Advances in Understanding Tungsten Disulfide Dynamics during the Hydrogen-Evolution Reaction: An Initial Step in Elucidating the Mechanism.

Tungsten disulfide (WS2), a promising electrocatalyst made from readily available materials, demonstrates significant effectiveness in the hydrogen-evolution reaction (HER). The study conducts a thorough investigation using various analytical methods such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), and in situ Raman spectroscopy. These techniques have uncovered changes in the WS2 particle structure during HER. Through employing EPR, XAS, and in situ Raman spectroscopy, the research reveals structural and chemical transformations. This includes the formation of novel W species and signs of W-O bond formation. Moreover, significant changes in the morphology of the particles were observed. These findings offer enhanced insights into the mechanisms of WS2 under HER conditions, highlighting its catalytic performance and durability.

Lead in the Presence of Iron under Alkaline Conditions for the Oxygen-Evolution Reaction.

The oxygen-evolution reaction (OER) is a bottleneck in water splitting, which is a critical process for energy storage. In this study, the electrochemistry of Pb in the absence or presence of K2FeO4, as a soluble Fe source, is examined at pH ≈ 13. Our findings indicate that Pb exhibits limited catalytic activity for the OER under alkaline conditions. However, upon the addition of K2FeO4 to the electrolyte, a significant enhancement in the OER activity is observed in the presence of Pb. A notable observation in this study is the formation of stable Fe(IV) species following the OER during chronoamperometry experiments conducted in an alkaline solution. In addition to in situ Raman and visible spectroscopies, the operated electrodes have been characterized by high-resolution transmission electron microscopy, scanning electron microscopy, electron spin resonance spectroscopy, X-ray diffraction, electrochemical methods, electron paramagnetic resonance, and X-ray absorption spectroscopy. Through our experimental investigations, it is consistently observed that the presence of Fe ions on the surface of Pb/PbOx serves as an effective catalyst for the OER. However, it is important to note that this heightened OER activity is only temporary due to the low adhesion of Fe ions on the surface of Pb/PbOx.

Reaction between Nickel Hydroxide and Cerium(IV) Ammonium Nitrate in Aqueous Solution.

Cerium(IV) ammonium nitrate (CAN) has been extensively used as a sacrificial oxidant to study water-oxidation catalysts (WOCs). Although nickel hydroxide has been extensively investigated as WOCs, the water-oxidation reaction (WOR) and mechanistic studies in the presence of CAN and nickel hydroxide were rarely performed. Herein, using in situ Raman spectroscopy, in situ X-ray absorption spectroscopy, and in situ electron paramagnetic resonance spectroscopy, WOR in the presence of CAN and ß-Ni(OH)2 was investigated. The proposed WOR mechanism involves the oxidation of ß-Ni(OH)2 by CAN, leading to the formation of γ-NiO(OH). γ-NiO(OH), in the presence of acidic conditions, evolves oxygen and is reduced to Ni(II). In other words, the role of ß-Ni(OH)2 is the storage of four oxidizing equivalents by CAN, and then a four-electron reaction could result in a WOR with low activation energy. ß-Ni(OH)2 in CAN at concentrations of 0.10 M shows WOR with a maximum turnover frequency and a turnover number (for 1000 s) of 5.5 × 10-5/s and 2.0 × 10-2 mol (O2)/mol(Ni), respectively. In contrast to ß-Ni(OH)2, Ni(OH2)62+ (aq) could not be oxidized to γ-NiO(OH). Indeed, Ni(OH2)62+ (aq) is the decomposition product of ß-Ni(OH)2/CAN.

Oxygen-evolution reaction in the presence of cerium(IV) ammonium nitrate and iron (hydr)oxide: old system, new findings.

Solar fuel production by photosynthetic systems strongly relies on developing efficient and stable oxygen-evolution catalysts (OECs). Cerium(IV) ammonium nitrate (CAN) has been the most commonly used sacrificial oxidant to investigate OECs. Although many metal oxides have been extensively investigated as OECs in the presence of CAN, mechanistic studies were rarely reported. Herein, first, Fe(III) (hydr)oxide (FeOxHy) was prepared by the reaction of Fe(ClO4)3 and KOH solution and characterized by some methods. Then, changes in Fe oxide in the presence of CAN during the OER were tracked using in situ Raman spectroscopy, in situ X-ray absorption spectroscopy, in situ visible spectroscopy, and in situ electron paramagnetic resonance spectroscopy. FeOxHy in the presence of CAN and during the OER converted to γ-Fe2O3 and [Fe(H2O)6]3+, and a small amount of oxygen was formed. A maximum turnover frequency and turnover number of 10-6 s-1 and 1.3 × 10-3 mol(O2)/mol(Fe) (for half an hour) in the presence of CAN (0.20 M) and FeOxHy were observed.

Role of decomposition products in the oxidation of cyclohexene using a manganese(III) complex.

Metal complexes are extensively explored as catalysts for oxidation reactions; molecular-based mechanisms are usually proposed for such reactions. However, the roles of the decomposition products of these materials in the catalytic process have yet to be considered for these reactions. Herein, the cyclohexene oxidation in the presence of manganese(III) 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine chloride tetrakis(methochloride) (1) in a heterogeneous system via loading the complex on an SBA-15 substrate is performed as a study case. A molecular-based mechanism is usually suggested for such a metal complex. Herein, 1 was selected and investigated under the oxidation reaction by iodosylbenzene or (diacetoxyiodo)benzene (PhI(OAc)2). In addition to 1, at least one of the decomposition products of 1 formed during the oxidation reaction could be considered a candidate to catalyze the reaction. First-principles calculations show that Mn dissolution is energetically feasible in the presence of iodosylbenzene and trace amounts of water.

A copper(II) coordination compound under water-oxidation reaction at neutral conditions: decomposition on the counter electrode.

In the context of energy storage, the oxygen-evolution reaction (OER, 2H2O → O2 + 4H+ + 4e-) through the water-oxidation reaction is a thermodynamically uphill reaction in overall water splitting. In recent years, copper(II) coordination compounds have been extensively used for the OER. However, challenges remain in finding the mechanism of the OER in the presence of these metal coordination compounds. Herein, the electrochemical OER activity is investigated in the presence of a copper(II) coordination compound at pH ≈ 7. While the investigations on finding true catalysts for the OER are focused on the working electrode, herein, for the first time, the focus is on the decomposition of copper(II) coordination compound (CuL3, L: 2,2'-bipyridine N,N'-dioxide) during the OER on the counter electrode toward the precipitation of copper(I) oxide and metallic Cu.

Finding the True Catalyst for Water Oxidation at Low Overpotential in the Presence of a Metal Complex.

The design of molecular-based catalysts for oxygen-evolution reaction (OER) requires more investigations for the true catalyst to be found. First-row transition metal complexes are extensively investigated for OER, but the role of these metal complexes as a true catalyst is doubtful. Some doubts have been expressed about the role of first-row transition metal complexes for OER at high overpotentials (η > 450). Generally, the detection of the true catalyst has so far been focused on high overpotentials (η > 450) because at low overpotentials (η < 450), many methods are not sensitive enough to detect small amounts of heterogeneous catalysts on the electrode surface during the first seconds of the reaction. Ni(II) phthalocyanine-tetra sulfonate tetrasodium (1) is in moderate conditions (at 20-50 °C and pH 5-13) in the absence of electrochemical driving forces, which could make it noteworthy for OER. Herein, the results of OER in the presence of 1 at low overpotentials under alkaline conditions are presented. In addition, in the presence of Ni complexes, using an Fe ion is introduced as a new method for detecting Ni (hydr)oxide under OER. Our experiments indicate that in the presence of a homogeneous OER (pre)catalyst, a deep investigation is necessary to rule out the heterogeneous catalysts formed. Our approach is a roadmap in the field of catalysis to understand the OER mechanism in the presence of a molecular Ni-based catalyst design. Our results shown in this study are likely to open up new perspectives and discussion on many molecular catalysts in a considerable part of the chemistry community.

Surprisingly Low Reactivity of Layered Manganese Oxide toward Water Oxidation in Fe/Ni-Free Electrolyte under Alkaline Conditions.

So far, many studies on the oxygen-evolution reaction (OER) by Mn oxides have been focused on activity; however, the identification of the best performing active site and corresponding catalytic cycles is also of critical importance. Herein, the real intrinsic activity of layered Mn oxide toward OER in Fe/Ni-free KOH is studied for the first time. At pH ≈ 14, the onset of OER for layered Mn oxide in the presence of Fe/Ni-free KOH happens at 1.72 V (vs reversible hydrogen electrode (RHE)). In the presence of Fe ions, a 190 mV decrease in the overpotential of OER was recorded for layered Mn oxide as well as a significant decrease (from 172.8 to 49 mV/decade) in the Tafel slope. Furthermore, we find that both Ni and Fe ions increase OER remarkably in the presence of layered Mn oxide, but that pure layered Mn oxide is not an efficient catalyst for OER without Ni and Fe under alkaline conditions. Thus, pure layered Mn oxide and electrolytes are critical factors in finding the real intrinsic activity of layered Mn oxide for OER. Our results call into question the high efficiency of layered Mn oxides toward OER under alkaline conditions and also elucidate the significant role of Ni and Fe impurities in the electrolyte in the presence of layered Mn oxide toward OER under alkaline conditions. Overall, a computational model supports the conclusions from the experimental structural and electrochemical characterizations. In particular, substitutional doping with Fe decreases the thermodynamic OER overpotential up to 310 mV. Besides, the thermodynamic OER onset potential calculated for the Fe-free structures is higher than 1.7 V (vs RHE) and, thus, not in the stability range of Mn oxides.

Role of Pt and PtO2 in the Oxygen-Evolution Reaction in the Presence of Iron under Alkaline Conditions.

The oxygen-evolution reaction (OER) through water oxidation is an inevitable reaction for water splitting toward storing energy. However, OER is a four-electron and slow reaction, which is also a bottleneck for water splitting. To find the role of Pt and PtO2 on the OER in the presence of Fe, the electrochemistry of Pt foil and PtO2 is investigated in the absence/presence of K2FeO4 as a soluble Fe salt at pH ≈ 13. After the addition of K2FeO4, a remarkable increase in the OER is recorded in the presence of Pt or PtO2. The obtained catalysts were characterized by operando visible spectroscopy, high-resolution transmission electron microscopy, scanning electron microscopy, electron-spin resonance spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and electrochemical methods. KOH solutions usually contain Fe and/or Ni impurities. It is found that neither Pt nor PtO2 is an OER catalyst in a Ni/Fe-free KOH, and even at an overpotential of 570 mV in purified KOH (pH ≈ 13), no clear OER was observed.

Oxygen-Evolution Reaction by a Palladium Foil in the Presence of Iron.

Herein, we investigate the oxygen-evolution reaction (OER) and electrochemistry of a Pd foil in the presence of iron under alkaline conditions (pH ≈ 13). As a source of iron, K2FeO4 is employed, which is soluble under alkaline conditions in contrast to many other Fe salts. Immediately after reacting with the Pd foil, [FeO4]2- causes a significant increase in OER and changes in the electrochemistry of Pd. In the absence of this Fe source and under OER, Pd(IV) is stable, and hole accumulation occurs, while in the presence of Fe this accumulation of stored charges can be used for OER. A Density Functional Theory (DFT) based thermodynamic model suggests an oxygen bridge vacancy as an active site on the surface of PdO2 and an OER overpotential of 0.42 V. A substitution of Pd with Fe at this active site reduces the calculated OER overpotential to 0.35 V. The 70 mV decrease in overpotential is in good agreement with the experimentally measured decrease of 60 mV in the onset potential. In the presence of small amounts of Fe salt, our results point toward the Fe doping of PdO2 rather than extra framework FeOx (Fe(OH)3, FeO(OH), and KFeO2) species on top of PdO2 as the active OER sites.

Aggregated manganese complex-nanolayered manganese oxide: a new hybrid molecular-inorganic material.

Layered materials such as clays, layered double hydroxides, and layered hydroxides are promising compounds for material science applications because, in addition to their structural and functional properties, the aggregation of these compounds with others results in new structural and functional characteristics. Notably, the aggregation of a metal complex and nanolayered material leads to new structures and properties. Mn oxides and complexes are different compounds, which show promising properties. Herein, a new hybrid molecular-inorganic material was synthesized by the aggregation of a manganese complex with a 2,4,6-tris(2-pyridyl)-1,3,5-triazine ligand and monolayers of Mn oxide. This new hybrid molecular-inorganic material was characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis, microanalysis, UV-Vis spectroscopy, nitrogen adsorption-desorption isotherm, magnetic properties, and electron paramagnetic resonance spectroscopy. All these methods showed that the aggregation of the manganese complex and layered Mn oxide occurred. A larger extent of aggregation for this hybrid molecular-inorganic material was observed compared to monolayered Mn oxide. The new material constitutes a new family of hybrid molecular-inorganic materials, in which transition metal complexes could be placed in a new environment.

Novel tetrahedral cobalt(ii) silanethiolates: structures and magnetism.

Three heteroleptic complexes of Co(ii) tri-tert-butoxysilanethiolates have been synthesized with piperidine [Co{SSi(OtBu)3}2(ppd)2] 1, piperazine [Co{SSi(OtBu)3}2(NH3)]2(µ-ppz)·2CH3CN 2, and N-ethylimidazole [Co{SSi(OtBu)3}2(etim)2] 3. The complexes have been characterized by a single-crystal X-ray, revealing their tetrahedral geometry on Co(ii) coordinated by two nitrogen and two sulfur atoms. Complexes 1 and 3 are mononuclear, whereas 2 is binuclear. The spectral properties and thermal properties of 1-3 complexes were established by FTIR spectroscopy for solid samples and TGA. The magnetic properties of complexes 1, 2, and 3 have been investigated by static magnetic measurements and X-band EPR spectroscopy. These studies have shown that 1 and 3, regardless of the similarity in structure of CoN2S2 cores, demonstrate different types of local magnetic anisotropy. Magnetic investigations of 2 reveal the presence of weak antiferromagnetic intra-molecular Co(ii)-Co(ii) interactions that are strongly influenced by the local magnetic anisotropy of individual Co(ii) ions.

Rigorous broadband study of the intrinsic ferromagnetic linewidth of monocrystalline garnet spheres.

This work demonstrates the first application of direct broadband (1 GHz-30 GHz) quality (Q) factor measurements of the uniform precession mode in magnetised garnet spheres for the accurate determination of the room-temperature intrinsic ferromagnetic linewidth (ΔH). The spheres were enclosed in a subwavelength cavity, so that the measured Q-factor depended mainly on their magnetic losses and the conduction losses of the cavity walls. The contribution of the latter is assessed by means of the recently proposed magnetic plasmon resonance model and has been found to be negligible. A total of 10 samples made from commercially available pure yttrium iron garnet (YIG) and gallium-substituted YIG have been measured, differing in diameter and/or saturation magnetisation Ms. The dependence of the intrinsic ΔH on the internal magnetic field is found to have near-perfect linear dependence, which cannot be said about the typically studied extrinsic ΔH even at high frequencies. It is found that the difference between the two linewidths, which becomes significant at low frequencies, can be attributed to a geometric effect. Due to its fundamental nature, this work is applicable not only to magnetic material characterization, but also to the study of the origins of losses in magnetic materials.

Ferromagnetic Resonance Revised - Electrodynamic Approach.

Resonance in a ferromagnetic sphere, known in the body of literature as the mode of uniform precession, has recently been proven to be magnetic plasmon resonance (MPR). This finding has prompted research which is presented in this paper on the relation between the Q-factor at the MPR and the ferromagnetic resonance (FMR) linewidth ΔH, which is a parameter of magnetized gyromagnetic materials. It is proven in this paper that ΔH can be unequivocally determined from the Q-factor measured at the MPR, if all losses in the resonance system are properly accounted for. It can be undertaken through a rigorous but simple electrodynamic study involving the transcendental equation, as proposed in this paper. The present study also reveals that electric losses have a substantially reduced impact on ΔH due to the large magnetic to electric energy storage ratio at the MPR. Theoretical results are supported by measurements of the Q-factors on a monocrystalline yttrium iron garnet (YIG) sphere.

EMR studies of the internal motion of Mn(4+) ions in the Sr overdoped (La(1-x)Sr(x))(Ga(1-y)Mn(y))O3 (x/y up to 8) supplemented by magnetic and optical spectroscopy measurements.

The effect of the Sr doping on electronic structure in single crystals of (La(1-x)Sr(x))(Ga(1-y)Mn(y))O3 solid solutions (LSGM) is investigated by means of electron magnetic resonance (EMR). The EMR results are supplemented by magnetic susceptibility and optical spectroscopy measurements. The compositions with small concentration of Mn doping (y<1%) and overdoped content of Sr (the ratio x(Sr)/y(Mn) up to 8) are used to maximally enhance the role of divalent doping. The experimental results provide evidence of the holes delocalization in the overdoped compound (x(Sr)/y(Mn)>1). This delocalization is accompanied by appearance of the new charge transfer transitions in the optical spectrum and dynamical valence change of manganese atoms. Additionally we observe the thermally activated narrowing of resonance EMR lines due to the internal motion, which is characterized by the energy barrier depending strongly on the ratio x(Sr)/y(Mn). The energy barrier is found to be associated with the charge carrier (hole) self-trapped energy. Fitting the EMR spectra in three orthogonal planes to an orthorhombic spin Hamiltonian enables extracting the zero-field splitting (ZFS) parameters and the Zeeman g-factors for Mn(4+) (S=3/2) ions in LSGM. The experimental ZFS parameters are modeled using superposition model analysis based on an orthorhombic symmetry approximation.

Synthesis, structure, magnetic properties and EPR spectroscopy of a copper(II) coordination polymer with a ditopic hydrazone ligand and acetate bridges.

A new one dimensional coordination polymer of copper(II), [Cu4(L)2(µ2-1,1-OAc)2(µ2-1,3-OAc)4]n (1), has been synthesized and characterized by spectroscopic methods and single crystal X-ray analysis [HL = (E)-N'-(phenyl(pyridin-2-yl)methylene)isonicotinhydrazide, OAc = acetate anion]. The coordination polymer contains two kinds of Cu(II) dimers which are connected by two types of acetate (µ2-1,1- and µ2-1,3-) bridging groups. The ditopic isonicotinhydrazone ligand coordinates to the Cu1 center through the N2O-donor set and connects to the Cu2 center by a pyridine group of the isonicotine part. The EPR and magnetic susceptibility measurements confirm the existence of two kinds of Cu(II) dimers. The intradimer isotropic exchange was estimated to be +0.80(1) cm(-1) for the ferromagnetic Cu1···Cu1 dimeric unit and -315 (1) cm(-1) for the antiferromagnetic Cu2···Cu2 dimeric unit.

Supramolecular control over molecular magnetic materials: γ-cyclodextrin-templated grid of cobalt(II) single-ion magnets.

Single-ion magnets (SIMs) are potential building blocks of novel quantum computing devices. Unique magnetic properties of SIMs require effective separation of magnetic ions and can be tuned by even slight changes in their coordination sphere geometry. We show that an additional level of tailorability in the design of SIMs can be achieved by organizing magnetic ions into supramolecular architectures, resulting in gaining control over magnetic ion packing. Here, γ-cyclodextrin was used to template magnetic Co(II) and nonmagnetic auxiliary Li(+) ions to form a heterometallic {Co, Li, Li}4 ring. In the sandwich-type complex [(γ-CD)2Co4Li8(H2O)12] spatially separated Co(II) ions are prevented from superexchange magnetic coupling. Ac/dc magnetic and EPR studies demonstrated that individual Co(II) ions with positive zero-field splitting exhibit field-induced slow magnetic relaxation consistent with the SIMs' behavior, which is exceptional in complexes with easy-plane magnetic anisotropy.

EPR of Mn2+ in the kagomé staircase compound Mg2.97Mn0.03V2O8.

The electron paramagnetic resonance (EPR) of Mn2+ in geometrically frustrated Mg2.97Mn0.03V2O8 single crystals is reported. The complex EPR spectrum shows resonance lines associated with two crystallographically nonequivalent lattice positions of Mn ions that are known in the kagomé staircase system as "cross-tie" and "spine" sites. Additionally, strongly anisotropic resonances of various Mn2+-Mn2+ pairs are observed. The signs and values of the crystal field parameters are determined from EPR spectra. The local magnetic symmetry details of magnetic ions and components of the hyperfine structure tensor are determined for various nonequivalent manganese positions. The exchange coupling between Mn ions in "cross-tie" and "spine" sites is found to be J=41 K.