Proton-coupled Electron Transfer Steps Research Articles

The role of proton dynamics on the catalyst-electrolyte interface in the oxygen evolution reaction

The development of non-precious metal catalysts that facilitate the oxygen evolution reaction (OER) is important for the widespread application of hydrogen production by water splitting. Various perovskite oxides have been employed as active OER catalysts, however, the underlying mechanism that occurs at the catalyst-electrolyte interface is still not well understood, prohibiting the design and preparation of advanced OER catalysts. Here, we report a systematic investigation into the effect of proton dynamics on the catalyst-electrolyte interfaces of four perovskite catalysts: La 0.5 Sr 0.5 CoO 3-δ (LSCO), LaCoO 3 , LaFeO 3 , and LaNiO 3 . The pH-dependent OER activities, H/D kinetic isotope effect, and surface functionalization with phosphate anion groups were investigated to elucidate the role of proton dynamics in the rate-limiting steps of the OER. For oxides with small charge-transfer energies, such as LSCO and LaNiO 3 , non-concerted proton-coupled electron transfer steps are involved in the OER, and the activity is strongly controlled by the proton dynamics on the catalyst surface. The results demonstrate the important role of interfacial proton transfer in the OER mechanism, and suggest that proton dynamics at the interface should carefully be considered in the design of future high-performance catalysts. A systematical study was conducted to probe the role of proton dynamics in the multiple PCET steps during OER on the catalyst-electrolyte interfaces using perovskite oxides as model catalysts.

Combining Metal Sulfides and Organic Cocatalysts to Overcome the Challenges of Electrochemical CO2 Reduction

Electrocatalytic reduction of CO2 is a potential method for converting CO2 to desirable products such as methanol and ethanol. Due to unfavorable scaling relations between the binding energies of catalytic intermediates on transition metals, such catalysts suffer from low current densities, high overpotentials, and low Faradaic efficiencies. Transition metal chalcogenides such as MoS2 have been shown to possess more favorable scaling relations for CO2 reduction – indeed, doped MoS2 in the presence of an electrolyte containing N-substituted imidazolium cations has been experimentally shown to have one of the highest reported current densities for CO2 reduction to CO. However, our density functional theory (DFT) calculations show that CO2 reduction pathways on this surface are hindered by high activation barriers for C-H bond formation. We hypothesize that a lower barrier pathway exists that involves the imidazolium cation functioning a cocatalyst by inducing polarity reversal (umpolung) on the carbon atom. As such, C-H bond formation can occur by proton transfer rather than by hydride transfer.We have used DFT to calculate barriers on such a cocatalyst, 1,3-dimethylimidazolium (DMIM), and found that CO2 can be activated by this molecule through electron-coupled addition to the carbon in the 2-position of the aromatic ring. The activated complex then undergoes several proton-coupled electron transfer steps and decomposes to eliminate CO. However, the activation barriers for this pathway are also too high for it to proceed at reasonable temperatures when the cocatalyst is not electronically coupled to the electrode. We further hypothesize that the DMIM cocatalyst chemisorbs on the MoS2 surface so that hybridization between the cocatalyst and surface orbitals lowers the electron transfer barriers enough for the proposed pathway to occur. Unfortunately, modeling these electron transfer reactions with DFT in the presence of the surface and electrochemical interface is challenging and we are currently extending the capabilities of an existing implicit solvation model (VASPsol) to be able to carry these out.

Isolation and Identification of Pseudo Seven-Coordinate Ru(III) Intermediate Completing the Catalytic Cycle of Ru-bda Type of Water Oxidation Catalysts

Isolation and Identification of Pseudo Seven-Coordinate Ru(III) Intermediate Completing the Catalytic Cycle of Ru-bda Type of Water Oxidation Catalysts

Simultaneous Manipulation of Bulk Excitons and Surface Defects for Ultrastable and Highly Selective CO2 Photoreduction.

The objective of photocatalytic CO2 reduction (PCR) is to achieve high selectivity for a single energy-bearing product with high efficiency and stability. The bulk configuration usually determines charge carrier kinetics, whereas surface atomic arrangement defines the PCR thermodynamic pathway. Concurrent engineering of bulk and surface structures is therefore crucial for achieving the goal of PCR. Herein, an ultrastable and highly selective PCR using homogeneously doped BiOCl nanosheets synthesized via an inventive molten strategy is presented. With B2 O3 as both the molten salt and doping precursor, this new doping approach ensures boron (B) doping from the surface into the bulk with dual functionalities. Bulk B doping mitigates strong excitonic effects confined in 2D BiOCl by significantly reducing exciton binding energies, whereas surface-doped B atoms reconstruct the BiOCl surface by extracting lattice hydroxyl groups, resulting in intimate B-oxygen vacancy (B-OV) associates. These exclusive B-OV associates enable spontaneous CO2 activation, suppress competitive hydrogen evolution and promote the proton-coupled electron transfer step by stabilizing *COOH for selective CO generation. As a result, the homogeneous B-doped BiOCl nanosheets exhibit 98% selectivity for CO2 -to-CO reduction under visible light, with an impressive rate of 83.64µmol g-1 h-1 and ultrastability for long-term testing of 120 h.

Reversible Proton-Coupled Reduction of an Iron Nitrosyl Porphyrin within [DBU-H]+-Based Protic Ionic Liquid Nanodomains.

The reduction of [Fe(OEP)(NO)] has been studied in the presence of aprotic room-temperature ionic liquids (RTIL) and protic (PIL) ionic liquids dissolved within a molecular solvent (MS). The cyclic voltammetric results showed the formation of RTIL nanodomains at low concentrations of the RTIL/PIL solutions. The pKa values of the two PILs studied (i.e., trialkylammonium and [DBU-H]+-based ionic liquids) differed by four units in THF. While voltammetry in solutions containing all three RTILs showed similar potential shifts of the first reduction of [Fe(OEP)(NO)] to [Fe(OEP)(NO)]- at low concentrations, significant differences were observed at higher concentrations for the ammonium PIL. The trialkylammonium cation had previously been shown to protonate the {FeNO}8 species at room temperature. Visible and infrared spectroelectrochemistry revealed that the [DBU-H]+-based PIL formed hydrogen bonds with [Fe(OEP)(NO)]- rather than formally protonating it. Despite these differences, both PILs were able to efficiently reduce the nitrosyl species to the hydroxylamine complex, which could be further reduced to ammonia. On the voltammetric time scale and when the switching potential was positive of the Fe(II)/Fe(I) potential, the hydroxylamine complex was re-oxidized back to the NO complex via direct oxidation of the coordinated hydroxylamine at low scan rates or initial oxidation of the ferrous porphyrin at high scan rates. The results of this work show that, while [DBU-H]+ does not protonate electrochemically generated [Fe(OEP)(NO)]-, it still plays an important role in efficiently reducing the nitroxyl ligand via a series of proton-coupled electron transfer steps to generate hydroxylamine and eventually ammonia. The overall reaction rates were independent of the PIL concentration, consistent with the nanodomain formation being important to the reduction process.

Strategy for improving the activity and selectivity of CO2 electroreduction on flexible carbon materials for carbon neutral

Selective electrochemical reduction of CO2 is of interest in relieving the accumulation of CO2 in the atmosphere and the global energy crisis. This study investigates active sites and reaction mechanisms for CO2 electroreduction over metal N-doped flexible carbon materials using Phthalocyanine (Pc) and Iron Phthalocyanine (FePc) molecules as model catalysts. Both the FePc molecule and Pc molecule show catalytic activity for CO production and H2 evolution based on experimental results and DFT calculations. The presence of Fe atom results in a significantly higher activity for CO production and a little lower activity for H2 evolution. Calculated pKa and redox potentials (Ure) are introduced for the multi-step proton-coupled electron transfer (PCET) steps of two competitive reactions. Specifically, a comparison between them is conducted to explain the competition between CO2 electroreduction and H2 evolution reaction with the support of energy diagram. The rate-determining steps for CO2 reduction and H2 evolution are also investigated. Both the CO and H2 formation on Pc molecule depend on electron transfer process. Besides, the Ure of two rate-determining steps are close, which is related to the closed Eonset for CO and H2 formation in the experiment. This study demonstrates that the mechanism for CO2 reduction from DFT calculation is in agreement with the experimental results. The technological approach and reaction mechanism can be applied to other types of CO2 electroreduction on carbon materials. Thus, this work provides a guidance in the rational design of the green catalyst and the implementation of carbon neutral.

Observation of a potential-dependent switch of water-oxidation mechanism on Co-oxide-based catalysts

Summary O–O bond formation is a key elementary step of the water-oxidation reaction. However, it is still unclear how the mechanism of O–O coupling depends on the applied electrode potential. Herein, using water-in-salt electrolytes, we systematically altered the water activity, which enabled us to probe the O–O bond-forming mechanism on heterogeneous Co-based catalysts as a function of applied potential. We discovered that the water-oxidation mechanism is sensitive to the applied potential: At relatively low driving force, the reaction proceeds through an intramolecular oxygen coupling mechanism, whereas the water nucleophilic attack mechanism prevails at high driving force. The observed mechanistic switch has major implications for the understanding and control of the water-oxidation reaction on heterogeneous catalysts.

Proton Removal Kinetics That Govern the Hydrogen Peroxide Oxidation Activity of Heterogeneous Bioinorganic Platforms.

Precise regulation of proton-coupled electron-transfer (PCET) rates holds the key to simultaneously optimizing the turnover frequency and product selectivity of redox reactions that are central to the realization of renewable energy schemes in a sustainable future. In this work, a self-assembled monolayer (SAM) of a Ru complex electrografted onto a glassy carbon (GC) electrode was prepared as a heterogeneous electrocatalytic interface to facilitate the hydrogen peroxide (H2O2) oxidation half-cell reaction of a direct hydrogen peroxide/hydrogen peroxide fuel cell. A functional lipid membrane embedded with catalytic amounts of proton carriers was appended on top of the Ru SAM to construct a hybrid bilayer membrane (HBM) platform that can modulate the thermodynamics and kinetics of proton- and electron-transfer steps independently. The performances of the as-prepared Ru SAMs and HBMs toward H2O2 oxidation were investigated using electrochemical means, kinetic isotope effect (KIE) studies, and Tafel analyses. Proton carriers featuring borate, phosphate, and nitrile headgroups were found to dictate the transmembrane proton removal rate, thereby controlling the H2O2 oxidation activity. The first significance of this work was the expansion of HBM platforms to GC substrates to overcome the limited redox potential window on gold thiol systems, thereby enabling electrochemical investigations of anodic reactions at the SAM-lipid interface. The second highlight of this work was demonstrating for the first time that deprotonation kinetics can be taken advantage of to enhance the electrocatalytic oxidation performance of a metal complex anchored at the SAM-lipid interface of a HBM platform. When the knowledge gaps regarding how PCET steps govern redox pathways are closed, the advances achieved using our unique bioinorganic platform are envisioned to accelerate the understanding and optimization of electrocatalytic processes involving proton- and electron- transfer steps that are fundamental to the development of high-performance energy devices.

Bioinorganic Platforms for Sensing, Biomimicry, and Energy Catalysis

Bioinorganic chemistry is a broad field with multiple branches. In this Highlight Review, the newest excitements in the branch of Bioinorganic Platforms are reviewed. Artificial bioinorganic platforms come in mesmerizing flavors, in particular the following 5 types are the major focus of this Highlight Review. First, single-stranded and double-stranded DNA, RNA, PNA, and their hybrids can be attached to Au electrodes for in vitro biosensing purposes as well as real-time monitoring of DNA repair processes via magnetoelectrochemistry. Second, Pseudomonas aeruginosa biofilms can be grown into interdigitated array (IDA) electrodes to investigate electron transport mechanisms in vivo. Third, protein films can be formed on glassy carbon (GC) electrodes to examine the effect of electrostatics on the redox behavior of metalloenzymes. Fourth, lipid-covered hybrid bilayer membrane (HBM) systems can be constructed to control the thermodynamics and kinetics of proton-coupled electron transfer (PCET) steps important to renewable energy schemes. Fifth, metal complexes can be physisorbed onto highly porous carbon powder as facile oxygen reduction reaction (ORR) catalysts.

Kinetics of chain reaction driven by proton-coupled electron transfer: α-hydroxyethyl radical and bromoacetate in buffered aqueous solutions.

We measured and computed the rate constants of the reaction between the α-hydroxyethyl radical (˙CH(CH3)OH) and bromoacetate (BrCH2CO2-) in the non-buffered (NB), as well as in the bicarbonate (HCO3-) and hydrogen phosphate (HPO42-) buffered aqueous solutions in the presence of ethanol. These complex multistep reactions are initiated by the proton-coupled electron transfer (PCET) which reduces BrCH2CO2- and incites its debromination. The PCET is followed by the step in which the resulting carboxymethyl radical propagates a radical chain reaction thus recovering ˙CH(CH3)OH and enhancing the debromination yields. It is found that the rate constants for the initial PCET step (k1) are raised by ca. an order of magnitude in the presence of the buffers (k1(NB) = 1.4 × 105 dm3 mol-1 s-1; k1(HCO3-) = 1.4 × 106 dm3 mol-1 s-1; k1(HPO42-) = 1.1 × 106 dm3 mol-1 s-1). To rationalize this, we used density functional theory at the M06-2X-D3/6-311+G(2d,p) level in conjunction with the polarizable continuum model (PCM) for an implicit description of the aqueous environment. To acceptably reproduce the measured rate constants, the minimal solute, consisting of ˙CH(CH3)OH, BrCH2CO2- and the buffer anion, has to be expanded by at least 2-3 explicit molecules of the water solvent. The used kinetic model consisting of a set of coupled differential equations indicates the sigmoid dependence of yields vs. k1 thereby confirming the autocatalytic trait of these reactions. The computations unravel the profound influence of the presence of buffers on these reaction systems. On the one hand, the buffer anions promote the PCET by accelerating the proton transfer; on the other hand, they slow down the propagation step by forming the strong hydrogen bonds with the carboxymethyl radical. The two opposing effects cancel out and cause the Br- yields to remain approximately comparable in the non-buffered and buffered media.

Role of Intact Hydrogen-Bond Networks in Multiproton-Coupled Electron Transfer.

The essential role of a well-defined hydrogen-bond network in achieving chemically reversible multiproton translocations triggered by one-electron electrochemical oxidation/reduction is investigated by using pyridylbenzimidazole-phenol models. The two molecular architectures designed for these studies differ with respect to the position of the N atom on the pyridyl ring. In one of the structures, a hydrogen-bond network extends uninterrupted across the molecule from the phenol to the pyridyl group. Experimental and theoretical evidence indicates that an overall chemically reversible two-proton-coupled electron-transfer process (E2PT) takes place upon electrochemical oxidation of the phenol. This E2PT process yields the pyridinium cation and is observed regardless of the cyclic voltammogram scan rate. In contrast, when the hydrogen-bond network is disrupted, as seen in the isomer, at high scan rates (∼1000 mV s-1) a chemically reversible process is observed with an E1/2 characteristic of a one-proton-coupled electron-transfer process (E1PT). At slow cyclic voltammetric scan rates (<1000 mV s-1) oxidation of the phenol results in an overall chemically irreversible two-proton-coupled electron-transfer process in which the second proton-transfer step yields the pyridinium cation detected by infrared spectroelectrochemistry. In this case, we postulate an initial intramolecular proton-coupled electron-transfer step yielding the E1PT product followed by a slow, likely intermolecular chemical step involving a second proton transfer to give the E2PT product. Insights into the electrochemical behavior of these systems are provided by theoretical calculations of the electrostatic potentials and electric fields at the site of the transferring protons for the forward and reverse processes. This work addresses a fundamental design principle for constructing molecular wires where protons are translocated over varied distances by a Grotthuss-type mechanism.

Spectroscopic and Electrokinetic Evidence for a Bifunctional Mechanism of the Oxygen Evolution Reaction*.

A bifunctional oxygen evolution reaction (OER) mechanism, in which the energetically demanding step of the attack of hydroxide on a metal oxo unit is facilitated by a hydrogen atom transfer to a second site, has the potential to circumvent the scaling relationship. However, the bifunctional mechanism has hitherto only been supported by theoretical computations. Here we describe an operando Raman spectroscopic and electrokinetic study of two highly active OER catalysts, FeOOH-NiOOH and NiFe layered double hydroxide (LDH). The data support two distinct mechanisms for the two catalysts: FeOOH-NiOOH operates by a bifunctional mechanism where the rate-determining O-O bond forming step is the OH- attack on a Fe=O coupled with a hydrogen atom transfer to a NiIII -O site, whereas NiFe LDH operates by a conventional mechanism of four consecutive proton-coupled electron transfer steps. The experimental validation of the bifunctional mechanism enhances the understanding of OER catalysts.

Redefinition of the Active Species and the Mechanism of the Oxygen Evolution Reaction on Gold Oxide

Accurately identifying the active species of catalytic materials and understanding how they catalyze the oxygen evolution reaction (OER) are critical for the development of energy storage technologies. In this contribution, we identify two pH-dependent active oxides by mapping the reduction behavior of gold oxide and by in situ surface-enhanced Raman spectroscopy. It was found that α-oxide is preferentially formed in an acidic solution, whereas β-oxide, Au(OH)3, is preferably formed in an alkaline solution. In line with the presence of two different surface structures on gold, there are two OER mechanisms: one mechanism wherein water splitting occurs via proton-coupled electron-transfer steps mediated by α-oxide and the other mechanism wherein electron transfer and proton transfer are decoupled and mediated by a deprotonated form of Au(OH)3. This identification of pH-dependent oxides offers a different perspective in our understanding of the OER mechanism on metal oxides in a full pH scale range.

Construction and Regulation of a Surface Protophilic Environment to Enhance Oxygen Reduction Reaction Electrocatalytic Activity.

Pyrolytic transition metal nitrogen-carbon (M-N/C) materials are considered as the most promising alternatives for platinum-based catalysts toward oxygen reduction reaction (ORR). As the proton-coupled electron transfer step in ORR has been proven to be a rate-determining step in the M-N/C catalysts, we envisaged that building a protophilic surface might be helpful to enhance the ORR activity. Herein, a polyaniline decoration strategy was put forward and realized to confer the Fe-N/C catalyst with a surface protophilic environment. A 20 mV positive shift in half-wave potential was observed owing to the enriched interfacial proton concentration, corresponding to a tripled turnover frequency under acidic conditions (from 0.46 to 1.28 e·s-1·sites-1). Our work blazed a new path toward the design of M-N/C ORR catalysts, commencing via the ORR kinetics.

Proton Relay Network in the Bacterial P450s: CYP101A1 and CYP101D1.

Cytochrome P450s are among nature's most powerful catalysts. Their ability to activate molecular dioxygen to form high-valent ferryl intermediates (Compounds I and II) enables a wide array of chemistries ranging from simple epoxidations to more complicated C-H bond oxidations. Oxygen activation is achieved by reduction of the ferrous dioxygen complex, which requires the transfer of an electron from a redox partner and subsequent double protonation to yield a water molecule and a ferryl porphyrin π-cation radical (Compound I). Previous studies of the CYP101 family of cytochrome P450s demonstrated the importance of the conserved active site Asp25X residue in this protonation event, although its precise role is yet to be unraveled. To further explore the origin of protons in oxygen activation, we analyzed the effects of an Asp to Glu mutation at the 25X position in P450cam and in CYP101D1. This mutation inactivates P450cam but not CYP101D1. A series of mutagenic, crystallographic, kinetic, and molecular dynamics studies indicate that this mutation locks P450cam into a closed, inactive conformation. In CYP101D1, the D259E mutant changes the rate-limiting step to reduction of the P450-oxy complex, thus opening a window into the critical proton-coupled electron transfer step in P450 catalysis.

Cover Feature: Mechanism of Diol Dehydration by a Promiscuous Radical‐SAM Enzyme Homologue of the Antiviral Enzyme Viperin (RSAD2) (ChemBioChem 11/2020)

Nucleotide analogues lacking the 3’-hydroxyl group have antiviral activity. The image illustrates the finding that a thermostable radical-SAM enzyme catalyses transformation of natural and unnatural nucleotides to their 3’-deoxy analogues by a mechanism requiring a proton-coupled electron transfer step from a tyrosine residue. A nucleotide analogue synthesised by the enzyme is known to have broad-spectrum antiviral activity in humans. The findings suggest a new enzymatic route for green biosynthesis of antiviral nucleotide analogues. More information can be found in the full paper by K. Honarmand Ebrahimi et al.

Characterization of fluorescent chlorophyll charge-transfer states as intermediates in the excited state quenching of light-harvesting complex II.

Light-harvesting complex II (LHCII) is the major antenna complex in higher plants and green algae. It has been suggested that a major part of the excited state energy dissipation in the so-called "non-photochemical quenching" (NPQ) is located in this antenna complex. We have performed an ultrafast kinetics study of the low-energy fluorescent states related to quenching in LHCII in both aggregated and the crystalline form. In both sample types the chlorophyll (Chl) excited states of LHCII are strongly quenched in a similar fashion. Quenching is accompanied by the appearance of new far-red (FR) fluorescence bands from energetically low-lying Chl excited states. The kinetics of quenching, its temperature dependence down to 4K, and the properties of the FR-emitting states are very similar both in LHCII aggregates and in the crystal. No such FR-emitting states are found in unquenched trimeric LHCII. We conclude that these states represent weakly emitting Chl-Chl charge-transfer (CT) states, whose formation is part of the quenching process. Quantum chemical calculations of the lowest energy exciton and CT states, explicitly including the coupling to the specific protein environment, provide detailed insight into the chemical nature of the CT states and the mechanism of CT quenching. The experimental data combined with the results of the calculations strongly suggest that the quenching mechanism consists of a sequence of two proton-coupled electron transfer steps involving the three quenching center Chls 610/611/612. The FR-emitting CT states are reaction intermediates in this sequence. The polarity-controlled internal reprotonation of the E175/K179 aa pair is suggested as the switch controlling quenching. A unified model is proposed that is able to explain all known conditions of quenching or non-quenching of LHCII, depending on the environment without invoking any major conformational changes of the protein.

First-principles microkinetics simulations of electrochemical reduction of CO2 over Cu catalysts

Electrochemical reduction of CO2 can contribute to the storage of excess renewable electricity in chemical bonds. Here we incorporate reaction energetics for CO2 reduction on Cu(111) and Cu(211) determined by DFT calculations in microkinetics simulations to predict the influence of surface topology, the presence of water and possible diffusion limitations on current density-potential curves and Faradaic efficiencies. A reaction-diffusion model was used that takes into account the effect of electrochemical potential on the stability of intermediates and associated activation barriers in proton-coupled electron transfer steps as well as diffusion of protons and CO2 from the bulk electrolyte to the electrode surface. The basic model can well reproduce hydrogen evolution including the effect of proton diffusion limitations and a shift of proton reduction (low potential) to water reduction (high potential). Considering CO2 electro-reduction, the stepped Cu(211) surface is more active than the Cu(111) terrace towards HCOO(H), CO and CH4. The presence of a catalytic H2O molecule increases the overall rate and selectivity to products (CO and CH4) derived from dissociated CO2. A catalytic H2O molecule facilitates the difficult electrochemical CO2 activation step to COOH and suppresses the competing activation step towards HCOO, which mainly yields HCOO(H). In general, the current densities increase at higher negative potential and the products follow the sequence CO2 → CO → CH4. That is to say, CO2 is converted to CO via COOH dissociation, followed by CO hydrogenation. Trendwise, the simulated product distribution follows the potential-dependent distribution observed in experiment. The low selectivity to CH3OH can be understood from the fast electrochemical steps that lead to CHx-OH dissociation. At high overpotentials the hydrogenation step from CO2 to COOH controls both activity and selectivity towards CH4. At high potential CO2 reduction becomes increasingly diffusion-limited, thus limiting the selectivity of CO2 reduction vs. hydrogen evolution. This aspect supports the need for better design of mass transfer in electrochemical reactors, which operate at high current density.

CoII(BPyPy₂COH)(OH₂)₂]2+: A Catalytic Pourbaix Diagram and AIMD Simulations on Four Key Intermediates.

Proton reduction by [CoII(BPyPy₂COH)(OH₂)₂]2+ (BPyPy₂COH = [2,2'-bipyridin]-6-yl-di[pyridin-2-yl]methanol) proceeds through two distinct, pH-dependent pathways involving proton-coupled electron transfer (PCET), reduction and protonation steps. In this account we give an overview of the key mechanistic aspects in aqueous solution from pH 3 to 10, based on electrochemical data, time-resolved spectroscopy and ab initio molecular dynamics simulations of the key catalytic intermediates. In the acidic pH branch, a PCET to give a CoIII hydride is followed by a reduction and a protonation step, to close the catalytic cycle. At elevated pH, a reduction to CoI is observed, followed by a PCET to a CoII hydride, and the catalytic cycle is closed by a slow protonation step. In our simulation, both CoI and CoII-H feature a strong interaction with the surrounding solvent via hydrogen bonding, which is expected to foster the following catalytic step.

Modeling Electrocatalytic Oxidation of Formic Acid at Platinum

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