Showing papers on "Reaction rate constant published in 2018"

Sulfate radical-based photo-Fenton reaction derived by CuBi2O4 and its composites with α-Bi2O3 under visible light irradiation: Catalyst fabrication, performance and reaction mechanism

Abstract: Sulfate radical-based photo-Fenton (SR-photo-Fenton) reaction, assisted by visible light irradiation, was achieved by CuBi2O4 and its composites with α-Bi2O3 for refractory chemical degradation in aqueous solution. Herein, this catalyst was fabricated by a sol-gel method and the fabrication conditions, including calcination temperature and molar ratio of Cu/Bi, were optimized according to the crystal phase composition, catalytic activity and toxic copper ion leaching. The optimal calcination temperature was 500 °C and molar ratio of Bi to Cu was 2.0. The catalyst containing CuBi2O4 and α-Bi2O3 showed a higher density of surface OH which might be the key surface active site than pure CuBi2O4. The influence of initial solution pH, PMS concentration, catalyst dosage and catalyst reuse on rhodamine B (RhB) degradation was investigated. Importantly, calcination at 500 °C reverted the catalytic activity of catalyst. Results of electron paramagnetic resonance, competitive radical experiments and surface chemical property characterization demonstrated that the reaction mechanism of this novel SR-photo-Fenton reaction is a combination of interface and solution reactions. In the interface reaction, the transfer of photogenerated electron/hole pairs drives the decomposition of PMS to produce SO4 − and OH. Furthermore, the cycling of Cu(I)/Cu(II) facilitated effective PMS activation to generate free radical that was responsible for the degradation of RhB. The second order reaction rate constant between RhB and SO4 − was determined to be 0.595–6.436 × 1010 M−1 S−1 based on the chemical reaction kinetics of radical, which was a first and important report for SO4 − chemistry.

Sulfate radical-mediated degradation and mineralization of bisphenol F in neutral medium by the novel magnetic Sr2CoFeO6 double perovskite oxide catalyzed peroxymonosulfate: Influence of co-existing chemicals and UV irradiation

Abstract: Metal-based catalysis has notably contributed to the chemical community mainly in environmental science. Various cobalt and iron based catalysts have shown great potential for powerful reactive species. However, the application of cobalt and iron based perovskite for aqueous phase oxidation still remains limited. In this study, Sr2FeCoO6 double perovskite oxide was synthesized and used for the first time as an effective heterogeneous magnetic catalyst for peroxymonosulfate activation to produce free reactive radicals. Bisphenol F (BPF), a new emergent compound, was used as a target contaminant to evaluate the performance of this combination. Sr2FeCoO6 exhibited superior catalytic performance to the single SrFeO3 and SrCoO3 nanocrystals. A synergetic catalytic effect was found between Co and Fe, probably due to the accelerated reduction of Fe. BPF removal depends on catalyst loading, temperature, BPF and PMS concentration. It was observed that Sr2FeCoO6 activates peroxymonosulfate heterogeneously, with its heterogeneity being more pronounced at neutral pH. Kinetics studies reveal that BPF degradation obeys pseudo First order kinetic with an activation energy of 14.085 KJ/mol and an apparent rate constant of 0.026 min−1. Under conditions of 0.45 g L−1 and 10−4 M PMS, the complete BPF degradation occurred within 90 min at neutral conditions. In addition, the degraded BPF had undergone more than 65% mineralization within 6 h. The combination of UV irradiation (254 nm) with the Sr2FeCoO6-PMS system induces a potential acceleration in the mineralization rate. The effects of humic acid, carbonate, bicarbonate and chloride were also evaluated. A total of 8 products were detected, and the associated degradation pathway was proposed. To identify the dominating reactive species generated in the Sr2CoFeO6/PMS system, radical quenching tests and in situ electron paramagnetic resonance analysis were performed. To our knowledge, this is the first study that documents the heterogeneous activation of peroxymonosulfate with cobalt-iron based double perovskite for the treatment of the new emergent compound Bisphenol F.

De-reducibility mechanism of titanium on maghemite catalysts for the SCR reaction: An in situ DRIFTS and quantitative kinetics study

Abstract: An environmental benign TiO2 doped maghemite catalyst, the γ-Fe95Ti5, was prepared via the precipitation microwave pyrolysis method for the NOx removal. The γ-Fe95Ti5 exhibited significantly higher catalytic activity and better N2 selectivity than the pure maghemite, γ-Fe100Ti0. The SCR active window of the catalyst is broadened and the resistances to H2O and SO2 are also preserved. Ti4+ cations could enter the lattice of γ-Fe100Ti0, forming the partial solid solution on the catalyst surface, as γ-Fe2-ζTiζO3+ξ. This structure improves the quantity and stability of both Lewis and Bronsted acid sites compared with the γ-Fe100Ti0. Meanwhile, the dopant cations suppress the reduction of Fe3+ and the percentage of active oxygen on the catalyst surface. These could suppress the N2O formation from NH3 oxidation and NOx reduction. By the combination of both DRIFTS and kinetic methods, the rate constants of the γ-Fe95Ti5 catalyst via the Eley-Rideal and Langmuir-Hinshelwood mechanisms increase simultaneously, while the rate constant via the catalytic oxidation of NH3 decreases compared with the γ-Fe100Ti0.

Aqueous Phase Photo-oxidation of Brown Carbon Nitrophenols: Reaction Kinetics, Mechanism, and Evolution of Light Absorption

Abstract: Light absorbing organic aerosol particles, referred to as brown carbon, are geographically widespread and can have an important climate impact through the absorption of solar radiation. Recent studies, both in the laboratory and the field, have shown that brown carbon aerosols can be bleached of their color by direct photolysis and photo-oxidation reactions on the time scale of hours to days. However, the photo-oxidation of nitrophenol molecules, which are colored compounds often associated with biomass burning organic aerosol, show an enhancement in light absorption before the color is lost. This study investigates the mechanism of color enhancement and the fate of three nitrophenol compounds, specifically nitrocatechol, nitroguaiacol, and dinitrophenol, in the aqueous phase using online aerosol chemical ionization mass spectrometry (aerosol-CIMS). The second-order rate constants for the three nitrophenols with OH radicals in the aqueous phase at pH 7 (298 K), were determined to be 5 × 109 M–1 s–1, 5.2 ×...

Kinetics of the Methanol Reaction with OH at Interstellar, Atmospheric, and Combustion Temperatures

Abstract: The OH radical is the most important radical in combustion and in the atmosphere, and methanol is a fuel and antifreeze additive, model biofuel, and trace atmospheric constituent. These reagents are also present in interstellar space. Here we calculate the rate constants, branching ratios, and kinetic isotope effects (KIEs) of the hydrogen abstraction reaction of methanol by OH radical in a broad temperature range of 30–2000 K, covering interstellar space, the atmosphere, and combustion by using the competitive canonical unified statistical (CCUS) model in both the low-pressure and high-pressure limits and, for comparison, the pre-equilibrium model. Coupled cluster CCSD(T)-F12a theory and multi-reference CASPT2 theory were used to carry out benchmark calculations of the stationary points on the potential energy surface to select the most appropriate density functional method for direct dynamics calculations of rate constants. We find a significant effect of the anharmonicity of high-frequency modes of tra...

Kinetic investigation for the catalytic reduction of nitrophenol using ionic liquid stabilized gold nanoparticles

Abstract: We demonstrate the synthesis of gold nanoparticles (AuNP) stabilized by 1-butyl-3-hexadecyl imidazolium bromide (Au@[C4C16Im]Br) and their use as a catalyst for the reduction of nitrophenol. The AuNPs show excellent stability in presence of [C4C16Im]Br ionic liquids for the reduction of 4-nitrophenol and 2-nitrophenol using NaBH4 as a reducing agent. The detailed kinetics for the reduction of 4-nitrophenol and 2-nitrophenol were investigated and the catalytic activity of Au@[C4C16Im]Br was evaluated. The pseudo first-order rate constant (kapp) values for 4-nitrophenol was observed to be greater than that of 2-nitrophenol and explained on the basis of hydrogen bonding present in 2-nitrophenol. Au@[C4C16Im]Br showed good separability and reusability and hence, it can be used for the complete reduction of nitrophenols in multiple cycles. The Langmuir–Hinshelwood reaction mechanism is elucidated for reduction of 4-nitrophenol by Au@[C4C16Im]Br nanocatalyst on the basis of the kapp values. The thermodynamic activation parameters such as activation energy, enthalpy of activation and entropy of activation were determined and explained using the temperature dependent kinetics for the reduction of nitrophenol using Au@[C4C16Im]Br. The above results reveal that the Au@[C4C16Im]Br nanocatalyst demonstrates excellent catalytic performance for the reduction of nitrophenol by NaBH4 at room temperature.

H-Abstraction reactions by OH, HO2, O, O2 and benzyl radical addition to O2 and their implications for kinetic modelling of toluene oxidation

Abstract: Alkylated aromatics constitute a significant fraction of the components commonly found in commercial fuels. Toluene is typically considered as a reference fuel. Together with n-heptane and iso-octane, it allows for realistic emulations of the behavior of real fuels by the means of surrogate mixture formulations. Moreover, it is a key precursor for the formation of poly-aromatic hydrocarbons, which are of relevance to understanding soot growth and oxidation mechanisms. In this study the POLIMI kinetic model is first updated based on the literature and on recent kinetic modelling studies of toluene pyrolysis and oxidation. Then, important reaction pathways are investigated by means of high-level theoretical methods, thereby advancing the present knowledge on toluene oxidation. H-Abstraction reactions by OH, HO2, O and O2, and the reactivity on the multi well benzyl-oxygen (C6H5CH2 + O2) potential energy surface (PES) were investigated using electronic structure calculations, transition state theory in its conventional, variational, and variable reaction coordinate forms (VRC-TST), and master equation calculations. Exploration of the effect on POLIMI model performance of literature rate constants and of the present calculations provides valuable guidelines for implementation of the new rate parameters in existing toluene kinetic models.

Oxygen Reduction Reaction Promoted by Manganese Porphyrins

Abstract: The oxygen reduction reaction (ORR) is catalyzed by manganese(II) porphyrins in the presence of Bronsted acids (HAs). Analyses of the catalytic cyclic voltammetric profiles have permitted the ORR mechanism to be constructed and rate constants to be extracted for both the formation of the initial oxygen adduct and the O–O bond cleavage event for a series of HAs. The dependence of the formation rate constant of the oxygen adduct on reactant concentrations reveals a rate law that is first order in Mn porphyrin and oxygen substrate. A second order dependence in HA is observed for unadorned Mn porphyrin platforms whereas with Mn hangman porphyrin, a proton is provided intramolecular to the oxygen adduct and consequently the HA order is reduced to unity. The stabilization of the oxygen adduct with an additional hydrogen bond from HA engenders a rate-determining step involving O–O bond cleavage, resulting in the rare instance where the activation of the O–O bond is directly observed.

Quantitative constraints on autoxidation and dimer formation from direct probing of monoterpene-derived peroxy radical chemistry

Abstract: Organic peroxy radicals (RO2) are key intermediates in the atmospheric degradation of organic matter and fuel combustion, but to date, few direct studies of specific RO2 in complex reaction systems exist, leading to large gaps in our understanding of their fate. We show, using direct, speciated measurements of a suite of RO2 and gas-phase dimers from O3-initiated oxidation of α-pinene, that ∼150 gaseous dimers (C16-20H24-34O4-13) are primarily formed through RO2 cross-reactions, with a typical rate constant of 0.75-2 × 10-12 cm3 molecule-1 s-1 and a lower-limit dimer formation branching ratio of 4%. These findings imply a gaseous dimer yield that varies strongly with nitric oxide (NO) concentrations, of at least 0.2-2.5% by mole (0.5-6.6% by mass) for conditions typical of forested regions with low to moderate anthropogenic influence (i.e., ≤50-parts per trillion NO). Given their very low volatility, the gaseous C16-20 dimers provide a potentially important organic medium for initial particle formation, and alone can explain 5-60% of α-pinene secondary organic aerosol mass yields measured at atmospherically relevant particle mass loadings. The responses of RO2, dimers, and highly oxygenated multifunctional compounds (HOM) to reacted α-pinene concentration and NO imply that an average ∼20% of primary α-pinene RO2 from OH reaction and 10% from ozonolysis autoxidize at 3-10 s-1 and ≥1 s-1, respectively, confirming both oxidation pathways produce HOM efficiently, even at higher NO concentrations typical of urban areas. Thus, gas-phase dimer formation and RO2 autoxidation are ubiquitous sources of low-volatility organic compounds capable of driving atmospheric particle formation and growth.

Self-Catalytic Reaction of SO3 and NH3 To Produce Sulfamic Acid and Its Implication to Atmospheric Particle Formation.

Abstract: Sulfur trioxide (SO3) is one of the most active chemical species in the atmosphere, and its atmospheric fate has profound implications to air quality and human health. The dominant gas-phase loss pathway for SO3 is generally believed to be the reaction with water molecules, resulting in sulfuric acid. The latter is viewed as a critical component in the new particle formation (NPF). Herein, a new and competitive loss pathway for SO3 in the presence of abundant gas-phase ammonia (NH3) species is identified. Specifically, the reaction between SO3 and NH3, which produces sulfamic acid, can be self-catalyzed by the reactant (NH3). In dry and heavily polluted areas with relatively high concentrations of NH3, the effective rate constant for the bimolecular SO3-NH3 reaction can be sufficiently fast through this new loss pathway for SO3 to become competitive with the conventional loss pathway for SO3 with water. Furthermore, this study shows that the final product of the reaction, namely, sulfamic acid, can enhance the fastest possible rate of NPF from sulfuric acid and dimethylamine (DMA) by about a factor of 2. An alternative source of stabilizer for acid-base clustering in the atmosphere is suggested, and this new mechanism for NPF has potential to improve atmospheric modeling in highly polluted regions.

Measuring Interfacial Polymerization Kinetics Using Microfluidic Interferometry

Abstract: A range of academic and industrial fields exploit interfacial polymerization in producing fibers, capsules, and films. Although widely used, measurements of reaction kinetics remain challenging and rarely reported, due to film thinness and reaction rapidity. Here, polyamide film formation is studied using microfluidic interferometry, measuring monomer concentration profiles near the interface during the reaction. Our results reveal that the reaction is initially controlled by a reaction–diffusion boundary layer within the organic phase, which allows the first measurements of the rate constant for this system.

Kinetics of the Reactions between the Criegee Intermediate CH2OO and Alcohols

Abstract: Reactions of the simplest Criegee intermediate (CH2OO) with a series of alcohols have been studied in a flash photolysis flow reactor. Laser photolysis of diiodomethane at 355 nm in the presence of molecular oxygen was used to produce CH2OO, and the absolute number densities were determined as a function of delay time from analysis of broadband transient absorption spectra obtained using a pulsed LED. The kinetics for the reactions of CH2OO with methanol, ethanol, and 2-propanol were measured under pseudo-first-order conditions at 295 K, yielding rate constants of (1.4 ± 0.4) × 10-13 cm3 s-1, (2.3 ± 0.6) × 10-13 cm3 s-1, and (1.9 ± 0.5) × 10-13 cm3 s-1, respectively. Complementary ab initio calculations were performed at the CCSD(T)/aug-cc-pVTZ//CCSD/cc-pVDZ level of theory to characterize stationary points on the reaction enthalpy and free energy surfaces and to elucidate the thermochemistry and mechanisms. The reactions proceed over free energy barriers of ∼8 kcal mol-1 to form geminal alkoxymethyl hydroperoxides: methoxymethyl hydroperoxide (MMHP), ethoxymethyl hydroperoxide (EMHP), and isopropoxymethyl hydroperoxide (PMHP). The experimental and theoretical results are compared to reactions of CH2OO with other hydroxylic compounds, such as water and carboxylic acids, and trends in reactivity are discussed.

Substituent Effects and Mechanism in a Mechanochemical Reaction

Abstract: We report the effect of substituents on the force-induced reactivity of a spiropyran mechanophore. Using single molecule force spectroscopy, force-rate behavior was determined for a series of spiropyran derivatives substituted with H, Br, or NO2 para to the breaking spirocyclic C-O bond. The force required to achieve the rate constants of ∼10 s-1 necessary to observe transitions in the force spectroscopy experiments depends on the substituent, with the more electron withdrawing substituent requiring less force. Rate constants at 375 pN were determined for all three derivatives, and the force-coupled rate dependence on substituent identity is well explained by a Hammett linear free energy relationship with a value of ρ = 2.9, consistent with a highly polar transition state with heterolytic, dissociative character. The methodology paves the way for further application of linear free energy relationships and physical organic methodologies to mechanochemical reactions, and the characterization of new force probes should enable additional, quantitative studies of force-coupled molecular behavior in polymeric materials.

Unimolecular reaction of acetone oxide and its reaction with water in the atmosphere.

Abstract: Criegee intermediates (i.e., carbonyl oxides with two radical sites) are known to be important atmospheric reagents; however, our knowledge of their reaction kinetics is still limited. Although experimental methods have been developed to directly measure the reaction rate constants of stabilized Criegee intermediates, the experimental results cover limited temperature ranges and do not completely agree well with one another. Here we investigate the unimolecular reaction of acetone oxide [(CH3)2COO] and its bimolecular reaction with H2O to obtain rate constants with quantitative accuracy comparable to experimental accuracy. We do this by using CCSDT(Q)/CBS//CCSD(T)-F12a/DZ-F12 benchmark results to select and validate exchange-correlation functionals, which are then used for direct dynamics calculations by variational transition state theory with small-curvature tunneling and torsional and high-frequency anharmonicity. We find that tunneling is very significant in the unimolecular reaction of (CH3)2COO and its bimolecular reaction with H2O. We show that the atmospheric lifetimes of (CH3)2COO depend on temperature and that the unimolecular reaction of (CH3)2COO is the dominant decay mode above 240 K, while the (CH3)2COO + SO2 reaction can compete with the corresponding unimolecular reaction below 240 K when the SO2 concentration is 9 × 1010 molecules per cubic centimeter. We also find that experimental results may not be sufficiently accurate for the unimolecular reaction of (CH3)2COO above 310 K. Not only does the present investigation provide insights into the decay of (CH3)2COO in the atmosphere, but it also provides an illustration of how to use theoretical methods to predict quantitative rate constants of medium-sized Criegee intermediates.

Dopamine Autoxidation Is Controlled by Acidic pH.

Abstract: We studied the reaction mechanism of dopamine autoxidation using quantum chemical methods. Unlike other biogenic amines important in the central nervous system, dopamine and noradrenaline are capable of undergoing a non-enzymatic autoxidative reaction giving rise to a superoxide anion that further decomposes to reactive oxygen species. The reaction in question, which takes place in an aqueous solution, is as such not limited to the mitochondrial membrane where scavenging enzymes such as catalase and superoxide dismutase are located. With the experimental rate constant of 0.147 s-1, the dopamine autoxidation reaction is comparably as fast as the monoamine oxidase B catalyzed dopamine decomposition with a rate constant of 1 s-1. By using quantum chemical calculations, we demonstrated that the rate-limiting step is the formation of a hydroxide ion from a water molecule, which attacks the amino group that enters intramolecular Michael addition, giving rise to a pharmacologically inert aminochrome. We have shown that for dopamine stability on a time scale of days, it is essential that the pH value of the synaptic vesicle interior is acidic. The pathophysiologic correlates of the results are discussed in the context of Parkinson’s disease as well as the pathology caused by long-term amphetamine and cocaine administration.

Biodiesel by Transesterification of Rapeseed Oil Using Ultrasound: A Kinetic Study of Base-Catalysed Reactions

Abstract: The objective of this work was to study the acceleration that ultrasound causes in the rate of biodiesel transesterification reactions. The effect of different operating variables, such as ultrasound power, catalyst (KOH) concentration and methanol:oil molar ratio, was studied. The evolution of the process was followed by gas chromatography, determining the concentration of methyl esters at different reaction times. The biodiesel was characterized by its density, viscosity, saponification and iodine values, acidity index, water content, flash and combustion points, cetane index and cold filter plugging point (CFPP), according to EN 14214 standard. High methyl ester yield and fast reaction rates were obtained in short reaction times. Ultrasound power and catalyst concentration had a positive effect on the yield and the reaction rate. The methanol:oil molar ratio also increased the yield of the reaction, but negatively influenced the process rate. The reaction followed a pseudo-first order kinetic model and the rate constants at several temperatures were determined. The activation energy was also determined using the Arrhenius equation. The main conclusion of this work is that the use of ultrasound irradiation did not require any additional heating, which could represent an energy savings for biodiesel manufacture.

Differential Kinetics of Two-Cysteine Peroxiredoxin Disulfide Formation Reveal a Novel Model for Peroxide Sensing.

Abstract: Two-cysteine peroxiredoxins (Prx) have a three-step catalytic cycle consisting of (1) reduction of peroxide and formation of sulfenic acid on the enzyme, (2) condensation of the sulfenic acid with a thiol to form disulfide, also known as resolution, and (3) reduction of the disulfide by a reductant protein. By following changes in protein fluorescence, we have studied the pH dependence of reaction 2 in human peroxiredoxins 1, 2, and 5 and in Salmonella typhimurium AhpC and obtained rate constants for the reaction and pKa values of the thiol and sulfenic acid involved for each system. The observed reaction 2 rate constant spans 2 orders of magnitude, but in all cases, reaction 2 appears to be slow compared to the same reaction in small-molecule systems, making clear the rates are limited by conformational features of the proteins. For each Prx, reaction 2 will become rate-limiting at some critical steady-state concentration of H2O2 producing the accumulation of Prx as sulfenic acid. When this happens, an a...