Showing papers on "Chemical kinetics published in 2002"
TL;DR: In this article, the effect of the crystalline defects on the rate of the reaction by using a series of stepped surfaces was investigated using chronoamperometry and an apparent intrinsic rate constant was determined.
Abstract: The kinetics of the electrochemical oxidation of a CO adlayer on Pt[n(111)×(111)] electrodes in 0.5 M H2SO4 has been studied using chronoamperometry. The objective is to elucidate the effect of the crystalline defects on the rate of the reaction by using a series of stepped surfaces. The reaction kinetics of the main oxidative process can be modeled using the mean-field approximation for the Langmuir−Hinshelwood mechanism, implying fast diffusion of adsorbed CO on the Pt[n(111)×(111)] surfaces under electrochemical conditions. The apparent rate constant for the electrochemical CO oxidation, determined by a fitting of the experimental data with the mean-field model, is found to be proportional to the step fraction (1/n) for the surfaces with n > 5, proving steps to be the active sites for the CO adlayer oxidation. An apparent intrinsic rate constant is determined. The potential dependence of the apparent rate constants is found to be structure insensitive with a Tafel slope of ca. 80 mV/dec, suggesting the...
356 citations
TL;DR: In this paper, a single particle mass spectrometer was coupled to a reaction flow tube to measure the uptake coefficient of ozone (O3) by oleic acid (9-octadecenoic acid) aerosol particles.
Abstract: The technique of single-particle mass spectrometry has been coupled to a reaction flow tube to measure the uptake coefficient, γ, of ozone (O3) by oleic acid (9-octadecenoic acid) aerosol particles. The reaction was followed by monitoring the decrease of oleic acid in the size-selected particles as a function of O3 exposure. The reactive uptake coefficient is found to depend on the size of the particle, with γmeas ranging from (7.3 ± 1.5) × 10-3 to (0.99 ± 0.09) × 10-3 for particles ranging in radius from 680 nm to 2.45 μm. It is suggested that the decrease in γmeas with increasing particle size results from the reaction being limited by the diffusion of oleic acid within the particle, and based on our measurements we estimate the value of γ to be (5.8−9.8) × 10-3 for particles that are not limited by oleic acid diffusion. A reaction model that includes simultaneous diffusion and reaction of both O3 and oleic acid is developed and used to fit the observed rates of reaction. Solutions obtained from this mo...
176 citations
TL;DR: In this article, the esterification kinetics of lactic acid with methanol without addition of catalyst and catalyzed by ion-exchange resins are presented, and the effect of catalyst type, stirrer speed, catalyst size, catalyst loading, initial reactant ratio, and temperature on reaction kinetics is evaluated.
Abstract: The esterification kinetics of lactic acid with methanol without addition of catalyst and catalyzed by ion-exchange resins are presented in this work. The effect of catalyst type, stirrer speed, catalyst size, catalyst loading, initial reactant ratio, and temperature on reaction kinetics was evaluated. Experimental reaction rates were correlated by some models based on homogeneous and heterogeneous (dual- and single-site mechanisms) approaches. Nonideality of the liquid phase was taken into account by using activities instead of mole fractions. Prediction of the activity coefficients was made by UNIFAC. Parameters of the different models were obtained by the simplex search method.
153 citations
TL;DR: Investigation of the H2O2 generation rate in phosphate buffer media over the range of pH 2-9 revealed a maximum yield at pH approximately 3.2, and the degradation rate of 4-NP under ultrasonic irradiation decreases with increasing pH.
152 citations
TL;DR: The functional and structural studies support the proposal that CHI accelerates the stereochemically defined intramolecular cyclization of chalcones into biologically active (2S)-flavanones by selectively binding an ionized chalcone in a conformation conducive to ring closure in a diffusion-controlled reaction.
149 citations
TL;DR: In this paper, the kinetics of the chemically activated reaction between the ethyl radical and molecular oxygen were analyzed using quantum Rice−Ramsperger−Kassel (QRRK) theory for k(E) with both a master equation analysis and a modified strong-collision approach to account for collisional deactivation.
Abstract: The kinetics of the chemically activated reaction between the ethyl radical and molecular oxygen are analyzed using quantum Rice−Ramsperger−Kassel (QRRK) theory for k(E) with both a master equation analysis and a modified strong-collision approach to account for collisional deactivation. Thermodynamic properties of species and transition states are determined by ab initio methods at the G2 and CBS-Q//B3LYP/6-31G(d,p) levels of theory and isodesmic reaction analysis. Rate coefficients for reactions of the energized adducts are obtained from canonical transition state theory. The reaction of C2H5 with O2 forms an energized peroxy adduct with a calculated well depth of 35.3 kcal mol-1 at the CBS-Q//B3LYP/6-31G(d,p) level of theory. The calculated (VTST) high-pressure limit bimolecular addition reaction rate constant for C2H5 + O2 is 2.94 × 1013T-0.44. Predictions of the chemically activated branching ratios using both collisional deactivation models are similar. All of the product formation pathways of ethyl...
137 citations
TL;DR: In this article, it is shown that it is possible to produce 1,2-propanediol (a high demand commodity chemical) in high yields via the vapor-phase catalytic hydrogenation of biomass-derived lactic acid.
Abstract: It is shown that it is possible to produce 1,2-propanediol (a high demand commodity chemical) in high yields via the vapor-phase catalytic hydrogenation of biomass-derived lactic acid. This catalytic process provides an environment-friendly route for the production of 1,2-propanediol from renewable resources. Reaction kinetics measurements were conducted for the vapor-phase hydrogenation of lactic acid over silica-supported copper at total pressures between 0.10 and 0.72 MPa and temperatures between 413 and 493 K. Lactic acid is hydrogenated over Cu/SiO2 under these reaction conditions to predominately 1,2-propanediol, with formation of smaller amounts of 2-hydroxy propionaldehyde, propionic acid, and propyl alcohols. Deactivation of the Cu/SiO2 catalyst does not appear to be significant under these reaction conditions. The production of 1,2-propandiol is favored at higher hydrogen partial pressures. At 473 K and a hydrogen partial pressure of 0.72 MPa, complete conversion of lactic acid was observed, with 88 mol% of the lactic acid converted to 1,2-propanediol. A reaction scheme for lactic acid conversion is proposed involving the dissociative adsorption of lactic acid to form an adsorbed α-hydroxy acyl species that undergoes successive hydrogenation steps to form adsorbed 2-hydroxy propionaldehyde species and then adsorbed α-hydroxy alkoxy species. These alkoxy species react with surface hydrogen to form 1,2-propanediol. The formation of 2-hydroxy propionaldehyde appears to be in equilibrium with 1,2-propanediol at the reaction conditions of the present study.
130 citations
TL;DR: In this paper, the unimolecular decomposition of CH3CH2OH has been investigated at the G2M (RCC2) level of theory and the decomposition reaction was found to be dependent strongly on pressure and temperature.
Abstract: The unimolecular decomposition of CH3CH2OH has been investigated at the G2M (RCC2) level of theory. The decomposition reaction was found to be dependent strongly on pressure and temperature. Among the eight product channels identified, the H2O-elimination process (1) via a four-member-ring transition state is dominant below 10 atm in the temperature range of 700–2500 K. At the high—pressure limit and over 1500 K, cleavage of the C–C bond by reaction (2) producing CH3+CH2OH is predicted to be dominant while the CH3CH2+OH channel (8) also becomes competitive. The predicted high-pressure rate constants for the two major product channels can be given by k1=7.0×1013 exp(−34 200/T) and k2=3.7×1026 T−2.95 exp(−45 600/T) s−1, which compare reasonably with earlier data and with our preliminary experimental result obtained in a shock tube and static cell study. At the internal energy corresponding to the O(1D)+C2H6 reaction (140.7 kcal/mol above C2H5OH), the predicted branching ratios for the production of CH3, C2H...
129 citations
TL;DR: A striking difference was found between aldose and ketose sugars as suggested by the modeling results: not the ketoses themselves but only their reaction products were found to be reactive in the Maillard reaction.
Abstract: In the present study, a kinetic model of the Maillard reaction occurring in heated monosaccharide-casein systems was proposed. Its parameters, the reaction rate constants, were estimated via multiresponse modeling. The determinant criterion was used as the statistical fit criterion instead of the familiar least squares to avoid statistical problems. The kinetic model was extensively tested by varying the reaction conditions. Different sugars (glucose, fructose, galactose, and tagatose) were studied regarding their effect on the reaction kinetics. This study has shown the power of multiresponse modeling for the unraveling of complicated reaction routes as occur in the Maillard reaction. The iterative process of proposing a model, confronting it with experiments, and criticizing the model was passed through four times to arrive at a model that was largely consistent with all results obtained. A striking difference was found between aldose and ketose sugars as suggested by the modeling results: not the ketoses themselves but only their reaction products were found to be reactive in the Maillard reaction.
126 citations
TL;DR: In this article, a mechanistic-like four-stage kinetic analysis consisting of multiple curve methods involving different heating rates (also known as isoconversional or apparent kinetic methods); analysis at the peak maximum of the differential scanning calorimetry (DSC) curves; integral and differential single curve methods; and modeling of the reaction and comparison of the model with the experiment was performed.
104 citations
TL;DR: In this paper, a gas−liquid reaction kinetics of the reactions between carbon dioxide and hydroxide ions is obtained via dynamic experiments in a stirred cell reactor. And a new expression for the reaction rate constant at infinite dilution is obtained.
Abstract: Gas−liquid reaction kinetics of the reactions between carbon dioxide and hydroxide ions is obtained via dynamic experiments in a stirred cell reactor. The evaluation of experiments is performed using a new technique enabling experimental series without time-consuming solvent purification after each experiment. A new expression for the reaction rate constant at infinite dilution is obtained. Furthermore, a kinetic description of aqueous potassium hydroxide and aqueous sodium hydroxide solutions in the temperature range between 20 and 50 °C is proposed.
TL;DR: In this article, the decomposition behavior of solid oxygenated complexes was examined under pure nitrogen, at temperatures between 60 and 110 °C, and the production rates of CO2 and CO depend on temperature, but are independent of the particle size of the samples.
TL;DR: The method to prepare tosylated cotton cloth for enzyme immobilization is simple, inexpensive, and scaleable for industrial applications, and can be used as a low cost support for highly active and stable biocatalyst.
TL;DR: X-ray diffraction analysis shows that the development of beta-TCP is not via direct reaction between Ca and P for all the different combinations betweenCa and P precursors, and the transition rate from interface-controlled to diffusion-controlled growth is precursor-dependent.
TL;DR: In this article, the authors present a general theory of the kinetics of reversible diffusion-influenced reactions that is exact both at short and long times for A+B⇌C and A+C⌌C+D. The formalism is based on an approximate set of reaction-diffusion equations for the pair distribution functions which incorporate the influence of the chemical reaction by using effective rate constants that are determined selfconsistently.
Abstract: The simplest general theory of the kinetics of reversible diffusion-influenced reactions that is exact both at short and long times for A+B⇌C and A+B⇌C+D is presented. The formalism is based on an approximate set of reaction-diffusion equations for the pair distribution functions which incorporate the influence of the chemical reaction by using effective rate constants that are determined self-consistently. For small deviations from equilibrium and contact reactivity, the relaxation function is given explicitly in the Laplace domain in terms of the Smoluchowski rate coefficient that describes the corresponding diffusion controlled irreversible reaction. Consequently, the kinetics can be easily obtained for arbitrary diffusion coefficients and equilibrium concentrations.
TL;DR: The light-off curve is the conversion-temperature plot of a catalytic reaction; it is usually used in catalyst development, and the possibility to deduce kinetic information such as the apparent reaction order or mass transfer limitation from the shape of the curve is examined in this article.
TL;DR: In this paper, the H-atom transfer reaction C2H2+1,J)+H2→C2H3++H is used to monitor the infrared excitation of acetylene ions.
Abstract: A sensitive experimental method for ion spectroscopy and state specific reaction dynamics is described, briefly called laser induced reactions (LIR). The technique is based on (i) trapping ions over a long time in a cold 22-pole rf ion trap followed by mass spectrometric detection, (ii) providing a suitable low density gas environment for collisions, (iii) modifying the low temperature chemical kinetics using selective excitation via a tunable radiation source. In this paper, the H-atom transfer reaction C2H2+ (v3=1,J)+H2→C2H3++H, is used to monitor the infrared excitation of acetylene ions. Rotationally resolved spectra are presented for the antisymmetric C–H stretching vibration. For recording a spectrum, it is sufficient to fill the trap with a few thousand parent ions. Differences with respect to conventional IR spectroscopy are discussed, especially the processes which influence the LIR signal. From the measured intensities and their dependence on parameters such as storage time, laser fluence and ta...
TL;DR: In this paper, the reactions of trifluoromethyl radicals with pyrene, phenanthrene, crotonic acid, and 2-propanol (2-PrOH) in the ionic liquid methyltributylammonium bis(trifluorsulfonyl)imide (R4NNTf2) are studied by pulse radiolysis.
Abstract: The reactions of trifluoromethyl radicals with pyrene, phenanthrene, crotonic acid, and 2-propanol (2-PrOH) in the ionic liquid methyltributylammonium bis(trifluoromethylsulfonyl)imide (R4NNTf2) are studied by pulse radiolysis. Radiolysis of R4NNTf2 leads to formation of solvated electrons and organic radicals, including •CF3. The solvated electrons do not react rapidly with the solvent and are reacted with CF3Br to produce additional •CF3 radicals. The rate constants for addition of •CF3 radicals to pyrene and phenanthrene are determined to be (1.1 ± 0.1) × 107 L mol-1 s-1 and (2.6 ± 0.4) × 106 L mol-1 s-1, respectively. By competition kinetics, the rate constant for reaction of •CF3 radicals with crotonic acid is determined to be (2.7 ± 0.4) × 106 L mol-1 s-1, and the reaction is predominantly addition to the double bond. Competition kinetics with 2-PrOH in the absence of CF3Br gives a rate constant of (4 ± 1) × 104 L mol-1 s-1 for H-abstraction from 2-PrOH, but in the presence of CF3Br, the rate consta...
TL;DR: In this paper, a review of experimental and theoretical studies of polymer reactions in polymer blends is presented, where the interchain interactions between the block copolymer and homopolymers are analyzed.
01 Jan 2002
TL;DR: The most important reaction in GB and DIMP that makes them more reactive than TMP or DMMP is found to be a six-center molecular elimination reaction producing propene.
Abstract: A detailed chemical kinetic reaction mechanism is developed to describe incineration of the chemical warfare nerve agent sarin (GB), based on commonly used principles of bond additivity and hierarchical reaction mechanisms. The mechanism is based on previous kinetic models of organophosphorus compounds such as trimethyl phosphate (TMP), dimethyl methyl phosphonate (DMMP), and di-isopropyl methyl phosphonate (DIMP) that are often used as surrogates to predict incineration of GB. Kinetic models of the three surrogates and GB are then used to predict their consumption in a perfectly stirred reactor fueled by natural gas to simulate incineration of these chemicals. Computed results indicate that DIMP is the only one of these surrogates that adequately describes combustion of GB under comparable conditions. The kinetic pathways responsible for these differences in reactivity are identified and discussed. The most important reaction in GB and DIMP that makes them more reactive than TMP or DMMP is found to be a six-center molecular elimination reaction producing propene.
TL;DR: In this paper, a methodology for including the effects of nonidealities, such as confinement in a porous solid or solvation, into the calculation of bimolecular reaction rate constants is presented.
Abstract: A methodology for including the effects of nonidealities, such as confinement in a porous solid or solvation, into the calculation of bimolecular reaction rate constants is presented. The method combines the transition-state theory formalism with the Reactive Monte Carlo simulation method. The approach is computationally efficient and accurate, within the approximations imposed by transition-state theory and the intermolecular potentials. Several applications of the method are presented for the decomposition reaction, 2HI→H2+I2, including effects due to confinement within carbon micropores and due to inert solvents. The method can be readily extended to other chemical reaction rate calculations in which the structure and the activation energy of the transition state is known a priori.
TL;DR: In this article, the growth kinetics of plasma-enhanced Ti atomic layer deposition (ALD) using a quartz crystal microbalance was investigated. But the growth rates were independent of TiCl4 exposure above 1×103 L, indicating typical ALD mode growth.
Abstract: We have investigated the growth kinetics of plasma-enhanced Ti atomic layer deposition (ALD) using a quartz crystal microbalance. Ti ALD films were grown at temperatures from 20 to 200 °C using TiCl4 as a source gas and rf plasma-produced atomic H as the reducing agent. Postdeposition ex situ chemical analyses of thin films showed that the main impurity is oxygen, mostly incorporated during the air exposure prior to analysis. The thickness per cycle, corresponding to the growth rate, was measured by quartz crystal microbalance as a function of various key growth parameters, including TiCl4 and H exposure time, rf plasma power, and sample temperature. The growth rates were independent of TiCl4 exposure above 1×103 L, indicating typical ALD mode growth. The key kinetic parameters for Cl extraction reaction and TiCl4 adsorption kinetics were obtained and the growth kinetics were modeled to predict the growth rates based upon these results. Also, the dependency of growth kinetics on different substrate materi...
TL;DR: In this paper, a model-free approach is proposed to predict the kinetics of complex reactions including the change from chemically controlled to diffusion-controlled kinetics, on the basis of a small number of experiments.
TL;DR: In this paper, a differential fixed-bed reactor was employed to study the reaction between Ca(OH)2/fly ash sorbent and SO2 under the conditions similar to those in the bag filters of the spray-drying flue gas desulfurization system.
TL;DR: In this paper, an extension of the force field based conformational flooding procedure is proposed to predict products, transition states, and reaction paths of unimolecular chemical reactions such as dissociation or rearrangement reactions of small to medium sized molecules.
Abstract: We present a method to predict products, transition states, and reaction paths of unimolecular chemical reactions such as dissociation or rearrangement reactions of small to medium sized molecules. The method thus provides the necessary input for established procedures to compute barrier heights and reaction rates, which conventionally have to be assumed heuristically. The method is an extension of the force field based conformational flooding procedure, but here aims at an accelerated barrier crossing of chemical reactions rather than conformational motions. Accordingly, it is now coupled to density functional molecular dynamics, such that the chemical reaction under study takes place at the picoseconds time scale set by todays computer technology. Barrier crossings are accelerated by means of an additional energy term (flooding potential) that locally destabilizes the educt conformation without affecting possible transition states or product states. The method is applied to two test systems, bicyclopropylidene and methylenecyclopropane, for which the reaction paths are predicted correctly. New details of reaction pathways are found, such as a transient concerted, but asynchronous rotation of the two methylene groups for the bicyclopropylidene→methylenespiropentane reaction. Our method can be applied to simulations in the gas phase as well as in solution and can be combined with force field simulations, e.g., in hybrid density functional/force field (QM/MM) computations.
TL;DR: In this paper, the authors used a soluble polymeric form of MnO2 and determined the stoichiometry of this reaction, as well as the chemical kinetics and the activation parameters.
Abstract: Manganese oxides are strong environmental oxidants recently found to be involvedin the nitrogen cycle. Of the several possible reactions with reduced nitrogen species,the reduction of MnO2 by nitrite has only received marginal attention. Yet, this reaction might explain why nitrification can occur in the absence of O2, observed in both sediments and water columns. We have determined the stoichiometry of this reaction, as well as the chemical kinetics and the activation parameters, using a soluble polymeric form of MnO2. The reaction rate decreases with increasing pH and decreasing temperature. The reaction is first order in each reactant with a second order rate constant (k) = 493 M-1 min-1 at 21.5 °C and pH = 5.00. The energy of activation (Ea = 9.370 kJ/mole) and the entropy of activation (Δ S‡ = -169.5 J/mole) show the reaction to be associative and diffusion controlled, occurring via an inner-sphere mechanism, likely with O atom transfer from MnO2 to HNO2. The reaction is proton assisted and slowsdown at pH ≥ 5.5 where NO2
- and MnO2 (unprotonated and negatively charged) become the dominant species. In natural waters and sediments where anaerobic nitrification has been observed the pH is higher than this. Thus, the thermodynamically favorable reaction will likely proceed by microbial mediation.
TL;DR: In this article, a model based on slurry hydrodynamics, mass transport, and reaction kinetics was developed to predict the removal rate of copper on a wafer-scale during chemical-mechanical planarization (CMP).
TL;DR: In this paper, the Pd/ACF were tested in liq.-phase hydrogenation of 2-butyne-1,4-diol showing a selectivity towards 2-butene-1-4-Diol up to 97% at conversions up to 80%.
TL;DR: In this paper, the reaction kinetics of drying, pyrolysis and combustion of paper sludge have been determined in a thermogravimetric analyzer (TGA).
TL;DR: In this paper, the authors measured the reaction rates of solvated electrons with oxygen and with sulfur hexafluoride in hydrothermal and supercritical water using transient absorption spectroscopy and electron pulse radiolysis.
Abstract: Reaction rates of solvated electrons with oxygen and with sulfur hexafluoride were measured in hydrothermal and supercritical water using transient absorption spectroscopy and electron pulse radiolysis. Under alkaline conditions, the reaction of hydrogen atoms with hydroxide ions to generate solvated electrons was also observed in the presence of the SF6 scavenger. At temperatures below 300 °C, the rate constants for scavenging by O2 or SF6 follow Arrhenius behavior but become increasingly dependent on water density (pressure) at higher temperatures. Above 100 °C, the rate constant for the H reaction with OH- falls well below the numbers extrapolated from the Arrhenius behavior in the one atmosphere liquid. At a fixed temperature above the water critical temperature (380 °C, T/Tc=1.01), rate constants for all three reactions reach a distinct minimum near 0.45 g/cm3. We propose an explanation for this behavior in terms of the potential of mean force separating an ion (OH- or (e-)aq) from a hydrophobic spec...