Proton-coupled Electron Transfer Process Research Articles

Crystal Phase Engineering of Ultrathin Alloy Nanostructures for Highly Efficient Electroreduction of Nitrate to Ammonia.

Electrocatalytic nitrate reduction reaction (NO3RR) toward ammonia synthesis is recognized as a sustainable strategy to balance the global nitrogen cycle. However, it still remains a great challenge to achieve highly efficient ammonia production due to the complex proton-coupled electron transfer process in NO3RR. Here, the controlled synthesis of RuMo alloy nanoflowers (NFs) with unconventional face-centered cubic (fcc) phase and hexagonal close-packed/fcc heterophase for highly efficient NO3RR is reported. Significantly, fcc RuMo NFs demonstrate high Faradaic efficiency of 95.2% and a large yield rate of 32.7mg h-1 mgcat -1 toward ammonia production at 0 and -0.1V (vs reversible hydrogen electrode), respectively. In situ characterizations and theoretical calculations have unraveled that fcc RuMo NFs possess the highest d-band center with superior electroactivity, which originates from the strong Ru─Mo interactions and the high intrinsic activity of the unconventional fcc phase. The optimal electronic structures of fcc RuMo NFs supply strong adsorption of key intermediates with suppression of the competitive hydrogen evolution, which further determines the remarkable NO3RR performance. The successful demonstration of high-performance zinc-nitrate batteries with fcc RuMo NFs suggests their substantial application potential in electrochemical energy supply systems.

The role of an intramolecular hydrogen bond in the redox properties of carboxylic acid naphthoquinones.

A bioinspired naphthoquinone model of the quinones in photosynthetic reaction centers but bearing an intramolecular hydrogen-bonded carboxylic acid has been synthesized and characterized electrochemically, spectroscopically, and computationally to provide mechanistic insight into the role of proton-coupled electron transfer (PCET) of quinone reduction in photosynthesis. The reduction potential of this construct is 370 mV more positive than the unsubstituted naphthoquinone. In addition to the reversible cyclic voltammetry, infrared spectroelectrochemistry confirms that the naphthoquinone/naphthoquinone radical anion couple is fully reversible. Calculated redox potentials agree with the experimental trends arising from the intramolecular hydrogen bond. Molecular electrostatic potentials illustrate the reversible proton transfer driving forces, and analysis of the computed vibrational spectra supports the possibility of a combination of electron transfer and PCET processes. The significance of PCET, reversibility, and redox potential management relevant to the design of artificial photosynthetic assemblies involving PCET processes is discussed.

Organophotocatalytic pyridination of N-arylglycines with 4-cyanopyridines by decarboxylative and decyanative radical-radical coupling.

A photocatalytic decarboxylative aminoalkylation of 4-cyanopyridines with N-arylglycines is achieved, providing 4-(aminomethyl)pyridine derivatives in moderate to good yields. This organic photocatalytic reaction undergoes a radical-radical cross-coupling process under redox-neutral conditions, featuring simple operation, readily available N-arylglycines and a broad substrate scope. Mechanistic investigations indicated that a proton-coupled electron-transfer process was involved to enable the single electron transfer between the reduced photocatalyst and 4-cyanopyridine in the presence of N-arylglycines.

C-H bond activation by high-valent iron/cobalt-oxo complexes: a quantum chemical modeling approach.

High-valent metal-oxo species serve as key intermediates in the activation of inert C-H bonds. Here, we present a comprehensive DFT analysis of the parameters that have been proposed as influencing factors in modeled high-valent metal-oxo mediated C-H activation reactions. Our approach involves utilizing DFT calculations to explore the electronic structures of modeled FeIVO (species 1) and CoIVO ↔ CoIII-O˙ (species 2), scrutinizing their capacity to predict improved catalytic activity. DFT and DLPNO-CCSD(T) calculations predict that the iron-oxo species possesses a triplet as the ground state, while the cobalt-oxo has a doublet as the ground state. Furthermore, we have investigated the mechanistic pathways for the first C-H bond activation, as well as the desaturation of the alkanes. The mechanism was determined to be a two-step process, wherein the first hydrogen atom abstraction (HAA) represents the rate-limiting step, involving the proton-coupled electron transfer (PCET) process. However, we found that the second HAA step is highly exothermic for both species. Our calculations suggest that the iron-oxo species (Fe-O = 1.672 Å) exhibit relatively sluggish behavior compared to the cobalt-oxo species (Co-O = 1.854 Å) in C-H bond activation, attributed to a weak metal-oxygen bond. MO, NBO, and deformation energy analysis reveal the importance of weakening the M-O bond in the cobalt species, thereby reducing the overall barrier to the reaction. This catalyst was found to have a C-H activation barrier relatively smaller than that previously reported in the literature.

Molecular enhancement of Cu-based catalysts for CO2 electroreduction.

The electrochemical carbon dioxide reduction reaction (eCO2RR) represents an effective means of achieving renewable energy storage and a supply of carbon-based raw materials. However, there are still great challenges in selectively producing specific hydrocarbon compounds. The unique ability of the copper (Cu) catalyst to promote proton-coupled electron transfer processes offers clear advantages in generating value-added products. This review presents molecular enhancement strategies for Cu-based catalysts for CO2 electroreduction. We also elucidate the principles of each strategy for enhancing eCO2RR performance, discuss the structure-activity relationships, and propose some promising molecular enhancement strategies. This review will provide guidance for the development of organic-inorganic hybrid Cu-based catalysts as high-performance CO2 electroreduction catalysts.

Controlled intermolecular proton-coupled electron transfer process: a novel design strategy for highly sensitive piezo-chromic fluorescent materials

Highly sensitive piezochromic fluorescent materials were successfully synthesized via a controlled intermolecular proton-coupled electron transfer process in single-benzene π-system.

The dual-functional role of carboxylate in a nickel-iron catalyst towards efficient oxygen evolution.

The efficiency of the oxygen evolution reaction (OER) is severely limited by the sluggish proton-coupled electron transfer processes and inadequate long-term stability. Herein, we introduce a carboxylate group (TPA) to modify NiFe layered double hydroxide (NiFe LDH@TPA), resulting in notable improvements in both activity and stability. A combination of spectroscopic and theoretical investigations reveals the dual-functional role of incorporated TPA. It facilitates the deprotonation of OER intermediates while strengthening the Fe-O bond and acting as a molecular fence, ensuring superior OER kinetics and anti-dissolution properties. NiFe LDH@TPA delivers a low overpotential of 200 mV at 10 mA cm-2 and an impressive long-term stability of 500 h at 150 mA cm-2, significantly outperforming its unmodified counterpart. Furthermore, operating in an anion exchange membrane water electrolyzer, it affords prolonged stability at an industrial-scale current density of 1 A cm-2, sustaining performance for over 120 hours. This strategy offers a promising avenue for the development of durable and efficient OER catalysts.

Mechanism of selective catalytic reduction on VOx/TiO2 catalyst by in situ ambient pressure X-ray photoelectron spectroscopy

Unveiling the relationship between NH3 adsorption/activation behaviors and SCR reaction mechanism is imperative for rational catalyst design, but is is still not established because of lacking methodology of in situ spectroscopies. Here, vanadia-based SCR catalysts were used to identify the electron transfer and dynamic evolutions of active sites in SCR reaction by in situ AP-XPS, Raman, and DRIFTS spectra. The activation of NH3 occurred at V5+O sites, and then they reduced to V4+OH species by proton-coupled electron transfer process at high temperature (350 °C). Reducibility of vanadium dominated the catalytic reaction and both lattice and surface oxygen can oxidize the V4+OH to fulfill redox cycles. Whereas, at low temperature (150 °C), the adsorptions of NH3 on Brønsted acid sites was the rate-determine step of NH3 activations. NO was stored as nitrate/nitrite with the help of surface chemisorbed oxygen. The NHx species reacted with adsorbed nitrite/nitrate to generate the NHxNOx intermediate that subsequently decomposed into N2 and H2O.

Tuning the inner- and outer-sphere electron transfer behavior of aqueous {CoW12} polyoxometalate clusters for redox flow batteries exceeding 1.5V

Redox active species as the key component of redox flow batteries (RFBs) directly determine battery performance. Herein, we report a unique redox active specie, {CoW12}, exhibits both outer-sphere electron transfer of the {CoO4} core and inner-sphere electron transfer of the {W12O36} shell in one cluster, which is capable of constructing a symmetric {CoW12}-based RFB with pH-controlled operating voltage output. The DFT calculation discloses the proton-coupled electron transfer process of the {W12O36} shell in thermodynamically and kinetically, explaining the negative shift of the redox potential of {W12O36} shell as the pH increasing from 1 to 4. Interestingly, the redox potential of outer-sphere controlled {CoO4} core is unchanged, which leads to an extension of voltage window from 1.31 to 1.56 V. Consequently, the cycling measurement of the symmetrical battery proves that low capacity decay rate of 0.009 % and high Coulombic efficiency over 99 % can be retained in 400 cycles at pH 4.

Unraveling the role of Co(II)-phosphate complex in trace Co(II) activated peroxymonosulfate process for water decontamination with phosphate surrounding

The evolution and role of Co(II)-phosphate complex in pH-dependent peroxymonosulfate (PMS) activation by trace Co(II) for phosphate-contained water decontamination was experimentally and theoretically investigated. The HPO42− species play the most positive effect through forming Co-HPO4 coordination for enhancing radical generation and pharmaceutical degradation (3.59 times) in neutral condition. The reduction of Co(III) to Co(II) was identified as a proton-coupled electron transfer process. In Co(II)/PMS system without phosphate, the protons from the neighboring H2O coordination will bind with the generated SO4•−, inducing proton poisoning effect and in situ radical quenching. While the HPO42− coordination not only modulates the Co 3d electronic structure to optimize the PMS interaction and charge transfer, but also alters the proton affinity to alleviate the proton poisoning effect, securing the radical generation. This study provides insights into the impacts of background factors on advanced oxidation processes and develops strategies for efficient remediation in real (waste)water matrix.

Surface-enhanced bimetallic effect of Au-Pd by internal electromagnetic fields from Au@Cu2O for efficient electrochemical nitrate reduction to ammonia

Electrochemical synthesis of ammonia from nitrate is an attractive approach to produce high-value “zero-carbon” fuel. However, this nitrate-to-ammonia technology is still restricted by its unsatisfactory ammonia conversion, which is resulted from the inevitable by-products formed during the complicated proton-coupled electron transfer process. In this study, Au NR@Cu2O-AuPd NPs were designed by taking advantage of the internal electromagnetic field derived from Au NR for enhanced nitrate-to-ammonia performance. The constructed Au NR@Cu2O-AuPd NPs exhibited a high yield of ammonia (4587.00 μg h−1mgcat.−1) and FE(93.09 %) in neutral solution (pH = 7), which is attributed to the different adsorption capacities of the surface bimetals for nitrate and the reaction intermediates of nitrate reduction to NH3. Under the irradiation of light, the surface bimetals showed different responsiveness to the generated internal electromagnetic field derived from Au NR, enabling a further increase in the yield of NH3 to 5328.99 μg h−1mgcat.−1. This work provides new insights into enhancing the conversion of nitrate to NH3 by electrocatalytic reduction through photo-modulation of the adsorption capacity on the catalyst surface.

Selective electrochemical methanation of carbon dioxide using a sulphide derived CuZn catalyst

Efficient catalysts facilitate the electrochemical reduction of CO2, enabling its conversion into fuels or chemical feedstocks through proton coupled electron transfer processes. Among these catalysts, Cu-based ones stand out for their unique ability to produce hydrocarbon products with high faradaic efficiencies. This study focuses on the use of sulphide-derived (SD)-CuxZny nanoparticle catalysts, which have been found to exhibit enhanced methane selectivity during CO2 reduction in an H-Cell with 0.10 M KHCO3 as the electrolyte. The composition of the catalysts plays a crucial role in determining their catalytic behavior. At an optimal Cu/Zn ratio of 1:1, the SD-CuZn catalyst demonstrates exceptional methane production, achieving a faradaic efficiency of 76 ± 3 % at a potential of −0.98 V vs RHE. Moreover, a partial current density of −4.5 mA cm−2 was achieved at a more negative potential of −1.09 V vs RHE. Ex situ characterization highlighted the significance of partially reduced CuS species in influencing the selectivity of hydrogenated carbon products. When CO2 reduction was performed in a flow cell equipped with a gas diffusion electrode with 1.0 M KHCO3 or 1.0 M KOH electrolytes in both anodic and cathodic compartments, the current density increased due to the enhanced mass transport rate. However, the altered conditions lead to a shift in selectivity, favoring carbon monoxide over methane. It is important to note that insights garnered from H-Cell experiments may not be directly extrapolated to flow cell setups.

Photo-induced versatile aliphatic C–H functionalization via electron donor–acceptor complex

The ability to selectively introduce diverse functionality onto hydrocarbons is of substantial value in the synthesis of both small molecules and pharmaceuticals. In this endeavour, as a photocatalyst- and metal-free process, the electron donor–acceptor (EDA) strategy has not been well explored. Here we report an approach to aliphatic carbon-hydrogen bond diversification through an EDA complex constituted by HCl and SIV=O groups. As an efficient hydrogen atom transfer (HAT) reagent, chlorine radical can be produced via a proton-coupled electron transfer process in this system. Based on this unusual path, a photo-promoted versatile aliphatic C–H functionalization is developed without photo- and metal-catalysts, including thiolation, arylation, alkynylation, and allylation. This conversion has concise and ambient reaction conditions, good functional group tolerance, and substrate diversity, and provides an alternative solution for the high value-added utilization of bulk light alkanes.

Carboxamide Fe(III) complex as the electrocatalyst of water oxidation reaction: WNA and I2M O–O bond formation pathways

A single-site Fe(III) carboxamide complex, [Fe(HbpH) (Cl)2] (1), (HbpH‾ = N,N′-Bis(picolinoyl)hydrazine) anion), was synthesized and used as the electrocatalyst of water oxidation reaction. Complex 1 catalyzes the oxidation of water to O2 at the pH of 8 in phosphate buffer solution with the onset of a catalytic wave at about 0.45 V versus RHE with the turnover frequency (TOF) of 5.8 s−1 at room temperature. The stability of the catalyst in water oxidation processes by electrochemical and UV–vis measurements, demonstrated the realistic molecular electrocatalysis of 1. Differential pulse voltammetry (DPV) of 1 in different pH values showed the dependency of oxidation potential to the pH of the environment, establishing the fact that the catalytic cycle involves the proton-coupled electron transfer (PCET) process. Density functional theory (DFT) calculations on the mechanism of water oxidation process of 1 shows that the oxygen evolution reaction is carried out through the water nucleophilic attack (WNA) to the metal center ion, resulted to the formation of FeIV(O) species. Also, DFT calculations showed that the formation of an O–O bond proceeds through the interaction two molecular mechanism (I2M), which compete with the WNA mechanism following from experimental analysis. The electroactivity of the metal centre and carboxamide ligand and the ability of the ligand to stabilize the high oxidation numbers of the metal-ion centre during the oxidation process, provide different oxidation pathways for an OER process.

Electrochemical proton-coupled electron transfer processes to form neutral radicals of azaphenalenes: pKa values of protonated radicals

Azaphenalenes are nitrogen substituted analogues of the phenalene ring system, and their radicals have attracted significant attention because of the novel electronic and magnetic properties. In the previous study, the neutral tris(tert-butyl) 1,3-diazaphenalenyl radical has been synthesized by chemical oxidation of the parent compound with PbO2. In this study, we focused on electrochemical proton-coupled electron transfer (PCET) process leading to the formation of neutral unsubstituted 1,3-diazaphenalenyl radical from 1,3-diazaphenalene (DAP) in solution. Additionally, this electrochemical approach was extended to study the redox behavior of newly synthesized 1,4,7-triazaphenalene (TAP) giving the access to the 1,4,7-triazaphenalenyl radical. Electrochemical oxidation potentials vs. pH plots (Pourbaix diagram) for DAP and TAP revealed that their one-electron oxidation to the corresponding neutral radicals occurred by a PCET process over a wide pH range. From the analysis of Pourbaix diagrams, the pKa values of the protonated radicals were determined to be pKa = 2.57 and 2.39 for the DAP and TAP radicals, respectively. Cyclic voltammograms of azaphenalenes in MeCN and MeCN/buffer solutions showed a characteristic ECE process, in which electrochemical oxidative dimerization took place following the electron transfer. The dimerization reaction pathway was rationalized by the digital simulations, DFT calculations and crystallization of a DAP dimer after chemical oxidation with PbO2. The spectroscopic properties of azaphenalenes and their dimerized products were characterized by in-situ UV–Vis and EPR spectroelectrochemistry techniques. The fundamental information about the pKa and redox potentials for a series of azaphenalenes (DAP, TAP, as well as 1,3,4,6,7,9-hexaazaphenalene, HAP) could be fed back into the molecular design of redox-active conductive coordination networks based on multi-interactive ligands.

Selective Electrochemical Oxidation of Benzylic C-H to Benzylic Alcohols with the Aid of Imidazolium Radical Mediators.

Selective oxidation of benzylic C-H to benzylic alcohols is a well-known challenge in the chemical community since benzylic C-H is more prone to be overoxidized to benzylic ketones. In this work, we report the highly selective electro-oxidation of benzylic C-H to benzylic alcohols in an undivided cell in ionic liquid-based solution. As an example, the selectivity toward xanthydrol could be as high as 95.7% at complete conversion of xanthene, a typical benzylic C-H compound, on gram-scale in imidazolium bromide/H2O/DMF. Mechanism investigation reveals that the imidazolium radical generated in situ participants in a proton-coupled electron transfer process and low-barrier hydrogen bonds stabilize the reaction intermediates, together steering the redox equilibrium, favoring benzylic alcohols over benzylic ketones.

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.

A cascade of in situ conversion of bicarbonate to CO2 and CO2 electroreduction in a flow cell with a Ni-N-S catalyst

Combination of CO2 capture using inorganic alkali with subsequently electrochemical conversion of the resultant HCO3- to high-value chemicals is a promising route of low cost and high efficiency. The electrochemical reduction of HCO3- is challenging due to the inaccessible of negatively charged molecular groups to the electrode surface. Herein, we adopt a comprehensive strategy to tackle this challenge, i.e., cascade of in situ chemical conversion of HCO3- to CO2 and CO2 electrochemical reduction in a flow cell. With a tailored Ni-N-S single atom catalyst (SACs), where sulfur (S) atoms located in the second shell of Ni center, the CO2 electroreduction (CO2ER) to CO is boosted. The experimental results and density functional theory (DFT) calculations reveal that the introduction of S increases the p electron density of N atoms near Ni atom, thereby stabilizing *H over N and boosting the first proton coupled electron transfer process of CO2ER, i.e., *+e–+*H+*CO2→*COOH. As a result, the obtained catalyst exhibits a high faradaic efficiency (FECO ∼98%) and a low overpotential of 425 mV for CO production as well as a superior turnover frequency (TOF) of 47397 h−1, outcompeting most of the reported Ni SACs. More importantly, an extremely high FECO of 90% is achieved at 50 mA cm−2 in the designed membrane electrode assembly (MEA) cascade electrolyzer fed with liquid bicarbonate. This work not only highlights the significant role of the second coordination on the first coordination shell of the central metal for CO2ER, but also provides an alternative and feasible strategy to realize the electrochemical conversion of HCO3- to high-value chemicals.

Sr2+-Doped CuO Nanoribbons with the Hydrophobic Surface Enabling CO2 Electroreduction to Ethane.

Electrochemical reduction of carbon dioxide to value-added multicarbon (C2+) products is a promising way to obtain renewable fuels of high energy densities and chemicals and close the carbon cycle. However, the difficulty of C-C coupling and complexity of the proton-coupled electron transfer process greatly hinder CO2 electroreduction into specific C2+ products with high selectivity. Here, we design an electrocatalyst of Sr-doped CuO nanoribbons with a hydrophobic surface for CO2 electroreduction to ethane with high selectivity. Sr doping enhances the chemical adsorption and activation of CO2 by inducing oxygen vacancies and increasing *CO coverage by stabilizing Cu2+ active sites, thus further boosting subsequent C-C coupling. The hydrophobic surface with dodecyl sulfate anions (DS-) adsorption increases the oxophilicity of the catalyst surface, enhancing the conversion of the *OCH2CH3 intermediate to ethane. As a result, the optimized Sr1.97%-CuO exhibits a Faradaic efficiency of 53.4% and a partial current density of 13.5 mA cm-2 for ethane under a potential of -0.8 V. This study provides a strategy to design a Cu-based catalyst by alkaline earth metal ions doping with the hydrophobic surface to engineer the evolution of the intermediates for a desired product during CO2RR.

Promoting oxygen reduction kinetics of single Fe sites for robust neutral Zn-air batteries via engineering synergistic Fe nanocluster as proton-feeding center

Constructing highly efficient and cost-effective catalysts for neutral oxygen reduction reaction (ORR) remains extremely challenging due to the sluggish reaction kinetics resulting from the low ionic conductivity and limited OH− concentration in the neutral electrolytes. Herein, we intentionally integrate the atomic Fe-N4 sites and Fe nanoclusters on N-doped multimodally porous carbon (FeSA+NC@NMPC) to achieve coherent optimization of rapid oxygen-containing intermediate conversions and fast water dissociation to provide abundant protons for boosting neutral ORR performance. As expected, the FeSA+NC@NMPC exhibits an excellent half-wave potential of 0.76 V in 0.1 M phosphate buffer solutions, outperforming that of commercial Pt/C (0.73 V). Theoretical calculations reveal the synergistic effect between atomic Fe-N4 sites and Fe nanoclusters, in which the former possess stable O2 adsorption and rapid intermediate conversion, while the latter facilitates fast water dissociation to supply protons for accelerating the proton-coupled electron transfer process. Moreover, the FeSA+NC@NMPC-based neutral zinc-air batteries afford a high open-circuit potential of 1.42 V and outstanding cycling stability at 5 mA cm−2 for 100 h. This work utilizes the advantages of both single sites and clusters of Fe to provide an in-depth understanding of the neutral ORR mechanism and advances the development of related energy storage and conversion technologies.