Rate-limiting Step Of Reaction Research Articles

Rapid and Highly Selective Fe(IV) Generation by Fe(II)-Peroxyacid Advanced Oxidation Processes: Mechanistic Investigation via Kinetics and Density Functional Theory.

High-valent iron (Fe(IV/V/VI)) has been widely applied in water decontamination. However, common Fe(II)-activating oxidants including hydrogen peroxide (H2O2) and persulfate react slowly with Fe(II) and exhibit low selectivity for Fe(IV) production due to the cogeneration of radicals. Herein, we report peroxyacids (POAs; R-C(O)OOH) that can react with Fe(II) more than 3 orders of magnitude faster than H2O2, with high selectivity for Fe(IV) generation. Rapid degradation of bisphenol A (BPA, an endocrine disruptor) was achieved by the combination of Fe(II) with performic acid (PFA), peracetic acid (PAA), or perpropionic acid (PPA) within one second. Experiments with phenyl methyl sulfoxide (PMSO) and tert-butyl alcohol (TBA) revealed Fe(IV) as the major reactive species in all three Fe(II)-POA systems, with a minor contribution of radicals (i.e., •OH and R-C(O)O•). To understand the exceptionally high reactivity of POAs, a detailed computational comparison among the Fenton-like reactions with step-by-step thermodynamic evaluation was conducted. The high reactivity is attributed to the lower energy barriers for O-O bond cleavage, which is determined as the rate-limiting step for the Fenton-like reactions, and the thermodynamically favorable bidentate binding pathway of POA with iron. Overall, this study advances knowledge on POAs as novel Fenton-like reagents and sheds light on computational chemistry for these systems.

Multivalent Pt/Ti3C2Tx Nanocomposite-Based Immunochromatographic Sensor for Colorimetric/Temperature/Pressure Trimodal Detection.

Multimodal immunochromatographic sensors (ICSs) have acquired extensive attention since they not only provide reliable results by comparing the different output signals but also flexibly respond to various application environments. Herein, an ICS with triple signal outputs including colorimetry, temperature, and pressure was developed for sensitive detection of chlorothalonil. The multivalent Pt/Ti3C2Tx nanoparticles as signal tags were facilely synthesized by loading PtNPs onto single-layer Ti3C2Tx nanosheets with high surface area. The acquired Pt/Ti3C2TxNPs accelerated the rate-limiting step of the aerogenesis reaction of H2O2 for producing intensive pressure signals due to their significant catalase-mimic activity. Meanwhile, they showed desirable photothermal conversion efficiency in the near-infrared region for producing significant temperature signals. Furthermore, their deep color also allowed facile colorimetry by using the naked eye. Based on a competitive immunoassay, chlorothalonil was detected as a model analyte on this trimodal ICS platform. The detection limits for pressure, temperature, and colorimetric modes were 0.04, 0.09, and 5 ng mL-1, respectively. The recoveries for detecting chlorothalonil supplemented in Astragalus and Honeysuckle with pressure mode were 84.0-110% and 108-114%, respectively. Therefore, the ICS presented a portable, sensitive, accurate, and flexible multimodal strategy suitable for point-of-care testing.

2,3-Arylacylation of allenes through synergetic catalysis of palladium and N-heterocyclic carbene

An unprecedented 2,3-arylacylation reaction of allenes with aryl iodides and aldehydes was developed by resorting to Pd/NHC synergetic catalysis. It's the first time that allene was introduced into transition metal and NHC synergetic catalysis, which demonstrated a versatile three-component reaction pattern, thus enabling two C-C bonds forged regioselectively in the reaction. The important reaction intermediates were successfully captured and characterized by HRMS analysis, and the migrative insertion of allene to the Ph-Pd species was identified as the reaction rate-limiting step by kinetic experiments.

Monolayers APS3 (A = Cr, Co) as cathode hosts for lithium/sodium-sulfur batteries with enhanced electrochemical performance

Metal-sulfur batteries (M–S batteries, M = Li, Na), because of their high energy density and low cost, are expected to become the next-generation energy storage systems. However, their commercial applications are impeded by the poor conductivity of polysulfides, the “shuttle effect” of soluble M2Sn (n = 4, 6, 8), and the slow reaction kinetics. In this work, we perform first-principles calculations to find that the two-dimensional APS3 (A = Cr, Co) monolayers are effective sulfur host materials for high-performance M–S batteries. The metallic electronic structures of APS3 can accelerate electron transfer in reaction processes. The shuttle effect is inhibited by moderate binding strength between APS3 and M2Sn. Remarkably, APS3, as a bifunctional electrocatalyst, can greatly reduce the rate-limiting step of sulfur reduction reaction (SRR) and the decomposition barriers of Li2S/Na2S. We further explored the effect of strain engineering on the catalytic activity of APS3 for SRR and found that the battery performance can be regulated by varying the applied strains ranging from −2 % to 5 %. These findings provide a new direction for the design of highly efficient catalysts for M–S batteries.

Aluminothermic reduction of CeO2: mechanism of an economical route to aluminum-cerium alloys.

Cerium oxide is a low-value byproduct of rare-earth mining yet constitutes the largest fraction of the rare earth elements. The reduction of cerium oxide by liquid aluminum is proposed as an energy- and cost-efficient route to produce high-strength Al-Ce alloys. This work investigated the mechanism of a multi-step reduction reaction to facilitate the industrial adaptation of the process. Differential scanning calorimetry in combination with time-resolved synchrotron diffraction data uncovered the rate-limiting reaction step as the origin of the reported temperature dependence of reduction efficiency. This is the first in situ study of a metallothermic reaction mechanism and will serve as guidance for cost- and energy efficient industrial process control.

Transferable Machine Learning Interatomic Potential for Bond Dissociation Energy Prediction of Drug-like Molecules.

We present a transferable MACE interatomic potential that is applicable to open- and closed-shell drug-like molecules containing hydrogen, carbon, and oxygen atoms. Including an accurate description of radical species extends the scope of possible applications to bond dissociation energy (BDE) prediction, for example, in the context of cytochrome P450 (CYP) metabolism. The transferability of the MACE potential was validated on the COMP6 data set, containing only closed-shell molecules, where it reaches better accuracy than the readily available general ANI-2x potential. MACE achieves similar accuracy on two CYP metabolism-specific data sets, which include open- and closed-shell structures. This model enables us to calculate the aliphatic C-H BDE, which allows us to compare reaction energies of hydrogen abstraction, which is the rate-limiting step of the aliphatic hydroxylation reaction catalyzed by CYPs. On the "CYP 3A4" data set, MACE achieves a BDE RMSE of 1.37 kcal/mol and better prediction of BDE ranks than alternatives: the semiempirical AM1 and GFN2-xTB methods and the ALFABET model that directly predicts bond dissociation enthalpies. Finally, we highlight the smoothness of the MACE potential over paths of sp3C-H bond elongation and show that a minimal extension is enough for the MACE model to start finding reasonable minimum energy paths of methoxy radical-mediated hydrogen abstraction. Altogether, this work lays the ground for further extensions of scope in terms of chemical elements, (CYP-mediated) reaction classes and modeling the full reaction paths, not only BDEs.

What Is the Rate-Limiting Step of Oxygen Reduction Reaction on Fe-N-C Catalysts?

Oxygen reduction reaction (ORR) is essential to various renewable energy technologies. An important catalyst for ORR is single iron atoms embedded in nitrogen-doped graphene (Fe-N-C). However, the rate-limiting step of the ORR on Fe-N-C is unknown, significantly impeding understanding and improvement. Here, we report the activation energies of all of the steps, calculated by ab initio molecular dynamics simulations under constant electrode potential. In contrast to the common belief that a hydrogenation step limits the reaction rate, we find that the rate-limiting step is oxygen molecule replacing adsorbed water on Fe. This occurs through concerted motion of H2O desorption and O2 adsorption, without leaving the site bare. Interestingly, despite being an apparent "thermal" process that is often considered to be potential-independent, the barrier reduces with the electrode potential. This can be explained by stronger Fe-O2 binding and weaker Fe-H2O binding at a lower potential, due to O2 gaining electrons and H2O donating electrons to the catalyst. Our study offers new insights into the ORR on Fe-N-C and highlights the importance of kinetic studies in heterogeneous electrochemistry.

Cloning and Functional Characterization of NADPH-Cytochrome P450 Reductases in Aconitum vilmorinianum

Diterpenoid alkaloids (DAs) are major pharmacologically active ingredients of Aconitum vilmorinianum, an important medicinal plant. Cytochrome P450 monooxygenases (P450s) are involved in the DA biosynthetic pathway, and the electron transfer reaction of NADPH-cytochrome P450 reductase (CPR) with P450 is the rate-limiting step of the P450 redox reaction. Here, we identified and characterized two homologs of CPR from Aconitum vilmorinianum. The open reading frames of AvCPR1 and AvCPR2 were found to be 2103 and 2100 bp, encoding 700 and 699 amino acid residues, respectively. Phylogenetic analysis characterized both AvCPR1 and AvCPR2 as class II CPRs. Cytochrome c and ferricyanide could be reduced with the recombinant proteins of AvCPR1 and AvCPR2. Both AvCPR1 and AvCPR2 were expressed in the roots, stems, leaves, and flowers of A. vilmorinianum. The expression levels of AvCPR1 and AvCPR2 were significantly increased in response to methyl jasmonate (MeJA) treatment. The yeasts co-expressing AvCPR1/AvCPR2/SmCPR1 and CYP76AH1 all produced ferruginol, indicating that AvCPR1 and AvCPR2 can transfer electrons to CYP76AH1 in the same manner as SmCPR1. Docking analysis confirmed the experimentally deduced functional activities of AvCPR1 and AvCPR2 for FMN, FAD, and NADPH. The functional characterization of AvCPRs will be helpful in disclosing molecular mechanisms relating to the biosynthesis of diterpene alkaloids in A. vilmorinianum.

Estimating Free Energy Barriers for Heterogeneous Catalytic Reactions with Machine Learning Potentials and Umbrella Integration.

Predicting the rate constants of elementary reaction steps is key for the computational modeling of catalytic processes. Within transition state theory (TST), this requires an accurate estimation of the corresponding free energy barriers. While sophisticated methods for estimating free energy differences exist, these typically require extensive (biased) molecular dynamics simulations that are computationally prohibitive with the first-principles electronic structure methods that are typically used in catalysis research. In this contribution, we show that machine-learning (ML) interatomic potentials can be trained in an automated iterative workflow to perform such free energy calculations at a much reduced computational cost as compared to a direct density functional theory (DFT) based evaluation. For the decomposition of CHO on Rh(111), we find that thermal effects are substantial and lead to a decrease in the free energy barrier, which can be vanishingly small, depending on the DFT functional used. This is in stark contrast to previously reported estimates based on a harmonic TST approximation, which predicted an increase in the barrier at elevated temperatures. Since CHO is the reactant of the putative rate limiting reaction step in syngas conversion on Rh(111) and essential for the selectivity toward oxygenates containing multiple carbon atoms (C2+ oxygenates), our results call into question the reported mechanism established by microkinetic models.

CoxFe1-xFeAlO4-δ as highly active oxygen carriers for enhanced chemical looping hydrogen production

The oxygen carrier is a key factor to enable efficient and high-quality chemical looping hydrogen production. Here, we prepared several oxygen vacancies engineered CoxFe1-xFeAlO4-δ spinel by tuning Co loading of and investigated the performance for hydrogen production through chemical looping. The results show that Co0.4Fe0.6FeAlO4-δ displays a high hydrogen yield of 10.56 mmol.g−1 with a stable trend for 100 cycles at 600 °C. Mechanism studies demonstrate that doping Co decreases the energy barriers for oxygen vacancy formation, resulting in a high oxygen vacancy concentration on the surface. The high oxygen vacancy concentration accelerates the bulk oxygen diffusion, which is the rate-limiting step of the redox reaction. The enhanced bulk oxygen diffusion improves the oxygen release and redeposition of the lattice oxygen during redox cycles, thus promoting the chemical looping hydrogen production performance. No obvious phase separation can be found for Co0.4Fe0.6FeAlO4-δ during 100 cycles. The observed doping effect can pave way to developing more advanced oxygen carriers with high reactivity and stability for chemical looping applications.

Insights into the Enhancement of the Poly(ethylene terephthalate) Degradation by FAST-PETase from Computational Modeling.

Polyethylene terephthalate (PET) is the most abundant polyester plastic, widely used in textiles and packaging, but, unfortunately, it is also one of the most discarded plastics after one use. In the last years, the enzymatic biodegradation of PET has sparked great interest owing to the discovery and subsequent mutation of PETase-like enzymes, able to depolymerize PET. FAST-PETase is one of the best enzymes hitherto proposed to efficiently degrade PET, although the origin of its efficiency is not completely clear. To understand the molecular origin of its enhanced catalytic activity, we have carried out a thorough computational study of PET degradation by the FAST-PETase action by employing classical and hybrid (QM/MM) molecular dynamics (MD) simulations. Our findings show that the rate-limiting reaction step for FAST-PETase corresponds to the acylation stage with an estimated free energy barrier of 12.1 kcal mol-1, which is significantly smaller than that calculated for PETase (16.5 kcal mol-1) and, therefore, supports the enhanced catalytic activity of FAST-PETase. The origin of this enhancement is mainly attributed to the N233K mutation, which, although sited relatively far from the active site, induces a chain folding where the Asp206 of the catalytic triad is located, impeding that this residue sets effective H-bonds with its neighboring residues. This effect makes Asp206 hold a more basic character compared to the wild-type PETase and boosts the interaction with the protonated His237 of the catalytic triad in the transition state of acylation, with the consequent decrease of the catalytic barrier and acceleration of the PET degradation reaction.

Transformation of Iminodi(methylene phosphonate) on Manganese Dioxides - Passivation of the Mineral Surface by (Formed) Mn2.

Aminopolyphosphonates (APPs) are strong chelating agents with growing use in industrial and household applications. In this study, we investigated the oxidation of the bisphosphonate iminodi(methylene phosphonate) (IDMP) - a major transformation product (TP) of numerous commercially used APPs and a potential precursor for aminomethylphosphonate (AMPA) - on manganese dioxide (MnO2). Transformation batch experiments at pH 6 revealed AMPA and phosphate as main TPs, with a phosphorus mass balance of 80 to 92% throughout all experiments. Our results suggest initial cleavage of the C-P bond and formation of the stable intermediate N-formyl-AMPA. Next, C-N bond cleavage leads to the formation of AMPA, which exhibits lower reactivity than IDMP. Reaction rates together with IDMP and Mn2+ sorption data indicate formation of IDMP-Mn2+ surface bridging complexes with progressing MnO2 reduction, leading to the passivation of the mineral surface regarding IDMP oxidation. Compound-specific stable carbon isotope analysis of IDMP in both sorbed and aqueous fractions further supported this hypothesis. Depending on the extent of Mn2+ surface concentration, the isotope data indicated either sorption of IDMP to the mineral surface or electron transfer from IDMP to MnIV to be the rate-limiting step of the overall reaction. Our study sheds further light on the complex surface processes during MnO2 redox reactions and reveals abiotic oxidative transformation of APPs by MnO2 as a potential process contributing to widespread elevated AMPA concentrations in the environment.

Mechanism study of the N-butoxycarbonyl-2,5-dimethylpyrrole synthesis reaction based on in-situ FTIR monitoring

The reaction pathway of many Paal-Knorr reactions for pyrrolic compounds hasn’t been thoroughly studied and remains controversial. In this work, the synthesis of an important anti-ultraviolet agent N-butoxycarbonyl-2,5-dimethylpyrrole based on the Paal-Knorr reaction was studied via in-situ Fourier transform infrared (FTIR) analysis combined with density functional theory (DFT) calculation. The online spectral data were analyzed using K-means clustering in order to find the characteristic wavenumbers of intermediates. It was found that the reaction with 2,5-hexadione and butyl carbamate are more favorable to form the imine and enamine intermediates by nucleophilic addition, which is different from another Paal-Knorr reaction with 2, 5-hexadione and ethanolamine. In addition, the reaction kinetic model was established through chemometric methods, with parameters of two rate-limiting reaction steps calculated. This approach of FTIR analysis is proven to be an effective tool for mechanism study of organic synthesis.

Ambient Carbon-Neutral Ammonia Generation via a Cyclic Microwave Plasma Process.

A novel reactor methodology was developed for chemical looping ammonia synthesis processes using microwave plasma for pre-activation of the stable dinitrogen molecule before reaching the catalyst surface. Microwave plasma-enhanced reactions benefit from higher production of activated species, modularity, quick startup, and lower voltage input than competing plasma-catalysis technologies. Simple, economical, and environmentally benign metallic iron catalysts were used in a cyclical atmospheric pressure synthesis of ammonia. Rates of up to 420.9 μmol min-1 g-1 were observed under mild nitriding conditions. Reaction studies showed that both surface-mediated and bulk-mediated reaction domains were found to exist depending on the time under plasma treatment. The associated density functional theory (DFT) calculations indicated that a higher temperature promoted more nitrogen species in the bulk of iron catalysts but the equilibrium limited the nitrogen converion to ammonia, and vice versa. Generation of vibrationally active N2 and, N2+ ions is associated with lower bulk nitridation temperatures and increased nitrogen contents versus thermal-only systems. Additionally, the kinetics of other transition metal chemical looping ammonia synthesis catalysts (Mn and CoMo) were evaluated by high-resolution time-on-stream kinetic analysis and optical plasma characterization. This study sheds new light on phenomena arising in transient nitrogen storage, kinetics, effect of plasma treatment, apparent activation energies, and rate-limiting reaction steps.

Chemical looping reforming characteristics of methane and toluene from biomass pyrolysis volatiles based on decoupling strategy: Embedding NiFe2O4 in SBA-15 as an oxygen carrier

To realize the directional conversion of biomass to produce hydrogen-rich syngas, biomass gasification with a complex reaction system was decoupled into two sub-processes. NiFe2O4@SBA-15 was prepared by an embedding strategy via the impregnation method to achieve high reactivity during the volatile chemical looping reforming process. The chemical looping reforming of methane and toluene was investigated based on a fixed-bed reactor and on-line mass spectrometry and gas chromatography. It was found that the selectivity and conversion were improved after embedding, and the conversion of toluene reached 97.5 % at 750 °C. The NiFe2O4@SBA-15 maintained good stability after 15 cycles, and the reaction performance had not obviously decreased. Real-time reaction performance showed that the conversion of lattice oxygen could reach 100% above 750 °C. The reaction system belonged to the phase boundary-controlled mechanism based on the conversion of lattice oxygen, and the activation energy was 43.10 kJ·mol−1. In addition, in-situ diffuse reflectance infrared spectroscopy(DRIFTS) study had shown that the dissociation of methane and toluene and the further conversion of formate might be the reaction rate-limiting steps. Although methane and toluene competed in adsorption, the reaction processes were independent.

First-principles prediction of two-dimensional MXene (V2C, Ti2C, and Mo2C) as the catalyst for ammonia decomposition

The geometric structure, electronic structure and dehydrogenation mechanism of NH3 adsorbed on three MXene (V2C, Ti2C, and Mo2C) surfaces were studied by combining density functional theory with periodic plate model. The calculation results of adsorption energy showed that the order of adsorption energy of NHx(x = 0–3) was Mo2C < V2C < Ti2C. The results of the transition states showed that the desorption of nitrogen atoms was the rate-limiting step of the whole reaction. The surface of V2C was more conducive to the decomposition of ammonia gas for hydrogen production, while the surface of Ti2C and Mo2C was susceptible to the adsorption poisoning of N atoms. The results of electron structure study showed that the strong interaction between d orbitals of transition metal atoms near Fermi level and molecular orbitals of ammonia made ammonia molecules activated in different degrees, showing high adsorption performance and catalytic activity.

Single Turnover of Transient of Reactants Supports a Complex Interplay of Conformational States in the Mode of Action of Mycobacterium tuberculosis Enoyl Reductase

The enoyl reductase from Mycobacterium tuberculosis (MtInhA) was shown to be a major target for isoniazid, the most prescribed first-line anti-tuberculosis agent. The MtInhA (EC 1.3.1.9) protein catalyzes the hydride transfer from the 4S hydrogen of β-NADH to carbon-3 of long-chain 2-trans-enoyl thioester substrates (enoyl-ACP or enoyl-CoA) to yield NAD+ and acyl-ACP or acyl-CoA products. The latter are the long carbon chains of the meromycolate branch of mycolic acids, which are high-molecular-weight α-alkyl, β-hydroxy fatty acids of the mycobacterial cell wall. Here, stopped-flow measurements under single-turnover experimental conditions are presented for the study of the transient of reactants. Single-turnover experiments at various enzyme active sites were carried out. These studies suggested isomerization of the MtInhA:NADH binary complex in pre-incubation and positive cooperativity that depends on the number of enzyme active sites occupied by the 2-trans-dodecenoyl-CoA (DD-CoA) substrate. Stopped-flow results for burst analysis indicate that product release does not contribute to the rate-limiting step of the MtInhA-catalyzed chemical reaction. The bearings that the results presented herein have on function-based anti-tuberculosis drug design are discussed.

Noble-Metal-Metalloid Alloy Architectures: Mesoporous Amorphous Iridium-Tellurium Alloy for Electrochemical N2 Reduction.

Amorphous noble metals with high surface areas have attracted significant interest as heterogeneous catalysts due to the numerous dangling bonds and abundant unsaturated surface atoms created by the amorphous phase. However, synthesizing amorphous noble metals with high surface areas remains a significant challenge due to strong isotropic metallic bonds. This paper describes the first example of a mesoporous amorphous noble metal alloy [iridium-tellurium (IrTe)] obtained using a micelle-directed synthesis method. The resulting mesoporous amorphous IrTe electrocatalyst exhibits excellent performance in the electrochemical N2 reduction reaction. The ammonia yield rate is 34.6 μg mg-1 h-1 with a Faradaic efficiency of 11.2% at -0.15 V versus reversible hydrogen electrode in 0.1 M HCl solution, outperforming comparable crystalline and Ir metal counterparts. The interconnected porous scaffold and amorphous nature of the alloy create a complementary effect that simultaneously enhances N2 absorption and suppresses the hydrogen evolution reaction. According to theoretical simulations, incorporating Te in the IrTe alloy effectively strengthens the adsorption of N2 and lowers the Gibbs free energy for the rate-limiting step of the electrocatalytic N2 reduction reaction. Mesoporous chemistry enables a new route to achieve high-performance amorphous metalloid alloys with properties that facilitate the selective electrocatalytic reduction of N2.

Tungsten disulfide (WS2) is a highly active co-catalyst in Fe(III)/H2O2 Fenton-like reactions for efficient acetaminophen degradation

The most important factor that restricts the decomposition of H2O2 in the Fe3+/H2O2 reaction is the slow cycling efficiency of reducing Fe3+ to Fe2+. In this study, the addition of tungsten disulfide (WS2) as a co-catalyst achieved a rapid cycling of the reaction rate-limiting step and a significant enhancement of H2O2 decomposition, which resulted in the effective degradation of acetaminophen (APAP). Results show that 99.6% of APAP (5 mg L−1) could be degraded by H2O2/Fe3+/WS2 system within 2.5 min. The conversion of Fe3+ to Fe2+ occurred mainly on the surface of WS2 due to the redox reaction of the exposed W4+ active sites with Fe3+ after the unsaturated S atoms were bound to protons. Electron paramagnetic resonance (EPR) and radical quenching experiments evaluated the contribution of hydroxyl radical (•OH) and superoxide radical (O2•−) in the degradation of pollutants. WS2 showed good recoverability after four cycles of the reaction. This study provides a new perspective to improve the efficiency of Fe3+/H2O2 and provides a reference for the involvement of transition metal sulfides in advanced oxidation processes (AOPs).

Unraveling the promotional role of BaCO3 in the electrode reaction kinetics of an SmBaFe2O5+δ air electrode of reversible solid oxide cells

BaCO3 decoration significantly accelerates the oxygen adsorption/dissociation kinetics, which is the rate-limiting step of SBF electrode reaction.