Fragments of Layered Manganese Oxide Are the Real Water Oxidation Catalyst after Transformation of Molecular Precursor on Clay

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 β-Ni(OH)₂ was investigated. The proposed WOR mechanism involves the oxidation of β-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 β-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. β-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 β-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 β-Ni(OH)₂/CAN.

Mesoporous Manganese Oxides with High-Valent Mn Species and Disordered Local Structures for Efficient Oxygen Electrocatalysis

Active and nonprecious-metal bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are vital components of clean energy conversion devices such as regenerative fuel cells and rechargeable metal-air batteries. Porous manganese oxides (MnO_x) are promising electrocatalyst candidates because of their high surface area and the abundance of Mn. MnO_x catalysts exhibit various oxidation states and crystal structures, which critically affect their electrocatalytic activity. These effects remain elusive mainly because the synthesis of oxidation-state-controlled porous MnO_x with similar structural properties is challenging. In this work, four different mesoporous manganese oxides (m-MnO_x) were synthesized and used as model catalysts to investigate the effects of local structures and Mn valence states on the activity toward oxygen electrocatalysis. The following activity trends were observed: m-Mn₂O₃ > m-MnO₂ > m-MnO > m-Mn₃O₄ for the ORR and m-MnO₂ > m-Mn₂O₃ > m-MnO ≈ m-Mn₃O₄ for the OER. These activity trends suggest that high-valent Mn species (Mn(III) and Mn(IV)) with disordered atomic arrangements induced by nanostructuring significantly influence electrocatalysis. In situ X-ray absorption spectroscopy was used to analyze the changes in the oxidation states under the ORR and OER conditions, which showed the surface phase transformation and generation of active species during electrocatalysis.

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.

Trends and progress in application of cobalt-based materials in catalytic, electrocatalytic, photocatalytic, and photoelectrocatalytic water splitting

There has been a growing interest in water oxidation in recent two decades. Along with that, remarkable discovery of formation of a mysterious catalyst layer upon application of an anodic potential of 1.13 V vs. standard hydrogen electrode (SHE) to an inert indium tin oxide electrode immersed in phosphate buffer containing Co^(II) ions by Nocera et.al, has greatly attracted researchers interest. These researches have oriented in two directions; one focuses on obtaining better understanding of the reported mysterious catalyst layer, further modification, and improved performance, and the second approach is about designing coordination complexes of cobalt and investigating their properties toward the application in water splitting. Although there have been critical debates on true catalysts that are responsible for water oxidation in homogeneous systems of coordination complexes of cobalt, and the case is not totally closed, in this short review, our focus will be mainly on recent major progress and developments in the design and the application of cobalt oxide-based materials in catalytic, electrocatalytic, photocatalytic, and photoelectrocatalytic water oxidation reaction, which have been reported since pioneering report of Nocera in 2008 (Kanan Matthew and Nocera Daniel in Science 321:1072-1075, 2008).

Investigation of electrocatalytic activity of a new mononuclear Mn(II) complex for water oxidation in alkaline media

Water splitting is a promising way to alleviate the energy crisis. In nature, water oxidation is done by a tetranuclear manganese cluster in photosystem II. Therefore, the study of water oxidation by Mn complexes is attractive in water splitting systems. In this report, a new mononuclear Mn(II) complex, MnL₂ (HL = (E)-3-hydroxy-N'-(pyridin-2-ylmethylene)-2-naphthohydrazide) was prepared and characterized by spectroscopic techniques and single-crystal X-ray diffraction. Crystallographic analysis indicated that the geometry around the Mn(II) ion is distorted octahedral. The MnN₄O₂ coordination moiety is achieved by bounding of oxygen and two nitrogen donor atoms of two hydrazone ligands. The synthesized complex was also investigated for electrochemical water oxidation using electrochemical techniques, scanning electron microscopy, energy dispersive spectrometry, and PXRD analysis. Linear sweep voltammetry experiment showed that the modified carbon paste electrode by the complex displays high activity for water oxidation reaction with an overpotential of 565 mV at a current density of 10 mA cm^-2 and Tafel slope of 105 mV dec^-1 in an alkaline solution. It was found that the complex structure finally changes during the reaction and converts to Mn oxide nanoparticles which act as active catalytic species and oxidize the water.

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.

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.

Soft x-ray signatures of ionic manganese-oxo systems, including a high-spin manganese(V) complex

Manganese-oxo species catalyze key reactions, including C–H bond activation or dioxygen formation in natural photosynthesis. To better understand relevant reaction intermediates, we characterize electronic states and geometric structures of [MnOn]+...

Homogeneous or heterogeneous electrocatalysis: reinvestigation of a cobalt coordination compound for water oxidation

A cobalt coordination compound with azo-ligand linkers combined with linked bisulfonate moieties has been argued to be an efficient catalyst for the oxygen-evolution reaction (OER) (H.-T. Shi, X.-X. Li, F.-H. Wu and W.-B. Yu, Dalton Trans., 2017, 46, 16321.). In the previously published report, this cobalt compound (compound 1) was believed to display a high turnover frequency (5 s^-1) at η = 720 mV at pH 9. Herein, the OER in the presence of compound 1 is reinvestigated. The nanosized oxide-based particles formed after the OER in the presence of compound 1 were tracked by electrochemical methods, scanning electron microscopy (SEM), energy dispersive spectrometry (EDX), X-ray diffraction studies (XRD), (High-resolution) transmission electron microscopy ((HR)TEM), Raman spectroscopy, X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS). Based on these experiments, it is proposed that a candidate for the true catalyst of the OER in the presence of compound 1 is cobalt oxide. During the OER and using chronoamperometry, the oxidation state of Co ions for the formed Co oxide is (III), but after consecutive CVs the oxidation states of Co ions for the formed Co oxide are (II) and (III). The results shed new light on the role of Co oxide nanoparticles formed in the presence of this Co coordination compound during the OER. Our experimental data also show that for the OER in the presence of a homogeneous (pre)catalyst, careful analyses to find the role of metal oxides are necessary for informed progress. The present findings also might help to find the mechanism of the OER in the presence of coordination compounds.

Voltage-Driven Molecular Catalysis of Electrochemical Reactions

Heterogeneous electrocatalysis and molecular redox catalysis have developed over several decades as two distinct ways to facilitate charge-transfer processes essential for energy conversion and storage. Whereas electrocatalytic reactions are driven by the applied voltage, molecular catalytic processes are driven by the difference between standard potentials of the catalyst and the reactant. Here, we demonstrate that the rate of electron transfer between a dissolved reactant and a molecular catalyst immobilized directly on the surface of a carbon nanoelectrode is governed by combination of chemical driving force and electrostatic potential drop across the double layer. DFT calculations show that varying the applied voltage alters the potential drop between the surface-bound and dissolved redox species. These results suggest a new route for designing next-generation hybrid molecular/electrocatalysts.

Design of molecular water oxidation catalysts with earth-abundant metal ions

The four-electron oxidation of water (2H2O → O2 + 4H+ + 4e-) is considered the main bottleneck in artificial photosynthesis. In nature, this reaction is catalysed by a Mn4CaO5 cluster embedded in the oxygen-evolving complex of photosystem II. Ruthenium-based complexes have been successful artificial molecular catalysts for mimicking this reaction. However, for practical and large-scale applications in the future, molecular catalysts that contain earth-abundant first-row transition metal ions are preferred owing to their high natural abundance, low risk of depletion, and low costs. In this review, the frontier of water oxidation reactions mediated by first-row transition metal complexes is described. Special attention is paid towards the design of molecular structures of the catalysts and their reaction mechanisms, and these factors are expected to serve as guiding principles for creating efficient and robust molecular catalysts for water oxidation using ubiquitous elements.

Ultra-small and highly dispersive iron oxide hydroxide as an efficient catalyst for oxidation reactions: a Swiss-army-knife catalyst

Ultra-small and highly dispersive (< 10 nm) iron oxide hydroxide is characterized by some methods. The compound is an efficient and stable catalyst for alcohol oxidation, organic sulfide oxidation, and epoxidation of alkenes in the presence of H₂O₂. The electrochemical oxygen-evolution reaction of the iron oxide hydroxide is also tested under acidic, neutral, and alkaline conditions. In the presence of the iron oxide hydroxide, excellent conversions (75–100%) and selectivities of substrates (92–97%), depending on the nature of the sulfide, were obtained. Benzylalcohols having electron-donating and-withdrawing substituents in the aromatic ring were oxidized to produce the corresponding aldehydes with excellent conversion (65–89%) and selectivity (96–100%) using this iron oxide hydroxide. The conversion of styrene and cyclooctene toward the epoxidation in the presence of this catalyst are 60 and 53%, respectively. Water oxidation for the catalysts was investigated at pH 2, 6.7, 12, and 14. The onset of OER at pH 14 is observed with a 475 mV overpotential. At 585 mV overpotential, a current density of more than 0.18 mA/cm² and a turnover frequency of 1.5/h is observed. Operando high-resolution visible spectroscopy at pH 14, similar to previously reported investigations, shows that Fe(IV)=O is an intermediate for water oxidation.

A synthetic manganese-calcium cluster similar to the catalyst of Photosystem II: challenges for biomimetic water oxidation

Herein, we report the synthesis, characterization, crystal structure, density functional theory calculations, and water-oxidizing activity of a pivalate Mn-Ca cluster. All of the manganese atoms in the cluster are Mn(iv) ions and have a distorted MnO6 octahedral geometry. Three Mn(iv) ions together with a Ca(ii) ion and four-oxido groups form a cubic Mn3CaO4 unit which is similar to the Mn3CaO4 cluster in the water-oxidizing complex of Photosystem II. Using scanning electron microscopy, transmission electron microscopy, energy dispersive spectrometry, extended X-ray absorption spectroscopy, chronoamperometry, and electrochemical methods, a conversion into nano-sized Mn-oxide is observed for the cluster in the water-oxidation reaction.

Core-shell nanostructured electrocatalysts for water splitting

As the cornerstone of the hydrogen economy, water electrolysis consisting of the hydrogen and oxygen evolution reactions (HER and OER) greatly needs cost-efficient electrocatalysts that can decrease the dynamic overpotential and save on energy consumption. Over past years, observable progress has been made by constructing core-shell structures free from or with few noble-metals. They afford particular merits, e.g., a highly-exposed active surface, modulated electronic configurations, strain effects, interfacial synergy, or reinforced stability, to promote the kinetics and electrocatalytic performance of the HER, OER and overall water splitting. So far, a large variety of inorganics (carbon and transition-metal related components) have been introduced into core-shell electrocatalysts. Herein, representative efforts and progress are summarized with a clear classification of core and shell components, to access comprehensive insights into electrochemical processes that proceed on surfaces or interfaces. Finally, a perspective on the future development of core-shell electrocatalysts is offered. The overall aim is to shed some light on the exploration of emerging materials for energy conversion and storage.

New findings and current controversies in the reaction of ruthenium red and ammonium cerium(iv) nitrate: focus on the precipitated compound

During water-oxidation reaction in the presence of RuR and CAN, a heterogeneous nano-sized Ru-Ce compound is detected, which is formed by the interaction of [(NH₃)₅RuORu(NH₃)₄ORu(NH₃)₅],^6+/7+ nitrate ions, and the products of the reduction of CAN.

Unique Ni Crystalline Core/Ni Phosphide Amorphous Shell Heterostructured Electrocatalyst for Hydrazine Oxidation Reaction of Fuel Cells

It is highly attractive but challenging to develop transition-metal electrocatalysts for direct hydrazine fuel cells (DHzFCs). In this work, a nickel crystalline core@nickel phosphide amorphous shell heterostructured electrocatalyst supported by active carbon (Ni@NiP/C) is developed. Ni@NiP/C with a P/Ni molar ratio of 3:100, Ni@NiP_3.0/C, exhibits outstanding catalytic activity for the hydrazine oxidation reaction (HzOR) in alkaline solution, achieving a much better catalytic activity (2675.1 A g_Ni^-1@0.25 V vs RHE) and high stability, as compared to Ni nanoparticles supported on carbon (Ni/C) and Pt/C catalysts. The results indicate that formation of the NiP amorphous shell effectively inhibits the passivation of the Ni core active sites and enhances the adsorption of hydrazine on Ni by improving the adsorption energy, leading to high electrochemical activity and stability of the Ni@NiP_3.0/C catalysts for HzOR. The density functional theory calculation confirms the structural and electrocatalytic effect of the core-shell heterostructure on the stability and activity of Ni active sites for HzOR. The unique crystalline core/amorphous shell-structured Ni@NiP/C demonstrates promising potential as an effective electrocatalyst for DHzFCs.

Electrochemical water oxidation by simple manganese salts

Recently, it has been great efforts to synthesize an efficient water-oxidizing catalyst. However, to find the true catalyst in the harsh conditions of the water-oxidation reaction is an open area in science. Herein, we showed that corrosion of some simple manganese salts, MnCO3, MnWO4, Mn3(PO4)2 · 3H2O, and Mn(VO3)2 · xH2O, under the water-electrolysis conditions at pH = 6.3, gives an amorphous manganese oxide. This conversion was studied with X-ray absorption spectroscopy (XAS), as well as, scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDXS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), spectroelectrochemistry and electrochemistry methods. When using as a water-oxidizing catalyst, such results are important to display that long-term water oxidation can change the nature of the manganese salts.

A new decomposition mechanism for metal complexes under water-oxidation conditions

Herein, water-oxidation reaction by cobalt(II) phthalocyanine, N,N′-bis (salicylidene) ethylenediamino cobalt(II), nickel(II) Schiff base (N,N′-bis (salicylidene)ethylenediamino nickel(II), nickel(II)) phthalocyanine-tetrasulfonate tetrasodium, manganese(II) phthalocyanine, 5,10,15,20-Tetraphenyl-21H,23H-porphine manganese(III) chloride, manganese(III) 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine chloride tetrakis(methochloride) was investigated using electrochemistry, UV-vis spectroscopy and spectroelectrochemistry. According to our results, a new decomposition pathway for these metal complexes under water-oxidation conditions was proposed. The produced metal oxide obtained by decomposition of metal complex under water -oxidation conditions not only catalyzes water-oxidation reaction but this metal oxide also accelerates decomposition of the corresponding complex to form higher amounts of the metal oxide. We hypothesize that such a mechanism could be investigated for many metal complexes under different oxidation or reduction reactions.

A nickel(ii) complex under water-oxidation reaction: what is the true catalyst?

A Ni(ii) complex as a water-oxidizing catalyst under electrochemical conditions was studied and the role of Ni oxide as a true catalyst was investigated.

A manganese(ii) phthalocyanine under water-oxidation reaction: new findings

The decomposition reaction for a manganese complex under water oxidation was investigated.

Facile Formation of Nanostructured Manganese Oxide Films as High-Performance Catalysts for the Oxygen Evolution Reaction

The development of inexpensive, earth abundant, and bioinspired oxygen evolution electrocatalysts that are easily accessible and scalable is a principal requirement with regard to the feasibility of water splitting for large-scale chemical energy storage. A unique, versatile, and scalable approach has been developed to fabricate manganese oxide films from single layers to multilayers with a controlled thickness and high reproducibility. The produced MnO_x films are composed of small nanostructures that are assembled closely in the form of porous sponge-like layers. The films were investigated for the electrochemical oxygen evolution reaction in alkaline media and demonstrate a remarkable activity as well as a superior stability of over 60 h. To elucidate the catalytically active species, as well as the striking structural characteristics, the films were further examined in depth by using SEM, TEM, and X-ray photoelectron spectroscopy, as well as quasi in situ extended X-ray absorption fine structure and X-ray absorption near edge structure analysis. The MnO_x catalyst films excel because of a favorably high fraction of Mn³⁺ ions that are retained even after operation at oxidizing potentials. Upon exposure to oxidizing potentials in strongly alkaline aqueous electrolyte, the catalyst material maintains its structural integrity at the nanostructural, morphological, and atomic level.

Nano-Sized Inorganic Energy-Materials by the Low-Temperature Molecular Precursor Approach

The low-temperature synthesis of inorganic materials and their interfaces at the atomic and molecular level provides numerous opportunities for the design and improvement of inorganic materials in heterogeneous catalysis for sustainable chemical energy conversion or other energy-saving areas. Using suitable molecular precursors for functional inorganic nanomaterial synthesis allows for facile control over uniform particle size distribution, stoichiometry, and leads to desired chemical and physical properties. This Minireview outlines some advantages of the molecular precursor approach in light of selected recent developments of molecule-to-nanomaterials synthesis for renewable energy applications, relevant for the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water-splitting.

Recent advances in the field of light-driven water oxidation catalyzed by transition-metal substituted polyoxometalates

Transition-metal (TM) substituted polyoxometalates (POMs) have been demonstrated to be excellent water oxidation catalysts (WOCs) under visible light due to their unique structural features, tunable band gap structures and high stability. Herein, we present a brief review on recent developments in the field of TM substituted POM water oxidation catalysts (TSPWOCs) including both homogeneous and heterogeneous TSPWOCs. We systematically highlight the composition and structure of these different materials for higher photochemical performances and summarize recent advancements, which will provide readers valuable guidance for further discovery and improvements in efficient and robust TSPWOCs in future.

Toward Escherichia coli bacteria machine for water oxidation

Nature uses a Mn oxide-based catalyst for water oxidation in plants, algae, and cyanobacteria. Mn oxides are among major candidates to be used as water-oxidizing catalysts. Herein, we used two straightforward and promising methods to form Escherichia coli bacteria/Mn oxide compounds. In one of the methods, the bacteria template was intact after the reaction. The catalysts were characterized by X-ray photoelectron spectroscopy, visible spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy, diffuse reflectance infrared Fourier transform spectroscopy, Raman spectroscopy, and X-ray diffraction spectrometry. Electrochemical properties of the catalysts were studied, and attributed redox potentials were assigned. The water oxidation of the compounds was examined under electrochemical condition. Linear sweep voltammetry showed that the onsets of water oxidation in our experimental condition for bacteria and Escherichia coli bacteria/Mn oxide were 1.68 and 1.56 V versus the normal hydrogen electrode (NHE), respectively. Thus, the presence of Mn oxide in the catalyst significantly decreased (~ 120 mV) the overpotential needed for water oxidation.

Monodisperse Ultrasmall Manganese-Doped Multimetallic Oxysulfide Nanoparticles as Highly Efficient Oxygen Reduction Electrocatalyst

The highly efficient and cheap non-Pt-based electrocatalysts such as transition-based catalysts prepared via facile methods for oxygen reduction reaction (ORR) are desirable for large-scale practical industry applications in energy conversion and storage systems. Herein, we report a straightforward top-down synthesis of monodisperse ultrasmall manganese-doped multimetallic (ZnGe) oxysulfide nanoparticles (NPs) as an efficient ORR electrocatalyst by simple ultrasonic treatment of the Mn-doped Zn-Ge-S chalcogenidometalate crystal precursors in H₂O/EtOH for only 1 h at room temperature. Thus obtained ultrasmall monodisperse Mn-doped oxysulfide NPs with ultralow Mn loading level (3.92 wt %) not only exhibit comparable onset and half-wave potential (0.92 and 0.86 V vs reversible hydrogen electrode, respectively) to the commercial 20 wt % Pt/C but also exceptionally high metal mass activity (189 mA/mg at 0.8 V) and good methanol tolerance. A combination of transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and electrochemical analysis demonstrated that the homogenous distribution of a large amount of Mn(III) on the surface of NPs mainly accounts for the high ORR activity. We believe that this simple synthesis of Mn-doped multimetallic (ZnGe) oxysulfide NPs derived from chalcogenidometalates will open a new route to explore the utilization of discrete-cluster-based chalcogenidometalates as novel non-Pt electrocatalysts for energy applications and provide a facile way to realize the effective reduction of the amount of catalyst while keeping desired catalytic performances.

Electrocatalytic water oxidation by a molecular catalyst incorporated into a metal-organic framework thin film

A molecular water oxidation catalyst, [Ru(tpy)(dcbpy)(OH₂)](ClO₄)₂ (tpy = 2,2':6',2''-terpyridine, dcbpy = 2,2'-bipyridine-5,5'-dicarboxylic acid) [1], has been incorporated into FTO-grown thin films of UiO-67 (UiO = University of Oslo), by post-synthetic ligand exchange. Cyclic voltammograms (0.1 M borate buffer at pH = 8.4) of the resulting UiO67-[RuOH₂]@FTO show a reversible wave associated with the Ru^III/II couple in the anodic scan, followed by a large current response that arises from electrocatalytic water oxidation beyond 1.1 V vs. Ag/AgCl. Water oxidation can be observed at an applied potential of 1.5 V over the timescale of hours with a current density of 11.5 μA cm^-2. Oxygen evolution was quantified in situ over the course of the experiment, and the Faradaic efficiency was calculated as 82%. Importantly, the molecular integrity of [1] during electrocatalytic water oxidation is maintained even on the timescale of hours under turnover conditions and applied voltage, as evidenced by the persistence of the wave associated with the Ru^III/II couple in the CV. This experiment highlights the capability of metal organic frameworks like UiO-67 to stabilize the molecular structure of catalysts that are prone to form higher clusters in homogenous phase.

Water oxidation by a copper(ii) complex: new findings, questions, challenges and a new hypothesis

We propose that an uncomplexed Cu(ii) ion or oxide is a candidate as a contributor to the observed catalysis in the presence of a Cu(ii) complex under water oxidation.

Water oxidation by a manganese–potassium cluster: Mn oxide as a kinetically dominant “true” catalyst for water oxidation

Herein, a manganese–potassium cluster was investigated for electrochemical water oxidation to find the true, kinetically dominant, catalyst.

Nanosized silver bromide: an efficient catalyst for alcohol oxidation in the presence of a multinuclear silver complex

We detected AgBr as a true catalyst for alcohol oxidation in the presence of a silver complex.

Water oxidation by simple manganese salts in the presence of cerium(iv) ammonium nitrate: towards a complete picture

For the first time, using some methods, we showed that under the water-oxidation conditions and in the presence of cerium(iv) ammonium nitrate, some manganese salts are converted to Mn oxide.

Water oxidation catalyzed by two cobalt complexes: new challenges and questions

Herein, two cobalt complexes as true catalysts for water oxidation were investigated.

The application of a nickel(ii) Schiff base complex in water oxidation: the importance of nanosized materials

The role of Ni oxide in the electrocatalytic water oxidation of a nickel(ii) Schiff base (N,N′-bis (salicylidene) ethylenediamino nickel(ii)) is investigated.

A mononuclear cobalt complex for water oxidation: new controversies and puzzles

Herein the role of a mononuclear cobalt(iii) complex, [Co^III(DPKOH)₂]ClO₄ (DPK = di(2-pyridyl)ketone), in the water electrooxidation process is reinvestigated.

Water oxidation by Ni(1,4,8,11-tetraazacyclotetradecane)2+ in the presence of carbonate: new findings and an alternative mechanism

The role of nanosized particles or Ni ions on the surface of the electrode under water-oxidation conditions in the presence of Ni(1,4,8,11-tetraazacyclotetradecane)²⁺ was investigated.

A DFT/B3LYP study of the mechanisms of the O2 formation reaction catalyzed by the [(terpy)(H2O)MnIII(O)2MnIV(OH2)(terpy)](NO3)3 complex: A paradigm for photosystem II

We present a theoretical study of the reaction pathway for dioxygen molecular formation catalyzed by the [(terpy)(H₂O)Mn^III(O)₂Mn^IV(OH₂) (terpy)](NO₃)₃ (terpy=2,2':6',2″-terpyridine) complex based on DFT-B3LYP calculations. In the initial state of the reaction, a partial oxido radical (0.44 spins) is formed ligated to Mn. This radical is involved in a nucleophylic attack by bulk water in the OO bond reaction formation step, in which the oxido fractional unpaired electron is delocalized toward the outermost Mn of the μ-oxo bridge, instead of the ligated Mn center. The reaction then follows with a series of proton-coupled electron transfer steps, in which the oxidation state, as well as the bond strength of the OO moiety increase, while the OOMn(1) bond gets weaker until O₂ is released. In this model, basic acetate ions from the buffer solution capture protons in the proton-transfer steps. In each step there is reduction of the OOMn(1) binding strength, with concomitant increase of the OO bond strength, which culminates with the release of O₂ in the last step. This last step is entropy driven, while formation of hydroperoxide and superoxide moieties is enthalpy driven. According with experiments, the rate-limiting step is the double oxidation of Mn(IV,III) or peroxymonosulfate binding, which occur prior to the OO bond formation step. This supports our findings that the barriers of all intermediate steps are below the experimental barrier of 19-21kcal/mol. The implications of these findings for understanding photosynthetic water-splitting catalysis are also discussed.

Nanosized manganese oxide/holmium oxide: a new composite for water oxidation

Ho₂O₃ as a support for nanosized Mn oxide was used for the synthesis of a new water-oxidizing catalyst.

Earth-Abundant Heterogeneous Water Oxidation Catalysts

Water oxidation is a key chemical transformation for the conversion of solar energy into chemical fuels. Our review focuses on recent work on robust earth-abundant heterogeneous catalysts for the oxygen-evolving reaction (OER). We point out that improvements in the performance of OER catalysts will depend critically on the success of work aimed at understanding reaction barriers based on atomic-level mechanisms. We highlight the challenge of obtaining acid-stable OER catalysts, with proposals for elements that could be employed to reach this goal. We suggest that future advances in solar fuels science will be accelerated by the development of new methods for materials synthesis and characterization, along with in-depth investigations of redox mechanisms at catalytic surfaces.