(II) chloride complexes have been reassessed in light of the new data. Thermodynamic calculations reveal that the identity of the Eu(III) aqueous complexes (e.g., chloride or hydroxide species) have a significant impact on the oxidation state of Eu in hydrothermal fluids at elevated temperatures.