Interfacial Proton-coupled Electron Transfer Research Articles

Modeling Interfacial Proton-Coupled Electron Transfer Reactions Involving Imidazolium Proton Donors

Proton-coupled electron transfer (PCET) is an elementary reaction that plays a pivotal role in various electrochemical energy conversion and storage processes. PCET reactions involving nonaqueous proton donors are of interest for applications in energy storage such as redox flow batteries, as well as in CO2 and O2 reduction catalysis. Yet, there remains much to be understood about the molecular reactivity of these systems. In this talk, I will describe periodic density functional theory (DFT) studies of interfacial PCET reactions involving different imidazolium proton donors on Pt(111) electrode surfaces.The imidazole conjugate bases adsorb strongly to the electrode surface; however, this adsorption behavior varies with isomers that feature distinct proton positions and with different functional substituent groups. Because the adsorption of imidazole conjugate bases could affect reactivity, these models are used as a foundation to understand the intrinsic PCET kinetics of these systems. The Volmer and Heyrovsky steps of hydrogen evolution reaction are two of the most basic heterogeneous electrochemical PCET reactions and are therefore chosen as model interfacial PCET reactions in this study. Potential-dependent reaction energies and activation barriers are calculated using the charge extrapolation method, where the electrode potential is tuned by modifying proton coverage for different-sized unit cells. This approach is used to develop relationships between PCET reaction thermodynamics and kinetics through Brønsted–Evans–Polanyi (BEP) relationships for different imidazoles, where BEP slopes are analogs to charge transfer coefficients in the Butler–Volmer equation. Investigating reaction parameters influencing charge transfer kinetics at the electrode-electrolyte interface is crucial for designing efficient energy storage systems, impacting overall device performance. These studies show trends in the kinetic behavior of different functionalized imidazoles, which can aid in the design of catalytic and energy storage systems.

Magnetic porous microspheres altering interfacial thermodynamics of sewage sludge to drive metabolic cooperation for efficient methanogenesis

Controllable and recyclable magnetic porous microspheres (MPMs) have been proposed as a means for enhancing the anaerobic digestion (AD) of sludge, as they do not require continuous replenishment and can serve as carriers for anaerobes. However, the effects of MPMs on the interfacial thermodynamics of sludge and the biological responses triggered by abiotic effects in AD systems remain to be clarified. Herein, the underlying mechanisms by which MPMs alter the solid–liquid interface of sludge to drive methanogenesis were investigated. A significant increase in the contents of 13C and 2H (D) in methane molecules was observed in the presence of MPMs, suggesting that MPMs might enhance the CO2-reduction methanogenesis and participation of water in methane generation. Experimental results demonstrated that the addition of MPMs did not promote the anaerobic bioconversion of soluble organics for methanogenesis, suggesting that the enhanced methanogenesis and water participation were not achieved through promotion of the bioconversion of original liquid-state organics in sludge. Analyses of the capillary force, surface adhesion force, and interfacial proton-coupled electron transfer (PCET) of MPMs revealed that MPMs can enhance mass transfer, effective contact, and electron–proton transfer with sludge. These outcomes were confirmed by the statistical analyses of variations in the interfacial thermodynamics and PCET of sludge with and without MPMs during AD. It was thus proposed that the MPMs enhanced the PCET of sludge and PCET-driven release of protons from water by promoting the interfacial Lewis acid–base interactions of sludge, thereby resulting in the enrichment of free and attached methanogenic consortia and the high energy-conserving metabolic cooperation. This proposition was further confirmed by identifying the predominant syntrophic partners, suggesting that PCET-based efficient methanogenesis was attributable to the enrichment of genomes harbouring CO2-reducing pathway and genes encoding water-mediated proton transfer. These findings offer new insights into how substrate properties can be altered by exogenous materials to enable highly efficient methanogenesis.

Interfacial Proton-Coupled Electron Transfer via Localized Trap States on Metal Oxide Surfaces

Interfacial Proton-Coupled Electron Transfer via Localized Trap States on Metal Oxide Surfaces

Electrokinetic Analyses Uncover the Rate‐Determining Step of Biomass‐Derived Monosaccharide Electroreduction on Copper

AbstractElectrochemical biomass conversion holds promise to upcycle carbon sources and produce valuable products while reducing greenhouse gas emissions. To this end, deep insight into the interfacial mechanism is essential for the rational design of an efficient electrocatalytic route, which is still an area of active research and development. Herein, we report the reduction of dihydroxyacetone (DHA)—the simplest monosaccharide derived from glycerol feedstock—to acetol, the vital chemical intermediate in industries, with faradaic efficiency of 85±5 % on a polycrystalline Cu electrode. DHA reduction follows preceding dehydration by coordination with the carbonyl and hydroxyl groups and the subsequent hydrogenation. The electrokinetic profile indicates that the rate‐determining step (RDS) includes a proton‐coupled electron transfer (PCET) to the dehydrated intermediate, revealed by coverage‐dependent Tafel slope and isotopic labeling experiments. An approximate zero‐order dependence of H+ suggests that water acts as the proton donor for the interfacial PCET process. Leveraging these insights, we formulate microkinetic models to illustrate its origin that Eley–Rideal (E−R) dominates over Langmuir–Hinshelwood (L−H) in governing Cu‐mediated DHA reduction, offering rational guidance that increasing the concentration of the adsorbed reactant alone would be sufficient to promote the activity in designing practical catalysts.

Electrokinetic Analyses Uncover the Rate-Determining Step of Biomass-Derived Monosaccharide Electroreduction on Copper.

Electrochemical biomass conversion holds promise to upcycle carbon sources and produce valuable products while reducing greenhouse gas emissions. To this end, deep insight into the interfacial mechanism is essential for the rational design of an efficient electrocatalytic route, which is still an area of active research and development. Herein, we report the reduction of dihydroxyacetone (DHA)-the simplest monosaccharide derived from glycerol feedstock-to acetol, the vital chemical intermediate in industries, with faradaic efficiency of 85±5 % on a polycrystalline Cu electrode. DHA reduction follows preceding dehydration by coordination with the carbonyl and hydroxyl groups and the subsequent hydrogenation. The electrokinetic profile indicates that the rate-determining step (RDS) includes a proton-coupled electron transfer (PCET) to the dehydrated intermediate, revealed by coverage-dependent Tafel slope and isotopic labeling experiments. An approximate zero-order dependence of H+ suggests that water acts as the proton donor for the interfacial PCET process. Leveraging these insights, we formulate microkinetic models to illustrate its origin that Eley-Rideal (E-R) dominates over Langmuir-Hinshelwood (L-H) in governing Cu-mediated DHA reduction, offering rational guidance that increasing the concentration of the adsorbed reactant alone would be sufficient to promote the activity in designing practical catalysts.

Probing Nonadiabaticity of Proton-Coupled Electron Transfer in Ribonucleotide Reductase.

The enzyme ribonucleotide reductase, which is essential for DNA synthesis, initiates the conversion of ribonucleotides to deoxyribonucleotides via radical transfer over a 32 Å pathway composed of proton-coupled electron transfer (PCET) reactions. Previously, the first three PCET reactions in the α subunit were investigated with hybrid quantum mechanical/molecular mechanical (QM/MM) free energy simulations. Herein, the fourth PCET reaction in this subunit between C439 and guanosine diphosphate (GDP) is simulated and found to be slightly exoergic with a relatively high free energy barrier. To further elucidate the mechanisms of all four PCET reactions, we analyzed the vibronic and electron-proton nonadiabaticities. This analysis suggests that interfacial PCET between Y356 and Y731 is vibronically and electronically nonadiabatic, whereas PCET between Y731 and Y730 and between C439 and GDP is fully adiabatic and PCET between Y730 and C439 is in the intermediate regime. These insights provide guidance for selecting suitable rate constant expressions for these PCET reactions.

A molecular-level mechanistic framework for interfacial proton-coupled electron transfer kinetics.

Electrochemical proton-coupled electron transfer (PCET) reactions can proceed via an outer-sphere electron transfer to solution (OS-PCET) or through an inner-sphere mechanism by interfacial polarization of surface-bound active sites (I-PCET). Although OS-PCET has been extensively studied with molecular insight, the inherent heterogeneity of surfaces impedes molecular-level understanding of I-PCET. Herein we employ graphite-conjugated carboxylic acids (GC-COOH) as molecularly well-defined hosts of I-PCET to isolate the intrinsic kinetics of I-PCET. We measure I-PCET rates across the entire pH range, uncovering a V-shaped pH-dependence that lacks the pH-independent regions characteristic of OS-PCET. Accordingly, we develop a mechanistic model for I-PCET that invokes concerted PCET involving hydronium/water or water/hydroxide donor/acceptor pairs, capturing the entire dataset with only four adjustable parameters. We find that I-PCET is fourfold faster with hydronium/water than water/hydroxide, while both reactions display similarly high charge transfer coefficients, indicating late proton transfer transition states. These studies highlight the key mechanistic distinctions between I-PCET and OS-PCET, providing a framework for understanding and modelling more complex multistep I-PCET reactions critical to energy conversion and catalysis.

Proton-Coupled Electron Transfer Involving Localized Electronic Defects on Solid Surfaces

Proton-coupled electron transfer (PCET) is a critical elementary step in many (photo)electrocatalytic transformations. PCET reactions on semiconducting metal oxide surfaces involve proton transfer that is charge compensated by electronic defects, such as electrons or holes. When highly localized as small polarons, these electronic defects are coupled with local bond distortions that accompany the excess or depleted charge. Using anatase TiO2 as a model system, I will describe recent first-principles modeling studies of interfacial PCET on metal oxide surfaces. The PCET reaction free energies involving the cleavage of O–H bonds on TiO2 surfaces varies widely depending on whether the charge-compensating electronic defects are conduction d-band electrons or valence p-band holes. Differences in PCET thermochemistry are accounted for using a Marcus theory framework based on defect energy levels and inner-sphere reorganization energies. The PCET rate constants associated with the cleavage of O–H bonds in the presence of a nitroxyl radical oxidant are calculated using vibronically nonadiabatic PCET theory, where both the transferring electron and proton are treated quantum mechanically. Input parameters to the rate constant expressions are calculated using hybrid periodic density functional theory calculations. These studies highlight the effects of local electronic defects on the PCET reactivity of semiconductor surfaces.

Supercapacitor based on the redox of quinone-carbon negative electrode with high cycle performance

A high specific surface area sucrose-derived porous carbon (SPC) using ZnCl2 as a salt template was successfully synthesized. Anthraquinone (AQ) molecules were then immobilized onto the SPC surface via the non-covalent functionalization approach to prepare the quinone-carbon (AQ/SPC) composites. The synthesized composites exhibited excellent self-synergy properties because of SPC's effective three-dimensional (3D) electron transmission network and the rapid-reversible interfacial proton-coupled electron transfer reaction of AQ molecules. Our experimental results indicated that the AQ/SPC-3 electrode in the three-electrode system with a 1 M H2SO4 electrolyte gave the highest specific capacitance (400.7 F g−1), more than twice that of SPC (155.1 F g−1) at a 1 A g−1 current density, exhibiting excellent rate capability and electrochemical reversibility. The cyclic voltammetry of AQ/SPC-3 also exhibited a symmetric and reversible redox peak at negative potential. Hence, we assembled an asymmetric supercapacitor (SPC@AQ/SPC-3) with a two-electrode system and tested it under the same conditions as the three-electrode system. The SPC@AQ/SPC system yielded 71.3 F g−1 and 11.8 Wh kg−1 at 50 W kg−1. At a 5 A g−1 current density, all the SPC@AQ/SPC-3, SPC@DCAQ/SPC-1, and SPC@DMAQ/SPC-1 systems could establish at least 96.94 % of high cycle stability after 10,000 cycles. Notably, by comparing the electron-withdrawing group (Cl) with the electron-donating group (NH2) at the same substitution position of AQ, we found that the electron-withdrawing group has a noticeable promotion effect on redox reversibility and long-term cycle stability.

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.

Reorganization Energies for Interfacial Proton-Coupled Electron Transfer to a Water Oxidation Catalyst.

The reorganization energy (λ) for interfacial electron transfer (ET) and proton-coupled ET (PCET) from a conductive metal oxide (In2O3:Sn, ITO) to a surface-bound water oxidation catalyst was extracted from kinetic data measured as a function of the thermodynamic driving force. Visible light excitation resulted in rapid excited-state injection (kinj > 108 s-1) to the ITO, which photo-initiated the two interfacial reactions of interest. The rate constants for both reactions increased with the driving force, -ΔG°, to a saturating limit, kmax, with rate constants consistently larger for ET than for PCET. Marcus-Gerischer analysis of the kinetic data provided the reorganization energy for interfacial PCET (0.90 ± 0.02 eV) and ET (0.40 ± 0.02 eV), respectively. The magnitude of kmax for PCET was found to decrease with pH, behavior that was absent for ET. Both the decrease in kmax and the larger reorganization energy for an unwanted competing PCET reaction from the ITO to the oxidized catalyst showcases a significant kinetic advantage for driving solar water oxidation at high pH. Computational analysis revealed a larger inner-sphere reorganization energy contribution for PCET than for ET arising from a more significant change in the Ru-O bond length for the PCET reaction. Extending the Marcus-Gerischer theory to PCET by including the excited electron-proton vibronic states and the proton donor-acceptor motion provided an apparent reorganization energy of 1.01 eV. This study demonstrates that the Marcus-Gerischer theory initially developed for ET can be reliably extended to PCET for quantifying and interpreting reorganization energies observed experimentally.

Rate and Mechanism of Electrochemical Formation of Surface-Bound Hydrogen on Pt(111) Single Crystals.

The formation of surface-bound hydrogen from one proton and one electron plays an enabling role in renewable hydrogen production. Quantifying the surface-bound hydrogen formation, however, requires decoupling the delicate interplay of numerous processes. We study cyclic voltammetry (CV) at fast scan rates to characterize the rate constant for the surface-bound hydrogen formation (also known as underpotential deposition hydrogen, UPD Had). We find that the formation of Had on Pt(111) single crystals is ∼100× faster in acid than in base. Reaction-order analysis indicates that the formation of Had occurs as a standard proton-coupled electron transfer (PCET) reaction in acid, whereas in base, it displays a pH-independent rate constant, indicating the presence of a chemical step such as the reorganization of interfacial water. Our results provide a methodology for quantifying the interfacial PCET kinetics and reveal the mechanistic nature of the UPD Had formation as the reason the hydrogen evolution electrocatalysis on Pt is faster in acid than in base.

Proton-Coupled Defects Impact O-H Bond Dissociation Free Energies on Metal Oxide Surfaces.

Proton-coupled electron transfer (PCET) reactions on metal oxides require coupling between proton transfer at the solid-liquid interface and electron transfer involving defects at or near the band edge. Herein, hybrid functional periodic density functional theory is used to elucidate the impact of proton-coupled defects on the bond dissociation free energies (BDFEs) of O-H bonds on anatase TiO2 surfaces. These O-H BDFEs are directly related to interfacial PCET thermochemistry. Comparison between geometrically similar O-H bonds associated with different defect types, namely conduction d-band electrons or valence p-band holes, reveals that the BDFEs differ by ∼81 kcal/mol (3.50 eV), comparable to the wide TiO2 band gap. These differences are shown to be determined primarily by differences in electron transfer driving forces, which are analyzed by using band energies and inner-sphere reorganization energies within a Marcus theory framework. These fundamental insights about the impact of proton-coupled defects on PCET thermochemistry at semiconductor surfaces have broad implications for electrocatalysis.

Interfacial Field-Driven Proton-Coupled Electron Transfer at Graphite-Conjugated Organic Acids.

Interfacial proton-coupled electron transfer (PCET) reactions are central to the operation of a wide array of energy conversion technologies, but molecular-level insights into interfacial PCET are limited. At carbon surfaces, designer sites for interfacial PCET can be incorporated by conjugating organic acid functional groups to graphite edges though aromatic phenazine linkages. At these graphite-conjugated catalysts (GCCs) bearing organic acid moieties, PCET is driven by complex interfacial electrostatic and field gradients that are difficult to probe experimentally. Herein, the spatially inhomogeneous interfacial electrostatic potentials and electric fields of GCC organic acids are computed as functions of applied potential. The calculated proton-coupled redox potentials for the PCET reactions at the GCC phenazine bridges and organic acid sites are in agreement with cyclic voltammetry measurements for a series of GCC acids. The trends in these redox potentials are explained in terms of the acidity of the molecular analogues and continuous conjugation between the acid and the graphite surface. The calculations illustrate that this conjugation is interrupted in a GCC acetic acid system, providing an explanation for the absence of a cyclic voltammetry peak corresponding to PCET at this acid site. This combined theoretical and experimental study demonstrates the critical role of continuous conjugation and strong electronic coupling between the GCC acid site and the graphite to enable interfacial field-driven PCET at the acid site. Understanding the connection between the atomic structure of the surface and the interfacial electrostatic potentials and fields that govern PCET thermochemistry may guide heterogeneous catalyst design.

Superior surface electron energy level endows WP2 nanowire arrays with N2 fixation functions

Nitrogen reduction reaction (NRR) under ambient conditions is always a long-standing challenge in science, due to the extreme difficulty in breaking the strong NN triple bond. The key to resolving this issue undoubtedly lies in searching superior catalysts to efficiently activate and hydrogenate the stable nitrogen molecules. We herein evaluate the feasibility of WP2 for N2 activation and reduction, and first demonstrate WP2 with an impressive ammonia yield rate of 7.13 μg h−1 cm−2, representing a promising W-based catalyst for NRR. DFT analysis further reveals that the NRR catalysis on WP2 proceeds in a distal reaction pathway, and the exceptional NRR activity is originated from superior surface electron energy level matching between WP2 and NRR potential which facilitates the interfacial proton-coupled electron transfer dynamics. The successfully unraveling the intrinsic catalytic mechanism of WP2 for NRR could offer a powerful platform to manipulate the NRR activity by tuning the electron energy levels.

Conformational Motions and Water Networks at the α/β Interface in E. coli Ribonucleotide Reductase.

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.

Determination of Proton-Coupled Electron Transfer Reorganization Energies with Application to Water Oxidation Catalysts.

The reorganization energy, λ, for interfacial electron transfer (ET) and for proton-coupled electron transfer (PCET) between a water oxidation catalyst and a conductive In2O3:Sn (ITO) oxide were extracted from kinetic data by application of Marcus-Gerischer theory. Specifically, light excitation of the water oxidation catalyst [RuII(tpy)(4,4'-(PO3H2)2-bpy)OH2]2+ (RuII-OH2), where tpy is 2,2':6',2″-terpyridine and bpy is 2,2'-bipyridine, anchored to a mesoporous thin film of ITO nanocrystallites resulted in rapid excited-state injection ( kinj > 108 s-1). The subsequent reaction of the injected electron (ITO(e-)) and the oxidized catalyst was quantified spectroscopically on nanosecond and longer time scales. The metallic character of ITO allowed potentiostatic control of the reaction free energy change -Δ Go over a 1 eV range. At pH values below the p Ka = 1.7 of the oxidized catalyst, ET was the primary reaction. Within the pH range 2 ≤ pH ≤ 5, an interfacial PCET reaction (ITO(e-) + RuIII-OH + H+→ RuII-OH2) occurred with smaller rate constants. Plots of the rate constants versus -Δ Go provided a reorganization energy of λPCET = 0.9 eV and λET = 0.5 eV. A second water oxidation catalyst provided similar values and demonstrated generality. The utilization of conductive oxides is shown to be a powerful tool for quantifying PCET reorganization energies at oxide surfaces for the first time.

Graphite-Conjugated Acids Reveal a Molecular Framework for Proton-Coupled Electron Transfer at Electrode Surfaces.

Proton-coupled electron-transfer (PCET) steps play a key role in energy conversion reactions. Molecular PCET reactions are well-described by “square schemes” in which the overall thermochemistry of the reaction is broken into its constituent proton-transfer and electron-transfer components. Although this description has been essential for understanding molecular PCET, no such framework exists for PCET reactions that take place at electrode surfaces. Herein, we develop a molecular square scheme framework for interfacial PCET by investigating the electrochemistry of molecularly well-defined acid/base sites conjugated to graphitic electrodes. Using cyclic voltammetry, we first demonstrate that, irrespective of the redox properties of the corresponding molecular analogue, proton transfer to graphite-conjugated acid/base sites is coupled to electron transfer. We then show that the thermochemistry of surface PCET events can be described by the pKa of the molecular analogue and the potential of zero free charge (zero-field reduction potential) of the electrode. This work provides a general framework for analyzing and predicting the thermochemistry of interfacial PCET reactions.

Controlling Electrons and Protons through Theory: Molecular Electrocatalysts to Nanoparticles.

The development of renewable energy sources that are environmentally friendly and economical is of critical importance. The effective utilization of such energy sources relies on catalysts to facilitate the interconversion between electrical and chemical energy through multielectron, multiproton reactions. The design of effective catalysts for these types of energy conversion processes requires the ability to control the localization and movement of electrons and protons, as well as the coupling between them. Theoretical calculations, in conjunction with experimental validation and feedback, are playing a key role in these catalyst design efforts. A general theory has been developed for describing proton-coupled electron transfer (PCET) reactions, which encompass all reactions involving the coupled transfer of electrons and protons, including sequential and concerted mechanisms for multielectron, multiproton processes. In addition, computational methods have been devised to compute the input quantities for the PCET rate constant expressions and to generate free energy pathways for molecular electrocatalysts. These methods have been extended to heterogeneous PCET reactions to enable the modeling of PCET processes at electrode and nanoparticle surfaces. Three distinct theoretical studies of PCET reactions relevant to catalyst design for energy conversion processes are discussed. In the first application, theoretical calculations of hydrogen production catalyzed by hangman metalloporphyrins predicted that the porphyrin ligand is reduced, leading to dearomatization and proton transfer from the carboxylic acid hanging group to the meso carbon of the porphyrin rather than the metal center, producing a phlorin intermediate. Subsequent experiments isolated and characterized the phlorin intermediate, validating this theoretical prediction. These molecular electrocatalysts exemplify the potential use of noninnocent ligands to localize electrons and protons on different parts of the catalyst and to direct their motions accordingly. In the second application, theoretical calculations on substituted benzimidazole phenol molecules predicted that certain substituents would lead to multiple intramolecular proton transfer reactions upon oxidation. Subsequent experiments verified these multiproton reactions, as well as the predicted shifts in the redox potentials and kinetic isotope effects. These bioinspired molecular systems demonstrate the potential use of multiproton relays to enable the transport of protons over longer distances along specified pathways, as well as the tuning of redox potentials through this movement of positive charge. In the third application, theoretical studies of heterogeneous PCET in photoreduced ZnO nanoparticles illustrated the significance of proton diffusion through the bulk of the nanoparticle as well as interfacial PCET to an organic radical in solution at its surface. These theoretical calculations were consistent with prior experimental studies of this system, although theoretical methods for heterogeneous PCET have not yet attained the level of predictive capability highlighted for the molecular electrocatalysts. These examples suggest that theory will play a significant role in the design of both molecular and heterogeneous catalysts to control the movement and coupling of electrons and protons. The resulting catalysts will be essential for the development of renewable energy sources to address current energy challenges.

Theoretical Insights into Proton-Coupled Electron Transfer from a Photoreduced ZnO Nanocrystal to an Organic Radical.

Proton-coupled electron transfer (PCET) at metal-oxide nanoparticle interfaces plays a critical role in many photocatalytic reactions and energy conversion processes. Recent experimental studies have shown that photoreduced ZnO nanocrystals react by PCET with organic hydrogen atom acceptors such as the nitroxyl radical TEMPO. Herein, the interfacial PCET rate constant is calculated in the framework of vibronically nonadiabatic PCET theory, which treats the electrons and transferring proton quantum mechanically. The input quantities to the PCET rate constant, including the electronic couplings, are calculated with density functional theory. The computed interfacial PCET rate constant is consistent with the experimentally measured value for this system, providing validation for this PCET theory. In this model, the electron transfers from the conduction band of the ZnO nanocrystal to TEMPO concertedly with proton transfer from a surface oxygen of the ZnO nanocrystal to the oxygen of TEMPO. Moreover, the proton tunneling at the interface is gated by the relatively low-frequency proton donor-acceptor motion between the TEMPO radical and the ZnO nanocrystal. The ZnO nanocrystal and TEMPO are found to contribute similar amounts to the inner-sphere reorganization energy, implicating structural reorganization at the nanocrystal surface. These fundamental mechanistic insights may guide the design of metal-oxide nanocatalysts for a wide range of energy conversion processes.