Mechanism Of Water Oxidation Reaction Research Articles

Trinuclear Nickel Catalyst for Water Oxidation: Intramolecular Proton-Coupled Electron Transfer Triggered Trimetallic Cooperative O–O Bond Formation

Trinuclear Nickel Catalyst for Water Oxidation: Intramolecular Proton-Coupled Electron Transfer Triggered Trimetallic Cooperative O–O Bond Formation

Exploring the mechanism of electrocatalytic water oxidation on CoO decorated Ti3C2Tx nanoplatelets

In this work, we report the facile preparation of ultrathin cobalt oxide layer decorated Ti3C2Tx nanoplatelets (CoO@Ti3C2Tx) and explore the mechanism of water oxidation reaction. Cobalt ions form complexes in the presence of ammonia, which lead to the uniform anchoring of the cobalt precursor on the surface of the nanoplatelets. Randles-Sevcik analysis revealed that water oxidation on CoO@Ti3C2Tx likely first undergoes a one-electron transfer pathway to form hydroxyl radical, two of which then combine to produce hydrogen peroxide. The latter, successfully detected via strip colorimetry, further disproportionately or electrochemically transforms to gaseous oxygen. The water oxidation reaction on CoO@Ti3C2Tx nanoplatelets with a low amount of ∼4%w.t. cobalt loading reaches 10 mA cm−2 at 1.68 V with a small Tafel slope of 72 mV dec−1 and robust stability with only 5.14% decay over 12 h. This work not only provides an important implication on the design and preparation of the cost-effective, efficient and durable MXene based electrocatalysts but also demonstrated the specific pathway of water oxidation reaction.

Operando time-resolved diffuse reflection spectroscopy: The origins of photocatalytic water-oxidation activity of bismuth vanadate

Photocatalytic water splitting has been believed to be a key technology for the construction of a sustainable society based on a renewable energy source. Understanding the exact mechanisms of oxygen evolution by water-splitting reaction, which has been regarded as a bottleneck to realize the water splitting in practical basis, should help us to enhance solar-to-hydrogen energy conversion efficiency. Here, we present a detailed investigation based on two types of bismuth vanadate (BiVO4) crystals. One is a monoclinic scheelite crystal that forms a truncated tetragonal bipyramidal shape (MS), and the other is a tetragonal zircon crystal that forms an octahedral shape (TZ). MS, which has relatively higher photocatalytic activity of water oxidation, rarely adsorbed reduced mediator (Fe2+). Operando time-resolved diffuse reflection spectral measurement demonstrated, for the first time, that the surface holes of MS crystals show extremely high reactivity, while those of TZ crystals show poor reactivity. Our investigation would promote an essential understanding of the photocatalytic reaction mechanism of water oxidation.

The influence of the pH on the reaction mechanism of water oxidation by a Ru(bda) catalyst

Recent results of Concepcion’s group (Chem. Com51 (2015) 4105) on water oxidation catalysis (WOC) by a ruthenium complex suggest that, at pH = 8, O2 release takes place after formation of a rhomboid bis(μ-oxo)-RuV2 species and not after generation of the typical μ-η1:η1-peroxo-RuIV2 intermediate, coming from the coupling of two RuV=O moieties (I2M mechanism), which is widely accepted to be formed at pH = 1. To analyze the differences between the reaction mechanisms of this WOC at different pHs, we performed DFT calculations of the full mechanism at pH = 1 and 8 of the WOC process catalyzed by the 2,2′-bipyridine-6,6′-dicarboxylate Ru complex. At pH = 8, we found that barriers leading to the hypothetic formation of rhombic RuV2O2 species are higher than those involved in the canonical I2M mechanism. The rate determining step at the latter pH is found to be the dimer formation while the bond cleavage for the O2 liberation process is barrierless. The computational results confirm that the most common I2M mechanism is preferred at both pHs, as the new proposal comprising formation of bis(μ-oxo)-RuV2 species involves higher energy barriers.

Capturing the Role of Phosphate in the Ni‐PY5 Catalyzed Water Oxidation

AbstractThe mononuclear Nickel complex Ni‐PY5 [PY5=2, 6‐bis(1,1‐bis(2‐pyridyl)ethyl)pyridine] has been disclosed to catalyze water oxidation electrochemically with an applied potential of 1.5 V at pH 10.8 in aqueous phosphate buffer solution. Density functional calculations were used to elucidate the reaction mechanism of water oxidation catalyzed by this nickel complex and to capture the role of the phosphate. The calculations demonstrated that the oxidations of the starting [OH2−NiII‐PY5]2+ complex by two sequential proton‐coupled electron‐transfer processes lead to the formation of a key intermediate [O=NiIV‐PY5]2+. O−O bond formation then takes place through a water nucleophilic attack on the high‐valent NiIV=O moiety of the catalyst, facilitated by a hydrogen phosphate anion, with a total barrier of 11.5 kcal mol−1. The calculated barrier agrees very well with the experimental turnover frequency of about 2000 s−1, which corresponds to a barrier of 12.9 kcal mol−1. The calculated deuterium kinetic isotope effect of 1.99 is also in excellent agreement with the experimental value of 2.06. Finally, we also predicted the catalytic activity of other PY5‐based first‐row transition metal complexes, namely, involving Mn, Fe, Co and Cu. The calculations showed that the Mn, Fe, Co complexes have higher barrier for water oxidation, while the Cu complex has lower barrier and higher water oxidation activity compared with the Ni complex.

A molecular precatalyst for water oxidation based on half-sandwich iridium fragment

A newly Ir-mediated complex constructed by treating [Cp*IrCl2]2 with an azo-compound [sodium (E)-4-((3-hydroxynaphthalen-2-yl)diazenyl)benzenesulfonate (HL)], was fully characterized by NMR, EA, TGA, IR and single crystal X-ray diffraction to reveal its molecular structure with mononuclear framework. Cyclic voltammetry (CV) was performed on detecting its electronic properties, affording potential application of catalyzing water oxidation to evolve oxygen. Moreover, the complex utilized as precatalyst of WOC was preformed under mild conditions, offering a high turnover frequency (TOF) which reached up to 4692.1 M−1 S−1. Meanwhile, the mechanism of water-oxidation reaction was elaborated in detail in this work.

Mechanism for O[sbnd]O bond formation in a biomimetic tetranuclear manganese cluster – A density functional theory study

Density functional theory calculations have been used to study the reaction mechanism of water oxidation catalyzed by a tetranuclear Mn-oxo cluster Mn4O4L6 (L=(C6H4)2PO4−). It is proposed that the OO bond formation mechanism is different in the gas phase and in a water solution. In the gas phase, upon phosphate ligand dissociation triggered by light absorption, the OO bond formation starting with both the Mn4(III,III,IV,IV) and Mn4(III,IV,IV,IV) oxidation states has to take place via direct coupling of two bridging oxo groups. The calculated barriers are 42.3 and 37.1kcal/mol, respectively, and there is an endergonicity of more than 10kcal/mol. Additional photons are needed to overcome these large barriers. In water solution, water binding to the two vacant sites of the Mn ions, again after phosphate dissociation triggered by light absorption, is thermodynamically and kinetically very favorable. The catalytic cycle is suggested to start from the Mn4(III,III,III,IV) oxidation state. The removal of three electrons and three protons leads to the formation of a Mn4(III,IV,IV,IV)-oxyl radical complex. The OO bond formation then proceeds via a nucleophilic attack of water on the MnIV-oxyl radical assisted by a Mn-bound hydroxide that abstracts a proton during the attack. This step was calculated to be rate-limiting with a total barrier of 29.2kcal/mol. This is followed by proton-coupled electron transfer, O2 release, and water binding to start the next catalytic cycle.

Which Oxidation State Leads to O–O Bond Formation in Cp*Ir(bpy)Cl-Catalyzed Water Oxidation, Ir(V), Ir(VI), or Ir(VII)?

Density functional calculations are used to revisit the reaction mechanism of water oxidation catalyzed by the Cp*Ir(bpy)Cl (Cp* = pentamethylcyclopentadienyl, bpy = 2,2′-bipyridine) complex. One of the experimentally suggested active species [(bpy)Ir(H2O)2(HCOO)Cl]+ can undergo very facile intramolecular formate oxidation at higher oxidation state even though it can also promote O–O bond formation. Therefore, [(bpy)Ir(H2O)2(CH3COO)Cl]+ is here proposed to be the most likely precatalyst as acetate was also experimentally observed after Cp* oxidation. O–O bond formation takes place at the high formal oxidation states of IrVI and IrVII, rather than that of IrV, as suggested before. Three sequential proton-coupled electron transfer oxidations result in the formation of a highly oxidized intermediate, [(bpy)IrVIO(OH)(CH3COO)Cl]+. From this formal IrVI intermediate, O–O bond formation takes place by a water attack on the IrVI=O moiety assisted by the acetate ligand, which abstracts a proton during the attack. The barrier was calculated to be very facile, being 14.7 kcal/mol, in good agreement with experimental kinetic results, which gave a barrier of around 18 kcal/mol. The attack leads to the formation of an IrIV-peroxide intermediate, which undergoes proton-coupled electron transfer to form an IrIII–O2 intermediate. Finally, O2 can be released, coupled with the binding of another water molecule, to regenerate the catalytic IrIII species. Water oxidation at IrVII has a slightly higher barrier, but it may also contribute to the activity. However, water oxidation at IrV has a significantly higher barrier. Acetate oxidation by C–H activation was found to have a much higher barrier, suggesting that [(bpy)Ir(H2O)2(CH3COO)Cl]+ is a remarkably stable catalyst. The possible catalytic species [(bpy-dc)IrIII(H2O)3Cl]2+ without acetate coordination has also been considered and also gave a reasonably feasible barrier for the water oxidation. O–O bond formation at IrVII is slightly preferred compared with at IrVI, which is different from the case with acetate.

Reaction Mechanism of Water Oxidation Catalyzed by Iron Tetraamido Macrocyclic Ligand Complexes – A DFT Study

AbstractDensity functional calculations are used to elucidate the reaction mechanism of water oxidation catalyzed by iron tetraamido macrocyclic ligand (TAML) complexes. The oxidation of the starting TAML–Fe3+–OH2 complex by removing three electrons and two protons leads to the formation of a key intermediate, TAML·–Fe5+=O, which can undergo nucleophilic attack by either a water molecule or a nitrate ion. Both pathways involve attack on the oxo group and lead to the production of O2. The water attack is more favoured and has a total barrier of 15.4 kcal/mol. The alternative nitrate attack pathway has a barrier of 19.5 kcal/mol. Nitrate functions as a cocatalyst by first donating an oxygen atom to the oxo group to form O2 and a nitrite ion, which can then be re‐oxidized to regenerate a nitrate ion. Three possible competing pathways result in ligand modification, namely, water and nitrate attack on the ligand, as well as ligand amide oxidation. The water attack on the ligand has a low barrier of only 10.9 kcal/mol and leads to the opening of the benzene ring, which explains the observation of fast catalyst degradation. The lack of activity or lower activity of other catalysts with different substituents is also rationalized.

Direct oxygen and hydrogen production by photo water splitting using a robust bioinspired manganese-oxo oligomer complex/tungsten oxide catalytic system

The n-type semiconductor tungsten oxide is readily dissolved in aqueous solution at pH > 4 and may be problematic in water splitting catalysis. We have reported that a tungsten oxide photoanode prepared by atomic layer deposition can be stabilized with a Mn-oxo compound for efficient photo water splitting at pH 4 and pH 7. However the molecular mechanism of water oxidation reaction in this robust catalytic system is not known. In this work, the mechanism for oxygen and hydrogen production by photo water splitting using Mn-oxo complex/tungsten oxide heteronanostructures was examined under different experimental conditions by X-ray photoelectron spectroscopy as well as gas chromatographic analysis, O-18 isotope measurements, and pH dependence of photocurrent. We found that the Mn(II) species plays an important role in the catalytic cycle of water oxidation in the Mn-oxo oligomer complex/tungsten oxide system and propose a working model of the Mn-oxo oligomer complex/tungsten oxide catalytic system in photo water splitting.

Molecular Mechanism of Water Oxidation Reaction at Photo-Irradiated TiO<sub>2</sub> and Related Metal Oxide Surfaces

Recent studies on the molecular mechanism of water photooxidation (or oxygen photoevolution) reaction on TiO2 and related metal oxides or oxynitrides are reviewed. It is shown that a lot of experimental and theoretical studies give definite support to our recently proposed new mechanism, called “nucleophilic attack of H2O” or “Lewis acid-base” mechanism. The new mechanism has the prominent features that it possesses energetic and kinetics different from the conventional electron-transfer mechanism and can explain water photooxidation reaction on visible-light responsive metal oxides or oxynitrides, contrary to the conventional one. The result indicates that the new mechanism is useful for searching for new efficient visible-light responsive materials for solar water splitting.