Reaction Kinetics Research Articles

Mechanistic study of β-Ga2O3 nitridation by RF nitrogen plasma for GaN heteroepitaxy

The transformation of 2¯01β-Ga2O3 to h-GaN under exposure to RF nitrogen plasma was monitored in situ by reflection high-energy electron diffraction. Analysis of the reaction kinetics reveals that the nitridation process is initiated by the formation of an oxynitride phase and proceeds via two-dimensional nucleation and growth of wurtzite GaN grains. X-ray photoelectron spectra suggest a Ga−(NxO1−x) type configuration dominates the surface early in the nitridation process. The surface restructuring is followed by a diffusion-fed phase transformation, which propagates the wurzite GaN structure into the substrate upon reaching 70% nitrogen anion site occupation, corresponding to the oxygen solubility in h-GaN. A direct correlation is observed between the nitridated film morphology and the epitaxial film crystallinity, demonstrating control of the residual strain, lateral coherence, and mosaicity in subsequent GaN epitaxy by the nitridation conditions. This study provides mechanistic details of the nitridation reaction of 2¯01β-Ga2O3 facilitating the optimization of the nitridation process toward improving GaN-2¯01β-Ga2O3 heterojunctions.

Role of Oxide-Derived Cu on the Initial Elementary Reaction Intermediate During Catalytic CO2 Reduction.

The catalytic role of oxide-derived Cu (OD-Cu) in promoting CO2 reduction (CO2R) to C2+ products has been appreciated for decades. However, the dynamic evolution of the surface oxidation states, together with their real correlation to the binding of reaction intermediates, remains unclear due to technical challenges. Here, we show the time-resolved spectroscopic signatures of key OD-Cu-CO2•- intermediates during catalytic CO2 reduction through one electron transfer from nanoseconds to seconds time scale. We generated the initial intermediate CO2•- radicals in the bulk solution and monitored the interfacial reaction kinetics with well-defined OD-Cu (Cu(0), Cu(I), and Cu(II)) nanoparticles. Combined with molecular simulations, transient absorption profiles analysis reveals that Cu(I) induced a faster CO2•- radical coupling reaction than Cu(0), whereas Cu(II) is only reduced to Cu(I) by the CO2•- radical. Furthermore, the newly developed multistep cumulative pulse methodology uncovered the transition in chemical states of mixed OD-Cu during radical coupling reactions. This pulse radiolysis study provides compelling evidence for the beneficial role of subsurface oxides in early time catalytic CO2 transformation.

Atomically Asymmetrical Ir–O–Co Sites Enable Efficient Chloride‐mediated Ethylene Electrooxidation in Neutral Seawater

The chloride‐mediated ethylene oxidation reaction (EOR) of ethylene chlorohydrin (ECH) via electrocatalysis is practically attractive because of its sustainability and mild reaction conditions. However, the chlorine oxidation reaction (COR), which is essential for the above process, is commonly catalyzed by dimensionally stable anodes (DSAs) with high contents of precious Ru and/or Ir. The development of highly efficient COR electrocatalysts composed of nonprecious metals or decreased amounts of precious metals is highly desirable. Herein, we report a modified Co3O4 with a single‐atom Ir substitution (Ir1/Co3O4) as a highly efficient COR electrocatalyst for chloride‐mediated EOR to ECH in neutral seawater. Ir1/Co3O4 achieves a Faradaic efficiency (FE) of up to 94.8% for ECH generation and remarkable stability. Combining experimental results and density functional theory (DFT) calculations, the unique atomically asymmetrical Ir‐O‐Co configuration with a strong electron coupling effect in Ir1/Co3O4 can accelerate electron transfer to increase the reaction kinetics and maintain the structural stability of Co3O4 during COR. Moreover, a coupling reaction system integrating the anodic chloride‐mediated and cathodic H2O2‐mediated EOR show a total FE of ~170% for paired electrosynthesis of ECH and ethylene glycol (EG) using ethylene as the raw material. The technoeconomic analysis highlights the promising application prospects of this system.

A Piezocatalysis Strategy to Enable Efficient Redox in Solid-State Battery.

Piezocatalysis is considered to be a favorable catalytic technology by utilizing external mechanical stimulation to promote the specific redox reactions. The application of piezocatalysis in batteries is promising but faces formidable challenges. Herein, the operation principles of piezocatalysis were successfully demonstrated by constructing a solid-state Li-Se (S) battery model with interfacial stress accumulation. Lead zirconate titanate with an ultra-high piezoelectric coefficient was applied as the piezoelectric catalyst. The uniformity of the dipole orientation in materials was essential for achieving piezocatalysis in batteries. The accumulated high-stress converts to piezopotential for catalyzing most reaction processes in batteries, while the rapid stress change prevents the internal charge reorganization, ensuring continuous efficient catalysis. The internal stress change alters the internal polarisation strength, driving excess screening charge to participate in the electrochemical reaction. Under piezocatalysis, solid-state Li-Se batteries exhibited a initial discharge capacity of 670.9 mAh g-1 (99.4% of the theoretical value) at 0.1C, and Li-S batteries showed a capacity of 1463 mAh g-1 at 0.2C, which was still maintained up to 1084 mAh g-1 at an elevated rate of 0.5C. This piezocatalysis strategy provides a strong theoretical basis and design specification for enhancing Li-Se (S) cell reaction kinetics.

Explosion limits of binary mixtures of ammonia and additives (hydrogen, natural gas, and diethyl ether)

This study investigates the explosion limits of ammonia (NH3) and the effect of three different active fuels, hydrogen (H2), natural gas (NG), and diethyl ether (C2H5OC2H5, DEE), on the explosive characteristics of NH3 through comprehensive chemical kinetics analysis. The results reveal that the explosion limits of NH3 exhibit the narrowest explosive range and display comparatively weakest explosive reactivity among the four fuels, presenting a monotonic changing trend. Reaction pathway analysis unveils that with increasing temperature, NH2 tends to oxidize more predominantly through the NH-N2H2-NNH-N2 pathway, while the conversion of NH2 to H2NO pathway decreases. Therefore, the introduction of active fuels significantly enhances the explosive reactivity of NH3. However, their impact under various temperature-pressure conditions presents nuanced and diversified characteristics. When a 0.5 % mole fraction of active fuel is added to NH3, DEE exhibits the most significant enhancement in medium to high-pressure conditions, while H2 predominates at low pressure, with NG consistently demonstrating the least pronounced enhancement. Upon the active fuel proportion being increased to 20 %, the promoting effect of DEE remains the most significant in the medium to high-pressure region, while NG has surpassed H2 as the secondary dominant fuel. In the low-pressure region, NH3-H2 still demonstrates the highest reactivity, followed by NH3-DEE, while NH3-NG exhibits the least reactivity. These findings provide valuable insights into the widespread utilization and scientific exploration of ammonia as a fuel source.

Residual stress and deformation of welded T-joint between low carbon steel and stainless steel

The welded structures between carbon steel and stainless steel are widely used in many sectors such as pedestrian bridges, food industry, chemical industry, petroleum, and thermal power plants, … These composite structures take advantages of each material for different parts of structure. In the welded structures between carbon steel and stainless steel, the structure is made by T-joint that is the most common. The calculation of residual stress and welding strain of the T-joint between low-carbon and stainless steel materials is the basis to evaluate the working ability of this structure. In this study, the residual stress and welding deformation of the T-joint are determined by the imaginary force method. The double-sided weld of T-joint between low carbon steel and stainless steel is welded simultaneously by MIG welding process. The obtained results of residual stress and welding deformation in this case are compared with the case of single-sided weld so that we can find the difference between residual stress and welding deformation in two cases.

Dynamic Lock-And-Release Mechanism Enables Reduced ΔG at Low Temperatures for High-Performance Polyanionic Cathode in Sodium-Ion Batteries.

Low-temperature synthesis of polyanionic cathodes for sodium-ion batteries is highly desirable but often plagued by prolonged reaction times and suboptimal crystallinity. To address these challenges, a novel self-adaptive coordination field regulation (SACFR) strategy based on a dynamic lock-and-release (DLR) mechanism is introduced. Specifically, urea is used as a DLR carrier during synthesis, which dynamically "locks" and "releases" vanadium ions for controlled release, simultaneously "locking" H+ ions to enhance phosphate group release, thereby creating a self-adaptive coordination field that can intelligently respond to real-time demands of the reaction system. This dynamic coordination behavior contributes to both an improvement in reaction kinetics and a significant reduction in Gibbs free energy change (ΔG). As a result, the kinetic efficiency and thermodynamic spontaneity of the reaction are greatly enhanced, enabling the efficient synthesis of high-crystalline Na3V2O2(PO4)2F (NVOPF) at 90 °C within just 3 hours. The as-prepared NVOPF cathode exhibits exceptional rate performance and ultra-stable cycling stability across a broad temperature range. Furthermore, the successful kilogram-scale synthesis underscores the practical potential of the innovative strategy. This work pioneers the regulation of coordination field chemistry for polyanionic cathode synthesis, providing transformative insights into material design.

Reaction kinetics and product evolution of hydrolysis in copper-chlorine thermochemical cycle for hydrogen production

By-products of the hydrolysis reaction in the copper-chlorine thermochemical cycle for hydrogen production are key to the final thermal and hydrogen efficiency. The current study focuses on the influences of reaction time, temperature, and steam partial pressure on the hydrolysis reaction and by-product formation mechanism via experimental and multiple analytical methods. The result demonstrates that the hydrolysis reaction of CuCl2 and steam converts to CuCl and Cu2OCl2, with a high conversion of 97%, while the Cu2OCl2 and CuCl2-CuO product mixtures exhibit interconversion capabilities. The increase of temperature increases the CuCl product content, while the decrease of steam partial pressure increases the by-product CuCl2. Particle morphology and reaction thermodynamics are analyzed for main and by-products formation. Finally, the homogeneous reaction model and shrinking core model are used and compared for the activation energy of the formation of Cu2OCl2 for the hydrolysis reaction and the effect of by-products.

Tailoring Cationic Cobalt Vacancies in Molybdenum-Cobalt Selenide Derived from POM@ZIF-67 for Enhanced Electrocatalysis in Lithium-Oxygen Batteries.

Slow reaction kinetics during redox reactions limits the utilization of the high theoretical energy density of lithium-oxygen batteries (LOBs). Vacancy engineering, a potential strategy for modulating active sites, is critical in the development of high performance catalysts. This study investigates cobalt vacancies in Mo-CoSe2 nanoparticles created by selenization of phosphomolybdic acid (POM) embedded into zeolitic imidazolate framework-67 (ZIF-67). The nanomaterial exhibits an outstanding electrochemical performance, characterized by high specific capacitance and excellent cycle durability. The LOBs with cobalt vacancies in the Mo-CoSe2 electrode material exhibit a discharge capacity of 21 836 mAh g-1 at a current density of 100 mA g-1 and exhibit stable cycling performance over 194 cycles at 300 mA g-1. Additionally, density functional theory (DFT) calculations suggest that the presence of cobalt vacancies increases the distance between the surface selenium atoms and the subsurface cobalt atoms. In addition, cobalt vacancies modify the electronic structure of the d-orbitals, lowering the energy barriers of the reaction and accelerating the reaction kinetics by improving the adsorption of the reactants. The research introduces a strategy for the rational design of efficient cathode materials in LOBs.

Carbon Nanoparticle Oxidation by NO2 and O2: Chemical Kinetics and Reaction Pathways

Carbon nanoparticle interactions with gases are central to many environmental and technical processes, but the underlying reaction kinetics and mechanisms are not well understood. Here, we investigate the oxidation and gasification of carbon nanoparticles by NO2 and O2 under combustion exhaust conditions. We build on a comprehensive experimental data set and use a kinetic multilayer model (KM‐GAP‐CARBON) to trace the uptake and release of gas molecules alongside the temporal evolution of particle size and surface composition. The experimental results are captured by a model mechanism that involves different types of carbon atoms (edge/plane‐like) and the formation of a reactive oxygen intermediate (activated CO complex) as the rate‐limiting step. A transition between distinct chemical regimes driven by NO2 at lower temperatures and O2 at higher temperatures is reflected by an increase in the observable activation energy from ~60 kJ/mol to ~130 kJ/mol. We derive energy profiles for three alternative reaction pathways that involve uni‐ or bimolecular decomposition of reactive oxygen intermediates.

Carbon Nanoparticle Oxidation by NO2and O2: Chemical Kinetics andReaction Pathways.

Carbon nanoparticle interactions with gases are central to many environmental and technical processes, but the underlying reaction kinetics and mechanisms are not well understood. Here, we investigate the oxidation and gasification of carbon nanoparticles by NO2and O2under combustion exhaust conditions. We build on a comprehensive experimental data set and use a kinetic multilayer model (KM-GAP-CARBON) to trace the uptake and release of gas molecules alongside the temporal evolution of particle size and surface composition. The experimental results are captured by a model mechanism that involves different types of carbon atoms (edge/plane-like) and the formation of a reactive oxygen intermediate (activated CO complex) as the rate-limiting step. A transition between distinct chemical regimes driven by NO2at lower temperatures and O2at higher temperatures is reflected by an increase in the observable activation energy from ~60 kJ/mol to ~130 kJ/mol. We derive energy profiles for three alternative reaction pathways that involve uni- or bimolecular decomposition of reactive oxygen intermediates.

Regulating oxygen vacancies to optimize the electronic structure and catalytic activity of tungsten oxides for hydrogen evolution reaction

The undesirable intrinsic activity of tungsten oxides derived from poor conductivity and adverse hydrogen absorption energy is insufficient for their practical application. Herein, the tungsten oxides with adjustable oxygen vacancies were synthesized by a facile pyrolysis of ammonium paratungstate hydrate in controllable atmospheres. The structural characterizations confirmed that oxygen vacancies and crystalline phases in tungsten oxides depend on the pyrolysis atmosphere. The electrochemical test indicated a strong dependence between catalytic activity and oxygen vacancies in tungsten oxides. Combining experimental results and density functional theory calculations verified that introducing oxygen vacancies into tungsten oxides effectively modulates the surface electronic structure. The enhanced electronic conductivity by reducing band gap accelerates the electron transfer from catalysts into the reactive species. The optimized hydrogen adsorption energy by electron migration from O into W promotes the activation of reactive species at W sites and the desorption from O sites, thereby accelerating the reaction kinetics.

Selective Activation of Peptide‐Thioester Precursors for Templated Native Chemical Ligations

AbstractChemical protein synthesis enables access to proteins that would otherwise be difficult or impossible to obtain with traditional means such as recombinant expression. Chemoselective ligations provide the ability to join peptide segments prepared by solid‐phase peptide synthesis. While native chemical ligation (NCL) is widely used, it is limited by the need for C‐terminal thioesters with suitable reaction kinetics, properly placed native Cys or thiolated derivatives, and peptide segment solubility at low mM concentrations. Moreover, repetitive purifications to isolate ligated products are often yield‐sapping, hampering efficiency and progress. In this work, we demonstrate the use of Controlled Activation of Peptides for Templated NCL (CAPTN). This traceless multi‐segment templated NCL approach permits the one‐pot synthesis of proteins by harnessing selective thioester activation and orthogonal conjugation chemistries to favor formation of the full‐length ligated product while minimizing side reactions. Importantly, CAPTN provides kinetic enhancements allowing ligations at sterically hindered junctions and low peptide concentrations. Additionally, this one‐pot approach removes the need for intermediate purification. We report the synthesis of two E. coli ribosomal subunits S16 and S17 enabled by the chemical tools described herein. We anticipate that CAPTN will expedite the synthesis of valuable proteins and expand on templated approaches for chemical protein synthesis.

A novel simultaneous abatement of bromate and diphenyl phosphate using the freezing process

The purification of bromate (BrO3-)-contaminated water has become a challenge because of its persistence and adverse effects. Furthermore, there has been concern over the release of byproducts, such as diphenyl phosphate (DPHP), from flame retardants in wastewater treatment plant (WWTP). In this study, we designed the water treatment system for the oxidation of DPHP accompanied by bromate (BrO3-) reduction via freezing the solution. A sample containing 10 μM DPHP, 100 μM Br-, and 50 μM BrO3-, with a pH of 3 was frozen at -20oC, approximately 25 μM BrO3- was reduced, and DPHP was fully eliminated after a 0.5 h reaction time. Conversely, these reactions did not advance in water at 20oC. This increase in the rate of chemical reaction in ice is the consequence of the freeze concentration effect, which refers to the extraction of dissolved chemical species into the liquid-like regions of the polycrystalline ice micro-structure during the freezing of the solution. The redox reactions among DPHP, Br-, and BrO3- become thermodynamically favorable due to the distinctive environment in the liquid brine in ice. The efficiency of the DPHP oxidation significantly increased with an increase in BrO3- concentration, and vice versa. The Br-/BrO3--induced HOBr production is proposed as a primary oxidant for DPHP degradation. The proton activity (pH) has a significant influence on the reaction efficiency. The low freezing temperature accelerated the reaction kinetics of DPHP degradation and BrO3- reduction. The results of this study indicate the possibility of utilizing ice chemistry for the BrO3- reduction that concomitantly removes DPHP for water treatment. This environmentally friendly water treatment method can be considered to implement in regions with a cold climate.

Scalable Fabrication of High-Performance Perovskite Solar Cell Modules by Mediated Vapor Deposition.

Perovskite solar cells (PSCs) can enable renewable electricity generation at low levelized costs, subject to the invention of an economically feasible technology for their large-scale fabrication, like vapor deposition. This approach is effective for the fabrication of small area (<1 cm2) PSCs, but its scale-up to produce high-efficiency larger area modules has been limited by a severe imbalance between the vapor-solid reaction kinetics and the mass-transport of the volatile ammonium salt precursor. In this study, an amidine-based low-dimensional perovskite is introduced as an intermediate of the solid-vapor reaction to help resolve this limitation. This improves reaction pathway produces unique vertically monolithic grains with no detectable horizontal boundaries, which is used to produce 1.0 cm2 PSCs with an efficiency of 22.1%, as well as 12.5 and 48 cm2 modules delivering 21.1% and 20.1% efficiency, respectively. The modules retain ≈85% of their initial performance after 900 h of continuous operation (ISOS-L-1 protocol) and ≈100% after 2800 h of storage in an ambient environment (ISOS-D-1 protocol).

Reaction kinetics for catalytic hydrogenation of quinoline to decahydroquinoline as liquid organic hydrogen carrier

The catalytic hydrogenation of quinoline to decahydroquinoline (DHQ) formation is studied for various parameters such as temperature, hydrogen pressure, reaction time, solvents, reactant-to-solvent ratio, and catalyst loading over 5%Pd loaded on alumina (Pd/Al2O3) to obtain optimal reaction conditions. The hydrogen uptake of quinoline in the liquid phase reaction using isopropylalcohol (IPA) is studied in an autoclave reactor. The optimum reaction parameters of 50 bar H2 pressure and 175oC with reactant to solvent ratio of 1:9 for reaction time of 5 h are observed. The effect of IPA solvent showed that hydrogen uptake of 6.91 wt% with complete hydrogenation of quinoline and DHQ yield of more than 99% is observed. The reaction kinetic model is developed for a simplified reaction mechanism and is simulated using the Markov Chain Monte Carlo (MCMC) simulation tool to predict the rate constants and the experimental observations are validated with the model predictions. The activation energy for quinoline hydrogenation to py-THQ formation is estimated to be 136.57 kJ/mol. It is envisaged that quinoline hydrogenation to pz-THQ is the rate-limiting step in the DHQ formation.

Exploring pyrolusite β-MnO2 as a robust support for noble metal catalysts in proton exchange membrane water electrolysis

Proton exchange membrane water electrolysis (PEMWE) represents a promising avenue for producing hydrogen sustainably. Nonetheless, its widespread adoption encounters hurdles owing to the sluggishness of the oxygen evolution reaction (OER) and the expensive noble metal catalysts, particularly iridium (Ir). Our investigation addresses these challenges by examining pyrolusite β-MnO2 as a robust substrate for noble metal catalysts, including iridium (Ir) and ruthenium (Ru), within PEMWE systems. We illustrate that nanostructured β-MnO2 effectively supports Ir and Ru catalysts, resulting in significantly improved catalytic activity for acidic OER compared to conventional IrO2 catalysts. The morphology shows nanorod structures with thicknesses ranging of 30–60 nm. The catalytic activity exhibits a lower overpotential of 215 mV and a higher Ir mass activity of 2268.7 A·gIr−1. The catalyst also exhibits lower Tafel slope value (37.82 mV/dec) and higher reaction kinetics. The catalyst also shows durability for the 12h under corrosive acidic OER conditions. The enhancements in performance can be attributed to the distinctive nanostructure of the Mn1-xTixO2 (MTO) support, which facilitates the even distribution of Ir and Ru catalysts. Additionally, the incorporation of titanium in pyrolusite β-MnO2 enhances the electrochemical durability of the MTO support under challenging acidic OER conditions.

Na-doped cobalt spinel electrocatalysts prepared by programmed electrostatic spraying for improving OER activity and durability in acidic media

The daunting challenge to establish a trade-off among the activity, durability, and cost of electrocatalysts for oxygen evolution reaction (OER) with sluggish reaction kinetics impedes the practical green energy conversion. Hereby, a non-noble metal-based electrocatalyst (i.e., Na-doped Co3O4) was in-situ prepared by programmed electrostatic spraying as an advanced alternative to the costly and scarce state-of-the-art IrO2 and RuO2-based electrocatalysts. The utilization of this sprayer facilitates intimate contact with the substrate electrode and provides short ion diffusion paths, facilitating efficient electron transport. Additionally, the doping of Na ions into the Co3O4 structure results in structural distortion and charge redistribution, leading to the generation of oxygen vacancies (OVs). These OVs enhance the activity of lattice oxygen atoms, thus enabling rapid and durable oxygen evolution in an acidic media (0.5 M H2SO4). The optimal Na-Co3O4 (AA) exhibits a notably low overpotential of 360 mV@10 mA·cm−2 and at least 200 h stability. In addition, density functional theory (DFT) calculations confirm that the incorporation of Na ions significantly elevates electrical conductivity by decreasing the energy band gap and reduces the free energy barrier for the conversion of *O to *OOH, thereby improving the intrinsic OER activity. Finally, by implanting the catalyst in a single-cell proton exchange membrane water electrolyzer (PEMWE), the current density of 100 mA·cm−2 is achieved at a voltage of 1.62 V. This innovative non-noble metal catalyst holds great potential for scalable PEMWE anodes.

Experimental and Kinetic Simulations of Technetium-Catalyzed Hydrazine Oxidation in Nitric Acid Solution

The catalytic reaction of Tc plays a significant role in the chemical separation process during spent fuel reprocessing. However, few studies have been conducted on the chemical reaction mechanism between Tc and hydrazine. Moreover, the instability of Tc(V) and Tc(VI) makes their measurement difficult, rendering many aspects of the reaction process and mechanism unclear. This study investigates the catalytic reaction between Tc and hydrazine in a nitric acid solution. To this end, we obtained the kinetic laws of the reaction under various conditions of acidity, hydrazine concentration, and Tc concentration by monitoring the concentrations of hydrazine and Tc(VII) over time. The reaction kinetics model demonstrated that numerical simulations could effectively predict the reaction process. Results indicated that hydrazine promotes the reduction of Tc(VII) to Tc(IV), constituting the basis for establishing the Tc(IV, V, VI, VII) catalytic cycle. Among these, Tc(V) and Tc(VI) were important active intermediates and the main consumers of hydrazine. The research results may be useful for actinide separation processes based on valence control.

Recent advances in electrocatalytic C-N coupling for urea synthesis.

Urea, one of the most widely used nitrogen-containing fertilizers globally, is essential for sustainable agriculture. Improving its production is crucial for meeting the increasing demand for fertilizers. Electrocatalytic co-reduction of CO₂ and nitrogenous compounds (NO₂⁻/NO₃⁻) has emerged as a promising strategy for green and energy-efficient urea synthesis. However, challenges such as slow reaction kinetics and complex multi-step electron transfers have hindered the development of efficient urea synthesis methods. This review explores recent advances in the electrocatalytic C-N coupling process, focusing on bimetallic catalysts, metal oxide/hydroxide catalysts, and carbon-based catalysts. The review also discusses the future prospects of designing effective catalysts for electrocatalytic C-N coupling to improve urea synthesis.