Proton-coupled Electron Transfer Steps Research Articles

Modeling Electrocatalytic Oxidation of Formic Acid at Platinum

The formic acid oxidation reaction (FAOR) is a model electrocatalytic reaction involving multiple proton-coupled electron transfer steps. Despite of repeated research extending over more than fifty years, the FAOR is still an active research field in which several important questions including reaction mechanisms, the activity dependence on the electrode structure, the hysteresis between positive- and negative-going scan in cyclic voltammetry (CV), and, especially, the pH effect, remain elusive yet. To shed some light on these puzzles, we herein develop a microkinetic model for the FAOR at Pt(111) which uses a reaction mechanism supported by microscopic and mechanistic information from density functional theory calculations and spectroscopic characterizations, formulates the mechanism using fully microkinetic modeling without designating a rate-determining step, and incorporates double-layer effects by means of a mean-field description for the electrode-electrolyte interphase. Moreover, chemisorbed intermediates play multifaceted roles in this formulism: they are reactants with lateral interactions, site-blockers, as well as modifiers of the double-layer structure and properties. The model is parameterized using CV data of Pt(111) in perchlorate electrolyte with different pHs, revealing that HCOOm is the main active intermediate with HCOO− as the main precursor. COad on defect sites induces the voltage hysteresis through modifying surface charging relation (the main effect) and blocking adsorption sites (the minor effect). It is also found that the higher current density as the pH increases from 0.11 to 1.42 is the result of two opposing factors: higher concentration of HCOO− in bulk solution and stronger double layer effects that suppress HCOOm formation. The presented work demonstrates that consideration of double-layer effects and an integrated view of multifaceted roles of reaction intermediates are a sheer necessity for FAOR.

Inner-Sphere Oxygen Activation Promoting Outer-Sphere Nucleophilic Attack on Olefins.

Alkoxylation and hydroxylation reactions of 1,5-cyclooctadiene (cod) in an iridium complex with alcohols and water promoted by the reduction of oxygen to hydrogen peroxide are described. The exo configuration of the OH/OR groups in the products agrees with nucleophilic attack at the external face of the olefin as the key step. The reactions also require the presence of a coordinating protic acid (such as picolinic acid (Hpic)) and involve the participation of a cationic diolefin iridium(III) complex, [Ir(cod)(pic)2 ]+ , which has been isolated. Independently, this cation is also involved in easy alkoxy group exchange reactions, which are very unusual for organic ethers. DFT studies on the mechanism of olefin alkoxylation mediated by oxygen show a low-energy proton-coupled electron-transfer step connecting a superoxide-iridium(II) complex with hydroperoxide-iridium(III) intermediates, rather than peroxide complexes. Accordingly, a more complex reaction, with up to four different products, occurred upon reacting the diolefin-peroxide iridium(III) complex with Hpic. Moreover, such hydroperoxide intermediates are the origin of the regio- and stereoselectivity of the hydroxylation/alkoxylation reactions. If this protocol is applied to the diolefin-rhodium(I) complex [Rh(pic)(cod)], free alkyl ethers ORC8 H11 (R=Me, Et) resulted, and the reaction is enantioselective if a chiral amino acid, such as l-proline, is used instead of Hpic.

Theory on optimizing the activity of electrocatalytic proton coupled electron transfer reactions

Understanding the effects of pH and potential on proton-coupled electron transfer (PCET) steps and optimizing the reaction activity are of key importance for efficient applications of energy conversion processes. In this work, we develop a simple theory to optimize the electrocatalytic PCET reactions, in which electron and proton transfer take place in sequential steps, by combining an energy level diagram and micro-kinetic theories. Our theory suggests matching conditions on PCET energetics (redox potentials and pKa’s) and reaction environments (potential and pH) in order to achieve maximum kinetics. Consequently, a descriptor representing deviation from the ideal condition, i.e. |ΔUre|+ 2.30kBT/e |ΔpKa|, is proposed to measure catalyst (in)activity. To test our theory, we further investigate CO2 electroreduction on a model catalyst both computationally and experimentally, and show that the activity for CO2 reduction on FePc (Iron Phthalocyanine) molecule is higher than H2 evolution at near neutral condition. Our theory not only identifies the individual contribution of electron and proton transfer to overpotentials but also provides simple guidelines for optimizing the experimental conditions and searching for efficient catalysts.

Ligand Noninnocence in Nickel Porphyrins: Nickel Isobacteriochlorin Formation under Hydrogen Evolution Conditions.

An electron-deficient nickel porphyrin complex undergoes facile ring reduction to form a nickel isobacteriochlorin complex under hydrogen evolution conditions. Spectroscopic experiments indicate that the reduced nickel porphyrin undergoes subsequent reduction and protonation to form a phlorin anion rather than a metal hydride, demonstrating that the key initial proton-coupled electron transfer step is directed toward the ligand versus the metal. The phlorin anion facilely converts to the isobacteriochlorin in the presence of two-electron and three-proton equivalents. Cyclic voltammetry (CV) and spectroscopic experiments reveal that the four-electron, four-proton electrochemical reduction of nickel porphyrin to isobacteriochlorin occurs promptly in the presence of the strong proton donor tosic acid, followed by hydrogen evolution reaction (HER) catalysis at slightly more negative potentials. CVs of independently synthesized Ni isobacteriochlorin show a catalytic HER at the same potentials as those observed for the HER in CVs of the Ni porphyrin. We find that, under strongly acidic conditions, the HER catalysis arises from conversion of the Ni isobacteriochlorin into a nickel-containing, catalytically active electrode-adsorbed species. These results show that Ni porphyrin converts to Ni isobacteriochlorin under HER catalysis conditions via a ligand-based PCET process and that it is the isobacteriochlorin complex which gives rise to an active HER catalysis.

Catalytic alkene cross-coupling

Catalysis Hydrogen atom transfer (HAT) from a metal-hydride complex to an alkene, which produces a free radical responsible for subsequent bond formation, is becoming an increasingly useful type of alkene coupling reaction. Using kinetic, spectroscopic, and computational analyses, Kim et al. pinpoint the key organometallic species and proton-coupled electron transfer steps responsible for regulating selectivity and reactivity in this reaction class. They explain several observations that were previously incomprehensible and demonstrate the intimate involvement of metal species throughout the catalytic cycle. The methods for the present mechanistic work may be applicable to various HAT alkene coupling reactions and should be useful for more expedient design of metal catalysts. J. Am. Chem. Soc. 141 , 7473 (2019).

Roles of Iron Complexes in Catalytic Radical Alkene Cross-Coupling: A Computational and Mechanistic Study.

A growing and useful class of alkene coupling reactions involve hydrogen atom transfer (HAT) from a metal-hydride species to an alkene to form a free radical, which is responsible for subsequent bond formation. Here, we use a combination of experimental and computational investigations to map out the mechanistic details of iron-catalyzed reductive alkene cross-coupling, an important representative of the HAT alkene reactions. We are able to explain several observations that were previously mysterious. First, the rate-limiting step in the catalytic cycle is the formation of the reactive Fe-H intermediate, elucidating the importance of the choice of reductant. Second, the success of the catalytic system is attributable to the exceptionally weak (17 kcal/mol) Fe-H bond, which performs irreversible HAT to alkenes in contrast to previous studies on isolable hydride complexes where this addition was reversible. Third, the organic radical intermediates can reversibly form organometallic species, which helps to protect the free radicals from side reactions. Fourth, the previously accepted quenching of the postcoupling radical through stepwise electron transfer/proton transfer is not as favorable as alternative mechanisms. We find that there are two feasible pathways. One uses concerted proton-coupled electron transfer (PCET) from an iron(II) ethanol complex, which is facilitated because the O-H bond dissociation free energy is lowered by 30 kcal/mol upon metal binding. In an alternative pathway, an O-bound enolate-iron(III) complex undergoes proton shuttling from an iron-bound alcohol. These kinetic, spectroscopic, and computational studies identify key organometallic species and PCET steps that control selectivity and reactivity in metal-catalyzed HAT alkene coupling, and create a firm basis for elucidation of mechanisms in the growing class of HAT alkene cross-coupling reactions.

Bimetallic Cu-based hollow fibre electrodes for CO2 electroreduction

• Porous Cu-based hollow fibre electrodes were synthesised for CO 2 electroconversion. • Cu hollow fibres were functionalised with Au and Ni via electrodeposition technique. • Preliminary results in cell showed promising results in terms of Faraday efficiency. • This work may provide new insights for developing highly efficient electrocatalysts. The electrochemical reduction of CO 2 represents an attractive alternative to both, i) satisfy the increasing energy demand, and ii) to help closing the carbon cycle. However, the energy required for CO 2 activation and the subsequent required multiple number of proton-coupled electron transfer steps, makes this process very challenging. Besides, catalytic material limitations hamper the application of this technology in the short term. Consequently, in this work we synthesise, characterise and preliminarily evaluate bimetallic Cu-based hollow fibre electrodes with a compact three-dimensional geometry to overcome mass transfer limitations and to enhance the electrochemical conversion of CO 2 . The Cu hollow fibres are functionalised with Au in an attempt to tune the binding energy of the CO* intermediate, which appears to be key in the reduction of CO 2 . The Cu fibres are also functionalised with Ni, aiming to decrease the reaction overpotential, resulting in beneficial energy efficiency. The so prepared Cu-based porous hollow fibre electrodes are obtained by dry-wet spinning and electrodeposition procedures. The materials are then characterised by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction analyses and cyclic voltammetry. Finally, preliminary results of CO 2 electroreduction in a divided three-electrode cell are reported. The results show the potential of highly active, bimetallic hollow fibre-based electrocatalysts for enhanced conversion of CO 2 into value-added products. Deposition of particles should be performed with care, not to affect pore characteristics and thus mass transfer properties.

Theoretical study of the mechanism of the manganese catalase KatB.

The mechanism of the H2O2 disproportionation catalyzed by the manganese catalase (MnCat) KatB was studied using the hybrid density functional theory B3LYP and the quantum chemical cluster approach. Compared to the previous mechanistic study at the molecular level for the Thermus thermophilus MnCat (TTC), more modern methodology was used and larger models of increasing sizes were employed with the help of the high-resolution X-ray structure. In the reaction pathway suggested for KatB using the Large chemical model, the O-O homolysis of the first substrate H2O2 occurs through a μ-η1:η1 coordination mode and requires a barrier of 10.9kcal/mol. In the intermediate state of the bond cleavage, two hydroxides form as terminal ligands of the dimanganese cluster at the Mn2(III,III) oxidation state. One of the two Mn(III)-OH- moieties and a second-sphere tyrosine stabilize the second substrate H2O2 in the second-sphere of the active site via hydrogen bonding interactions. The H2O2, unbound to the metals, is first oxidized into HO2· through a proton-coupled electron transfer (PCET) step with a barrier of 9.5kcal/mol. After the system switches to the triplet surface, the uncoordinated HO2· replaces the product water terminally bound to the Mn(II) and is then oxidized into O2 spontaneously. Transition states with structural similarities to those obtained for TTC, where μ-η2-OH-/O2- groups play important roles, were found to be higher in energy.

Switching between Stepwise and Concerted Proton-Coupled Electron Transfer Pathways in Tungsten Hydride Activation.

Catalytic processes to generate (or oxidize) fuels such as hydrogen are underpinned by multiple proton-coupled electron transfer (PCET) steps that are associated with the formation or activation of metal-hydride bonds. Fully understanding the detailed PCET mechanisms of metal hydride transformations holds promise for the rational design of energy-efficient catalysis. Here we investigate the detailed PCET mechanisms for the activation of the transition metal hydride complex CpW(CO)2(PMe3)H (Cp = cyclopentadienyl) using stopped-flow rapid mixing coupled with time-resolved optical spectroscopy. We reveal that all three limiting PCET pathways can be accessed by changing the free energy for elementary proton, electron, and proton-electron transfers through the choice of base and oxidant, with the concerted pathway occurring exclusively as a secondary parallel route. Through detailed kinetics analysis, we define free energy relationships for the kinetics of elementary reaction steps, which provide insight into the factors influencing reaction mechanism. Rate constants for proton transfer processes in the limiting stepwise pathways reveal a large reorganization energy associated with protonation/deprotonation of the metal center (λ = 1.59 eV) and suggest that sluggish proton transfer kinetics hinder access to a concerted route. Rate constants for concerted PCET indicate that the concerted routes are asynchronous. Additionally, through quantification of the relative contributions of parallel stepwise and concerted mechanisms toward net product formation, the influence of various reaction parameters on reactivity are identified. This work underscores the importance of understanding the PCET mechanism for controlling metal hydride reactivity, which could lead to superior catalyst design for fuel production and oxidation.

Approach to the Mechanism of Hydrogen Evolution Electrocatalyzed by a Model Co Clathrochelate: A Theoretical Study by Density Functional Theory.

The hydrogen evolution reaction (HER) has attracted much attention within the scientific community because of increasing demands of modern society for clean and renewable energy sources. Molecular complexes of 3d-transition metals, such as cobalt, hold potential to replace platinum for the HER in acidic media. Among these, cage complexes such as tris-glyoximate metal clathrochelates, have demonstrated promising catalytic properties towards the HER. However, it is not clear whether the catalytic activity of this molecule stems from metal-centered activation of H+ , due to a low oxidation state of the metal stabilized by the surrounding organic cage, or if it is the organic cage playing a further cooperative role in bringing protons together. Herein, we report on a density functional theory study of two possible mechanisms for the HER catalyzed by a model Co clathrochelate. To assess the putative ligand involvement in the mechanism, several combinations of single and double protonation sites were investigated. The structural and energetic analysis of relevant intermediates suggests that the electrocatalytic mechanism is not based on the cooperation between the ligand and the metal. Instead, it is mainly due to the activation of H+ by the Co metallocenter. Our calculations further suggest that the last step in the mechanism is a proton coupled electron transfer step.

Accelerating proton-coupled electron transfer of metal hydrides in catalyst model reactions

Metal hydrides are key intermediates in catalytic proton reduction and dihydrogen oxidation. There is currently much interest in appending proton relays near the metal centre to accelerate catalysis by proton-coupled electron transfer (PCET). However, the elementary PCET steps and the role of the proton relays are still poorly understood, and direct kinetic studies of these processes are scarce. Here, we report a series of tungsten hydride complexes as proxy catalysts, with covalently attached pyridyl groups as proton acceptors. The rate of their PCET reaction with external oxidants is increased by several orders of magnitude compared to that of the analogous systems with external pyridine on account of facilitated proton transfer. Moreover, the mechanism of the PCET reaction is altered by the appended bases. A unique feature is that the reaction can be tuned to follow three distinct PCET mechanisms-electron-first, proton-first or a concerted reaction-with very different sensitivities to oxidant and base strength. Such knowledge is crucial for rational improvements of solar fuel catalysts.

Conformationally Dynamic Radical Transfer within Ribonucleotide Reductase.

Ribonucleotide reductases (RNR) catalyze the reduction of nucleotides to deoxynucleotides through a mechanism involving an essential cysteine based thiyl radical. In the E.coli class 1a RNR the thiyl radical (C439•) is a transient species generated by radical transfer (RT) from a stable diferric-tyrosyl radical cofactor located >35 Å away across the α2:β2 subunit interface. RT is facilitated by sequential proton-coupled electron transfer (PCET) steps along a pathway of redox active amino acids (Y122β ↔ [W48β?] ↔ Y356β ↔ Y731α ↔ Y730α ↔ C439α). The mutant R411A(α) disrupts the H-bonding environment and conformation of Y731, ostensibly breaking the RT pathway in α2. However, the R411A protein retains significant enzymatic activity, suggesting Y731 is conformationally dynamic on the time scale of turnover. Installation of the radical trap 3-amino tyrosine (NH2Y) by amber codon suppression at positions Y731 or Y730 and investigation of the NH2Y• trapped state in the active α2:β2 complex by HYSCORE spectroscopy validate that the perturbed conformation of Y731 in R411A-α2 is dynamic, reforming the H-bond between Y731 and Y730 to allow RT to propagate to Y730. Kinetic studies facilitated by photochemical radical generation reveal that Y731 changes conformation on the ns-μs time scale, significantly faster than the enzymatic kcat. Furthermore, the kinetics of RT across the subunit interface were directly assessed for the first time, demonstrating conformationally dependent RT rates that increase from 0.6 to 1.6 × 104 s-1 when comparing wild type to R411A-α2, respectively. These results illustrate the role of conformational flexibility in modulating RT kinetics by targeting the PCET pathway of radical transport.

Selectivity and Efficiency of Heat Treated Sputter Deposited Thin Films of Copper towards the Carbon Dioxide Electro-reduction

The CO2 electro-reduction has the potential to store energy from intermittent renewable sources and to produce valuable carbon-neutral fuels or chemical feedstocks. However, current CO2 reduction technologies are challenged by high overpotentials, poor overall faradaic efficiencies (FE) and low selectivity, producing a mixture of carbon monoxide, methane, ethylene, formate, methanol, ethanol, and others. Catalyst performance is a significant contributor to these factors due to multiple proton-coupled electron transfer steps of CO2 electro-reduction reaction [1-3]. Recent experimental studies on Cu electrocatalysts have shown that higher CO2 and CO reduction efficiencies and selectivities towards certain valuable products could be achieved by modifying these metallic electrodes [4, 5]. For example, Cu thin films prepared by electrochemically reduced thermally grown Cu oxide (Cu2O) layers exhibit dramatically improved selectivity towards ethanol at significantly lower potentials (up to 50% FE at -0.35 V vs. RHE) in comparison to non-treated Cu catalysts (»3% FE at -0.35 V vs. RHE) [5]. Herein, inspired by these studies, we will present detailed examination of modified Cu thin film electrodes for CO2 reduction. Cu thin films are fabricated by reactive sputter deposition and subsequently modified by heat treatment varying the temperature and time of exposure. A combination of SEM/EDX, XRD and XPS were used for physico-chemical characterization. Whereas, CO2 electro-reduction products were monitored using in line GC for gas products and capillary GC, IC, NMR for liquid products. The results show that these modified Cu surfaces can improve the selectivity and efficiency of the catalyst. Acknowledgements Financial support of this work by the Swiss Competence Center for Energy Research Heat and Electricity Storage, the Competence Center Energy and Mobility, and Swiss Electric Researach and the Swiss Federal Office of Energy are greatly acknowledged.

Interfacial proton-coupled electron transfer in metal oxide semiconductor photocatalysis

Metal oxide semiconductor-based photo(electro)catalysis has drawn increasing attention due to its prominent applications in solar energy conversion and environmental remediation. The photogenerated valence-band hole and conduction-band electron diffuse to the metal oxide/solution interface and trigger subsequent chemical conversions involving multiple proton-coupled electron (hole) transfer (PCET) steps to reconstruct chemical bonds. We review herein our recent work on regulating such PCET processes in photocatalytic oxygen reduction and water oxidation. The reaction thermodynamics, kinetics, and PCET pathways can be tuned by controlling the crystal facet and surface structure. Reductive conversion of organic halides by PCET on pristine or modified TiO2 as well as its unique kinetic features are also discussed. Such mechanistic fundamentals of interfacial reactions could guide development of photocatalysts with improved efficiency and desired selectivity.

A DFT/B3LYP study of the mechanisms of the O2 formation reaction catalyzed by the [(terpy)(H2O)MnIII(O)2MnIV(OH2)(terpy)](NO3)3 complex: A paradigm for photosystem II

We present a theoretical study of the reaction pathway for dioxygen molecular formation catalyzed by the [(terpy)(H2O)MnIII(O)2MnIV(OH2) (terpy)](NO3)3 (terpy=2,2′:6′,2″-terpyridine) complex based on DFT-B3LYP calculations. In the initial state of the reaction, a partial oxido radical (0.44 spins) is formed ligated to Mn. This radical is involved in a nucleophylic attack by bulk water in the OO bond reaction formation step, in which the oxido fractional unpaired electron is delocalized toward the outermost Mn of the μ-oxo bridge, instead of the ligated Mn center. The reaction then follows with a series of proton-coupled electron transfer steps, in which the oxidation state, as well as the bond strength of the OO moiety increase, while the OOMn(1) bond gets weaker until O2 is released. In this model, basic acetate ions from the buffer solution capture protons in the proton-transfer steps. In each step there is reduction of the OOMn(1) binding strength, with concomitant increase of the OO bond strength, which culminates with the release of O2 in the last step. This last step is entropy driven, while formation of hydroperoxide and superoxide moieties is enthalpy driven. According with experiments, the rate-limiting step is the double oxidation of Mn(IV,III) or peroxymonosulfate binding, which occur prior to the OO bond formation step. This supports our findings that the barriers of all intermediate steps are below the experimental barrier of 19–21kcal/mol. The implications of these findings for understanding photosynthetic water-splitting catalysis are also discussed.

Proton‐Coupled Electron Transfer in Photoredox Catalytic Reactions

Proton‐coupled electron transfer (PCET) is studied in different research domains of chemistry. Many reports involve biochemical processes. Although related phenomena have often been investigated in photochemical electron‐ and hydrogen‐transfer processes, only recently have a larger variety of such steps come to be discussed under the comprehensive concept of PCET. In photoredox catalytic reactions applied to organic synthesis, various PCET steps have been identified in cases in which redox or simple electron‐transfer steps are estimated to be endergonic by simple comparison of redox potentials. Such steps become exergonic when the electron transfer is coupled with proton transfer. In this article, examples of photoredox catalytic reactions of synthetic interest are reported, with the PCET discussed in detail.

The Nickel-Pincer Complex in Lactate Racemase Is an Electron Relay and Sink that acts through Proton-Coupled Electron Transfer.

QM/MM calculations reveal that the nickel pincer complex in lactate racemase functions as a reversible "single-center electrode" that accepts and donates back an electron. In this way, it catalyzes the isomerization process d-lactate⇌l-lactate through successive proton-coupled electron-transfer steps.

Evidence for photosensitised hydrogen production from water in the absence of precious metals, redox-mediators and co-catalysts.

The water-soluble zinc porphyrin complex Zn(TPPS)4- with TPPS = tetrakis-(4-sulfonatophenyl)porphyrin surprisingly was found to produce significant amounts of hydrogen from aqueous sulfite or amine solutions under visible-light exposure without requiring any other components such as electron relays or additional proton reduction catalysts. Although the production rates and total amounts of chemically stored fuel obtained under these conditions are still much too low to be relevant for practical applications, the background of this unprecedented observation was further studied in its own right. Since the central metal zinc is unlikely to be involved in proton-coupled electron transfer steps upon long-wavelength irradiation and the process does not seem to be much affected by variations of the electron donor added, the mechanism of photocatalytic H2 release is suggested to involve previously neglected redox features of the in situ generated hydroporphyrin ligand system in aqueous solution.

Orientation Sensitivity of Oxygen Evolution Reaction on Hematite

The sensitivity of the surface orientation on photoelectrochemical water oxidation has recently been reported by experimental studies. However, a detailed theoretical understanding is still missing. Density functional theory + Hubbard U (DFT + U) calculations are therefore carried out in order to investigate the oxygen evolution reaction (OER) on hematite (Fe2O3) surfaces for five surface orientations, namely (100), (210), (101), (021), and (211). The free energies of four proton-coupled electron transfer steps and the OER overpotential were calculated, and the trend in activity was analyzed. For the (100) orientation, two adsorbate–adsorbate distances were studied. Interestingly, a very low overpotential of 0.52 V was found for the (100) surface with a bridge site (adsorbate on a bridge of two Fe atoms) configuration that benefited from adsorbate–adsorbate interactions.

Insights into the Mechanism of Aromatic Ring Cleavage of Noncatecholic Compound 2-Aminophenol by Aminophenol Dioxygenase: A Quantum Mechanics/Molecular Mechanics Study

2-Aminophenol 1,6-dioxygenase (APD) is an extradiol dioxygenase responsible for the ring cleavage of 2-aminophenol (2AP) at the position ortho to the hydroxyl substituent. To elucidate the reaction mechanism, we conducted quantum mechanical/molecular mechanical (QM/MM) calculations. The mode of binding of the substrate (monodentate or bidentate) to the iron center was found to have a crucial role in dioxygen activation. The Fe–O2 adducts with 2AP bound bidentately has a quintet ground state having a FeIII–superoxo character, while the Fe–O2 adducts with a monodentately bound substrate has been characterized as a substrate radical–FeII–superoxide. Unlike other extradiol dioxygenases that cleave catechol analogues using the superoxo moiety of the Fe–O2 adducts to attack the substrate, we found here an FeII–O(H)O intermediate formed through two sequential proton-coupled electron transfer steps from the initial FeIII–superoxo species is responsible for the attack. Importantly, the second-sphere His195 residue...