Computer modeling for analysis of complex reaction kinetics

A new method for computer prediction of the catalytic activity dependence on service life and technological conditions for bifunctional Pt catalysts has been proposed. The proposed method is based on physical and chemical laws of multicomponent hydrocarbon transformations on polymetallic catalysts and takes into account deactivation of acidic (Al) and metallic (Pt) sites. It allows to calculate the product composition and catalytic activity level for real industrial units as a function of their technological parameters and raw material characteristics.     

Characterizing complex reaction mechanisms using machine learning clustering techniques

A machine learning conceptual clustering method applied to reaction mechanisms provides an automatic and, hence, unbiased means to differentiate between reactive phases within a total reactive process. Similar reactive phases were defined by means of local reaction sensitivity values. The method was applied to the Hochgreb and Dryer aldehyde combustion mechanism of 36 reactions. Three major time ranges were found and characterized: an initial phase of aldehyde reaction, an intermediate phase where only a small amount of aldehyde is left, and an end phase of reactions to final products. Further refinements of these phases into subtime intervals were found. All ranges found could be chemically justified. This method is meant as a supplement to existing methods of mechanism analysis and its main purpose is the automatic characterization of existing mechanisms and can potentially be used for mechanism reduction. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 107–118, 2004     

Solid-phase reaction kinetics

It is argued that, for the macroscopic parameters of conventional kinetic models to become meaningful, they may be and must be expressed in terms of elementary single-barrier processes. To accomplish this means to associate some (external) extensional measure with a single-barrier elementary act, remaining within the logic of the existing geometrical-probabilistic scheme. A manner of doing this involving the use of Dirichlet fragmentations is suggested.     

Identifiability classes in reaction kinetics

Some difficulties of eliciting reliable rate constants from experimental data are considered. Different classes of ill-posed estimation problems are given and demonstrated by simple examples.

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A user-friendly programme 'SIMKINERSQUO; for simulation of kinetics involving complex reaction mechanisms

A programme, 'SIMKINERSQUO; is developed using a semi-implicit extrapolation method (SIEM) which uses the implicit midpoint rule and extrapolation to simulate complex mechanisms based on the kinetics of homogeneous chemical systems. The chemical kinetics pre-processor code is designed to translate a user-specified system of chemical rate equations into a system of chemical kinetic differential equations. The developed programme is applied to the 13-step mechanism of the reaction between Nile blue and acidic bromate. The results obtained compare well with the curves drawn using the other method, reported in literature.     

An inverse problem in reaction kinetics

In this paper we investigate the problem of extracting information about chemical reactions involving multiple species from the time history of the concentration of each species. The mathematical model of the kinetic system leads to a system of ordinary differential equations. Our focus is to examine whether the species’ concentrations as functions of time are sufficient to determine what chemical reactions, and at what reaction rates, have occurred. We show that within the limitation of our model, there may be many candidate reaction systems that could explain the data. Using the notion of sparsity, we provide a quantitative assessment of the question of distinguishability. We further demonstrate that sparsity enforcing approaches, such as minimizing the ℓ ₁ or the ℓ ₀ norms are not reliable. Our conclusion is that additional knowledge about the kinetic system will be necessary to reliably solve this inverse problem.     

Chelate ring closure kinetics in the reaction of a platinum(II) complex with diphosphines

The reactions of PPh2(CH2)nPPh2 (n=1–4, P-P) and PPh3 with cis- [PtPh2(CO)(SEt2)] have been studied in chloroform-d by 1H- and 31P{1H}-n.m.r. When n=2 or3 the first product observed is [PtPh2(P-P)], where the diphosphine is acting as a chelate, and ring closure is fast compared to the rate of entry of the phosphine into the complex. When n=1 or 4 the first observed product is [PtPh2(CO)(P-P)], with P-P acting as monodentate, and the second observable stage of reaction is ring closure. The rate constants and activation parameters kc at 298K (s–1), H (kJmol–1), S (JK–1mol–1) for the dppm and dppb complexes are 0.0198, 88±1, +17±3; and 0.00273, 38±2, –169±6, respectively. The formation of the large seven-membered ring is a strainless process, comparable to the intermolecular process. The increase in the enthalpy of activation as ring size decreases is due to ring strain and inter-atomic repulsions associated to the conformation of the four-membered chelate ring.     

Dehydration kinetics by non-isothermal techniques

The kinetics of thermal degradation of some cementitious calcium aluminates, sulfoaluminates, sulfoferrites and carboaluminates were studied. Activation energies, obtained by Morris and Rogers and Kissinger techniques, were found to be in good agreement with each other.

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Zusammenfassung Die thermische Zersetzungskinetik einiger im Zement befindlicher Kalziumaluminate, Sulfoaluminate, Sulfoferrite und Karboaluminate wurde untersucht. Die nach den Methoden von Morris und Rogers, sowie Kissinger erhaltenen Aktivierungsenergien stimmten gut überein.

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Résumé On a étudié la cinétique de la décomposition thermique de plusieurs constituants du ciment tels les aluminates, sulfoaluminates, sulfoferrites et carboaluminates de calcium. Les énergies d'activation obtenues en appliquant la méthode de Morris et Rogers ainsi que celle de Kissinger sont concordantes.

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Non-meanfield deterministic limits in chemical reaction kinetics

A general mechanism is proposed by which small intrinsic fluctuations in a system far from equilibrium can result in nearly deterministic dynamical behaviors which are markedly distinct from those realized in the meanfield limit. The mechanism is demonstrated for the kinetic Monte Carlo version of the Schnakenberg reaction where we identified a scaling limit in which the global deterministic bifurcation picture is fundamentally altered by fluctuations. Numerical simulations of the model are found to be in quantitative agreement with theoretical predictions.     

Approximation of slow attracting manifolds in chemical kinetics by trajectory-based optimization approaches

Many common kinetic model reduction approaches are explicitly based on inherent multiple time scales and often assume and directly exploit a clear time scale separation into fast and slow reaction processes. They approximate the system dynamics with a dimension-reduced model after eliminating the fast modes by enslaving them to the slow ones. The corresponding restrictive assumption of full relaxation of fast modes often renders the resulting approximation of slow attracting manifolds inaccurate as a representation of the reduced model and makes the numerical solution of the nonlinear "reduction equations" particularly difficult in many cases where the gap in intrinsic time scales is not large enough. We demonstrate that trajectory optimization approaches can avoid such severe restrictions by computing numerical solutions that correspond to "maximally relaxed" dynamical modes in a suitable sense. We present a framework of trajectory-based optimization for model reduction in chemical kinetics and a general class of reduction criteria characterizing the relaxation of chemical forces along reaction trajectories. These criteria can be motivated geometrically exploiting ideas from differential geometry and fundamental physics and turn out to be highly successful in example applications. Within this framework, we provide results for the computational approximation of slow attracting low-dimensional manifolds in terms of families of optimal trajectories for a six-component hydrogen combustion mechanism.     

Theoretical reaction kinetics of reversible polymerization

The kinetics of reversible polymerization of a polymer system without transfer and termination reactions is investigated theoretically. An analytic solution is obtained that enables one to predict the variation of monomer concentration with respect to time in a closed system. When the polymer system reaches equilibrium, three different relationships are derived between the equilibrium monomer concentration and the ceiling temperature under different conditions. The classical relationship by Dainton and Ivin is a special case of the three derived cases.     

Concerning some widespread errors in diffusion-controlled reaction kinetics

It is stressed that the “long-time” rate constant of a diffusion-mediated bimolecular reaction does not equal the product of the reaction probability per encounter with k_D, the encounter rate constant, and that k_D ≠ 4πD? where D is the sum of the diffusion coefficients of the reactants and ? the encounter diameter. Neglect of these points has vitiated the analysis used by Stevens and his associates for estimating diffusive displacement parameters.     

热重法研究煤的燃烧反应及其动力学

我国是个多煤少油的国家,煤炭是我国的主要能源,在我国的一次能源中所占比列高达70%[1].煤炭被广泛用于人类生产与生活的各个方面,且它的利用主要是通过燃烧直接获得能量.然而由于先进技术的欠缺,我国在煤炭的利用中一直存在着燃烧效率不高和环境污染严重等缺点,这与现今能源短缺和环境保护日益受重视的现状不相适应.本文通过研究分析煤的燃烧反应及燃烧反应动力学,为煤炭在利用时燃烧的清洁化与高效化提供科学依据.     

Profiling single-molecule reaction kinetics under nanopore confinement

The study of a single-molecule reaction under nanoconfinement is beneficial for understanding the reactive intermediates and reaction pathways. However, the kinetics model of the single-molecule reaction under confinement remains elusive. Herein we engineered an aerolysin nanopore reactor to elaborate the single-molecule reaction kinetics under nanoconfinement. By identifying the bond-forming and non-bond-forming events directly, a four-state kinetics model is proposed for the first time. Our results demonstrated that the single-molecule reaction kinetics inside a nanopore depends on the frequency of individual reactants captured and the fraction of effective collision inside the nanopore confined space. This insight will guide the design of confined nanopore reactors for resolving the single-molecule chemistry, and shed light on the mechanistic understanding of dynamic covalent chemistry inside confined systems such as supramolecular cages, coordination cages, and micelles.

A four-state kinetics model is proposed to reveal the kinetics of a single-molecule reaction under nanopore confinement.     

Spectroscopy and reaction kinetics of HS radicals

A laser-induced fluorescence technique is used to monitor the HS radical concentration. The rate constants at room temperature have been determined for the reaction of HS radical with various added scavengers.     

Water dependence of the HO2 self reaction: kinetics of the HO2-H2O complex

Transient absorption spectra and decay profiles of HO2 have been measured using cw near-IR two-tone frequency modulation absorption spectroscopy at 297 K and 50 Torr in diluent of N2 in the presence of water. From the depletion of the HO2 absorption peak area following the addition of water, the equilibrium constant of the reaction HO2 + H2O <--> HO2-H2O was determined to be K2 = (5.2 +/- 3.2) x 10(-19) cm3 molecule(-1) at 297 K. Substituting K2 into the water dependence of the HO2 decay rate, the rate coefficient of the reaction HO2 + HO2-H2O was estimated to be (1.5 +/- 0.1) x 10(-11) cm3 molecule(-1) s(-1) at 297 K and 50 Torr with N2 as the diluent. This reaction is much faster than the HO2 self-reaction without water. It is suggested that the apparent rate of the HO2 self-reaction is enhanced by the formation of the HO2-H2O complex and its subsequent reaction. Results are discussed with respect to the kinetics and atmospheric chemistry of the HO2-H2O complex. At 297 K and 50% humidity, the concentration ratio of [HO2-H2O]/[HO2] was estimated from the value of K2 to be 0.19 +/- 0.11.     

Measuring reaction kinetics in a lab-on-a-chip by microcoil NMR

A microfluidic chip with an integrated planar microcoil was developed for Nuclear Magnetic Resonance (NMR) spectroscopy on samples with volumes of less than a microliter. Real-time monitoring of imine formation from benzaldehyde and aniline in the microreactor chip by NMR was demonstrated. The reaction times in the chip can be set from 30 min down to ca. 2 s, the latter being the mixing time in the microfluidic chip. Design rules will be described to optimize the microreactor and detection coil in order to deal with the inherent sensitivity of NMR and to minimize magnetic field inhomogeneities and obtain sufficient spectral resolution.