Water oxidation by manganese oxides

CHAPTER THREE

Water oxidation by manganese oxides Mina Tavakoliana,†, Payam Salimia,†, Zahra Zanda,†, Mohammad Mahdi Najafpoura,b,c,* a

Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, Iran Center of Climate Change and Global Warming, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, Iran c Research Center for Basic Sciences & Modern Technologies (RBST), Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, Iran *Corresponding author e-mail address: [email protected] b

Contents 1. Introduction 2. Water-oxidizing complex in photosystem II 3. Manganese oxide 3.1 First experiments 3.2 Nanostructured Mn oxide clusters supported on mesoporous silica 3.3 Mn(III) oxide with both oxygen reducing and water-oxidizing activities 3.4 λ-MnO2 3.5 Amorphous Mn oxides 3.6 Electrodeposition of Mn oxides 3.7 Atomic layer deposition MnOx/glassy carbon 3.8 Mn oxide supported on carbon nanotubes 3.9 Pure Mn oxides 3.10 Layered Mn oxides 4. Expanding a hypothesis 4.1 Mn oxides with organic compounds 4.2 Nanoscale manganese oxide within Faujasite zeolite 4.3 Manganese oxide-coated montmorillonite 4.4 Gold or silver deposited on layered manganese oxide 4.5 Induction potential 4.6 Important factors in water oxidation by Mn oxides 5. The role of redox-inert ions in layered manganese oxide 6. Manage damage: Self-healing in manganese oxide 7. Water oxidation by Mn complexes 8. The true catalyst in water oxidation

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These authors contributed equally to this work.

Advances in Inorganic Chemistry, Volume 74 ISSN 0898-8838 https://doi.org/10.1016/bs.adioch.2019.03.003

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2019 Elsevier Inc. All rights reserved.

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9. Mn oxide under acidic condition 10. Conclusions Acknowledgments References Further reading

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Abstract Sustainable energies are usually intermittent and if solar energy is to become an energy source in future, energy-storage systems are necessary. A reasonable solution to store solar energy is artificial photosynthesis with energy stored in chemical bonds such as molecular hydrogen. Among different strategies, water splitting toward hydrogen and oxygen is very promising. This article provides a short review of the current status of manganese compounds as water-oxidizing catalysts in artificial photosynthesis. Generalization and inductive reasoning have been criticized, but at least many manganese complexes and salts convert to Mn oxide during water oxidation. Thus, we focused on Mn oxides and discuss the sophisticated design strategies for manganese oxides as water-oxidizing catalysts. Since an Mn oxide-cluster acts as the biological site for water oxidation, Mn oxides are counted as structural and functional models for the wateroxidizing cluster in photosystem II.

1. Introduction The recognized limitations of fossil-based fuels, lead to the realization there is a growing need to convert electrical energy from renewable sources to store energy.1–20 Hydrogen formation by water splitting is a good strategy to store renewable energies.13–19 It is important to note that hydrogen would be the simplest fuel to make, but the protons and electrons from water oxidation could be also used to produce hydrocarbons and alcohols from carbon dioxide or ammonia from nitrogen.17 To find a very efficient water-oxidizing compound, an important strategy is to learn from the composition, structure, and chemistry of the water-oxidizing complex (WOC) of photosynthesis (Fig. 1).20

2. Water-oxidizing complex in photosystem II Natural photosynthetic system is a great form of life, and an artificial photosynthetic system is usually a device to store energy. The wateroxidizing complex of photosystem II (PSII) is a unique catalyst to oxidize water in Nature and serves as a model for developing artificial models for water oxidation.21–48

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Fig. 1 The manganese-calcium cluster in photosystem II in the oxidized form.20

The WOC is an Mn4CaO5(H2O)4 cluster in a protein environment in PSII and somehow this protein manages to assemble oxygen, protons, and water around the active site. In 2011, Shen and his co-workers described the detail of the atomic structure of the Mn4CaO5(H2O)4 cluster,37 in which four manganese ions and one calcium, are bridged by five oxygen atoms. Among the four water molecules in this structure, one or two of them are proposed as the substrates for oxygen evolution.37,41 A structural variation was considered for the WOC. Specifically, it was shown that the electron paramagnetic resonance multiline signal may derive from two different structures that differed in the position of O5.42 Recently, the details of the structure of the WOC and the water-oxidation mechanism by the biological complex were considered by the Zouni, Messinger and Yano groups.20 It is important to note that in this structure of the WOC, manganese is specific for water oxidation and no other ions could be substituted for it in the structure. However, strontium is able to play partially the role of Ca(II) in the structure.38,43–47 The WOC and other photosynthetic systems are promising to be investigated toward developing artificial photosynthesis.17,48–56

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3. Manganese oxide 3.1 First experiments Glikman and Shcheglova reported that Mn oxides are catalysts for water oxidation, in the presence of Ce(IV) as an oxidant.57 Morita et al. in 1977 used MnO2 electrochemically for water oxidation.58 The active sites for the oxygen-evolution reaction were suggested as the Mn(III) sites on the oxide surfaces.58 From the kinetic parameters, Morita et al. found that the oxidation process of the electrode surface is a rate-determining step on the massive Mn oxide electrodes.58 Shilov’s group59 reported a catalyst by dispersion of dipalmitoylDL-α-phosphatidylcholine in Mn(II) sulfate (borate buffer, pH
8.3). The catalyst contains unilamellar vesicles with strongly bound manganese(III). The yield of oxygen per oxidant molecule reaches 60–65% in the presence of [Ru(bpy)3]3+ (103 M, pH
3–4).59 Nakamura et al. found that the reaction of amines with Mn oxide effectively stabilized the Mn(III) species by the formation of NdMn bonds with Mn oxide and a
500 mV decrease of the onset potential for the water oxidation at neutral pH was observed. The overpotential is as low as the overpotential used by Nature in PSII.60 Shilov and co-workers introduced a photocatalytic system consisting of
the photosensitizer RuðbpyÞ3 2 + , an electron acceptor (manganese(IV) pyrophosphate) and MnO2 as a catalyst to generate O2 continuously from water. The oxygen yield was reported as 30% based on the reduction of the electron acceptor Mn(IV) pyrophosphate. Visible light with wavelength 452 nm was applied to transform the excited state of RuðbpyÞ3 2 + .59 In comparison with other 3d metal ions, manganese is the most effective catalyst in water oxidation in the “vesicular” system. Interestingly, in contrast to Mn(IV) oxides, RuO2 was reported to oxidize the lipids rather than water in the applied system.59 It was also indicated that Mn oxides compared to Fe, Co and Ru oxides are less active in the degradation of organic compounds.61,62 On the other hand, it was shown that calcium ion decreases the potential for Mn oxidation in an MndCa oxide compared to Mn oxide without calcium ion.62 The selectivity toward water oxidation in the presence of organic compounds was also increased in the presence of calcium.62 In other words, Nature may use manganese within the water-oxidizing complex because it specifically oxidizes water instead of amino acid side-chains. This and other factors may be important for selection of manganese and calcium for water oxidation by Nature.63,64

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Harriman᾽s group was one of the first that systematically considered metal oxides as water-oxidizing catalysts in the presence of oxidants (Ce(IV) or [Ru(bpy)3]3+).65 The group found manganese, cobalt, iridium and ruthenium as efficient catalysts for water oxidation. Among Mn oxides, Mn(III) oxide was shown to be an efficient catalyst for water oxidation.65

3.2 Nanostructured Mn oxide clusters supported on mesoporous silica Mn oxide clusters supported on mesoporous silica were studied as efficient catalysts for water oxidation.66 It was observed that the mean diameters of Mn oxides were in the range 73–86 nm and they are exclusively formed inside the silica host. Manganese K-edge X-ray absorption spectroscopy showed that materials calcined at 500, 600 and 900 °C are mainly Mn(IV), Mn(III) and Mn(II,III). The authors found that Mn oxide inside silica clusters are efficient catalysts for water oxidation at low overpotential of 350 mV (Eo([Ru(bpy)3]+3/ [Ru(bpy)3]+2) ¼ 1.23 V), Eo (O2/H2O) ¼ 0.89 V at pH 5.6).66 In addition, it was found that Mn2O3 is the most efficient catalyst toward water oxidation (TOF ¼ 3330 s1 per Mn oxide nanocluster). However, an interesting question would be the comparison of water oxidation of a solid that is heated at 900 °C with another heated in 500 or 600 °C, and the answer. High temperature not only decreases Mn-OH as active site, but also decreases the surface of the compound. The high surface area silica support is suggested to be important for the integrity of the catalyst.66 The group proposes that the silica environment stabilized Mn oxides by protecting the active manganese sites and could prevent leaching of the manganese catalyst.66

3.3 Mn(III) oxide with both oxygen reducing and water-oxidizing activities In 2010, Jaramillo and co-workers demonstrated that a nanostructured Mn(III) oxide bifunctional catalyst, yielded both oxygen reduction and water oxidation very favorably in comparison with popular precious metal nanoparticle catalysts such as Pt, Ir and Ru.67 The impressive efficiency of the catalyst was attributed to its nanostructured nature; this led to the existence of MnxOy active sites that performed the oxygen reduction reaction and the oxygen evolution reaction.67

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3.4 λ-MnO2 Dismukes’s group in 2010 prepared nano-sized λ-MnO2 by the reaction of Mn(OAc)2 and LiNO3 at 350 °C and introduced it as an efficient catalyst   +3 for water oxidation in the photochemical RuðbpyÞ3 =S2 O8 2 system at pH 5.8.68 Delithiation of LiMn2O4 by dilute HNO3 solution produced nano-sized λ-MnO2 (20–100 nm). Dismukes’s group reported the cubical Mn4O4 units in λ-MnO2 as indirectly similar to the WOC of PSII.68

3.5 Amorphous Mn oxides In another study, Dutta and Suib’s group used three forms of manganese oxides, amorphous manganese oxides (AMO), cryptomelane type tunnel manganese oxides (OMS-2) and layered birnessite (OL-1), as a catalyst for water oxidation.69 Evaluation of results obtained, showed higher turnovers for AMO (290 mmol O2/mol Mn) compared to OMS-2 (110 mmol O2/mol Mn) and layered structure OL-1 (27 mmol O2/mol Mn) to water oxidation, in the presence of Ce(IV). In addition, the AMO catalyst showed a reusable property and, after washing with water, could be used in the reaction without loss of catalytic activity. Kurz and Dau’s groups proposed that cation vacancies in AMO are important in catalytic activity due to coordinatively unsaturated oxygens that are sites for proton binding.70

3.6 Electrodeposition of Mn oxides In 2012, the group of Zaharieva and Dau,71 used an electrodeposition method to synthesize an active Mn oxide catalyst for oxidation of water. For this purpose, three protocols based on electrodeposition were tested. (1) In 0.1 M MgSO4 solution, the anode potential was changed stepwise between +1.4 V (for 29 s) and +0.25 V (for 29 s). (2) In de-ionized water, the anode potential was changed stepwise between +2.15 V (for 29 s) and 0.75 V (for 29 s). (3) In de-ionized water, the potential was cycled by continuous variation of the anode voltage between +2.15 and 0.75 V. The results indicated that among these protocols, protocol 3 (cyclic sweeping in de-ionized water) is the best choice, and for protocols 1 and 2 lower activities than 3 were reported. For further studies, a comparison between active synthesized Mn oxide using protocol 3, and inactive Mn oxide (deposited at constant potential) was carried out. Cyclic voltammograms (CVs) of electrodeposited Mn films showed for the inactive film, at least

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two redox transitions at about 0.8 and 1.15 V are visible that are attributed to MnII-MnIII and MnIII-MnIV transitions, respectively. In the active site, separated redox transitions are not readily determined. The electronic interactions of Mn sites in the amorphous material result in broadening of these redox transitions.71 As well as SEM images of two forms of Mn oxide (active and inactive), disclosed that these two materials have different morphology and the active site is composed of sand-rose like structures while the inactive form is like a fluffy network. The high performance of active Mn oxide has been attributed to the use of cycling the electrode potential during electrodeposition, and that prevents the formation of stable and unreactive MnIVO2 oxide.

3.7 Atomic layer deposition MnOx/glassy carbon In 2012, an MnOx/glassy carbon catalyst was prepared by atomic layer deposition (ALD) for both water oxidation and oxygen reduction.72 The ALD technique is a method to create conformal thin films over complex substrates. In this study, at first, ALD-MnO on glassy carbon was applied to the oxygen evolution reaction (OER) and to oxygen reduction (ORR). Although it showed a good efficiency for OER, it could not carry out the ORR process. In order to be able to perform both the ORR and OER, this research group focused on the synthesis of ALD-Mn2O3 by controlled annealing of the films of MnO. The synthesized ALD-Mn2O3 acted as a highly active catalyst for both of the OER and the ORR processes.72 In another study from 2012,73 the groups of Spiccia and MacFarlane published an electrodeposition method for preparation of a catalyst for water oxidation. In this new approach,73 the manganese oxide (MnOx) films were electrodeposited on a fluorine doped tin oxide (FTO) glass substrate at high temperature (120 °C) from an ionic liquid electrolyte (ethylammonium nitrate). A change in acidity or basicity of the electrolyte led to manganese oxide with different phases and probably mixtures of these being produced. The Mn3O4 (hausmannite, spinel structure) was obtained from the most basic electrolyte and showed poor efficiency for water oxidation.73 In contrast, the Mn2O3 and birnessite-like manganese oxide from acidic, neutral and slightly basic electrolytes, showed higher catalytic activity for water oxidation.73 This observation (high catalytic activity) has been attributed to highly porous structures of birnessite-like phase and Mn2O3.73 In fact, increasing porosity in deposited films in acidic, neutral and slightly basic electrolytes caused an increase in electrode/electrolyte contact area and easy ion/oxygen diffusion pathways from the internal catalytic sites in the material.73

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3.8 Mn oxide supported on carbon nanotubes The groups of Behrens and Strasser described two different methods, incipient wetness impregnation sample and a novel deposition synproportionation precipitation for dispersion of nanostructured manganese oxide on commercial carbon nanotubes and synthesis of MnOx/CNT electrocatalysts.74 Various techniques were employed for the determination of the Mn oxidation state, and it was found that the oxidation state is higher for the synproportionated MnOx (near +4) compared to the impregnated sample (+2). In both methods, it is possible to adjust the Mn oxidation state as a function of oxygen partial pressure and temperature. Generally, the 5 wt% MnO/CNT sample obtained by conventional impregnation, provided active and stable catalytic anode material for water electrolysis at neutral pH. In addition, the authors noted that the combination of this material with d0 and d10 transition metal oxides or oxynitrides (photoactive semiconductors), could act as a surface electrocatalytic component in photoelectrochemical water splitting devices. Recently, Messinger and Kurz indicated that the selection of carbon-based material is important for the synthesis of Mn oxide supported carbon-based catalysts for water oxidation.75 Buckypaper and carbon felt are unsuitable anode supports because these materials are mechanically unstable and/or are not stable under the water-oxidation conditions. However, graphite sheets and graphitized carbon fiber paper were reported to be surprisingly well suited for Mn-based water oxidation catalysis.75 Generally, as discussed by Messinger and Kurz, the supports containing sp3-C were found to be unsuitable for the water-oxidation reaction as it is accompanied by carbon corrosion to deep oxidation (CO2).75 The carbon-based supports containing sp2-C are more stable for water oxidation.75

3.9 Pure Mn oxides In 2013, Dismukes’ group studied very pure β-MnO2, R-MnO2, α-MnO2, δ-MnO2, λ-MnO2, LiMn2O4, Mn2O3 and Mn3O4, and found that Mn2O3 and Mn3O4 are the most active materials.76 It was hypothesized that in edge-sharing octahedra, Mn(III)-O bonds are weaker and flexible. In contrast, the MnO2 has shorter and stronger Mn(IV)-O bonds, which are more stable for water oxidation.76 Mn(III) was also proposed as the active sites for water oxidation by other research groups.58,60 As we discuss in the next sections, it is important to note that complex relationships between properties are among the factors in water oxidation.77 For example, the calcination temperature has effects upon water content,

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oxidation number, phase, crystallinity and surface. Thus, to compare the water-oxidizing activity of Mn oxides, all factors should be carefully analyzed; on the other hand, one Mn oxide may have high surface area and many active sites, but the question is whether enough oxidant molecules are present to react with all sites? If not, we cannot observe real activity for the phase; and if we use high concentrations of oxidant, decomposition may occur. Crystallinity is another important factor. It is difficult to synthesize Mn oxides with similar crystallinity for comparison of the wateroxidizing activity. A series of manganese oxide-minerals (marokite (CaMn2O4), pyrolusite (MnO2), hollandite (Ba0.2Ca0.15K0.3Mn6.9Al0.2Si0.3O16), hausmannite (Mn3O4), Mn2O3H2O and synthetic marokite (CaMn2O4)) were reported as water-oxidizing catalysts.78 Generally, high crystallinity of these minerals decreases the extent of water oxidation compared to amorphous Mn oxide.78 However, manganese oxide-minerals containing M(III) ions are more efficient catalysts for water oxidation. A water-soluble Mn oxide was reported by Perez-Benito et al.79 The compound is produced by the reaction of MnO4  and S2 O3 2 79: 8MnO4  ðaqÞ + 3S2 O3 2 ðaqÞ + 2H + ðaqÞ ! 8MnO2 ðsÞ + 6SO4 2 ðaqÞ + H2 O ðlÞ

The radius of the colloidal particles is around 50 nm and their negative electrostatic charge stabilizes them in solution. The colloidal manganese(IV) was reported as a catalyst for oxygen evolution in the presence of oxone, H2O2, cerium(IV) ammonium nitrate, and [Ru(bpy)3]3+.80 The small size, dispersivity and charge of particles are among important factors with respect to water oxidation. The efficiency of layered Mn oxide toward water oxidation is related to low amounts of Mn(III) in this structure. However, the solution containing the water-soluble Mn oxide is transparent, thus it may be useful for mechanistic studies. In some cases, Mn oxides could be prepared very easily. For example, it is reported the decomposition of an aqueous solution of manganese nitrate at 100 °C results in α-Mn2O3 (ca. <100 nm).81

3.10 Layered Mn oxides In 2010, it was reported that by incorporating Ca ions in Mn oxides, an efficient catalyst is produced.82 This idea was taken from earlier suggestions that billions of years ago, the PSII proto-enzyme might have originated from naturally occurring manganese oxide minerals.35 To achieve the goal

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Scheme 1 Synthetic procedure for the synthesis of CaMn2O4xH2O.82

of preparing this efficient catalyst, synthetic routes were designed and α-Mn2O3 and CaMn2O4xH2O with enhanced surface areas were prepared (Scheme 1). The BET (Brunauer-Emmett-Teller method) results indicated the highest surface area for the synthesized CaMn2O4xH2O with 303 m2/g (2) and 205 m2/g for compound 3.82 More evaluation of this new system confirmed this difference in the surface area did not affect the performance of the oxides to act as oxygen-evolving catalysts. By employing a Clark electrode, the ability of manganese oxides to generate oxygen with various oxidants such as H2O2, HSO5  , Ce(IV) and [Ru(bipy)3]2+, was studied and it was found, among these Mn oxides, that 1 and 3 are the most efficient catalysts.82 Generally, CaMn2O4xH2O oxide materials of this study are more appropriate for large-scale applications with respect to well-known, but expensive, IrO2, RuO2, and Rh2O3 catalysts. The compounds were introduced as the structural and functional models to the biological water-oxidizing complex in PSII.82 Two different Ca-containing motifs were found in MndCa oxides.70 One of them results in the formation of Mn3CaO4 cubes, these are also proposed for the WOC.70 Other calcium ions are likely to form interconnected oxide-layer fragments. The average oxidation state of manganese is 3.8.70 Neighboring manganese ions are interconnected by two bridging oxygens, as in layered Mn oxides.70 According to these similarities, in a new assessment of the WOC

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of PSII, protocyanobacteria in the Archaean ocean in the presence of Mn and Ca ions, and under alkaline conditions, may enable the assembly of oxide minerals as functional compliments of the early active site of PSII.70 Synthesis of such compounds is also observed on the cell walls of the Algae Chara corallina.83 Growing in Mn-rich media, Chara corallina forms brown deposits of Mn oxides on their cell wall surfaces. These deposits are volcano shaped and exhibit 3–5 μm craters in their centers.83 The structure of this oxide was detected as birnessite containing di-μ-oxido-bridged MnIII/IVO6 octahedra as central building units. The motif in the structure is similar to the synthetic MndCa oxide and the MndCa cluster in the WOC. The oxide also shows the water-oxidizing activity.83 Regarding the WOC structures, molecular modeling of the Mn4O5Ca cluster in PSII shows that it has a dimension of about
0.5 nm.20 Thus, nano, and more interesting A˚ngstr€ om-scale, Mn or MndCa oxides may be better structural and functional models for the Mn4O5Ca in PSII. In continuation of previous work,82 nano-sized amorphous MndCa oxide as a catalyst was synthesized for water oxidation in the presence of Ce(IV) as an oxidant.84 Synthesized nano-size amorphous calcium–manganese oxide has features that are akin to those of the water-oxidizing cluster in photosystem II.84 The most important feature is the small particle size with respect to previously reported micro-size amorphous calcium–manganese oxides. This catalytic system showed high activity toward water oxidation compared to other reported manganese compounds. In addition, increasing the concentration of Ce(IV) up to 0.45 M causes an increase in the rate of oxygen evolution, but at higher concentration of Ce(IV) (>0.45 M), further enhancement in the rate of oxygen evolution is not observed.84 The catalyst was used for several times (at least five) while maintaining its reactivity.84 The TOF of the compound in the presence a solution of Ce(IV) (0.45 M) was
0.002 s1 and could be used several times (at least for five repetitions) without any significant loss in its reactivity.84 The use of different ions between layered Mn oxide was reported.85 These inert redox ions give different characterizations to Mn oxide. For example, CadMn oxides are very amorphous up to
500 °C, but similar compounds with Zn showed a crystalline phase even at
300 °C.85 Using H18 2 O isotope-labeling experiments coupled with membrane inlet mass spectrometry, the mechanism of oxygen evolution of Mn oxides has also been studied by Messinger and Kurz.54 These oxygen evolution results

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are from experiments carried out in the presence of single-electron oxidants such as cerium (IV) or RuðbpyÞ3 3 + and showed54: – Oxygen evolution in the presence of single-electron oxidants such as cerium (IV) or RuðbpyÞ3 3 + is “real” water oxidation and the two oxygen atoms in dioxygen originate from water molecules. – The involvement of surface oxido species in the oxygen evolution reaction from water was not observed. Because of the structures attributed to these calcium Mn oxides and the potential comparability with the WOC in PSII, similar investigations may help in further understanding of the mechanism of oxygen evolution in Nature. Possible mechanisms for oxygen evolution and water oxidation by these manganese (calcium) oxides are presented in Scheme 2 with details. This mechanism for water oxidation is correct only if μ-oxido groups exchange with bulk water very slowly.54 In some cases, μ-oxido species do not exchange with bulk water rapidly.86 In contrast, Rapatskiy et al. reported a μ-oxo species that can exchange rapidly with bulk water.87 It is also important to note that μ-oxo on the surface of oxides may exchange faster than μ-oxo in complexes. More experiments should be performed to find out more about the exchange of μ-oxo in complexes or metal oxides. However, as suggested by Messinger, these experiments are usually performed with very low water concentrations in organic solvents and therefore are likely to underestimate significantly the absolute magnitude of the exchange rate.88 As depicted in Scheme 3 for a cobalt compound, usually two mechanisms were proposed for water oxidation by terminal water molecules.89 One possible pathway is via the LH (two-site) mechanism that is facilitated by the formation and bridging of two oxo groups of terminally coordinated high valent metal ions, while in the ER (mono-site) mechanism, the water oxidation proceeded through the formation of peroxide groups which act as the precursor of oxygen.89 Using H18 2 O exchange for μ-O groups on the surface of nanolayered MndK oxide by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), a slow water exchange was estimated.90–92 CaMn3O6 and CaMn4O8 were synthesized and characterized by SEM, XRD, FTIR and N2 adsorption isotherms. The mixed-valence MndCa oxides are efficient catalysts for oxygen evolution in the presence of H2O2, oxone, Ce(IV) and [Ru(bpy)3]3+ in comparison with efficiencies of MnO2, CaMnO3 and Ca2Mn3O8.93 Thus, it is suggested: – Incorporation of calcium into mixed-valence Mn oxides increases water oxidation activity of these Mn oxides in the presence of Ce(IV), and also oxygen evolution in the presence of oxone.

A

O

Kremer-stein pathway MnIII O O

–

–H2O

O O

+H2O2 –OH–

Oxide+I

–H+/–O2 2e– - Reduction

A (MnIII)

–I H O

O–I

MnIII O O

+H2O2

Oxide

–H+

Mn

O O Oxide

Oxide

from H2O2

2e - Oxidation

Haber-Weiss pathway

Start :

+I

O

MnIV

O O

+H2O2

–I H O

O–I

MnIV

Oxide

A

from H2O2

O–II

+H2O2 1e– - Oxidation

B (MnII)

MnI

MnII

O O

O O

Oxide+I

Oxide

+H

O–I

MnIII

MnIII

O O

O O

Oxide

+H

O–I +H2O2 –H2O

Oxide

O0

+H

O–I

O0

B (MnII)

Scheme 2 The two mechanisms proposed for the decomposition of hydrogen peroxide by transition metals (A). Two possible mechanisms for oxygen evolution by manganese oxide in the presence of HSO5  . The upper one is similar to the mechanism suggested by Brudvig’s group36 (B). Alternatively, three proposed mechanisms for the OdO bond formation as part of the manganese catalyzed water oxidation in the presence of Mn oxide (C). The images are a modification from Shevela, D.; Koroidov, S.; Najafpour, M. M.; Messinger, J.; Kurz, P. Chem. Eur. J. 2011, 17, 5415.

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Scheme 3 LH (two-site) mechanism (top) and ER (mono-site) mechanism (bottom) for OdO bond formation. The image and caption were modified from Jia, H.; Stark, J.; Zhou, L. Q.; Ling, C.; Sekito, T.; Markin, Z. RSC Adv. 2012, 2, 10874.

– Manganese ions with the oxidation state of III are better catalysts for water oxidation. – In another view, it is possible that calcium ions on the surface of oxides can be removed to the solution and this phenomenon increases the surface of catalysts; however, we have found that the surface of the catalyst is not a very important factor in water oxidation.93 Electrochemical water oxidation by the MndCa oxides was extended.87 Linear sweep voltammograms were investigated for the layered manganese-calcium oxide coated glassy carbon (GC) electrode at pH 1, 7, and 14.94 The results showed a pronounced effect of pH in the activity of the layered MndCa oxide powder with respect to relative oxygen evolution. For this catalyst, the highest catalytic activity was observed under acidic conditions. Interestingly, the WOC in the PSII works under acidic conditions. In the pH 7 solution, the water oxidation for the layered MndCa oxide was observed to be only a slight improvement over the reaction with the glassy carbon electrode. In addition, water oxidation decreases when the pH was increased to 14; therefore, the layered manganese-calcium oxide coated electrode has a lower activity for oxygen evolution than the glassy carbon electrode.94

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In order to find an answer for the variation in efficiencies, Birkner et al. studied surface enthalpy,95 formation enthalpy, and internal oxidation enthalpy of several synthetic MndCa oxides in an endeavor to find microscopic reasons for such improved catalytic activity, using a combination of inductively coupled plasma–optical emission spectroscopy (ICPOES), thermogravimetry and differential scanning calorimetry (TG-DSC), XRD, SEM, TEM. A Murray titration was also employed to determine manganese oxidation states in the solids. Measurements of formation and surface enthalpy employed high-temperature oxide-melt solution calorimetry.95 These samples were genetically similar, but vary in their calcium, manganese, oxygen, water content, and average particle size and surface area. The results showed that the compounds (synthetic MndCa oxides) have smaller surface enthalpies than those of Mn3O4, Mn2O3, and MnO2 and, most strikingly, the enthalpy of oxidation of Mn(III) to Mn(IV) appears constant along the series.95 The probable reasons for enhancement of water oxidation catalysis by these samples involve a combination of several factors: the open and layered structure with thermodynamic stability and high surface area, mixed valent manganese structure with internal oxidation enthalpy smaller in magnitude than for the Mn2O3-MnO2 couple, and low surface enthalpy, which suggests loose binding of H2O on the nanoparticle surfaces. These factors increase catalytic activity by providing easy access for solutes and water molecules to specific sites on the catalyst and facile electron transfer between manganese in different oxidation states.95 To assess the stability of these compounds with respect to leaching of manganese and calcium in water, Ce(IV), H2O2 and RuðbpyÞ3 3 + were investigated.96 For this purpose, CaMn3O6, CaMn4O8, CaMnO3 and Ca2Mn3O8 were studied for water oxidation. The results indicated a little manganese dissolved under the reaction conditions, even in the strongly acidic media containing Ce(IV) and, within 24 h. In contrast, a much larger fraction of the calcium was found in solution after water oxidation.96

4. Expanding a hypothesis In addition to the MndCa oxide, which was designed based on CaMn4(H2O)4O5 in PSII, D€ orr et al.97 reported a procedure to produce ammonia from dinitrogen using H2S as a reductant and iron sulfide, which could be considered as a biomimetic model for the inorganic core of nitrogenase.

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Yuhas et al. studied chalcogens with photocatalytic hydrogen production under solar illumination.98 It is important to note that in many cases the metal oxide or sulfidecluster similar to ones which could be found in enzymes are efficient and selective catalysts for the corresponded reactions in vitro. These clusters, are similar to minerals, and are formed in no special or preorganized precursors, and display structures similar to inorganic cores of enzymes. Thus, it is concluded that “Both inorganic cores of enzymes and related inorganic compounds catalyze similar reactions.”99,100

4.1 Mn oxides with organic compounds Hundreds of amino acids exist in PSII. Although, only a small fraction of the residues of amino acids come in direct contact with the calcium and manganese ions in the water-oxidizing cluster.20 Others are involved in proton, water and oxygen transfer. The MndCa oxides are structural and functional models for the native water-oxidizing complex in PSII, but these models contain no organic groups to stabilize Mn oxide or (and) decrease the activation energy for water oxidation, whereas organic groups are present in PSII.20 These uncoordinated groups play very important roles in decreasing the activation energy of water oxidation. Hydrogen bonds around the WOC are very important and play a crucial role in decreasing the activation energy of water oxidation. Only a few changes in these hydrogen bonds decrease water oxidation activity in PSII dramatically.101–103 With the aim of simulating the Mn4O5Ca cluster in PSII,101–103 it is necessary to understand how the Mn oxide layers are located close to guanidinium, imidazole or other significant groups. A report of the insertion of organic groups between layers of Mn oxides has appeared and is schematically displayed.104 (Fig. 2). Thus, guanidinium and imidazolium each with a positive charge form a self-assembled layered manganese(III,IV) oxide.104 The compounds could be detected by FTIR, SEM, TEM and XRD. Electrochemical experiments showed that the overvoltage of water oxidation for the manganese(III,IV) oxide monosheets under these conditions is
600 mV.104 These catalysts could be considered as the first step toward preparation of a hybrid of organic compounds and manganese as water-oxidizing catalysts to mimic the WOC of PSII. The organic groups could have important roles upon water oxidation by stabilizing manganese oxide, optimizing proton, water and oxygen transfer (Fig. 3).104–106 The stability of these organic groups during the water

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A

B

C

D

E

Fig. 2 Schematic representation of the structure of guanidinium (A), imidazolium (B), 4-aminophenol (C) and 2-(2-Hydroxyphenyl)-1H-benzimidazole (D) and RuðbpyÞ3 2 + (E) between manganese layers.

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Fig. 3 A special protein in PSII manages proton, oxygen and water. A few groups around manganese compounds could perform the task.

oxidation reaction should be considered, as it is known that organic compounds such as Nafion are stable during the water oxidation process. Using in situ spectro-electrochemical techniques, it was shown that the stabilization of Mn(III) ions on the surface of Mn oxide is an effective way to decrease the overpotential for water oxidation by MnO2. It was reported that NdMn bond formation via the coordination of amine groups of poly(allylamine hydrochloride) to the surface Mn sites of MnO2 electrodes, effectively decreases the onset potential for water oxidation at neutral pH.60 A self-assembled layered hybrid of phenol-manganese ions as exists in the WOC of PSII was reported.105 In the next step, the characterization and electrochemistry of a 2-(2-hydroxyphenyl)-1H-benzimidazole-Mn oxide hybrid were determined.106 2-(2-hydroxyphenyl)-1H-benzimidazole (IP) was used as a model for tyrosine-161 and histidine-190 in PSII.107,108 The effect of treatment of nanolayered manganese oxide by oxidizable compounds was investigated.109 It was found that the treatment of the synthetic Mn oxides with oxidizable compounds increases the water-oxidizing activity in the presence of Ce(IV) >25-fold. On the basis of X-ray absorption investigations, this effect was attributed to the increased amount of Mn(III) ions.109 In a strategy, RuðbpyÞ3 2 + was placed between layers of Mn oxide.110 The electron transfer from the catalyst to the oxidized sensitizer ([Ru(bpy)3]3+) is the rate-determining step in this system.111 In 2011, the water oxidation reaction was studied employing RuðbpyÞ3 2 + as a photosensitizer, and manganese(III,IV) oxide monosheets as a catalyst.110 XRD, SEM and TEM analysis of the catalyst disclosed the birnessite structure with the arrangement of RuðbpyÞ3 2 + between layers. Therefore, a photosensitizer with a positive charge could form a self-assembled layered hybrid [photosensitizer]n+/manganese(III,IV) oxide.110 It was proposed that incorporating a photosensitizer between layers of manganese oxide,

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led to a constant and low concentration of photosensitizer in solution.110 In
addition, a faster reaction between the photosensitizer RuðbpyÞ3 2 + and catalyst due to their respective cationic and anionic character, rather than elec
tron transfer from the catalyst to the oxidized sensitizer RuðbpyÞ3 3 + , was a notable finding. These functions could stabilize RuðbpyÞ3 2 + in solution and increase the yield of oxygen evolution.110 The rate of water oxidation of this catalyst with a 250 W tungstenmercury lamp in the presence of [Co(NH3)5Cl]2+ (10.0 mM) as the sacri1 110 ficial electron acceptor was 7.0 μmolO2.mol1 Mn.s . In PSII, the manganese stabilizing protein is around the WOC. The protein, in addition to stabilizing Mn oxides, decreases the activation energy for water oxidation, and is important in a proton transfer network. Manganese oxide-bovine serum albumin (BSA) as a structural and functional model for the WOC in Nature was reported and investigated.112,113 In other words, BSA is soluble in water, has strong interactions with inorganic compounds and has a flexible structure,114 and thus it is of interest to be used within the composite as a promising model for stabilizing the protein in PSII. The water-oxidation activity of the compound was studied in the presence of Ce(IV) and RuðbpyÞ3 3 + as powerful and non-oxo transfer oxidants. The composite is a water-oxidizing catalyst, but decomposition of the protein under this condition is observed. In biological water oxidation, the YZ/YZ. potential is only
1.0 V vs SHE. In other words, there is a competition between organic and water oxidation in all these reactions in the presence of Ce(IV).112,113 With the aim of synthesizing a heterogeneous catalyst based on an Mn oxide and organic compounds combination for water oxidation by a low overpotential, nanolayered manganese oxide/poly(4-vinylpyridine) (PVP) as a biomimetic and efficient water-oxidizing catalyst was prepared in 2013.115 PVP compounds are promising materials for linking the Mn oxide to Pt. The polymer was used as a model for the Mn-stabilizing protein in PSII. Pyridine groups in the polymer played essential roles as proton transfer, and acceptor, and provide a buffered environment for Mn oxide.

4.2 Nanoscale manganese oxide within Faujasite zeolite Faujasite zeolites possess supercages with a diameter of ca. 1.3 nm and the structure is amenable for metal or metal oxide nanoparticles with sizes <2 nm.116–119 Nanoscale Mn oxides within Faujasite zeolite have been synthesized by exchanging Mn(II) in the zeolite structure, followed by reaction with

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MnO4  .120 These oxides indicated efficient water-oxidizing activity in the presence of Ce(IV) as a non-oxo transfer oxidant. It was proposed that immobilization of Mn was performed by zeolites in a hydrophilic and highly oxidation stable environment, similar to a mesoporous silica or clay support.66,121 Since the high activity of nano-sized particles is concomitant with fast decomposition reactions, for a larger scale operation it would be necessary and important to use nanosize material. In 2014, synthesis ˚ ngstr€ of a form of A om-scale Mn oxide within an HY zeolite was reported and characterized by SEM, TEM, XRD, and AAS.122 The compound shows water oxidation in the presence of Ce(IV).122 However, the rate of decomposition to MnO4  is also high under this condition. The data showed that this Angstr€ om-scale Mn oxide is different from the nano-scale version. The Angstr€ om-scale Mn oxide is very fragile in the presence of Ce(IV) and decomposes to MnO4  much faster than nano-scale manganese oxide.122 The Angstr€ om-scale Mn oxides, at least under our experimental conditions, behave more like Mn ions in solution. In other words, although nano-scale Mn oxides show high similarity with bulk Mn oxide, the Angstr€ om-scale Mn oxide shows similarity to the molecular scale. A similar characterization may extend to other Angstr€ om-scale compounds. These results could indicate that a special sized (10–50 nm) scale may be a good scale for Mn oxide-based catalysts toward water oxidation. Aluminum-free ZSM-5 type manganosilicate was synthesized and tested for water oxidation.123 After elimination of extra-framework manganese oxides, no oxygen evolution was observed. Thus, Mn ions which are dispersed in an ZSM-5 lattice are not active sites for the water-oxidation reaction, but there is a suggestion that cooperation between Mn ions is important in order to observe water oxidation activity.123 In 2017, an Mn-containing ZSM-5 type zeolite was synthesized and investigated for water oxidation catalytic activity.124 The activity for water oxidation could be attributed to the amorphous phase of the extra-framework manganese oxides.124 Under the electrochemical conditions, the catalyst with lowest manganese oxide contents is more active in low overpotentials. This effect is attributed to a high activity of the smaller particles.

4.3 Manganese oxide-coated montmorillonite Hydrous Mn oxide (HMO), δ-MnO2, and birnessite are commonly found in soils125 and sediments. HMO has an amorphous structure and a very large surface area66,126–133 and thus is a promising compound for application in the water oxidation reaction.

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In 2012, amorphous manganese oxide-coated montmorillonite was used as an efficient catalyst for water oxidation in the presence of Ce(IV).121 For this catalyst water oxidation increases with an increasing manganese content of montmorillonite. The result indicates that MnOx-clay hybrid operates cooperatively for the catalysis. The phenomenon may take place because of a redox accumulation by Mn ions toward a four-electron reaction by collecting four electrons from four one-electron reactions. For Mn
1.0 mmol/clay (gr) a high activity for oxygen evolution was observed.121

4.4 Gold or silver deposited on layered manganese oxide Layered Mn oxides upon which gold or silver has been deposited are very interesting compounds for water oxidation because these two metals improve many catalysts.134 Such Mn oxides were synthesized and characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction spectrometry, atomic absorption spectroscopy and energy-dispersive X-ray mapping.135 The gold treated layered Mn oxide shows efficient catalytic activity toward water oxidation in the presence of Ce(IV). The group of Bell also observed the effect of gold on water oxidation by manganese oxide. They suggested that gold, as the most electronegative transition metal, could act as an electron sink to perform easily the oxidation of low valence metal ions to higher valences; and therefore as high valence metal ions are usually responsible for water oxidation, this deposition increase water oxidation activity.136,137 However, the electronegativity of Mn(IV) rather than Mn should be considered, and it seems gold is not significantly more electronegative than Mn(IV). Wang et al. reported a comparison of gold/Mn2O3 partially oxidized gold species with the fresh catalyst.138 However, the effect of organic compounds, which are used to reduce gold salt in water oxidation by Mn oxides should be carefully reinvestigated.109 It is important to note that silver shows no effect on water oxidation of layered manganese-potassium oxides. In the case of silver, a decrease was observed in water oxidation as compared with layered Mn oxides, most probably because of the lower number of manganese ions on the surface of the compound.135

4.5 Induction potential So far, many strategies such as adding gold, platinum, organic compounds, using fluoride and metal cations were used by many research groups to

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improve manganese oxides by the induction of Mn(III) toward the goal of successful water oxidation. Recently, the effect of an induction potential before the electrolysis potential on the water-oxidizing activity of Mn oxide for the water oxidation reaction, was used.139 At the electrolysis potential, Mn ions are oxidized to Mn(IV), thus the improvement in water oxidation by an induction potential before the electrolysis potential was considered. In phosphate buffers at pH values of 11, 7 and 3, and using the MnO2/FTO combination, many different induction potentials for 5 min periods, and also operation potentials for 15 min periods, were investigated. At least in some cases the electrochemical induction approach could be promising for improvement of the water oxidation system.139

4.6 Important factors in water oxidation by Mn oxides 4.6.1 Surface The active sites are on the surface of a compound and usually, increasing the surface of a catalyst results in increasing the number of active sites. This implies that if all other factors with regard to reaction are equal, then increasing the surface area causes increased activity. In considering applying the (BET) method to calculate the surface of catalysts, it would be advantageous if the surface area between layers in clay or birnessite could be calculated. 4.6.2 Water content A Mn oxide prepared for the water oxidation reaction yields favorable results when the water content is minimal.140 4.6.3 Oxidation state The oxidation state of Mn ions is an important factor in water oxidation by Mn oxides: in general the order is Mn(III,IV) > Mn(III) > Mn(IV), Mn(II) and Mn(II,III). An analysis of reports in which different conditions are extant, and in which the objective is to increase the efficiency of water oxidation by Mn oxides, leads to the conclusion that a reduction from oxidation state IV to III for Mn ions occurs.140 4.6.4 Structure, shape and size Structure, shape and size of metal oxides are important in the water oxidation reaction. It is suggested that an open structure, observed in layered or tunneled structures, is an important contributing factor for efficiency of Mn oxide in the water oxidation reaction.

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4.6.5 The amount of crystallized Mn oxides Some reports indicated that amorphous Mn oxides are better catalysts for water oxidation than crystalline phases.141 Zaharieva et al. suggested that the low water oxidation by crystalline MnO2 is related to the absence of readily (de)protonatable μ2-O(H) groups and the lack of terminal water coordination sites.70,71 Furthermore, the energetic stability of well-ordered MnO2 impedes water oxidation catalysis.70,71 4.6.6 Calcination temperature At temperatures higher than 500 °C, a large amount of crystallized Mn oxides with a lower surface area, together with dehydration compounds, are produced. A consequence could be significant in reducing wateroxidizing activity in the presence of Mn oxides. In an example, Zaharieva et al., by using XAS, found that CaMnIV 1:6 MnIII 0:4 O4.5(OH)0.5.3H2O prepared at 60 °C with a higher surface (303 m2/g) and a lower activity (0.325 mmolO2.molMn1.s1) changed to CaMnIV 1:6 MnIII 0:4 O4.5(OH)0.5 with lower surface (205 m2/g) and higher activity (0.54 mmolO2.molMn1. s1).70 The results showed that both compounds contained Mn(IV) cations with only a small fraction of the manganese (around 20%) present as Mn(III) ions. Its high water content in CaMnIV 1:6 MnIII 0:4 O4.5(OH)0.5.3H2O implies that in the oxide the space between the MnO6 layers is filled not only by Ca2+ ions, but also by many water molecules. The results also showed that for layered Mn oxides, heat treatment in the 300–500 °C range increases water oxidation activity in the presence of Ce(IV).82 Recently, Spiccia and MacFarlane reported a thermal treatment at 120 °C for half an hour that transfers the MnOx films deposited from aqueous electrolytes to highly catalytic films with comparable activity to ionicliquid-deposited films.140 The XAS results indicated the growth of small amounts (3–10%) of reduced Mn species (Mn(II) or Mn(III) after heat treatment.140 They concluded that their thermal treatment is mainly a dehydration process that results in a permanent loss of structural water and hydroxyl groups.142 However, as was shown by Nakmura’s group,60 treatment by amines, reduced Mn(IV) to Mn(III), at moderate temperatures, and similar increases in water oxidation were observed. Thus, heat treatment changes the oxidation state of manganese ions, changes the status of water and hydroxyl groups, and the crystallinity levels and surface area of oxides. Thus, finding a potent factor that will give rise to increase in water oxidation is not straightforward. However, regarding the experiments of Nakamura, change of the oxidation state of manganese ions seems to be the most important factor in heat treatment.60

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4.6.7 Manganese content When we consider layered Mn oxides, the manganese content may be a factor relating to the water-oxidizing activity. With a low manganese content and a high content of redox-inert ions, then Mn2O3 or MnO2 is a major phase. Thus, a layered Mn oxide phase as an efficient catalyst for water oxidation is not generated under these conditions. For three Mg, Cd and CadMn layered oxides,85 a simple relationship between water oxidation and manganese content is observed. However, it was indicated that for a low manganese content, a low water-oxidizing activity (mol O2/mol Mn) is observed. Our new results show that in the presence of a very low amount of manganese, water oxidation increases with manganese content very sharply, but in the presence of a moderate content of manganese this increase becomes linear.121 4.6.8 Dispersion The dispersion of Mn oxide on supports, electrodes and solution has a significant influence on increasing the water-oxidizing activity. 4.6.9 Charge Mn oxides with negative or positive charge have been investigated. In these compounds, electrostatic interactions have an influence on the rate of electron transfer between oxidant and Mn oxide.110 4.6.10 pH pH is an important factor in all stages of water oxidation from the synthesized catalyst to the oxygen-evolving reaction. The pH value in which the Mn oxide has been prepared, has a critical effect on the water-oxidizing activity.

5. The role of redox-inert ions in layered manganese oxide Different layered Mn oxides with different redox-inert ions were synthesized: Ca(II), Zn(II), Al(III), Cd(II), K(I) or Mg(II) all displayed similar water oxidation activity.85 There was no special effect for the redox-inert ions, but a redox-inert ion increases the water oxidation activity85,143. However, layered Mn oxides with different ions have different characterizations. For example, CadMn oxides are very amorphous up to
500 °C, but a similar compound with Cd(II) showed a crystalline phase even at
300 °C.77 Thus, the few different

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Scheme 4 The most critical factors for manganese oxides toward water oxidation. The image is from Najafpour, M. M.; Sedigh, D. J.; Pashaei, B.; Nayeri, S. New J. Chem. 2013, 37, 2448.

water oxidation activities of these compounds could be related to different conditions of preparation than to those for redox-inert ions. It has been noted that for water oxidation, comprehending the role of metal ions between Mn layers is a challenging issue.127 It was shown that the Mn layers require calcium ions in their structures to achieve maximum catalytic activity.127 As noted earlier there are many factors (Scheme 4) involved in the water oxidation reaction by Mn oxides, and the relationships between these factors are also complex. For example, the calcination temperature has an effect on the water content, oxidation number, phase, crystallinity and surface area. However, it seems that the presence of Mn(III) ions is a critical factor to increase the water-oxidizing activity by Mn oxides, but how exactly Mn(III) ions improve water oxidation yields remains to be established. Extended X-ray absorption fine structure (EXAFS) spectra of the Mn(III) activated films indicate a decrease in the MndO coordination number, and Raman microspectroscopy displayed the presence of distorted MndO environments.144 Computational studies suggested that Mn(III) is kinetically trapped in tetrahedral sites in a fully oxidized structure. In a reduced form, computation showed that Mn(III) states are stabilized relative to those of oxygen and that the highest occupied conduction band is thus dominated by oxygen

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states.144 Furthermore, the presence of Mn(II) results in a reduced gap between the conduction and the valence bands. The influence of a reduced conduction band–valence band gap and oxygen-based conduction band, results in the facilitation of water oxidation.144 No disproportionation is possible in the oxidized δ-MnO2, thus it is speculated that the Mn is kinetically locked in its metastable Td site.144

6. Manage damage: Self-healing in manganese oxide Long-term stability of a water-oxidizing catalyst is important for commercial application.145–147 Thus, self-healing (the ability to repair damages automatically and autonomously) catalysts is important and of considerable interest. Nature has an interesting strategy to stabilize the water-oxidizing catalyst; PSII replaces the entity of the D1 polypeptide approximately every 30 min in order to repair itself.148 The catalysts for multielectron reactions are prone to instability, during the reaction. Thus, catalysts with self-healing properties are very significant.145–147 A very interesting self-healing water-oxidizing catalyst was introduced by the Nocera group145–147; it was a cobalt-phosphate heterogeneous catalyst for the water oxidation reaction. To probe the dynamics of the catalyst during water oxidation, the group investigated the electrosynthesis of the catalyst using radioactive 57Co and 32P isotopes. To perform the experiments, a phosphate solution containing 0.5 mM Co(NO3)2 was enriched with 57Co(NO3)2.145–147 The chemical stability and decomposition pathways for the Mn compounds under different conditions were studied.149–152 The leaching of Mn(II) ions to the solution in the presence of Ce(IV) was easily detected by EPR. In addition, MnO4 was detected by UV-Vis spectroscopy when the oxides were treated with Ce(IV).151,152 The presence of both MnO4  and Mn(II) was surprising as usually they react together to form Mn oxide151: MnO4  + MnðIIÞ ! Mn oxide

(1)

It was proposed that a self-repair mechanism operates for Mn oxide in the presence of Ce(IV) (Scheme 5). The mechanistic scheme could show that a few MnO4  ions formed by the decomposition of Mn oxide, react with Mn(II) ions to form Mn oxides.151 And the effect of Ce(IV) on the layered manganese-potassium catalyst was also studied during a lengthy time period.152

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Scheme 5 Self-healing in water oxidation by Mn oxides in the presence of Ce(IV). 1: Oxygen evolution was detected by an oxygen meter. The origin of oxygen is water. 2: Mn(II) was detected by EPR. 3: MnO4  formation could be detected by UV–vis spectrophotometry in reacting Mn(II) and Ce(IV). 4: It is known that in the reaction of Mn(II) and MnO4  at different pH values, Mn oxide is produced. 5: MnO4  in the presence of Mn oxide oxidizes water. In the reaction, MnO4  reduces to Mn oxide. 6: Mn(II) in the presence of Ce(IV) forms Mn oxide. In a typical experiment, the reaction of MnSO4 in the presence of Ce(IV) (1.0 M) forms MnO2, which can be detected by XRD. Images and captions reprinted with permission from Najafpour, M. M.; Kompany-Zareh, M.; Zahraei, A.; Sedigh, D. J.; Jaccard, H.; Khoshkam, M.; Britt, R. D.; Casey, W. H. Dalton Trans. 2013, 42, 14603. Copyright (2013) by the Royal Society of Chemistry.

The catalyst is stable even after 15 days under the water oxidation conditions (Ce(IV); 1.8 M). Using EDX, it is found that the formula of the compound changes from K0.25MnO2 before reaction with Ce(IV), to Ce0.048MnO2, which shows that many potassium ions are removed from the surface of the catalyst. However, no change in water-oxidizing activity was observed. The observation also shows that replacing potassium with cerium has a few effects on water oxidation by layered Mn oxides.77 An increase in the surface of the Mn oxide was observed from 32.4 to 130.2 m2/g after 15 days reaction with Ce(IV) (1.8 M).

7. Water oxidation by Mn complexes In addition to its use in Nature,20 manganese is both low cost and environmentally friendly, thus it is attractive for use as a water-oxidizing catalyst in artificial photosynthesis. Obviously, there have been many attempts to design nonbiomimetic catalysts for water oxidation. Learning different strategies from biological systems is a sensible approach since the biological

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systems (cyanobacteria, algae and plants) have used manganese successfully for millions of years; the strategies may be relevant and applied in artificial systems directly or indirectly.82 Water oxidation by manganese complexes is a challenging issue, regarding the role of Mn oxide instead of an Mn complex toward water oxidation,153–156 topics reviewed extensively.157–159

8. The true catalyst in water oxidation As shown by the group of Spiccia, the structure of some manganese complexes embedded in Nafion and deposited on glassy carbon electrode was found to be very different from that of the cuban complex recorded in solution and similar to those of mononuclear Mn(II) compounds. This study thus demonstrated that the synthetic [Mn4O4(O2PPh2)6] cuban-like complex is decomposed to Mn(II) species which are then oxidized into an Mn oxide phase during water oxidation.149 Phthalocyanines are among the most stable classes of ligands for metal ions because of their high binding affinity. Phthalocyanines tightly bind Mn ion and significantly stabilize it under moderate conditions. Recently, manganese(II) phthalocyanine was studied as a reagent within the electrochemical water oxidation process.160 By applying several techniques, it was demonstrated that for the stable manganese complex under moderate conditions, the conversion into nanosized Mn-oxide occurs during the wateroxidation reaction. Such nanosized Mn oxides are regarded as one of the true catalysts for water oxidation.160 Generalization and the inductive reasoning has been criticized,161 but when a very stable complex was observed to be decomposed under the water oxidation conditions, statistically it is possible that other Mn complexes decompose under the water-oxidation condition. The experiments also showed that a stable complex under moderate conditions could easily decompose under the oxidizing condition. In 2013, it was reported that many Mn oxides applied in the wateroxidation reaction, transform to layered Mn oxide after a few hours.162,163 After the conversion, an improvement in water-oxidation reaction by Mn oxides was also observed.162,163 Similar to biological evolution, the phenomenon can be considered as a catalyst evolution.162,163 A self-improvement during the water oxidation reaction was also observed by such conversion.162,163 In addition to this phenomenon, it is suggested that leaching and redeposition cause many hybrids, salts and composites to transform to Mn oxide under the water-oxidation reaction (Scheme 6).164–166

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Scheme 6 Conversion of different manganese-based materials to amorphous manganese oxide under chemical or electrochemical conditions. *In the case of Oxone, the molecular-based mechanism was also reported.36

9. Mn oxide under acidic condition Water oxidation under acidic conditions is very promising in proton exchange membrane (PEM) electrolyzers. In comprehensive electrokinetical experiments in the Nocera group,146 the oxygen evolution mechanism of MnOx was studied in a wide range of pH, acidic, neutral, and alkaline conditions. In the alkaline pH regime, a
60 mV/decade Tafel slope and inverse first-order dependence on proton concentration were observed, while the water oxidation reaction under acidic pH regime showed a quasi-infinite Tafel slope and zeroth-order dependence on proton concentration. The results demonstrated two competing mechanisms: a proton-coupled electron transfer pathway under alkaline conditions and an Mn(IV) disproportionation process, under acidic conditions. Using data for a point of zero charge, it was found in the alkaline regime from pH 11.35 to 13.30 that the MnOx surface has partial negative charge, suggesting a dominance of terminal hydroxo and oxo groups and a proton-coupled electron transfer pathway under acidic conditions, bridging di-μ-hydroxyl and terminal hydroxyl groups (not terminal oxos) are preferred. Therefore, the oxygen-evolution reaction formation would have to be accompanied by the electrode, which cannot generally be accommodated by an infinite Tafel slope. Comparing the rate laws of these

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two oxygen-evolution reaction processes with that of MnOx electrodeposition, clarified the self-healing characteristics of these catalyst films. This work highlighted the employing self-healing process for operating non-noble metal oxide oxygen-evolution reaction catalyst in acidic conditions.146 In 2015, Stephens and co-workers addressed the subject of water oxidation by MnOx under acidic conditions, and used a technique based on occupation of undercoordinated surface sites of MnO2, with a TiO2 as a stable oxide.167 Initially, the oxides in the form of sputtered thin films were synthesized and their catalytic activity and stability toward the oxygen evolution reaction were examined in 0.05 M H2SO4.167 The thin films showed unprecedented activity for non-noble metal oxides in acidic solution. By employing inductively coupled plasma mass spectrometry and electrochemical quartz crystal microbalance analysis, it was found that incorporating Ti into the catalyst caused moderate decreases in mass losses and a slight decrease in catalytic activity toward water oxidation.167 Using XRD analysis, no peaks were observed, indicating that the films are highly disordered and possibly amorphous. In addition, XPS diagrams revealed that the Mn exists in the MnO2 form. Furthermore, by using XPS, the ratio of Mn:Ti in the surface was calculated to be 80:20, which was in good agreement with the measured deposition rates. The SEM images of Ti–MnO2 films as prepared, and after the electrochemical test, were similar to their pure MnO2 counterparts and no significant reconstruction was observed for the two types of thin films during the stability tests, indicating that Ti had no effect on the overall morphology. This approach is wide-ranging, and could be used to stabilize all kinds of catalysts, including metals, against corrosion in electrochemical devices.167

10. Conclusions Manganese-based catalysts toward water oxidation were discussed and reviewed. Among different Mn compounds, Mn oxides are the best candidates for water oxidation. Generalizations may oversimplify, nevertheless many Mn complexes and Mn salts within the water oxidation reaction convert to Mn oxides; even such conversion is observed for Mn oxides. Manganese was used for water oxidation in biological systems and may be applied for water oxidation under special conditions. Learning from Nature is prudent because Nature successfully uses an Mn cluster for water oxidation for millions of years.

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Acknowledgments The authors are grateful to the Institute for Advanced Studies in Basic Sciences and the National Elite Foundation for financial support. M. M. N. would like to acknowledge the members of his research group and collaborators who have contributed to the work described herein. M. M. N. is also very grateful to Prof. Philipp Kurz for the valuable guidance and mentoring that he provided while undertaking research on water oxidation catalysis in his laboratories.

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Further reading 168. Najafpour, M. M. C. R. Chim. 2017, 20, 243.