Reaction Pathways Research Articles

Finite Difference Model of Substrate Channeling in TCA Cycle Enzymes

The Tricarboxylic Acid (TCA) cycle is a highly optimized metabolic pathway. Of particular interest in the TCA cycle is the malate dehydrogenase—citrate synthase (MDH-CS) complex, which accomplishes citrate production from malate with an oxaloacetate intermediate. This complex was studied experimentally by Bulutoglu et al, who showed that mutation of positively-charged residues on the complex surface effected the dynamic response of citrate production as measured by lag time experiments.1 More recently, we have created a Markov State model that was used to predict the transfer efficiency of each complex and determine the reaction pathways on the surface.2 Utilizing the kinetic parameters of both the recombinant and mutant complexes determined experimentally1and Markov state transition matrix produced by previous modeling2, we report a finite difference model to calculate time trajectories of the surface and bulk occupancies by OAA. Solution of a system of differential mass balance equations, based on the transient matrix, provide the occupancy probability at each node in the network. The lag time of MDH-CS was determined computationally to be comparable to experiment for both the recombinant and mutant complex with lag times of 0.44 and 2.1 seconds, respectively.1 Using the model, the dynamics of each of the four possible reaction pathways between the the two source (MDH) active sites and the two sink (CS) sites could be studied independently. This analysis provides a dynamic model for intermediate transport in an electrostatically channeled system, and can be used as a predictive tool to provide mechanistic insight into pathway dominance. We gratefully acknowledge support from the Army Research Office MURI (#W911NF1410263) via The University of Utah. References B. Bulutoglu, K. E. Garcia, F. Wu, S. D. Minteer, and S. Banta, ACS Chemical Biology, 11, 2847–53 (2016). doi:10.1021/acschembio.6b00523Y. Xie, S. D. Minteer, S. Banta, and S. Calabrese Barton, ACS Nanoscience Au, 2, 414-421 (2022). doi:10.1021/acsnanoscienceau.2c00011 Figure 1

Biocatalytic Carbenoid N-H Insertions Via Engineered Myoglobins: A Systematic Reaction Mechanism Study

Recently engineered myoglobins have demonstrated excellent biocatalytic activities for a broad range of carbenoid N-H insertion reactions with up to >99% yield. Such biocatalysts offer various tuning points beyond protein residues, such as metal center, and axial ligand, to regulate different chemical reactivities. However, there is no computational mechanistic study to reveal its basic reaction mechanism, which could be utilized to study effects of different catalyst component for rational biocatalyst design. The reported computational mechanistic papers of other heme protein catalyzed N-H insertion have limited information without a careful comparison of all possible pathways and just focused on certain steps. Building on our group’s recent mechanistic work on a number of heme carbene transfer reactions, we performed a quantum chemical study to systematically study all possible reaction pathways for myoglobin catalyzed N-H insertions, which starts from the reactants to final released products and thus constitute a complete reaction pathway study. We examined ylide (proton transfer), hydrogen atom transfer, and hydride transfer pathways. In the favored ylide pathways, we calculated the direct proton transfer from amine to carbene and indirect paths with both concerted and stepwise proton transfer and dissociation components. For the indirect pathways in which the proton resides on the carbonyl oxygen of the original carbene substituent, subsequent water-assisted proton transfer from this carbonyl oxygen to carbene’s carbon to form the final product was also investigated. The most favorable pathway from this systematic reaction mechanism study was further supported by new experimental work from our collaborators, the original developer of myoglobin-based biocatalysts. These novel mechanistic results provide important mechanistic information for future biocatalyst design with enhanced reactivities.

Reactant Discovery with an Ab Initio Nanoreactor: Exploration of Astrophysical N-Heterocycle Precursors and Formation Pathways.

The incorporation of nitrogen atoms into cyclic compounds is essential for terrestrial life; nitrogen-containing (N-)heterocycles make up DNA and RNA nucleobases, several amino acids, B vitamins, porphyrins, and other components of biomolecules. The discovery of these molecules on meteorites with non-terrestrial isotopic abundances supports the hypothesis of exogenous delivery of prebiotic material to early Earth; however, there has been no detection of these species in interstellar environments, indicating that there is a need for greater knowledge of their astrochemical formation and destruction pathways. Here, we present results of simulations of gas-phase pyrrole and pyridine formation from an ab initio nanoreactor, a first-principles molecular dynamics simulation method that accelerates reaction discovery by applying non-equilibrium forces that are agnostic to individual reaction coordinates. Using the nanoreactor in a retrosynthetic mode, starting with the N-heterocycle of interest and a radical leaving group, then considering the discovered reaction pathways in reverse, a rich landscape of N-heterocycle-forming reactivity can be found. Several of these reaction pathways, when mapped to their corresponding minimum energy paths, correspond to novel barrierless formation pathways for pyridine and pyrrole, starting from both detected and hypothesized astrochemical precursors. This study demonstrates how first-principles reaction discovery can build mechanistic knowledge in astrochemical environments as well as in early Earth models such as Titan's atmosphere where N-heterocycles have been tentatively detected.

Understanding Lithium-Gold Electrochemical Reactions Using Operando X-Ray Diffraction

Gold electrodes are used in lithium-based battery research as catalysts and micro / quasi-reference electrodes despite their high cost and unclear reactivity with lithium. Here we present an investigation of the electrochemical reactions of gold-lithium (Au-Li) mixtures using variable-temperature electrochemistry and operando X-ray diffraction.Multiple solid solutions (alpha, beta, delta) and intermetallic phases (AuLi3, Au4Li15) are expected based on the equilibrium phase diagram.1 Previous electrochemical investigations conclusively identified only AuLi3;2,3 alpha and beta may be bypassed during lithium insertion, and the structure of delta is unclear.4 We are unaware of any previous reports of the electrochemical formation of Au4Li15. Metastable crystallographic solutions have been proposed.5 X-ray diffraction patterns collected during the galvanostatic lithiation of gold leaf (at 60 °C and a rate of ca. C/50) are provided in Figure 1. Reference diffraction patterns and the associated Powder Diffraction File card number are indicated, along with proposed phases.4,5 X-ray and electrochemical data align with earlier results suggesting that the equilibrium alpha and beta phases are seemingly bypassed during the first insertion. Delta is the first phase to form at a potential of 0.25 V vs. Li/Li+, followed by the formation of AuLi3 at 0.15 V vs. Li/Li+ at all conditions and Au4Li15 at 0.05 V vs. Li/Li+ in select conditions. Electrochemical data suggests AuLi3 may also have a small range of solubility. Peaks associated with AuLi3 and Au4Li15 align with existing literature but peaks associated with delta do not. Reactions are reversible to delta, followed by the formation of another phase near 0.3 V vs. Li/Li+ and alpha at potentials above 0.4 V vs. Li/Li+ during lithium removal. Significant hysteresis is present in cell potential and reaction pathways in the low-lithium content region. The second cycle (not shown) is reversible between alpha (not pure gold) and the terminal phase without a significant loss in capacity. Alternative crystallographic unit cell(s) for delta and other gold-lithium phases will be described.While the delta / AuLi3 reaction exhibited a consistent potential during lithium insertion and removal, the potential otherwise varied strongly with temperature, rate, and composition, implying that gold quasi-reference electrodes may not be suitable for lithium-ion battery research.

(Invited) Plasmon-Driven Nitrogen Photofixation

Ammonia is one of the most important chemicals as well as an efficient energy carrier. The nitrogen element is indispensable for nearly all living species on the earth. However, dinitrogen molecules that are abundant in the atmosphere cannot be used directly by living species. They need to be converted into ammonia or nitrate for use. Such conversion is known as nitrogen fixation. Because natural nitrogen fixation is insufficient for sustaining all population on the earth, the industrial Haber-Bosch process has been developed for producing ammonia from dinitrogen and dihydrogen. The process consumes a large amount of energy and generates a large amount of carbon dioxide. Photocatalytic nitrogen fixation can operate under mild conditions and uses solar energy and water. It offers an intriguing alternative approach for the conversion of atmospheric dinitrogen into ammonia.Plasmonic metal nanocrystals can effectively absorb light and efficiently generate hot charge carriers. Compared to traditional interband excitation in semiconductors, plasmon excitation offers a new approach for generating hot charge carriers, which can thereafter be utilized for various physical and chemical processes. The use of plasmonic hot charge carriers for photocatalysis has become a hot research topic in the recent years. Plasmonic hot charge carriers can not only enhance the reaction yield and selectivity, but also introduce new reaction pathways. Moreover, the plasmon resonance wavelengths of various nanoparticles can be synthetically adjusted over a broad range that surpasses the solar spectrum.We have demonstrated the powerfulness of localized plasmon resonance on driving photocatalytic reduction of atmospheric nitrogen, where plasmonic hot electrons play an essential role. First, Au nanoparticles are anchored on ultrathin titania nanosheets with oxygen vacancies. The oxygen vacancies chemisorb and activate nitrogen molecules, which are subsequently reduced to ammonia by plasmonic hot electrons generated from the Au nanoparticles. Our synthesized all-inorganic hybrid structures mimic the function of biological nitrogenase enzymes, where the plasmonic Au nanoparticles play the role of the Fe proteins and the titania nanosheets play the role of the MoFe proteins. Second, we have extended the biomimetic idea into another popular semiconductor. A plasmonic hybrid catalyst is produced by uniformly embedding Au nanoparticles in the mesopores of nitrogen-deficient hollow carbon nitride spheres. The carbon nitride spheres possess abundant nitrogen vacancies, which serve as the sites for nitrogen chemisorption and activation, and capture photoexcited electrons stemmed from the carbon nitride spheres and the plasmon resonance of the embedded Au nanoparticles for the efficient reduction of the adsorbed nitrogen molecules to ammonia. Third, when metal-semiconductor hybrid nanostructures are designed as photocatalysts for nitrogen fixation, the introduction of a Schottky barrier at the junction is unavoidable. This Schottky barrier will undoubtedly lower the utilization efficiency of plasmonic charge carriers because only plasmonic hot charge carriers with sufficient energies and appropriate momenta can overcome the barrier and inject into the semiconductor. In addition, noble metals are well-known to be expensive. We have synthesized different metal oxide nanoparticles that are degenerately doped and thus exhibit strong plasmon resonance and demonstrated Schottky-barrier-free plasmonic photocatalysts for nitrogen fixation. The excitation of the plasmon resonance in these metal oxide nanoparticles can efficiently generate hot charge carriers. Because of the abundance of defects in these degenerately doped semiconductor nanoparticles, the photogenerated hot charge carriers can rapidly migrate to the defect sites and get trapped there. In this way, the lifetime of the plasmonic hot charge carriers is Moreover, these metal oxide nanoparticles possess active sites for adsorbing and activating dinitrogen molecules. Therefore, they function in a “one-stone-two-birds” manner. As a result, a record-high solar-to-chemical energy conversion efficiency has been achieved.

Computational Characterization of the On-Surface Synthesis of Low-Dimensional Nanostructures

On-surface chemistry represents a fast-growing field allowing to synthesize molecular structures not available by traditional wet chemistry. In this context, high-resolution scanning probe microscopy techniques are able to provide unprecedented spatial resolution, allowing to precisely identify individual products of the reactions. Nevertheless, a deep understanding of the reaction mechanism under the conditions imposed by the substrate remains unknown.Nowadays, various computational strategies are employed to characterize the optimal reaction pathways of the chemical processes that take place on the surfaces of solids. For example, one of the most popular approaches is the nudged elastic band (NEB) as well as different transition state theory methods beyond it. These approaches had demonstrated their capability to characterize the potential energy landscape of the reaction pathways [1-2]. Another approach that had attracted the interest of the on-surface synthesis community is the application of molecular dynamics (MD) methods, within the canonical ensemble, and based on the QM/MM (quantum mechanics/molecular mechanics) of the interatomic forces [3-4]. This approach, which combines Density functional theory (DFT) and empirical force fields to describe the interaction forces, allows to simulate very large systems, at a very low computational cost while still accounting for the crucial quantum mechanics interactions. Also, such temperature-dependent simulations include the effect of entropy, vibrations modes, concerted motion, etc.I will present the experimental and computational results of different projects in which we have made use of the aforementioned computational approaches to achieve a better understanding of the on-surface synthesis experiments performed by the group. For example, the rationalization of the selective cyclodehydrogenation of non-benzenoid nanographenes deposited on a Au(111) substrate by means of temperature-dependent MD calculations and the analysis of the obtained vibrational modes (See Figure 1a). The group had also studied the stepwise transformation of coronene-based organometallic networks into two-dimensional covalent patches through intermolecular homocoupling (See Figure 1b). We had as well investigated the transformation of indenyl precursors, through a triple C-H cleavage followed by a peripheral directed homocoupling, to form a polyacetylene backboned polymer that features a parity dependent and highly delocalized soliton in-gap state (See Figure 1c).

Electrochemical Plasma-Activated CO2 Reduction at a Plasma-Water Interface

The development of technologies and processes to decarbonize chemical and fuel production is key to reducing global anthropogenic greenhouse gas emissions. Electrochemical carbon dioxide reduction enables a sustainable pathway to circularize carbon-based chemical products by upgrading recovered carbon dioxide using renewable energy sources. Electrochemical CO2 reduction has successfully been employed to produce chemicals such as methanol and ethylene, but it has been hindered by dependence on elevated temperature to achieve high yields and limited ability to generate higher order carbon products. Non-thermal carbon dioxide plasma can be generated at ambient temperature and pressure and contains energetically excited oxygen and carbon species. When introduced to an electrocatalyst, these reactive species could more readily participate in reaction pathways with higher activation barriers towards forming higher order carbon species compared to electrochemical reactions involving ground-state reactants. Further, plasma discharges over water in an electrochemical cell generate plasma-activated water, introducing solvated electrons and secondary reactive oxygen species.In this work, we investigate how the use of plasma-activated carbon dioxide and a plasma-water interface impacts electrochemical product generation. We design a novel non-thermal plasma electrode to discharge carbon dioxide plasma into water in an electrochemical cell with a Cu nanoparticle electrocatalyst. Results suggest the potential for a plasma-water electrochemical system to generate value-added products that are less commonly generated in electrochemical processes at ambient pressure and temperature.

(Invited) Thermal and Plasmon-Mediated Catalytic Transfer Hydrogenation Reactions on Nanostructured Metal Surfaces

Catalytic hydrogenation reactions play crucial roles in fine chemical production and pharmaceutical synthesis. Compared to direct hydrogenation of organic compounds with pressurized hydrogen gas, catalytic transfer hydrogenation reactions using inexpensive and readily accessible small molecules as hydrogen-donors has been considered as a more versatile, scalable, and sustainable pathway toward enhanced chemoselectivity under mild reaction conditions. Noble metal nanoparticles, especially those within the sub-5 nm size regime, can efficiently catalyze the dehydrogenation of a series of hydrogen-storing molecules, such as ammonia borane, hydrazine, formaldehyde, formic acid, and isopropanol, to produce surface-adsorbed hydrogen species that actively hydrogenate a variety of organic substrate molecules. The transfer hydrogenation/hydrogenolysis reactions are mechanistically complex, and may occur selectively along multiple distinct pathways, exhibiting kinetic features signifying the Langmuir–Hinshelwood, Eley–Rideal, autocatalysis, and reversible reaction mechanisms, respectively, depending on the compositions of the metal catalysts and the chemical nature of the hydrogen donors. In this talk, I will share with the audience some new insights concerning the detailed mechanisms of transfer hydrogenation reactions over metal nanocatalyst surfaces. We employ surface-enhanced Raman scattering (SERS) as an in situ fingerprinting spectroscopic tool to precisely resolve the detailed structural evolution of molecular adsorbates during catalytic reactions, based upon which the key intermediates along different reaction pathways are unambiguously identified. The results of deliberately designed in situ SERS measurements show that the chemoselective transfer hydrogenation of nitrophenyl isocyanide by ammonia borane may proceed on noble metal nanocatalyst surfaces selectively through either a unimolecular or a bimolecular pathway, depending on how the nitrophenyl isocyanide adsorbates interact with the metal nanoparticle surfaces. The experimental observations are corroborated by density functional theory calculations, which shed light on the underlying relationships between catalyst-adsorbate interactions and reaction pathway selection. We have further demonstrated that the photoexcited plasmonic hot carriers in the metal nanoparticles can be effectively harnessed to fine-regulate the activation energy barriers associated with the rate-limiting steps for the transfer hydrogenation of nitrophenyl isocyanide on Pd nanocatalyst surfaces when ammonium formate serves as the hydrogen donor. Our results clearly demonstrate the feasibility of using plasmonic hot carriers to kinetically modulate catalytic molecule-transforming processes.

(Invited) Integrated Chemocatalytic Conversion of Cellulosic Biomass to Gasoline Blended Fuel Using a Multifunctional Nano-Catalysts

The Ministry of New and Renewable Energy (MNRE) in India has initiated several programs to promote effective ways of utilizing biomass energy to boost the country's economy. In order to improve the effective use of biomass resources, the initiative for biomass power and cogeneration has adopted the use of bagasse-based cogeneration and biomass power generation in sugar mills. Because of its remarkable characteristics, including its high-octane number, low cetane number, and significant heat of vaporization, bioethanol is considered as a desirable biofuel. In addition, the increased biofuel production, in particular bioethanol, satisfies the criteria of the energy demand. These criteria include the fuel's ability to be blended with gasoline and diesel, as well as standards for low-carbon fuels.Chemocatalytic conversion using nanowire catalysts reduce side reactions caused on by an intermediate product while facilitating cascade reactions that lead to a desired product. In this work, metal oxide nanowires (TiO2 and WO3) were synthesised using a plasma-assisted method and metal nanoparticles (Pt and Ru) were impregnated on the as-synthesised NWs. In a high-pressure slurry batch reactor, reactions were carried out to obtain the optimal conditions for the desired product. The catalysts were characterised to reveal the catalysts-activity relationship. Various analytical techniques were used to quantify the liquid and gaseous products collected after the reactions.The direct conversion of cellulose to C2-C3 alcohols using tungsten-based co-catalysts was enhanced even at low temperatures. X-ray photoelectrons spectroscopy (XPS) and Raman analysis show the oxygen vacancy (Ov) enrichment on the surface of Pt/TiO2 in presence of tungsten co-catalysts which improved their catalytic activity. The role of metallic platinum (Pto) was also investigated and found to have a linear relationship with the activity as follows: H2WO4 > (NH4)6H2W12O40.xH2O > H3PW12O40 . Maximum yields of 32.33% and 51.52% of ethanol and propane-2-ol at optimum temperatures of 220℃ and 250℃, respectively, were obtained with H2WO4. Catalytic reaction performed using tandem catalytic system gives a high ethanol yield of 25.56%, which is low as compared to an integrated catalytic system. Based on the experimental results, the reaction pathway is proposed which elaborates the activation and cleavage of specific C-C and C-O bonds.Moreover, the dual functionality of tungsten oxide (WO3) nanorods as support and co-catalyst was also studied, enhances the catalytic behaviour, and the promotional study of W was studied for delivering a high yield of ethanol to 43.98% under optimum reaction conditions. The cellulose conversion was calculated to be 98.5%. The XPS study reveals the oxidation state of WO3 employing the change in catalytic activity before and after the addition of co-catalysts. The electron-deficient state of the metallic component (Ruo) is favourable for H2 adsorption on the surface, producing more active hydrogen species and thus promoting hydrogenation. As a result of these electronic understandings of Ruo and W, the reason for the higher activity of these nano synthesised Ru/WO3 compared to WO3 was concluded. Furthermore, a series of characterizations were performed to reveal the catalysts-activity relationship. Our characterization and activity test suggested that the synergistic effect of W6+ and Ruo facilitate the formation of ethanol whereas, Bronsted acid sites from W-based catalysts and Bronsted base from hot compressed water (HCW) collectively participate in the hydrolysis of cellulose and C-C cleavage of glucose to ethanol via selective hydrogenolysis of C-OH. The bond functionality was also investigated by performing reactions with various reactants under the same reaction conditions. This presentation will provide our understanding of the mechanism and kinetic correlation of the integrated cellulose-to-ethanol synthesis important for sustainable ethanol production.Correspondence should be addressed to: sreedevi@chemical.iitd.ac.in. Acknowledgments The authors thankfully acknowledge funding support from TUD-IITD MFIRP project. References Governement of India of India, BioEnergy, Ministry of New and Renewable Energy. https://mnre.gov.in/bio-energy/current-status (accessed Dec. 27, 2023).S. Achinas and G. J. W. Euverink, Consolidated briefing of biochemical ethanol production from lignocellulosic biomass, Electron. J. Biotechnol., 2016, 23: 44–53Beena Patel and Bharat Gami, Biomass Characterization and its Use as Solid Fuel for Combustion, Iran. J. Energy Environ., Iranica Journal of Energy & Environment, 2012; 3 (2):123-128N. Wei, J. Quarterman, S. R. Kim, J. H. D. Cate, and Y. S. Jin, Enhanced biofuel production through coupled acetic acid and xylose consumption by engineered yeast, Nat. Commun., 2013; 4:1–8 Figure 1

Modeling Silver Corrosion Mechanisms in Marine Atmospheric Environments

Measures that can assess the severity of the local environment are an important tool for combating atmospheric corrosion. One approach for determining the environmental severity employs electrochemical analysis alongside surface characterization on silver coupons exposed to the environment for varying durations. However, despite the long use of silver as an environmental severity monitor, the underlying reaction mechanisms that give rise to the observed corrosion products are not well understood. In addition, the measured ratios of silver corrosion products appear to be unique to the location where the silver coupons were exposed.The goal of this work is to develop models of silver oxidation reactions that can explore pathways for the formation of silver corrosion products. We use density functional theory (DFT) calculations in nudged elastic band (NEB) approximations to determine electrochemical and chemical reaction along with surface diffusion energy barriers. An example reaction is shown in Figure 1a and b, in which an O2 molecule is adsorbed on an ‘on-top’ site on a relaxed Ag (100) crystal structure with periodic boundary conditions in the x and y-planes. The O2 molecule then dissociates into individual O atoms. The reaction pathway is approximated using the NEB approach to determine the dissociation energy barrier in Figure 1c. These reaction energy barriers are incorporated into a kinetic Monte Carlo (KMC) model of silver reactions to estimate the corrosion products present in marine atmospheric environments for comparison with experimental data. Figure 1

On-Surface Chemistry: Probing the Presence of Adatoms on Differently Terminated Surfaces by Porphyrin Metalation

Reactive adsorbates in on-surface reactions and in heterogeneous catalysis on different substrates are of emerging importance. This is for the crucial roles they take in reaction pathways and for their decisive influence on reaction kinetics. [1] The thermally activated 2D mobility [2] of reaction precursors and reactive adatoms as well as their spatial and temporal coincidence at reaction sites is of mechanistic importance for many interface-specific reactions. In our work, we use the well-studied metalation reaction of H2-porphyrins to investigate the action of adatoms on metallic and passivated surface substrates. Porphyrin metalation has been established on atomically clean metallic substrates [3] as well as on metal-oxides [4] and metallic surfaces modified by oxygen [5].We perform spectro-microscopy correlation experiments combining X-ray Photoelectron Spectroscopy, Ultraviolet Photoelectron Spectroscopy, Low Energy Electron Diffraction and Scanning Tunnelling Microscopy on Cl- and N terminated Cu(001). Thereby we find extended layers of adsorbate-induced superstructures that have decisive and contrasting impact on the reactivity of the surface, as well as on molecular self-assembly. The O and N termination of Cu facilitates the metalation reaction and self-assembled domains of CuTPP are formed at room temperature. Cl termination on the contrary, fully inhibits the metalation reaction and causes 2HTPP to assemble into small ‘magic’ clusters.

Electrocatalytic Conversion of Plasma-Activated CO2 Toward Multicarbon Products

Renewable electricity-powered CO2 reduction reaction (CO2RR) toward chemicals and fuels has been gaining significant attention not only for achieving net-zero carbon emissions but also for enabling long-term energy storage. In electrochemical conversion, CO2 can be upgraded into single- or multicarbon products such as carbon monoxide, formate, and ethylene with remarkable selectivity. However, challenges still exist in terms of high overpotential, limited product range, and low production rates. Moreover, most reported CO2RR to multicarbon products which used Cu as catalysts have predominantly generated C2 products like ethylene or ethanol. However these systems require further performance improvement to target C3+ products which have higher energy density and market price. Another electrified conversion technique involves non-thermal plasma, which can activate CO2 into radical, ionized, and vibrationally excited species at atmospheric pressure and near-ambient temperature. However, plasma processes are non-selective, which poses limitations for controlling reaction pathways and necessitates H2 or light alkanes as co-reactants to produce hydrocarbons.We synergized plasma and electrocatalytic conversion to demonstrate a combined CO2RR process that overcomes the limitations of each individual method. Through the integration of a dielectric barrier discharge plasma and an electrochemical flow reactor, pre-activated CO2 was electrochemically converted on a Cu electrode to produce multicarbon products. The combination enhanced the faradaic efficiency and reaction rate of desirable products and opened new reaction pathways by increasing the activity of CO2 prior to adsorption. The production rates of ethanol, propanol, and acetaldehyde were significantly increased, and new products such as methanol, acetylene, and ethane that were formed only in the combined reaction were detected. The product distribution was studied according to plasma-activated states of CO2 and the electrochemical applied potential. To elucidate the synergistic effects in the combined system, the selectivity and production rates were compared with those in the plasma-only and electrochemical-only reactions. Further control experiments using ground-state reactants were conducted, and the excited states during plasma activation were identified by optical electron spectroscopy. This work represents not only electrocatalysis of pre-activated reactants for enhanced generation of multicarbon products during electrified conversion of CO2, but also demonstrates synergistic convergence of plasma and electrocatalysis for converting reactants possessing strong chemical bonds.

Early-Stage Evaluation of Battery Safety Based on Materials, Component, and Microcell Calorimetry Measurements

There are numerous “beyond lithium ion” battery chemistries under development at universities, national labs, companies, and other organizations. Early-stage battery chemistry work typically focuses on demonstrating performance at the materials, coin, and pouch cell level, with evaluation and design for safety left for later stages.1 In this talk, we will describe the opportunities for assessing the safety of a battery chemistry at the earliest stages of its development, as soon as the active materials and electrolyte have been identified.2 With carefully designed and quantitative experiments, key information such as the amounts of heat released by the reactions of and among cell components, the temperatures at which that heat is released, the release of toxic or otherwise dangerous gases, the material heat capacities, and other key properties can be measured with differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), and other approaches. We have demonstrated the value of this approach for a LixCoO2+C+PVDF cathode sheet /LLZO/Li metal material set, where we identified the previously under-appreciated role of the cathode sheet conductive additive and binder in the reaction pathways upon heating to 500°C in a DSC pan.3 Despite this material set containing no liquid electrolyte, delithiated LixCoO2 releases oxygen upon heating to >~230°C, and that oxygen can react exothermically with the Li metal and conductive carbon. In addition, PVDF binder releases HF gas at >400°C, which can also react exothermically with Li metal.This talk will focus on the methodology required to obtain accurate measurements on milligram-scale samples, with an emphasis on the sample preparation and DSC methods to obtain accurate, repeatable results. Approaches associated with DSC baselining and integration methods will be presented. The talk will also present results from our recent work on material sets that are more commercially relevant for electric vehicle applications than the LixCoO2 cathode sheet/ LLZO / Li material set. In particular, results from material sets such as NMC532 cathode sheet / LLZO / Li, NMC811 cathode sheet / LLZO / Li, and NMC811 cathode sheet / LPSCl / Li metal will be presented. Finally, we will discuss the implications of our early-stage, material-scale work for future large-format cells.1. See Category 3 in the ARPA-E EVS4ALL program, https://arpa-e.energy.gov/technologies/programs/evs4all2. Joule 6, 742–755, April 20, 2022.3. ACS Appl. Mater. Interfaces 2023, 15, 49, 57134–57143

(Invited) Elucidating the Cation Effect in CO2 Electroreduction Using Non-Metal Cations

Electrochemical reduction of CO2 may enable a promising route for the recycling of CO2 and the sustainable production of chemicals and fuels. Due to the complex electrochemical environment, a deeper understanding of the reaction mechanism, particularly the role of cations in the process, is still needed. It was recently demonstrated that metal cations are essential for the CO2 reduction to CO, by stabilizing the CO2 – intermediate via electrostatic interaction. Here we investigate the cation effect in CO2 reduction using non-metal cations, particularly quaternary ammonium cations, whose size and shape symmetry can be tuned. We select CO2 reduction on Bi catalyst that produces both CO and formate, thus to reveal and compare the effect of cations on the two reaction pathways. Interestingly, the Faradaic efficiency for CO production increased apparently for the electrolyte with larger cations, while the formate Faradaic efficiency showed a weak dependence on the cation size, indicating a reaction-pathway-dependent effect of the cations. We further compared cations with different symmetries and observed a more profound effect of substituted ammonium cations on the CO2 reduction activity and selectivity, which was attributed to the charge distribution and weak hydration shells of the cations that help stabilize reaction intermediates. Our work elucidates the critical role of cations in the CO2 electroreduction catalysis and suggests a method to improve the electrocatalysis with optimized electrolytes.

Material Dynamics of Nickel (oxy)Hydroxide As an Alcohol Oxidation Electrocatalyst

Electrochemical alcohol oxidation is an attractive reaction for renewable-energy driven production of value-added chemicals, like precursors for acrylic (acrolein from glycerol) and nylon (adipic acid from cyclohexanol). Implementing selective, earth-abundant catalysts for alcohol oxidation can improve electrolyzer efficiency and economics. For example, replacing the energy intensive oxygen evolution reaction (OER) in CO2 electrolyzers with alcohol oxidation can decrease operating voltage by about 0.85 V and reduce electricity requirements up to 53% while also reducing carbon emissions from chemical production.1 Electrochemical alcohol oxidation catalyzed by as synthesized nickel oxyhydroxide (NiOOH) is hypothesized to proceed through two pathways, a chemical, redox pathway preferred by aldehydes (geminal diols in alkaline electrolyte), and a potential dependent pathway preferred by alcohols.2 Many studies have focused on understanding reaction mechanisms and device performance, however, transition metal alcohol oxidation catalysts, such as NiOOH, are known to undergo reduction/oxidation, phase changes, and even dissolution in the same potential window as alcohol oxidation.3 It is still unclear how these processes affect catalyst material stability and electrochemical performance.In this work, we probe the performance stability and material stability of Ni(OH)2 for alcohol oxidation in alkaline media through electrochemical cycling and chronoamperometry. We use a rigorously Fe-free environment to mitigate performance impacts from Fe-contamination common in non-purified alkaline electrochemical studies.4 We show that the rate of change of alcohol oxidation current density (mA cm-2) depends on reaction pathway (redox vs. potential dependent), alcohol concentration, and side chain structure. Using EC-MS and NMR, we also probe changes in performance in yield and selectivity by quantifying oxidation products, including oxygen gas. These changes in performance are correlated to material changes through monitoring dissolution with on-line ICP-MS. Furthermore, to disentangle the role of potential and reactant (e.g. alcohol or water) on stability, we elucidate oxidation state and local structural changes using in situ X-ray Absorption Spectroscopy (XAS). Insights from this rigorous model system are directly translatable to the broader goal of developing efficient, long-lasting catalysts for industrial deployment of electrochemical organic reactions powered by clean energy. References. Verma, Sumit, Shawn Lu, and Paul JA Kenis. Nature Energy 4.6 (2019): 466-474.2.Bender, Michael T., and Kyoung‐Shin Choi. ChemSusChem 15.13 (2022): e202200675.Klaus, Shannon, et al. The Journal of Physical Chemistry C 119.13 (2015): 7243-7254Wei, Lingze, et al. ACS Catalysis 13.7 (2023): 4272-4282.

Heterogeneous Electrosynthesis of Nitriles through Coupling of Primary Alcohols and Ammonia on Noble-Metal-Free Monometallic Catalyst

Despite increasing interest devoted to the electrocatalytic refinery of renewable feedstocks for the sustainable production of value-added chemicals, the direct electrosynthesis of nitriles from biomass-derived alcohols is rarely studied. Nitriles are highly versatile intermediates for producing a wide range of chemicals, including biological materials, pharmaceuticals and polymers. The conventional chemical methods for nitrile synthesis, such as the Sandmeyer and the Rosenmund-von Braun reactions, are not benign as they utilize toxic starting materials, require severe reaction conditions and generate large amounts of chemical waste. More environmentally friendly chemical routes using alcohols and ammonia as the substrates, via oxidation or oxidant-free dehydrogenation coupled with imination, have been recently reported. However, they face certain issues, including the need for oxidants or high reaction temperatures, as well as poor selectivity due to possible over-oxidation and other side reactions. On the electrocatalysis front, the synthesis of hydrogen cyanide, an analogue of nitrile, has been demonstrated using methanol and ammonia as the substrates, albeit at an elevated temperature with costly solid electrolytes. Here, we report a benign one-pot synthesis of nitriles from primary alcohols and ammonia, in the presence of a simple nickel electrocatalyst under ambient temperature using aqueous electrolytes without the need for oxidants.In the initial screening stage, nine species of metal-based catalysts which were reported to be active in the thermocatalytic pathway of the same reaction, including Mn, Fe, Co, Ni, Cu, Zn, Ru, Pd and Pt, as well as carbon paper were studied using benzyl alcohol (BnOH) as a model compound (Figure 1a). Among them, Ni foam, bearing good resistances towards dissolution under oxidative potentials and poisoning by ammonia, has the capacity to produce benzonitrile as the main product under lower overpotentials. Several control experiments were conducted, revealing that the reaction pathway proceeds via two sequential steps: (1) The alcohol first undergoes dehydrogenation to produce an aldehyde intermediate, which equilibrates in the presence of ammonia to afford the corresponding imine. (2) The imine is subsequently dehydrogenated to form the nitrile product (Figure 1b). Based on the linear sweep voltammetry (LSV) results and in-situ Raman analyses (Figure 1c, d), we propose that the in-situ formed NiII/NiIII redox species serve as the active sites for the nitrile production through a hydrogen atom transfer (HAT) mechanism. The influences of applied potentials, alcohol/ammonia concentrations and pH on the reaction were also systematically investigated. A high benzonitrile faradaic efficiency (FE) of 63.0% and a formation rate of 90.8 mmol m-2 h-1 were achieved at applied potentials of 1.375 V and 1.425 V vs. RHE, respectively, under the optimum conditions: 20 mM BnOH, 1 M ammonia and pH 13 (Figure 1e). Subsequently, kinetic studies and mathematical modelling were performed, suggesting that the dehydrogenation from alcohol to aldehyde displays the smallest rate constant. Isotope labelling experiments (Figure 1f) further confirmed that the rate-determining step (RDS) likely involves the cleavage of α-carbon C-H bond in the hydroxyl group of the alcohol. Encouragingly, various aromatic, aliphatic and heterocyclic primary alcohols were transformed to the corresponding nitriles, exhibiting the broad feasibility of our strategy. This study offers a promising electrocatalytic system for the low-cost, green synthesis of high-value nitriles in the chemical industry. Figure 1

Changing Mechanisms for CO2 Reduction to Formate Via Protonation of the Catalyst

Electrochemical reduction reaction of CO2 (CO2RR) is a promising method to transform CO2 to useful products for consumption such as formic acid, carbon monoxide, or methane. Simply increasing the driving force of a catalytic process to speed up the rate of CO2 reduction is unviable because of the competing Hydrogen Evolution Reaction (HER) pathway and of increased energy cost. Controlling the reactivity of active intermediates is of substantial importance for such processes. [Fe4N(CO)12]− is a formate selective electrocatalyst previously studied by our group and was modified through protonation and replacement of several CO ligands with triethylphosphines (PEt3) to synthesize [HFe4N(PEt3)4(CO)8] . This PEt3 substituted, pre-protonated analog of [Fe4N(CO)12]− exhibits altered rates compared to a non-protonated analog which we attribute to a switch in mechanism. We further explore explanations for different rates despite similar “active intermediates” and similar driving force using Cyclic Voltammetry (CV).

Multiscale Synchrotron Characterization of Electrode-Electrolyte Interfaces in Multivalent Metal-Ion Batteries

The development of multivalent metal-ion battery chemistries (Zn2+, Mg2+, Al3+, etc.) is still impeded by fundamental scientific questions and engineering technical challenges. Particularly, the debate persists on the charge storage mechanisms of the cathode and their implications for achievable battery performance. The charge density of multivalent metal ions offers advantages in multielectron redox processes but is accompanied by strong Coulombic interactions within the bulk electrolyte, at electrode–electrolyte interfaces, and during solid-state diffusion in the electrode(1). Herein, we have designed operando electrochemical batteries and developed operando and ex situ synchrotron X-ray techniques based on beamlines in Shanghai Synchrotron Radiation Facility (SSRF) for multiscale characterization of multivalent metal-ion batteries. In this work, we employed multiscale synchrotron X-ray characterization to probe the electrode-electrolyte interface in aqueous zinc batteries and rechargeable magnesium batteries, respectively; and further in conjunction with electrochemical measurement to reveal their charge storage mechanisms.In aqueous zinc batteries, the addition of MnSO4 to the ZnSO4 electrolyte commonly exhibit an initial-cycle increase in capacity. To better understand the role of Mn2+ additive in the electrolytes, operando synchrotron X-ray diffraction, ex situ X-ray absorption spectroscopy and electrochemical analyses were carried out to elucidate the reaction mechanism and pathway. A reversible Mn2+/MnO2 deposition/dissolution reaction occurred at the surface of cathode, in which the change in the electrolyte environment enabled Zn2+/Zn4SO4(OH)6∙5H2O deposition/dissolution. The chemical reactions of Zn2+/Zn4SO4(OH)6∙5H2O contributed no capacity and hindered the diffusion kinetics of the Mn2+/MnO2 reaction, which prevented operation of the ZIBs at high currents(2).In rechargeable magnesium batteries, the utilization of copper current collector presented a remarkable capacity and improved cycling performance. To understand the role of the copper current collector in battery performance, we conducted multiscale operando X-ray characterization, coupled with electrochemical analyses, to investigate the interfacial and bulk reactions between the electrolyte and electrode. Operando synchrotron X-ray diffraction was employed during battery operation to track the real-time structural evolution of cathode and copper current collector at the atomic scale. Additionally, operando synchrotron X-ray imaging was used to observe the morphological changes in the cathode, current collector, and anode at the microscale. The results and discussion reveal that the rechargeable magnesium batteries were driven by a redox chemistry of copper, in which the characteristic charge and discharge plateau represent redox reactions between copper and copper ions. Nevertheless, battery failure was also caused by depletion of copper current collector, which is attributed to an irreversible process whereby copper ions migrate towards the magnesium anode and are reduced to copper deposits. Reference M. A. Schroeder, L. Ma, G. Pastel and K. Xu, Current Opinion in Electrochemistry, 29, 100819 (2021). Z. Li, Y. Li, X. Ren, Y. Zhao, Z. Ren, Z. Yao, W. Zhang, H. Xu, Z. Wang, N. Zhang, Y. Gu, X. Li, D. Zhu and J. Zou, Small, 19, 2301770 (2023). Figure 1

(Invited) Thermally Selective Skeletal Rearrangements at Interfaces

Thermal rearrangements, in which an atom, ion or chemical unit of a molecule is rearranged to obtain a structural isomer of the original molecule under a thermal stimulus, represents an excellent alternative to induce the formation of novel antiaromatic PCHs and conjugated polymers not accessible otherwise. Only recently, a few strain-induced rearrangement reactions of precursors that are composed of rigid and strained three dimensional (3D) structures have shown skeletal rearrangements in their backbone after planarization on metal surfaces. In this respect, the selectivity of on-surface chemical reaction pathways is a promising approach to increase synthetic versatility, though remains largely underexplored. In this talk I will review our recent discoveries regarding the exploitation of thermal rearrangements to form 0D, 1D and 2D unique molecular nanocarbons.

Modeling Stochastic Kinetics of Cathodic Cu Surface Corrosion and Ionic Interaction in Nanoscopic Water Pools

Cathodic Cu corrosion has been reported to occur in both the presence and absence of carbon dioxide in aqueous electrolytes and while the mechanism is not understood well it implies carbide-, hydride- and hydroxide-formation mediated pathways. Corrosion leads to roughening, which creates local pockets of electrolyte that are somewhat disconnected from the bulk. In this talk, we describe a combination of theoretical studies to identify reaction pathways, and kinetics modeling of them to obtain a working description of corrosion in water. Corrosion at metal surfaces is both influenced by and influences the formation of nanoscopic aqueous pools by trapping electrolyte solution in microscopic pockets. Such electrolyte nano-pools exhibit non-bulk behavior, and the lifetime of locally trapped ions can be significantly different than in the bulk. This can potentially have an effect on the chemistry in the local microenvironment. Stochastic reaction-diffusion models can be used to model kinetics of electrochemical processes at length and time scales relevant to laboratory experiments, allowing a direct comparison of transient observable quantities to time-resolved experimental data. Atomistic simulations employ methods of electronic structure theory (such as density functional theory (DFT)) and molecular dynamics (both classical and ab initio) to describe a detailed picture of the (electro-)chemistry at microscopic time and length scales. Scaling up these methods to macroscopic levels is not trivial but required for direct comparison to experiment. By using Gibbs free energies obtained from atomistic simulations it is possible to calculate rate constants for individual elementary steps in reaction schemes describing kinetics of relevant processes. We use Kinetiscope to implement stochastic kinetics models of two reaction schemes describing: 1. cathodic Cu surface corrosion through formation of copper hydroxides, through which we aim to describe essential steps in the cathodic corrosion mechanism of metal surfaces; 2. interactions between bicarbonate and hydronium ions, both present in the bicarbonate salt buffers used routinely in electrochemical reduction at copper surfaces, in a nanoscopic water droplet to understand time-dependent changes in the local concentrations of electrolytes trapped at or near metal surfaces. Figure 1