Home  About us  Contact
 

Complex Reaction Mechanism (complex + reaction_mechanism)

Distribution by Scientific Domains
Chemistry100%

Selected Abstracts

Comparison of global and local sensitivity techniques for rate constants determined using complex reaction mechanisms

INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 12 2001
James J. Scire Jr.
Many rate constant measurements, including some "direct" measurements, involve fitting a complex reaction mechanism to experimental data. Two techniques for estimating the error in such measurements were compared. In the first technique, local first-order elementary sensitivities were used to rapidly estimate the sensitivity of the fitted rate constants to the remaining mechanism parameters. Our group and others have used this technique for error estimation and experimental design. However, the nonlinearity and strong coupling found in reaction mechanisms make verification against globally valid results desirable. Here, the local results were compared with analogous importance-sampled Monte Carlo calculations in which the parameter values were distributed according to their uncertainties. Two of our published rate measurements were examined. The local uncertainty estimates were compared with Monte Carlo confidence intervals. The local sensitivity coefficients were compared with coefficients from first and second-degree polynomial regressions over the whole parameter space. The first-order uncertainty estimates were found to be sufficiently accurate for experimental design, but were subject to error in the presence of higher order sensitivities. In addition, global uncertainty estimates were found to narrow when the quality of the fit was used to weight the randomly distributed points. For final results, the global technique was found to provide efficient, accurate values without the assumptions inherent in the local analysis. The rigorous error estimates derived in this way were used to address literature criticism of one of the studies discussed here. Given its efficiency and the variety of problems it can detect, the global technique could also be used to check local results during the experimental design phase. The global routine, coded using SENKIN, can easily be extended to different types of data, and therefore can serve as a valuable tool for assessing error in rate constants determined using complex mechanisms. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 784,802, 2001 [source]

Kinetic Reaction Models for the Selective Reduction of NO by Methane over Multifunctional Zeolite-based Redox Catalysts

CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 12 2004
T. Sowade
Abstract Kinetic measurements of the selective catalytic reduction (SCR) of NO by methane were performed over CeO2/H-ZSM-5, In-ZSM-5, and CeO2/In-ZSM-5 catalysts. The parameter space covered NO, CH4, and O2 concentrations varying from 250 to 1000 ppm, from 500 to 2000 ppm, and from 0.5 to 10,vol.-%, respectively, space velocities between 5000 and 90000 h,1 and temperatures between 573 and 873 K depending on the catalyst activities. With CeO2/In-ZSM-5 an additional series of measurements was performed with moistened feed gas (0.5,10,vol.-% H2O). On the basis of a pseudo-homogeneous, one-dimensional fixed-bed reactor model, the data were fitted to a kinetic model that includes power rate laws for the reduction of NO and for the unselective total oxidation of methane. From analyses of isothermal data sets, almost all reaction orders were found to vary significantly with changing temperature, which indicates that the simple kinetic model cannot reflect the complex reaction mechanism correctly. Nevertheless, the data measured with In-ZSM-5 could be modeled with good accuracy over a wide range of reaction temperatures (150 K) while the accuracy was less satisfactory with the remaining data sets, in particular for data with the moist feed over CeO2/In-ZSM-5. With the latter catalyst it was not possible to represent the data measured in dry and in moist feed in a single model even upon confinement to fixed reaction temperatures. A comparison of the separate models established showed strong changes in the reaction orders in the presence of water, which occur apparently already at a very low water content (,,0.5,vol.-%). The kinetic parameters found are in agreement with earlier conclusions about the reaction mechanisms. With In-ZSM-5, both reaction orders and the activation energy show a rate-limiting influence of NO oxidation on the NO reduction path which is removed by the presence of the CeO2 promoter. A difference in the reaction mechanism over CeO2/In-ZSM-5 and CeO2/H-ZSM-5 is reflected in different kinetic parameters. The differences of the kinetic parameters between dry-feed and moist-feed models for CeO2/In-ZSM-5 reflect adsorption competition between the reactants and water. [source]

Comparison of global and local sensitivity techniques for rate constants determined using complex reaction mechanisms

INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 12 2001
James J. Scire Jr.
Many rate constant measurements, including some "direct" measurements, involve fitting a complex reaction mechanism to experimental data. Two techniques for estimating the error in such measurements were compared. In the first technique, local first-order elementary sensitivities were used to rapidly estimate the sensitivity of the fitted rate constants to the remaining mechanism parameters. Our group and others have used this technique for error estimation and experimental design. However, the nonlinearity and strong coupling found in reaction mechanisms make verification against globally valid results desirable. Here, the local results were compared with analogous importance-sampled Monte Carlo calculations in which the parameter values were distributed according to their uncertainties. Two of our published rate measurements were examined. The local uncertainty estimates were compared with Monte Carlo confidence intervals. The local sensitivity coefficients were compared with coefficients from first and second-degree polynomial regressions over the whole parameter space. The first-order uncertainty estimates were found to be sufficiently accurate for experimental design, but were subject to error in the presence of higher order sensitivities. In addition, global uncertainty estimates were found to narrow when the quality of the fit was used to weight the randomly distributed points. For final results, the global technique was found to provide efficient, accurate values without the assumptions inherent in the local analysis. The rigorous error estimates derived in this way were used to address literature criticism of one of the studies discussed here. Given its efficiency and the variety of problems it can detect, the global technique could also be used to check local results during the experimental design phase. The global routine, coded using SENKIN, can easily be extended to different types of data, and therefore can serve as a valuable tool for assessing error in rate constants determined using complex mechanisms. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 784,802, 2001 [source]

Applications of non-steady-state kinetics in physical organic chemistry: guidelines for the resolution of the kinetics of complex reaction mechanisms

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 9 2001
Vernon D. Parker
Abstract The resolution of the kinetics of the reversible consecutive second-order reaction mechanism involving the formation of a kinetically significant intermediate, which does not reach steady state before late in the first half-life, followed by an irreversible product-forming reaction is discussed. It is shown that an apparent second-order rate constant kapp and an extent of reaction,time profile are the only experimental data necessary for the evaluation of kf and kb (the forward and reverse rate constants) as well as kp (the microscopic rate constant for the product forming reaction). When the product-forming step involves the cleavage of a CH bond, for which there is a deuterium kinetic isotope effect on kp, the resolution of the kinetics is enhanced. In this case, the experimental data include two apparent rate constants ( and ) and two extent of reaction,time profiles, one for normal reactants and the other for isotopically substituted reactants. Under these circumstances, a unique highly resolved experimental to theoretical data fit is found that results in the evaluation of all four microscopic rate constants: and . An alternative, when a kinetic isotope effect is not involved, is to fit the extent of reaction,time profiles for two or more concentrations of reactants concurrently. This procedure results in the resolution of the three microscopic rate constants for the reaction. Copyright © 2001 John Wiley & Sons, Ltd. [source]