Bulk X-ray Absorption Spectroscopy Research Articles

Front Cover: Modeling Heterogeneity in UO2 Nanoparticles Using X‐ray Absorption Spectroscopy (Eur. J. Inorg. Chem. 7/2023)

The Front Cover shows UO2 nanoparticles with a pictorial representation of the atoms (uranium in blue and oxygen in red). In this work, user-defined X-ray absorption spectroscopy models informed by surface relaxation effects and inherent surface disorder were generated and preferentially fit to experimentally derived data. The developed method enables representative local surface structure attributes to be extracted from bulk X-ray absorption spectroscopy data. Results show that the UO2 surface is highly disordered compared with the bulk structure, and this cannot be explained through surface relaxation effects alone. Additionally, this trend is notably not observed in 8 nm nanoparticles (which resemble the bulk), as it is in 1.4 and 4.7 nm particles. This suggests that there may be a critical domain size for ordering within UO2 nanoparticle surfaces. More information can be found in the Research Article by L. M. Moreau and co-workers.

Understanding the Stability of Manganese Chromium Antimonate Electrocatalysts through Multimodal In Situ and Operando Measurements.

Improving electrocatalyst stability is critical for the development of electrocatalytic devices. Herein, we utilize an on-line electrochemical flow cell coupled with an inductively coupled plasma-mass spectrometer (ICP-MS) to characterize the impact of composition and reactant gas on the multielement dissolution of Mn(-Cr)-Sb-O electrocatalysts. Compared to Mn2O3 and Cr2O3 oxides, the antimonate framework stabilizes Mn at OER potentials and Cr at both ORR and OER potentials. Furthermore, dissolution of Mn and Cr from Mn(-Cr) -Sb-O is driven by the ORR reaction rate, with minimal dissolution under N2. We observe preferential dissolution of Cr totaling 13% over 10 min at 0.3, 0.6, and 0.9 V vs RHE, with only 1.5% loss of Mn, indicating an enrichment of Mn at the surface of the particles. Despite this asymmetric dissolution, operando X-ray absorption spectroscopy (XAS) showed no measurable changes in the Mn K-edge at comparable potentials. This could suggest that modification to the Mn oxidation state and/or phase in the surface layer is too small or that the layer is too thin to be measured with the bulk XAS measurement. Lastly, on-line ICP-MS was used to assess the effects of applied potential, scan rate, and current on Mn-Cr-Sb-O during cyclic voltammetry and accelerated stress tests. With this deeper understanding of the interplay between oxygen reduction and dissolution, testing procedures were identified to maximize both activity and stability. This work highlights the use of multimodal in situ characterization techniques in tandem to build a more complete model of stability and develop protocols for optimizing catalyst performance.

Elucidating Fundamental Degradation Mechanisms in Polymer Electrolyte Membrane Water Electrolyzers

Hydrogen is an attractive option for renewable energy applications due to its ability for large quantities to be stored over a long period of time, but current H2 production from fossil fuels is a major CO2 emitter; consequently, there is a need for efficient, renewable H2 production.1 Polymer electrolyte membrane water electrolysis (PEMWE) is a promising strategy, but major degradation issues and the slow kinetics of the anodic oxygen evolution reaction (OER) limit commercialization.2 Ir has been shown to be the most active metal towards the OER, while IrO2 is typically more stable in the harsh oxidizing conditions of the OER. High cost/limited availability of noble metals require a transition to lower loadings; however, low-loading Ir- and IrO2-based electrodes are not yet thoroughly understood,3 motivating further studies. The interfaces and interactions within the catalyst layer (CL) and between the neighboring membrane or diffusion media impact initial and long-term performance and need to be investigated in order to fully understand degradation mechanisms and propose potential pathways towards optimization.Commercial Ir and IrO2 black catalysts were obtained and studied as catalyst powders, fresh membrane electrode assemblies (MEAs), and tested MEAs. Electrochemical durability testing was performed on PEMWEs to replicate the start-up and shut-down process and an extended period of operation, following established protocols for catalysts4 and electrodes.3 Extensive physicochemical characterization was utilized in this study to correlate CL composition, structure, and morphology to activity and durability in order to deconvolute the impact of individual constituents within the CL on overall electrode structure and stability. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) combined with energy-dispersive x-ray spectroscopy (EDS) mapping were used to image powders, top-down MEAs, and cross-sectioned MEAs to elucidate morphological and structural information as well as elemental distribution as a function of electrochemical testing. Further chemical and structural information was obtained through surface-sensitive x-ray photoelectron spectroscopy (XPS) and bulk x-ray absorption spectroscopy (XAS). Finally, transmission x-ray microscopy (TXM) was used to visualize element-sensitive morphology information at a large scale. Preliminary results illuminate significant changes in the catalyst composition, catalyst-ionomer interface, and CL structure, as well as delamination of the CL from the membrane and overall CL collapse after cycling. These findings demonstrate a promising systematic characterization approach to isolate contributions of the catalyst, ionomer, and overall CL morphology, composition, and structure on the resulting electrode, in order to advance fundamental understanding of preparing uniform, stable CLs for PEMWE development.(1) Ayers, K. E. Curr. Opin. Electrochem. 2019, 18, 9–15.(2) Spöri, C.; Kwan, J. T. H.; Bonakdarpour, A.; Wilkinson, D. P.; Strasser, P. Angew. Chemie Int. Ed. 2017, 56 (22), 5994–6021.(3) Alia, S. M.; Stariha, S.; Borup, R. L. J. Electrochem. Soc. 2019, 166 (15), F1164–F1172.(4) Alia, S.M.; Anderson, G.C. J. Electrochem. Soc. 2019, 166 (4), F282-F294.

Actualizing In Situ X-ray Absorption Spectroscopy Characterization of PEMFC-Cycled Pt-Electrodes

The family of the PtM (M represents transition metals such as Co, Ni, Pd, etc.) alloys is the most promising cathode electrocatalysts for proton exchange membrane fuel cells (PEMFCs) owing to their superior oxygen reduction reaction (ORR) activity to pure Pt. However, the activity gain fades with long-term PEMFC operation, and the degradation mechanism is not yet fully understood. To truly understand the degradation mechanism of the carbon supported PtM nanoparticles (PtM/C) in the cathode of a membrane electrode assembly (MEA) upon long-term PEMFC operation, it is essential to characterize the PEMFC-cycled electrode under working conditions. Herein, we showed that operando X-ray absorption spectroscopy (XAS) characterization of PtM/C electrocatalysts cycled in a PEMFC has inherent difficulties since Pt and especially M dissolve during PEMFC operation and migrate into the membrane; the bulk XAS spectrum is an average of the signals from the electrode and the membrane. Alternatively, we developed a method that allows for in situ XAS characterization on PEMFC-cycled PtM/C electrocatalysts. We justified the method by showing that the dissolved species in the membrane were separated from the PtM/C electrocatalyst in the cathode, and the in situ XAS signals arose exclusively from the electrocatalyst.

Coordination of arsenic and nickel to aluminum and magnesium phases in uranium mill raffinate precipitates

The Key Lake U mill uses a stepwise neutralization process (pH 4.0, 6.5, 9.5, and 10.5) to treat raffinate (acidic, metal-rich wastewater) prior to safely releasing effluent to the environment. This process generates a complex mixture of precipitates that are deposited to a tailings facility. In this study, the coordination environments of As and Ni with respect to Al-Mg phases precipitated in the presence and absence of Fe in mill-generated and synthetic precipitates were defined using bulk X-ray absorption spectroscopy complemented with bulk X-ray diffraction. In low pH (pH 4.0–4.6) samples, As(V) precipitates as ferric arsenate and adsorbs to AlOHSO4 (an amorphous hydrobasaluminite-like phase) and ferrihydrite via bidentate-binuclear complexes. Nickel(II) predominantly adsorbs to amorphous Al(OH)3 via edge-sharing bidentate-mononuclear complexes. In high pH (pH 9.5–9.9) samples, As(V) adsorbs to amorphous Al(OH)3, ferrihydrite, and MgAlFe-hydrotalcite (bidentate complex). Nickel(II) octahedra adsorb to amorphous Al(OH)3 and likely form a Ni-Al layered double hydroxide (LDH) surface precipitate on MgAlFe-hydrotalcite via Al dissolution-precipitation. In the final solids (blended low and high pH precipitates) discharged at ∼ pH 10.5, As(V) adsorbs to amorphous Al(OH)3, ferrihydrite, and MgAlFe-hydrotalcite. Nickel(II) adsorbs to amorphous Al(OH)3 and forms Ni-Al LDH surface precipitates on hydrotalcite. This study demonstrates that neutralization of chemically complex wastewater precipitates multiple phases capable of controlling dissolved As and Ni concentrations. Knowledge gained from this study will aid investigations in understanding the long-term fate of these potential contaminants in the environment and can be applied to other industries and environmental systems with similar conditions.

Synchrotron X-ray absorption spectroscopy analysis of arsenic chemical speciation in human nail clippings

Environmental context Chronic ingestion of arsenic leads to its accumulation in keratinous tissues, which can represent a risk factor for developing cancer. We use synchrotron X-ray absorption spectroscopy to investigate chemical bonding of arsenic in the keratins from nail clippings of volunteers from areas in Atlantic Canada with low-to-moderate arsenic contamination of drinking water. The study helps our understanding of arsenic metabolism and its role in cancer development. Abstract Drinking water aquifers in many areas of the world have naturally elevated levels of inorganic arsenic exceeding the World Health Organization limit. Arsenic concentrations in human nail clippings are commonly used as a biomarker of exposure to this toxic element. However, the chemical form of arsenic accumulated in nail tissues is not well determined. We employed synchrotron microprobe and bulk X-ray absorption spectroscopy techniques to analyse the concentration and chemical speciation of arsenic in the finger- and toenail clippings of volunteers living in the vicinity of Sackville, New Brunswick, Canada. This area is known to have low-to-moderately elevated levels of arsenic in ground water. Arsenic species in clippings were represented by three main groups, distinguished by the As-K near-edge X-ray absorption fine structure spectra: (1) AsIII type, which can be fitted as a mixture of As bound to thiols, and also to oxygen or methyl groups, with a small contribution from AsV species, (2) AsV type, best represented by fitting arsenate in aqueous solution and (3) The AsIII+AsV mixture type. The high proportion (%) of sulfur-bound arsenic species most likely corresponds to binding between arsenic (in its trivalent and, to a lesser extent, pentavalent forms) and cysteine residues in the sulfur-rich fraction of keratin and keratin-associated proteins. Further work is needed to explore whether these chemical species could be used as toxicity biomarkers of human exposure to elevated levels of As in drinking water.

Arsenic speciation in sinter mineralization from a hydrothermal channel of El Tatio geothermal field, Chile

Summary El Tatio geothermal field is the principal natural source of arsenic for the Loa River, the main surface water resource in the hyper-arid Atacama Desert (Antofagasta Region, Northern Chile). Prior investigations by bulk X-ray absorption spectroscopy have identified hydrous ferric oxides as the principal arsenic-containing phase in sinter material from El Tatio, suggesting sorption as the main mechanism for arsenic scavenging by the solid phases of these hot spring environments. Here we examine siliceous sinter material sampled from a hydrothermal channel using synchrotron based X-ray micro-probe techniques, including As and Fe Kα X-ray fluorescence (μ-XRF), As K-edge X-ray absorption near edge structure (μ-XANES), and X-ray diffraction (μ-XRD). Least-squares linear fitting of μ-XANES spectra shows that arsenic is predominantly present as arsenate sorbed on hydrous ferric oxides (63% molar proportion), but we also identify nodular arsenide micro-mineralizations (37% molar proportion) similar to loellingite (FeAs 2 ), not previously detected during bulk-scale analysis of the sinter material. Presence of arsenide mineralizations indicates development of anoxic environments on the surface of the siliceous sinter, and suggests a more complex biogeochemistry for arsenic than previously observed for circum-neutral pH brine hot spring environments.

Effects of ZnO nanoparticles in alfalfa, tomato, and cucumber at the germination stage: Root development and X-ray absorption spectroscopy studies

Abstract Past reports indicate that some nanoparticles (NPs) affect seed germination; however, the biotransformation of metal NPs is still not well understood. This study investigated the toxicity on seed germination/root elongation and the uptake of ZnO NPs and Zn2+ in alfalfa (Medicago sativa), cucumber (Cucumis sativus), and tomato (Solanum lycopersicum) seedlings. Seeds were treated with ZnO NPs at 0–1600 mg L–1 as well as 0–250 mg L–1 Zn2+ for comparison purposes. Results showed that at 1600 mg L–1 ZnO NPs, germination in cucumber increased by 10 %, and alfalfa and tomato germination were reduced by 40 and 20 %, respectively. At 250 mg Zn2+ L–1, only tomato germination was reduced with respect to controls. The highest Zn content was of 4700 and 3500 mg kg–1 dry weight (DW), for alfalfa seedlings germinated in 1600 mg L–1 ZnO NPs and 250 mg L–1 Zn2+, respectively. Bulk X-ray absorption spectroscopy (XAS) results indicated that ZnO NPs were probably biotransformed by plants. The edge energy positions of NP-treated samples were at the same position as Zn(NO3)2, which indicated that Zn in all plant species was as Zn(II).

Arsenic and iron speciation in uranium mine tailings using X-ray absorption spectroscopy

Abstract In northern Saskatchewan, Canada, high-grade U ores and the resulting tailings can contain high levels of As. An environmental concern in the U mining industry is the long-term stability of As within tailings management facilities (TMFs) and its potential transfer to the surrounding groundwater. To mitigate this problem, U mill effluents are neutralized with lime to reduce the aqueous concentration of As. This results in the formation of predominantly Fe 3+ –As 5+ secondary mineral phases, which act as solubility controls on the As in the tailings discharged to the TMF. Because the speciation of As in natural systems is critical for determining its long-term environmental fate, characterization of As-bearing mineral phases and complexes within the deposited tailings is required to evaluate its potential transformation, solubility, and long-term stability within the tailings mass. In this study, synchrotron-based bulk X-ray absorption spectroscopy (XAS) was used to study the speciation of As and Fe in mine tailings samples obtained from the Deilmann TMF at Key Lake, Saskatchewan. Comparisons of K-edge X-ray absorption spectra of tailings samples and reference compounds indicate the dominant oxidation states of As and Fe in the mine tailings samples are +5 and +3, respectively, largely reflecting their generation in a highly oxic mill process, deposition in an oxidized environment, and complexation within stable oxic phases. Linear combination fit analyses of the K-edges for the Fe X-ray absorption near edge spectra (XANES) to reference compounds suggest Fe is predominantly present as ferrihydrite with some amount of the primary minerals pyrite (8–15% in some samples) and chalcopyrite (5–15% in some samples). Extended X-ray absorption fine structure (EXAFS) analysis of As K-edge spectra indicates that As 5+ (arsenate) present in tailings samples is adsorbed to the ferrihydrite though an inner-sphere bidentate linkage.

X-ray absorption and emission spectroscopy of ZnO nanoparticle and highly oriented ZnO microrod arrays

The electronic structures of ZnO nanoparticles and microrod arrays are studied by O 1s X-ray absorption spectroscopy (XAS) and O Kα X-ray emission spectroscopy (XES). We show that the present LDA+U SIC calculation approach is suitable to correct the LDA self-interaction error of the cation d-states. The atomic eigenstates of 3d in zinc and 2p in oxygen are energetically close, which induces strong Zn-3d-O-2p hybridization. This anomalous valence band cation-d-anion-p hybridization is affected when the localization of the Zn 3d-states is taken into account. Experimentally, the XES spectra show energy dependence in the spectral shape revealing selected excitations to the Zn 3d, 4s and 4p states, hybridized with O 2p states. Strong anisotropic effects are observed for the highly oriented ZnO rods, but not for the isotropic spherical nanoparticles. The nanostructured ZnO has primarily bulk XAS and XES properties.

Spatial and Temporal Variability of Arsenic Solid-State Speciation in Historically Lead Arsenate Contaminated Soils

The arsenic (As) solid-state speciation (i.e., oxidation state, precipitates, and adsorption complexes) is one of the most important factors controlling dissolved As concentrations at As contaminated sites. In this case study, two representative subsurface samples (i.e., oxidized and semi-reduced sites) from former lead arsenate contaminated soils in the northeastern United States were chosen to investigate the effects of aging on As retention mechanisms using multiscale spectroscopic techniques. X-ray powder diffraction (XRD), synchrotron based microfocused (micro) XRD, in situ micro-synchrotron based X-ray fluorescence spectroscopy (SXRF), and micro-X-ray absorption near edge structure (XANES) spectroscopy were used to compliment the final bulk X-ray absorption spectroscopy (XAS) analyses. In the sample from an oxic area, As is predominantly (approximately 71%) present as As(V) adsorbed onto amorphous iron oxyhydroxides with a residue (approximately 29%) of an original contaminant, schultenite (PbHAsO4). Contrarily, there is no trace of schultenite in the sample from a semi-reduced area. Approximately 25% of the total As is present as adsorbed phases on amorphous iron oxyhydroxide and amorphous orpiment (As2S3). The rest of the fractions (approximately 46%) were identified as As(V)-Ca coprecipitates. This study shows that aging effects can significantly alter the original chemical constituent (schultenite) in soils, resulting in multi and site-specific As solid-state speciation. The variability in spatial and temporal scale may be important in assessing the environmental risk and in developing in situ remediation technologies.