Reaction Mechanism Of Oxidation Research Articles

Reaction Mechanisms of H2S Oxidation by Naphthoquinones.

1,4-naphthoquinones (NQs) catalytically oxidize H2S to per- and polysufides and sulfoxides, reduce oxygen to superoxide and hydrogen peroxide, and can form NQ-SH adducts through Michael addition. Here, we measured oxygen consumption and used sulfur-specific fluorophores, liquid chromatography tandem mass spectrometry (LC-MS/MS), and UV-Vis spectrometry to examine H2S oxidation by NQs with various substituent groups. In general, the order of H2S oxidization was DCNQ ~ juglone > 1,4-NQ > plumbagin >DMNQ ~ 2-MNQ > menadione, although this order varied somewhat depending on the experimental conditions. DMNQ does not form adducts with GSH or cysteine (Cys), yet it readily oxidizes H2S to polysulfides and sulfoxides. This suggests that H2S oxidation occurs at the carbonyl moiety and not at the quinoid 2 or 3 carbons, although the latter cannot be ruled out. We found little evidence from oxygen consumption studies or LC-MS/MS that NQs directly oxidize H2S2-4, and we propose that apparent reactions of NQs with inorganic polysulfides are due to H2S impurities in the polysulfides or an equilibrium between H2S and H2Sn. Collectively, NQ oxidation of H2S forms a variety of products that include hydropersulfides, hydropolysulfides, sulfenylpolysulfides, sulfite, and thiosulfate, and some of these reactions may proceed until an insoluble S8 colloid is formed.

A review on Fe2O3‐based catalysts for toluene oxidation: catalysts design and optimization with the formation of abundant oxygen vacancies

Volatile organic compounds (VOCs) are typical pollutants with hazards for humans and the environment, and can be efficiently mitigated by catalytic combustion. Fe2O3‐based catalysts are a promising choice due to their low cost and strong redox ability. Several attempts have been made to promote the catalytic performance of Fe2O3 at low temperatures. This review summarizes the research progress on Fe2O3‐based catalysts for the oxidation of toluene, one of the most common and harmful VOC. Firstly, the properties and catalytical performance for Fe2O3‐based catalysts were summarized, and the reaction mechanisms for toluene oxidation on the surface of Fe2O3 were detailed to comprehend the role of oxygen vacancy. Then, the modification for single Fe2O3 catalysts, including synthesis parameters, structure and morphology control, has been introduced to reveal the correlation between physicochemical properties of catalysts and their activity. In addition, composite Fe2O3 catalysts, which can promote catalytic performance significantly due to the synergetic effect between different components, were comprehended. Finally, waste‐derived Fe2O3 catalysts with sustainable merit as converting waste into worth have been discussed. Moreover, the advanced machine learning tools, which are helpful in accelerating catalyst design, configuration optimization and reactivity prediction, have been introduced as an emerging research opportunity for the future.

Theoretical study on the reaction mechanism of Dakin oxidation: influence of methoxy groups

Theoretical study on the reaction mechanism of Dakin oxidation: influence of methoxy groups

Catalytic combustion of toluene over highly dispersed Pt NPs embedded in porous TiO2 via in situ pyrolysis of MOFs

In this work, a series of x wt% Pt@TiO2 (x = 0.18, 0.42, and 0.84) catalysts with highly dispersed Pt nanoparticles (NPs) are synthesized via pyrolysis method using Ti-based metal-organic frameworks (MOFs) as precursors. Physicochemical properties of the catalysts are characterized by a number of analytical techniques. It is shown that all samples possess mesoporous structure with surface area of 59–75.6 m2/g, thus can suppress the aggregation of Pt NPs, and enhance toluene adsorption and diffusion. The 0.84 wt% Pt@TiO2 catalyst exhibits the best catalytic performance for toluene oxidation (T90% =150 °C at space velocity = 20,000 mL/ (g h)), which is attributed to the higher surface area, abundant adsorbed oxygen and Pt0 species, and strong interaction between Pt NPs and TiO2. In addition, the possible reaction mechanism of toluene oxidation is proposed based on in situ DRIFTS technique.

Simultaneous removal of ciprofloxacin and Cr (VI) from contaminated water by polymer-supported nanoscale zero-valent copper: Performance and mechanism

Coexistence of organic compounds and heavy metals in aquatic environments is a widespread problem. In this study, the applicability of resin-supported nano zero-valent copper (nZVC@D201) for the simultaneous removal of ciprofloxacin (CIP) and Cr (VI) was systematically investigated. nZVC with sizes of 10–20 nm was uniformly dispersed on the resin using the ion exchange-liquid phase reduction method. The CIP and Cr (VI) removal rates of the nZVC@D201 composite were 78.68% and 94.98%, respectively. Practicability experiments of nZVC@D201 exhibited high efficiency even after five cycles and a generalized removal capacity for different pollutants. Combined with electron spin resonance (ESR), free radical quenching experiments, X-ray photoelectron spectroscopy (XPS), and liquid chromatography-mass spectrometry (LC-MS), a possible reaction pathway and mechanism for CIP oxidation and Cr (VI) removal were proposed. Cr (VI) was removed through a comprehensive process including adsorption/coprecipitation and reduction, whereas CIP was effectively degraded by hydroxyl radicals (·OH) produced from the Fenton-like reactions, producing a series of intermediates. These results demonstrate that nZVC@D201 is a promising alternative composite for the simultaneous removal of Cr (VI) and the degradation of CIP from wastewater.

Transient simulation of conjugate heat transfer of kerosene with thermal oxidative and surface coking reactions at a supercritical pressure

The regenerative cooling technology in aerospace applications often involves supercritical-pressure heat transfer of a hydrocarbon fuel, in which thermal oxidative reactions of the fuel would lead to surface coke accumulation on the cooling channel wall. In this paper, the long-term transient conjugate heat transfer of kerosene has been numerically investigated in a circular mini tube at a supercritical pressure of 3 MPa. The modified chemical mechanism and kinetics of thermal oxidative and surface coking reactions of kerosene are proposed, which, with an additional surface reaction, are capable of treating large coke deposition in regions with sharp temperature gradients. Transient surface coke accumulation and its effect on heat transfer are handled by a dynamic mesh technique. A series of numerical studies are conducted to elucidate the influences of surface heat flux, dissolved oxygen mass fraction, inlet fuel mass flow rate, and coke layer thermal conductivity on the supercritical-pressure heat transfer and thermal oxidative reactions. The effects of different reaction pathways on surface coke accumulation are analyzed comprehensively, and it is revealed that the oxygen-consuming surface coke deposition is initiated only at the high fuel temperature at around 590 K. The present numerical results would enhance fundamental understanding of the long-term transient heat transfer process of a hydrocarbon fuel at supercritical pressures in the regenerative cooling applications.

Computational Insights into the Mechanism of Boyland-Sims Oxidation Reaction

The Boyland-Sims oxidation is the reaction between peroxydisulfate ions and arylamines to form the aryl amine ortho sulfate with minor para isomer. The long-standing mechanism involves a nucleophilic attack by the amine on peroxide oxygen to form arylhydroxylamine-O-sulfonate intermediate, which subsequently rearranges to the arylamine o-sulfate. Marjanović et al. have challenged this long-standing mechanism, proposing a nitrenium ion intermediate as the reactive species instead of the uncharged amine. The nitrenium ion proposal was based on relatively low-level calculations. We previously computed the energetics of reaction intermediates in both mechanisms for aniline, 2,4-dinitroaniline, and N,N-dimethylaniline using high-level of density functional theory (B3LYP/6-311++G**) calculations. We present here new computations using density functional theory (B3LYP/6-311++G**) to model both arylhydroxylamine-O-sulfonate and nitrenium ion pathways in a series of aromatic amines. The transition state calculations revealed two possible energetically feasible pathways for the rearrangement of arylhydroxylamine-O-sulfonate to arylamine o-sulfate. Additionally, we computationally modeled the feasibility of the reaction of aniline and tetrathionate to yield 2-aminothiophenol product via Boyland-Sims Oxidation reaction. The results indicate tetrathionate ion is less reactive with aniline than the peroxydisulfate ion.

Resolving the Reaction Mechanism and Active Sites for Oxidation–Hydration of Ethylene toward Ethylene Glycol Synthesis over Bifunctional Al/TS-1 Catalysts

Resolving the Reaction Mechanism and Active Sites for Oxidation–Hydration of Ethylene toward Ethylene Glycol Synthesis over Bifunctional Al/TS-1 Catalysts

Steady-State and transient kinetic investigations of the oxidation of NO over Pt/SiO2

The reaction mechanism of the oxidation of NO under conditions of high NO concentration over a 2 wt% Pt/SiO2 was studied using both kinetic and transient isotopic tracing experiments. The reaction mechanism was found to involve both NO adsorption and oxygen adsorption, contrary to the situation under low NO concentration where NO reacts from the gas phase. The kinetic rate data could be fitted to the Langmuir-Hinshelwood model permitting the estimation of rate and equilibrium constants of elementary steps. These steps were investigated using transient responses following independent reactant gas isotopic switches for tracing the nitrogen path and the oxygen path during NO oxidation. These transients could be sufficiently described by a microkinetic model based on two pools of NO intermediates, one pool each for the adsorbed molecular oxygen and atomically adsorbed oxygen. Surface atomic oxygen was found to be formed by two different routes: via a direct dissociation of molecularly adsorbed oxygen and also via an assisted pathway that leads to NO2 formation. The latter surface reaction between adsorbed NO and adsorbed molecular oxygen comprised the kinetically relevant step. Insights from the two experimental and model approaches were in good agreement and provided a coherent reaction mechanism for NO oxidation under high NO reactant concentration.

Mechanistic Studies of Continuous Partial Methane Oxidation on Cu−Zeolites Using Kinetic and Spectroscopic Methods

AbstractOver the past few decades, a significant amount of research effort has focused on investigating the active site requirements and reaction mechanisms for partial methane oxidation (PMO) to methanol over copper–exchanged zeolites during stoichiometric and stepwise chemical looping routes. More recently, research efforts have expanded to include investigating the PMO reaction in a continuous catalytic regime, primarily focusing on determining the influence of catalyst composition on Cu speciation and structure and, in turn, on PMO rate and selectivity. The structures of candidate Cu active sites are commonly studied using a combination of ex situ and in situ spectroscopic approaches. In this perspective, we critically examine the prior literature on catalytic PMO over Cu–zeolites to identify key knowledge gaps that remain in our understanding as motivation for future research efforts. We identify opportunities for future research to address these gaps by adapting analogous interrogation techniques that have been successfully used to elucidate the active site requirements and mechanistic details of another catalytic redox reaction cycle on Cu–zeolites, the selective catalytic reduction (SCR) of nitrogen oxides (NOx).

The oxidation reaction mechanism and its kinetics for a carbonaceous precursor prepared from ethylene tar for use as an anode material for lithium-ion batteries

The oxidation reaction mechanism and its kinetics for ethylene tar were investigated in order to obtain a suitable anode material for Li-ion batteries. The oxidation of ethylene tar was divided into 3 stages (350–550, 550–700 and 700–900 K) according to the thermogravimetric curve. To reveal the oxidation reaction mechanism, the components of the gases evolved at different stages were analyzed by mass spectrometry and infrared technology. Based on these results the reaction was divided into 4 stages (323–400, 400–605, 605–750 and 750–860 K) to perform simulation calculations of the kinetics. Using the iso-conversion method (Coats-Redfern) to analyze the linear regression rates (R2) between 17 common reaction kinetics models and experimental data, an optimum reaction kinetics model for expressing the oxidation of ethylene tar was determined and the results were as follows. (1) During oxidation, the side chains of aromatic compounds first react with oxygen to form alcohols and aldehydes, leaving peroxy-radicals on aromatic rings. Subsequently, the aromatic compounds with peroxy-radicals undergo polymerization/condensation reactions to form larger molecules. (2) A fourth-order reaction model was used to describe the first 3 stages in the oxidation process, and the activation energies are 47.33, 18.69 and 9.00 kJ·mol−1 at 323–400, 400–605, 605–750 K, respectively. A three-dimensional diffusion model was applied to the fourth stage of the oxidation process, and the activation energy is 88.37 kJ·mol−1 at 750–860 K. A high softening point pitch was also produced for use as a coating of the graphite anode, and after it had been applied the capacity retention after 300 cycles increased from 51.54% to 79.07%.

Reaction mechanism and light gas conversion in pyrolysis and oxidation of dimethyl ether (DME): A ReaxFF molecular dynamics study

To uncover the microscopic reaction mechanisms of dimethyl ether (DME) pyrolysis and oxidation, reactive molecular dynamics simulations were employed to investigate the reaction processes under various O2/DME ratios and impurity environments at 2200 K–3200 K. The results show that DME pyrolysis begins with a decomposition reaction of CH3OCH3 → CH3O + CH3, ultimately leading to the formation of CO, H2 and PAHs. The reaction pathways of DME oxidation contain light gas conversion and combustion. In fuel rich conditions, the oxidation produces mainly H2 and CO, while in fuel lean conditions, the oxidation products tend to be H2O and CO2. The carbon components of DME are completely converted into CO2 and CO regardless of O2 content. In the presence of impurities H2O or CO2, the oxidation reaction pathway is altered, causing consumption and transformation of these impurities into active radicals such as OH, further promoting the progress of the reaction. The top three light gases produced are hydrogen, methanol and methane, and increasing H2 production in the DME oxidation process can be achieved through both oxygen subtraction and water addition. The rate of formaldehyde generation during pyrolysis is higher than that during oxidation. However, a higher oxygen content leads to increased formaldehyde production.

Phase engineering of Fe2O3 nanocrystals for the direct oxidation of CH4 to HCOOH

The direct oxidation of methane (CH4) under mild conditions is a highly desirable technology to achieve the low-carbon and atom-economic production of high-value-added C1 oxygenates. However, the difficult activation of the CH bond of CH4 and the overoxidation of C1 oxygenates to carbon dioxide (CO2) are still the major bottlenecks. Herein, we systematically investigate the phase engineering of Fe2O3 in dependence on the performance of the direct oxidation of CH4. The cubic phase of Fe2O3 (Fe2O3-c) nanocrystals enables the direct oxidation of CH4 to HCOOH (DOMF) under mild aqueous conditions, and the HCOOH yield and selectivity could reach 6.315 mmol gcat−1 h−1 and 89.4%, which are 69.3 and 1.34 times higher than those of the hexagonal phase of Fe2O3 (Fe2O3-h) nanocrystals. Mechanism experiments indicate that Fe2O3-c nanocrystals can optimal CH4 adsorption behavior, favoring to the occurrence of the DOMF. In addition, in situ DRIFTS demonstrates that the DOMF is the radical reaction process that the activation of CH bond is caused by the ·OH radical. Subsequently, the unstable CH3OOH and stable CH3OH intermediates undergo further oxidation to form HCOOH. This work offers the phase engineering strategy to enhance the catalytic performance and reveal the reaction mechanism for the direct oxidation of CH4.

An innovative method for predicting oxidation reaction rate constants by extracting vital information of organic contaminants (OCs) based on diverse molecular representations

The reaction rate constant (k) of oxidants with organic contaminants (OCs) is an important parameter to assess the efficiency of oxidants in removing contaminants. In this study, the degradation of OCs in three oxidation systems was evaluated. The modeling process applied three molecule representations (molecular descriptors (MD), quantum chemical descriptors (QCD) and MACCS fingerprints) and their variable integrations. Models based on integration molecule representations show significant performance improvements. Eventually, the optimal models for ozone, chlorine dioxide and hypochlorite were found to be (MD+QCD)-XGBoost (R2tra = 0.982, Q2tra = 0.715), (MD+QCD+MACCS)-XGBoost (R2tra = 0.982, Q2tra = 0.778), and (MD+QCD+MACCS)-CatBoost (R2tra = 0.856, Q2tra = 0.709) model, respectively. Here, we introduced a new perspective that differed from focusing on machine learning (ML) algorithm optimization. This perspective centered on the input variables (i.e., molecular representations) of models to improve model performance by capturing the key properties of OCs comprehensively. Furthermore, the key effects of pH, ionization potential, orbital energy, polarizability and electronegativity on the oxidation reaction in different oxidation systems were clarified. We hope that the mechanism explanation in this study can provide valuable insights for understanding the mechanism of various oxidation reactions of complex OCs.

Construction of Pd@K-Silicalite-1 via in situ encapsulation and alkali metal modification for catalytic elimination of formaldehyde at room temperature

The Silicalite-1-encapsulated Pd catalyst with alkali metal K modification (Pd@K-Silicalite-1) was prepared via the hydrothermal method with ethylenediamine as ligand and KOH as precursor of K promoter. The introduction of K partially replaced H of silanol in Silicalite-1 and induced the formation of Si-O-K-Pd species, which were beneficial for the improvement of Pd dispersion. The Pd nanoparticles with a size of ca. 1.9 nm were uniformly encapsulated in the matrix of Silicalite-1 zeolite. The catalytic activity of Pd@K-Silicalite-1 was much better than those of Pd/Silicalite-1 and Pd@Silicalite-1 for HCHO oxidation, HCHO conversion could reach 100 % over Pd@K-Silicalite-1 (Pd content = 0.15 wt%) at room temperature under space velocity (SV) of 120,000 mL/(g × h) and relative humidity (RH) of 35 %. Based on the results of various characterization techniques, one can conclude that the improvement in catalytic activity of Pd@K-Silicalite-1 was attributable to high dispersion and small size of Pd NPs and enhanced HCHO adsorption capacities. In addition, we investigated the reaction mechanism of HCHO oxidation over the Pd@K-Silicalite-1 catalyst by in situ DRIFTS spectra. This work provides some guidance on developing high-performance catalysts for HCHO elimination via in situ encapsulation and alkali metal modification.

Enhanced Catalytic Activity of a Copper(II) Metal-Organic Framework Constructed via Semireversible Single-Crystal-to-Single-Crystal Dehydration.

Herein, we present a copper(II) metal-organic framework, [Cu2(btec)(OH2)4]·2H2O (1) [(btec)4- = 1,2,4,5-benzenetetracarboxylate], that undergoes single-crystal-to-single-crystal transformations into two anhydrous phases 2' and 2″ with the chemical formula [Cu2(btec)], triggered by two-step dehydration at 403 and 433 K, respectively. After immersion in water for 3 days at room temperature, 2' transformed into [Cu2(btec)(OH2)] (3), while both 2' and 2″ took 1 week to revert to 1. Dynamic vapor sorption studies validated water-induced reversible structural transformations at 70% relative humidity (RH). According to single-crystal X-ray diffraction (SC-XRD), the local coordination geometry of the Cu2+ ion in 2' changed from a saturated octahedron to a coordinatively unsaturated square-based pyramid in 3, manifested by changes in color and dimensionality. From a topological point of view, all of the scaffolds show a binodal (3,6)-connected kgd topology with the point symbol {43}2{46}. In addition, the materials were thoroughly characterized using routine spectroscopic data and various analytical techniques. The catalytic activity of the microporous materials in the liquid-phase oxidation of styrene in acetonitrile, using 30% (wt) H2O2 as the oxidant, was investigated. The excellent performance of the monohydrous phase 3 was shown to be superior to the pristine framework and the anhydrous counterparts, as evidenced by a good turnover number (TON) and turnover frequency (TOF) = 82.6 and 21.0 h-1, respectively. Within 4 h, the substrates were catalytically oxidized to the desired products with up to 67% conversion and 100% benzaldehyde selectivity. It is worth noting that the accessible active metal sites and higher surface area enhanced the catalytic properties of 3. Furthermore, the maintenance of catalytic efficiency over five cycles and reusability are illustrated and discussed in terms of the structural differences of the microporous frameworks. Thus, a preliminary reaction mechanism for the selective oxidation of styrene is proposed. This study not only provides a fascinating example of MOF chromism achieved by thermal activation and rehydration but also sheds some light on the relationship between pore-surface- or metal-engineered sites in MOFs and their heterogeneous catalytic performances.

Modification of SmMn2O5 Catalyst with Silver for Soot Oxidation: Ag Loading and Metal–Support Interactions

A series of Ag-modified manganese-mullite (SmMn2O5) catalysts with different Ag contents (1, 3, and 6 wt.%) were prepared via a citric acid sol–gel method for catalytic soot oxidation. The catalysts were characterized by powder X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), Raman spectroscopy, transmission electron microscopy (TEM), high-resolution transmission electron microscopy analysis (HRTEM), X-ray photoelectron spectroscopy (XPS), and H2 temperature-programmed reduction (H2-TPR). The soot oxidation activity of the mullite was significantly promoted by the addition of silver and affected by the loading amount of the metal. Herein, the influences of silver loading on the metal size distribution and its interactions with the mullite were studied. Based on these characterizations, a possible soot oxidation reaction mechanism was proposed for silver-modified SmMn2O5.

Unraveling the electro-oxidation steps of methanol on a single nanoparticle by in situ nanoplasmonic scattering spectroscopy

Understanding the mechanism of methanol oxidation reaction (MOR) remains a challenge in the development of direct methanol fuel cells. Large-scale investigations of the MOR encounter issues related to mass transfer and averaging effects. To address these limitations, exploring the MOR on the surfaces of individual nanocatalyst and precisely identifying the reaction steps can yield valuable insights into the underlying pathways. In this study, we employed in situ nanoplasmonic resonance scattering spectroscopy to dynamically monitor the MOR process on single gold nanorod particles (GNPs) and Pt-coated gold nanoparticles (Pt-GNPs). We observed the evolution of metal hydroxides, which was assumed as the active species. Notably, the dynamic behavior of the surface atomic layers revealed the rate-determining steps for both the GNPs and Pt-GNPs, indicating competitive adsorption of intermediates on the nanocatalyst surface. The resulting inherent reaction mechanism highlights the thermodynamics-dependent catalysts’ redox processes and their surface adsorptions, which holds significance for advancing highly active MOR catalysts.

Removal of CO in flue gas by catalytic oxidation: a review

Abstract Most coal-fired industrial flue gases contained low concentration CO. How to deal with it effectively was a research hotspot in recent years. Catalytic oxidation was considered as the most promising method in the 21st century for the removement of CO with the high efficiency, environmentally friendly, easy to operate and low cost. In this review, the reaction mechanisms of CO oxidation were described, which could provide ideas for the development of new catalysts. The effects of supports and preparation methods on catalysts activity was also reviewed systematically. In addition, some suggestions and outlooks were provided for future development of CO catalytic oxidation.

Synergistic hydroxyl mechanism on halloysite-confined PtFe alloy boosting low-temperature CO-PROX performance

Both structural and adsorbed surface hydroxyl groups in solid materials are important carriers for oxygen species transport. The role of identifiable structural hydroxyl groups in catalytic reactions, however, is often underestimated. Here, we propose a synergistic hydroxyl reaction mechanism on Pt1Fe0.33/HNTs catalyst in CO-PROX reaction, which is critical to reveal the new way of oxygen transport in the oxidation reaction. Particularly, the Pt1Fe0.33 nanoparticles (NPs) have been confined in the cavity of halloysite nanotubes (HNTs) to activate structural hydroxyl groups linked to the AlVI via a two-step reduction treatment. The activated structural hydroxyl groups exhibit sensitivity to CO, as revealed by in situ DRIFTS and MS data. The experimental and DFT data demonstrate that the cooperation of Pt, Fe, and hydroxyl sites in the active units on Pt1Fe0.33/HNTs can induce a synergistic participation of structural and adsorbed hydroxyl groups in CO oxidation through the formate pathway. In such catalytic process, the oxygen mobility is dominated by oxygen-containing hydroxyl species rather than reactive oxygen species. This work contributes to a deeper understanding of the role of hydroxyl groups in catalysis and lays the foundation for establishing a comprehensive oxidation reaction mechanism in the future.