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

Exploring the Linear Energy Relationships between Activation Energy and Reaction Energy under an Electric Field

Electric-field (EF)-mediated chemistry has recently garnered increasing attention partly owing to its capability to catalyze a broad range of chemical reactions. How the EF affects the kinetics and thermodynamics of target reactions is a critical question. Herein, both density functional theory (DFT) and MP2 calculations suggest that the change of activation energy ΔΔE‡ and the change of reaction energy ΔΔErxn under an EF display a linear energy relationship (LER) ΔΔE‡ = mΔΔErxn. This has been tested against several reactions such as SN2 and proton transfer reactions, including neutral and charged systems and endothermic and exothermic processes. The linear coefficient m approximates to the ratio of the dipole moment change, i.e., Δμ‡/Δμrxn, of the studied reactions. The LER holds well at EF strengths up to ≈1 V/nm but deviates from the DFT-calculated results at larger EFs. Such deviations are mainly caused by the molecular geometry changes under an EF. Systems with larger polarizability experience greater geometry changes under an EF, thus leading to larger deviations. In addition, we propose that the reaction barrier can be predicted by -Δμ‡F - 0.5Δα‡F2, while it is well approximated by -Δμ‡F for small EF strengths. The proposed LER and the field-dependent barrier estimation promise broad applicability in EF-mediated chemical reactions.

Ab initio deep neural network simulations reveal that carbonic acid dissociation is dominated by minority cis-trans conformers

Carbonic acid (H2CO3), rather than water, serves as the primary protonating buffer regulating pH in biological systems and oceans. Its dissociation dynamics, driven by three conformers-cis-cis (CC), cis-trans (CT), and trans-trans (TT)-pose substantial experimental and theoretical challenges. Using deep potential molecular dynamics simulations with ab initio accuracy, we explored the dissociation dynamics of H2CO3 in solution on the nanosecond timescale. While the CC conformer is the most abundant, the CT conformer is the dominant proton donor. This enhanced deprotonation ability arises from the CT conformer's involvement in more hydrogen-bonding ring structures, enabling diverse proton transfer pathways, and its greater electronic asymmetry, which increases hydrophilicity and destabilizes the hydroxyl group. Furthermore, protons dissociated from the CT conformer demonstrate a stronger preference for the homing pathway. Our findings underscore the critical role of the topology and electronic properties of the CT conformer in aqueous H2CO3 dissociation and proton transfer.

Proton-coupled electron transfer controls peroxide activation initiated by a solid-water interface

Decentralized water treatment technologies, designed to align with the specific characteristics of the water source and the requirements of the user, are gaining prominence due to their cost and energy-saving advantages over traditional centralized systems. The application of chemical water treatment via heterogeneous advanced oxidation processes using peroxide (O-O) represents a potentially attractive treatment option. These processes serve to initiate redox processes at the solid-water interface. Nevertheless, the oxidation mechanism exemplified by the typical Fenton-like persulfate-based heterogeneous oxidation, in which electron transfer dominates, is almost universally accepted. Here, we present experimental results that challenge this view. At the solid-liquid interface, it is demonstrated that protons are thermodynamically coupled to electrons. In situ quantitative titration provides direct experimental evidence that the coupling ratio of protons to transferred electrons is almost 1:1. Comprehensive thermodynamic analyses further demonstrate that a net proton-coupled electron transfer occurs, with both protons and electrons entering the redox cycle. These findings will inform future developments in O-O activation technologies, enabling more efficient redox activity via the tight coupling of protons and electrons.

Non-Nernstian Effects in Theoretical Electrocatalysis

Electrocatalysis is one of the principal pathways for the transition to sustainable chemistry, promising greater energy efficiency and reduced emissions. As the field has grown, our theoretical understanding has matured. The influence of the applied potential on reactivity has developed from the first-order predictions based on the Nernst equation to the implicit inclusion of second-order effects including the interaction of reacting species with the interfacial electric field. In this review, we explore these non-Nernstian field effects in electrocatalysis, aiming to both understand and exploit them through theory and computation. We summarize the critical distinction between Nernstian and non-Nernstian effects and outline strategies to address the latter in theoretical studies. Subsequently, we examine the specific energetic contributions of the latter on capacitive and faradaic processes separately. We also underscore the importance of considering non-Nernstian effects in catalyst screening and mechanistic analysis. Finally, we provide suggestions on how to experimentally unravel these effects, offering insights into practical approaches for advancing the field.

Bias Dependence of the Transition State of the Hydrogen Evolution Reaction

The hydrogen evolution reaction (HER) is one of the most prominent electrocatalytic reactions of green energy transition. However, the kinetics across materials and electrolyte pH and the impact of hydrogen coverage at high current densities remain poorly understood. Here, we study the HER kinetics over a large set of nanoparticle catalysts in industrially relevant acidic and alkaline membrane electrode assemblies that are only operated with pure water humidified gases. We discover distinct kinetic fingerprints between the iron triad (Fe, Ni, Co), coinage (Au, Cu, Ag), and platinum group metals (Ir, Pt, Pd, Rh). Importantly, the applied bias changes not only the activation energy (EA) but also the pre-exponential factor (A). We interpret these changes as entropic changes in the interfacial solvent that differ between acid and base and entropic changes on the surface due to a changing hydrogen coverage. Finally, we observe that anions can induce Butler-Volmer behavior for the coinage metals in acid. Our results provide a new foundation to understand HER kinetics and, more broadly, highlight the pressing need to update common understanding of basic concepts in the field of electrocatalysis.

Electric-Field Catalysis on Carbon Nanotubes in Electromicrofluidic Reactors: Monoterpene Cyclizations

The control over the movement of electrons during chemical reactions with oriented external electric fields (OEEFs) has been predicted to offer a general approach to catalysis. Recently, we suggested that many problems to realize electric-field catalysis in practice under scalable bulk conditions could possibly be solved on multiwalled carbon nanotubes in electromicrofluidic reactors. Here, we selected monoterpene cyclizations to assess the scope of our system in organic synthesis. We report that electric-field catalysis can function by stabilizing both anionic and cationic transition states, depending on the orientation of the applied field. Moreover, electric-field catalysis can promote reactions which are barely accessible by general Brønsted and Lewis acids and field-free anion-π and cation-π interactions, and drive chemoselectivity toward intrinsically disfavored products without the need for pyrene interfacers attached to the substrate to prolong binding to the carbon nanotubes. Finally, interfacing with chiral organocatalysts is explored and evidence against contributions from redox chemistry is provided.

An Organic Molecular Mimetic Metal-Free Heterogeneous Catalyst for Electrocatalytic Alkyne Semihydrogenation

The direct construction of metal-free catalysts on conductive substrates for electrocatalytic organic hydrogenation reactions is significant but still unexplored. Here, learning from the homogeneous molecular catalysts, an organic molecular mimetic metal-free heterogeneous catalyst is designed and constructed in situ on a graphite flake electrode via a mild electrochemical oxidation-reduction relay strategy. The as-prepared -COOH- and -OH-functionalized metal-free catalyst exhibits an electrocatalytic alkyne semihydrogenation performance with a 72 % Faradaic efficiency, 99 % selectivity and 96 % yield of the alkene product, which is comparable to that of noble metal catalysts. The removal of these oxygen-containing groups leads to negligible activity. The experimental and calculation results reveal that the origin of the high activity can be assigned to the -COOH and -OH groups on graphite. A flow electrolytic cell delivers ten grams of hydrogenated products with 81 % Faradaic efficiency. This metal-free catalyst is also suitable for gas-phase acetylene semihydrogenation and other electrocatalytic hydrogenation reactions.

Multiplexed Fluoro-electrochemical Single-Molecule Counting Enabled by SiC Semiconducting Nanofilm

A major challenge for ultrasensitive analysis is the high-efficiency determination of different target single molecules in parallel with high accuracy. Herein, we developed a quantitative fluoro-electrochemical imaging approach for direct multiplexed single-molecule counting with a SiC-nanofilm-modified indium tin oxide transparent electrode. The nanofilm could control local pH through proton-coupled electron transfer in a lower potential range and further induce direct electrochemical oxidation of the dye molecules with a higher applied potential. The fluoro-electrochemical responses of immobilized single molecules with different pH values and redox behaviors could thus be distinguished within the same fluorescence channels. This method yields nonamplified direct counting of single molecules, as indicated by excellent linear responses in the picomolar range. The successful distinction of seven different randomly mixed dyes underscores the versatility and efficacy of the proposed method in the highly accurate determination of single dye molecules, paving the way for highly parallel single-molecule detection for diverse applications.

Activating TiO2 through the Phase Transition-Mediated Hydrogen Spillover to Outperform Pt for Electrocatalytic pH-Universal Hydrogen Evolution

Endowing conventional materials with specific functions that are hardly available is invariably of significant importance but greatly challenging. TiO2 is proven to be highly active for the photocatalytic hydrogen evolution while intrinsically inert for electrocatalytic hydrogen evolution reaction (HER) due to its poor electrical conductivity and unfavorable hydrogen adsorption/desorption behavior. Herein, the first activation of inert TiO2 for electrocatalytic HER is demonstrated by synergistically modulating the positions of d-band center and triggering hydrogen spillover through the dual doping-induced partial phase transition. The N, F co-doping-induced partial phase transition from anatase to rutile phase in TiO2 (AR-TiO2|(N,F)) exhibits extraordinary HER performance with overpotentials of 74, 80, and 142 mV at a current density of 10 mA cm-2 in 1.0 M KOH, 0.5 M H2SO4, and 1.0 M phosphate-buffered saline electrolytes, respectively, which are substantially better than pure TiO2, and even superior to the benchmark Pt/C catalysts. These findings may open a new avenue for the development of low-cost alternative to noble metal catalysts for electrocatalytic hydrogen production.

Ultrafast Interfacial Self-Assembly toward Bioderived Polyester COF Membranes with Microstructure Optimization

The precise manipulation of the microstructure (pore size, free volume distribution, and connectivity of the free-volume elements), thickness, and mechanical characteristics of membranes holds paramount significance in facilitating the effective utilization of self-standing membranes. In this contribution, the synthesis of two innovative ester-linked covalent-organic framework (COF) membranes is first reported, which are generated through the selection of plant-derived ellagic acid and quercetin phenolic monomers in conjunction with terephthaloyl chloride as a building block. The optimization of the microstructure of these two COF membranes is systematically achieved through the application of three different interfacial electric field systems: electric neutrality, positive electricity, and negative electricity. It is observed that the positively charged system facilitates a record increase in the rate of membrane formation, resulting in a denser membrane with a uniform pore size and enhanced flexibility. In addition, a correlation is identified wherein an increase in the alkyl chain length of the surfactants leads to a more uniform pore size and a decrease in the molecular weight cutoff of the COF membrane. The resulting COF membrane exhibits an unprecedented combination of high water permeance, superior sieving capability, robust mechanical strength, chemical robustness for promising membrane-based separation science and technology.

Highly Selective Electrochemical Baeyer-Villiger Oxidation through Oxygen Atom Transfer from Water

The Baeyer-Villiger oxidation of ketones is a crucial oxygen atom transfer (OAT) process used for ester production. Traditionally, Baeyer-Villiger oxidation is accomplished by thermally oxidizing the OAT from stoichiometric peroxides, which are often difficult to handle. Electrochemical methods hold promise for breaking the limitation of using water as the oxygen atom source. Nevertheless, existing demonstrations of electrochemical Baeyer-Villiger oxidation face the challenges of low selectivity. We report in this study a strategy to overcome this challenge. By employing a well-known water oxidation catalyst, Fe2O3, we achieved nearly perfect selectivity for the electrochemical Baeyer-Villiger oxidation of cyclohexanone. Mechanistic studies suggest that it is essential to produce surface hydroperoxo intermediates (M-OOH, where M represents a metal center) that promote the nucleophilic attack on ketone substrates. By confining the reactions to the catalyst surfaces, competing reactions (e.g., dehydrogenation, carboxylic acid cation rearrangements, and hydroxylation) are greatly limited, thereby offering high selectivity. The surface-initiated nature of the reaction is confirmed by kinetic studies and spectroelectrochemical characterizations. This discovery adds nucleophilic oxidation to the toolbox of electrochemical organic synthesis.