Active Intermediates Research Articles

In situ exploration of oxygen electrocatalysis using core-shell nanostructure-enhanced Raman spectroscopy

Advancements in fuel cells and water electrolyzers have significantly bolstered the utilization of hydrogen energy. Notably, the oxidation and reduction processes of oxygen at the electrode—termed oxygen evolution reaction (OER) and oxygen reduction reaction (ORR)—manifest sluggish reaction kinetics, thus requiring noble metals as catalysts, which considerably impedes system efficiency and cost. The imperative for enhancing reaction rates and diminishing overpotential necessitates the development of effective catalysts, which strongly depends on the mechanistic understanding of these reactions at the molecular level. Therefore, this review summarizes our recent efforts in utilizing in situ enhanced Raman spectroscopy, especially the borrowing surface-enhanced Raman spectroscopy (SERS) strategy, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), and the SHINERS-satellite strategy, to capture oxygen intermediate species as a bridge to investigate the molecular mechanisms of OER and ORR. Combining in situ SERS with other characterization techniques and theoretical simulation, the structural evolution of active sites and intermediates, including ∗OOH, ∗OH, ∗OO, etc., during OER/ORR has been monitored under reaction conditions, and the reaction mechanisms together with structure-activity correlations have been identified at the molecular level. These findings may provide a pivotal scientific foundation towards the discovery of better materials for electrochemical hydrogen energy.

NHC-Mediated Photochemical/Electrochemical Synthesis of Carbonyl Compounds

AbstractGreat progress has been made in the photochemical/electrochemical reactions of carbonyl compounds catalyzed by N-heterocyclic carbenes (NHCs), allowing for many incredible reactions. The form of active intermediates hugely varies in different reactions. Under light or electricity, different active intermediates can be generated during NHC-catalyzed reactions depending on their interaction with different substrates and the reaction conditions. Intermediates can be converted into α,β-unsaturated acylazoliums, Breslow intermediates, homoenolates, and acylazoliums and subsequently undergo single- or double-electron conversions. This study reviews the different active intermediates formed by NHC in photochemical/electrochemical catalysis.1 Introduction2 NHC-Mediated Photoreactions3 NHC-Mediated Electrochemical Reactions4 Conclusion and Prospects

Electrochemical Generation of Te Vacancy Pairs in PtTe for Efficient Hydrogen Evolution.

Two-dimensional (2D) van der Waals materials are increasingly seen as potential catalysts due to their unique structures and unmatched properties. However, achieving precise synthesis of these remarkable materials and regulating their atomic and electronic structures at the most fundamental level to enhance their catalytic performance remain a significant challenge. In this study, we synthesized single-crystal bulk PtTe crystals via chemical vapor transport and subsequently produced atomically thin, large PtTe nanosheets (NSs) through electrochemical cathode intercalation. These NSs are characterized by a significant presence of Te vacancy pairs, leading to undercoordinated Pt atoms on their basal planes. Experimental and theoretical studies together reveal that Te vacancy pairs effectively optimize and enhance the electronic properties (such as charge distribution, density of states near the Fermi level, and d-band center) of the resultant undercoordinated Pt atoms. This optimization results in a significantly higher percentage of dangling O-H water, a decreased energy barrier for water dissociation, and an increased binding affinity of these Pt atoms to active hydrogen intermediates. Consequently, PtTe NSs featuring exposed and undercoordinated Pt atoms demonstrate outstanding electrocatalytic activity in hydrogen evolution reactions, significantly surpassing the performance of standard commercial Pt/C catalysts.

Regulating Reactive Oxygen Species over M-N-C Single-Atom Catalysts for Potential-Resolved Electrochemiluminescence.

The development of potential-resolved electrochemiluminescence (ECL) systems with dual emitting signals holds great promise for accurate and reliable determination in complex samples. However, the practical application of such systems is hindered by the inevitable mutual interaction and mismatch between different luminophores or coreactants. In this work, for the first time, by precisely tuning the oxygen reduction performance of M-N-C single-atom catalysts (SACs), we present a dual potential-resolved luminol ECL system employing endogenous dissolved O2 as a coreactant. Using advanced in situ monitoring and theoretical calculations, we elucidate the intricate mechanism involving the selective and efficient activation of dissolved O2 through central metal species modulation. This modulation leads to the controlled generation of hydroxyl radical (·OH) and superoxide radical (O2·-), which subsequently trigger cathodic and anodic luminol ECL emission, respectively. The well-designed Cu-N-C SACs, with their moderate oxophilicity, enable the simultaneous generation of ·OH and O2·-, thereby facilitating dual potential-resolved ECL. As a proof of concept, we employed the principal component analysis statistical method to differentiate antibiotics based on the output of the dual-potential ECL signals. This work establishes a new avenue for constructing a potential-resolved ECL platform based on a single luminophore and coreactant through precise regulation of active intermediates.

Efficient Electrochemical Nitrate Removal by Ordered Ultrasmall Intermetallic AuCu3 via Enhancing Nitrate Adsorption.

Developing a high-performance electrocatalyst for synthesizing ammonia from nitrate represents a promising solution for addressing wastewater pollution and achieving sustainable ammonia production. However, it remains a formidable challenge. Herein, an intermetallic AuCu3 electrocatalyst with high-density active sites is designed and prepared for an efficient nitrate electroreduction to generate ammonia. Remarkably, the Faraday efficiency and yield rate of ammonia at -0.9 V are 97.6% and 75.9 mg h-1 cm-2, respectively. More importantly, after 10 cycles of testing, the removal rate of nitrate can still reach 95.2%. Electrochemical in situ Fourier transform infrared analysis indicates that AuCu3 IM can promote the adsorption of nitrate and enhance ammonia production from nitrate. *NH3, *NO, and *NO2 have been proven to be active intermediates. Theoretical and experimental studies show that the Au site can provide a large amount of *H for nitrate reduction, and the Cu site is conducive to the reduction of nitrate to produce nitrogen-containing products. Meanwhile, AuCu3 intermetallic compounds (AuCu3 IM) can inhibit the dimerization of *H. The power density and ammonia yield of the assembled Zn-nitrate battery reached 2.17 mW cm-2 and 71.2 mg h-1 cm-2, respectively.

Orienting Electron Fillings in d Orbitals of Cobalt Single Atoms for Effective Zinc–Air Battery at a Subzero Temperature

AbstractThe missions in extreme environments such as the moon, the Arctic/Antarctic, and the plateau require the operation of batteries at low temperatures even below the freezing point of water. Herein, it is reported that compressively stressed Co single atoms exhibit enhanced oxygen reduction reaction (ORR) activity and enable the effective operation of zinc–air battery at a subzero temperature. The compressive strain is generated by depositing Co single atoms on highly arced carbon layers with ultra‐small curvature radii ≈2 nm. The locally compressive strain on Co single atoms redistributes the electron fillings in d orbitals with different spatial orientations, thereby strengthening the adsorption of active intermediates and enhancing the activity toward ORR. As expected, compressively stressed Co single atoms outperform Co single atoms on a flat support without strain in terms of kinetic current density (31.09 mA cm−2 vs 0.35 mA cm−2) at 0.85 V during ORR. The integration of the catalyst into a Zn–air battery generates a superior power density of 54.8 mW cm−2 than the commercial Pt/C counterpart (24.1 mW cm−2) at −40 °C.

Ligand-engineered Cu-based halide perovskite for highly efficient near-infrared photocatalytic CO2 reduction

A key challenge in harvesting solar energy for efficient chemical conversion is the lack of photocatalysts with wide activation wavelengths. Herein, we propose for the first time to utilize halide perovskite to absorb the full spectrum (200–2500 nm), including ultraviolet (UV), visible (Vis), and near-infrared (NIR) light, to directly power photocatalytic CO2 reduction. This full-spectrum light-responsive occurs on metal halide perovskite Cs2CuCl4 microcrystals (MCs), and the ligand soybean lecithin is applied to optimize the active phase of Cs2CuCl4 photocatalyst, such as morphology, particle size, crystal face and electronic structure. As revealed by optical absorption analysis, the Cs2CuCl4 and ligand soybean lecithin modified Cs2CuCl4 (SL-Cs2CuCl4) MCs exhibit significant optical absorption in the UV, Vis light and NIR light regions. The photocatalytic CO2 reduction performance was assessed under simulated sunlight (200–2500 nm), and the SL-Cs2CuCl4 MCs achieved a CO fuel yield of 254.46 µmol g−1, which increased the yield by 5 times relative to the initial sample. Based on in-situ Fourier transform infrared, electron spin resonance and X-ray photoelectron spectroscopy, the active substances and reaction intermediates at the active site of SL-Cs2CuCl4 MCs were dynamically monitored, and the photocatalytic mechanism was revealed together with density functional theory (DFT) calculations. The DFT calculation shows that the photocatalytic reduction of CO2 to CO by Cs2CuCl4 is proposed to involve the concerted action of both Cs and Cu sites.

Oxygen vacancy-dependent chemical intermediates on Ru/MnO catalysts dictate the selectivity of CO2 reduction

Structural dependence of the formation and transformation of the surface intermediates is the key to controlling the catalytic selectivity. Especially a highly controversial issue is whether formate species are spectator or active intermediates for CO2 reduction. Herein, the Ru/MnOx catalysts with different oxygen vacancy densities were developed to clarify these aspects. The high-defective RuMn-Ov showed high CH4 selectivity (i.e. 89.3 % for CH4 at 320 °C), while the low-defective RuMn-C mainly produced CO (i.e. 100 % for CO at 320 °C). By using multiple characterizations combined with theoretical modelling, it can be proved that the surface oxygen vacancies on MnOx induces the generation of the formate intermediate, followed by hydrogenated to methane via spillover H derived from Ru nanoparticle. Remarkably, the RuMn-C catalyst at high Ru loading (c.a. 5 wt%) without formate intermediates achieved over 93 % CO selectivity. The results give an instructive way to understand the selectivity regulation driven by response reaction pathways.

Ethanolysis of enzymatic hydrolysis lignin with Ni catalysts on different supports: the roles of catalytic sites

Catalytic lignin solvolysis (CLS) holds promise for efficient lignin utilization, yielding small molecules with minimal or no char formation. However, the role of different active sites of catalyst in CLS are rarely discussed. Here, Ni catalysts on different supports, i.e., SiO2, Al2O3, MgO, and ZrO2, were prepared with the aim of manipulating the relative importance of metal and acid-base functionalities to investigate the role of different active sites in the enzymatic hydrolysis of lignin (EHL) ethanolysis. The main ether linkage, i.e., β-O-4, in EHL is cleaved without the participation of a catalyst. Metal sites suppress repolymerization through hydrogenating active intermediates, while acid and base sites facilitate the conversion of phenolic monomers into complex alkylated and etherified products and also promote repolymerization reactions. Among the catalysts, Ni/SiO2 demonstrated the highest hydrogenation activity and yielded the most monomers (24.7 wt%) at 280 oC for 6 h under 2 MPa H2 in ethanol. These findings shed light on the catalytic mechanisms in CLS, offering valuable insights for future catalyst design.

Stabilizing BiVO4 Photoanode in Bicarbonate Electrolyte for Efficient Photoelectrocatalytic Alcohol Oxidation.

In order to expand the application of bismuth vanadate (BiVO4) to the field of photoelectrochemistry, researchers have explored the potential of BiVO4 in catalyzing or degrading organic substances, potentially presenting a green and eco-friendly solution. A study was conducted to investigate the impact of electrolytes on the photocatalysis of benzyl alcohol by BiVO4. The research discovered that, in an acetonitrile electrolyte (pH 9) with sodium bicarbonate, BiVO4 catalyzed benzyl alcohol by introducing saturated V5+. This innovation addressed the issue of benzyl alcohol being susceptible to catalysis in an alkaline setting, as V5+ was prone to dissolution in pH 9 on BiVO4. The concern of the photocorrosion of BiVO4 was mitigated through two approaches. Firstly, the incorporation of a non-aqueous medium inhibited the formation of active material intermediates, reducing the susceptibility of the electrode surface to photocorrosion. Secondly, the presence of saturated V5+ further deterred the leaching of V5+. Concurrently, the production of carbonate radicals by bicarbonate played a vital role in catalyzing benzyl alcohol. The results show that, in this system, BiVO4 has the potential to oxidize benzyl alcohol by photocatalysis.

Chemoselective Transfer Hydrogenation of Nitroarenes with Ammonia Borane Catalyzed by Copper N‐heterocyclic Carbene Complexes

Comprehensive SummaryHerein, we present a method for the homogeneous hydrogenation of nitroarenes to produce anilines using low catalyst loading (1 mo%) of copper N‐heterocyclic carbene complexes as the catalyst and ammonia borane as the source of hydrogen. A wide range of nitroarenes, featuring diverse functional groups, were selectively transformed into their corresponding primary aromatic amines with high yields. This process can be readily scaled up and exhibits compatibility with various sensitive functional groups, including halogen, trifluoromethyl, aminomethyl, alkenyl, cyano, ester, amide, and hydroxyl. Notably, this catalytic methodology finds application in the synthesis of essential drug compounds. Mechanistic investigations suggest that the in‐situ‐generated Cu‐H species may serve as active intermediates, with reduction pathways involving species such as azobenzene, 1,2‐diphenylhydrazine, nitrosobenzene, and N‐phenylhydroxylamine.

Integrated Process for Schizochytrium Oil Extraction, Enzymatic Modification of Lipids and Concentration of DHA Fatty Acid Esters Using Alternative Methodologies.

Marine microalgae Schizochytrium sp. have a high content of docosahexaenoic acid (DHA), an omega-3 fatty acid that is attracting interest since it prevents certain neurodegenerative diseases. The obtention of a bioactive and purified DHA fatty acid ester using a whole-integrated process in which renewable sources and alternative methodologies are employed is the aim of this study. For this reason, lyophilized Schizochytrium biomass was used as an alternative to fish oil, and advanced extraction techniques as well as enzymatic modification were studied. Microalgal oil extraction was optimized via a surface-response method using pressurized liquid extraction (PLE) obtaining high oil yields (29.06 ± 0.12%) with a high concentration of DHA (51.15 ± 0.72%). Then, the enzymatic modification of Schizochytrium oil was developed by ethanolysis using immobilized Candida antarctica B lipase (Novozym® 435) at two reaction temperatures and different enzymatic loads. The best condition (40 °C and 200 mg of lipase) produced the highest yield of fatty acid ethyl ester (FAEE) (100%) after 8 h of a reaction attaining a cost-effective and alternative process. Finally, an enriched and purified fraction containing DHA-FAEE was obtained using open-column chromatography with a remarkably high concentration of 93.2 ± 1.3% DHA. The purified and bioactive molecules obtained in this study can be used as nutraceutical and active pharmaceutical intermediates of marine origin.

Photoinduced Generation of Active Intermediates from Unmodified 1,3‐Dicarbonyl Compounds for Organic Transformations

AbstractPhotocatalysis utilizes light as the initial driving force for chemical reactions, enabling reactions with high energy barriers in traditionally thermal chemistry to proceed under mild conditions. 1,3‐Dicarbonyl compounds occur as important active molecules in nature and are intermediates in pharmaceutical syntheses. In recent years, there have been numerous studies of these compounds, and it is common practice to pre‐modify them before the reaction, which is, however, unfavorable for the atom economy of the process. This paper provides a comprehensive review of the latest advancements in the site‐specific activation of unmodified 1,3‐dicarbonyl compounds by photoinduction. The focus is on elucidating the mechanisms of these reactions to provide insights and directions for future research in designing novel reactions.

Theoretical study on oxygen evolution reaction mechanism of double rare earth europium-doped graphene under hydroxyl modification in alkaline environment

The development and design of high performance OER catalyst is the key to electrocatalysis technology. Herein, based on density functional theory (DFT), the oxygen evolution reaction mechanism of double rare earth europium-doped graphene under hydroxyl modification in alkaline environment has been systematically studied. Through thermodynamic and kinetic stability analysis, and the optimal reaction path and different adsorption sites of intermediates were compared. Four catalysts with good catalytic activity for OER reaction were selected. The results show that when the two hydroxyl groups are modified, the overpotentials on the optimal pathway for the four catalyst configurations are 0.54 V, 0.50 V, 0.54 V, and 0.61 V, respectively. These catalysts demonstrate excellent catalytic activity for the oxygen evolution reaction (OER). Moreover, these catalysts have good electrical conductivity, and the active site and adsorption intermediates can be stably bonded together. In addition, the scaling relationship between oxygen adsorption free energy and overpotential is described. This work may provide new insights and guidance for future research on rare-earth atom-based OER catalysts.

Modulating the acidity and reducibility of FeMn/TiO2 to boost the low-temperature selective catalytic reduction of NOx

The design and fabrication of selective catalytic reduction (SCR) catalyst with ultra-low-temperature activity is diligent, however, remains challenging hitherto. Herein, a variety of TiO2-based catalysts with various Fe/Mn ratios were fabricated using a coprecipitation technique and tested for SCR of NOx with ammonia. The experimental results demonstrate that the Fe/Mn ratio considerably affects the acidity, redox property and thereafter the NH3-SCR behavior of FeMn/TiO2 catalysts. Therein, Fe0.3Mn0.7/TiO2 exhibits the broadest activity temperature window from 75 to 210 °C because of its more reactive oxygen species and higher Mn4+ ratio on the catalyst surface, leading to improved oxidation performance and surface acidity, thereby promoting the generation of active NO2 and HONO intermediates, and thereafter enabling the catalyst to adsorb more NH3 to generate active nitrate species and increasing the NH3-SCR performance. Further increasing or decreasing the Fe/Mn ratio would downgrade the SCR performances of corresponding catalysts due to the mismatch between acidity and redox ability, which inhibits the formation of active NO2 and HONO intermediates. In-situ DRIFTS results further illustrate that the SCR of NOx over FeMn/TiO2 catalysts follow both the L-H and E-R mechanisms at the low temperature, however, the types of active nitrates flourish at high temperatures, primarily via the E-R mechanism.

Construction of In2O3@CN S-type heterojunctions with defect engineering for highly efficient photocatalytic degrade atrazine: Efficacy and mechanism evaluation

Developing exceptional photocatalysts with a high ability for photocarrier dissociation and photoredox reactions is crucial for effectively eliminate water pollutants, yet it remains a significant challenge. In this study, a S-type heterojunction with N defect, 2In2O3@CN, is constructed by incorporating In2O3 nanoparticles onto g-C3N4 nanosheet. The S-type charge transfer mode markedly promotes the space detachment of high-energy electron/hole and preservation on 2In2O3@CN, effectively suppressing the photocorrosion of In2O3. The optimized 2In2O3@CN heterojunction demonstrates superior performance. In visible light-driven photocatalytic degradation of atrazine, the efficiency reaches an impressive 81.5%. The pseudo-first-order kinetics are 12.7 times higher than those of pure CN, while maintaining excellent stability and reusability. Calculations and liquid mass measurements have revealed the primary active substances, intermediates, and photodegradation pathways in the photocatalytic process. This study not only offers a potential photocatalyst for safeguarding aquatic environments but also encourages the exploration of novel and potent CN-based S-type photocatalysts for environmental protection.

C-H Bond Activation by a Seven-Coordinate Bipyridine-Bipyrazole Ruthenium(IV) Oxo Complex.

Ruthenium-oxo species with high coordination numbers have long been proposed as active intermediates in catalytic oxidation chemistry. By employing a tetradentate bipyridine-bipyrazole ligand, we herein reported the synthesis of a seven-coordinate (CN7) ruthenium(IV) oxo complex, [RuIV(tpz)(pic)2(O)]2+ (RuIVO) (tpz = 6,6'-di(1H-pyrazol-1-yl)-2,2'-bipyridine, pic = 4-picoline), which exhibits high activity toward the oxidation of alkylaromatic hydrocarbons. The large kinetic isotope effects (KIE) for the oxidation of DHA/DHA-d4 (KIE = 10.3 ± 0.1) and xanthene/xanthene-d2 (KIE = 17.2 ± 0.1), as well as the linear relationship between log (rate constants) and bond dissociation energies of alkylaromatics, confirmed a mechanism of hydrogen atom abstraction.

Inhibitory effect of thiol compounds on browning precursors and intermediates in sorbitol/glycine system.

Sorbitol as a sweetener is often thought to be unable to participate in the Maillard reaction causing browning. However, browning of a system was found to be significant when sorbitol was mixed with glycine and heated. The thiol compounds glutathione and cysteine were added to the system, and the inhibition mechanism of the two on the browning of the system was studied by combining the changes of precursor substances, intermediate products and browning degree. When the concentration of thiol compounds reached 25 mg mL-1 , both could make the browning inhibition of the system more than 80%, and the accumulated glucose concentration was reduced to <35% of the control. The production of 3-deoxyglucosone, a precursor of melanoidin, was significantly reduced. Glutathione and cysteine directly inhibited the production of substrates in the sorbitol/glycine system, reduced glucose accumulation through competitive consumption and captured highly active intermediates through sulfhydryl groups. This has implications for the browning control of food systems containing sugar alcohols. © 2024 Society of Chemical Industry.

Geometric and Electronic Engineering in Co/VN Nanoparticles to Boost Bifunctional Oxygen Electrocatalysis for Aqueous/Flexible Zn-Air Batteries.

Modulating metal-metal and metal-support interactions is one of the potent tools for augmenting catalytic performance. Herein, highly active Co/VN nanoparticles are well dispersed on three-dimensional porous carbon nanofoam (Co/VN@NC) with the assistance of dicyandiamide. Studies certify that the consequential disordered carbon substrate reinforces the confinement of electrons, while the coupling of diverse components optimizes charge redistribution among species. Besides, theoretical analyses confirm that the regulated electron configuration can significantly tune the binding strength between the active sites and intermediates, thus optimizing reaction energy barriers. Therefore, Co/VN@NC exhibits a competitive potential difference (ΔE, 0.65 V) between the half-wave potential of ORR and OER potential at 10 mA cm-2 , outperforming Pt/C+RuO2 (0.67 V). Further, catalyst-based aqueous/flexible ZABs present superior performances with peak power densities of 156 and 85 mW cm-2 , superior to Pt/C-based counterparts (128 and 73 mW cm-2 ). This research provides a pivotal foundation for the evolution of bifunctional catalysts in the energy sector.

Pd-Catalyzed Cascade Cyclization/Thiocarbonylation with Thioformates: Synthesis of Thioester-Functionalized Oxindoles.

A Pd-catalyzed thiocarbonylative cyclization of N-(o-iodoaryl)acrylamides with easily accessible thioformates has been developed. The reaction has a wide substrate scope with good yields and represents a powerful route to the synthesis of thioester-functionalized oxindoles. Both S-aryl and alkyl thioformates as the thioester sources were well tolerated. The active Pd-CO intermediate may play an important role in the transformation process.