Stability Of Reaction Intermediates Research Articles

CO2 Reduction by Transition‐Metal Complex Systems: Effect of Hydrogen Bonding on the Second Coordination Sphere

AbstractHomogeneous electrocatalysts typified by transition‐metal complex show transcendent potency in efficient energy catalysis through molecular design. For example, metal complexes with elaborate design performed wonderful activity and selectivity for electrocatalytic CO2 reduction. Primary coordination sphere of metal complexes plays a key role in regulating its intrinsic redox properties and catalytic activity. However, the overall reduction efficiency of CO2 is also bound up with the substrate activation process. Transition‐metal complexes are hoped to exhibit reasonable redox potential, reactive activity, and stability, while binding and activating CO2 molecules to achieve efficient CO2 reduction. Construction of second coordination sphere, especially hydrogen‐bonding network of transition‐metal complexes, is reported to be the “kill two birds with one stone” strategy to realize efficient CO2 reduction catalysis via systematic catalyst properties modulation and substrate activation. Herein, we present recent progress on the construction of hydrogen‐bonding network in the second coordination sphere of metal complexes by ligand modification or the introduction of exogenous organic ligand, and the resulted productive enhancement of the catalytic performance of metal complexes by the improvement of adsorption capacity and activation of CO2, proton transfer rate, and stability of reaction intermediates, and so forth.

Utilization of Cobalt and its Oxide/Hydroxide Mediated by Ionic Liquids/Deep Eutectic Solvents as Catalysts in Water Splitting.

With the ever-growing global demand for sustainable energy solutions, hydrogen has garnered significant attention as a clean, efficient, and renewable energy source. In the field of hydrogen production, catalyst research stands out as one of the foremost areas of focus. In recent years, the preparation of electrocatalysts using ionic liquids (ILs) and deep eutectic solvents (DESs) has attracted widespread attention. ILs and DESs possess unique physicochemical properties and are recognized as green media as well as functional materials. Cobalt-based catalysts have proven to be efficient electrocatalysts for water splitting. Incorporating ILs or DESs into the preparation of cobalt-based catalysts offers a remarkable advantage by allowing precise control over their structural design and composition. This control directly influences the adsorption properties of the catalyst's surface and the stability of reaction intermediates, thereby enabling enhanced control over reaction pathways and product selectivity. Consequently, the catalytic activity and stability of cobalt-based catalysts can be effectively improved. In the process of preparing cobalt-based catalysts, ILs and DESs can serve as solvents and templates. Owing to the good solubility of ILs and DESs, they can efficiently dissolve raw materials and provide a special nucleation and growth environment, obtaining catalysts with novel-structures. The main focus of this review is to provide a detailed introduction to metal cobalt and its oxide/hydroxide derivatives in the field of water splitting, with a particular emphasis on the research progress achieved through the utilization of IL and DES. The aim is to assist readers in designing and synthesizing novel and high-performance electrochemical catalysts.

Electrified Operando-Freezing of Electrocatalytic CO2 Reduction Cells for Cryogenic Electron Microscopy.

The ability to freeze and stabilize reaction intermediates in their metastable states and obtain their structural and chemical information with high spatial resolution is critical to advance materials technologies such as catalysis and batteries. Here, we develop an electrified operando-freezing methodology to preserve these metastable states under electrochemical reaction conditions for cryogenic electron microscopy (cryo-EM) imaging and spectroscopy. Using Cu catalysts for CO2 reduction as a model system, we observe restructuring of the Cu catalyst in a CO2 atmosphere while the same catalyst remains intact in air at the nanometer scale. Furthermore, we discover the existence of a single valence Cu (1+) state and C-O bonding at the electrified liquid-solid interface of the operando-frozen samples, which are key reaction intermediates that traditional ex situ measurements fail to detect. This work highlights our novel technique to study the local structure and chemistry of electrified liquid-solid interfaces, with broad impact beyond catalysis.

Concept of Utilizing Ionic Liquids for the Co‐Electroreduction of Carbon Dioxide and Nitrogen‐Containing Compounds

AbstractFormation of C−N bonds through the electrochemical utilization of CO2 and nitrogen containing compounds (N‐compounds) is appealing for the purpose of converting waste and readily available sources or pollutants into value added chemicals at ambient conditions. Existing research predominantly explores these electrochemical reactions independently, often in aqueous electrolytes, leading to challenges associated with competitive hydrogen evolution reaction (HER), low product selectivity, and yield. Functional electrolytes such as those containing ionic liquids (ILs) present selective solubility to the solute reactants and present unique interactions with the electrode surface that can suppress the undesired side reactions such as HER while simultaneously co‐catalyzing the conversion of CO2 and N‐compounds such as N2, NO, NO2, and NO3. In this concept paper, we discuss how the microenvironment enabled by ILs can be leveraged to stabilize reaction intermediates at the electrode‐electrolyte interface, thereby promoting C−N bond formation on an active electrode surface at reduced overpotential, with the case study of CO2 and N‐compounds co‐catalysis to generate urea.

(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.

Recent advances in polyethylene glycol as a dual‐functional agent in heterocycle synthesis: Solvent and catalyst

AbstractReactant solubility, which dictates achievable concentrations, and the stability of reaction intermediates (excited states), solvents modulate the potential energy landscape and influence reaction rates. Consequently, solvent selection is pivotal in optimizing process productivity, economic feasibility, and environmental footprint. At present, organic synthesis pivots around the idea of sustainability. In particular, PEG‐400, a popular solvent and phase transfer catalyst, is considered greener as it can be reused several times without significant loss in its catalytic activity, which checks the box regarding sustainability. This review highlights the emerging potential of Polyethylene Glycol 400 (PEG‐400) as a dual‐threat agent in sustainable organic synthesis. We explore its efficacy as a catalyst, promoting various reactions under mild conditions and often eliminating the need for traditional metal catalysts. Additionally, PEG‐400's role as a green solvent is addressed, emphasizing its biodegradability, low toxicity, and ability to facilitate reactions without hazardous Volatile Organic Compounds (VOCs). The review examines recent research on PEG‐400 mediated reactions, showcasing its effectiveness in diverse transformations, thus exploring the potential of PEG 400 as a facilitator for heterocycle synthesis in both multicomponent reactions and stepwise approaches. It identifies exciting research directions that promise to expand the boundaries of polymer‐based solvents in heterocyclic chemistry.

First-principle molecular dynamics and in situ DRIFTS study the stability of the intermediates of CO2 hydrogenation to methanol on ZnZrOx solid solution catalyst

Among the many catalysts used for CO2 hydrogenation to methanol, ZnZrOx solid solution is a highly efficient catalyst. Although the reaction mechanism has been widely explored at 0 K using DFT, critical aspects such as the atomic structure of the solid solution and the stability of reaction intermediates during CO2 reduction at experimental temperatures remain unclear. This study employs first-principle molecular dynamics (MD) simulations, FTIR and DRIFTS characterization to address these important questions. During the MD simulation, oxygen vacancies are formed in the system and same as the doped Zn atoms. The formation of oxygen vacancies was further confirmed by the Transmission FTIR and in situ DRIFTS. Due to the creating of oxygen vacancies, the coordination of Zn and Zr in the bulk is reduced to five and seven, respectively. And on the surface, their coordination is reduced to three、four and six、seven, respectively. Subsequently, the hydrogenation of CO2 to methanol using the formate and CO* pathways was studied. In the formate pathway, all intermediates remained stable at 593 K. However, in the CO* pathway, we observe instability of key intermediates which also confirmed by transmission FTIR and in situ DRIFTS characterization.

Local CO2 reservoir layer promotes rapid and selective electrochemical CO2 reduction

Electrochemical CO2 reduction reaction in aqueous electrolytes is a promising route to produce added-value chemicals and decrease carbon emissions. However, even in Gas-Diffusion Electrode devices, low aqueous CO2 solubility limits catalysis rate and selectivity. Here, we demonstrate that when assembled over a heterogeneous electrocatalyst, a film of nitrile-modified Metal-Organic Framework (MOF) acts as a remarkable CO2-solvation layer that increases its local concentration by ~27-fold compared to bulk electrolyte, reaching 0.82 M. When mounted on a Bi catalyst in a Gas Diffusion Electrode, the MOF drastically improves CO2-to-HCOOH conversion, reaching above 90% selectivity and partial HCOOH currents of 166 mA/cm2 (at −0.9 V vs RHE). The MOF also facilitates catalysis through stabilization of reaction intermediates, as identified by operando infrared spectroscopy and Density Functional Theory. Hence, the presented strategy provides new molecular means to enhance heterogeneous electrochemical CO2 reduction reaction, leading it closer to the requirements for practical implementation.

Exploring Tunable Properties, Solvent-Modulated Dynamics, and Novel C(sp3)-H Activation Mechanisms in Electron Donor-Acceptor Complexes.

Electron donor-acceptor (EDA) complex photochemistry has emerged as a vibrant area in visible-light-mediated synthetic radical chemistry. However, theoretical insights into the reaction mechanisms remain limited. Our study investigates the influence of solvent polarity and halogen atom types on radical reaction pathways in EDA complexes. We demonstrate that solvent polarity modulates the charge transfer and spatial arrangement within EDA complexes, thereby influencing their stability and reaction kinetics. Iodide ions play a crucial role in facilitating free radical generation and stabilizing reaction intermediates. Different halogen atom types exhibit distinct effects on radical reactions. Variations in radical concentration and solvent environment further affect the pathway selectivity. Additionally, light conditions influence the free radical generation and pathway selectivity. Our findings enhance the understanding of EDA complex photochemistry and radical reactions, offering insights for organic synthesis and photochemistry applications.

High-performance and selective catalyst for the chemical fixation of CO2 into cyclic carbonates based on pyridinium-based deep eutectic solvent

Rising atmospheric CO2 levels have prompted interest in chemical fixation routes to convert CO2 into value-added cyclic carbonates. This study reports a high-performance catalyst based on a pyridinium-based deep eutectic solvent (DES) for CO2 cycloaddition with epoxides under mild conditions. The DES was prepared by mixing 1-(carboxymethyl)pyridinium bromide ([CMPy]Br) as a hydrogen bond acceptor with maleic acid as an inexpensive and eco-friendly hydrogen bond donor [HBD]. The melting point depression enables the DES to act as a homogeneous catalyst without additional solvents. A diverse range of cyclic carbonates were synthesized in high yields from various epoxides and CO2 under atmospheric pressure at 90–110 °C temperatures. Compared to other catalytic systems, the offered DES significantly reduced reaction times of 1–3 h to achieve best quantitative conversions. The DES was recyclable for at least 5 runs without loss of activity. The high performance was attributed to the DES' ability to activate epoxides via hydrogen bonding while stabilizing reaction intermediates. This metal-free and solvent-free approach provides an efficient strategy for CO2 utilization under green conditions using readily available pyridinium-based DES.

Recognizing the reactive sites of SnFe2O4 for the oxygen evolution reaction: the synergistic effect of SnII and FeIII in stabilizing reaction intermediates.

Among the reported spinel ferrites, the p-block metal containing SnFe2O4 is scarcely explored, but it is a promising water-splitting electrocatalyst. This study focuses on the reaction kinetics and atomic scale insight of the reaction mechanism of the oxygen evolution reaction (OER) catalyzed by SnFe2O4 and analogous Fe3O4. The replacement of FeIIOh sites with SnIIOh in SnFe2O4 improves the catalytic efficiency and various intrinsic parameters affecting the reaction kinetics. The variable temperature OER depicts a low activation energy (Ea) of 28.71 kJ mol-1 on SnFe2O4. Experimentally determined second-order dependence on [OH-] and the prominent kinetic isotope effect observed during the deuterium labelling study implies the role of hydroxide ions in the rate-determining step (RDS). Using density functional theory, the reaction mechanism on the (001) surface of SnFe2O4 and Fe3O4 is modelled. The DFT simulated free energy diagram for the reaction intermediates shows an adsorbate evolution mechanism (AEM) on both the ferrites' surfaces where the formation of *OOH is the RDS on SnFe2O4 while *O formation is the RDS on Fe3O4. In contrast to other spinel ferrites, where individual metal sites act independently, in case of SnFe2O4, a synergy between FeIIIOh and the neighbouring SnIIOh atoms is responsible for stabilizing the OER intermediates, enhancing the catalytic OER activity of SnFe2O4 as compared to isostructural Fe3O4.

Photocatalytic production of ethylene and propionic acid from plastic waste by titania-supported atomically dispersed Pd species

Current chemical recycling of bulk synthetic plastic, polyethylene (PE), operates at high temperature/pressure and yields a complex mixture of products. PE conversion under mild conditions and with good selectivity toward value-added chemicals remains a practical challenge. Here, we demonstrate an atomic engineering strategy to modify a TiO 2 photocatalyst with reversible Pd species for the selective conversion of PE to ethylene (C 2 H 4 ) and propionic acid via dicarboxylic acid intermediates under moderate conditions. TiO 2 -supported atomically dispersed Pd species exhibits C 2 H 4 evolution of 531.2 μmol g cat −1 hour −1 , 408 times that of pristine TiO 2 . The liquid product is a valuable chemical propanoic acid with 98.8% selectivity. Plastic conversion with a C 2 hydrocarbon yield of 0.9% and a propionic acid yield of 6.3% was achieved in oxidation coupled with 3 hours of photoreaction. In situ spectroscopic studies confirm a dual role of atomic Pd species: an electron acceptor to boost charge separation/transfer for efficient photoredox, and a mediator to stabilize reaction intermediates for selective decarboxylation.

Understanding the reaction mechanism and kinetics of photocatalytic oxygen evolution on CoOx-loaded bismuth vanadate.

Photocatalytic water splitting for green hydrogen production is hindered by the sluggish kinetics of oxygen evolution reaction (OER). Loading a co-catalyst is essential for accelerating the kinetics, but the detailed reaction mechanism and role of the co-catalyst are still obscure. Here, we focus on cobalt oxide (CoOx) loaded on bismuth vanadate (BiVO4) to investigate the impact of CoOx on the OER mechanism. We employ photoelectrochemical impedance spectroscopy and simultaneous measurements of photoinduced absorption and photocurrent. The reduction of V5+ in BiVO4 promotes the formation of a surface state on CoOx that plays a crucial role in the OER. The third-order reaction rate with respect to photohole charge density indicates that reaction intermediate species accumulate in the surface state through a three-electron oxidation process prior to the rate-determining step. Increasing the excitation light intensity onto the CoOx-loaded anode improves the photoconversion efficiency significantly, suggesting that the OER reaction at dual sites in an amorphous CoOx(OH)y layer dominates over single sites. Therefore, CoOx is directly involved in the OER by providing effective reaction sites, stabilizing reaction intermediates, and improving the charge transfer rate. These insights help advance our understanding of co-catalyst-assisted OER to achieve efficient water splitting.

ABZYMES: A Comprehensive Review of Applications and Advances

Abzymes, or antibody enzymes, are antibodies that have enzymatic activity. They can catalyze chemical reactions, similar to traditional enzymes. Abzymes have potential applications in medicine and biotechnology, as they can be engineered to target specific molecules, making them promising for therapies and diagnostics. They often called catmab (from catalytic monoclonal antibody).They can be artificial or naturally occurring, even in autoimmune diseases. They function by stabilizing reaction intermediates, making them valuable tools in biotechnology

Direct Conversion of Methane to Ethylene and Acetylene over an Iron-Based Metal-Organic Framework.

Conversion of methane (CH4) to ethylene (C2H4) and/or acetylene (C2H2) enables routes to a wide range of products directly from natural gas. However, high reaction temperatures and pressures are often required to activate and convert CH4 controllably, and separating C2+ products from unreacted CH4 can be challenging. Here, we report the direct conversion of CH4 to C2H4 and C2H2 driven by non-thermal plasma under ambient (25 °C and 1 atm) and flow conditions over a metal-organic framework material, MFM-300(Fe). The selectivity for the formation of C2H4 and C2H2 reaches 96% with a high time yield of 334 μmol gcat-1 h-1. At a conversion of 10%, the selectivity to C2+ hydrocarbons and time yield exceed 98% and 2056 μmol gcat-1 h-1, respectively, representing a new benchmark for conversion of CH4. In situ neutron powder diffraction, inelastic neutron scattering and solid-state nuclear magnetic resonance, electron paramagnetic resonance (EPR), and diffuse reflectance infrared Fourier transform spectroscopies, coupled with modeling studies, reveal the crucial role of Fe-O(H)-Fe sites in activating CH4 and stabilizing reaction intermediates via the formation of an Fe-O(CH3)-Fe adduct. In addition, a cascade fixed-bed system has been developed to achieve online separation of C2H4 and C2H2 from unreacted CH4 for direct use. Integrating the processes of CH4 activation, conversion, and product separation within one system opens a new avenue for natural gas utility, bridging the gap between fundamental studies and practical applications in this area.

PbHPO4 derived Pb catalysts modified by polyethylene glycol toward efficient CO2 electroreduction to formate

Surface ligands of catalytic materials can create a favorable reaction microenvironment which stabilizes reaction intermediates and enriches solvated reactants. Herein, a polyethylene glycol-modified Pb nanosheet (Pb-PEG) catalyst is developed through in situ electrochemical reconstruction of PEG-modified PbHPO4, for efficient CO2 electroreduction. Formate is selectively produced with a Faradaic efficiency (FE) of 93.2 ± 2.9% and a remarkable partial current density of 326.0 ± 10.3 mA cm−2. The Pb-PEG catalyst also exhibits a stable formate FE of over 80% for 100 h, resulting in the production of ∼693 mmol formate. The morphology engineering by PEG maintains the lamellar morphology of Pb nanosheets to expose more active sites. Density functional theory calculations reveal that the tailored reaction microenvironment created by vicinal PEG ligands acts as a catalytic reservoir which facilitates CO2 adsorption and stabilizes the formation of the *OCOH intermediate, thus resulting in the impressive formate production.

Spontaneous Oxygen Reduction on Perovskite Oxide Surface in Aqueous Medium

Oxygen reduction/evolution reactions (OER/ORR) do not normally occur spontaneously at room temperature. Here, we will demonstrate that a perovskite oxide can drive a spontaneous ORR in aqueous media. Our experiments show that spontaneous ORR, while taking place at open-circuit conditions, results in a severe oxidation of the surface and causes cation leaching, oxygen intercalation and gradual dissolution of the perovskite. Using DFT and molecular dynamics simulations of the perovskite-electrolyte system, we propose a possible mechanism for this highly unusual phenomenon. Likely, a spontaneous ORR is connected with the emergence of a specific atomic configuration at the surface that stabilizes reaction intermediates and reduces activation barrier(s) for the ORR.

Atomic-scale surface restructuring of copper electrodes under CO2 electroreduction conditions

AbstractPotentiodynamic methods that induce structural changes in Cu catalysts for the electrochemical reduction of CO2 (CO2RR) have been identified as a promising strategy for steering the catalyst selectivity towards the generation of multi-carbon products. In current approaches, active species are created via a sequential Cu oxidation–reduction process. Here we show by in situ scanning tunnelling microscopy, surface X-ray diffraction and Raman spectroscopy measurements that low-coordinated Cu surface species form spontaneously near the onset of CO2 electrocatalytic reduction. This process starts by CO-induced Cu nanocluster formation in the initial stages of the reaction, leading to irreversible surface restructuring that persists over a wide potential range. On subsequent potential increase, the nanoclusters disperse into Cu adatoms, which stabilize reaction intermediates on the surface. The observed self-induced formation of undercoordinated sites on the CO2-converting Cu catalyst surface can account for its reactivity and may be exploited to (re)generate active CO2RR sites by potentiodynamic protocols.

Size-Dependent Reactivity of Con- (n = 5-25) Cluster Anions toward Carbon Dioxide.

A fundamental understanding of the reactivity evolution of nanosized clusters at an atomically precise level is pivotal to assemble desired materials with promising candidates. Benefiting from the tandem mass spectrometer coupled with a high-temperature ion-trap reactor, the reactions of mass-selected Con- (n = 5-25) clusters with CO2 were investigated and the increased reactivity of Co20-25- was newly discovered herein. This finding marks an important step to understand property evolution of subnanometer metal clusters (Co25-, ∼0.8 nm) atom-by-atom. The reasons behind the increased reactivity of Co20-25- were proposed by analyzing the reactions of smaller Co6-8- clusters that exhibit significantly different reactivity toward CO2, in which a lower electron affinity of Con contributes to the capture of CO2 while the flexibility of Con- could play vital roles to stabilize reaction intermediates and suppress the barriers of O-CO rupture and CO desorption.

Exploring The Synergistic Effect Of CoSeP/CoP Interface Catalyst For Efficient Urea Electrolysis.

Electrocatalytic oxidation of urea (UOR) is a potential energy-saving hydrogen production technology that can replace oxygen evolution reaction (OER). Therefore, CoSeP/CoP interface catalyst is synthesized on nickel foam using hydrothermal, solvothermal, and in situ template methods. The strong interaction of tailored CoSeP/CoP interface promotes the hydrogen production performance of electrolytic urea. During the hydrogen evolution reaction (HER), the overpotential can reach 33.7mV at 10mA cm-2 . The cell voltage can reach 1.36V at 10mA cm-2 in the overall urea electrolytic process. Notably, the overall urine electrolysis performance of the catalyst in the human urine medium can reach 1.40V at 10mA cm-2 and can exhibit durable cycle stability at 100mA cm-2 . Density functional theory (DFT) proves that the CoSeP/CoP interface catalyst can better adsorb and stabilize reaction intermediates CO* and NH* on its surface through a strong synergistic effect, thus enhancing the catalytic activity.