Water oxidation by manganese oxides formed from tetranuclear precursor complexes: the influence of phosphate on structure and activity

Electrochemical Water Oxidation and CO2 Reduction with a Nickel Molecular Catalyst

Mimicking the photosynthesis of green plants to combine water oxidation with CO2 reduction is of great significance for solving energy and environmental crises. In this context, a trinuclear nickel complex, [NiII3(paoH)6(PhPO3)2]·2ClO4 (1), with a novel structure has been constructed with PhPO32- (phenylphosphonate) and paoH (2-pyridine formaldehyde oxime) ligands and possesses a reflection symmetry with a mirror plane revealed by single-crystal X-ray diffraction. Bulk electrocatalysis demonstrates that complex 1 can homogeneously catalyze water oxidation and CO2 reduction simultaneously. It can catalyze water oxidation at a near-neutral condition of pH = 7.45 with a high TOF of 12.2 s-1, and the Faraday efficiency is as high as 95%. Meanwhile, it also exhibits high electrocatalytic activity for CO2 reduction towards CO with a TOF of 7.84 s-1 in DMF solution. The excellent electrocatalytic performance of the water oxidation and CO2 reduction of complex 1 could be attributed to the two unique µ3-PhPO32- bridges as the crucial factor for stabilizing the trinuclear molecule as well as the proton transformation during the catalytic process, while the oxime groups modulate the electronic structure of the metal centers via π back-bonding. Therefore, apart from the cooperation effect of the three Ni centers for catalysis, simultaneously, the two kinds of ligands in complex 1 can also synergistically coordinate the central metal, thereby significantly promoting its catalytic performance. Complex 1 represents the first nickel molecular electrocatalyst for both water oxidation and CO2 reduction. The findings in this work open an avenue for designing efficient molecular electrocatalysts with peculiar ligands.

Water oxidation reaction in the presence of a dinuclear Mn(II)-semicarbohydrazone coordination compound

Water splitting, producing of oxygen, and hydrogen molecules, is an essential reaction for clean energy resources and is one of the challenging reactions for artificial photosynthesis. The Mn₄Ca cluster in photosystem II (PS-II) is responsible for water oxidation in natural photosynthesis. Due to this, water oxidation reaction by Mn coordination compounds is vital for mimicking the active core of the oxygen-evolving complex in PS-II. Here, a new dinuclear Mn(II)-semicarbohydrazone coordination compound, [Mn(HL)(µ-N₃)Cl]₂ (1), was synthesized and characterized by various methods. The structure of compound 1 was determined by single crystal X-ray analysis, which revealed the Mn(II) ions have distorted octahedral geometry as (MnN₄OCl). This geometry is created by coordinating of oxygen and two nitrogen donor atoms from semicarbohydrazone ligand, two nitrogen atoms from azide bridges, and chloride anion. Compound 1 was used as a catalyst for electrochemical water oxidation, and the surface of the electrode after the reaction was investigated by scanning electron microscopy, energy dispersive spectrometry, and powder X-ray diffraction analyses. Linear sweep voltammetry (LSV) experiments revealed that the electrode containing 1 shows high activity for chemical water oxidation with an electrochemical overpotential as low as 377 mV. Although our findings showed that the carbon paste electrode in the presence of 1 is an efficient electrode for water oxidation, it could not withstand water oxidation catalysis under bulk electrolysis and finally converted to Mn oxide nanoparticles which were active for water oxidation along with compound 1.

Regulating the Coordination of Co sites in Co3 O4 /MnO2 Compounding for Facilitated Oxygen Reduction Reaction

Binary transition metal oxides as a promising oxygen reduction reaction (ORR) catalyst have received significant attention. However, their exact reaction mechanisms are often too complex to be discussed. Herein, novel Co-Mn composites with a well-defined nanostructure were developed for understanding the role of each component. The growth pattern of cobalt oxide and the effects of the coordination environment of Co sites during growth on the overall activity were investigated. Based on experimental and density functional theory studies, it was found that the decaying coordination number directly affected the expression of crystal planes of cobalt oxide, which further had a great influence upon limiting current density of Co-Mn catalysts. The cuboid-Co/Mn catalyst exhibited outstanding limiting current density and showed good stability, related to more highly active (110) planes exposed in Co₃ O₄ . These provided many references for the preparation of related nonprecious catalysts in various domains.

Light-driven formation of manganese oxide by today's photosystem II supports evolutionarily ancient manganese-oxidizing photosynthesis

Water oxidation and concomitant dioxygen formation by the manganese-calcium cluster of oxygenic photosynthesis has shaped the biosphere, atmosphere, and geosphere. It has been hypothesized that at an early stage of evolution, before photosynthetic water oxidation became prominent, light-driven formation of manganese oxides from dissolved Mn(2+) ions may have played a key role in bioenergetics and possibly facilitated early geological manganese deposits. Here we report the biochemical evidence for the ability of photosystems to form extended manganese oxide particles. The photochemical redox processes in spinach photosystem-II particles devoid of the manganese-calcium cluster are tracked by visible-light and X-ray spectroscopy. Oxidation of dissolved manganese ions results in high-valent Mn(III,IV)-oxide nanoparticles of the birnessite type bound to photosystem II, with 50-100 manganese ions per photosystem. Having shown that even today’s photosystem II can form birnessite-type oxide particles efficiently, we propose an evolutionary scenario, which involves manganese-oxide production by ancestral photosystems, later followed by down-sizing of protein-bound manganese-oxide nanoparticles to finally yield today’s catalyst of photosynthetic water oxidation.

Photosynthetic formation of manganese (Mn) oxides from dissolved Mn ions was proposed to occur in ancestral photosystems before oxygenic photosynthesis evolved. Here, the authors provide evidence for this hypothesis by showing that photosystem II devoid of the Mn cluster oxidises Mn ions leading to formation of Mn-oxide nanoparticles.

Water-Oxidation Electrocatalysis by Manganese Oxides: Syntheses, Electrode Preparations, Electrolytes and Two Fundamental Questions

The efficient catalysis of the four-electron oxidation of water to molecular oxygen is a central challenge for the development of devices for the production of solar fuels. This is equally true for artificial leaf-type structures and electrolyzer systems. Inspired by the oxygen evolving complex of Photosystem II, the biological catalyst for this reaction, scientists around the globe have investigated the possibility to use manganese oxides (“MnOx”) for this task. This perspective article will look at selected examples from the last about 10 years of research in this field. At first, three aspects are addressed in detail which have emerged as crucial for the development of efficient electrocatalysts for the anodic oxygen evolution reaction (OER): (1) the structure and composition of the “MnOx” is of central importance for catalytic performance and it seems that amorphous, MnIII/IV oxides with layered or tunnelled structures are especially good choices; (2) the type of support material (e.g. conducting oxides or nanostructured carbon) as well as the methods used to immobilize the MnOx catalysts on them greatly influence OER overpotentials, current densities and long-term stabilities of the electrodes and (3) when operating MnOx-based water-oxidizing anodes in electrolyzers, it has often been observed that the electrocatalytic performance is also largely dependent on the electrolyte’s composition and pH and that a number of equilibria accompany the catalytic process, resulting in “adaptive changes” of the MnOx material over time. Overall, it thus has become clear over the last years that efficient and stable water-oxidation electrolysis by manganese oxides can only be achieved if at least four parameters are optimized in combination: the oxide catalyst itself, the immobilization method, the catalyst support and last but not least the composition of the electrolyte. Furthermore, these parameters are not only important for the electrode optimization process alone but must also be considered if different electrode types are to be compared with each other or with literature values from literature. Because, as without their consideration it is almost impossible to draw the right scientific conclusions. On the other hand, it currently seems unlikely that even carefully optimized MnOx anodes will ever reach the superb OER rates observed for iridium, ruthenium or nickel-iron oxide anodes in acidic or alkaline solutions, respectively. So at the end of the article, two fundamental questions will be addressed: (1) are there technical applications where MnOx materials could actually be the first choice as OER electrocatalysts? and (2) do the results from the last decade of intensive research in this field help to solve a puzzle already formulated in 2008: “Why did nature choose manganese to make oxygen?”.

Electrocatalytic water oxidation studies of a tetranuclear Cu(ii) complex with cubane-like core Cu4(μ3-O)4

A bio-inspired cubane-like tetranuclear cluster [Cu₄(pdmH)₄(OAc)₂](NO₃)₂·3H₂O can electrocatalyze water oxidation under aqueous alkaline conditions through a PCET process.

Oxidative Deposition of Manganese Oxide Nanosheets on Nitrogen-Functionalized Carbon Nanotubes Applied in the Alkaline Oxygen Evolution Reaction

The development of nonprecious catalysts for water splitting into hydrogen and oxygen is one of the major challenges to meet future sustainable fuel demand. Herein, thin layers of manganese oxide nanosheets supported on nitrogen-functionalized carbon nanotubes (NCNTs) were formed by the treatment of NCNTs dispersed in aqueous solutions of KMnO₄ or CsMnO₄ under reflux or under hydrothermal (HT) conditions and used as electrocatalysts for the oxygen evolution reaction (OER) in alkaline media. The samples were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Our results show that the NCNTs treated under reflux were covered by partly amorphous and birnessite-type manganese oxides, while predominantly crystalline birnessite manganese oxide was observed for the hydrothermally treated samples. The latter showed, depending on the temperature during synthesis, an electrocatalytically favorable reduction from birnessite-type MnO₂ to γ-MnOOH. OER activity measurements revealed a decrease of the overpotential for the OER at a current density of 10 mA cm^-2 from 1.70 V_RHE for the bare NCNTs to 1.64 V_RHE for the samples treated under reflux in the presence of KMnO₄. The hydrothermally treated samples afforded the same current density at a lower potential of 1.60 V_RHE and a Tafel slope of 75 mV dec^-1, suggesting that the higher OER activity is due to γ-MnOOH formation. Oxidative deposition under reflux conditions using CsMnO₄ along with mild HT treatment using KMnO₄, and low manganese loadings in both cases, were identified as the most suitable synthetic routes to obtain highly active MnO _x /NCNT catalysts for electrochemical water oxidation.

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.

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.

Size-Dependent Reactivity of Nano-Sized Neutral Manganese Oxide Clusters toward Ethylene

Neutral manganese oxide clusters with the general composition Mn_2 N O_3 N+x (N=2-22; x=-1, 0, 1) with dimensions up to a nanosize were prepared by laser ablation and reacted with C₂ H₄ in a fast flow reactor. The size-dependent reactivity of C₂ H₄ adsorption on these clusters was experimentally identified and the adsorption reactivity decreases generally with an increase of the cluster size. Density functional theory calculations were performed to study the geometrical and electronic structures of the Mn_2 N O_3 N (N=1-6) clusters. The calculated results indicated that the coordination number and the charge distribution of the metal centers are responsible for the experimentally observed size-dependent reactivity. The highly charged Mn atoms with low coordination are preferential to adsorb C₂ H₄ . In contrast, the neutral manganese oxide clusters are completely inert toward the saturated hydrocarbon molecule C₂ H₆ . This work provides new perspectives to design related materials in the separation of hydrocarbon molecules.

Pt/MnO2nanosheets: facile synthesis and highly efficient catalyst for ethylene oxidation at low temperature

The obtained catalyst Pt/MnO₂-48 h demonstrated complete removal of 20 ppm C₂H₄at 50 °C for at least 12 h due to the large amount of adsorbed oxygen species and synergetic catalytic effect between Pt and the MnO₂support.

δ-MnO2-Mn3O4 Nanocomposite for Photochemical Water Oxidation: Active Structure Stabilized in the Interface

Pure phase manganese oxides have been widely studied as water oxidation catalysts, but further improvement of their activities is much challenging. Herein, we report an effective method to improve the water oxidation activity by fabricating a nanocomposite of Mn₃O₄ and δ-MnO₂ with an active interface. The nanocomposite was achieved by a partial reduction approach which induced an in situ growth of Mn₃O₄ nanoparticles from the surface of δ-MnO₂ nanosheets. The optimum composition was determined to be 38% Mn₃O₄ and 62% δ-MnO₂ as confirmed by X-ray photoelectron spectra (XPS) and X-ray absorption spectra (XAS). The δ-MnO₂-Mn₃O₄ nanocomposite is a highly active water oxidation catalyst with a turnover frequency (TOF) of 0.93 s^-1, which is much higher than the individual components of δ-MnO₂ and Mn₃O₄. We consider that the enhanced water oxidation activity could be explained by the active interface between two components. At the phase interface, weak Mn-O bonds are introduced by lattice disorder in the transition of hausmannite phase to birnessite phase, which provides active sites for water oxidation catalysis. Our study illustrates a new view to improve water oxidation activity of manganese oxides.

Electrosynthesis of Biomimetic Manganese-Calcium Oxides for Water Oxidation Catalysis--Atomic Structure and Functionality

Water-oxidizing calcium-manganese oxides, which mimic the inorganic core of the biological catalyst, were synthesized and structurally characterized by X-ray absorption spectroscopy at the manganese and calcium K edges. The amorphous, birnesite-type oxides are obtained through a simple protocol that involves electrodeposition followed by active-site creation through annealing at moderate temperatures. Calcium ions are inessential, but tune the electrocatalytic properties. For increasing calcium/manganese molar ratios, both Tafel slopes and exchange current densities decrease gradually, resulting in optimal catalytic performance at calcium/manganese molar ratios of close to 10 %. Tracking UV/Vis absorption changes during electrochemical operation suggests that inactive oxides reach their highest, all-Mn(IV) oxidation state at comparably low electrode potentials. The ability to undergo redox transitions and the presence of a minor fraction of Mn(III) ions at catalytic potentials is identified as a prerequisite for catalytic activity.

Manganese Compounds as Water-Oxidizing Catalysts: From the Natural Water-Oxidizing Complex to Nanosized Manganese Oxide Structures

All cyanobacteria, algae, and plants use a similar water-oxidizing catalyst for water oxidation. This catalyst is housed in Photosystem II, a membrane-protein complex that functions as a light-driven water oxidase in oxygenic photosynthesis. Water oxidation is also an important reaction in artificial photosynthesis because it has the potential to provide cheap electrons from water for hydrogen production or for the reduction of carbon dioxide on an industrial scale. The water-oxidizing complex of Photosystem II is a Mn-Ca cluster that oxidizes water with a low overpotential and high turnover frequency number of up to 25-90 molecules of O2 released per second. In this Review, we discuss the atomic structure of the Mn-Ca cluster of the Photosystem II water-oxidizing complex from the viewpoint that the underlying mechanism can be informative when designing artificial water-oxidizing catalysts. This is followed by consideration of functional Mn-based model complexes for water oxidation and the issue of Mn complexes decomposing to Mn oxide. We then provide a detailed assessment of the chemistry of Mn oxides by considering how their bulk and nanoscale properties contribute to their effectiveness as water-oxidizing catalysts.

Electrochemical deposition of manganese oxide–phosphate–reduced graphene oxide composite and electrocatalysis of the oxygen evolution reaction

The MnO_x–Pi–rGO OER catalyst is prepared by the simultaneous electrochemical reduction of MnO₄⁻ ions and GO in a neutral phosphate electrolyte.

Enhancement Effect of Noble Metals on Manganese Oxide for the Oxygen Evolution Reaction

Developing improved catalysts for the oxygen evolution reaction (OER) is key to the advancement of a number of renewable energy technologies, including solar fuels production and metal air batteries. In this study, we employ electrochemical methods and synchrotron techniques to systematically investigate interactions between metal oxides and noble metals that lead to enhanced OER catalysis for water oxidation. In particular, we synthesize porous MnOx films together with nanoparticles of Au, Pd, Pt, or Ag and observe significant improvement in activity for the combined catalysts. Soft X-ray absorption spectroscopy (XAS) shows that increased activity correlates with increased Mn oxidation states to 4+ under OER conditions compared to bare MnOx, which exhibits minimal OER current and remains in a 3+ oxidation state. Thickness studies of bare MnOx films and of MnOx films deposited on Au nanoparticles reveal trends suggesting that the enhancement in activity arises from interfacial sites between Au and MnOx.

Uncovering structure-activity relationships in manganese-oxide-based heterogeneous catalysts for efficient water oxidation

Artificial photosynthesis by harvesting solar light into chemical energy could solve the problems of energy conversion and storage in a sustainable way. In nature, CO2 and H2 O are transformed into carbohydrates by photosynthesis to store the solar energy in chemical bonds and water is oxidized to O2 in the oxygen-evolving center (OEC) of photosystem II (PS II). The OEC contains CaMn4 O5 cluster in which the metals are interconnected through oxido bridges. Inspired by biological systems, manganese-oxide-based catalysts have been synthesized and explored for water oxidation. Structural, functional modeling, and design of the materials have prevailed over the years to achieve an effective and stable catalyst system for water oxidation. Structural flexibility with eg(1) configuration of Mn(III) , mixed valency in manganese, and higher surface area are the main requirements to attain higher efficiency. This Minireview discusses the most recent progress in heterogeneous manganese-oxide-based catalysts for efficient chemical, photochemical, and electrochemical water oxidation as well as the structural requirements for the catalyst to perform actively.

Cerium(IV)-driven water oxidation catalyzed by a manganese(V)-nitrido complex

The study of manganese complexes as water-oxidation catalysts (WOCs) is of great interest because they can serve as models for the oxygen-evolving complex of photosystem II. In most of the reported Mn-based WOCs, manganese exists in the oxidation states III or IV, and the catalysts generally give low turnovers, especially with one-electron oxidants such as Ce(IV) . Now, a different class of Mn-based catalysts, namely manganese(V)-nitrido complexes, were explored. The complex [Mn(V) (N)(CN)4 ](2-) turned out to be an active homogeneous WOC using (NH4 )2 [Ce(NO3 )6 ] as the terminal oxidant, with a turnover number of higher than 180 and a maximum turnover frequency of 6 min(-1) . The study suggests that active WOCs may be constructed based on the Mn(V) (N) platform.

Surfactant-mediated electrodeposition of a water-oxidizing manganese oxide

Use of sodium dodecyl sulfate (SDS) during electrodeposition improves the mechanical stability and catalytic activity of manganese dioxide for electrocatalytic water oxidation.