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17 pages, 1006 KiB

Open AccessArticle

Determination of pK_a Values via ab initio Molecular Dynamics and its Application to Transition Metal-Based Water Oxidation Catalysts

by Mauro Schilling and Sandra Luber

Inorganics 2019, 7(6), 73; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics7060073 - 12 Jun 2019

Cited by 21 | Viewed by 4524

Abstract

The

p K_{a}

values are important for the in-depth elucidation of catalytic processes, the computational determination of which has been challenging. The first simulation protocols employing ab initio molecular dynamics simulations to calculate

p K_{a}

values appeared almost two decades ago. [...] Read more.

The

p K_{a}

values are important for the in-depth elucidation of catalytic processes, the computational determination of which has been challenging. The first simulation protocols employing ab initio molecular dynamics simulations to calculate

p K_{a}

values appeared almost two decades ago. Since then several slightly different methods have been proposed. We compare the performance of various evaluation methods in order to determine the most reliable protocol when it comes to simulate

p K_{a}

values of transition metal-based complexes, such as the here investigated Ru-based water oxidation catalysts. The latter are of high interest for sustainable solar-light driven water splitting, and understanding of the underlying reaction mechanism is crucial for their further development. Full article

(This article belongs to the Special Issue Recent Advances in Water Oxidation Catalysis)

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7 pages, 2287 KiB

Open AccessArticle

Modeling the Catalyst Activation Step in a Metal–Ligand Radical Mechanism Based Water Oxidation System

by Nitish Govindarajan and Evert Jan Meijer

Inorganics 2019, 7(5), 62; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics7050062 - 09 May 2019

Cited by 9 | Viewed by 3659

Abstract

Designing catalysts for water oxidation (WOCs) that operate at low overpotentials plays an important role in developing sustainable energy conversion schemes. Recently, a mononuclear ruthenium WOC that operates via metal–ligand radical coupling pathway was reported, with a very low barrier for O–O bond [...] Read more.

Designing catalysts for water oxidation (WOCs) that operate at low overpotentials plays an important role in developing sustainable energy conversion schemes. Recently, a mononuclear ruthenium WOC that operates via metal–ligand radical coupling pathway was reported, with a very low barrier for O–O bond formation, that is usually the rate-determining step in most WOCs. A detailed mechanistic understanding of this mechanism is crucial to design highly active oxygen evolution catalysts. Here, we use density functional theory based molecular dynamics (DFT-MD) with an explicit description of the solvent to investigate the catalyst activation step for the [Ru(bpy)

_{2}

(bpy–NO)]

^{2 +}

complex, that is considered to be the rate-limiting step in the metal–ligand radical coupling pathway. We find that a realistic description of the solvent environment, including explicit solvent molecules and thermal motion, is crucial for an accurate description of the catalyst activation step, and for the estimation of the activation barriers. Full article

(This article belongs to the Special Issue Recent Advances in Water Oxidation Catalysis)

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10 pages, 2095 KiB

Open AccessCommunication

Water Network Dynamics Next to the Oxygen-Evolving Complex of Photosystem II

by Krystle Reiss, Uriel N. Morzan, Alex T. Grigas and Victor S. Batista

Inorganics 2019, 7(3), 39; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics7030039 - 11 Mar 2019

Cited by 14 | Viewed by 4280

Abstract

The influence of the environment on the functionality of the oxygen-evolving complex (OEC) of photosystem II has long been a subject of great interest. In particular, various water channels, which could serve as pathways for substrate water diffusion, or proton translocation, are thought [...] Read more.

The influence of the environment on the functionality of the oxygen-evolving complex (OEC) of photosystem II has long been a subject of great interest. In particular, various water channels, which could serve as pathways for substrate water diffusion, or proton translocation, are thought to be critical to catalytic performance of the OEC. Here, we address the dynamical nature of hydrogen bonding along the water channels by performing molecular dynamics (MD) simulations of the OEC and its surrounding protein environment in the S₁ and S₂ states. Through the eigenvector centrality (EC) analysis, we are able to determine the characteristics of the water network and assign potential functions to the major channels, namely that the narrow and broad channels are likely candidates for proton/water transport, while the large channel may serve as a path for larger ions such as chloride and manganese thought to be essential during PSII assembly. Full article

(This article belongs to the Special Issue Recent Advances in Water Oxidation Catalysis)

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11 pages, 993 KiB

Open AccessArticle

Computational Characterization of Single-Electron Transfer Steps in Water Oxidation

by Adiran de Aguirre, Ignacio Funes-Ardoiz and Feliu Maseras

Inorganics 2019, 7(3), 32; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics7030032 - 01 Mar 2019

Cited by 13 | Viewed by 5197

Abstract

The presence of single-electron transfer (SET) steps in water oxidation processes catalyzed by first-row transition metal complexes has been recently recognized, but the computational characterization of this type of process is not trivial. We report a systematic theoretical study based on density functional [...] Read more.

The presence of single-electron transfer (SET) steps in water oxidation processes catalyzed by first-row transition metal complexes has been recently recognized, but the computational characterization of this type of process is not trivial. We report a systematic theoretical study based on density functional theory (DFT) calculations on the reactivity of a specific copper complex active in water oxidation that reacts through two consecutive single-electron transfers. Both inner-sphere (through transition state location) and outer-sphere (through Marcus theory) mechanisms are analyzed. The first electron transfer is found to operate through outer-sphere, and the second one through inner-sphere. The current work proposes a scheme for the systematic study of single-electron transfer in water oxidation catalysis and beyond. Full article

(This article belongs to the Special Issue Recent Advances in Water Oxidation Catalysis)

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11 pages, 1445 KiB

Open AccessArticle

Modelling the (Essential) Role of Proton Transport by Electrolyte Bases for Electrochemical Water Oxidation at Near-Neutral pH

by Holger Dau and Chiara Pasquini

Inorganics 2019, 7(2), 20; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics7020020 - 13 Feb 2019

Cited by 16 | Viewed by 5182

Abstract

The oxygen-evolution reaction (OER) in the near-neutral pH-regime is of high interest, e.g., for coupling of OER and CO₂-reduction in the production of non-fossil fuels. A simple model is proposed that assumes equal proton activities in the catalyst film and the [...] Read more.

The oxygen-evolution reaction (OER) in the near-neutral pH-regime is of high interest, e.g., for coupling of OER and CO₂-reduction in the production of non-fossil fuels. A simple model is proposed that assumes equal proton activities in the catalyst film and the near-surface electrolyte. Equations are derived that describe the limitations relating to proton transport mediated by fluxes of molecular “buffer bases” in the electrolyte. The model explains (1) the need for buffer bases in near-neutral OER and (2) the pH dependence of the catalytic current at high overpotentials. The latter is determined by the concentration of unprotonated buffer bases times an effective diffusion constant, which can be estimated for simple cell geometries from tabulated diffusion coefficients. The model predicts (3) a macroscopic region of increased pH close to the OER electrode and at intermediate overpotentials, (4) a Tafel slope that depends on the reciprocal buffer capacity; both predictions are awaiting experimental verification. The suggested first-order model captures and predicts major trends of OER in the near-neutral pH, without accounting for proton-transport limitations at the catalyst–electrolyte interface and within the catalyst material, but the full quantitative agreement may require refinements. The suggested model also may be applicable to further electrocatalytic processes. Full article

(This article belongs to the Special Issue Recent Advances in Water Oxidation Catalysis)

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14 pages, 2428 KiB

Open AccessArticle

Design of Molecular Water Oxidation Catalysts Stabilized by Ultrathin Inorganic Overlayers—Is Active Site Protection Necessary?

by Laurent Sévery, Sebastian Siol and S. David Tilley

Inorganics 2018, 6(4), 105; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics6040105 - 29 Sep 2018

Cited by 8 | Viewed by 4740

Abstract

Anchored molecular catalysts provide a good step towards bridging the gap between homogeneous and heterogeneous catalysis. However, applications in an aqueous environment pose a serious challenge to anchoring groups in terms of stability. Ultrathin overlayers embedding these catalysts on the surface using atomic [...] Read more.

Anchored molecular catalysts provide a good step towards bridging the gap between homogeneous and heterogeneous catalysis. However, applications in an aqueous environment pose a serious challenge to anchoring groups in terms of stability. Ultrathin overlayers embedding these catalysts on the surface using atomic layer deposition (ALD) are an elegant solution to tackle the anchoring group instability. The propensity of ALD precursors to react with water leads to the question whether molecules containing aqua ligands, such as most water oxidation complexes, can be protected without side reactions and deactivation during the deposition process. We synthesized two iridium and two ruthenium-based water oxidation catalysts, which contained an aqua ligand (Ir–OH₂ and Ru–OH₂) or a chloride (Ir–Cl and Ru–Cl) that served as a protecting group for the former. Using a ligand exchange reaction on the anchored and partially embedded Ru–Cl, the optimal overlayer thickness was determined to be 1.6 nm. An electrochemical test of the protected catalysts on meso-ITO showed different behaviors for the Ru and the Ir catalysts. The former showed no onset difference between protected and non-protected versions, but limited stability. Ir–Cl displayed excellent stability, whilst the unprotected catalyst Ir–OH₂ showed a later initial onset. Self-regeneration of the catalytic activity of Ir–OH₂ under operating conditions was observed. We propose chloride ligands as generally applicable protecting groups for catalysts that are to be stabilized on surfaces using ALD. Full article

(This article belongs to the Special Issue Recent Advances in Water Oxidation Catalysis)

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Review

Jump to: Research

30 pages, 4236 KiB

Open AccessReview

The S₃ State of the Oxygen-Evolving Complex: Overview of Spectroscopy and XFEL Crystallography with a Critical Evaluation of Early-Onset Models for O–O Bond Formation

by Dimitrios A. Pantazis

Inorganics 2019, 7(4), 55; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics7040055 - 22 Apr 2019

Cited by 27 | Viewed by 5888

Abstract

The catalytic cycle of the oxygen-evolving complex (OEC) of photosystem II (PSII) comprises five intermediate states S_i (i = 0–4), from the most reduced S₀ state to the most oxidized S₄, which spontaneously evolves dioxygen. The precise geometric [...] Read more.

The catalytic cycle of the oxygen-evolving complex (OEC) of photosystem II (PSII) comprises five intermediate states S_i (i = 0–4), from the most reduced S₀ state to the most oxidized S₄, which spontaneously evolves dioxygen. The precise geometric and electronic structure of the S_i states, and hence the mechanism of O–O bond formation in the OEC, remain under investigation, particularly for the final steps of the catalytic cycle. Recent advances in protein crystallography based on X-ray free-electron lasers (XFELs) have produced new structural models for the S₃ state, which indicate that two of the oxygen atoms of the inorganic Mn₄CaO₆ core of the OEC are in very close proximity. This has been interpreted as possible evidence for “early-onset” O–O bond formation in the S₃ state, as opposed to the more widely accepted view that the O–O bond is formed in the final state of the cycle, S₄. Peroxo or superoxo formation in S₃ has received partial support from computational studies. Here, a brief overview is provided of spectroscopic information, recent crystallographic results, and computational models for the S₃ state. Emphasis is placed on computational S₃ models that involve O–O formation, which are discussed with respect to their agreement with structural information, experimental evidence from various spectroscopic studies, and substrate exchange kinetics. Despite seemingly better agreement with some of the available crystallographic interpretations for the S₃ state, models that implicate early-onset O–O bond formation are hard to reconcile with the complete line of experimental evidence, especially with X-ray absorption, X-ray emission, and magnetic resonance spectroscopic observations. Specifically with respect to quantum chemical studies, the inconclusive energetics for the possible isoforms of S₃ is an acute problem that is probably beyond the capabilities of standard density functional theory. Full article

(This article belongs to the Special Issue Recent Advances in Water Oxidation Catalysis)

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37 pages, 14744 KiB

Open AccessReview

Water Oxidation Catalysts: The Quest for New Oxide-Based Materials

by Christos K. Mavrokefalos and Greta R. Patzke

Inorganics 2019, 7(3), 29; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics7030029 - 26 Feb 2019

Cited by 19 | Viewed by 6189

Abstract

The expected shortage of fossil fuels as well as the accompanying climate change are among the major challenges of the 21st century. A global shift to a sustainable energy landscape is, therefore, of utmost importance. Over the past few years, solar technologies have [...] Read more.

The expected shortage of fossil fuels as well as the accompanying climate change are among the major challenges of the 21st century. A global shift to a sustainable energy landscape is, therefore, of utmost importance. Over the past few years, solar technologies have entered the energy market and have paved the way to replace fossil-based energy sources, in the long term. In particular, electrochemical solar-to-hydrogen technologies have attracted a lot of interest—not only in academia, but also in industry. Solar water splitting (artificial photosynthesis) is one of the most active areas in contemporary materials and catalysis research. The development of low-cost, efficient, and stable water oxidation catalysts (WOCs) remains crucial for artificial photosynthesis applications, because WOCs still represent a major economical and efficient bottleneck. In the following, we summarize recent advances in water oxidation catalysts development, with selected examples from 2016 onwards. This condensed survey demonstrates that the ongoing quest for new materials and informed catalyst design is a dynamic and rapidly developing research area. Full article

(This article belongs to the Special Issue Recent Advances in Water Oxidation Catalysis)

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20 pages, 2287 KiB

Open AccessReview

Tracing the Pathways of Waters and Protons in Photosystem II and Cytochrome c Oxidase

by Divya Kaur, Xiuhong Cai, Umesh Khaniya, Yingying Zhang, Junjun Mao, Manoj Mandal and Marilyn R. Gunner

Inorganics 2019, 7(2), 14; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics7020014 - 31 Jan 2019

Cited by 15 | Viewed by 5346

Abstract

Photosystem II (PSII) uses water as the terminal electron donor, producing oxygen in the Mn₄CaO₅ oxygen evolving complex (OEC), while cytochrome c oxidase (CcO) reduces O₂ to water in its heme–Cu binuclear center (BNC). Each protein is oriented in [...] Read more.

Photosystem II (PSII) uses water as the terminal electron donor, producing oxygen in the Mn₄CaO₅ oxygen evolving complex (OEC), while cytochrome c oxidase (CcO) reduces O₂ to water in its heme–Cu binuclear center (BNC). Each protein is oriented in the membrane to add to the proton gradient. The OEC, which releases protons, is located near the P-side (positive, at low-pH) of the membrane. In contrast, the BNC is in the middle of CcO, so the protons needed for O₂ reduction must be transferred from the N-side (negative, at high pH). In addition, CcO pumps protons from N- to P-side, coupled to the O₂ reduction chemistry, to store additional energy. Thus, proton transfers are directly coupled to the OEC and BNC redox chemistry, as well as needed for CcO proton pumping. The simulations that study the changes in proton affinity of the redox active sites and the surrounding protein at different states of the reaction cycle, as well as the changes in hydration that modulate proton transfer paths, are described. Full article

(This article belongs to the Special Issue Recent Advances in Water Oxidation Catalysis)

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