Mechanism Of Oxygen Evolution Research Articles

Oxygen evolution from water oxidation on molecular catalysts confined in the nanocages of mesoporous silicas

Here, we report that the water oxidation activity can be significantly increased by confining ruthenium molecular catalysts, such as RuII(bda)(pic)2, in the nanocage of SBA-16. The TOF of RuII(bda)(pic)2 confined in the nanocage increased from 1.2 to 8.7 s−1 by simply increasing the number of RuII(bda)(pic)2 molecules from one to seven in each nanocage, which is direct evidence for the “cooperative activation” mechanism involved in a binuclear reaction pathway for water oxidation reactions. The TOF of RuII(bda)(pic)2 confined in the nanocage can be as high as two times that of the homogeneous RuII(bda)(pic)2 due to the enhanced “cooperative activation” in the limited space of nanocages. Moreover, preliminary kinetic studies suggest that the stability of the molecular catalysts can be greatly improved after confinement in the nanocage. This strategy not only provides a new strategy for the preparation of highly efficient solid-hosted catalysts for water oxidation, but also gives direct evidence for the oxygen evolution mechanism.

Kinetics and mechanism of light-driven oxygen evolution at thin film α-Fe2O3 electrodes

Rate constants for recombination and hole transfer during oxygen evolution at illuminated α-Fe(2)O(3) electrodes were measured by intensity-modulated photocurrent spectroscopy and found to be remarkably low. Treatment of the electrode with a Co(II) solution suppressed surface recombination but did not catalyse hole transfer. Intermediates in the reaction were detected spectroscopically.

Mn–Mo–W-oxide anodes for oxygen evolution during seawater electrolysis for hydrogen production: Effect of repeated anodic deposition

Abstract MnMoW-triple oxide catalyst for chlorine-less oxygen generating electrodes from seawater electrolysis were prepared by repeated anodic deposition of their oxides on Ti/IrO 2 substrate. The durability of the electrodes were examined by electrolysis in 0.5 M NaCl solution at current density of 1000 Am −2 and compared with their corresponding electrodes that were prepared by continuous anodic deposition. The electrodes repeatedly and continuously deposited for 90 min kept the oxygen evolution efficiency higher than 99% for 2600 h and 120 h of electrolysis, respectively. The improvement in the electrode durability by repeated deposition was attributed to the formation of single phase oxide deposits of MnMoW-oxides with optimized thickness, composition and structural water content, which enables the reduction of the electrode overpotential for the oxygen evolution reaction and increase overall stability and adhesion of the oxide deposits to Ti/IrO 2 substrate. A mechanism for oxygen evolution at the anode surface during electrolysis was proposed.

Mechanism of PSII oxygen evolution predicted from its 1.9 Å resolution structure

Mechanism of PSII oxygen evolution predicted from its 1.9 Å resolution structure

ChemInform Abstract: Oxygen Evolution from Water Catalyzed by Mononuclear Aquaruthenium Complexes

AbstractReview: 57 refs.

ルテニウム単核錯体を触媒とする水からの酸素発生反応

[Ru(terpy)(bpy)(OH2)]2+ and its analogues were found to be highly active as catalysts toward water oxidation in the presence of Ce4+ as an oxidizing reagent in acidic aqueous media. These findings were considered as a significant breakthrough in this field because there had been a long belief that the four-electron process (i.e., 2H2O→O2+4H++4e−) is much more effectively accelerated by dinuclear or tetranuclear metal complexes. The kinetics of O2 evolution is investigated as a function of either the catalyst concentration or the oxidant concentration, revealing that these catalysts can be classified into two groups exhibiting different rate laws for O2 evolution. Moreover, the singlet biradical character of the hydroxocerium(IV) ion is realized, indicating that the radical coupling of the oxygen atoms of a RuV=O species and a hydroxocerium(IV) ion is the key step for the catalysis. Several important insights into the mechanism of oxygen evolution from water catalyzed by the mononuclear aquaruthenium complexes will also be discussed.

Redox switching and oxygen evolution at oxidized metal and metal oxide electrodes: iron in base

Outstanding issues regarding the film formation, redox switching characteristics and the oxygen evolution reaction (OER) electrocatalytic behaviour of multicycled iron oxyhydroxide films in aqueous alkaline solution have been revisited. The oxide is grown using a repetitive potential multicycling technique, and the mechanism of the latter hydrous oxide formation process has been discussed. A duplex layer model of the oxide/solution interphase region is proposed. The acid/base behaviour of the hydrous oxide and the microdispersed nature of the latter material has been emphasised. The hydrous oxide is considered as a porous assembly of interlinked octahedrally coordinated anionic metal oxyhydroxide surfaquo complexes which form an open network structure. The latter contains considerable quantities of water molecules which facilitate hydroxide ion discharge at the metal site during active oxygen evolution, and also charge compensating cations. The dynamics of redox switching has been quantified via analysis of the cyclic voltammetry response as a function of potential sweep rate using the Laviron-Aoki electron hopping diffusion model by analogy with redox polymer modified electrodes. Steady state Tafel plot analysis has been used to elucidate the kinetics and mechanism of oxygen evolution. Tafel slope values of ca. 60 mV dec(-1) and ca. 120 mV dec(-1) are found at low and high overpotentials respectively, whereas the reaction order with respect to hydroxide ion activity changes from ca. 3/2 to ca. 1 as the potential is increased. These observations are rationalised in terms of a kinetic scheme involving Temkin adsorption and the rate determining formation of a physisorbed hydrogen peroxide intermediate on the oxide surface. The dual Tafel slope behaviour is ascribed to the potential dependence of the surface coverage of adsorbed intermediates.

Mechanism of oxygen reactions at porous oxide electrodes. Part 2—Oxygen evolution at RuO2, IrO2 and IrxRu1−xO2 electrodes in aqueous acid and alkaline solution

The kinetics and mechanism of the oxygen evolution reaction at a series of RuO(2)/IrO(2) mixed oxides in aqueous acid and alkaline solution has been examined using a variety of electrochemical methods. Factors affecting the electrocatalytic activity have been elucidated and novel oxygen evolution mechanisms in terms of reactive oxyruthenium and oxyiridium surface groups are proposed.

Theory of chemical bonds in metalloenzymes. XV. Local singlet and triplet diradical mechanisms for radical coupling reactions in the oxygen evolution complex

AbstractReaction mechanisms of oxygen evolution in native and artificial photosynthesis II (PSII) systems have been investigated on the theoretical grounds, together with experimental results. First of all, our previous broken‐symmetry (BS) molecular orbitals (MO) calculations are reviewed to elucidate the instability of the dπ‐pπ bond in high‐valent (HV) Mn(X)O systems and the dπ‐pπ‐dπ bond in HV MnOMn systems. The triplet instability of these bonds entails strong or intermediate diradical characters: •Mn(IV)O• and •MnOMn•; the BS MO resulted from strong electron correlation, leading to the concept of electron localizations and local spins. The BS computations have furthermore revealed guiding principles for derivation of selection rules for radical reactions of local spins. As a continuation of these theoretical results, the BS MO interaction diagrams for oxygen‐radical coupling reactions in the oxygen evolution complex (OEC) in the PSII have been depicted to reveal scope and applicability of local singlet diradical (LSD) and local triplet diradical (LTD) mechanisms that have been successfully utilized for theoretical understanding of oxygenation reactions mechanisms by p450 and methane monooxygenase (MMO). The manganese‐oxide cluster models examined are London, Berlin, and Berkeley models of CaMn4O4 and related clusters Mn4O4 and Mn3Ca. The BS MO interaction diagrams have revealed the LSD and/or LTD mechanisms for generation of molecular oxygen in the total low‐, intermediate and high‐spin states of these clusters. The spin alignments are found directly corresponding to the spin‐coupling mechanisms of oxygen‐radical sites in these clusters. The BS UB3LYP calculations of the clusters have been performed to confirm the comprehensive guiding principles for oxygen evolution; charge and spin densities by BS UB3LYP are utilized for elucidation and confirmation of the LSD and LTD mechanisms. Applicability of the proposed selection rules are examined in comparison with a lot of accumulated experimental and theoretical results for oxygen evolution reactions in native and artificial PSII systems. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Mechanism and Activity of Photocatalytic Oxygen Evolution on Titania Anatase in Aqueous Surroundings

Due to its high overpotential and low efficiency, the conversion of water to O(2) using solar energy remains a bottleneck for photocatalytic water splitting. Here the microscopic mechanisms of the oxygen evolution reaction (OER) on differently structured anatase surfaces in aqueous surroundings, namely, (101), (001), and (102), are determined and compared systematically by combining first-principles density functional theory calculations and a parallel periodic continuum solvation model. We show that OER involves the sequential removal of protons from surface oxidative species, forming surface peroxo and superoxo intermediates. The initiating step, the first proton removal, dictates the high overpotential. Only at an overpotential of 0.7 V (1.93 V vs SHE) does this rate-controlling step become surmountable at room temperature: the free energy change of the step is 0.69, 0.63, and 0.61 eV for (101), (102), and (001) surfaces, respectively. We therefore conclude that (i) OER is not sensitive to the local surface structure of anatase and (ii) visible light (<∼590 nm) is, in principle, capable of driving the photocatatlytic OER on anatase kinetically. By co-doping high-valent elements into the anatase subsurface, we demonstrate that the high overpotential of the OER can be significantly reduced, with extra occupied levels above the valence band.

Orientations of Carboxylate Groups Coupled to the Mn Cluster in the Photosynthetic Oxygen-Evolving Center As Studied by Polarized ATR-FTIR Spectroscopy

It is essential to clarify the structures and interactions of amino acids surrounding the Mn cluster in photosystem II (PSII) to understand the molecular mechanism of photosynthetic oxygen evolution. In this study, polarized attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) was applied for the first time to PSII to investigate the orientation of carboxylate groups coupled to the oxygen-evolving Mn cluster. PSII membranes from spinach were oriented on the surface of a silicon ATR crystal, and flash-induced polarized ATR-FTIR difference spectra for the S(1) --> S(2) transition (S(2)/S(1) spectra) were obtained. The distribution of membrane orientations by mosaic spread was estimated from the semiquinone CO peak in polarized Q(A)(-)/Q(A) difference spectra recorded using the same oriented sample by buffer exchange. The orientations of carboxylate groups coupled to the Mn cluster were estimated from the dichroic ratios of the symmetric COO(-) bands in the polarized S(2)/S(1) ATR-FTIR spectra. We found that most of the carboxylate groups perturbed during the S(1) --> S(2) transition, due to direct ligation to the Mn cluster or though a hydrogen bond network, have orientations in a relatively narrow angle range of 34-48 degrees with respect to the membrane normal. Implications of the obtained orientations and the changes upon formation of S(2) are discussed on the basis of the information from previous FTIR studies and the X-ray structures. The results in this study show that polarized ATR-FTIR difference spectroscopy is a fruitful method for investigating the orientations and their reaction-induced changes in redox cofactors and coupled amino acid side chains in photosynthetic proteins.

On the Minimum Quantum Requirement of Photosynthesis

An analysis of the shape of photosynthetic light curves is presented and the existence of the initial non-linear part is shown as a consequence of the operation of the non-cooperative (Kok's) mechanism of oxygen evolution or the effect of dark respiration. The effect of nonlinearity on the quantum efficiency (yield) and quantum requirement is reconsidered. The essential conclusions are: 1) The non-linearity of the light curves cannot be compensated using suspensions of algae or chloroplasts with high (>1.0) optical density or absorbance. 2) The values of the maxima of the quantum efficiency curves or the values of the minima of the quantum requirement curves cannot be used for estimation of the exact value of the maximum quantum efficiency and the minimum quantum requirement. The estimation of the maximum quantum efficiency or the minimum quantum requirement should be performed only after extrapolation of the linear part at higher light intensities of the quantum requirement curves to "0" light intensity.

Effect of a Single-Amino Acid Substitution of the 43 kDa Chlorophyll Protein on the Oxygen-Evolving Reaction of the Cyanobacterium Synechocystis sp. PCC 6803: Analysis of the Glu354Gln Mutation

We constructed a mutant (CP43-Glu354Gln) of the cyanobacterium Synechocystis sp. PCC 6803 in which the glutamic acid at position 354 of the 43 kDa chlorophyll protein (CP43) was replaced with glutamine. To determine the effect of this mutation on the reaction processes of the Mn cluster in the oxygen-evolving complex, we mainly analyzed the spectroscopic properties, including Fourier transform infrared (FTIR) spectroscopy, of photosystem II core complexes. Mutant cells exhibited a lower oxygen-evolving activity than wild-type cells, and an altered pattern of flash-dependent delayed luminescence. This phenotype differed somewhat from an earlier report of the same mutant [Strickler, M. A., et al. (2008) Philos. Trans. R. Soc. London, Ser. B 363, 1179-1187]. FTIR difference spectroscopy revealed that CP43-Glu354 functions as a ligand to the Mn cluster, most likely with bridging bidentate coordination to two Mn ions in the S(1) state and chelating bidentate coordination to a single Mn ion in the S(2) state. A single water molecule was bound to the same Mn atom to which CP43-Glu354 was ligated, and this Mn atom was oxidized in the S(1)-to-S(2) transition. This is the first report on a binding site of a water molecule relevant to a specific amino acid ligand. We found that the Mn ion or ligand that is oxidized in the S(2)-to-S(3) transition was not directly coupled to CP43-Glu354. While the definitive assignment of ligation to the Mn atoms is still under debate, our identification of a novel water binding site will lead to new insights into the oxygen evolution mechanism.

Redox switching and oxygen evolution electrocatalysis in polymeric iron oxyhydroxide films

Outstanding issues regarding the redox switching characteristics and the oxygen evolution reaction (OER) electrocatalytic behaviour of multicycled iron oxyhydroxide films in aqueous alkaline solution have been examined. Charge percolation through the hydrous layer has been quantified, using cyclic voltammetry, in terms of a charge transport diffusion coefficient D(CT) which admits a value of ca. 3 x 10(-10) cm2 s(-1). Steady-state Tafel plot analysis and electrochemical impedance spectroscopy have been used to elucidate the kinetics and mechanism of oxygen evolution. Tafel slope values of ca. 60 mV dec(-1) and ca. 120 mV dec(-1) are found at low and high overpotentials respectively, whereas the reaction order with respect to hydroxide ion activity changes from ca. 3/2 to ca. 1 as the potential is increased. These observations are rationalised in terms of a kinetic scheme involving Temkin adsorption and the rate determining formation of a physisorbed hydrogen peroxide intermediate on the oxide surface. The dual Tafel slope behaviour is ascribed to the potential dependence of the surface coverage of adsorbed intermediates.

Light-induced FTIR difference spectroscopy as a powerful tool toward understanding the molecular mechanism of photosynthetic oxygen evolution

The molecular mechanism of photosynthetic oxygen evolution remains a mystery in photosynthesis research. Although recent X-ray crystallographic studies of the photosystem II core complex at 3.0-3.5 A resolutions have revealed the structure of the oxygen-evolving center (OEC), with approximate positions of the Mn and Ca ions and the amino acid ligands, elucidation of its detailed structure and the reactions during the S-state cycle awaits further spectroscopic investigations. Light-induced Fourier transform infrared (FTIR) difference spectroscopy was first applied to the OEC in 1992 as detection of its structural changes upon the S(1)-->S(2) transition, and spectra during the S-state cycle induced by consecutive flashes were reported in 2001. These FTIR spectra provide extensive structural information on the amino acid side groups, polypeptide chains, metal core, and water molecules, which constitute the OEC and are involved in its reaction. FTIR spectroscopy is thus becoming a powerful tool in investigating the reaction mechanism of photosynthetic oxygen evolution. In this mini-review, the measurement method of light-induced FTIR spectra of OEC is introduced and the results obtained thus far using this technique are summarized.

The electrocatalytic reactions of adenine, guanine, H2O, H2O2, N2H4, and l-cysteine catalyzed by poly(Ni(4-TMPyP)) film-modified electrodes

Electrochemical preparation of poly(nickel tetrakis(N-methyl-4-pyridyl)porphyrin) tetratosylate (poly-Ni(4-TMPyP)) produces stable and electrochemically active films in strong and weak basic aqueous solutions. These films were produced on glassy carbon and gold electrodes. The electrochemical quartz crystal microbalance and cyclic voltammetry were used to study the in situ growth of poly(Ni(4-TMPyP)) films. The electrochemical properties of poly(Ni(4-TMPyP)) films indicate that the redox process was confined in to the immobilized film. The electrochemical quartz crystal microbalance results showed an ion exchange reaction for the redox couple. The polymer films showed one new redox couple when transferred to strong and weak basic aqueous solutions and the formal potential was found to be pH dependent. The electrocatalytic oxidation of H2O by a nickel tetrakis(N-methyl-4-pyridyl)porphyrin film-modified electrode was also performed. The mechanism of oxygen evolution was determined by cyclic voltammetry, chronoamperometry and rotating ring disc electrode methods. The oxygen evolution was determined by a bicatalyst system using hemoglobin, and iron tetrakis (N-methyl-2-pyridyl)porphyrin as catalyst to detect the oxygen by electrocatalytic reduction. The electrocatalytic oxidations of adenine, guanine, H2O2, N2H4, NH2OH, and l-cysteine by the film-modified electrode obtained from water-soluble nickel porphyrin were also investigated.

Current molecular mechanisms of photosynthetic oxygen evolution

The recently published 3.5-Å resolution X-ray crystal structure of a cyanobacterial Photosystem II provides a detailed architecture of the oxygen-evolving complex (OEC) and the surrounding amino acids. Despite intense interest in the mechanism of dioxygen evolution by the OEC of Photosystem II in green plants, the precise mechanism involved remains a matter of debate. In this paper two mechanisms are compared, and a new mechanism is described.

A time-resolved vibrational spectroscopy glimpse into the oxygen-evolving complex of photosynthesis

Oxygenic photosynthesis is arguably one of the most important biochemical processes occurring in the biosphere. Photons produced by solar fusion are harvested by photosynthetic organisms and are used to drive the production of reducing equivalents in the form of NADPH and energy equivalents in the form of ATP, both of which are subsequently used in carbohydrate biosynthesis. Central to this process is the extraction of electrons from water with the concomitant production of molecular oxygen. This extraction is carried out by an exquisite molecular machine called photosystem II. Photosynthetic organisms consequently generate virtually all of Earth’s atmospheric oxygen and produce the carbohydrates that lie at the base of nearly all food chains. In this issue of PNAS, Barry et al. (1) describe the use of time-resolved vibrational spectroscopy to detect protein-centered intermediates in the oxygen-evolving process. This study paves the way for the identification of the protein domains responsible for these transient signals, which should in turn provide critical details for unraveling the mechanism for oxygen evolution.

A diruthenium-substituted polyoxometalate as an electrocatalyst for oxygen generation.

Transition metal heteropolyanions have been used to catalyze a variety of organic oxidations but have not previously been used for O2 generation, despite sharing some structural similarities with dioxoruthenium water-oxidation catalysts. In this study, we report that the di-Ru-substituted polyoxometalate (POM) [Ru2Zn2(H2O)2(ZnW9O34)2]14- can be used to catalyze the electrochemical generation of O2. By comparing the behavior of this compound to that observed using a mono-Ru-substituted POM catalyst, we show that adjacent Ru sites are necessary to observe O2 generation. These observations suggest a reaction pathway involving two Ru-bound oxygen species combining to form O2 and are consistent with the accepted mechanism of electrochemical oxygen evolution. Finally, analysis of the observed electrode kinetics yields a Tafel slope of roughly 120 mV, which is similar to values reported previously for perovskite anodes.

Homologs of plant PsbP and PsbQ proteins are necessary for regulation of photosystem ii activity in the cyanobacterium Synechocystis 6803.

The mechanism of oxygen evolution by photosystem II (PSII) has remained highly conserved during the course of evolution from ancestral cyanobacteria to green plants. A cluster of manganese, calcium, and chloride ions, whose binding environment is optimized by PSII extrinsic proteins, catalyzes this water-splitting reaction. The accepted view is that in plants and green algae, the three extrinsic proteins are PsbO, PsbP, and PsbQ, whereas in cyanobacteria, they are PsbO, PsbV, and PsbU. Our previous proteomic analysis established the presence of a PsbQ homolog in the cyanobacterium Synechocystis 6803. The current study additionally demonstrates the presence of a PsbP homolog in cyanobacterial PSII. Both psbP and psbQ inactivation mutants exhibited reduced photoautotrophic growth as well as decreased water oxidation activity under CaCl(2)-depleted conditions. Moreover, purified PSII complexes from each mutant had significantly reduced activity. In cyanobacteria, one PsbQ is present per PSII complex, whereas PsbP is significantly substoichiometric. These findings indicate that both PsbP and PsbQ proteins are regulators that are necessary for the biogenesis of optimally active PSII in Synechocystis 6803. The new picture emerging from these data is that five extrinsic PSII proteins, PsbO, PsbP, PsbQ, PsbU, and PsbV, are present in cyanobacteria, two of which, PsbU and PsbV, have been lost during the evolution of green plants.