Electrically driven proton transfer promotes Brønsted acid catalysis by orders of magnitude.

Electric fields play a key role in enzymatic catalysis and can enhance reaction rates by 100,000-fold, but the same rate enhancements have yet to be achieved in thermochemical heterogeneous catalysis. In this work, we probe the influence of catalyst potential and interfacial electric fields on heterogeneous Brønsted acid catalysis. We observed that variations in applied potential of ~380 mV led to a 100,000-fold rate enhancement for 1-methylcyclopentanol dehydration, which was catalyzed by carbon-supported phosphotungstic acid. Mechanistic studies support a model in which the interfacial electrostatic potential drop drives quasi-equilibrated proton transfer to the adsorbed substrate prior to rate-limiting C-O bond cleavage. Large increases in rate with potential were also observed for the same reaction catalyzed by Ti/TiOyHx and for the Friedel Crafts acylation of anisole with acetic anhydride by carbon-supported phosphotungstic acid.

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.

Tunable Electrochemical Entropy through Solvent Ordering by a Supramolecular Host.

An aqueous electrochemically controlled host-guest encapsulation system demonstrates a large and synthetically tunable redox entropy change. Electrochemical entropy is the basis for thermally regenerative electrochemical cycles (TRECs), which utilize reversible electrochemical processes with large molar entropy changes for thermogalvanic waste-heat harvesting and electrochemical cooling, among other potential applications. A supramolecular host-guest system demonstrates a molar entropy change of 4 times that of the state-of-the-art aqueous TREC electrolyte potassium ferricyanide. Upon encapsulation of a guest, water molecules that structurally resemble amorphous ice are displaced from the host cavity, leveraging a change in the degrees of freedom and ordering of the solvent rather than the solvation of the redox-active species to increase entropy. The synthetic tunability of the host allows rational optimization of the system's ΔS, showing a range of -51 to -101 cal mol-1 K-1 (-2.2 to -4.4 mV K-1) depending on ligand and metal vertex modifications, demonstrating the potential for rational design of high-entropy electrolytes and a new strategy to overcome theoretical limits on ion solvation reorganization entropy.

Weakly Coordinating Organic Cations Are Intrinsically Capable of Supporting CO2 Reduction Catalysis.

The rates and selectivity of electrochemical CO2 reduction are known to be strongly influenced by the identity of alkali metal cations in the medium. However, experimentally, it remains unclear whether cation effects arise predominantly from coordinative stabilization of surface intermediates or from changes in the mean-field electrostatic environment at the interface. Herein, we show that Au- and Ag-catalyzed CO2 reduction can occur in the presence of weakly coordinating (poly)tetraalkylammonium cations. Through competition experiments in which the catalytic activity of Au was monitored as a function of the ratio of the organic to metal cation, we identify regimes in which the organic cation exclusively controls CO2 reduction selectivity and activity. We observe substantial CO production in this regime, suggesting that CO2 reduction catalysis can occur in the absence of Lewis acidic cations, and thus, coordinative interactions between the electrolyte cations and surface-bound intermediates are not required for CO2 activation. For both Au and Ag, we find that tetraalkylammonium cations support catalytic activity for CO2 reduction on par with alkali metal cations but with distinct cation activity trends between Au and Ag. These findings support a revision in electrolyte design rules to include water-soluble organic cation salts as potential supporting electrolytes for CO2 electrolysis.

Metal nanoparticles supported on a nonconductive oxide undergo pH-dependent spontaneous polarization.

Electrochemical polarization, which often plays a critical role in driving chemical reactions at solid-liquid interfaces, can arise spontaneously through the exchange of ions and/or electrons across the interface. However, the extent to which such spontaneous polarization prevails at nonconductive interfaces remains unclear because such materials preclude measuring and controlling the degree of interfacial polarization via standard (i.e., wired) potentiometric methods. Herein, we circumvent the limitations of wired potentiometry by applying infrared and ambient pressure X-ray photoelectron spectroscopies (AP-XPS) to probe the electrochemical potential of nonconductive interfaces as a function of solution composition. As a model class of macroscopically nonconductive interfaces, we specifically probe the degree of spontaneous polarization of ZrO2-supported Pt and Au nanoparticles immersed in aqueous solutions of varying pH. Shifts in the Pt-adsorbed CO vibrational band position evince electrochemical polarization of the Pt/ZrO2-water interface with changing pH, and AP-XPS reveals quasi-Nernstian shifts of the electrochemical potential of Pt and Au with pH in the presence of H2. These results indicate that spontaneous proton transfer via equilibrated H+/H2 interconversion spontaneously polarizes metal nanoparticles even when supported on a nonconductive host. Consequently, these findings indicate that solution composition (i.e., pH) can be an effective handle for tuning interfacial electrical polarization and potential at nonconductive interfaces.

Concerted Proton-Coupled Electron Transfer to a Graphite Adsorbed Metalloporphyrin Occurs by Band to Bond Electron Redistribution.

Surface immobilized catalysts are highly promising candidates for a range of energy conversion reactions, and atomistic mechanistic understanding is essential for their rational design. Cobalt tetraphenylporphyrin (CoTPP) nonspecifically adsorbed on a graphitic surface has been shown to undergo concerted proton-coupled electron transfer (PCET) in aqueous solution. Herein, density functional theory calculations on both cluster and periodic models representing π-stacked interactions or axial ligation to a surface oxygenate are performed. As the electrode surface is charged due to applied potential, the adsorbed molecule experiences the electrical polarization of the interface and nearly the same electrostatic potential as the electrode, regardless of the adsorption mode. PCET occurs by electron abstraction from the surface to the CoTPP concerted with protonation to form a cobalt hydride, thereby circumventing Co(II/I) redox. Specifically, the Co(II) d-state localized orbital interacts with a proton from solution and an electron from the delocalized graphitic band states to produce a Co(III)-H bonding orbital below the Fermi level, corresponding to redistribution of electrons from the band states to the bonding states. These insights have broad implications for electrocatalysis by chemically modified electrodes and surface immobilized catalysts.

Organic Non-Nucleophilic Electrolyte Resists Carbonation during Selective CO2 Electroreduction.

The spontaneous reaction of CO2 with water and hydroxide to form (bi)carbonates in alkaline aqueous electrolytes compromises the energy and carbon efficiency of CO2 electrolyzers. We hypothesized that electrolyte carbonation could be mitigated by operating the reaction in an aprotic solvent with low water content, while also employing an exogenous non-nucleophilic acid as the proton donor to prevent parasitic capture of CO2 by its conjugate base. However, it is unclear whether such an electrolyte design could simultaneously engender high CO2 reduction selectivity and low electrolyte carbonation. We herein report selective CO2 electroreduction with low carbonate formation on a polycrystalline Au catalyst using dimethyl sulfoxide as the solvent and acetic acid/acetate as the proton-donating medium. CO2 is reduced to CO with over 90% faradaic efficiency at potentials relative to the reversible hydrogen electrode that are comparable to those in neutral aqueous electrolytes. 1H and 13C NMR studies demonstrate that only millimolar concentrations of bicarbonates are reversibly formed, that the proton activity of the medium is largely unaffected by exposure to CO2, and that low carbonation is maintained upon addition of 1 M water. This work demonstrates that electrolyte carbonation can be attenuated and decoupled from efficient CO2 reduction in an aprotic solvent, offering new electrolyte design principles for low-temperature CO2 electroreduction systems.

Electrolytic Synthesis of White Phosphorus Is Promoted in Oxide-Deficient Molten Salts.

Elemental white phosphorus (P4) is a key feedstock for the entire phosphorus-derived chemicals industry, spanning everything from herbicides to food additives. The electrochemical reduction of phosphate salts could enable the sustainable production of P4; however, such electrosynthesis requires the cleavage of strong, inert P-O bonds. By analogy to the promotion of bond activation in aqueous electrolytes with high proton activity (Brønsted-Lowry acidity), we show that low oxide anion activity (Lux-Flood acidity) enhances P-O bond activation in molten salt electrolytes. We develop electroanalytical tools to quantify the oxide dependence of phosphate reduction, and find that Lux acidic phosphoryl anhydride linkages enable selective, high-efficiency electrosynthesis of P4 at a yield of 95% Faradaic efficiency. These fundamental studies provide a foundation that may enable the development of low-carbon alternatives to legacy carbothermal synthesis of P4.

Neutralization Short-Circuiting with Weak Electrolytes Erodes the Efficiency of Bipolar Membranes.

Bipolar membranes (BPMs) are critical components of a variety of electrochemical energy technologies. Many electrochemical applications require the use of buffers to maintain stable, nonextreme pH environments, yet the impact of buffers or weak acids/bases on the electrochemical behavior of BPMs remains poorly understood. Our data for a cell containing weak electrolytes is consistent with internal pH gradients within the anion exchange membrane (AEM) or cation exchange membrane (CEM) component of the BPM that form via ionic short-circuiting processes at open-circuit. Short-circuiting results from the coupling of co-ion crossover and parasitic neutralization and leads to buffering of the bipolar interface. This phenomenon, which we term neutralization short-circuiting, serves to erode BPM efficiency by attenuating the open-circuit membrane voltage and introducing parasitic reverse bias currents associated with weak acid/base dissociation at the interface. These findings establish a mechanistic basis for the operation of BPM cells in the presence of weak acid/base electrolytes.

Correlation between Electronic Descriptor and Proton-Coupled Electron Transfer Thermodynamics in Doped Graphite-Conjugated Catalysts.

Graphite-conjugated catalysts (GCCs) provide a powerful framework for investigating correlations between electronic structure features and chemical reactivity of single-site heterogeneous catalysts. GCC-phenazine undergoes proton-coupled electron transfer (PCET) involving protonation of phenazine at its two nitrogen atoms with the addition of two electrons. Herein, this PCET reaction is investigated in the presence of defects, such as heteroatom dopants, in the graphitic surface. The proton-coupled redox potentials, EPCET, are computed using a constant potential periodic density functional theory (DFT) strategy. The electronic states directly involved in PCET for GCC-phenazine exhibit the same nitrogen orbital character as those for molecular phenazine. The energy εLUS of this phenazine-related lowest unoccupied electronic state in GCC-phenazine is identified as a descriptor for changes in PCET thermodynamics. Importantly, εLUS is obtained from only a single DFT calculation but can predict EPCET, which requires many such calculations. Similar electronic features may be useful descriptors for thermodynamic properties of other single-site catalysts.

Thermal Hydroquinone Oxidation on Co/N-doped Carbon Proceeds by a Band-Mediated Electrochemical Mechanism.

Molecular metal complexes catalyze aerobic oxidation reactions via redox cycling at the metal center to effect sequential activation of O2 and the substrate. Metal surfaces can catalyze the same transformations by coupling independent half-reactions for oxygen reduction and substrate oxidation mediated via the exchange of band-electrons. Metal- and nitrogen-doped carbons (MNCs) are promising catalysts for aerobic oxidation that consist of molecule-like active sites embedded in conductive carbon hosts. Owing to their combined molecular and metallic features, it remains unclear whether they catalyze aerobic oxidation via the sequential redox cycling pathways of molecules or band-mediated pathways of metals. Herein, we simultaneously track the potential of the catalyst and the rate of turnover of aerobic hydroquinone oxidation on a cobalt-based MNC catalyst in contact with a carbon electrode. By comparing operando measurements of rate and potential with the current-voltage behavior of each constituent half-reaction under identical conditions, we show that these molecular materials can display the band-mediated reaction mechanisms of extended metallic solids. We show that the action of these band-mediated mechanisms explains the fractional reaction orders in both oxygen and hydroquinone, the time evolution of catalyst potential and rate, and the dependence of rate on the overall reaction free energy. Selective poisoning experiments suggest that oxygen reduction proceeds at cobalt sites, whereas hydroquinone oxidation proceeds at native carbon-oxide defects on the MNC catalyst. These findings highlight that molecule-like active sites can take advantage of band-mediated mechanisms when coupled to conductive hosts.

Aprotic Solvent Exposes an Altered Mechanism for Copper-Catalyzed Ethylene Electrosynthesis.

The selectivity and efficiency of Cu-catalyzed CO2 or CO electroreduction are known to be sensitive to the electrolyte composition. However, in aqueous media, changes to pH and ionic composition do not alter the electrokinetic profile of C2 product formation, commonly invoked to proceed via a rate-limiting pH-independent C-C coupling step to form an oxyanionic *CO dimer. We hypothesize that new mechanistic pathways can be exposed in an aprotic solvent-based electrolyte, where inhibited interfacial charge stabilization can favor pathways with electroneutral intermediates resulting from proton-coupled electron-transfer (PCET) steps from an exogenous donor. We herein report CO electroreduction to higher-order products on a polycrystalline Cu catalyst with dimethyl sulfoxide as the solvent and phenol as the proton donor. CO is reduced principally to C2 products including ethylene, acetate, ethylene glycol, and ethane with negligible methane production. In stark contrast to aqueous electrolytes, we observe a low Tafel slope (27 ± 1 mV dec-1) and Nernstian dependence on proton activity for ethylene formation, suggesting a dramatically different mechanism involving quasi-equilibrated PCET steps. This work highlights the critical role of the solvent environment and proton donor in dictating the mechanistic landscape of CO electroreduction, exposing new strategies for tuning product selectivity in hydrocarbon electrosynthesis.

Spontaneous Electric Fields Play a Key Role in Thermochemical Catalysis at Metal-Liquid Interfaces.

Large oriented electric fields spontaneously arise at all solid-liquid interfaces via the exchange of ions and/or electrons with the solution. Although intrinsic electric fields are known to play an important role in molecular and biological catalysis, the role of spontaneous polarization in heterogeneous thermocatalysis remains unclear because the catalysts employed are typically disconnected from an external circuit, which makes it difficult to monitor or control the degree of electrical polarization of the surface. Here, we address this knowledge gap by developing general methods for wirelessly monitoring and controlling spontaneous electrical polarization at conductive catalysts dispersed in liquid media. By combining electrochemical and spectroscopic measurements, we demonstrate that proton and electron transfer from solution controllably, spontaneously, and wirelessly polarize Pt surfaces during thermochemical catalysis. We employ liquid-phase ethylene hydrogenation on a Pt/C catalyst as a thermochemical probe reaction and observe that the rate of this nonpolar hydrogenation reaction is significantly influenced by spontaneous electric fields generated by both interfacial proton transfer in water and interfacial electron transfer from organometallic redox buffers in a polar aprotic ortho-difluorobenzene solvent. Across these vastly disparate reaction media, we observe quantitatively similar scaling of ethylene hydrogenation rates with the Pt open-circuit electrochemical potential (E OCP). These results isolate the role of interfacial electrostatic effects from medium-specific chemical interactions and establish that spontaneous interfacial electric fields play a critical role in liquid-phase heterogeneous catalysis. Consequently, E OCP-a generally overlooked parameter in heterogeneous catalysis-warrants consideration in mechanistic studies of thermochemical reactions at solid-liquid interfaces, alongside chemical factors such as temperature, reactant activities, and catalyst structure. Indeed, this work establishes the experimental and conceptual foundation for harnessing electric fields to both elucidate surface chemistry and manipulate preparative thermochemical catalysis.

Rapid Electrochemical Methane Functionalization Involves Pd-Pd Bonded Intermediates.

High-valent Pd complexes are potent agents for the oxidative functionalization of inert C-H bonds, and it was previously shown that rapid electrocatalytic methane monofunctionalization could be achieved by electro-oxidation of PdII to a critical dinuclear PdIII intermediate in concentrated or fuming sulfuric acid. However, the structure of this highly reactive, unisolable intermediate, as well as the structural basis for its mechanism of electrochemical formation, remained elusive. Herein, we use X-ray absorption and Raman spectroscopies to assemble a structural model of the potent methane-activating intermediate as a PdIII dimer with a Pd-Pd bond and a 5-fold O atom coordination by HxSO4(x-2) ligands at each Pd center. We further use EPR spectroscopy to identify a mixed-valent M-M bonded Pd2II,III species as a key intermediate during the PdII-to-PdIII2 oxidation. Combining EPR and electrochemical data, we quantify the free energy of Pd dimerization as <-4.5 kcal/mol for Pd2II,III and <-9.1 kcal/mol for PdIII2. The structural and thermochemical data suggest that the aggregate effect of metal-metal and axial metal-ligand bond formation drives the critical Pd dimerization reaction in between electrochemical oxidation steps. This work establishes a structural basis for the facile electrochemical oxidation of PdII to a M-M bonded PdIII dimer and provides a foundation for understanding its rapid methane functionalization reactivity.

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.

A pyridinic Fe-N4 macrocycle models the active sites in Fe/N-doped carbon electrocatalysts.

Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum catalysts for the oxygen reduction reaction (ORR) in fuel cells; however, their active site structures remain poorly understood. A leading postulate is that the iron-containing active sites exist primarily in a pyridinic Fe-N4 ligation environment, yet, molecular model catalysts generally feature pyrrolic coordination. Herein, we report a molecular pyridinic hexaazacyclophane macrocycle, (phen2N2)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for ORR to a typical Fe-N-C material and prototypical pyrrolic iron macrocycles. N 1s XPS and XAS signatures for (phen2N2)Fe are remarkably similar to those of Fe-N-C. Electrochemical studies reveal that (phen2N2)Fe has a relatively high Fe(III/II) potential with a correlated ORR onset potential within 150 mV of Fe-N-C. Unlike the pyrrolic macrocycles, (phen2N2)Fe displays excellent selectivity for four-electron ORR, comparable to Fe-N-C materials. The aggregate spectroscopic and electrochemical data demonstrate that (phen2N2)Fe is a more effective model of Fe-N-C active sites relative to the pyrrolic iron macrocycles, thereby establishing a new molecular platform that can aid understanding of this important class of catalytic materials.

Using nature's blueprint to expand catalysis with Earth-abundant metals.

Numerous redox transformations that are essential to life are catalyzed by metalloenzymes that feature Earth-abundant metals. In contrast, platinum-group metals have been the cornerstone of many industrial catalytic reactions for decades, providing high activity, thermal stability, and tolerance to chemical poisons. We assert that nature's blueprint provides the fundamental principles for vastly expanding the use of abundant metals in catalysis. We highlight the key physical properties of abundant metals that distinguish them from precious metals, and we look to nature to understand how the inherent attributes of abundant metals can be embraced to produce highly efficient catalysts for reactions crucial to the sustainable production and transformation of fuels and chemicals.

Polarization-Induced Local pH Swing Promotes Pd-Catalyzed CO2 Hydrogenation.

Electrochemical polarization can dramatically promote the rate of concurrent nonfaradaic catalytic reactions, but the mechanistic basis for these promotion effects at solid-liquid interfaces remains poorly understood. Herein, we establish a mechanistic framework for nonfaradaic promotion in aqueous media that operates via a local pH swing induced by a concurrent faradaic reaction. As a model system, we examined the kinetics of nonfaradaic Pd-catalyzed CO2 hydrogenation to formate and find that the reaction can be promoted by a combination of high alkalinity and high CO2 concentration. In bulk electrolyte, alkalinity and CO2 concentration are inversely correlated to each other as set by the CO2/bicarbonate equilibrium. We show that this impasse can be overcome by using electrical polarization to generate a nonequilibrium local environment that has both high alkalinity and high CO2 concentration. We find that this local pH swing promotes the rate of nonfaradaic CO2 hydrogenation to formate by nearly 3 orders of magnitude at modest potential bias. The work establishes a rigorous mechanistic model of nonfaradaic promotion in aqueous media and provides a basis for enhancing hydrogenation catalysis under mild conditions via electrical polarization.

Molecular Control of Heterogeneous Electrocatalysis through Graphite Conjugation.

The efficient interconversion of electrical and chemical energy requires catalysts capable of accelerating multielectron reactions at or near electrified interfaces. These reactions can be performed at metallic surface sites on heterogeneous electrocatalysts or through redox mediation at molecular electrocatalysts. The relative ease of synthesis and characterization for homogeneous catalysts has allowed for molecular-level control over the active site and permitted systematic tuning of activity and selectivity. Similar control is difficult to achieve with heterogeneous electrocatalysts, because they typically exhibit a distribution of active site geometries and local electronic structures, which are challenging to modify with molecular precision. However, metallic heterogeneous electrocatalysts benefit from a continuum of electronic states that distribute the redox burden of multielectron transformations, enabling more efficient catalysis. We envisioned that we could combine the attractive properties of molecular and heterogeneous catalysts by integrating tunable molecular active sites into the delocalized band states of a conductive solid. The Surendranath group has developed a class of electrocatalysts in which molecules are strongly electronically coupled to graphitic electrodes through a conductive, aromatic pyrazine linkage such that they behave like metallic surface active sites. In this Account, we discuss the dual role of these graphite-conjugated catalysts (GCCs) as a platform with which to answer molecular-level questions of metallic active sites and as a tool with which to fundamentally alter the mechanism and enhance the performance of molecular active sites. We begin by describing the electrochemical and spectroscopic studies that demonstrated that GCC sites behave like metallic active sites rather than simply as redox mediators attached to electrode surfaces. We then discuss how electrochemical studies of a series of graphite-conjugated acids enabled the construction of a molecular model for the thermochemistry of proton-coupled electron transfer reactions at GCC sites based on the pKa of the molecular analogue of the conjugated site and the potential of zero free charge of the electrode. In the final section, we discuss the effects of graphite conjugation on the mechanism and rate of oxygen reduction, hydrogen evolution, and carbon dioxide reduction catalysis across four different GCC platforms involving N-heterocycle, organometallic, and metalloporphyrin active sites. We discuss how molecular-level tuning at graphite-conjugated active sites directly correlates to changes in catalytic activity for the oxygen reduction reaction. We demonstrate that graphite-conjugated porphyrins show enhanced catalytic oxygen reduction activity over amide-linked porphyrins. Lastly, we describe how catalysis at graphite-conjugated sites proceeds through mechanisms involving concerted electron transfer and substrate activation, in stark contrast to the mechanisms observed for molecular analogues. Overall, we showcase how GCCs provide a rich platform for controlling heterogeneous catalysis at the molecular level.

Tracking Electrical Fields at the Pt/H2O Interface during Hydrogen Catalysis.

We quantify changes in the magnitude of the interfacial electric field under the conditions of H2/H+ catalysis at a Pt surface. We track the product distribution of a local pH-sensitive, surface-catalyzed nonfaradaic reaction, H2 addition to cis-2-butene-1,4-diol to form n-butanol and 1,4-butanediol, to quantify the concentration of solvated H+ at a Pt surface that is constantly held at the reversible hydrogen electrode potential. By tracking the surface H+ concentration across a wide range of pH and ionic strengths, we directly quantify the magnitude of the electrostatic potential drop at the Pt/solution interface and establish that it increases by ∼60 mV per unit increase in pH. These results provide direct insight into the electric field environment at the Pt surface and highlight the dramatically amplified field existent under alkaline vs acidic conditions.