Surface chemistry of aerosolized nanoparticles:thermal oxidation of silicon

The kinetics of reaction between silicon nanoparticles and molecular oxygen were studied by tandem differential mobility analysis. Aerosolized silicon nanoparticles were extracted from a low-pressure silane plasma into an atmospheric pressure aerosol flow tube reactor. Particles were initially passed through a differential mobility analyzer that was set to transmit only those particles having mobility diameters of approximately 10 nm. The monodisperse particle streams were mixed with oxygen/nitrogen mixtures of different oxygen volume fractions and allowed to react over a broad temperature range (600-1100 degrees C) for approximately one second. Particles were size-classified after reaction with a second differential mobility analyzer. The particle mobility diameters increased upon oxidation by up to 1.3 nm, depending on the oxygen volume fraction and the reaction temperature. Oxidation is described by a kinetic model that considers both oxygen diffusion and surface reaction, with diffusion becoming important after formation of a 0.5 nm thick oxide monolayer.     

Kinetic study on TEMPO-mediated selective oxidation of regenerated cellulose

The kinetics of the TEMPO-mediated oxidation of regenerated celluloses has been studied. It is revealed that the oxidation reaction of the regenerated celluloses by the 2,2,6,6-tetramethyl-piperidine-1-oxyl radical (TEMPO)–NaBr–NaOCl system can be approximately described by first-order kinetics with respect to substrate. In the concentration range used the rate constant k is directly proportional to the concentration of TEMPO, while it is proportional to the concentration of NaBr in a relatively lower range and tends to level off at higher concentration. The effect of temperature on the rate constant can be well described by the Arrhenius equation, the apparent activation energy measured is about 66.2 kJ/mol. The effect of the pH of the reaction solution, the crystallinity and morphology of the substrates on the oxidation rate is also discussed.     

Surface chemistry of nanometer-sized aerosol particles: reactions of molecular oxygen with 30 nm soot particles as a function of oxygen partial pressure

The kinetics of the reaction between soot nanoparticles and molecular oxygen were studied by tandem differential mobility analysis (TDMA). The particles were extracted from the tip of an ethene diffusion flame. Reactions were studied at atmospheric pressure in mixtures of nitrogen and oxygen. The studies involved particles of an initial mobility diameter of 30 nm over broad ranges of temperature (500-1100 degrees C) and oxygen volume fraction (0-1). Measurements as a function of oxygen partial pressure establish that the oxidation kinetics are not first-order in oxygen volume fraction (F(O2)). Rather, the oxidation rate increases rapidly and linearly with F(O2) between 0 and 0.05 and then more slowly but still linearly between 0.05 and 1. Temperature dependent measurements are consistent with a reaction pathway involving two kinetically distinguishable oxidation sites which interconvert thermally and through oxidation. Results and conclusions are compared to those of earlier studies on the oxidation of soot.     

多相条件下亚硫酸镁非催化氧化反应动力学及机理

副产物的氧化回收利用是影响镁法脱硫工艺发展的关键.本文通过真空旋转蒸发的方式制备了高纯度的亚硫酸镁样品,并利用鼓泡式反应装置,研究了亚硫酸镁非催化氧化的反应动力学,考察了pH、氧分压、亚硫酸镁浓度、气体流量、温度等因素对亚硫酸镁氧化反应速率的影响,得到了各反应物的分级数及表观活化能.结合三相反应过程的数学模型推断出亚硫酸镁氧化的本征反应在慢反应区进行,且氧的传质扩散是总反应的速率控制步骤.研究结果为氧化回收镁法脱硫副产物提供了理论依据.     

HEMATOPORPHYRIN PHOTOSENSITIZATION OF SERUM ALBUMIN and SUBTILISIN BPN'

—The photosensitized inactivation of subtilisin BPN' by free hematoporphyrin (HP) followed exponential kinetics with positive mechanistic tests for the involvement of singlet oxygen (¹O₂) as the principal intermediate. The photoinactivation quantum yield was 0.029 at 390 nm in oxygen-saturated, D₂O buffer at pH 7.0. The effects of HP binding were investigated for tryptophan oxidation in bovine serum albumin (BSA) and human serum albumin (HSA) at high protein:HP concentration ratios where the HP was > 97% complexed. The reaction kinetics were non-exponential and mimick a second-order process in the initial stages. The rate of HP photobleaching was 30-fold faster for complexed HP compared with free HP, which was shown to account for the observed kinetics. Mechanistic tests showed that ¹O₂ was the dominant photooxidizing intermediate of tryptophan residues and that it was not involved in the accompanying photobleaching of HP. The quantum yield for tryptophan oxidation in BSA was 0.11 at 390 nm in oxygen-saturated, D₂O buffer at pH 8.0. The reactivity of HSA was approximately 2-fold lower than BSA for equivalent conditions. Estimates of the reaction cross sections led to 3 Ų for the inactivation of subtilisin BPN' by ¹O₂ and 20 Ų for the oxidation of tryptophan in BSA.     

Kinetic and mechanism study of the oxidative decolorization of neutral red by bromate in micellar medium

The kinetics of the oxidation of neutral red by bromate was investigated in the presence of cationic surfactant (N-dodecylpyridinium chloride) and strongly acidic media over the temperature range 22-35 °C. The kinetic studies were carried out as a function of different variables like concentration and temperature. The described reaction rate is greatly catalyzed by the presence of trace amount of Fe(II) ions and followed by pseudo-third-order kinetics with respect to dye concentration. A possible mechanism for the catalyzed reaction is offered which is found to be coincidentally congruent with experimental findings. The derived rate equation is in good agreement with the experimental observations. Thermodynamic parameters were also evaluated.     

Degradation kinetics of telithromycin determined by HPLC method

The degradation kinetics of the antibiotic telithromycin using a stability-indicating high-performance liquid chromatography (HPLC) method is demonstrated. The photodegradation is performed by UVC lamp-254 nm (15W), installed in a chamber internally coated with mirrors, where telithromycin solutions prepared from coated tablets are placed in quartz cells. To promote oxidation, the reaction between the telithromycin solution and 3% hydrogen peroxide solution is carried out. The kinetics parameters of order of reaction and the rate constants of the degradation are determined for both conditions. The degradation process of telithromycin can be described by first-order kinetics under both experimental conditions used in this study. The results reveal the photo and oxidation lability of the drug and confirm the reliability of HPLC method for telithromycin in the presence of its degradation products.     

在Pd/TiO2上CO的光催化增强效应

在Pd(2％)/TiO2上CO催化氧化的研究中发现：室温下， 以黑光灯（λ=365 nm）照时， CO的氧化活性比暗态显著提高，产生了明显的光催化增强效应（两者速率常数比约为15）.但在TiO2上，无论暗态还是黑光灯（λ=365 nm）照，均无CO氧化反应发生.这可能是由于氧分子解离吸附时的键能减弱，使Pd表面氧原子（或O－）浓度增加和氧原子的溢流（oxygen spillover）两种效应结合产生的.     

Reactivity of Arylhydrazones toward Molecular Oxygen

The kinetics and mechanism of reaction of arylhydrazones with molecular oxygen were studied by gas volumetry. The reaction rate was studied in relation to the structure of arylhydrazone and kind of the solvent. The inhibiting power of the compounds toward initiated oxidation of ethylbenzene was evaluated, and the most effective compounds were found, judging from the ratio of the rate constants of the reactions with molecular oxygen and with peroxy radicals arising in the course of ethylbenzene oxidation.     

Simulation of the effect of surface-oxide formation on bistability in CO oxidation on Pt-group metals

The kinetics of CO oxidation on Pt-group metals are known to often exhibit bistability. During the low-reactive regime observed at relatively high CO pressure, the surface is primarily covered by CO and the reaction rate is controlled by O2 dissociation. During the high-reactive regime at relatively low CO pressure, in contrast, the surface is mainly covered by oxygen and the reaction rate is proportional to CO pressure. In the latter case, the adsorbed oxygen may be in the chemisorbed state and/or may form surface oxide. The experiments indicate that the formation of surface oxide often occurs via the island growth and accordingly should be described in terms of the theory of first-order phase transitions. Here, the author proposes a generic lattice-gas model satisfying this requirement and allowing one to execute the corresponding Monte Carlo simulations. Systematically varying the model parameters determining the oxide stability, he classifies the likely scenarios of the bistable reaction kinetics complicated by oxide formation.     

Propane oxidation on nickel in a self-oscillation mode

The kinetics of the catalytic oxidation of propane with oxygen on nickel in a self-oscillation mode was studied. A comparative analysis of changes in reaction rate oscillations with time in the presence of nickel wire and foil was performed. It was found that the reaction medium influenced the morphology of the catalyst surface. With the use of X-ray photoelectron spectroscopy, it was demonstrated that a NiO layer no less than 100 nm in thickness was formed on the catalyst surface in the course of the reaction of propane oxidation. The mechanism of the appearance of self-oscillations in the test system is discussed.__________Translated from Kinetika i Kataliz, Vol. 46, No. 2, 2005, pp. 269–277.Original Russian Text Copyright © 2005 by Gladky, Kaichev, Bukhtiyarov, Parmon.Deceased.     

Kinetics of polymeric radical oxidation with molecular oxygen in poly(methyl methacrylate) films

In poly(methyl methacrylate) films, the kinetics of the oxidation of polymeric radicals and azobenzenenitrenes with molecular oxygen dissolved in the polymer is studied. The free radicals are produced at 77 K by irradiating the polymer with UV light, fast electrons, or γ rays. The concentration of oxygen is varied from 4.5 × 10¹⁸ to 3.1 × 10¹⁹ cm^?3; the temperature of the reaction, from 90 to 130 K. The reaction is carried out in excess oxygen. The kinetics of radical oxidation is shown to be independent of the type of radiation that stimulates the formation of radicals and coincides with the kinetics of the oxidation of azobenzenenitrenes, which are uniformly dissolved in the polymer. It is concluded that the structure of the polymer in the vicinity of the radicals is virtually the same as the structure of the polymer bulk. The activation energy of the oxygen diffusion coefficient calculated according to the radical oxidation kinetics amounts to ～30 kJ/mol.     

Oxidation of malic acid by vanadium (V) in sulphuric acid medium: A kinetic and mechanistic investigation

The oxidation kinetics of malic acid by vanadium(V) in aqueous sulphuric acid was investigated at 303 K by monitoring the appearance of vanadium(IV) at 760 nm. The reaction showed first-order behavior with respect to vanadium(V), malic acid, and hydrogen ion concentration, respectively, regardless of the ionic strength. The reaction rate is enhanced by an increase in ionic strength and a decrease of the dielectric constant of the medium by addition of methanol. The activation parameters were estimated by varying the temperature in the range of 293 K to 313 K to 313 K. An oxidation mechanism is proposed, involving different vanadium(V) species produced in the presence of sulphuric acid. © 1995 John Wiley & Sons, Inc.     

Evaluation of antioxidants using oxidation reaction rate constants

An evaluation method for the capacity of antioxidants to protect drugs against oxidation is presented. As a new viewpoint, to determine the priority of the competitive oxidations between the antioxidant and the protected drug, and to compare the drug-protection capacity of antioxidants, it is important to determine their oxidation rate constants using chemical kinetics instead of standard oxidation (or reduction) potentials. Sodium sulfite, sodium bisulfite and sodium pyrosulfite were used as models for the determination of oxidation reaction rate constants in aqueous solutions. In the experiments, sufficient air was continually infused into the solution to keep the concentration of dissolved oxygen constant. The residual concentrations of the antioxidants were determined by iodimetry, and the concentration of dissolved oxygen by oxygen electrode. The data were fitted by linear regressions to obtain the reaction rate constants. It was found that the degradation of sodium sulfite, sodium bisulfite or sodium pyrosulfite obeyed pseudo zero-order kinetics in the buffer solutions. Because of the ionization equilibrium, these three antioxidants have the same ion form in solutions at a definite pH value and therefore their apparent rate constants were essentially the same. The average apparent rate constants of the three antioxidants at 25°C are (1.34 ± 0.03) × 10⁻³ at pH 6.8, (1.20 ± 0.02) × 10⁻³ at pH 4.0 and (6.58 ± 0.02) × 10⁻³ mol·L⁻¹·h⁻¹ at pH 9.2, respectively. Translated from Acta Chimica Sinica, 2006, 64(6): 496–500 (in Chinese)     

Kinetics of Ir (III)-catalyzed Oxidation of D-glucose by Potassium Iodate in Aqueous Alkaline Medium

The kinetics of Ir (III) chloride-catalyzed oxidation of D-glucose by iodate in aqueous alkaline medium was investigated at 45°C. The reaction follows first-order kinetics with respect to potassium iodate in its low concentration range but tends to zero order at its higher concentration. Zero-order kinetics with respect to [D-glucose] was observed. In the lower concentration range of Ir (III) chloride, the reaction follows first kinetics, while the order shifts from first to zero at its higher concentration range. The reaction follows first-order kinetics with respect to [OH^?] at its low concentration but tends towards zero order at higher concentration. Variation in [Cl^?] and ionic strength of the medium did not bring about any significant change in the rate of reaction. The first-order rate constant increased with a decrease in the dielectric constant of the medium. The values of rate constants observed at four different temperatures were utilized to calculate the activation parameters. Sodium salt of formic acid and arabinonic acid have been identified as the main oxidation products of the reaction. A plausible mechanism from the results of kinetic studies, reaction stoichiometry, and product analysis has been proposed.     

Oxidation kinetics of crystal violet by potassium permanganate in acidic medium

The oxidation kinetics of crystal violet (a triphenylmethane dye) by potassium permanganate was focused in an acidic medium by the spectrophotometric method at 584 nm. The oxidation reaction of crystal violet by potassium permanganate is carried out in an acidic medium at different temperatures ranging within 298–318 K. The kinetic study was carried out to investigate the effect of the concentration, ionic strength and temperature. The reaction followed first order kinetics with respect to potassium permanganate and crystal violet and the overall rate of the reaction was found to be second order. Thermodynamic activation parameters like the activation energy (E_a), enthalpy change (ΔH*), free energy change (ΔG*), and entropy change (ΔS*) have also been evaluated.     

Intrinsic kinetic study on the oxidation of 6-pentyl-1,2,3,4-tetrahydroanthacene-9,10-diol

A new working solution consisting of 2-pentylanthraquinone (PAQ) and 6-pentyl-1,2,3,4-tetrahydroantraquinone (4HPAQ) was hydrogenated and then oxidized by O₂ to produce H₂O₂. The oxidation reaction was conducted in a well-stirred batch reactor at 30~50^oC and 0.10~0.20 MPa. By measuring the concentrations of generated H₂O₂ by iodometry, the intrinsic kinetics for the gas-liquid oxidation of 6-pentyl-1,2,3,4-tetrahydroanthacene- 9,10-diol (4HPAQH₂, the only hydrogenated product in the hydrogenated working solution) with molecular oxygen were studied. An exponential model was employed to describe the reaction rate and the kinetic parameters were obtained. The results show that the reaction rate is 0.7 and 1.4 order in the concentration of 4HPAQH₂ and oxygen pressure respectively, and the activation energy for oxidation is 41.3 kJ/mol. This revised version was published online in June 2006 with corrections to the Cover Date.     

Micellar catalysis of oxidation of glycolic acid by N-bromophthalimide

Catalysis of oxidation of glycolic acid by N-bromophthalimide by micelles of cetyltrimethylammonium bromide (CTAB) at 318 K was investigated. The observed value of critical micelle concentration of CTAB in the presence of other components was lower than those reported in the literature. The oxidation reaction was strongly catalyzed by cationic micelles of CTAB. The reaction rate increased with CTAB concentration until the steady state was achieved. The reaction kinetics corresponded to first, fractional and inverse fiactional orders with respect to changes of concentration of reaction components. Effects of solvent, phthalimide, mercuric acetate, and potassium chloride on the reaction kinetics were also studied. The micelle-catalyzed oxidation reaction was shown to fit Arrhenius equation. The experimental data were rationalized in terms of Menger-Portnoy model considering a distribution of the reactants between the micellar and aqueous phases.     

Enzymic oxidation of dopamine by polyphenol oxidaze

The kinetics of the enzymic oxidation of dopamine (A) by polyphenol oxidaze (E) was studied by spectrophotometrically monitoring the concentration of the reaction product, 2,3-dihydroindole-5, 6 quinone, at 470 nm. The kinetics model proposed for the reaction and the effect of pH agreed with the experimental data and the resulting rate equation is given by where [A] is the concentration of dopamine (mM), [E_o] the concentration of polyphenol oxidaze (mg/1) and [H⁺] the concentration of hydrogen ion (M) in the reaction mixture. © John Wiley & Sons, Inc.     

Photo-oxidative degradation of bisphenol A polycarbonate and its possible initiation processes

During UV degradation of bisphenol A polycarbonate (BPA-PC), photo-Fries rearrangements and photo-oxidation reactions take place, however, in outdoor exposure conditions the photo-oxidation reaction is the most dominant one. To initiate this autocatalytic oxidation process, an initiating radical is required. In this research two possible sources for photo-initiation are explored, viz. hydroperoxides and peroxides formed by thermo-oxidation and charge transfer complexes between the polymer and oxygen. A comparison of the photodegradation rate of thermo-oxidized and undegraded polycarbonate samples was made. It was shown that the formed thermo-oxidation products in BPA-PC did not affect the photo-oxidation rate. Charge transfer complexes (CTCs) between oxygen and polycarbonate were studied by UV absorption spectroscopy at different air pressures. The concentration of CTCs increased with oxygen pressures. These CTCs absorb wavelengths in the region of terrestrial sunlight and could cause the initiation of the photo-oxidation. The influence of oxygen pressure on the photodegradation of BPA-PC was studied by irradiating the samples with a by the polymer absorbing wavelength, viz. 250 nm. The absorption of this wavelength leads to photo-Fries products. It was shown that at higher oxygen pressures the rate of the photo-Fries reaction is reduced, which was explained by quenching of the photo-Fries reaction by oxygen.