Toward Finding the Role of Surface Iron Ions in Enhancing Oxygen-Evolution Reaction.

The oxygen evolution reaction (OER) in alkaline media is crucial for energy conversion technologies, and Fe-based catalysts have garnered significant attention for their efficacy. In this study, we provide an investigation of Fe-based catalysts under OER conditions using some techniques. Our findings reveal minimal structural alterations in the bulk FeHxOy framework during OER, indicating that the bulk structure remains largely intact. Instead, the catalytic activity is primarily localized on the material's surface. This conclusion is corroborated by the measured low electrochemical surface area (ECSA) of FeHxOy, suggesting that its limited OER performance stems from a paucity of active sites rather than low intrinsic activity. The superior efficiency of Fe ions in conductive oxides or on conductive metal substrates (e.g., copper and gold) in OER is attributed to the increased availability of surface-active sites in these systems. To further elucidate this phenomenon, we investigated two additional systems: NiFe and VFe (hydr)oxides. For NiFe (hydr)oxides, we demonstrate that only surface Fe ions contribute actively to OER. In contrast, in VFe (hydr)oxides, the removal of vanadium resulted in a marked increase in ECSA and the generation of defect sites, significantly enhancing OER activity. These findings provide critical insights into the surface-specific role of Fe ions in catalytic activity and could inform future design strategies for more efficient OER catalysts by optimizing the availability and reactivity of surface-active Fe sites.

Innovative Insights into Water-Oxidation Mechanism: Investigating Birnessite's Reaction with Cerium(IV) Ammonium Nitrate.

Developing Mn-based water-oxidation reaction (WOR) catalysts is key for renewable energy storage, utilizing Mn's abundance, cost-effectiveness, and natural role. Cerium(IV) ammonium nitrate (CAN) has been widely utilized as a sacrificial oxidant in the exploration of WOR catalysts. In this study, advanced techniques, such as X-ray absorption spectroscopy (XAS), in situ Raman spectroscopy, and in situ electron paramagnetic resonance (EPR), to delve into the WOR facilitated by CAN and birnessite were employed. XANES analysis has demonstrated that the average oxidation states (AOSs) of Mn in birnessite, a birnessite/CAN mixture, and in the birnessite/CAN mixture postwater addition are 3.7, 3.8, and 3.9, respectively. In situ Raman spectroscopy performed in the presence of birnessite and CAN revealed a distinct peak at 784 cm-1, which is attributed to Mn(IV)═O. A shift of this peak to 769 cm-1 in H218O confirms its association with Mn(IV)═O. No change in this peak was observed in D2O, further supporting the notion that it is linked to Mn(IV)═O rather than Mn-OH (D). Furthermore, EPR spectroscopy shows the presence of Mn(IV). It is suggested that the WOR mechanism initiates with the oxidation of birnessite by CAN, which enhances the concentration of Mn(IV) sites in the birnessite structure. Under acidic conditions, birnessite, enriched in Mn(IV), facilitates oxygen evolution and subsequently transitions into a form with reduced Mn(IV) levels. This process highlights the critical function of the Mn (hydr)oxide structure, similar to its role in the water-oxidizing complex of Photosystem II, where it serves as charge storage for oxidizing equivalents from CAN, paving the way for a four-electron reaction that drives the WOR.

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.

Iron Integration in Nickel Hydroxide Matrix vs Surface for Oxygen-Evolution Reaction: Where the Nernst Equation Does Not Work.

This study focuses on the oxygen-evolution reaction (OER) activity comparison between two forms of NiFe (hydr)oxides: compound 1, where Fe ions are applied on the surface of nickel (hydr)oxide, and compound 2, with Fe ions incorporated into the structural matrix of nickel (hydr)oxide. The observed exponential link between Coulombic energy and the total charge of the system points to a direct proportionality between the potential and the concentration of oxidized nickel ions (e.g., V ∝ [oxidized Ni]), diverging from the logarithmic relationship outlined in the Nernst equation or its modifications, which is not evident in this case. Initial visible spectroscopy indicates a notable trend toward oxidation. As, during the oxidation, more Ni is oxidized, a repulsion effect develops, diminishing the likelihood of further oxidation, and a distinct linear correlation emerges between the quantity of oxidized Ni(II) and the applied potentials.

Enhancing catalysis studies with chat generative pre-trained transformer (ChatGPT): Conversation with ChatGPT.

The progress made in natural language processing (NLP) and large language models (LLMs), such as generative pre-trained transformers, (GPT) has provided exciting opportunities for enhancing research across various fields. Within the realm of catalysis studies, GPT-driven models present valuable support in expediting the exploration and comprehension of catalytic processes. This research underscores the significance of ChatGPT in catalysis research, emphasizing its prowess as a valuable tool for furthering scientific inquiries. It suggests that for an outstanding oxygen evolution reaction (OER) catalyst as a case study, scientists can leverage ChatGPT to extract deeper insights and brainstorm innovative approaches to grasp the mechanism better and refine current systems.

Exploring an Electrochemical Route for Water-Enhanced Oxygenation Reactions Utilizing Nickel Molecular Structures: A Case Study.

Recently, Ni molecular catalysis has been extensively applied in oxygenation reactions. This work is underpinned by the characterization techniques and the discovered instability of the Ni-bipyridine/phenanthroline system, which results in Ni (hydr)oxide production under oxidative conditions. The practical applications of this mechanism by employing a prepared Ni (hydr)oxide-based electrode specifically in the oxygenation of sulfides, achieving noteworthy yields in contrast to noncatalyst control experiments, are explored. Thus, a Ni (hydr)oxide-based material is proposed as a candidate for the true catalyst for sulfide oxidation in the presence of the Ni-bipyridine/phenanthroline system. The findings of this study are expected to stimulate discussion and encourage new viewpoints within the chemical community regarding the potential applications and mechanisms of molecular catalysts in oxidation reactions.

Decoding Natural Strategy: Oxygen-Evolution Reaction on the Surface of Nickel Oxyhydroxide at Extremely Low Overpotential.

Although nickel (hydr)oxides in the absence of other metal ions are conventionally deemed inefficient catalysts for the oxygen-evolution reaction (OER) under alkaline conditions, this study reveals that nickel oxyhydroxide displays an OER activity at the associated peak for Ni(II) to Ni(III) oxidation postcharge accumulation. This occurs with only 90-120 mV overpotentials (at a low current density) and a Tafel slope of 297 mV/decade in a 0.10 M KOH solution. In the initial seconds, the Faraday efficiency lingers at a relatively low 20%, which can be attributed to charge storage. Nonetheless, as the duration extends to reach the 200 s mark, the efficiency notably escalates, exceeding 80%. Additionally, a mechanism for the OER in this low-overpotential zone is proposed, grounded in our investigation of the Ni(II) to Ni(III) peak and the OER region through in situ Raman spectroscopy. Taking into account the quantity of oxygen generated and the concentrations of redox-active Ni ions in the region of the redox peak, a turnover frequency of at a potential of 4.3 × 10-4 s-1 at 1.37 V was calculated. The documented reduction in overpotential during the OER may be ascribed to the complex interplay between the process of the OER and charge accumulation. The convergence of these reciprocally influencing factors facilitates a notably low overpotential in the OER. Our findings bear substantial implications for developing highly efficient and stable electrocatalysts for the OER in water-splitting applications.

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.

Unveiling the Oxygen Evolution Mechanism with FeNi (Hydr)oxide under Neutral Conditions.

Large-scale solar-driven water splitting is a way to store energy, but it requires the development of practical and durable oxygen evolution reaction (OER) catalysts. The present paper aims to investigate the mechanism of the OER, local pH, high-valent metal ions, limitations, conversions, and details during the OER in the presence of FeNi foam using in situ surface-enhanced Raman spectroscopy. This research also explores the use of in situ surface-enhanced Raman spectroscopy for detecting species on foam surfaces during the OER. The acidic media around the electrode not only limit the process but also affect the phosphate ion protonation and overall catalysis effectiveness. The study proposes that FeNi hydroxides serve as true catalysts for OER under neutral conditions, rather than FeNi phosphates. However, phosphate species remain crucial for proton transfer and water molecule adsorption. Changes observed in pH at the open-circuit potential suggest new insights concerning the coordination of Ni(II) to phosphate ions under certain conditions. By extrapolating the Tafel plot, the overpotential for the onset of OER was determined to be 470 mV. Furthermore, the overpotentials for current densities of 1 and 5 mA/cm2 were 590 and 790 mV, respectively. These findings offer valuable insights into the advancement of the OER catalysts and our understanding of the underlying mechanism for efficient water splitting; both are crucial elements for the purpose of energy storage.

Dynamic Changes of an Anodized FeNi Alloy during the Oxygen Evolution Reaction under Alkaline Conditions.

An efficient and durable oxygen evolution reaction (OER) catalyst is necessary for the water-splitting process toward energy conversion. The OER through water oxidation reactions could provide electrons for H2O, CO2, and N2 reduction and produce valuable compounds. Herein, the FeNi (1:1 Ni/Fe) alloy as foam, after anodizing at 50 V in a two-electrode system in KOH solution (1.0 M), was characterized by Raman spectroscopy, diffuse reflectance spectroscopy (DRS), X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDX), transmission electron microscopy (TEM), high-angle annular dark-field imaging (HAADF)-scanning transmission electron microscopy (STEM) and used as an efficient and durable OER electrocatalyst in KOH solution (1.0 M). The overpotential for the onset of the OER based on extrapolation of the Tafel plot was 225 mV. The overpotentials for the current densities of 10 and 30 mA/cm2 are observed at 270 and 290 mV, respectively. In addition, a low Tafel slope is observed, 38.0 mV per decade, for the OER. To investigate the mechanism of the OER, in situ surface-enhanced Raman spectroscopy was used to detect FeNi hydroxide and characteristic peaks of H2O. Impurities in KOH can adsorb onto the electrode surface during the OER. Peaks corresponding to Ni(III) (hydr)oxide and FeO42- can be detected during the OER, but high-valent FeNi (hydr)oxides are unstable and reduce under the open circle potential. Metal hydroxide transformations during the OER and anion adsorption should be carefully considered. In addition, Fe3O4 may convert to γ-Fe2O3 during the OER. This study aims to offer logical perspectives on the dynamic changes that occur during the OER under alkaline conditions in an anodized FeNi alloy. These changes encompass variations in morphology, surface oxidation, the generation of high-valent species, and phase conversion during the OER.

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.

Water Oxidation by a Copper(II) Complex with 6,6'-Dihydroxy-2,2'-Bipyridine Ligand: Challenges and an Alternative Mechanism.

Recently, copper(II) complexes have been extensively investigated as oxygen-evolution reaction (OER) catalysts through a water-oxidation reaction. Herein, new findings regarding OER in the presence of a Cu(II) complex with 6,6'-dihydroxy-2,2'-bipyridine ligand are reported. Using scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, Raman spectroscopy, in situ visible microscopy, in situ visible spectroelectrochemistry, X-ray absorption spectroscopy, and electrochemistry, it is hypothesized that the film formed on the electrode's surface in the presence of this complex causes an appropriated matrix to produce Cu (hydr)oxide. The resulting Cu (hydr)oxide could be a candidate for OER catalysis. The formed film could form Cu (hydr)oxide and stabilize it. Thus, OER activity increases in the presence of this complex.

Water oxidation couples to electrocatalytic hydrogenation of carbonyl compounds and unsaturated carbon-carbon bonds by nickel.

Artificial photosynthesis, an umbrella term, is a chemical process that biomimetics natural photosynthesis. In natural photosynthesis, electrons from the water-oxidation reaction are used for carbon dioxide reduction. Herein, we report the reducion of aldehydes and ketones to corresponding alcohols in a simple undivided cell. This reaction utilized inexpensive nickel foam electrodes (1 cm2) and LiClO4 (0.05 M) as a commercially accessible electrolyte in an aqueous medium. Under electrochemical conditions, a series of alcohols (21 examples) produces high selectivity in good yields (up to 100%). Usage the current method, 10 mmol (1060 mg) of benzaldehyde is also successfully reduced to benzyl alcohol (757 mg, 70% isolated yield) without any by­products. This route to alcohols matched several green chemistry principles: (a) atom economy owing to the use of H2O as the solvent and the source of hydrogen, (b) elimination of the homogeneous metal catalyst, (c) use of smooth reaction conditions, (d) waste inhibition due to low volumetric of by-products, and (e) application of safe EtOH co-solvent. Moreover, the ability of the system to operate with alkyne and alkene compounds enhanced the practical efficiency of this process.

Unsupervised recognition of components from the interaction of BSA with Fe cluster in different conditions utilizing 2D fluorescence spectroscopy.

The excitation-emission fluorescence spectroscopy combined with three-way analysis was applied for discriminating the pure BSA and BSA/Fe3O(OAc)6ClO4 (Fe) using unsupervised classification methods. Herein, the interaction of bovine serum albumin (BSA) and Fe clusters as an artificial enzyme is studied by extracting the intrinsic excitation-emission (EEM) fluorescence of BSA. The conformation of BSA changes with pH, temperature, and Fe concentration. Three-way fluorescence data were recorded for BSA and BSA/Fe during different days. The obtained results showed that the Fe clusters cause changes in the structure of BSA conformation as a function of pH, temperature, and Fe concentration. Also, the denaturation pathway of the BSA molecule is significantly different in the presence of Fe clusters. Both techniques of PARAFAC and PCA were used in the excitation-emission fluorescence matrices (EEM) of solutions at three different pH (5.0, 7.0, and 9.0) and temperatures (15.0, 25.0, and 35.0 °C) values. Also, we reported the results of the change in concentrations of Fe (4.0, 6.0, and 8.0 mg) using these methods. These three amino acids (tyrosine, tryptophan, and phenylalanine) indicate all datasets and their similarities and differences. The spectral differences were more remarkable in different pH values compared to different temperatures. Also, we could distinguish between the groups of protein samples properly in different concentrations of Fe using low-cost EEM spectral images and PARAFAC.

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.

Candidate for Catalyst during Water-Oxidation Reaction in the Presence of Manganese Compounds, from Nanosized Particles to Impurities: Sleep with One Eye Open.

Water-oxidation reaction (WOR) catalysts are critical for energy conversion. WOR is a four-electron oxidation and sluggish reaction. WOR needs a high thermodynamic driving force; it is also a kinetically slow reaction. Different compounds have been used for WOR; among these compounds, Mn materials have proven to be interesting because Mn is low-cost and also nontoxic, at least compared to many transition metals. Naturally, it has also been used in the biological water-oxidizing complex (WOC). Indeed, WOR has occurred on a huge scale in natural photosynthesis.For a long time, efforts have been made to design and synthesize various ligands and generate Mn compounds toward WOR catalysts. However, the addition or removal of electrons inside Mn compounds during harsh WOR conditions can lead to the formation or the breakage of bonds and result in the conversion of a precatalyst to a catalyst.Here, our findings on the conversion of Mn compounds to catalysts during WOR are presented. Many Mn compounds have been claimed to be catalysts for WOR in the presence of various chemical oxidants or under electrophotochemical conditions. Currently, the advances in characterization techniques and different spectroscopic methods have enabled a better understanding of catalysts. Different conversions such as that of the Mn complex to Mn oxide and Mn salts to Mn oxide during WOR have been explained. Indeed, the morphology and size of the Mn oxide formed depend on several factors such as the origin compounds, pH, ligands, and conditions. Thus, different Mn compounds show different activities toward WOR. The biomimetic models with Mn-Ca clusters are also decomposed during WOR. On the other hand, stable Mn complexes such as Mn phthalocyanines, which are very stable in the absence of potential, are easily decomposed during WOR. It is noted that for many of these Mn compounds, two steps result in the formation of Mn oxide during WOR: (i) Mn(II) or (III) leaching into the electrolyte and (ii) deposition of the leached Mn ions into the solution.Considering these steps, it can be seen that challenges remain in the area of Mn compounds, given the fact that even in the catalytic cycle at low oxidation numbers no Mn(II) or (III) should be leached to the electrolyte.

Catalysis of the Water Oxidation Reaction in the Presence of Iron and a Copper Foil.

The oxygen evolution reaction (OER) can provide electrons for reducing water, carbon dioxide, and ammonia. On the other hand, copper compounds are among the most interesting OER catalysts. In this study, water oxidation of a Cu foil in the presence of K2FeO4, a soluble Fe source, under alkaline conditions (pH ≈ 13) is investigated using electrochemical methods, X-ray diffraction, X-ray photoelectron spectroscopy, in situ visible spectroelectrochemistry, Raman spectroscopy, and scanning electron microscopy. After the reaction of the Fe salt with the Cu foil, a remarkable improvement for OER is recorded, which indicates that either the Fe ions on the copper foil directly participate in OER or these ions are critical for activating copper ions on the surface toward OER. Indeed, a remarkable decrease (130 mV) in the overpotential is recorded for the Cu foil in the presence of [FeO4]2-. Tafel slopes for the Cu foil in the absence and presence of K2FeO4 are 113.2 and 46.4 mV/decade, respectively. X-ray photoelectron spectroscopy shows that there is a strong interaction between Cu(II) and Fe(III) on the surface of the Cu foil. During OER in the presence of Cu(II) (hydr)oxide, Cu(III) is detected. In situ visible spectroelectrochemistry shows that Cu and Fe ions are dynamically active and precipitate on the surface of the counter electrode during cyclic voltammetry (CV). The isotopic experimental data using H218O based on Raman spectroscopy show that there is no change in the lattice oxygen. All of these experiments adopt a new perspective on the role of Fe in OER in the presence of a Cu foil under alkaline conditions.