Higher Oxidation States Research Articles

Chromium-Induced High Covalent Co-O Bonds for Efficient Anodic Catalysts in PEM Electrolyzer.

The proton exchange membrane water electrolyzer (PEMWE), crucial for green hydrogen production, is challenged by the scarcity and high cost of iridium-based materials. Cobalt oxides, as ideal electrocatalysts for oxygen evolution reaction (OER), have not been extensively applied in PEMWE, due to extremely high voltage and poor stability at large current density, caused by complicated structural variations of cobalt compounds during the OER process. Thus, the authors sought to introduce chromium into a cobalt spinel (Co3O4) catalyst to regulate the electronic structure of cobalt, exhibiting a higher oxidation state and increased Co-O covalency with a stable structure. In-depth operando characterizations and theoretical calculations revealed that the activated Co-O covalency and adaptable redox behavior are crucial for facilitating its OER activity. Both turnover frequency and mass activity of Cr-doped Co3O4 (CoCr) at 1.67V (vs RHE) increased by over eight times than those of as-synthesized Co3O4. The obtained CoCr catalyst achieved 1500mA cm-2 at 2.17V and exhibited notable durability over extended operation periods - over 100h at 500 mA cm-2 and 500h at 100 mA cm-2, demonstrating promising application in the PEMWE industry.

Flame-retardant vinyl ester resins enabled by phosphorus-containing 1-vinylimidazole salts with different phosphorus oxidation states

Different reactive flame retardants have been extensively developed for vinyl ester resins (VERs), but very few of them can yield a flame-retardant resin that meets defined standards (e.g. UL-94 V-0 rating). In this work, phosphorous-containing 1-vinylimidazole salts (called VIDHP and VIDPP) were synthesized through the facile neutralization of the acid and 1-vinylimidazole. VIDHP and VIDPP were then applied as flame-retardant crosslinking agents of VERs, by which phosphorus-containing groups could be incorporated into the resin chain via ionic bonds. VIDHP/VER and VIDPP/VER showed a high curing activity and can be well cured in moderate temperatures. With 20 wt.% additions of VIDHP and VIDPP, VIDHP20/VER, and VIDPP20/VER presented a limiting oxygen index value of 29.7 % and 28.4 %, respectively, with the latter achieving a UL 94 V0 rating. In the cone calorimetric test, compared to the unmodified VERs, VIDPP20/VER exhibited large reductions in the peak heat release rate, total heat release rate, and total smoke release rate while VIDHP20/VER demonstrated comparatively inferior performance in terms of the heat release. VIDHP20/VER and VIDPP20/VER showed good thermal stability and presented a little lower glass transition temperature than the control sample. VIDPP with a low phosphorus oxidation state (+1) demonstrated high flame-retardant activities in the gaseous phase, whereas VIDHP with a high phosphorus oxidation state (+5) primarily exhibited efficacy in the condensed phase.

Solution blow spun Co3O4 nanofibers as electrocatalyst for oxygen evolution processes

In this study, Co3O4 nanofibers were produced using the solution blow spinning (SBS) method. The investigation focused on studying their structure, morphology, and electrocatalytic properties in relation to the oxygen evolution reaction (OER) in alkaline solution (KOH). Calcination at 600 °C resulted in Co3O4 fibers without the presence of secondary phases (NF600), while calcined fibers at 400 °C showed a Co3O4–CoO composite phase (NF400), confirmed by XRD. The size of the crystallites, determined through Rietveld refinement analysis of XRD patterns, ranged from 42.6 nm to 56.2 nm for the calcined fibers at 400 °C and 600 °C, respectively, confirming the nanometric scale of the cobalt oxide fibers. Morphological analyses confirmed the production of rough nanofibers with monomodal distribution, with mean diameter of 385 nm for NF600 and 485 nm for NF400. The EDS confirms the presence of the chemical elements of cobalt and oxygen, estimating their abundance in the sample. The XPS analysis showed that the concentration of oxygen vacancies and the presence of species with higher oxidation states (Co3+) were beneficial to improve OER. Nanofibers calcined at 600 °C showed an overpotential of 330 mV at 10 mA cm−2, more efficient Tafel kinetics and low resistance to charge transfer (0.37 Ω) when compared to nanofibers calcined at 400 °C (338 mV). This result demonstrates the competitive catalytic activity of cobalt oxide nanofibers produced by SBS compared to other systems mentioned in the literature. Therefore, this work offers a new perspective on the development of electrocatalysts based on the structure and properties of cobalt oxide electrodes by the SBS method.

Synthesis, Characterization, and the Effect of Lewis Bases on the Nuclearity of Iron Alkoxide Complexes.

Inspired by the potential of alkoxides as weak-field ligands and their ability to bridge, we report herein a series of high-spin iron complexes supported by a bis-alkoxide framework PhDbf. A diiron complex [Fe2(PhDbf)2] (1a) is obtained upon metalation of the ligand, whereas addition of substituted pyridines affords five-coordinate mononuclear iron complexes [(R-Py)2Fe(PhDbf)] (2a-4a, R = H, p-tBu, p-CF3). The potential for nuclearity control of the metal complexes via auxiliary ligands is highlighted by the formation of asymmetric diiron species [(p-CF3-Py)Fe2(PhDbf)2] (5a) and [(m-CF3-Py)Fe2(PhDbf)2] (6a) with trifluoromethyl substituted pyridines, while electron-rich pyridines only produced monomeric species. Electronic properties analysis via UV-vis, electron paramagnetic resonance, 57Fe Mössbauer spectroscopy, and time-dependent density functional theory, along with redox capabilities of these complexes are reported to illustrate the effect of nuclearity on reactivity and the potential of these complexes to access higher oxidation states relevant in oxidative chemistry. Species 1a-5a, [(THF)2Fe(PhDbf)][PF6] (7), [PyFe(PhDbf)Cl] (2b), and [Py2Fe(PhDbf)][PF6] (2c) were characterized via SCXRD. Indirect evidence for the formation of dimeric Fe(III) species (1b, 5b, and 6b) is discussed.

Polyaniline Induced Trivalent Ni in Laser-Fabricated Nickel Oxides for Efficient Oxygen Evolution Reaction.

Although it is generally acknowledged that transition metals at high oxidation states represent superior oxygen evolution reaction (OER) activity, the preparation and stability of such a high-valence state are still a challenge, which requires relatively harsh reaction conditions and is unstable under ambient conditions. Herein, we report the formation of trivalent nickel (Ni3+) in laser-fabricated nickel oxides induced by polyaniline (PANI) under electrochemical activation via a significant charge transfer between Ni and N, as confirmed by X-ray photoelectron spectroscopy and density functional theory calculations. Thereafter, the presence of Ni3+ and the improved conductivity by PANI effectively increase the electrochemical OER activity of the samples together with excellent long-term stability. This work provides new insights for the rational manufacture of high-valence metal for electrochemical reactions.

Photochemical evolution of the molecular composition of dissolved organic carbon and dissolved brown carbon from wood smoldering

Recently, extreme wildfires occur frequently around the world and emit substantial brown carbon (BrC) into the atmosphere, whereas the molecular compositions and photochemical evolution of BrC remain poorly understood. In this work, primary smoke aerosols were generated from wood smoldering, and secondary smoke aerosols were formed by the OH radical photooxidation in an oxidation flow reactor, where both primary and secondary smoke samples were collected on filters. After solvent extraction of filter samples, the molecular composition of dissolved organic carbon (DOC) was determined by Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS). The molecular composition of dissolved BrC was obtained based on the constraints of DOC formulae. The proportion of dissolved BrC fractions accounted for approximately 1/3–1/2 molecular formulae of DOC. The molecular characteristics of dissolved BrC showed higher levels of carbon oxidation state, double bond equivalents, and modified aromaticity index than those of DOC, indicating that dissolved BrC fractions were a class of organic structures with relatively higher oxidation state, unsaturated and aromatic degree in DOC fractions. The comparative analysis suggested that aliphatic and olefinic structures dominated DOC fractions (contributing to 70.1%-76.9%), while olefinic, aromatic, and condensed aromatic structures dominated dissolved BrC fractions (contributing to 97.5%-99.9%). It is worth noting that dissolved BrC fractions only contained carboxylic-rich alicyclic molecules (CRAMs)-like structures, unsaturated hydrocarbons, aromatic structures, and highly oxygenated compounds. CRAMs-like structures were the most abundant species in both DOC and dissolved BrC fractions. Nevertheless, the specific molecular characteristics for DOC and dissolved BrC fractions varied with subgroups after aging. The results highlight the similarities and differences in the molecular compositions and characteristics of DOC and dissolved BrC fractions with aging. This work will provide insights into understanding the molecular composition of DOC and dissolved BrC in smoke.

Boosting hydrogen production via water and urea oxidation using a facile fabricated alkali-etched ZIF-67@Ni(OH)2 electrocatalyst

Designing an operative and stable metal electrocatalyst for hydrogen production via water and urea oxidation has been in the spotlight for the past few years. In this work, we have deposited Ni(OH)2 at various time spans of 1, 3 and 5 h over alkali-etched ZIF-67 via facile strategy. The optimum Ni(OH)2@etched ZIF-67 (5 h) (NEZ-3) heterostructure demonstrates excellent OER and UOR performance with a lower overpotential of 240 mV and 1.32 V, respectively at 10 mA cm−2. Moreover, the NEZ-3 electrocatalyst attains smaller Tafel values of 106 and 35 mV dec−1 with remarkable stability of 60 and 48 h for OER/UOR performance at 10 mA cm−2. Specifically, the two-cell electrolyzer requires a lower cell voltage of 1.73 V (OER) and 1.48 V (UOR) with an excellent durability of 20 h at 10 mA cm−2, respectively. Additionally, the water displacement method is used to examine the 4-electron transfer process (OER) of NEZ-3 electrocatalyst with a Faradaic efficiency of 87.53 %. Notably, the porous structure, introduction of oxygen defects (Ov), higher oxidation state of Ni3+, and abundant active sites in NEZ-3 electrocatalyst are the foremost reasons for the superior electrocatalytic water and urea oxidation reactions. Therefore, this research work delivers a simple methodology in emerging a highly active, stable, and cost-effective multifunctional electrocatalysts towards energy conversion applications.

Modeling Complex Ligands for High Oxidation State Catalysis: Titanium Hydroamination with Unsymmetrical Ligands.

A method for modeling high oxidation state catalysts is used on precatalysts with unsymmetrical and symmetrical bidentate ligands to get a more detailed understanding of how changes to ancillary ligands affect the hydroamination of alkynes catalyzed by titanium. To model the electronic donor ability, the ligand donor parameter (LDP) was used, and to model the steric effects, percent buried volume (% Vbur) was employed. For the modeling study, 7 previously unpublished unsymmetrical Ti(XX')(NMe2)2 precatalysts were prepared, where XX' is a chelating ligand with pyrrolyl/indolyl linkages. The rates of these unsymmetrical and 10 previously reported symmetrical precatalysts were used with the model kobs = a + b(LDP)1 + c(LDP)2 + d(% Vbur)1 + e(% Vbur)2, where a-e were found through least-squares refinement. The model suggests that (1) the two attachment points of the bidentate ligand XX' are in different environments on the metal (e.g., axial and equatorial in a trigonal bipyramidal or square pyramidal structure), (2) the position of the unsymmetrical ligand on the metal is determined by the electronics of the ligand rather than the sterics, and (3) that one side of the chelating ligand's electronics strongly influences the rate, while the other side's sterics more strongly influences the rate. From these studies, we were able to generate catalysts fitting to this model with rate constants larger than the fastest symmetrical catalyst tested.

High-Valence W6+ Ions Boost Cr2+ Activity in CrWO4 for Ideal Water Oxidation.

Electrocatalytic activity of multi-valence metal oxides for oxygen evolution reaction (OER) arises from various interactions among the constituent metal elements. Although the high-valence metal ions attract recent attentions due to the interactions with their neighboring 3d transition metal catalytic center, atomic-scale explanations for the catalytic efficiencies are still lacking. Here, by employing density functional theory predictions and experimental verifications, unprecedented electronic isolation of the catalytic 3d center (M2+) induced by the surrounding high-valence ions such as W6+ is discovered in multivalent oxides MWO4 (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn). Due to W6+'s extremely high oxidation state with the minimum electron occupations (d0), the surrounding W6+ blocks electron transfer toward the catalytic M2+ ions and completely isolates the ions electronically. Now, the isolated M2+ ions solely perform OER without any assistant electron flow from the adjacent metal ions, and thus the original strong binding energies of Cr with OER intermediates are effectively moderated. Through exploiting "electron isolators" such as W6+ surrounding the catalytic ion, exploring can be done beyond the conventional materials such as Ni- or Co-oxides into new candidate groups such as Cr and Mn on the left side of the periodic table for ideal OER.

Polyphenols-Enrichment of Vienna Sausages Using Microcapsules Containing Acidic Aqueous Extract of Boletus edulis Mushrooms.

Polyphenols are ubiquitous by-products in many plant foods. Their intake has been linked to health benefits like the reduced incidence of cardiovascular disease, diabetes, and cancer. These bioactive compounds can be successfully extracted from Boletus edulis mushrooms with acidic water. However, such extract could influence the sensory or textural properties of the product to be enriched; this inconvenience can be avoided by microencapsulating it using spray drying. In this study, the Vienna sausages were reformulated by replacing 2% of the cured meat with microcapsules containing an acidic aqueous extract of Boletus edulis mushrooms and by replacing ice flakes, an ingredient that represents 22.9% of the manufacturing recipe, with ice cubes from the same extract aiming to obtain a polyphenol enriched product. The results showed a higher content of polyphenols in sausages with extract (VSe; 568.92 μg/g) and microcapsules (VSm; 523.03 μg/g) than in the control ones (455.41 μg/g), with significant differences for 2,4-dihydroxybenzoic acid, protocatechuic acid, and 1-O-galloyl-β-D-glucose. However, because of the oxidative stress caused to the microcapsules by the extract's spray drying, VSm had the highest oxidation state. PV and TBARS levels varied with storage time in all formulations, but given the short period tested, they were well below the allowed/recommended limit. The extract, as such, negatively affected the appearance, odor, and taste of Vienna sausages. The microcapsules, instead, determined an increase in their acceptance rate among consumers; they also prevented moisture loss and color changes during storage. In conclusion, microcapsules are more suitable for use as a polyphenol enrichment ingredient in Vienna sausages than the extract.

Fertile, alkalic intrusions, Karari–Whirling Dervish gold deposit, Yilgarn Craton, Western Australia

Some Archean and Phanerozoic gold deposits demonstrate a close spatial relationship with alkalic intrusions, although the genetic connotations of this association are contentious. Only a few of the alkalic intrusions in the Kurnalpi Terrane of the Archean Kalgoorlie–Kurnalpi Rift are associated with economic gold mineralisation (e.g. at Wallaby, Karari). The subvolcanic syenitic intrusions at the 2 Moz Karari–Whirling Dervish gold deposit are markedly different from most other alkalic rocks in the Kalgoorlie–Kurnalpi Rift. Although preserving pristine to slightly modified igneous textures, the syenitic intrusions are thoroughly altered by hypogene hydrothermal fluids, such that igneous minerals, comprising apatite micro-phenocrysts and relicts of Ba-rich K-feldspar, Ba-rich biotite, sodic amphibole and Cr-rich magnetite, total <5 modal %. Elevated SO3 (up to 1 wt%) in igneous apatite shows that the alkalic magmas were S-bearing and oxidised, and a high oxidation state carried through to the hydrothermal stages that followed, including potassic (biotite) alteration, at 650 °C (apatite–biotite geothermometry). Oxidised hydrothermal activity was interrupted by periods of sodic and sodic–calcic alteration in structurally controlled zones, interpreted to record the influx of reduced, external fluids at around 400–450 °C. A second, volumetrically minor stage of potassic (K-feldspar–pyrite–hematite) alteration in the intrusions is nevertheless significant, as it matches the proximal alteration assemblage in volcaniclastic country rock-hosted gold lodes. Overall, the hydrothermal system in the intrusions evolved from pervasive, high-temperature to more focussed, structurally controlled, mesothermal alteration. The source of the high-temperature, oxidised, alkaline hydrothermal fluid is considered to be the oxidised, alkalic magma itself, including an alkalic magma chamber underlying the gold deposits. Based on evidence from Karari–Whirling Dervish, gold fertility indicators for alkalic intrusions include: (i) widespread high-temperature alkaline metasomatism, including coarsening of alkali-feldspar intergrowths, abundant hydrothermal magnesian biotite and advanced to total replacement of igneous minerals; (ii) low SiO2 and high K2O, relative to unmineralised alkalic rocks in the region; (iii) alteration of igneous titanite to hydrothermal rutile with concomitant depletion of Nb; (iv) low rare earth element/P owing to depletion of rare earth element in apatite by the potassic (biotite) alteration fluid; and (v) high V/Th relative to regional alkalic rocks.

Lattice contraction-driven design of highly efficient and stable O–NiFe layered double hydroxide electrocatalysts for water oxidation

The slow progress of the oxygen evolution reaction (OER) is a key challenge in advancing water splitting as a viable technology for sustainable hydrogen production. Utilizing a self-standing NiFe layered double hydroxide (LDH) is a promising approach to address the slow kinetics of OER. Herein, the oxidized Fe2+-containing NiFe-LDH supported on Ni foam (O–NiFe-LDH@NF) is developed through corrosion engineering and aging. The growth mechanism is studied by adjusting urea content, hydrothermal temperature and time. It is revealed that the transformation of Fe2+ to Fe3+ in Fe2+-containing NiFe-LDH and the increase of Fe content leads to lattice contraction. Furthermore, detailed experiments and theoretical simulations illustrate that the substantial Fe content and lattice shrinkage promote the generation of Ni in high oxidation state and enhance the adsorption affinity toward the reaction species. Consequently, O–NiFe-LDH@NF exhibits enhanced OER activity and stability, with an exceptionally low overpotential of 197 mV and 354 mV observed at the current densities of 10 and 500 mA cm−2, respectively, and remarkable stability over 300 h. This approach can be generalized for designing advanced electrocatalysts.

Anion modulate the morphological and electronic structure of NiFe-based electrocatalyst for efficient urea oxidation-assisted water electrolysis

Renewable energy-driven water electrolysis is considered as an environmentally friendly hydrogen (H2) production technology. Replacing the oxygen evolution reaction (OER) with the urea oxidation reaction (UOR) is a more effective way to improve the energy efficiency of H2 generation. Herein, a highly efficient 2D NiFeMo-based UOR catalyst and 1D NiFeMo-based HER catalyst are prepared by adjusting the concentration of MoO4‒. The MoO4‒ can serve as the key regulator to adjust the balance between the electrolytic dissociation (α) of the reactants and the supersaturation (S) to modulate the morphological and electronic structure. The prepared 2D NiFeMo nanosheet UOR catalyst and 1D NiFeMo nanorod HER catalyst can achieve a current density of 100 mA cm−2 at a potential of 1.36 and 0.062 V, respectively. In a HER/UOR system, a cell voltage of 1.58 V is needed to achieve a current density of 100 mA cm−2. The HER/UOR system operated stably for over 60 h with 3 times the direct water electrolysis current density. Moreover, the in situ Raman characterization coupled with XPS analysis clarifies that the addition of high-valence Mo can lower the transition energy barrier between the low and high oxidation state of Ni, which in turn lowers the overpotential of UOR. This work provides a novel strategy for synthesizing morphology-dependent electrocatalysts for different catalytic systems.

Synthesis and Properties of Cobalt Complexes with [B11H11]4- Ligands.

The unusually high oxidation state + IV of cobalt is stabilized by ligands based on [B11H11]4- in dark blue colored Cs4[Co(B11H10.11(OH)0.75)2]·4.56H2O, K4[Co(B11H9.19(OH)1.81)2]·2H2O, Cs8[Co{(B11H6)2(O)(O2BOH)4}]2·4H2O and K4[Co{(B11H6)2(O2BOH)5}]·7H2O. These compounds were obtained by reacting Co2+ salts with [B11H14]- under alkaline conditions. In the absence of oxygen, Co(+III) compounds such as the light brownish K4[Co(B11H11)(CN)3]·KCl·2.5H2O are formed. The title compounds were characterized by X-ray crystallography. Cs8[Co{(B11H6)2(O)(O2BOH)4}]2·4H2O and K4[Co(B11H11)(CN)3]·KCl·2.5H2O were also characterized using IR-, UV-vis and cyclovoltammetry. Magnetic measurements of Cs4[Co(B11H10.11(OH)0.75)2]·4.56H2O and ESR measurements of Cs8[Co{(B11H6)2(O)(O2BOH)4}]2·4H2O show that in these Co(+IV) low-spin d5 complexes the unpaired electron is on the dx2-y2, dxy (E2g) orbitals.

Fundamental Structural and Electronic Understanding of Palladium Catalysts on Nitride and Oxide Supports.

The nature of the support can fundamentally affect the function of a heterogeneous catalyst. For the novel type of isolated metal atom catalysts, sometimes referred to as single-atom catalysts, systematic correlations are still rare. Here, we report a general finding that Pd on nitride supports (non-metal and metal nitride) features a higher oxidation state compared to that on oxide supports (non-metal and metal oxide). Through thorough oxidation state investigations by X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), CO-DRIFTS, and density functional theory (DFT) coupled with Bader charge analysis, it is found that Pd atoms prefer to interact with surface hydroxyl group to form a Pd(OH)x species on oxide supports, while on nitride supports, Pd atoms incorporate into the surface structure in the form of Pd-N bonds. Moreover, a correlation was built between the formal oxidation state and computational Bader charge, based on the periodic trend in electronegativity.

Fundamental structural and electronic understanding of palladium catalysts on nitride and oxide supports

The nature of the support can fundamentally affect the function of a heterogeneous catalyst. For the novel type of isolated metal atom catalysts, sometimes referred to as single‐atom catalysts, systematic correlations are still rare. Here, we report a general finding that Pd on nitride supports (non‐metal and metal nitride) always features a higher oxidation state compared to that on oxide supports (non‐metal and metal oxide). Through thorough oxidation state investigations by X‐ray absorption spectroscopy (XAS), X‐ray photoelectron spectroscopy (XPS), CO‐DRIFTS, and density functional theory (DFT) coupled with Bader charge analysis, it is found that Pd atoms prefer to interact with surface hydroxyl group to form a Pd(OH)x species on oxide supports, while on nitride supports, Pd atoms incorporate into the surface structure in the form of Pd−N bonds. Moreover, a correlation was built between the formal oxidation state and the computational Bader charge, based on the periodic trend in electronegativity.

Synergistic Effect of Co-Mo Pinning in Lay-Structured Oxide Cathode for Enhancing Stability toward Potassium-Ion Batteries.

Owing to the high economic efficiency and energy density potential, manganese-based layer-structured oxides have attracted great interests as cathode materials for potassium ion batteries. In order to alleviate the continuous phase transition and K+ re-embedding from Jahn-Teller distortion, the [Mn-Co-Mo]O6 octahedra are introduced into P3-K0.45MnO2 herein to optimize the local electron structure. Based on the experimental and computational results, the octahedral center metal molybdenum in [MoO6] octahedra proposes a smaller ionic radius and higher oxidation state to induce second-order JTE (pseudo-JTE) distortion in the adjacent [MnO6] octahedra. This distortion compresses the [MnO6] octahedra along the c-axis, leading to an increased interlayer spacing in the K+ layer. Meanwhile, the Mn3+/Mn4+ is balanced by [CoO6] octahedra and the K+ diffusion pathway is optimized as well. The proposed P3-K0.45Mn0.9Co0.05Mo0.05O2 cathode material shows an enhanced cycling stability and rate performance. It demonstrates a high capacity of 80.2mAhg-1 at 100mAhg-1 and 77.3mAhg-1 at 500mAhg-1. Furthermore, it showcases a 2000 cycles stability with a 59.6% capacity retention. This work presents a promising solution to the challenges faced by manganese-based layered oxide cathodes and offers a deep mechanism understanding and improved electrochemical performance.

Formation of Fe4+ based on the Fe-O bond lengths in perovskite Eu1-xSrxFeO3

The physical properties of high oxidation state of transition metal oxide compounds have attracted many research interest for its potential applications. A systematic investigation of Fe4+ in Eu1-xSrxFeO3 with x = 0, 0.1, 0.2, 0.3, and 0.4 has been carried out in polycrystalline form obtained using sol-gel method. The single phase of Eu1-xSrxFeO3 is observed up to x = 0.3. The X-Ray powder pattern for low Sr content of x = 0.1, is best refined by using EuFeO3 crystal structure with Sr occupied the Eu-site. On the other hand, for higher Sr contents, two phases of EuFeO3 and Eu1/3Sr2/3FeO3 are required to obtained the best refinement. The result of the refinements shows that the volume and the average bond lengths of Fe-O decreases as increasing the Sr content. These results are consistent to support the existence of Fe4+ upon Sr doping in Eu1-xSrxFeO3. For the Sr content of x = 0.1, an anomalous change of the volume is observed.

Interplay between protonation and Jahn-Teller effects in a manganese vanadium cubane water oxidation catalyst.

Understanding the protonation behavior of metal-oxo water oxidation catalysts is essential to improve catalyst efficiency and long-term performance, as well as to tune their properties for specific applications. In this work, we explore the basicity and protonation effects of the highly active water oxidation catalyst [(Mn4O4) (V4O13) (OAc)3]3- using density functional theory. We computed the relative free energies of protonation in a systematic fashion for all symmetry-inequivalent O atoms, where the presence of multiple oxidation states from Mn4IV to Mn4III and a rich Jahn-Teller isomerism adds a significant amount of complexity. For high oxidation states, the compound behaves like some other polyoxometalates, showing protonation preferably at the terminal and μ2-bridging O atoms of the vanadate cap. However, upon reduction, eventually, the protonation preference switches to the cubane O atoms, mostly driven by a strong increase in basicity for O atoms located along the Jahn-Teller axes. Our work further evidences that protonation can potentially lead to several chemical transformations, like disproportionation and charge transfer to vanadium, dissociation of ligands, or the opening of the cubane structure. Our simulated UV/Vis absorption spectra additionally provide valuable insights about how the protonation of the catalyst could be tracked experimentally. Overall, our analysis highlights the complexity involved in the protonation of heterometallic polyoxometalate clusters.

Self-supported ultrathin NiMn-LDH nanosheets for highly active and robust urea oxidation

Due to its low equilibrium potential, urea oxidation reaction (UOR) is an advantageous alternative to the conventional anodic water oxidation reaction (OER) for energy-saving hydrogen production. Developing energy- and time-saving methods to synthesize active and stable UOR catalysts is significant. Herein, we present a simple and time-efficient method for the direct growth of defect-rich few-layered NiMn-layered double hydroxide (NiMn-LDH) nanosheets on nickel foam at ambient temperature and pressure. The defect-rich, few-layered structure exposes numerous active sites, promoting the formation of high-oxidation state Ni species at low overpotentials. Additionally, the introduction of Mn facilitates Ni oxidation and stabilizes in-situ formed NiOOH species. The optimized f-NiMn-LDH catalyst exhibits compelling UOR activity, requiring only 1.310/1.330/1.387 V for 10/100/400 mA cm−2 UOR, respectively. When used directly in urea-assisted hydrogen production, it achieves a low cell voltage of 1.436 V for a high current density of 100 mA cm−2, representing a substantial 249 mV reduction compared to water electrolysis. This work offers a simple strategy for preparing high-performance electrocatalysts for energy-efficient hydrogen production via urea-assisted water splitting.