X-ray Absorption Spectroscopy Study Research Articles

Origin of magnetism via cation vacancy defects in non-stoichiometric NiCo2O4 nanoparticles: A synchrotron-based x-ray absorption and x-ray magnetic circular dichroism spectroscopic study

The present study probes the effect of cation distributions in the structural, optical, electronic, and magnetic properties of mixed-valent inverse-spinel NiCo2O4 (NCO) nanoparticles (NPs). NCO NPs were prepared using the sol–gel combustion method and the grain size was obtained in the magnetic exchange length range assumed to be from single-ion anisotropy. The Raman and photoluminescence spectroscopies confirm the presence of an inverse-spinel structure with different oxidation states, and vibrating sample magnetometry clarifies the existence of ferromagnetism with the presence of magnetic anisotropy among the cations. These NPs annealed at a higher grain-growth temperature accumulate ferrimagnetic properties and produce magneto-crystalline anisotropy making NCO an assuring material for spintronic applications. A detailed x-ray absorption spectroscopy and x-ray magnetic circular dichroism studies reveal an indestructible correlation between the distribution of the present cations and the element-specific origin of ferrimagnetic behavior. Ni is found to be accountable for the magnetic moment and electronic conduction, whereas Co is associated mainly with the generation of the magnetic anisotropy even in the polycrystalline NP form. This describes the anti-ferromagnetic coupling between Co and Ni ions that is pivotal in demonstrating the exchange interaction between these cations. The above result signifies the site-dependent cation valence states for the magnetic properties, and the extent of growing conditions are related to such cation-site dysfunction. This depicts further tunability in NCO as a functional oxide material.

Convergent Active Site Evolution in Platinum Single Atom Catalysts for Acetylene Hydrochlorination and Implications for Toxicity Minimization.

Platinum single atoms anchored onto activated carbon enable highly stable Hg-free synthesis of vinyl chloride (VCM) via acetylene hydrochlorination. Compared to gold-based alternatives, platinum catalysts are in initial phases of development. Most synthetic approaches rely on chloroplatinic acid, presenting opportunities to explore other precursors and their impact on catalyst structure, reactivity, and toxicity aspects. Here, we synthesize platinum single atom catalysts (Pt SACs, 0.2-0.8 wt % Pt) employing diverse Pt2+ and Pt4+ complexes with ammine, hydroxyl, nitrate, and chloride ligands, following a scalable impregnation protocol on activated carbon extrudates. X-ray absorption spectroscopy (XAS) reveals that Pt4+ species reduce to Pt2+ upon deposition onto the support. Despite similar oxidation states, the initial activity is precursor dependent, with tetraammine-derived Pt SACs displaying 2-fold higher VCM yield than chlorinated counterparts, linked to superior hydrogen chloride binding abilities by density functional theory (DFT) simulations. Their activity gradually converges due to dynamic active site restructuring, delivering remarkable precursor-independent stability over 150 h. Operando XAS and DFT studies uncover reaction-induced ligand exchange, generating common active and stable Pt-Cl x (x = 2-3) species. Convergent active site evolution enables flexibility in metal precursor selection and thus toxicity minimization through multiparameter assessment. This study advances safe-by-design catalysts for VCM synthesis, highlighting the importance of toxicity analyses in early-stage catalyst development programs.

In Situ X-ray Absorption Spectroscopy Study of the Deactivation Mechanism of a Ni-SrTiO3 Photocatalyst Slurry Active in Water Splitting.

We used in situ X-ray absorption spectroscopy (XAS) to investigate the composition-performance correlation of Ni-SrTiO3 photocatalysts active for water splitting. After preparation and exposure to ambient conditions, the Ni particles on SrTiO3 consist of Ni(0) and Ni(II) phases, with a 4:1 at % ratio, in a metal/oxide core/shell configuration, as confirmed by XPS and TEM-EDX. In situ XAS experiments using an aqueous slurry of the Ni-SrTiO3 photocatalyst and simultaneous continuous exposure to 365 nm light with a power density of 100 mW cm-2 and the X-rays do not reveal significant changes in oxidation state of the Ni particles. Contrarily, when the X-rays are discontinuously applied, UV excitation leads to oxidation of a significant fraction of Ni(0) to Ni(II), specifically to NiO and Ni(OH)2 phases, along with cocatalyst restructuring. Ni dissolution or oxidation to higher valence states (e.g., Ni(III)) was not observed. The UV light-induced oxidation of Ni(0) causes the hydrogen evolution rate to drop to similar rates as observed for pristine SrTiO3, suggesting that Ni(0) is the active phase for H2 generation. Our results underscore the importance of assessing the effects of (continuous) X-ray exposure to (photo)catalyst-containing aqueous slurries during in situ XAS experiments, which can significantly influence the observation of compositional and structural changes in the (photo)catalysts. We ascribe this to X-ray induced water photolysis and formation of free electrons, which in this study quench SrTiO3 photoholes and prevent Ni oxidation.

Influence of local structure and metal-oxygen hybridization on the electrical and magnetic properties of alkaline earth metal (Mg2+, Ca2+, Sr2+) substituted LaFeO3 ceramics

Pristine and alkaline-earth metal-substituted LaFeO3 (La1−xAxFeO3−δ; x = 0 and 0.2; A = Mg2+, Ca2+, and Sr2+) sintered ceramics are prepared from nanoparticles synthesized via a low-temperature citrate sol–gel technique. X-ray diffraction studies confirm the formation of a phase-pure LaFeO3 structure without any secondary phases for all the La1−xAxFeO3−δ compositions. LaFeO3 and La0.8Mg0.2FeO3−δ ceramics show Raman active modes related to La vibration, oxygen octahedral tilting, bending, and stretching. The optical bandgap is estimated to be 2.34 eV for pure LaFeO3 and reduces to 2.23 eV for La0.8Mg0.2FeO3−δ ceramics. On the contrary, La0.8Ca0.2FeO3−δ and La0.8Sr0.2FeO3−δ ceramics show no features in Raman spectra, consistent with the observation of metallic nature and diffuse band edge without any indication of sharp band edge noted. X-ray absorption spectroscopy (XAS) studies on La-L3 and Fe-K-edges confirm the oxidation states of La3+ and Fe3+ in all these ceramics. Local structural distortions and formation of oxygen vacancies in La0.8A0.2FeO3−δ (A = Mg2+, Ca2+, and Sr2+) ceramics are discerned from XAS structure analysis compared to the pristine LaFeO3 ceramics. Magnetic measurements of La1−xAxFeO3−δ reveal weak ferromagnetic nature except for La0.8Mg0.2FeO3−δ, which shows a large magnetization of 4.6 (6.7) emu/g at 300 (5) K. The ferromagnetic behavior of La0.8Mg0.2FeO3−δ ceramics seems to originate from the modification of hybridization between Fe(3d)–O(2p), La(5d)–O(2p), and Fe(4sp)–O(2p) orbitals. An anomalous magnetic transition observed only in zero-field-cooled curves at 88 K in La0.8Ca0.2FeO3−δ and La0.8Sr0.2FeO3−δ ceramics is correlated to the formation of new electronic states containing O 2p character as discerned from pre-peak O-K-edge.

Addressing the Challenge of a Simultaneous Destructive Assay of Plutonium and Uranium: Highly Precise, Robust, Universal, and Reagent-Free Differential Pulse Voltametric Method Development in Biodegradable Methanesulfonic Acid Medium.

The destructive assay of bulk uranium and plutonium, a cornerstone for chemical quality control and nuclear material accounting of fuel matrices, mandates robust and precise methodologies. Despite ongoing research, simultaneous, matrix independent determination of U and Pu has eluded solution owing to inherent limitations in aqueous acid medium, viz., coexistence of multiple oxidation states, coupled electrochemical reactions, smaller potential window, and requirement for multistep sample preconditioning and tedious electrode modification. The present study addresses this challenge wherein non-aqueous methanesulfonic acid (MSA) served the dual role of solvent and analyte media with a bare glassy carbon (GC) electrode. Fuel matrices, viz., (U, Pu)C, (U, Pu)O2, PuO2, UO3, UO2, and U3O8, were quantitatively dissolved in biodegradable MSA, without using any additives. Redox speciation of the analytes, U and Pu, in MSA was probed by ultraviolet-visible spectrophotometry and electrometry, revealing the absence of electrocatalytic regeneration and stabilization of single oxidation state, viz., U(VI) and Pu(IV), along with relevant redox-energetic (electron transfer and reversibility) and kinetic data. Bidentate coordination of MSA with the U analyte was indicated by X-ray absorption spectroscopy studies and was corroborated by density functional theory-based investigations, providing the optimized structure, viz., [UO2(MSA)2] and [Pu(MSA)4], binding modes and energy, partial charges, and molecular orbital diagrams. Based on these insights, the feasibility of differential pulse voltammetry (DPV)-based assay method development for U and Pu separately and in different U/Pu ratios, representing assorted fuel matrices, was probed. Analytical figures of merit for both U and Pu (detection limit of ∼10-5 M, precision of ∼0.2%, accuracy of ∼0.2%, high sensitivity, repeatability, and non-influence of relevant interferences) were determined, method validated employing actual fuel samples, and compared with the established, multi-step biamperometry method. Hence, a universal, simultaneous U and Pu destructive assay method in non-aqueous MSA media based on DPV with a commercial GC electrode was demonstrated.

Operando and Ex-Situ Studies of Columbite Electrode Materials

Storing and utilising electricity from renewable sources is of growing importance as we move away from our reliance on fossil fuels. Batteries and supercapacitors are becoming ever more present in our day to day lives; in order to improve the next generation of batteries and supercapacitors, we must understand on an atomic scale what happens to electrode materials upon charge and discharge. In order to do this, we must combine structural and electrochemical studies, using techniques which use both different length scales and different timescales. These experiments are known as operando experiments and are a valuable tool in analysing new materials for energy storage.Transition metal niobates of the columbite structure (MNb2O6, M = transition or alkali earth metal) have previously been studied for their photocatalytic and magnetic properties but recently have garnered interest as negative electrode materials for batteries and supercapacitors.1 In this study, a series of columbite materials have been studied using a range of operando and ex-situ techniques. Samples were prepared using both solid-state synthesis2 and solution methods. Ex-situ X-ray diffraction and scanning electron microscopy were used to characterise the columbites prior to electrochemical testing. Operando X-ray absorption spectroscopy and operando Raman studies have been used to probe the changes in oxidation state of the transition metals and local structure upon charge/discharge. References Yang, X. Zhao, F. Liao, Q. Cheng, L. Mao, H. Fa and L. Chen, Sustain. Energy Fuels, 2021, 5, 3039Garía Alvarado, A. Orera, J. Canales-Vázquez, and J. T. S. Irvine, Chem. Mater., 2006, 18, 3827-3834 Figure 1

Electrocatalysis for Water Splitting Using Layered Zirconium Phosphate Nanomaterials

We are studying new applications of layered zirconium phosphate (ZrP) inorganic nanomaterials [1]. The θ phase of ZrP can be directly ion-exchanged with metal complexes and catalysts, producing intercalated phases useful for artificial photosynthesis schemes for water splitting, amperometric biosensors, and drug delivery applications. Recently, we have demonstrated improved electrocatalytic activity of ZrP nanomaterials loaded with metal ions suitable for the oxygen evolution reaction of water splitting. Electrocatalysts have been incorporated as intercalated species, surface bound, on exfoliated layers, and on nanoparticles of different morphologies (hexagonal platelets, cubes, rods, and spheres). Single and bimetallic electrocatalysts based on earth-abundant materials have been studied. Reduction in overpotentials and increases in mass activity have been achieved. Mixed metal NiFe-intercalated ZrP electrocatalysts at 90% Fe metal content proved to have superior OER electrocatalytic performance (decreased overpotentials, increased mass activities, reduced Tafel slopes) compared to adsorbed counterparts. We have recently prepared CoFe electrocatalysts systems and again the intercalated bimetallic system is more active than the surface-adsorbed one. We are starting to work also with Mn electrocatalysts. We are exploring OER activities of other mixed-metal catalysts on ZrP, bifunctional catalysts, and operando synchrotron X-ray absorption spectroscopy studies to elucidate the nature of the active species. We are also exploring efficient solar H2 production using these supports.Reference[1] M. V. Ramos-Garcés; J. González-Villegas; A. López-Cubero; J. L. Colón, Acc. Mater. Res. 2021, 2, 793-803. DOI: 10.1021/accountsmr.1c00102.

Chemical Substitution Strategies for Tailoring P2- and O3-Type Layered Oxide Cathodes in Sodium-Ion Batteries

Among cathodes for sodium-ion batteries (SIBs), layered transition metal oxides Na x MO2 (M = transition metal) are very promising due to their easy synthesis and high theoretical capacity.1,2 In this class, Ni/Mn/Fe-based layered oxides are attractive due to the high redox potential of Ni2+/ Ni4+ and Jahn Teller inactive centers (Ni2+, Fe3+,and Mn4+).3 However, complex phase transitions, Na+/vacancy ordering and transition-metal (TM) migration degrade their electrochemical performances.4 To address the issues, a widely studied cathode- P2-type Na0.67[Ni0.33Mn0.67]O2 is chosen and Li+ is substituted in the TM-layer to tailor a series of high Na-content cathodes. Among them, Na0.85[Li0.14Ni0.29Mn0.57]O2 cathode with optimal Li-substitution exhibits a reversible capacity of 168 mAh g-1 at 0.1 C rate and good cycling stability (82% retention after 100 cycles).5 In-situ XRD measurement reveals a complete solid-solution formation and X-ray absorption spectroscopy studies confirm the participation of Ni4+/Ni2+ and Mn4+/Mn3+ redox couples during Na+-extraction/insertion. Lastly, a full Na-ion cell with hard carbon is demonstrated with an energy density of 420 Wh kg-1. In subsequent work, a Li and Ti co-substitution strategy is applied to design a high-entropy O3-type NaLixNiyFezMn0.5Ti0.5-aO2 cathode.6 It retains 77% of its initial capacity after 400 cycles at a 2 C rate with a facile O3-P3-OP2-P3-O3 phase transition. The co-substitution strategy uplifts the average voltage of the system, improves the reversibility of the high-voltage OP2 phase, and enhances the Na-ion diffusion rate. A mere 1.32% unit cell volume change and diminishing electrochemical cell resistance over cycling ensure its superior long-term cycling performance. A full cell is fabricated, which holds 62% of its initial capacity even after 1000 cycles at a 0.5 C rate. Therefore, mono- and di-ion substitution strategies hold enormous potential for designing high-performing P2- and O3-type cathode materials for practical SIBs. References T. Hosaka, K. Kubota, A. S. Hameed, and S. Komaba, Chem. Rev., 120, 6358–6466 (2020).J. Y. Hwang, S. T. Myung, and Y. K. Sun, Chem. Soc. Rev., 46, 3529–3614 (2017).F. Zhang, J. Liao, L. Xu, W. Wu, and X. Wu, ACS Appl. Mater. Interfaces, 13, 40695–40704 (2021).Z. Lu and J. R. Dahn, J. Electrochem. Soc., 148, A1225 (2001).A. Ghosh, B. Senthilkumar, S. Ghosh, P. Amonpattaratkit, and P. Senguttuvan, J. Electrochem. Soc., 170, 030538 (2023).A. Ghosh, R. Hegde, and P. Senguttuvan, J. Mater. Chem. A., submitted(2023).

In Situ Soft XAS Study of Electrocatalysts with an Ultrahigh Vacuum-Compatible Liquid Flow Cell

Synchrotron X-ray absorption spectroscopy offers a unique tool for investigating the element-specific electronic structure of materials in chemistry and quantum physics fields. In particular, in situ/operando X-ray absorption spectroscopy (XAS) has yielded valuable atomic-level insight into electrocatalysts in operation. While most studies have employed "hard" X-rays that are higher than ~5 keV in energy, "soft" X-rays with energies around or below 1-2 keV have the unique capability to probe core-level excitations of light elements such as oxygen or p→d excitations of 1st-row transition metals. The low-energy soft X-rays easily become absorbed and scattered by ambient air, however, requiring ultrahigh vacuum (UHV) environments. To acquire soft XAS spectra of electrocatalysts in action within a UHV chamber, we employ home-fabricated SiNx membranes and a sealed electrochemical liquid flow cell. We demonstrate the use of our technique with canonical water oxidation catalysts RuO2 and MnOx.

Engineering of Soft Urea Synthesized Ni3n Nanomaterials for Oxygen Evolution Reaction

Most active electrocatalysts include platinum-group metals (PGMs) and their alloys for electrochemical water splitting, but they are strongly limited by material cost and scarcity. [1] On the other hand, transition metal nitrides overcome this bottleneck and show high electrical conductivity, making them catalytically active for water electrolysis. The high conductivity is based on the incorporation of nitrogen atoms into the unit cell of metal, leading to their d-band contraction with electron density enhancement below Fermi level. [2] For instance, metallic nickel nitride (Ni3N) has been widely reported as a promising electrocatalyst material to accelerate the oxygen evolution reaction (OER) by in-situ forming a nickel (oxy)hydroxide [Ni(OH)2 and NiOOH] layer. [3] The Ni3N materials can be prepared by single step soft-urea route to obtain pure crystal phase [4] or a composite with carbon [5], which rises the following two research questions: 1. What are the optimal synthesis parameters of the soft-urea route to control the structure of Ni3N? 2. What is the relationship between structure and OER activity for Ni3N materials?In this work, we used the soft-urea route to prepare Ni3N materials by varying the ratios of NiCl2, urea and ethanol as precursor chemicals. The precursors were made into a gel by magnetic stirring having metal:urea ratio of 1:5, and then heated up to 400 ºC in a tubular furnace with constant flow of argon. The as-prepared Ni3N were then characterized by X-ray diffraction (XRD), elemental analysis (CHN), transmission electron microscopy (TEM) equipped with an energy-dispersive X-ray spectroscopy (EDX) and X-ray absorption spectroscopy (XAS).Based on the quantitative Rietveld refinement, the formation of pristine Ni3N materials is strongly controlled by the annealing temperature. In addition to the Ni3N phase, we observed the formation of the fcc Ni phase (0 – 10 wt.%) as a minor phase. The TEM images show individual particles of below 100 nm in size. Rotating disc electrode (RDE) technique was employed to establish the electrochemical OER activity of Ni3N materials by sweeping the potential from 1.3 to 1.8 VRHE at 10 mV s-1 in Ar-saturated 0.1 M KOH. Polycrystalline Ni and Ni particles are taken as reference materials. We observed an improvement of the OER activity as the overpotential at 10 mA cm-2 (normalized by the geometric surface area) reduced from 379 mV (poly-Ni) to 358 mV (Ni3N). Furthermore, in-situ XAS studies were performed to better understand the nature of the redox reactions between Ni species and the in-situ formation of nickel (oxy)hydroxide [Ni(OH)2 and NiOOH] layers before and during the OER as a function of the applied potential. Based on the in-situ XANES data, we correlated the structure of Ni3N/fcc-Ni/carbon composite with observed OER activity under alkaline conditions.To summarize, we were able to prepare a composite of Ni3N/fcc-Ni/carbon material with improved OER activity via soft-urea route. The in-situ formation of the layered nickel (oxy)hydroxide [Ni(OH)2 and NiOOH] layers as the catalytically active OER species were probed by in-situ XANES data.

Electrocatalytic CH4 production from CO2 by SiO2-induced amorphous CuOx

CH4, as an important clean fuel and hydrogen carrier, its production from CO2 driven by renewable energy could simultaneously achieve energy supply, CO2 emission reduction, and renewable energy consumption, which is of great significance. Electrocatalytic CH4 production relies on high-valence Cu species for realizing multielectron reduction, while facing inadequate performance stability due to the reconstruction tendency of Cu. Herein, an amorphous CuOx-SiO2 catalyst (amorCuOx-SiO2) is synthesized via an electrostatic interaction strategy to construct stable Cu2+ active sites for efficient electrocatalytic CH4 production. AmorCuOx-SiO2 shows a good reconstruction resistance during electroreduction, confirmed via X-ray absorption spectroscopy studies, endowing a 63.7% CH4 faradaic efficiency at 500 mA cm−2, which is 7.8 folds of crystalized CuOx-SiO2 (crysCuOx-SiO2). Theoretical calculations revealed that stable Cu2+ active sites enhanced *CO adsorption and were more susceptible for further hydrogenation, thus promoting the CO2-to-CH4 conversion. Our findings provide a feasible strategy for developing reconstruction-resistant electrocatalysts for advanced CH4 production performance.

Sustainable Electrosynthesis of N,N-Dimethylformamide via Relay Catalysis on Synergistic Active Sites.

Electrified synthesis of high-value organonitrogen chemicals from low-cost carbon- and nitrogen-based feedstocks offers an economically and environmentally appealing alternative to traditional thermocatalytic methods. However, the intricate electrochemical reactions at electrode surfaces pose significant challenges in controlling selectivity and activity, especially for producing complex substances such as N,N-dimethylformamide (DMF). Herein, we tackle this challenge by developing relay catalysis for efficient DMF production using a composite WO2-NiOOH/Ni catalyst with two distinctive active sites. Specifically, WO2 selectively promotes dimethylamine (DMA) electrooxidation to produce strongly surface-bound (CH3)2N*, while nearby NiOOH facilitates methanol electrooxidation to yield more weakly bound *CHO. The disparity in binding energetics of the key C- and N-intermediates expedites C-N coupling at the WO2-NiOOH interface. In situ infrared spectroscopy with isotope-labeling experiments, quasi-in situ electron paramagnetic resonance trapping experiments, and electrochemical operating experiments revealed the C-N coupling mechanism and enhanced DMF-synthesis selectivity and activity. In situ X-ray absorption spectroscopy (XAS) and postreaction transmission electron microscopy (TEM) studies verified the stability of WO2-NiOOH/Ni during extended electrochemical operation. A Faradaic efficiency of ∼50% and a production rate of 438 μmol cm-2 h-1 were achieved at an industrially relevant current density of 100 mA cm-2 over an 80 h DMF production period. This study introduces a new paradigm for developing electrothermo relay catalysis for the sustainable and efficient synthesis of valuable organic chemicals with industrial potential.

Synthesis and Catalytic Performance of Bimetallic Oxide-Derived CuO-ZnO Electrocatalysts for CO2 Reduction.

Steering the selectivity of electrocatalysts toward the desired product is crucial in the electrochemical reduction of CO2. A promising approach is the electronic modification of the catalyst's active phase. In this work, we report on the electronic modification effects on CuO-ZnO-derived electrocatalysts synthesized via hydrothermal synthesis. Although the synthesis method yields spatially separated ZnO nanorods and distinct CuO particles, strong restructuring and intimate atomic mixing occur under the reaction conditions. This leads to interactions that have a profound effect on the catalytic performance. Specifically, all of the bimetallic electrodes outperformed the monometallic ones (ZnO and CuO) in terms of activity for CO production. Surprisingly, on the other hand, the presence of ZnO suppresses the formation of ethylene on Cu, while the presence of Cu improves CO production of ZnO. In situ X-ray absorption spectroscopy studies revealed that this catalytic effect is due to enhanced reducibility of ZnO by Cu and stabilization of cationic Cu species by the intimate contact with partially reduced ZnO. This suppresses ethylene formation while favoring the production of H2 and CO on Cu. These results show that using mixed metal oxides with different reducibilities is a promising approach to alter the electronic properties of electrocatalysts (via stabilization of cationic species), thereby tuning the electrocatalytic CO2 reduction reaction performance.

Incorporation of atomic Fe-oxide triggers a quantum leap in the CO2 methanation performance of Ni-hydroxide

The heterogeneous catalytic conversion of carbon dioxide (CO2) to methane (CH4) via CO2 methanation offers a promising avenue for establishing the closed carbon loop. Nevertheless, the lack of effective catalysts limits its industrial applications. Considering this, we developed a novel heterogeneous catalyst comprising oxygen vacancies enriched atomic Fe-oxide clusters confined in the TiO2-supported Ni-hydroxide (denoted as NiFe-TiO2) via wet chemical reduction method. This material delivers an unprecedently high CH4 productivity of ∼24,358 mmol g-1h−1 in CO2 methanation at 300 °C, surpassing the Ni-TiO2 (12,481 mmol g-1h−1) by ∼ 95 %. On top of that, the high structural reliability of the Fe-oxide atomic clusters endows the NiFe-TiO2 catalyst with outstanding durability, where it achieves an optimum CH4 productivity of ∼ 36,399 mmol g-1h−1 after 116 cycles (155 h) with CH4 selectivity of 90.5 % and retains the pristine performance up to 220 cycles (330 h) in the stability test. With evidence from in-situ X-ray absorption and ambient pressure X-ray photoelectron spectroscopy studies, the performance descriptors and reaction pathways were unveiled, where the oxygen vacancies in the atomic Fe-oxide clusters and the adjacent Ni-hydroxide domains synergistically boost the CO2 activation and the H2 dissociation, respectively. Such a potential synergy enables the simultaneous operation of all intermediate steps for enhanced CO2 methanation kinetics on the NiFe-TiO2 surface. Most importantly, these findings not only unravel the merits of oxygen vacancies in transition metals for CO2 methanation but mark a step ahead for the rational design of heterogeneous catalysts in various catalytic applications.

Steering the Reaction Pathway of CO2 Electroreduction by Tuning the Coordination Number of Copper Catalysts.

Cu-based catalysts are optimal for the electroreduction of CO2 to generate hydrocarbon products. However, controlling product distribution remains a challenging topic. The theoretical investigations have revealed that the coordination number (CN) of Cu considerably influences the adsorption energy of *CO intermediates, thereby affecting the reaction pathway. Cu catalysts with different CNs were fabricated by reducing CuO precursors via cyclic voltammetry (Cyc-Cu), potentiostatic electrolysis (Pot-Cu), and pulsed electrolysis (Pul-Cu), respectively. High-CN Cu catalysts predominantly generate C2+ products, while low-CN Cu favors CH4 production. For instance, over the high-CN Pot-Cu, C2+ is the main product, with the Faradaic efficiency (FE) reaching 82.5% and a partial current density (j) of 514.3 mA cm-2. Conversely, the low-CN Pul(3)-Cu favors the production of CH4, achieving the highest FECH4 value of 56.7% with a jCH4 value of 234.4 mA cm-2. In situ X-ray absorption spectroscopy and Raman spectroscopy studies further confirm the different *CO adsorptions over Cu catalysts with different CN, thereby directing the reaction pathway of the CO2RR.

Ni-rich LiNi0.905Co0.043Al0.052O2 cathode material for high-energy density Li-ion cells: Tuning lithium content, structural evolution, and full-cell performance

Ni-rich LiNi0.905Co0.043Al0.052O2 oxides with the Li excess (x) up to 7 mol.% used in the synthesis are prepared by a facile dry-mixing method. It is shown that the Li/Ni mixing level and crystallite size are dependent on the Li amount, while Al from the precursor acts as a scavenger of the excessive Li. Operando X-ray diffraction (XRD) studies enable explaining differences in the structural evolution of materials exhibiting high and low Li/Ni mixing, especially in the H2↔H3 transition region. Soft X-ray absorption spectroscopy (XAS) studies show differences in the charge state evolution of the surface and bulk Ni. The optimized oxide with the Li excess x = 2 mol.% delivers excellent electrochemical performance, with an initial discharge capacity of 205.5 mAh g−1 at 0.1C, and 91.4 % capacity retention after 100 cycles at 1C. The unmodified in any other way cathode material allows achieving the discharge capacity of 178.5 mAh g−1 at the high rate of 5C, and works well in the full-cells.

Novel high-pressure/high-temperature reactor cell for in situ and operando x-ray absorption spectroscopy studies of heterogeneous catalysts at synchrotron facilities.

This paper presents the development of a novel high-pressure/high-temperature reactor cell dedicated to the characterization of catalysts using synchrotron x-ray absorption spectroscopy under operando conditions. The design of the vitreous carbon reactor allows its use as a plug-flow reactor, monitoring catalyst samples in a powder form with a continuous gas flow at high-temperature (up to 1000 °C) and under high pressure (up to 1000bar) conditions, depending on the gas environment. The high-pressure/high-temperature reactor cell incorporates an automated gas distribution system and offers the capability to operate in both transmission and fluorescence detection modes. The operando x-ray absorption spectroscopy results obtained on a bimetallic InCo catalyst during CO2 hydrogenation reaction at 300 °C and 50bar are presented, replicating the conditions of a conventional microreactor. The complete setup is available for users and permanently installed on the Collaborating Research Groups French Absorption spectroscopy beamline in Material and Environmental (CRG-FAME) sciences and French Absorption spectroscopy beamline in Material and Environmental sciences at ultra-high dilution (FAME-UHD) beamlines (BM30 and BM16) at the European Synchrotron Radiation Facility in Grenoble, France.

Probing the Dynamic Landscape: From Static to Time-Resolved X-Ray Absorption Spectroscopy to Investigate Copper Redox Chemistry in Neurodegenerative Disorders.

Copper (Cu), with its ability to exist in various oxidation states, notably Cu(I) and Cu(II), plays a crucial role in diverse biological redox reactions. This includes its involvement in pathways associated with oxidative stress in neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, and Transmissible Spongiform Encephalopathies. This paper offers an overview of X-ray Absorption Spectroscopy (XAS) studies designed to elucidate the interactions between Cu ions and proteins or peptides associated with these neurodegenerative diseases. The emphasis lies on XAS specificity, revealing the local coordination environment, and on its sensitivity to Cu oxidation states. Furthermore, the paper focuses on XAS applications targeting the characterization of intermediate reaction states and explores the opportunities arising from recent advancements in time-resolved XAS at ultrabright synchrotron and Free Electron Laser radiation sources.

Aluminium, Nitrogen-Dual-Doped Reduced Graphene Oxide Co-Existing with Cobalt-Encapsulated Graphitic Carbon Nanotube as an Activity Modulated Electrocatalyst for Oxygen Electrocatalyst for Oxygen Electrochemistry Applications.

There is a rising need to create high-performing, affordable electrocatalysts in the new field of oxygen electrochemistry. Here, a cost-effective, activity-modulated electrocatalyst with the capacity to trigger both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in an alkaline environment is presented. The catalyst (Al, Co/N-rGCNT) is made up of aluminium, nitrogen-dual-doped reduced graphene oxide sheets co-existing with cobalt-encapsulated carbon nanotube units. Based on X-ray Absorption Spectroscopy (XAS) studies, it is established that the superior reaction kinetics in Al, Co/N-rGCNT over their bulk counterparts can be attributed to their electronic regulation. The Al, Co/N-rGCNT performs as a versatile bifunctional electrocatalyst for zinc-air battery (ZAB), delivering an open circuit potential ≈1.35V and peak power density of 106.3mWcm-2, which are comparable to the system based on Pt/C. The Al, Co/N-rGCNT-based system showed a specific capacity of 737mAhgZn -1 compared to 696mAhgZn -1 delivered by the system based on Pt/C. The DFT calculations indicate that the adsorption of Co in the presence of Al doping in NGr improves the electronic properties favoring ORR. Thus, the Al, Co/N-rGCNT-based rechargeable ZAB (RZAB) emerges as a highly viable and affordable option for the development of RZAB for practical applications.

Operando double-edge high-resolution X-ray absorption spectroscopy study of BiVO4 photoanodes.

High energy resolution fluorescence detected X-ray absorption spectroscopy is a powerful method for probing the electronic structure of functional materials. The X-ray penetration depth and photon-in/photon-out nature of the method allow operando experiments to be performed, in particular in electrochemical cells. Here, operando high-resolution X-ray absorption measurements of a BiVO4 photoanode are reported, simultaneously probing the local electronic states of both cations. Small but significant variations of the spectral lineshapes induced by the applied potential were observed and an explanation in terms of the occupation of electronic states at or near the band edges is proposed.