Proton-coupled Electron Transfer Mechanism Research Articles

Synergy between Cu and Co in a Layered Double Hydroxide Enables Close to 100% Nitrate-to-Ammonia Selectivity.

Nitrate electroreduction (NO3RR) holds promise as an energy-efficient strategy for the removal of toxic nitrate to restore the natural nitrogen cycle and mitigate the adverse impacts caused by overfertilization from suboptimal agricultural practices. However, existing catalysts suffer from limited electrocatalytic activity, poor selectivity, inadequate durability, and low scalability. To address this quadrilemma, in this study, we developed a cost-effective layered double hydroxide (LDH) electrocatalyst with a lamellar structure that presents trimetallic CuCoAl active sites on the nanomaterial surface. This codoping design enabled electrochemical upcycling of nitrate into ammonia exclusively and efficiently with an onset potential at 0 V vs RHE, where the electrocatalytic process is less energy intensive and has a lower carbon footprint than conventional practices. The synergistic interaction among Cu, Co, and Al further afforded a 99.5% Faradic efficiency (FE) and a yield rate of 0.22 mol h-1 g-1 for nitrate-to-ammonia electroreduction, surpassing the performance of state-of-the-art nonprecious metal NO3RR electrocatalysts over an extended operation period. To gain insights into the origin of the catalytic performance observed on LDH, control materials were employed to elucidate the roles of Cu and Co. Cu was found to improve the NO3RR onset potential despite displaying limited FE for ammonia synthesis, while Co was discovered to suppress the formation of nitrite byproduct though requiring large overpotential. Simulated wastewater containing phosphate and sulfate, which are typically present in industrial effluents, was used to further investigate the effect of electrolytes on NO3RR. Intriguingly, the use of phosphate buffer resulted in a superior yield rate and FE for ammonia production while simultaneously inhibiting nitrite byproduct formation compared with the sulfate case. These experimental findings were supported by density functional theory (DFT) calculations, which explored the adsorption strength of nitrate adducts adjacent to coadsorbed electrolytes on the LDH surface. Additionally, the relative free energies of NO3RR species were also computed to examine the proton-coupled electron transfer (PCET) mechanism on CuCoAl LDH, shedding light on the potential-dependent step (PDS) and the exclusive selectivity for nitrate-to-ammonia conversion. The CuCoAl LDH developed here offers scalability by eliminating the need for precious metals, rendering this earth-abundant catalyst particularly appealing for sustainable nitrate electrovalorization technology.

Exceptionally high proton conductivity in Eu2O3 by proton-coupled electron transfer mechanism

Proton conductors are typically developed by doping to introduce structural defects such as oxygen vacancies to facilitate ionic transport through structural bulk conduction mechanism. In this study, we present a novel electrochemical proton injection method via an in situ fuel cell process, demonstrating proton conduction in europium oxide (Eu2O3) through a surficial conduction mechanism for the first time. By tuning Eu2O3 into a protonated form, H-Eu2O3, we achieved an exceptionally high proton conductivity of 0.16S cm-1. Distribution of relaxation time (DRT) analysis was employed to investigate the proton transport behavior and reveal the significant contribution of surface proton transport to the overall conductivity of Eu2O3. Remarkably, H-Eu2O3 exhibited a low activation energy for ionic transport, comparable to the best ceramic electrolytes available. The proton-coupled electron transfer (PCET) mechanism describes this novel surficial proton conduction mechanism. These findings provide new possibilities for developing advanced proton conductors with improved performance.

β-Scission by Direct Electrochemical Oxidation: Proton-coupled Electron Transfer Mechanism Dictated by Synthetic Study and Computations

β-Scission by Direct Electrochemical Oxidation: Proton-coupled Electron Transfer Mechanism Dictated by Synthetic Study and Computations

Cations Determine the Mechanism and Selectivity of Alkaline Oxygen Reduction Reaction on Pt(111).

The proton-coupled electron transfer (PCET) mechanism of the oxygen reduction reaction (ORR) is a long-standing enigma in electrocatalysis. Despite decades of research, the factors determining the microscopic mechanism of ORR-PCET as a function of pH, electrolyte, and electrode potential remain unresolved, even on the prototypical Pt(111) surface. Herein, we integrate advanced experiments, simulations, and theory to uncover the mechanism of the cation effects on alkaline ORR on well-defined Pt(111). We unveil a dual-cation effect where cations simultaneously determine i) the active electrode surface by controlling the formation of Pt-O and Pt-OH overlayers and ii) the competition between inner- and outer-sphere PCET steps. The cation-dependent transition from Pt-O to Pt-OH determines the ORR mechanism, activity, and selectivity. These findings provide direct evidence that the electrolyte affects the ORR mechanism and performance, with important consequences for the practical design of electrochemical systems and computational catalyst screening studies. Our work highlights the importance of complementary insight from experiments and simulations to understand how different components of the electrochemical interface contribute to electrocatalytic processes.

Concerted proton-coupled electron transfer promotes NiCoP nanowire arrays for efficient overall water splitting at industrial-level current density

Inspired by the oxygen-evolving complex (OEC) in photosystem II (PS-II), self-supported Fe-doped NiCoP nanowire arrays modified with carboxylate (NCFCP@NF) is constructed to boost industrial-level overall water splitting by the concerted proton-coupled electron transfer (c-PET) mechanism. NCFCP@NF exhibits 283 mV at the current density of 3000 mA cm−2 for HER, which is attributed to the H2O adsorption sites provided by Fe and Ni, as well as the electrons provided by carboxyl ligand. Also, NCFCP@NF shows remarkable OER catalytic activity with overpotential of 329 mV to afford 3000 mA cm−2. Herein, the introduction of Fe promotes the formation of *OOH, and carboxyl ligand accelerate the deprotonation process of OER. Furthermore, NCFCP@NF displays the ultralong durability (200 h) at 1000 mA cm−2 for overall water splitting. This study offers a new perspective on designing efficient overall water splitting catalyst at the industrial-level current density by the concerted proton-coupled electron transfer mechanism.

Electrocatalytic Hydrogen Evolution using a Nickel-based Calixpyrrole Complex: Controlling the Secondary Coordination Sphere on an Electrode Surface.

Incorporating design elements from homogeneous catalysts to construct well defined active sites on electrode surfaces is a promising approach for developing next generation electrocatalysts for energy conversion reactions. Furthermore, if functionalities that control the electrode microenvironment could be integrated into these active sites it would be particularly appealing. In this context, a square planar nickel calixpyrrole complex, Ni(DPMDA) (DPMDA=2,2'-((diphenylmethylene)bis(1H-pyrrole-5,2-diyl))bis(methaneylylidene))bis(azaneylylidene))dianiline) with pendant amine groups is reported that forms a heterogeneous hydrogen evolution catalyst using anilinium tetrafluoroborate as the proton source. The supported Ni(DPMDA) catalyst was surprisingly stable and displayed fast reaction kinetics with turnover frequencies (TOF) up to 25,900 s-1 or 366,000 s-1 cm-2 . Kinetic isotope effect (KIE) studies revealed a KIE of 5.7, and this data, combined with Tafel slope analysis, suggested that a proton-coupled electron transfer (PCET) process involving the pendant amine groups was rate-limiting. While evidence of an outer-sphere reduction of the Ni(DPMDA) catalyst was observed, it is hypothesized that the control over the secondary coordination sphere provided by the pendant amines facilitated such high TOFs and enabled the PCET mechanism. The results reported herein provide insight into heterogeneous catalyst design and approaches for controlling the secondary coordination sphere on electrode surfaces.

Reactions of a hydrogen atom with haloacetates in aqueous solutions: Computational evidence for proton-coupled electron transfer and competing mechanisms.

A computational study of the mechanisms and kinetics of the aqueous reactions of a hydrogen atom with haloacetates is presented. Several mechanisms in the close competition are observed, such as proton-coupled electron transfer (PCET), hydrogen atom transfer (HAT), and halogen abstraction (XA). Computations predict that dechlorination takes place via PCET mechanisms and not via XA, as stated earlier, while XA is the fastest mechanism for . The reaction rate constants are reasonably well predicted within the theoretically most reliable canonical variational transition state theory with small curvature tunneling corrections and compared with the experimental ones. To reproduce the experimental rate constants of the debromination process it is necessary to include the PCET and XA cumulative values. Small curvature tunneling corrections to the rate constants are the highest for HAT and PCET mechanisms, up to 70 times larger than the Wigner, while variational effects for XA mechanisms are very small.

Hydrocarbon Oxidation by a Porphyrin-π-Cation Radical Complex.

Heme and chlorin π-cation radical oxidants are widely implicated in biological and synthetic oxidation catalysis. Little insight into the role of π-cation radicals in proton coupled electron transfer (PCET) oxidation is available. We prepared a NiII-porphyrin-π-cation complex ([NiII(P•+)]) and found it to be capable of the oxidation of a variety of simple hydrocarbon substrates. Interestingly, some of the products were hydroxylated, with ([NiII(P•+)]) working in concert with atmospheric O2 to yield hydroxylated hydrocarbons. Kinetic data suggested that the porphyrin-π-cation radical species oxidised substrates via a concerted PCET mechanism, where the nickel porphyrin-π-cation radical accepted the electron, and the proton was transferred to a free anion. Our findings highlight the potential role of π-cation radicals as hydrocarbon activators, demonstrating that porphyrin ligand non-innocence could be a readily manipulated resource for oxidation catalyst development.

Hydrocarbon Oxidation by a Porphyrin‐π‐Cation Radical Complex

AbstractHeme and chlorin π‐cation radical oxidants are widely implicated in biological and synthetic oxidation catalysis. Little insight into the role of π‐cation radicals in proton coupled electron transfer (PCET) oxidation is available. We prepared a NiII‐porphyrin‐π‐cation complex ([NiII(P⋅+)]) and found it to be capable of the oxidation of a variety of simple hydrocarbon substrates. Interestingly, some of the products were hydroxylated, with ([NiII(P⋅+)]) working in concert with atmospheric O2 to yield hydroxylated hydrocarbons. Kinetic data suggested that the porphyrin‐π‐cation radical species oxidised substrates through a concerted PCET mechanism, where the porphyrin‐π‐cation radical accepted the electron, and the proton was transferred to a free anion. Our findings highlight the potential role of π‐cation radicals as hydrocarbon activators, demonstrating that porphyrin ligand non‐innocence could be a readily manipulated resource for oxidation catalyst development.

Tyrosine residues initiated photopolymerization in living organisms

Towards intracellular engineering of living organisms, the development of new biocompatible polymerization system applicable for an intrinsically non-natural macromolecules synthesis for modulating living organism function/behavior is a key step. Herein, we find that the tyrosine residues in the cofactor-free proteins can be employed to mediate controlled radical polymerization under 405 nm light. A proton-coupled electron transfer (PCET) mechanism between the excited-state TyrOH* residue in proteins and the monomer or the chain transfer agent is confirmed. By using Tyr-containing proteins, a wide range of well-defined polymers are successfully generated. Especially, the developed photopolymerization system shows good biocompatibility, which can achieve in-situ extracellular polymerization from the surface of yeast cells for agglutination/anti-agglutination functional manipulation or intracellular polymerization inside yeast cells, respectively. Besides providing a universal aqueous photopolymerization system, this study should contribute a new way to generate various non-natural polymers in vitro or in vivo to engineer living organism functions and behaviours.

Choice of Electrolyte Impacts the Selectivity of Proton-Coupled Electrochemical Reactions on Hydrogen Titanate

Proton-coupled electron transfer (PCET) reactions involving transition metal oxides are prevalent in aqueous electrochemical systems used for energy storage and conversion. Here, we elucidate the role of electrolyte on PCET mechanisms in transition metal oxides in aqueous acidic electrolytes using layered hydrogen titanate (H2Ti3O7) as an example. We identify three processes by which electrolyte protons interact with hydrogen titanate at the electrochemical interface: (1) adsorption at the surface and/or insertion into the bulk, (2) adsorption as part of the hydrogen evolution reaction (HER) at the surface, and (3) dissolution of the hydrogen titanate. We utilize a combined experimental and computational (ReaxFF) approach to probe how the competition for protons and electrons among these processes influences electrochemical properties, including the energy storage, Coulombic efficiency (CE), rate capability, and lifetime. In an acidic buffered electrolyte (1 M H3PO4), the CE increases from an average of 48% to 71% and the specific capacity increases from 83 to 90 mAh g–1 as compared to a strong acid electrolyte (1 M H2SO4). We propose that H3PO4 mitigates the HER and hydrogen titanate dissolution, thereby increasing the operating potential window for proton adsorption/insertion for charge storage in hydrogen titanate. Material characterization and computational results indicate that adsorption of phosphate species onto the surface of hydrogen titanate may decrease its dissolution upon reduction, thereby improving electrode performance. We offer a preliminary solution to improve energy storage performance via electrolyte tuning by decreasing the prevalence of the HER and electrode dissolution.

Integrated Computational Study of the Light-Activated Structure of the AppA BLUF Domain and Its Spectral Signatures.

We apply an integrated approach combining microsecond MD simulations and (polarizable) QM/MM calculations of NMR, FTIR, and UV-vis spectra to validate the structure of the light-activated form of the AppA photoreceptor, an example of blue light using flavin (BLUF) protein domain. The latter photoactivate through a proton-coupled electron transfer (PCET) that results in a tautomerization of a conserved glutamine residue in the active site, but this mechanism has never been spectroscopically proven for AppA, which has been always considered as an exception. Our simulations instead confirm that the spectral features observed upon AppA photoactivation are indeed directly connected to the tautomer form of glutamine as predicted by the PCET mechanism. In addition, we observe small but significant changes in the AppA structure, which are transmitted from the flavin binding pocket to the surface of the protein.

A DFT Study on the Kinetics of HOO•, CH3OO•, and O2•- Scavenging by Quercetin and Flavonoid Catecholic Metabolites.

Reaction kinetics have been theoretically examined to ascertain the potency of quercetin (Q) and flavonoid catecholic metabolites 1-5 in the inactivation of HOO•, CH3OO•, and O2•- under physiological conditions. In lipidic media, the koverallTST/Eck rate constants for the proton-coupled electron transfer (PCET) mechanism indicate the catecholic moiety of Q and 1-5 as the most important in HOO• and CH3OO• scavenging. 5-(3,4-Dihydroxyphenyl)-γ-valerolactone (1) and alphitonin (5) are the most potent scavengers of HOO• and CH3OO•, respectively. The koverallMf rate constants, representing actual behavior in aqueous media, reveal Q as more potent in the inactivation of HOO• and CH3OO• via single electron transfer (SET). SET from 3-O- phenoxide anion of Q, a structural motif absent in 1-5, represents the most contributing reaction path to overall activity. All studied polyphenolics have a potency of O2•- inactivation via a concerted two-proton-coupled electron transfer (2PCET) mechanism. The obtained results indicate that metabolites with notable radical scavenging potency, and more bioavailability than ingested flavonoids, may contribute to human health-promoting effects ascribed to parent molecules.

Gradient Alloyed Quantum Dots for Photocatalytic Lignin Valorization via Proton Coupled Electron Transfer

In the field of photocatalytic lignin valorization, simultaneously fulfilling the requirements of broad light harvesting and strong redox ability has been a key challenge for single binary quantum dot (QD) photocatalyst systems. In order to address the problem, gradient alloyed QDs with funnel-shaped energy structures have been developed. The gradient energy level increases the photon absorption range, while the periphery energy bands at the funnel edge are sufficiently wide to preserve effective redox capacity. In this work, the gradient alloyed QD photocatalyst along with proton donors have been employed in enhancing the efficiency and selectivity of β-O-4 bond cleavage in lignin, realizing yields of target aromatic products phenol and acetophenone to increase up to 70% and 65%, respectively. The mechanism of conversion from the ketone intermediate to target products and how to choose suitable proton donors have been systematically investigated. We propose a proton-coupled electron transfer (PCET) photocatalytic strategy through producing a Cα radical from ketone with the help of acid possessing appropriate bond dissociation free energy (BDFE) and strong electronic transmission facilitating ability. The knowledge acquired in this work is useful for designing photocatalysts and proton donors that can effectively cleave bonds in lignin via the PCET mechanism.

Mechanism of proton-coupled electron transfer described with QM/MM implementation of coupled-perturbed density-functional tight-binding.

Coupled-perturbed equations for degenerate orbitals were implemented for third order density-functional tight binding, which allowed the use of Mulliken charges as reaction coordinates. The method was applied to proton-coupled electron transfer (PCET) reactions in a model system and thoroughly tested for QM and QM/MM setups (i.e., coupled quantum and molecular mechanics). The performed enhanced sampling simulations were stable, and the obtained potentials of the mean force were able to address the thermodynamic and kinetic features of the reactions by showing the expected topography and energy barriers. Hence, this method has the potential to distinguish between concerted and sequential mechanisms and could next be applied to proton-coupled electron transfer reactions in more complex systems like proteins.

Introduction of Methyl Group in Substituted Isoselenazolones: Catalytic and Mechanistic Study.

Copper-catalyzed direct selenation of substituted 2-bromo-N-phenylbenzamide substrates with elemental selenium powder provided a series of methoxy-substituted isoselenazolones via the C-Se and Se-N bond formations. Phenolic substituted isoselenazolones have been obtained by O-demethylation of the corresponding methoxy-substituted analogues using boron tribromide. Some isoselenazolones have been structurally characterized by X-ray single-crystal analysis. The glutathione peroxidase (GPx)-like antioxidant activity of isoselenazolones has been evaluated both in thiophenol and coupled-reductase assays. All isoselenazolones showed good GPx-like activities in the coupled-reductase assay. The ferric-reducing antioxidant power of phenolic antioxidants has also been evaluated. The best phenolic antioxidants were found to be good ferric-reducing antioxidant power agents. The single electron transfer, hydrogen atom transfer, and proton-coupled electron transfer mechanisms for the antioxidant properties of all catalysts have been supported by density functional theory calculations. The catalytic cycle was proposed for one of the phenolic isoselenazolones involving diselenide, selenenyl sulfide, selenol, and selenenic acid as intermediates using 77Se{1H} NMR spectroscopy.

Reactivity of trans-Resveratrol toward Electrogenerated Superoxide in N,N-Dimethylformamide.

The reactivity of 5-[(E)-2-(4-hydroxyphenyl)ethen-1-yl]benzene-1,3-diol (trans-resveratrol) and related compounds toward electrogenerated superoxide radical anion (O2•-) were investigated using electrochemistry, in situ electrolytic electron spin resonance, and in situ electrolytic ultraviolet-visible spectral measurements, in N,N-dimethylformamide (DMF) with the aid of density functional theory (DFT) calculations. The quasi-reversible cyclic voltammogram of dioxygen/O2•- was modified by the presence of trans-resveratrol, suggesting that the electrogenerated O2•- was scavenged by trans-resveratrol through proton-coupled electron transfer (PCET) via three phenolic hydroxy groups (OH) on the stilbene moiety. The reactivity of trans-resveratrol toward O2•- characterized by the OHs was experimentally confirmed in comparative analyses using some related compounds, pinosylvin, pterostilbene, p-coumaric acid, and so on, in DMF. The electrochemical and DFT results suggested that a concerted PCET mechanism via 4'OH of trans-resveratrol proceeds, where the coplanarity of the two phenolic rings in the stilbene moiety linked by an ethylene bridge is essential for a successful O2•- scavenging.

Cover Feature: Photoelectrochemical C−H Activation Through a Quinacridone Dye Enabling Proton‐Coupled Electron Transfer (ChemSusChem 5/2023)

The Cover Feature shows the photoelectrochemical activation of C−H bonds with mesoporous SnO2 electrodes sensitized with a quinacridone dye (pigment violet 19, used in paints). The key feature of quinacridone is the possibility to operate through a proton-coupled electron transfer mechanism, generating a nitrogen-based radical characterized by a bond dissociation free energy (BDFE) of 80.5 kcal mol−1 and capable of hydrogen atom abstraction from allylic and benzylic C−H bonds. More information can be found in the Research Article by Y. Yang, G. A. Volpato, E. Rossin et al.

Direct Proton-Coupled Electron Transfer between Interfacial Tyrosines in Ribonucleotide Reductase.

Ribonucleotide reductase (RNR) regulates DNA synthesis and repair in all organisms. The mechanism of Escherichia coli RNR requires radical transfer over a proton-coupled electron transfer (PCET) pathway spanning ∼32 Å across two protein subunits. A key step along this pathway is the interfacial PCET reaction between Y356 in the β subunit and Y731 in the α subunit. Herein, this PCET reaction between two tyrosines across an aqueous interface is explored with classical molecular dynamics and quantum mechanical/molecular mechanical (QM/MM) free energy simulations. The simulations suggest that the water-mediated mechanism involving double proton transfer through an intervening water molecule is thermodynamically and kinetically unfavorable. The direct PCET mechanism between Y356 and Y731 becomes feasible when Y731 is flipped toward the interface and is predicted to be approximately isoergic with a relatively low free energy barrier. This direct mechanism is facilitated by the hydrogen bonding of water to both Y356 and Y731. These simulations provide fundamental insights into radical transfer across aqueous interfaces.

A Proton-Coupled Electron Transfer Strategy to the Redox-Neutral Photocatalytic CO2 Fixation.

Herein, we report our study on the design and development of a novel photocarboxylation method. We have used an organic photoredox catalyst (PC, 4CzIPN) and differently substituted dihydropyridines (DHPs) in combination with an organic base (1,5,7-triazabicyclodec-5-ene, TBD) to access a proton-coupled electron transfer (PCET) based manifold. In depth mechanistic investigations merging experimental analysis (NMR, IR, cyclic voltammetry) and density-functional theory (DFT) calculations reveal the key activity of a H-bonding complex between the DHP and the base. The thermodynamic and kinetic benefits of the PCET mechanism allowed the implementation of a redox-neutral fixation process leading to synthetically relevant carboxylic acids (18 examples with isolated yields up to 75%) under very mild reaction conditions. Finally, diverse product manipulations were performed to demonstrate the synthetic versatility of the obtained products.