Reaction Mechanism Research Articles

Recent Advances in Revealing the Electrocatalytic Mechanism for Hydrogen Energy Conversion System.

In light of the intensifying global energy crisis and the mounting demand for environmental protection, it is of vital importance to develop advanced hydrogen energy conversion systems. Electrolysis cells for hydrogen production and fuel cell devices for hydrogen utilization are indispensable in hydrogen energy conversion. As one of the electrolysis cells, water splitting involves two electrochemical reactions, hydrogen evolution reaction and oxygen evolution reaction. And oxygen reduction reaction coupled with hydrogen oxidation reaction, represent the core electrocatalytic reactions in fuel cell devices. However, the inherent complexity and the lack of a clear understanding of the structure-performance relationship of these electrocatalytic reactions, have posed significant challenges to the advancement of research in this field. In this work, the recent development in revealing the mechanism of electrocatalytic reactions in hydrogen energy conversion systems is reviewed, including in situ characterization and theoretical calculation. First, the working principles and applications of operando measurements in unveiling the reaction mechanism are systematically introduced. Then the application of theoretical calculations in the design of catalysts and the investigation of the reaction mechanism are discussed. Furthermore, the challenges and opportunities are also summarized and discussed for paving the development of hydrogen energy conversion systems.

Removal of Hg(II) with MgAl-layered double hydroxide functionalized by Schiff base ligands: Application and condition optimization

To effectively remove heavy metal Hg(II) from water bodies, a novel adsorbent of MgAl-layered double hydroxide (LDH) was designed and functionalized with Schiff base. The characterization results of the adsorbent (MgAl-LDH@SiO2-AG) show that the Schiff base polymer was successfully coated onto the outside surface of MgAl-LDH with hexagonal structure. The theoretical maximum adsorption capacity to Hg(II) is 228.46 mg/g at pH 7 and 298 K. The different pH solutions were investigated from pH 2 to 8, and the optimal capacity of MgAl-LDH@SiO2-AG toward Hg(II) achieves 268.7 mg/g at pH = 7.2, T = 36.8 °C, C0 = 32.1 mg/L and dosage = 0.083 g/L. In reality, the adsorbent not only exhibits efficient removal of Hg(II) in various water bodies, including lake water, river water, effluent from sewage treatment plant, but also has an excellent selectivity in electroplating wastewater containing different heavy metal ions. Low contents of TN and TP in real wastewater have less effect on the removal of Hg(II). Moreover, the prepared adsorbent had a good reusability and stability. The reaction mechanism mainly involves chelation with nitrogen/oxygen-containing groups and the predominant participation of nitrogen atoms in the Schiff base functional group. The removal of Hg(II) relies on the pseudo-second-order kinetics and Langmuir model, and is an endothermic and spontaneous chemical reaction. The present work offers a practical method for preparing highly effective adsorptive materials with the LDH composites and for the treatment of heavy metal Hg(II) from water bodies.

Computational Insights into Palladium-Catalyzed Site-Selective Anilide and Benzamide-Type [3+2] Annulation via Double C-H Bond Activation.

The mechanism of palladium-catalyzed annulation reactions of benzamide- and anilide-type aromatic systems with maleimides is investigated using density functional theory. Double C-H bond activation is key to forming the desired annulation product. The first C-H bond activation for anilide- and amide-type ligands can occur at the ortho and benzylic C-H bonds, while the second C-H activation occurs at the meta carbon of the aromatic rings. For the anilide-type system, ortho and benzylic C-H bond activations occur via four- and five-membered palladacycles, respectively. In contrast, for the benzamide-type system, ortho and benzylic C-H bond activations occur via five- and six-membered palladacycles, respectively. The energy span model suggests that the initial C-H bond activation step at the benzylic position determines the turnover frequency for both anilide- and benzamide-type systems. Energy decomposition analysis and distortion-interaction/activation-strain analyses are employed to understand the electronic and steric factors controlling the turnover frequency-determining transition state.

Methylation Modification in Ornamental Plants: Impact on Floral Aroma and Color

Methylation represents a crucial class of modification that orchestrates a spectrum of regulatory roles in plants, impacting ornamental characteristics, growth, development, and responses to abiotic stress. The establishment and maintenance of methylation involve the coordinated actions of multiple regulatory factors. Methyltransferases play a pivotal role by specifically recognizing and methylating targeted sites, which induces alterations in chromatin structure and gene expression, subsequently influencing the release of volatile aromatic substances and the accumulation of pigments in plant petals. In this paper, we review the regulatory mechanisms of methylation modification reactions and their effects on the changes in aromatic substances and pigments in plant petals. We also explore the potential of methylation modifications to unravel the regulatory mechanisms underlying aroma and color in plant petals. This aims to further elucidate the synthesis, metabolism, and regulatory mechanisms of various methylation modifications related to the aroma and color substances in plant petals, thereby providing a theoretical reference for improving the aroma and color of plant petals.

Exploring the Reaction Mechanism of Polyethylene Terephthalate Biodegradation through QM/MM Approach.

The enzyme PETase fromIdeonella sakaiensis (IsPETase) strain 201-F6 can catalyze the hydrolysis of polyethylene terephthalate (PET), mainly converting it into mono(2-hydroxyethyl) terephthalic acid (MHET). In this study, we used quantum mechanics/molecular mechanics (QM/MM) simulations to explore the molecular details of the catalytic reaction mechanism of IsPETase in the formation of MHET. The QM region was described with AM1d/PhoT and M06-2X/6-31+G(d,p) potential. QM/MM simulations unveil the complete enzymatic PET hydrolysis mechanism and identify two possible reaction pathways for acylation and deacylation steps. The barrier obtained at M06-2X/6-31+G(d,p)/MM potential for the deacylation step corresponds to 20.4 kcal/mol, aligning with the experimental value of 18 kcal/mol. Our findings indicate that deacylation is the rate-limiting step of the process. Furthermore, per-residue interaction energy contributions revealed unfavorable contributions to the transition state of amino acids located at positions 200-230, suggesting potential sites for targeted mutations. These results can contribute to the development of more active and selective enzymes for PET depolymerization.

Effects of different Mg/Ca molar ratio on the formation of carbonate minerals in microbially induced carbonate precipitation (MICP) process

Traditional MICP technology has been extensively applied to improve soil properties and solidify heavy metal contaminated soil. However, the traditional product performance of calcite is far less than that of huntite, dolomite, and other carbonate minerals. Therefore, the soil strength and environmental safety after curing cannot reach the safety index. This study has examined the distribution of carbonate minerals induced by microorganisms under different Mg/Ca ratio. Compared with the sample with 0 Mg/Ca content, the unconfined compressive strength (UCS) can be increased by 1–5 times by increasing the content. By X-ray diffraction (XRD) and thermogravimetric analysis (TGA), the contents of different carbonate minerals under different Mg/Ca content were determined semi-quantitatively. The microscopic results have also shown that the introduction of microorganisms can provide nucleation sites for the synthesis of dolomite and huntite. MICP experiment coupled with Quantum ESPRESSO calculations revealed the mechanism of microbial influence on the formation of carbonate minerals under different Mg/Ca conditions through first principles calculations and nucleation kinetics. These findings advance our understanding of the reaction mechanisms and pathways of biogenic precipitation of carbonate minerals, and provide insights into the artificial quantitative synthesis of dolomite.

Advancing Organic Photoredox Catalysis: Mechanistic Insight through Time-Resolved Spectroscopy.

The rapid development of light-activated organic photoredox catalysts has led to the proliferation of powerful synthetic chemical strategies with industrial and pharmaceutical applications. Despite the advancement in synthetic approaches, a detailed understanding of the mechanisms governing these reactions has lagged. Time-resolved optical spectroscopy provides a method to track organic photoredox catalysis processes and reveal the energy pathways that drive reaction mechanisms. These measurements are sensitive to key processes in organic photoredox catalysis such as charge or energy transfer, lifetimes of singlet or triplet states, and solvation dynamics. The sensitivity and specificity of ultrafast spectroscopic measurements can provide a new perspective on the mechanisms of these reactions, including electron-transfer events, the role of solvent, and the short lifetimes of radical intermediates.

Investigation of cation doping on the structure and electrochemical properties of K0.5MnO2 cathode materials based on first-principles calculation

In an effort to find high-energy-density and high-mobility cathode material for K-Ion batteries, this paper presents a systematic study of multiple doping concentrations of P3-type K0.5Mn1-xMxO2 (M = Fe, Ti, and Cu) materials using first-principles approach. The most thermodynamically stable phase of each material is predicted by the formation of mixing enthalpy, and the optimal doping concentration of each doping element is screened. It is found that the redox activity of K0.5Mn1-xMxO2 is derived from the anion redox of O2−/O1− during depotassium. At the same time, doped Mn-based cathode materials have the characteristics of large capacity, stable structure, good thermal stability and small diffusion energy barrier. Among them, Fe doping will promote the charge compensation of O 2p-electrons in the intercalation process and reduce the Jahn-Teller distortion caused by Mn3+ (ratio < 46.67 %). Ti doping can improve the lattice stability of O, inhibit the precipitation reaction of oxygen, make TiO more covalent, and reduce the structural change during depotassium. Therefore, the overall thermal stability of the material is improved. Cu doping reduces the diffusion barrier of the material (0.54 eV). Through the basic understanding of the redox reaction mechanism in K0.5Mn1-xMxO2, the work of this paper lays a foundation for the rational design of high-performance K-ion batteries.

Construction of diabatic potential energy surfaces for the SiH2+ system and dynamics studies of the Si+(2P1/2, 3/2) + H2 reaction.

The construction of diabatic potential energy surfaces (PESs) for the SiH2+ system, related to the ground (12A') and excited states (22A'), has been successfully achieved. This was accomplished by utilizing high-level abinitio energy points, employing a neural network fitting method in conjunction with a specifically designed function. The newly constructed diabatic PESs are carefully examined for dynamics calculations of the Si+(2P1/2, 3/2) + H2 reaction. Through time-dependent quantum wave packet calculations, the reaction probabilities, integral cross sections (ICSs), and differential cross sections (DCSs) of the Si+(2P1/2, 3/2) + H2 reaction were reported. The dynamics results indicate that the total ICS is in excellent agreement with experimental data within the collision energy range studied. The results also indicate that the SiH+ ion is hardly formed via the Si+(2P3/2) + H2 reaction. The results from the DCSs suggest that the "complex-forming" reaction mechanism predominates in the low collision energy region. Conversely, the forward abstraction reaction mechanism is dominant in the high collision energy region.

Mechanistic investigation of methanol-to-olefins conversion catalyzed by H-ZSM-5 zeolite: a DFT study.

The mechanisms for the formation of the first C - C bond and lower olefins on methanol to olefins (MTO) conversion on H-ZSM-5 had been focused in dispute. In this paper, density functional theory has been used to study the reaction mechanisms of methanol to olefins on ZSM-5. The configurations of reactants, intermediates, products and transition state of the numerous reactions involved in such a process have been optimized, as well as the elementary reactions related to these configurations were determined by the calculation of corresponding activation energy barriers and reaction heats. Here, two different kinds of the mechanisms were proposed for the formation of dimethylether (DME), one involving an associative interaction of two methanol molecules with the zeolite Brønsted acid sites and the other occurring via a surface methoxy species and a methanol molecule. A critical intermediate of the methoxy methyl cation was theoretically verified by the reaction of the methoxy species and dimethylether. Besides, it was found that the first intermediates containing a C - C bond were 1,2-dimethoxyethane and 2-methoxy-ethanolare, in which the former was formed from methoxy species with dimethyl ether and the latter was formed from methanol by onium ions((CH3)2O+CH2CH2OCH3), respectively. For the whole reaction mechanism, the results in this paper indicated that the ethene formation is more favorable than propylene formation due to the low activation energy barrier for ethene formation (123.49 vs. 162.09kJ.mol-1). From these calculations, it would be concluded that ethene is the first alkene product that induces the occurrence of the hydrocarbon pool mechanism. All the periodic density function theory (DFT) calculations were performed by the Vienna Ab Initio Simulation package (VASP). The interaction between nucleus and valence electron was described using the pseudopotentials found in the projector augmented wave (PAW) method. PBE-D3 was used in the whole DFT calculations and CI-NEB was used to locate transition state.

Highly Selective Boron-Wittig Reaction: a Practical Method to Synthesize Trans-Aryl Alkenes.

Olefins play an essential role in synthetic chemistry, serving not only as important synthons but also as key functional groups in numerous bio-active molecules. Consequently, there has been considerable interest in the development of more powerful methods for olefins. While the Wittig reaction stands as a prominent choice for olefin synthesis due to its simplicity and the ready availability of raw materials, its limitation lies in the challenge of controlling cis-trans selectivity, hampering its broader application. In this study, a novel Boron-Wittig reaction has been developed utilizing gem-bis(boryl)alkanes and aldehydes as starting materials. This method enables creating favourable intermediates, which possess less steric hindrance, and leading to trans-olefins via intramolecular O-B bonds elimination. Notably, synthesis studies have validated its good efficacy in modifying bioactive molecules and synthesizing drug molecules with great trans-selectivity. Furthermore, the reaction mechanism was elucidated based on intermediate trapping experiments, isotope labelling studies, and kinetic analyses.

Green electrosynthesis of bis(indolyl)methane derivatives in deep eutectic solvents

In this study, a new green method was developed for the synthesis of bis(indolyl)methane derivatives using electrochemical bisarylation reaction in deep eutectic solvents as a green alternative to traditional solvents and electrolytes. The effects of varying time, current, type of solvent and material of electrodes were all studied. The optimum reaction conditions involved the use of ethylene glycol/choline chloride with a ratio of 2:1 at 80 °C for 45 min. Graphite and platinum were used as cathode and anode, respectively. The newly developed method offered many advantages such as using mild reaction conditions, short reaction time and affording high product yields with a wide range of substituted aromatic aldehydes bearing electron donating or electron withdrawing substituents. In addition, the electrochemical method proved to be more effective than heating in deep eutectic solvents and afforded higher yields of products in shorter reaction time. The mechanism of the electrochemical reaction was proposed and confirmed using the cyclic voltammetry study.

Sustainable scalable solid-state synthesis of highly efficient synergetic 2D/0D micro/nanostructured g-C3N4/CdS photocatalysts for hydrogen production and water purification

Highly efficient synergetic photocatalysts based on graphitic carbon nitride (g-C3N4) and cadmium sulfide (CdS) semiconductors with 2D/0D micro/nanostructure were successfully prepared by easy scalable mechanochemical method and investigated. An in-depth study of electrons' binding energy of semiconductors and their influence on the photocatalytic activity of nanocomposites has been performed. The synthesized materials were applied for the photodegradation of Orange II dye and photocatalytic hydrogen evolution. The obtained experimental results revealed that nanocomposite with 20 wt% of g-C3N4 and 80 wt% of CdS is able to completely decompose Orange II molecules after two hours of visible light irradiation. The mechanism and pathways of photocatalytic reactions have been proposed. The nanocomposite composed of 60 wt% g-C3N4 and 40 wt% CdS, decorated by a platinum (Pt) co-catalyst, demonstrated the peak hydrogen evolution rate (HER) of 2254.54 μmolh−1 g−1 after 4th hour of solar light illumination. This remarkable achievement, with an apparent quantum efficiency (AQE) of 2.0%, occurred on the fourth hour of solar light irradiation. Furthermore, under continuous visible light irradiation, the sample with the same composition is capable of producing hydrogen. The peak HER rate recorded was 246.14 μmolh−1 g−1 (AQE = 0.44%) after 2.5 h, and this remained consistently the same throughout the entire duration of the process.

Charge carrier controlled free radical construction efficient photocatalytic self-Fenton system

The photocatalytic self-Fenton technology has demonstrated unique advantages in the low-cost and efficient removal of environmental pollutants. Designing an efficient photocatalytic self-Fenton system (PSFs) to enhance the synergistic effect between photocatalytic H2O2 production and pollutant mineralization processes, as well as understanding the regulatory mechanism of free radical reactions in this process, are crucial for advancing this technology. In this study, an efficient PSFs was developed by coating COFs (TpPa-1) onto the surface of MXene (Ti3C2). Under visible light irradiation, the generation rate of H2O2 reached 983.9 μmol·g−1·h−1, which is 9 times that of pure TpPa-1. Additionally, this led to a significant increase in the removal rate of sulfamethoxazole (SMX) from 25 % to 92 % within 100 min. Photoelectrochemical tests and density functional theory (DFT) calculations confirmed that the incorporation of Ti3C2 not only facilitates efficient charge carrier transport but also enhances the generation of hydroxyl radicals (·OH) and the production of H2O2, thereby improving the photocatalytic self-Fenton removal efficiency of SMX. This research is expected to have a positive impact on the development of efficient PSFs.

Electrosynthesis of Cyclic Isoureas and Ureas Through Contiguous Heterofunctionalizations.

An efficient synthetic protocol for the selenylated cyclic isoureas was developed using electrochemical activation of diselenides. This sustainable approach permitted transition metal and chemical oxidant-free difunctionalization of olefins and overall access to distinct 1,2,3 triheterofunctionalized carbon skeletons. Excellent functional group tolerance was noticed, allowing the synthesis of a series of cyclic isourea derivatives. In addition, an acid-triggered skeletal isomerization facilitated the synthesis of cyclic urea derivatives from the corresponding cyclic isoureas. Mechanistic investigations, along with voltammetric studies, enabled the postulation of the reaction mechanism.

Simultaneously Producing H2 and H2O2 by Photocatalytic Water Splitting: Recent Progress and Future.

The solar-driven overall water splitting (2H2O→2H2 + O2) is considered as one of the most promising strategies for reducing carbon emissions and meeting energy demands. However, due to the sluggish performance and high H2 cost, there is still a big gap for the current photocatalytic systems to meet the requirements for practical sustainable H2 production. Economic feasibility can be attained through simultaneously generating products of greater value than O2, such as hydrogen peroxide (H2O2, 2H2O→H2 + H2O2). Compared with overall water splitting, this approach is more kinetically feasible and generates more high-value products of H2 and H2O2. In several years, there has been an increasing surge in exploring the possibility and substantial progress has been achieved. In this review, a concise overview of the importance and underlying principles of PIWS is first provided. Next, the reported typical photocatalysts for PIWS are discussed, including commonly used semiconductors and cocatalysts, essential design features of these photocatalysts, and connections between their structures and activities, as well as the selected approaches for enhancing their stability. Then, the techniques used to quantify H2O2 and the operando characterization techniques that can be employed to gain a thorough understanding of the reaction mechanisms are summarized. Finally, the current existing challenges and the direction needing improvement are presented. This review aims to provide a thorough summary of the most recent research developments in PIWS and sets the stage for future advancements and discoveries in this emerging area.

Characterization of alane (AlH3) thin films grown by atomic layer deposition for hydrogen storage applications

Alane (AlH3) is an increasingly favored material for hydrogen and energy storage. It has demonstrated its potential in various applications, including rocket fuel, explosives, reducing agents, and serving as a viable source of hydrogen for portable fuel cells. In this study, we present the fabrication, morphology and elemental characterization of an AlH3 thin film grown through atomic layer deposition using a dimethylethylamine alane (DMEAA; AlH3N(CH3)2(CH2CH3)) precursor. The AlH3 thin film exhibited a rough surface with a white-like appearance, forming coarse grains as observed via scanning electron microscopy. X-ray diffraction confirmed the presence of AlH3 and it was compared with an aluminum thin film. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy confirmed the presence of pure AlH3 under a very thin aluminum oxide surface layer. Additionally, we propose a dissociation reaction mechanism based on the DMEAA precursor. Our work contributes to the advancement of hydrogen storage materials and energy-related applications that necessitate high hydrogen storage capacity and energy efficiency.

Coupling interface, electronic and conjugation effects in C-COFs/TiO2 composite structure toward boosting photocatalytic hydrogen evolution

Hydrogen energy, recognized as a pivotal clean energy source, has attracted considerable attention in the realm of photocatalytic hydrogen production. Nevertheless, inorganic materials were difficult to regulate band gaps, resulting in limited photogenerated carrier rates. Additionally, organic materials frequently suffer from insufficient stability and a propensity for electron recombination. Therefore, fully utilizing the interface effect will become an important breakthrough for novel and efficient catalysts. Herein, hydroxylate covalent organic frameworks (H-COFs) were formed by direct synthesis method, in addition, titanium dioxide (TiO2) was used as an inorganic substrate, and TiO2 was transformed titanium acid under weakly acidic conditions, meanwhile, chloride covalent organic frameworks (C-COFs) growth on TiO2 surface by interface effect when H-COFs were formed into C-COFs under titanium acid. The structure characteristics, photoelectric performance, bandgap width and stability of C-COFs/TiO2 were confirmed through characterization analysis. C-COFs/TiO2 exhibit strong stability, conjugation effects, electron deficiency effects, and catalytic efficiency. Hydrogen production rate up to 5598 μmol g−1 h−1 by C-COFs/TiO2 as photocatalyst. C-COFs/TiO2 lead to 161 times improvement in photocatalytic activity for TiO2 and 3.6 times improvement in photocatalytic activity for H-COFs. The catalytic reaction mechanism was obtained by utilizing experimental data and DFT calculation. This synergistic integration of organic inorganic combined catalyst provides a foundational framework for advancing research into novel photocatalysts.

Direct CO2 Hydrogenation over Bifunctional Catalysts to Produce Dimethyl Ether—A Review

Hydrogenation of CO2 represents a promising pathway for converting it into valuable hydrocarbons and clean fuels like dimethyl ether (DME). Despite significant research, several challenges persist, including a limited understanding of reaction mechanisms, thermodynamics, the necessity for catalyst design to enhance DME selectivity, and issues related to catalyst deactivation. The paper provides a comprehensive overview of recent studies from 2012 to 2023, covering various aspects of CO2 hydrogenation to methanol and DME. This review primarily focuses on advancing the development of efficient, selective, and stable innovative catalysts for this purpose. Recent investigations that have extensively explored heterogeneous catalysts for CO2 hydrogenation were summarized. A notable focus is on Cu-based catalysts modified with promoters such as Zn, Zr, Fe, etc. Additionally, this context delves into thermodynamic considerations, the impact of reaction variables, reaction mechanisms, reactor configurations, and recent technological advancements, such as 3D-printed catalysts. Furthermore, the paper examines the influence of different parameters on catalyst deactivation. The review offers insights into direct CO2 hydrogenation to DME and proposes paths for future investigation, aiming to address current challenges and advance the field.

Thermal decomposition behaviors of adamantane and 1-methyladamantane in oxygen atmosphere: ReaxFF molecular dynamics simulation

High energy density fuels (HEDFs) have garnered considerable interest in the aerospace applications due to their high density and effective heat dissipation capabilities. However, there is still no clear understanding of the pyrolysis and oxidation mechanisms of HEDFs. In this work, a computational strategy based on ReaxFF molecular dynamics reveals dehydrogenation and ring-opening reaction events for adamantane and 1-methyladamantane. A comparative analysis of the initial decomposition behavior, intermediate reaction mechanism, and final product distribution demonstrates that the decay of the reactants is dependent on temperature and oxygen concentration. The main decomposition pathways of adamantane and 1-methyladamantane involve multi-step dehydrogenation and ring opening by the following reaction: i. C10H16 → C10H15 → C7H10 + C3H5, ii. C10H16 → C10H12, iii. C11H18 → C11H16, iv. C11H18 → C11H15 → C2H4 + C9H11. C7H10, C3H5, and C9H11 undergo further bond-breaking reactions to form linear hydrocarbon chains. At low temperatures, C10H16 and C11H18 exhibit a preference for reacting with OH and HO2, accounting for 15 %–35 % of the reactions. At high temperatures, the preferred reaction for C10H16 and C11H18 is with O2, accounting for 33 %–64 % of the reactions. The bridge carbon atom acts as the initial dehydrogenation site for both adamantane and 1-methyladamantane, as well as the active site for ring opening. The decomposition energy barrier for 1-methyladamantane (108.16–130.39 kJ/mol) is lower than that for adamantane (130.72–142.61 kJ/mol). Our findings shed light on the complicated interplay of oxidation of adamantane and 1-methyladamantane.