Kinetics Of Proton Transfer Research Articles

Cobalt single-site molecular Catalyst-mediated electrochemical Hydrodechlorination for detoxification of halogenated Antibiotics: Performance, reaction pathway and mechanism

Electrocatalytic hydrodehalogenation (ECHD) offers a promising approach for detoxification of the halogenated antibiotics via hydrogenolysis of the C-X bonds (X = F, Cl or Br). Herein we demonstrated that the commercially-available cobalt phthalocyanine (CoPc) molecules with a high-loading of −[Co-N4]- moieties (Co wt%: 10.8 %) were exceptionally active for hydrogenolysis of the C-Cl bonds in florfenicol (FLO). It afforded an impressive mass activity of 6.2 gFLO h−1 g-1Co, a low Co leaching of 0.34 μg/L and a strong tolerance to interference from impurities in water, significantly surpassing reported Co/Fe/Ni-based particle and single-site counterparts. Mechanistic studies revealed that the ECHD on CoPc underwent a direct electron transfer manner rather than the atomic hydrogen (H*)-mediated indirect way, and the ECHD efficacy was affected by both the electron and proton transfer kinetics. The single-atom Co, with its d orbitals constituting the lowest unoccupied molecular orbitals of CoPc, served as the primary active site. It facilitated both electron and proton transfer, as well as the cleavage of the C-Cl bond but not the C-F bond. In a continuous-flow cell, the CoPc/C-driven ECHD reduced the antibacterial activity of a FLO-contaminated natural lake water sample by 90.8 % with an energy consumption of 0.19 kwh g-1FLO. This study highlighted great promise of the single-atom metal-bearing molecular catalyst toward the sustainable detoxification of halogenated antibiotics-contaminated water.

Influence of active hydrogen on pathway selection in electrochemical nitrate reduction

Electrochemical nitrate reduction reaction in alkaline condition involves two reactants, the nitrate () and the water (H2O). Although the significance of the active *H species produced from the dissociation of H2O has been proved, the correlation between the reaction pathways and the *H species is often overlooked. Herein, Co(OH)2-CoP supported Ru nanoclusters is designed for electrocatalytic nitrate reduction and shows a record-high faradaic efficiency of 99.7% at an ultralow potential of 0.1 V vs. RHE. Experiments and theoretical calculations reveal that in addition to the faster proton transfer kinetics, the reaction pathway is strongly correlated with *H supply with the aid of CoP, that is, the direct hydrogenation of *NOH instead of deprotonation over Ru sites with the lowest energy barrier is promoted with the moderate production of *H species. This work provides new insights into the impact of *H species on the thermodynamics and kinetics of electrocatalytic nitrate reduction.

Polyelectrolyte Additive-Modulated Interfacial Microenvironment Boosting CO2 Electrolysis in Acid.

Acidic CO2 electrolysis offers a promising strategy to achieve high carbon utilization and high energy efficiency. However, challenges still remain in suppressing the competitive hydrogen evolution reaction (HER) and improving product selectivity. Although high concentrations of potassium ions (K+) can suppress HER and accelerate CO2 reduction, they still inevitably suffer from salt precipitation problems. In this study, we demonstrate that the sulfonate-based polyelectrolyte, polystyrene sulfonate (PSS), enables to reconstruct the electrode-electrolyte interface to significantly enhance the acidic CO2 electrolysis. Mechanistic studies reveal that PSS induces high local K+ concentrations through the electrostatic interaction between PSS anions and K+. In situ spectroscopy reveals that PSS reshapes the interfacial hydrogen-bond (H-bond) network, which is attributed to the H-bonds between PSS anions and hydrated proton, as well as the steric hindrance of the additive molecules. This greatly weakens proton transfer kinetics and leads to the suppression of undesirable HER. As a result, a Faradaic efficiency of 93.9 % for CO can be achieved at 250 mA cm-2, simultaneous with a high single-pass carbon efficiency of 72.2 % on commercial Ag catalysts in acid. This study highlights the important role of the electrode-electrolyte interface induced by polyelectrolyte additives in promoting electrocatalytic reactions.

Enhancing Oxygen Reduction Kinetics and Proton Transfer of La0.6Sr0.4Co0.2Fe0.8O3−δ Cathode through Pr2Ni0.5Co0.5O4−δ Impregnation for Protonic Ceramic Fuel Cells

AbstractSluggish reaction kinetics in oxygen reduction reaction (ORR) is one of the most important challenges to the development of protonic ceramic fuel cells (PCFCs). La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) exhibits high mixed ionic–electronic conductivity in traditional solid oxide fuel cells, but their slow proton transfer and ORR kinetics impedes practical applications. Herein, composite Pr2Ni0.5Co0.5O4−δ (PNC) particles composed of a perovskite PrNi0.5Co0.5O3−δ phase and a PrO2 phase are impregnated into a LSCF cathode to enhance the ORR activity and proton transfer. The polarization tested in a symmetric cell with PNC‐impregnated LSCF cathode is 0.06 Ω cm2 at 700 °C. The fuel cell with this impregnated cathode shows maximum power densities of 1857 mW cm−2 at 700 °C. Moreover, the impregnated cathode exhibits a low degradation rate in the durability test. This work not only provides a novel and practical approach to improving the performance of current cathode materials for PCFCs but also highlights the potential for enhancing the commercial viability of PCFC technology.

Zn-O-Sn covalency interface governs the intrinsic activity of the Zn2SnO4/SnO2 heterostructure for boosting hydrogen peroxide production

Electrocatalytic production of hydrogen peroxide (H2O2) from oxygen is vital for sustainable clean fuel. Developing highly active, stable, and inexpensive catalysts for H2O2 remains a top priority but is still challenging. In this study, an atomic-shared heterointerface was constructed between Zn2SnO4 and SnO2 via a self-templating co-precipitation method. Theoretical and experimental analysis confirms the delocalization effects of electrons within the Zn2SnO4/SnO2 spinel structure. The resulting Zn-O-Sn bridge (asymmetric sites) modulates electronic transport at the interface, and the intensified Zn-O bond facilitates proton adsorption and transfer kinetics. Moreover, strong interface coupling between Zn2SnO4 and SnO2 enhances electron transfer, which effectively promotes the *OOH intermediate favorable for H2O2 production, boosting H2O2 selectivity of nearly 97 %. The high production rate of 8.19 mol gcat−1 h−1 and excellent stability during long-term operation over 24 h. This work provides strategic insights into developing efficient H2O2 catalysts and other catalytic applications.

Polyelectrolyte Additive‐Modulated Interfacial Microenvironment Boosting CO2 Electrolysis in Acid

Acidic CO2 electrolysis offers a promising strategy to achieve high carbon utilization and high energy efficiency. However, challenges remain in suppressing the competitive hydrogen evolution reaction (HER) and improving product selectivity. High concentrations of potassium ions (K+) can suppress HER and accelerate CO2 reduction, but they still inevitably suffer from salt precipitation problems. In this study, we demonstrate that the sulfonate‐based polyelectrolyte, polystyrene sulfonate (PSS), enables to reconstruct the electrode‐electrolyte interface to significantly enhance the acidic CO2 electrolysis. Mechanistic studies reveal that PSS induces high local K+ concentrations through electrostatic interaction between PSS anions and K+. In situ spectroscopy reveals that PSS reshapes the interfacial hydrogen–bond (H–bond) network, which is attributed to the H–bonds between PSS anions and hydrated proton as well as the steric hindrance of the additive molecules. This greatly weakens proton transfer kinetics and leads to the suppression of undesirable HER. As a result, a Faradaic efficiency of 93.9% for CO can be achieved at 250 mA cm–2, simultaneous with a high single‐pass carbon efficiency of 72.2% on commercial Ag catalysts in acid. This study highlights the important role of the electrode‐electrolyte interface induced by polyelectrolyte additives in promoting electrocatalytic reactions.

Proton Transfer Kinetics at a Defect-Free Liquid-Liquid Interface

Proton transfer at electrochemical interfaces is fundamentally important for many areas of science and technology, yet kinetic measurements of this elementary step are often convoluted by inhomogeneous electrode surface structures. We show that facilitated proton transfer at the interface between two immiscible electrolyte solutions (ITIES) can serve as a model system to study proton transfer kinetics in the absence of defects found at solid|electrolyte interfaces. Diffusion-controlled micropipette voltammetry revealed that 2,6-diphenylpyridine (DPP) facilitated direct proton transfer across the liquid|liquid interface and voltammetry at nanopipette-supported interfaces yielded activation-controlled ion transfer currents. The exchange current density for proton transfer at the ITIES is comparable to exchange current densities recently reported for H-UPD on Pt (111) and provides the first direct means of comparing ion transfer kinetics between solid|liquid and liquid|liquid interfaces, supporting the development of a general theory of ion transfer kinetics.

Proton Transfer Kinetics in Histidine Side Chains Determined by pH-Dependent Multi-Nuclear NMR Relaxation.

Histidine is a key amino-acid residue in proteins with unique properties engendered by its imidazole side chain that can exist in three different states: two different neutral tautomeric forms and a protonated, positively charged one with a pKa value close to physiological pH. Commonly, two or all three states coexist and interchange rapidly, enabling histidine to act as both donor and acceptor of hydrogen bonds, coordinate metal ions, and engage in acid/base catalysis. Understanding the exchange dynamics among the three states is critical for assessing histidine's mechanistic role in catalysis, where the rate of proton exchange and interconversion among tautomers might be rate limiting for turnover. Here, we determine the exchange kinetics of histidine residues with pKa values representative of the accessible range from 5 to 9 by measuring pH-dependent 15N, 13C, and 1H transverse relaxation rate constants for 5 nuclei in each imidazole. Proton exchange between the imidazole and the solvent is mediated by hydronium ions at acidic and neutral pH, whereas hydroxide mediated exchange becomes the dominant mechanism at basic pH. Proton transfer is very fast and reaches the diffusion limit for pKa values near neutral pH. We identify a direct pathway between the two tautomeric forms, likely mediated by a bridging water molecule or, in the case of high pH, hydroxide ion. For histidines with pKa 7, we determine all rate constants (lifetimes) involving protonation over the entire pH range. Our approach should enable critical insights into enzymatic acid/base catalyzed reactions involving histidines in proteins.

Manipulating electrochemical CO2 reduction pathway by engineering energy level of Cu/Zn dual-metal single atom catalysts

Dual-metal single atom catalysts (DSACs) demonstrate outstanding catalytic performance in the CO2 reduction reaction. However, the complexity of catalyst structures and interface environments has led to an unclear understanding of the synergistic catalysis mechanism involved. Here, we developed a series of catalysts with atomically dispersed Cu–N4 and Zn–N4 sites on N-doped carbon supports (Cu1-xZnx-NC), which can efficiently reduce CO2 to C2 products by optimizing the Zn loading. Specifically, a remarkable faradaic efficiency of 65 % at −1.1 VRHE for the C2 products on the Cu0.75Zn0.25-NC catalyst has been obtained. Structural characterizations and density functional theory calculations demonstrate that Cu/Zn DSACs synergistically lower the energy barrier of the rate-determining step and facilitate proton transfer kinetics. Among them, Zn atoms activate CO2, while Cu atoms aid in the adsorption and dissociation of intermediates. The electron energy redistribution induced by adjacent Cu–N4 and Zn–N4 site optimizes the 3d orbitals of Cu centers, thereby accelerating and promoting C–C coupling. The energy level optimization of Cu/Zn DSACs results in Cu0.75Zn0.25-NC with a high C2 selectivity and activity, improved kinetics. In summary, these findings provide new insights into the development of highly efficient DSACs and its synergistic catalysis mechanism in CO2 reduction.

Mechanistic Insights into Surfactant-Modulated Electrode-Electrolyte Interface for Steering H2O2 Electrosynthesis.

Electrocatalytic reactions taking place at the electrified electrode-electrolyte interface involve processes of proton-coupled electron transfer. Interfacial protons are delivered to the electrode surface via a H2O-dominated hydrogen-bond network. Less efforts are made to regulate the interfacial proton transfer from the perspective of interfacial hydrogen-bond network. Here, we present quaternary ammonium salt cationic surfactants as electrolyte additives for enhancing the H2O2 selectivity of the oxygen reduction reaction (ORR). Through in situ vibrational spectroscopy and molecular dynamics calculation, it is revealed that the surfactants are irreversibly adsorbed on the electrode surface in response to a given bias potential range, leading to the weakening of the interfacial hydrogen-bond network. This decreases interfacial proton transfer kinetics, particularly at high bias potentials, thus suppressing the 4-electron ORR pathway and achieving a highly selective 2-electron pathway toward H2O2. These results highlight the opportunity for steering H2O-involved electrochemical reactions via modulating the interfacial hydrogen-bond network.

Blinking characteristics of organic fluorophores for blink-based multiplexing

Single-molecule fluorescence experiments have transformed our understanding of complex materials and biological systems. Whether single molecules are used to report on their nano-environment or provide for localization, understanding their blinking dynamics (i.e., stochastic fluctuations in emission intensity under continuous illumination) is paramount. We recently demonstrated another use for blinking dynamics called blink-based multiplexing (BBM), where individual emitters are classified using a single excitation laser based on blinking dynamics, rather than color. This study elucidates the structure-activity relationships governing BBM performance in a series of model rhodamine, BODIPY, and anthraquinone fluorophores that undergo different photo-physical and-chemical processes during blinking. Change point detection and multinomial logistic regression analyses show that BBM can leverage spectral fluctuations, electron and proton transfer kinetics, as well as photostability for molecular classification—even within the context of a shared blinking mechanism. In doing so, we demonstrate two- and three-color BBM with ≥ 93% accuracy using spectrally-overlapped fluorophores.

Quantum tunnelling effect in the cis-trans isomerization of uranyl tetrahydroxide.

The role of quantum tunnelling (QT) in the proton transfer kinetics of the uranyl tetrahydroxide (UTH, [UO2(OH)4]2-) cis to trans isomerization was computationally studied under three possible reaction pathways. The first pathway involved a direct proton transfer from the hydroxide ligand to the oxo atom. In the other two pathways, one or two water molecules were added to the second sphere. The first H2O, bound by hydrogen bonds to the ligands, acts as a bridge enabling a proton shuttling, a concerted hopping of a proton from the hydroxide to the oxo atom similar to the Grotthuss mechanism. In the third pathway, the second water molecule does not participate in the H-transfer chain, but works as an anchor for the first water molecule, limiting its movement and therefore enhancing the QT. Since experimentally the reaction occurs in water, the first two pathways (no water or one H2O) serve only as models of the gas phase behaviour, while the third pathway will always be thermodynamically and kinetically preferred. The effects were investigated in the gas phase as well as in a continuum aqueous model, including the H/D Kinetic Isotope Effect (KIE). The results indicate that at very low temperatures, QT is the only mechanism that permits the reaction kinetics, consistent with the large computed KIE. At higher temperatures, thermally activated tunnelling competes with the classical crossing over the potential barrier.

The nature of extraframework aluminum species and Brønsted acid site interactions under catalytic operating conditions

A systematic investigation of hydrated extraframework aluminum (EFAl) species interacting with Brønsted acid sites (BAS) in H-ZSM-5 is presented to understand the cooperative active site structure under catalytic operating conditions. Static models of EFAl species confined in the H-ZSM-5 unit cell show that isolated BAS protonate neutral EFAl species to form cations. Ab-initio molecular dynamics (AIMD) simulations and enhanced sampling performed at the temperature for methanol-to-hydrocarbon conversion reveal two regimes of stable EFAl species, namely the [Al(OH)2]+ ion existing with two bonds to the zeolite scaffold or as a pore-guest in the form of [Al(OH)2(H2O)2]+. Our results indicate that hydrogen-bonding plays a significant role in BAS-EFAl active site structure, especially at higher BAS density and that EFAl species can function as both Bronsted and Lewis acidic components to alter proton transfer kinetics as well as shape selectivity within these microporous solids.

Design of a perovskite oxide cathode for a protonic ceramic fuel cell

High catalytic activity, low-cost and stable cathode in a temperature range 550–700 °C is essential for the development of protonic ceramic fuel cells (PCFCs). Doping nickel into perovskite La0.5Sr0.5MnO3-δ(LSM) is designed as a cobalt-free cathode based on theoretical calculations and experiments. La0.5Sr0.5Mn0.9Ni0.1O3-δ (LSMNi) as cathode shows higher proton conductivity and ORR activity than the undoped LSM. The PCFCs with LSMNi exhibit low polarization resistance and high peak power density 1.1 W cm−2 at 700 °C. The density functional theory simulations indicate that doping with nickel decreases the oxygen vacancy formation energy and promotes the formation of hydroxide defects. The decrease in proton transfer energy barriers and hydration energy improves the proton conductivity. The improved performance is attributed to fast proton transfer and rapid kinetics of oxygen reduction on the surface of LSMNi. This work provides a novel approach to design cobalt-free cathode for a protonic ceramic fuel cell.

Constructing asymmetric double-atomic sites for synergistic catalysis of electrochemical CO2 reduction

Elucidating the synergistic catalytic mechanism between multiple active centers is of great significance for heterogeneous catalysis; however, finding the corresponding experimental evidence remains challenging owing to the complexity of catalyst structures and interface environment. Here we construct an asymmetric TeN2–CuN3 double-atomic site catalyst, which is analyzed via full-range synchrotron pair distribution function. In electrochemical CO2 reduction, the catalyst features a synergistic mechanism with the double-atomic site activating two key molecules: operando spectroscopy confirms that the Te center activates CO2, and the Cu center helps to dissociate H2O. The experimental and theoretical results reveal that the TeN2–CuN3 could cooperatively lower the energy barriers for the rate-determining step, promoting proton transfer kinetics. Therefore, the TeN2–CuN3 displays a broad potential range with high CO selectivity, improved kinetics and good stability. This work presents synthesis and characterization strategies for double-atomic site catalysts, and experimentally unveils the underpinning mechanism of synergistic catalysis.

Modulating the proton transfer kinetics via Ru single atoms for highly efficient ammonia synthesis

Modulating the proton transfer kinetics via Ru single atoms for highly efficient ammonia synthesis

Discrete Dynamics of Warhead Modulation on Covalent Inhibition of Oxyr: A QM/MM Study.

The bacterial transcriptional factor OxyR, a peroxide sensor conserved in bacterial virulence pathways, has the capability to exhibit exceptional reactivity toward hydrogen peroxide (H2O2). H2O2 is essential for oxidizing cysteine thiolates to maintain cellular redox homeostasis and is dispensable for bacterial growth that can potentially mitigate drug resistance, thus underlining OxyR as a valuable target. We employ quantum mechanics/molecular mechanics (QM/MM) umbrella sampling (US) simulations at the DFTB3/MM level of theory and propose a reaction mechanism with four potential covalent inhibitors. The potential of mean force reveals the direct role of intrinsic reactivity of inhibitors, for instance, benzothiophenes and modified experimental inhibitors with methyl oxo-enoate warhead-activated carbonyl samples in the first step of reaction, which shed light on the significance of proton transfer indispensable for full inhibition, whereas the nitrile inhibitor undergoes a stepwise mechanism with a small proton-transfer energy barrier and lower imaginary frequencies that materialize instantly after nucleophilic attack. To unveil the molecular determinants of respective binding affinities, transition states along the reaction path are optimized and characterized with B3LYP 6-31+G(d,p). Furthermore, the post-simulation analysis indicates the catalytic triad (His130/Cys199/Thr129), thermodynamically favored for inhibition, which restricts water molecules from acting as the potential source of protonation/deprotonation. This study thus serves as a preamble to add variation in the proposed structures and unveils the impact of functional groups lying in warheads that modulate the kinetics of proton transfer, which will certainly aid to design more selective and efficient irreversible inhibitors of OxyR.

Proton transfer kinetics of transition metal hydride complexes and implications for fuel-forming reactions.

Proton transfer reactions involving transition metal hydride complexes are prevalent in a number of catalytic fuel-forming reactions, where the proton transfer kinetics to or from the metal center can have significant impacts on the efficiency, selectivity, and stability associated with the catalytic cycle. This review correlates the often slow proton transfer rate constants of transition metal hydride complexes to their electronic and structural descriptors and provides perspective on how to exploit these parameters to control proton transfer kinetics to and from the metal center. A toolbox of techniques for experimental determination of proton transfer rate constants is discussed, and case studies where proton transfer rate constant determination informs fuel-forming reactions are highlighted. Opportunities for extending proton transfer kinetic measurements to additional systems are presented, and the importance of synergizing the thermodynamics and kinetics of proton transfer involving transition metal hydride complexes is emphasized.

Electrochemical hydrogen isotope exchange of amines controlled by alternating current frequency.

Here, we report an electrochemical protocol for hydrogen isotope exchange (HIE) at α-C(sp3)-H amine sites. Tetrahydroisoquinoline and pyrrolidine are selected as two model substrates because of their different proton transfer (PT) and hydrogen atom transfer (HAT) kinetics at the α-C(sp3)-H amine sites, which are utilized to control the HIE reaction outcome at different applied alternating current (AC) frequencies. We found the highest deuterium incorporation for tetrahydroisoquinolines at 0 Hz (i.e., under direct current (DC) electrolysis conditions) and pyrrolidines at 0.5 Hz. Analysis of the product distribution and D isotope incorporation at different frequencies reveals that the HIE of tetrahydroisoquinolines is limited by its slow HAT, whereas the HIE of pyrrolidines is limited by the overoxidation of its α-amino radical intermediates. The AC-frequency-dependent HIE of amines can be potentially used to achieve selective labeling of α-amine sites in one drug molecule, which will significantly impact the pharmaceutical industry.

Effect of Proton Transfer on Electrocatalytic Water Oxidation by Manganese Phosphates

AbstractThe effect of proton transfer on water oxidation has hardly been measurably established in heterogeneous electrocatalysts. Herein, two isomorphous manganese phosphates (NH4MnPO4 ⋅ H2O and KMnPO4 ⋅ H2O) were designed to form an ideal platform to study the effect of proton transfer on water oxidation. The hydrogen‐bonding network in NH4MnPO4 ⋅ H2O has been proven to be solely responsible for its better activity. The differences of the proton transfer kinetics in the two materials indicate a fast proton hopping transfer process with a low activation energy in NH4MnPO4 ⋅ H2O. In addition, the hydrogen‐bonding network can effectively promote the proton transfer between adjacent Mn sites and further stabilize the MnIII−OH intermediates. The faster proton transfer results in a higher proportion of zeroth‐order in [H+] for OER. Thus, proton transfer‐affected electrocatalytic water oxidation has been measurably observed to bring detailed insights into the mechanism of water oxidation.