Computational Studies of the Geometry and Electronic Structure of an All-Inorganic and Homogeneous Tetra-Ru-Polyoxotungstate Catalyst for Water Oxidation and Its Four Subsequent One-Electron Oxidized Forms

Activation by oxidation and ligand exchange in a molecular manganese vanadium oxide water oxidation catalyst

Despite their technological importance for water splitting, the reaction mechanisms of most water oxidation catalysts (WOCs) are poorly understood. This paper combines theoretical and experimental methods to reveal mechanistic insights into the reactivity of the highly active molecular manganese vanadium oxide WOC [Mn4V4O17(OAc)3]3- in aqueous acetonitrile solutions. Using density functional theory together with electrochemistry and IR-spectroscopy, we propose a sequential three-step activation mechanism including a one-electron oxidation of the catalyst from [Mn2 3+Mn2 4+] to [Mn3+Mn3 4+], acetate-to-water ligand exchange, and a second one-electron oxidation from [Mn3+Mn3 4+] to [Mn4 4+]. Analysis of several plausible ligand exchange pathways shows that nucleophilic attack of water molecules along the Jahn-Teller axis of the Mn3+ centers leads to significantly lower activation barriers compared with attack at Mn4+ centers. Deprotonation of one water ligand by the leaving acetate group leads to the formation of the activated species [Mn4V4O17(OAc)2(H2O)(OH)]- featuring one H2O and one OH ligand. Redox potentials based on the computed intermediates are in excellent agreement with electrochemical measurements at various solvent compositions. This intricate interplay between redox chemistry and ligand exchange controls the formation of the catalytically active species. These results provide key reactivity information essential to further study bio-inspired molecular WOCs and solid-state manganese oxide catalysts.

Phosphate-Templated Encapsulation of a {CoII 4 O4 } Cubane in Germanotungstates as Carbon-Free Homogeneous Water Oxidation Photocatalysts

The ever-growing interest in sustainable energy sources leads to a search for an efficient, stable, and inexpensive homogeneous water oxidation catalyst (WOC). Herein, the PO₄ ^3- templated synthesis of three abundant-metal-based germanotungstate (GT) clusters Na₁₅ [Ge₄ PCo₄ (H₂ O)₂ W₂₄ O₉₄ ] ⋅ 38H₂ O (Co₄ ), Na_2.5 K_17.5 [Ge₃ PCo₉ (OH)₅ (H₂ O)₄ W₃₀ O₁₁₅ ] ⋅ 45H₂ O (Co₉ ), Na₆ K₁₆ [Ge₄ P₄ Co₂₀ (OH)₁₄ (H₂ O)₁₈ W₃₆ O₁₅₀ ] ⋅ 61H₂ O (Co₂₀ ) with non-, quasi-, or full cubane motifs structurally strongly reminiscent of the naturally occurring {Mn₄ Ca} oxygen evolving complex (OEC) in photosystem II was achieved. Under the conditions tested, all three GT-scaffolds were active molecular WOCs, with Co₉ and Co₂₀ outperforming the well-known Na₁₀ [Co₄ (H₂ O)₂ (PW₉ O₃₄ )₂ ] {Co₄ P₂ W₁₈ } by a factor of 2 as shown by a direct comparison of their turnover numbers (TONs). With TONs up to 159.9 and a turnover frequency of 0.608 s^-1 Co₉ currently represents the fastest Co-GT-based WOC, and photoluminescence emission spectroscopy provided insights into its photocatalytic WOC mechanism. Cyclic voltammetry, dynamic light scattering, UV/Vis and IR spectroscopy showed recyclability and integrity of the catalysts under the applied conditions. The experimental results were supported by computational studies, which highlighted that the facilitated oxidation of Co₉ was due to the higher energy of its highest occupied molecular orbital electrons as compared to Co₄ .

Comparative first principles-based molecular dynamics study of catalytic mechanism and reaction energetics of water oxidation reaction on 2D-surface

The study of the water-splitting process, which can proceed in 2e^- as well as 4e^- pathway, reveals that the process is entirely an uphill process, and the third step, that is, the oxooxo bond formation is the rate-determining step. The kinetic barrier of the oxygen evolution reaction (OER) on the 2D material catalysts in the presence of explicit solvents is scarcely studied. Here, we investigate the dynamics of the OER on the undoped graphene and the activation energy barrier of each step using first principles molecular dynamics simulations. Here we provide a detailed analysis of the kinetics of all the 4e^- transfer steps of OER on the graphene surface. We also compare the accuracy of one of the density functional theory (DFT) functionals and density functional based tight binding (DFTB) method in explaining the OER steps. The comparative study reveals that DFTB can be used for performing metadynamics simulations quipped with much less computational cost than DFT functionals. By both Perdew-Burke-Ernzerhof and DFTB methods, the third step is revealed to be the rate-determining step with an energy barrier of 21.19 ± 0.51 and 20.23 ± 0.20 kcal mol^-1 , respectively. DFTB gives an impression of being successful in predicting the energy barriers of OER in 4e- transfer pathway and comparable to the DFT method, and we would like to extend the use of DFTB for further studies with a sizable and complex system.

A Force Field for a Manganese-Vanadium Water Oxidation Catalyst: Redox Potentials in Solution as Showcase

We present a molecular mechanics force field in AMBER format for the mixed-valence manganese vanadium oxide cluster [Mn4V4O17(OAc)3]3−—a synthetic analogue of the oxygen-evolving complex that catalyzes the water oxidation reaction in photosystem II—with parameter sets for two different oxidation states. Most force field parameters involving metal atoms have been newly parametrized and the harmonic terms refined using hybrid quantum mechanics/molecular mechanics reference simulations, although some parameters were adapted from pre-existing force fields of vanadate cages and manganese oxo dimers. The characteristic Jahn–Teller distortions of d4 MnIII ions in octahedral environments are recovered by the force field. As an application, the developed parameters have been used to calculate the redox potential of the [MnIIIMn3IV] ⇌ [Mn4IV]+e− half-reaction in acetonitrile by means of Marcus theory.

The Reactivity and Stability of Polyoxometalate Water Oxidation Electrocatalysts

This review describes major advances in the use of functionalized molecular metal oxides (polyoxometalates, POMs) as water oxidation catalysts under electrochemical conditions. The fundamentals of POM-based water oxidation are described, together with a brief overview of general approaches to designing POM water oxidation catalysts. Next, the use of POMs for homogeneous, solution-phase water oxidation is described together with a summary of theoretical studies shedding light on the POM-WOC mechanism. This is followed by a discussion of heterogenization of POMs on electrically conductive substrates for technologically more relevant application studies. The stability of POM water oxidation catalysts is discussed, using select examples where detailed data is already available. The review finishes with an outlook on future perspectives and emerging themes in electrocatalytic polyoxometalate-based water oxidation research.

Photo-induced water oxidation: New photocatalytic processes and materials

New progress towards artificial photosynthetic methods and solar fuels will depend on the discovery of highly robust multi-electron catalysts and materials enabling light-activated water splitting with high quantum efficiency and low overpotential, thus mimicking the natural process.

Polyoxometalate water oxidation catalysts and the production of green fuel

In the last five years and currently, research on solar fuels has been intense and no sub-area in this field has been more active than the development of water oxidation catalysts (WOCs). In this timeframe, a new class of molecular water oxidation catalysts based on polyoxometalates have been reported that combine the advantages of homogeneous and heterogeneous catalysts. This review addresses central issues in green energy generation, the challenges in water oxidation catalyst development, and the possible uses of polyoxometalates in green energy science.

Structure, properties and reactivity of polyoxometalates: a theoretical perspective

In the thematic review dedicated to polyoxometalate (POM) chemistry published in Chemical Reviews in 1998, no contribution was devoted to theory. This is not surprising because computational modelling of molecular metal-oxide clusters was in its infancy at that time. Nowadays, the situation has completely changed and modern computational methods have been successfully applied to study the structure, electronic properties, spectroscopy and reactivity of POM clusters. Indeed, the progress achieved during the past decade has been spectacular and herein we critically review the most important papers to provide the reader with an almost complete perspective of the field.