Oxygen Evolution Step Research Articles

Mechanism and Dynamics of Formation of Bisoxo Intermediates and O–O Bond in the Catalytic Water Oxidation Process

This work elucidates the reactivity of water molecules toward the tridentate nitrogen-containing iron complex in the water oxidation process. Here, we consider the FeV-bisoxo complex {[FeV(Me3tacn)(OH2)(═O)2]+} to be responsible for the oxygen-oxygen bond formation. This O-O bond formation happens through the addition of water as a nucleophile. The transition state was determined by the synchronous transit-guided quasi-Newton method using reactants and products and verified by intrinsic reaction coordinates (IRCs). From the IRC calculations, we observe that the FeV═O moiety is attacked by water and assisted by the H-bonded interaction with the oxygen atom of the bisoxo complex. The hydrogen atom is transferred to the oxygen atom of the bisoxo complex through the transition state, and subsequently, the hydroxide is transferred to another oxygen of the bisoxo complex, resulting in the formation of the oxygen-oxygen bond. This work also explains the effect of explicit water molecules on the oxygen-oxygen bond formation. Our results also show how the formation of superoxide plays an essential role in O2 evolution. We used the potential energy scan method to compute the transition state in the oxygen evolution step. In the present work, we study the effect of chlorine on the formation of the oxygen-oxygen bond formation. In this study, the changes in the oxidation state, spin density, and spin multiplicity of the complexes are investigated for each successive step. Apart from these static theoretical calculations, we also studied the oxygen-oxygen bond formation through first-principles molecular dynamics with the aid of the well-tempered metadynamics sampling technique. From the observation of the free energy surfaces from metadynamics simulations, it is evident that the hydroxide transfer has a lesser free energetic reaction as compared to the proton transfer. This complete mechanistic study may give an idea to design a suitable water oxidation catalyst.

Chemical Hydrogenations Using an Electro-Generated Mediator

The Electron-Coupled-Proton Buffer (see: Nature Chem. 2013, 5, 403-409; Nature Chem. 2018, 10, 1042-1047; Joule, 2018, 2, 1390-1395) permits the electrochemical oxygen evolution reaction from water to be coupled to the reversible reduction and protonation of a redox mediator (rather than the coupling of the oxygen evolution reaction directly with hydrogen production). Careful selection of this redox mediator, along with judicious choice of the electrode material for the cathode reaction, can allow reduced species to be generated that are thermodynamically unstable with respect to hydrogen generation (Science, 2014, 345, 1326-1330). These reduced mediators can either be used for spontaneous hydrogen generation (decoupled from the oxygen evolution step), or they can be used as soluble sources of hydrogen for performing the hydrogenation of organic molecules (Chem. Commun. 2018, 54, 1093-1096). In this talk, we such discuss current progress and potential future avenues for the use of Electron-Coupled-Proton Buffers as reducing equivalents in organic synthesis.

Chemical Hydrogenations Using an Electro-Generated Mediator

The Electron-Coupled-Proton Buffer (see: Nature Chem. 2013, 5, 403-409; Nature Chem. 2018, 10, 1042-1047; Joule, 2018, 2, 1390-1395) permits the electrochemical oxygen evolution reaction from water to be coupled to the reversible reduction and protonation of a redox mediator (rather than the coupling of the oxygen evolution reaction directly with hydrogen production). Careful selection of this redox mediator, along with judicious choice of the electrode material for the cathode reaction, can allow reduced species to be generated that are thermodynamically unstable with respect to hydrogen generation (Science, 2014, 345, 1326-1330). These reduced mediators can either be used for spontaneous hydrogen generation (decoupled from the oxygen evolution step), or they can be used as soluble sources of hydrogen for performing the hydrogenation of organic molecules (Chem. Commun. 2018, 54, 1093-1096). In this talk, we such discuss current progress and potential future avenues for the use of Electron-Coupled-Proton Buffers as reducing equivalents in organic synthesis.

Oxygen-Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O2 Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical.

In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric reduction of O2 to H2O with 2 equiv of Co(II) and suggests a new pathway for selective reduction of O2 to water via Co(III)-O-O-Co(III) peroxo intermediates.

Rapid evolution of the Photosystem II electronic structure during water splitting.

Photosynthetic water oxidation is a fundamental process that sustains the biosphere. A Mn4Ca cluster embedded in the photosystem II protein environment is responsible for the production of atmospheric oxygen. Here, time-resolved x-ray emission spectroscopy (XES) was used to observe the process of oxygen formation in real time. These experiments reveal that the oxygen evolution step, initiated by three sequential laser flashes, is accompanied by rapid (within 50 μs) changes to the Mn Kβ XES spectrum. However, no oxidation of the Mn4Ca core above the all MnIV state was detected to precede O-O bond formation, and the observed changes were therefore assigned to O-O bond formation dynamics. We propose that O-O bond formation occurs prior to the transfer of the final (4th) electron from the Mn4Ca cluster to the oxidized tyrosine YZ residue. This model resolves the kinetic limitations associated with O-O bond formation, and suggests an evolutionary adaptation to avoid releasing of harmful peroxide species.

O-O Bond Formation in Photosystem II Oxygen Evolving Complex

Photosynthetic water oxidation is a fundamental process that sustains the biosphere. Light driven water oxidation is facilitated in Nature by the Oxygen Evolving Complex (OEC) - Mn4Ca cluster embedded in the photosystem II protein. During its function, the OEC cycles through five redox states termed S0-S4 after which, oxygen is evolved. Here, time-resolved x-ray emission spectroscopy (XES) was used to observe the process of oxygen formation in real time. These experiments reveal that the oxygen evolution step, initiated by three sequential laser flashes, is accompanied by rapid (within 50 μs) changes to the Mn Kβ XES spectrum. However, no oxidation of the Mn4Ca core above the all MnIV state was detected to precede O-O bond formation, and the observed changes were therefore assigned to O-O bond formation dynamics. Our mechanism proposes O-O bond formation occurs prior to the transfer of the final (4th) electron from the Mn4Ca cluster to the oxidized TyrZ. This model resolves the kinetic limitations associated with O-O bond formation, and suggests an evolutionary adaptation to avoid releasing of harmful peroxide species. Atomistic model for the S3 state reported by us [1, 2] is indistinguishable (within the resolution of XRD) from one of the recently reported by Shen and co-workers [3]. This S3-state model incorporates a MnIV=O fragment of radicaloid character and exhibits antiferromagnetic alignment with opposite spin orientations between two Mn centers and associated oxygens creating an opportunity for a low barrier to O-O bond formation. 1. Davis et al. 2015, arXiv:1506.08862 2. Jensen et al. J Phys. Chem. Letters, 2017, 8, 2584−2589. 3. Suga et al. Nature, 2017, 543(7643), 131-135.

Rational Ligand Design for an Efficient Biomimetic Water Splitting Complex.

Being an important biomimetic model catalyst for water oxidation, the dimanganese molecular complex [H2O(terpy)MnIII(μ-O)2MnIV(terpy)OH2]3+ (complex 1, terpy = 2,2':6',2″-terpyridine) has been investigated extensively by experimentalists. By carrying out density functional theory calculations, we explore theoretically the oxygen evolution mechanisms of complex 1. On the basis of understandings of the geometric and electronic structural features of complex 1, we explore the possibility of improving its catalytic efficiency through a rational design of ligands coordinated to the manganese ions. Recognizing that the rate-determining step of oxygen evolution is the formation of an O-O bond at a high-valent manganese center, we design a new complex, [H2O(2-bpnp)MnIII(μ-O)2MnIV(2-bpnp)OH2]3+ (complex 2, 2-bpnp = 2-([2,2'-bipyridin]-6-yl)-1,8-naphthyridine). It is verified that the proton-accepting 2-bpnp ligand leads to stabilized hydrogen bonding with surrounding water molecules, and hence, the barrier height associated with O-O bond formation is substantially reduced. Moreover, despite its larger size, the 2-bpnp ligand does not cause steric hindrance for the release of molecular oxygen. Consequently, the proposed complex 2 is expected to outperform the existing complex 1 regarding catalytic efficiency. This work highlights the potential usefulness of rational design toward reaching the high efficiency of the oxygen evolution center in photosystem II.

Effect of Water Adsorption on Carrier Trapping Dynamics at the Surface of Anatase TiO2 Nanoparticles.

Charge carrier trapping plays a vital role in heterogeneous photocatalytic water splitting because it strongly affects the dynamics of photogenerated charges and hence the photoconversion efficiency. Although hole trapping by water at water/photocatalyst interface is the first step of oxygen evolution in water splitting, little has been known on how water adsorbate itself is involved in hole trapping dynamics. To clarify this point, we have performed infrared transient and steady-state absorption spectroscopy of anatase TiO2 nanoparticles as a function of the number of water adsorbate layers. Here, we demonstrate that water molecules reversibly adsorbed in the first layer on TiO2 nanoparticles are capable to trap photogenerated holes, while water in the second layer hydrogen bonding to the first-layer water makes hole trapping less effective.

ORR on the Different Surfaces with Different Adsorbents: First Principles Molecular Dynamics Simulations

Oxygen Reduction Reaction (ORR) is the hard task in the fuel cell operations and has been studied extensively not only from practical point of view but also from fundamental one. Although various calculations by both MO and DFT calculations were reported, many of them are under the static conditions (i.e. at 0 K) or they are on simple surfaces. We are currently working for it with more realistic models at a certain temperature under the electric bias. First Principles Molecular Dynamics (FPMD) combined with Effective Screening Medium (ESM) [1,2] to apply the potential on the electrode in the similar manner as the real double layer region. We first used it for the simple electrode reaction of the first step of the hydrogen evolution reaction (Volmer reaction) on Pt(111) [3] and then we are extending it to ORR [4,5,6] on various surfaces with various adsorbents.Electronic structure calculations in the FPMD are based on DFT with ultrasoft-pseudopotentials and PBE functional for the exchange correlation. Program code "STATE" is used. MD is performed at constant temperature 353 K by a velocity scaling thermostat. Electrode potential is controlled by the charges added by the ESM method shown in the figure.The last step of ORR (=the first step of oxygen evolution) isH3O+ + OH(ad) + e– --> 2 H2OThe reversibility of this reaction was confirmed on Pt(111) surface with surface charge density σ = 7 µC/cm2(corresponding to ca. 0.6–0.7 V vs. SHE) [4,5] with a pre-adsorbed OH. With higher or lower σ, reaction was remained in only one side. If we introduce a defect (a pit) on the surface, the reaction did not take place [6].In order to see an effect of the different surface, we simulated a full ORR from O2adsorption to H2O formation on (111), (110) and (100) surfaces. The main reaction scheme observed in the several time simulations for each surface is (dissociation scheme):O2 --> 2O(ad)O(ad) + H3O+ + e– --> OH(ad) + H2OH3O+ + OH(ad) + e– --> 2 H2OIn some cases, following H2O2 formation takes place as an intermediate species (association scheme).2O(ad) + e– + H3O+ --> H2O2 We are also testing another kind of coadsorption species as a reaction promoter.In summary, we could confirm that water liquid layer (not only single or bilayer) on the Pt surface plays an important role to proceed to a complete ORR.[1] M. Otani and O. Sugino, Phys. Rev. B 73, 115407 (2006). [2] O. Sugino, I. Hamada, M. Otani, Y. Morikawa, T. Ikeshoji Y. Okamoto, Surf. Sci., 601 (2007) 5237. [3] M. Otani, I. Hamada, O. Sugino, Y. Morikawa, Y. Okamoto, T. Ikeshoji, J. Phys. Soc. Jpn.. 77 (2008) 024802, Phys. Chem. Chem. Phys., 10 (2008) 3609. [4] T. Ikeshoji, M. Otani, I. Hamada and Y. Okamoto, Phys. Chem. Chem. Phys., 13 (2011) 20223. [5] T. Ikeshoji, M. Otani, I. Hamada, O. Sugino, Y. Morikawa, Y. Okamoto, Y. Qian, and I. Yagi, AIP Advances, 2, 032182 (2012). [6] Y. Qian, I. Hamada, M. Otani, and T. Ikeshoji, Catal. Today, 202, 163-167 (2013).

Catalytic activities of cobalt, nickel and copper ferrospinels for sulfuric acid decomposition: The high temperature step in the sulfur based thermochemical water splitting cycles

The catalytic decomposition of sulfuric acid is the most endothermic step of the sulfur based water splitting thermochemical cycles, which are promising technologies for large scale hydrogen production in future. In the present study the catalytic activities of three ferrospinels AFe 2O 4 (A = Co, Ni, Cu) were evaluated for high temperature sulfuric acid decomposition reaction. Catalyst characterization by Mössbauer spectroscopy confirmed the occupancy of octahedral and tetrahedral sites by Fe 3+ ions in all three inverse spinels. The temperature programmed reduction studies revealed that the reducibility of Fe 3+ was greatly enhanced in CuFe 2O 4 as compared to other ferrites. Copper ferrite was found to be the most active catalyst for the reaction with ∼78% conversion at 800 °C. Presence of sulfate species on the spent catalysts was revealed by an ex situ analysis of the spent catalyst samples by FTIR, SEM and evolved gas analysis (EGA). FTIR spectra of all the three spent catalyst samples exhibit four prominent peaks in the region 950–1200 cm −1, which is an indicative of C 2v symmetry and bidentate sulfate coordination. A plausible mechanism for the sulfuric acid decomposition over spinel ferrites was proposed via the metal sulfate formation and then decomposition followed with an oxygen evolution step with the sulfate decomposition being the rate determining step at higher temperatures. EGA showed evolution of SO 2 as a decomposition product from existed sulfate of spent catalysts at high temperatures, with the rate of SO 2 evolution following the order: CuFe 2O 4 > NiFe 2O 4 > CoFe 2O 4. The enhanced rate of decomposition of the sulfates of copper ferrite can be attributed to the higher electronegativity of Cu 2+ as compared to Ni 2+ and Co 2+, which renders the S–O bond in the mixed metal sulfate weaker than others and thus more susceptible to dissociation. The lower thermal stability of sulfate and better reducibility are responsible for the improved catalytic properties of copper ferrite among the three ferrospinels investigated for sulfuric acid decomposition.

Kinetics of the oxygen evolution step in plants determined from flash-induced chlorophyll t a fluorescence

Photosystem II (PS II) of plants and cyanobacteria, which catalyzes the light-induced splitting of water and the release of oxygen, is the primary source of oxygen in the earth atmosphere. When activated by short light flashes, oxygen release in PS II occurs periodically with maxima after the third and the seventh flashes. Many other processes, including chlorophyll (Chl) t a fluorescence, are also modulated with period of four, reflecting their sensitivity to the activity of Photosystem II. A new approach has been developed for the analysis of the flash-induced fluorescence of Chl t a in plants, which is based on the use of the generalized Stern–Volmer equation for multiple quenchers. When applied to spinach thylakoids, this analysis reveals the presence of a new quencher of fluorescence whose amplitude is characterized by a periodicity of four with maxima after the third and the seventh flashes, in phase with oxygen release. The quencher appears with a delay of ≈ 0.5 ms followed by a rise time of 1.2–2 ms at pH 7, also in agreement with the expected time for oxygen evolution. It is concluded that the quencher is a product of the reaction leading to the oxygen evolution in PS II. The same quenching activity, maximal after the third flash, could be seen in dark adapted leaves, and provides the first fully time-resolved measurement of the kinetics of the oxygen evolution step in the leaf. Thus, the non-invasive probe of Chl t a fluorescence provides a new and sensitive method for measuring the kinetics of oxygen evolution with potential for use in plants and cyanobacteria t in vivo.

Kinetics of the Oxidation of Dithionite (S2O42−) by Manganese (III)-Hematoporphyrin

Abstract Manganese porphyrins were first prepared by Zaleski over sixty years ago,1 and can exist in the plus two, three, and four oxidation States. Calvin2 has suggested that the manganese required for the oxygen evolution step in plant photosynthesis is bound in a porphyrin type ligand. The synthetic, electrochemical and theoretical studies on manganese porphyrins have been recently reviewed in detail by Boucher.3 The kinetics of oxidation of Cr2+ and V2+ by Mn(III)-tetrapyridylporphin have been studied in acid solution,4 the main result being that such reactions are halide catalyzed, but much less so than their Fe(III) counterparts. The dithionite ion (S2O4 2−) is the most common reductant used to produce Mn(II) porphyrins in basic solution,5 presumably by the overall stoichiometry: We report the kinetics of oxidation of dithionite by Mn(III) hematoporphyrin with a view of establishing the mechanism of such metalloporphyrin reduction and to further explore the chemistry of dithionite.