An electrochemical anaerobic dynamic membrane bioreactor for enhanced sludge digestion: Unveiling molecular interactions and microbial mechanisms.

Hydrogenation of organic molecules requires sequential addition of two H atoms to the substrate. So far, two pathways have been identified for this reaction on transition metal particles in water: conventional hydrogenation (CH) and proton coupled electron transfer (PCET). Typically, CH takes place in thermocatalytic hydrogenation (TCH) in which organic substrate reacts with adsorbed H atom on the surface of transition metal (Rxn. 1). PCET is a classic pathway occurring in electrocatalytic hydrogenation (ECH) by the simultaneous or sequential attack of hydronium ions with electrons to organic substrate on an electrode under a negative overpotential (Rxn. 2).Phenol* + H*→ H-Phenol* + * (CH) Rxn. 1Phenol* + H+ + e-→ H-Phenol* (PCET) Rxn. 2In water, a metal particle itself is a hydrogen electrode in presence of H2 and at a certain pH, having a so-called open circuit potential (OCP). In this work, we will use the hydrogenation reaction of phenol on Pt to explore whether the OCP on a transition metal can induce a PCET pathway, in particular at which conditions PCET overtakes CH pathway at OCP.Figure 1(a) shows the turnover frequency of phenol hydrogenation catalyzed by carbon nanotube (CNT) supported Pt catalyst under different pH. The turnover frequency shows a clear increasing trend, growing by an order of magnitude as pH decreasing from 5.3 to 2. In order to determine the reaction pathway, kinetic experiments were designed based on the difference between two pathways. The H atom added to phenol molecule in CH pathway (Rxn. 1) is from dissociative adsorption of gaseous H2, whereas that in PCET pathway is from hydronium ions in water (Rxn. 2). If the reaction undergoes the CH pathway, a kinetic isotope effect (KIE) would be observed when changing reductant from H2 to D2; whereas if PCET pathway dominates, a KIE would be observed via replacing H2O by D2O. Figure 1(b) shows that the dominated pathway is CH under pH 5.3, a KIE of kH2/kD2 was obtained when changing between H2 and D2, while the dominated route under pH 2 shown in Figure 1(c) is PCET due to KIE of kH2O/kD2O when changing between H2O and D2O. The mentioned results reveal that both CH and PCET pathways participate in phenol hydrogenation, with the dominated route transforming from CH to PCET as pH decreasing. It should be noted that all the reactions were carried out in absence of over potential, that only OCP drove the PCET pathway.In conclusion, hydrogenation rate of phenol is greatly enhanced by decreasing pH in water. Two reaction routes, CH and PCET, are turned out to be involved in the reaction. As pH decreasing from 5.3 to 2, the main reaction pathway shifts from CH to PCET. The reaction rate of PCET is promoted by decrease of pH because of the largely increased hydronium ion concentration. These results demonstrate that an electrocatalytic hydrogenation reaction can still occur under OCP. Figure 1. TOFs of phenol hydrogenation at 313-333 K with 10 bar H2 on 1 wt% Pt/CNT (5 mg) in 100 mL H2O (0.106 M phenol) plotted as a function of pH (a). Comparison of TOF of phenol hydrogenation in H2O-H2, H2O-D2, D2O-H2, D2O-D2 on 1 wt% Pt/CNT (50 mg) under pH 5.3 (no HClO4) (b) and 1 wt% Pt/CNT (10 mg) under pH 2 (HClO4) (c),10 bar H2 or D2, 30 mL H2O or D2O, 313 K, 0.106 M phenol. Figure 1

Ribonucleotide reductases (RNRs) catalyze the conversion of all four ribonucleotides to deoxyribonucleotides and are essential for DNA synthesis in all organisms. The active form of E. coli Ia RNR is composed of two homodimers that form the active α2β2 complex. Catalysis is initiated by long-range radical translocation over a ∼32 Å proton-coupled electron transfer (PCET) pathway involving Y356β and Y731α at the interface. Resolving the PCET pathway at the α/β interface has been a long-standing challenge due to the lack of structural data. Herein, molecular dynamics simulations based on a recently solved cryogenic-electron microscopy structure of an active α2β2 complex are performed to examine the structure and fluctuations of interfacial water, as well as the hydrogen-bonding interactions and conformational motions of interfacial residues along the PCET pathway. Our free energy simulations reveal that Y731 is able to sample both a flipped-out conformation, where it points toward the interface to facilitate interfacial PCET with Y356, and a stacked conformation with Y730 to enable collinear PCET with this residue. Y356 and Y731 exhibit hydrogen-bonding interactions with interfacial water molecules and, in some conformations, share a bridging water molecule, suggesting that the primary proton acceptor for PCET from Y356 and from Y731 is interfacial water. The conformational flexibility of Y731 and the hydrogen-bonding interactions of both Y731 and Y356 with interfacial water and hydrogen-bonded water chains appear critical for effective radical translocation along the PCET pathway. These simulations are consistent with biochemical and spectroscopic data and provide previously unattainable atomic-level insights into the fundamental mechanism of RNR.

The intrinsic catalytic property of a Fe-S complex toward H2 evolution was investigated in a wide range of acids. The title complex exhibited catalytic events at -1.16 and -1.57 V (vs Fc+/Fc) in the presence of trifluoromethanesulfonic acid (HOTf) and trifluoroacetic acid (TFA), respectively. The processes corresponded to the single reduction of the Fe-hydride-S-proton and Fe-hydride species, respectively. When anilinium acid was used, the catalysis occurred at -1.16 V, identical with the working potential of the HOTf catalysis, although the employment of anilinium acid was only capable of achieving the Fe-hydride state on the basis of the spectral and calculated results. The thermodynamics and kinetics of individual steps of the catalysis were analyzed by density functional theory (DFT) calculations and electroanalytical simulations. The stepwise CCE or CE (C, chemical; E, electrochemical) mechanism was operative from the HOTf or TFA source, respectively. In contrast, the involvement of anilinium acid most likely initiated a proton-coupled electron transfer (PCET) pathway that avoided the disfavored intermediate after the initial protonation. Via the PCET pathway, the heterogeneous electron transfer rate was increased and the overpotential was decreased by 0.4 V in comparison with the stepwise pathways. The results showed that the PCET-involved catalysis exhibited substantial kinetic and thermodynamic advantages in comparison to the stepwise pathway; thus, an efficient catalytic system for proton reduction was established.

Oxidative C-H bond cleavage of toluene derivatives and sulfoxidation of thioanisole derivatives by a nonheme iron(IV)-oxo complex, [(N4Py)Fe(IV)(O)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), were remarkably enhanced by the presence of triflic acid (HOTf) and Sc(OTf)3 in acetonitrile at 298 K. All the logarithms of the observed second-order rate constants of both the oxidative C-H bond cleavage and sulfoxidation reactions exhibit remarkably unified correlations with the driving forces of proton-coupled electron transfer (PCET) and metal ion-coupled electron transfer (MCET) in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes between PCET and MCET were taken into account, respectively. Thus, the mechanisms of both the oxidative C-H bond cleavage of toluene derivatives and sulfoxidation of thioanisole derivatives by [(N4Py)Fe(IV)(O)](2+) in the presence of HOTf and Sc(OTf)3 have been unified as the rate-determining electron transfer, which is coupled with binding of [(N4Py)Fe(IV)(O)](2+) by proton (PCET) and Sc(OTf)3 (MCET). There was no deuterium kinetic isotope effect (KIE) on the oxidative C-H bond cleavage of toluene via the PCET pathway, whereas a large KIE value was observed with Sc(OTf)3, which exhibited no acceleration of the oxidative C-H bond cleavage of toluene. When HOTf was replaced by DOTf, an inverse KIE (0.4) was observed for PCET from both toluene and [Ru(II)(bpy)3](2+) (bpy =2,2'-bipyridine) to [(N4Py)Fe(IV)(O)](2+). The PCET and MCET reactivities of [(N4Py)Fe(IV)(O)](2+) with Brønsted acids and various metal triflates have also been unified as a single correlation with a quantitative measure of the Lewis acidity.

Primary and secondary radiation-induced damage to DNA, and chemical repair of the lesions on the nucleobases in solution involve a cascade of proton transfer (PT), electron transfer (ET), and proton-coupled electron transfer (PT-ET) reactions. The rate constants of these reactions depend on the standard Gibbs energy changes that can be derived from experiment. We here apply a first principles approach to calculate standard Gibbs energy changes of proton, electron, and proton-coupled electron transfer reactions in solution, wherein electrons and protons participate as independent ions; data that is fully compatible with that experimentally derived. Hence, the thermodynamic feasibility of ET and PT-ET pathways for these reactions depending on the effective concentration of hydrogen ions can be directly rationalized from first principles. The focus of this study is the primary and secondary ionization events in nucleobases in the presence of hydrogen atoms, solvated electrons and protons in aqueous solution, leading to the formation of nucleobase radical anions B˙−, radical cations B˙+ and their major deprotonated radical forms B(–H)˙. We also examine the chemical repair reaction by thiols, B(–H)˙(aq) + RSH(aq) = B(aq) + RS˙(aq), where B = A,G,C,T. Our results for the chemical repair of B(–H)˙ suggest that a PT-ET pathway should be favored for A and C at any pH, whereas for G and T, a PT-ET pathway is preferred at acidic and near neutral pH, but in the pH range 9-11, the ET pathway would dominate.

The electrochemical proton reactivity of transition metal complexes receives significant attentions. A thorough understanding of proton-coupled electron transfer (PCET) pathways is essential for elucidating the mechanism behind a proton reduction reaction, and controlling the pathway is a key focus in the field of the catalyst development. Spin interactions within complexes, which arise during electron transfer, can affect significantly the PCET pathway. Herein, we explore the phenomenon of spin rearrangement during the electrochemical reorganization of high-spin cobalt complexes. Our findings reveal that opposing spin interactions, induced by different coordination environments, can alter the PCET pathway. Finally, detailed analysis of the PCET pathway allows us to propose mechanisms for proton reduction in high-spin cobalt complexes.

The E. coli class Ia ribonucleotide reductase (RNR) catalyzes the de novo synthesis of deoxynucleoside 5ʹ‐diphosphates (dNDPs) using nucleoside 5ʹ‐diphosphates (NDPs) as substrates. The catalytic mechanism of RNR requires an active site thiyl radical, which is generated via reversible oxidation by a tyrosyl radical cofactor located over 35 Å away. The model for the long range oxidation involves multiple proton‐coupled electron transfer (PCET) steps through aromatic amino acid residues. Efforts to study the radical hopping mechanism have been challenging due to kinetic masking of the chemistry by a conformational gate. Lowering the midpoint potentials of pathway residues by using the amber stop codon suppression method to site‐specifically incorporate the unnatural amino acid (UAA) 3‐aminotyrosine (NH2Y) acts as a radical “sink” on pathway and changes the rate limiting step of the radical propagation mechanism from a conformational change to a PCET step. NH2Y‐incorporated RNR (NH2Y‐RNRs) have allowed the detection of radical intermediates on this PCET pathway and have established the role of three tyrosines, Y356, Y730 and Y731, in the PCET pathway. Despite the large perturbation in midpoint potential, NH2Y‐RNRs can also do multiple turnovers and produce dNDPs. The study of the rapid kinetics of NH2Y● and dNDP formation demonstrates the kinetic and chemical competence of the NH2Y● intermediates. Additionally, insight into the kinetic model of the NH2Y‐RNRs give rise to further details of multiple, more subtle conformational changes within the PCET pathway mechanism. Specifically, the NH2Y● populations exist in two distinct conformations, with one less competent in nucleotide reduction.

Molecular oxygen (O2), the greenest oxidant, is kinetically stable in the oxidation of organic substrates due to its triplet ground state. In nature, O2 is reduced by two electrons with two protons to produce hydrogen peroxide (H2O2) and by four electrons with four protons to produce water (H2O) by oxidase and oxygenase metalloenzymes. In the process of the two-electron/two-proton and four-electron/four-proton reduction of O2 by metalloenzymes and their model compounds, metal-oxygen intermediates, such as metal-superoxido, -peroxido, -hydroperoxido, and -oxido species, are generated depending on the numbers of electrons and protons involved in the O2 activation reactions. The one-electron reduction of metal-oxygen intermediates is coupled with the binding of one proton. Such a hydrogen atom transfer (HAT) is defined as proton-coupled electron transfer (PCET), and there is a mechanistic dichotomy whether HAT occurs via a concerted PCET pathway or stepwise pathways [i.e., electron transfer followed by proton transfer (ET/PT) or proton transfer followed by electron transfer (PT/ET)]. The metal-oxygen intermediates formed are oxidants that can abstract a hydrogen atom (H-atom) from substrate C-H bonds. The H-atom abstraction from substrate C-H bonds by the metal-oxygen intermediates can also occur via a concerted PCET or stepwise PCET pathways. In the PCET reactions, a proton can be provided not only by the substrate itself but also by an acid that is added to a reaction solution. This Account describes the reactivities of metal-oxygen intermediates, such as metal-superoxido, -peroxido, -hydroperoxido, and -oxido complexes, in HAT reactions, focusing on the mechanisms of PCET reactions of metal-oxygen intermediates and on the mechanistic dichotomy of concerted versus stepwise pathways. Recent developments in the reactivity studies of Cr-, Fe-, and Cu-superoxido complexes in H-atom and hydride transfer reactions are discussed. Reactivities of an iron(III)-hydroperoxido complex and an iron(III)-peroxido complex binding redox-inactive metal ions are also summarized briefly. Mononuclear nonheme iron(IV)- and manganese(IV)-oxido complexes have shown high reactivities in HAT reactions, and their chemistry in PCET reactions is discussed intensively. Acid-catalyzed HAT reactions of metal-oxygen intermediates are also discussed to demonstrate a unified driving force dependence of logarithm of the rate constants of acid-catalyzed oxidation of various substrates by an iron(IV)-oxido complex and that of PCET from one-electron donors to the iron(IV)-oxido complex. PCET reactions of metal-oxygen intermediates are shown to proceed via a concerted pathway (one-step HAT) or a stepwise ET/PT pathway depending on the ET and PCET driving forces (-Δ G). The boundary conditions between concerted versus stepwise PCET pathways are clarified to demonstrate a switchover of the mechanisms only by changing the reaction temperature in the boundary conditions. This Account summarizes recent developments in the HAT reactions by synthetic mononuclear nonheme metal-oxygen intermediates over the past 10 years.

The work describes the use of cyclic voltammetry to investigate the proton coupled electron transfer (PCET) pathways for oxidation of thymine in water. Biologically important compounds like DNA bases, and amino acids showed irreversible behavior when oxidized electrochemically. The reversible redox potentials for such systems (say, irreversible electrochcemical systems) were estimated by analyzing the cyclic voltammograms recorded at different scan rates. The approach presents here is relatively simple and easy for obtaining reversible redox potential of electrochemically irreversible systems. Moreover, for the case of thymine, the obtained reversible oxidation potential also used to establish the PCET-pathways for oxidation of thymine in water.

Biomass represents an untapped resource for achieving a circular carbon economy. Current processes for converting biomass are limited to centralized processing facilities. Electrochemical approaches are inherently modular and enable decentralized upgrading of pyrolysis oil to value-added chemicals and fuels (products). A primary component of the aqueous phase of such feedstocks are carbonyl functionalities, where benzaldehyde (BZ) represents a model compound. Selective reduction of BZ to benzyl alcohol (BnOH) can be used to understand the catalytic upgrading of pyrolysis oil to value-added products. Electrocatalytic hydrogenation (ECH) of BZ can be carried out at mild temperatures and pressure with the hydrogen required for BZ reduction coming from the protons in the solution. However, these protons can also combine in the hydrogen evolution reaction (HER) in a competitive process. Among the noble and base metals studied in the literature, palladium (Pd) is very active and has demonstrated a high faradaic efficiency (FE) for ECH of BZ relative to HER. Palladium also forms a thermodynamically stable hydride under these conditions. However, prior work has shown that BZ inhibits the formation of palladium hydride under reaction conditions, whereas the presence of weaker adsorbing organics does not. Together, these observations raise interesting questions whether ECH occurs via a proton coupled electron transfer (PCET) pathway or via addition of H from the catalyst surface directly (Langmuir-Hinshelwood, LH pathway). The specific aims of this work investigate the effect of catalyst morphology on the surface coverage of intermediates like H and adsorbed BZ, the relative rates of PCET and LH pathways, and their competition with HER and hydride formation. For this, we investigate high surface area Pd nanoparticle electrocatalysts and a gel network (Pd Gel) of them rich with grain boundaries and strain via cyclic voltammetry. Adsorption isotherms on these Pd catalysts show changes in hydride formation and the relative affinity of H and BZ on the Pd surface. We further study the reaction rate orders and FE as a function of applied potential to better understand the contributions of PCET and LH pathways. These findings point to catalyst morphology as a further handle to manipulate activity and FE for ECH of BZ. Furthermore, these findings will enhance the mechanistic understanding of BZ ECH kinetics and will contribute to improving the overall efficacy of biomass conversion to biofuels.

The mechanism of electrochemical reduction of acetophenone in 1-butyl-3-methylimidazolium tetrafluroborate ([BMIM][BF4]) under nitrogen (N2) and carbon dioxide (CO2) atmospheres have been investigated using transient voltammetry, steady-state voltammetry, bulk electrolysis and numerical simulation. Under a N2 atmosphere, acetophenone undergoes a one-electron reduction to the radical anion followed by rapid dimerization reactions with an apparent rate constant of 1.0 × 106 M−1s−1. In contrast, under a CO2 atmosphere, the electrochemical reduction of acetophenone is an overall two-electron transfer chemically irreversible process with the final electrolysis product being 1-phenylethanol, instead of the anticipated 2-hydroxy-2-phenylpropionic acid resulting from an electrocarboxylation reaction. A proton coupled electron transfer pathway leading to the formation of 1-phenylethanol requires the presence of a sufficiently strong proton donor which is not available in neat [BMIM][BF4]. However, the presence of CO2 enhances the C-2 hydrogen donating ability of [BMIM]+ due to strong complex formation between the deprotonated form of [BMIM]+, N-heterocyclic carbene, and CO2, resulting in a thermodynamically favorable proton coupled electron transfer pathway.

There is a growing appreciation of the important role that H-bond complexes can play in the mechanism of proton-coupled electron transfer (PCET) reactions. Until relatively recently it had been thought that PCET reactions always proceeded step-wise with sequential electron and proton transfer. Much of the recent fundamental interest in PCET stems from the realization that a third option is available, concerted electron and proton transfer or CPET in which the electron and proton both move in a single kinetic step. This interest in the concerted process has increased awareness of H-bonding states in PCET, since the concerted reaction occurs within a H-bonded intermediate. However, even if the proton and electron transfer is not concerted, the H-bonded complex formed in the process of proton transfer can play an important role in the PCET mechanism if it is sufficiently long-lived.Recently we have introduced a generally useful mechanistic framework with which to include H-bonding steps within an overall PCET pathway. This scheme, which for obvious reasons we call a “wedge”, is shown in Scheme 1 for the generic 1e−, 1H+ oxidation, AH + B = A + HB+ + e−. The front face in the wedge (in bold) is the standard electron transfer/proton transfer square scheme, with the two possible electron transfer reactions on the top and bottom edges, and the two possible proton transfers on the left and right edges. However, proton transfer reactions actually go through a H-bond intermediate, so a more accurate description of the proton transfer follows the dashed lines on the triangular sides of the wedge to and from the H-bond intermediates, A-H-B or A-H-B+, which are meant to represent the thermodynamically most stable H-bond complex in each oxidation state. If the H-bonded intermediate has sufficient lifetime, then electron transfer to/from the H-bond complex is also possible, represented by the rear edge of the wedge (thin solid line). If the proton moves from being more attached to A in A-H-B to being more attached to B in A-H-B+, then E° of this reaction is that of the CPET step, if the proton doesn’t move then the E° is simply that of oxidation of the H-bond complex. Either way, it is straightforward to show that E°(A-H-B0/+) has to have a value in between E°(AH0/+) and E°(A−/0). Thus the possibility of electron transfer through the H-bond complex opens up a pathway of intermediate potential for the overall reaction AH + B = A + HB+ + e−.The usefulness of the wedge scheme is demonstrated by its ability to explain the unusual electrochemistry of the phenylenediamine-based urea, U(H)H, which we have shown undergoes a self proton transfer upon oxidation to give half equivalent of the doubly oxidized quinoidal cation and half-equivalent of the electroinactive, protonated reduced urea, Scheme 2. The reaction gives chemically irreversible voltammetry in acetonitrile as would be expected given that the quinoidal cation is harder to reduce than the initially formed radical cation. However, it gives reversible voltammetry in methylene chloride, which can be explained by the greater stability of the H-bonded intermediate in this solvent. In addition, in methylene chloride, we are able to clearly observe a concentration and scan rate dependent conversion between two different reduction pathways on the return scan. This behavior cannot be explained by a simple square scheme, but is readily explained by the wedge scheme.In this presentation, we will report recent results on the voltammetry of U(H)H in the presence of guest molecules that H-bond to the starting, reduced state. We will show that their effect on the voltammetry can be explained in terms of two interlinked wedge schemes, one representing the electron transfer / H-bonding / proton transfer reactions of U(H)H with itself and the other representing the reactions with the added guest.

Discerning hydrogen atom transfer (HAT) from proton-coupled electron transfer (PCET) mechanisms in C-H oxidation by biomimetic Fe-oxo complexes remains a longstanding challenge, with a central question being how the electronic structure and spin state of the iron-oxo core dictate this mechanistic preference. To address this, we conducted detailed electronic structure analyses of the hydrogen atom abstraction (HAA) reaction of 1,4-cyclohexadiene by the tetragonal iron(IV)-oxo complex, [FeIVN4Py(O)]2+ (1) and its one-electron-reduced congener, [FeIIIN4Py(O)]+ (2) using DFT, CASSCF, NEVPT2, and IBO methods. Electronic structure analyses at varying Fe-O distances reveal that the emergence of "oxyl" character promotes HAT, while its absence favors PCET. Notably, the PCET pathway in the high-spin (S = 5/2) Fe(III)-oxo complex shifts to HAT in the intermediate-spin (S = 3/2) state, underscoring the pivotal role of electronic configuration and spin state. The electronic nature of the HAT or PCET transition states for complexes 1 (5TS1σ) and 2 (6TS2π and 4TS2σ) was verified using multiconfigurational CASSCF calculations, complemented by IBO analysis along the intrinsic reaction coordinate. Overall, this study clarifies the distinction between HAT and PCET mechanisms and highlights the iron-oxo core's decisive role in shaping the reactivity of synthetic Fe-oxo species.

Covalent organic frameworks (COFs) have emerged as promising photocatalysts for the artificial photosynthesis of hydrogen peroxide (H2O2), yet their practical application is hindered by limited photocatalytic efficiency. Herein, a bioinspired strategy is reported to enhance the photocatalytic performance of COFs by introducing an ortho-positioned arrangement of enol (OH) and imine (N) groups. Through rational molecular engineering, a concerted proton-coupled electron transfer (PCET) pathway is established, which facilitates the efficient separation and transfer of photoexcited charge carriers, thereby dramatically enhancing H2O2 production from water and oxygen. The TbDO COF, featuring the PCET pathway, demonstrates superior performance, achieving a remarkable H2O2 production rate of 7265 μmol g-1 h-1 in a nonsacrificial system, surpassing COFs with similar structure but lacking this pathway. This study advances the design of COFs and opens new avenues for sustainable solar-to-chemical energy conversion.

Producing high-purity oxygen (O2) has a wide range of applications across diverse sectors, such as medicine, tunnel construction, the chemical industry, and fermentation. However, current O2 production methods are burdened by complexity, heavy equipment, high energy consumption, and limited adaptability to harsh environments. Here, to address this grand challenge, the de novo design of Ru-doped metal hydroxide is proposed to serve as bioinspired O2-evolution catalysts with proton-coupled electron transfer (PCET) pathway for low-energy, environmentally friendly, cost-effective, and portable O2 generation. The comprehensive studies confirm that the lattice H species in Ru-Co(OH)x-based O2-evolution catalyst can trigger a PCET pathway to optimize Ru-oxygen intermediates interactions, thus ultimately reducing reaction energy barriers and improving the activities and durabilities. Consequently, the prepared Ru-Co(OH)x-loaded membrane catalysts exhibit rapid and long-term stable O2 production capabilities. Furthermore, the proposed material design strategy of lattice H-species shows remarkable universality and adaptability to broad Ru-doped metal hydroxides. This efficient, portable, and cost-effective O2 generation technique is suggested to ensure an uninterrupted O2 supply during emergencies and in regions with limited O2 availability or air pollution, thus offering significant societal benefits in broad applications.