Showing papers on "Elementary reaction published in 1983"

Hydrocarbon Oxidation at High Temperatures

Abstract: The detailed knowledge of combustion mechanisms is important, for example for the control of (kinetically determined) pollutant formation (e.g. NO, hydrocarbons, soot), for ignition problems, or for the extrapolation to technologically important but experimentally inaccessible condition. – In this review it is described how by suitable separation and elimination of unimportant reactions a mechanism is developed with the aid of the present kinetic data for the elementary reactions involved. This mechanism explains, without fitting, the currently available experimental results for laminar premixed flames of alkanes, alkenes and acetylene (flame velocity and structure of free flames, concentration and temperature profiles in burner-stabilized flames). These experimental results are simulated by the solution of the corresponding conservation equations with suitable models describing diffusion and heat conduction in the multicomponent mixture considered. – In lean and moderately rich flames the hydrocarbon is attacked by O, H, and OH in the first step. These radicals are produced by the chain-branching steps of the oxyhydrogen reaction. The alkyl radicals formed in this way always decompose to smaller alkyl radicals by fast thermal elimination of alkenes. Only the relatively slow thermal decomposition of the smallest alkyl radicals (CH3 and C2H5) competes with recombination and with oxidation reactions by O atoms and O2. This part of the mechanism is rate-controlling in the combustion of alkanes and alkenes, and is therefore the reason for the similarity of all alkane and alkene flames. – In rich flames of aliphatic fuels, acetylene becomes a very important intermediate leading to soot precursors and to Non-Zeldovich-NO. Details of the reaction mechanisms are not yet known.

The thermal decomposition of n‐hexane

Abstract: The kinetics of the pyrolysis of n-hexane was studied in a conventional static reactor over a temperature range of 650–840 K. The overall reaction is essentially first order with the kinetic parameters A = 1013.92 s−1 and EA = 260.3 kJ/mol. The distributions of the main products were analyzed by gas chromatography. A reaction model involving 240 elementary reactions was developed to describe the experimental rate data. The agreement of the model with experimental data was surprisingly good over a wide range of temperatures and pressures and up to medium extents of conversion. Methods for sensitivity studies based upon the quasi-stationary-state assumption (QSSA) were developed, and for a number of more detailed effects, such as self-inhibition, explanations could be given. It was also shown that the hexyl isomerization reactions influence strongly the product distribution. The outstanding capability of kinetic modeling with computer simulations in handling complex kinetic systems is demonstrated.

Analysis of the possible mechanisms for a catalytic reaction system

Abstract: Publisher Summary This chapter focuses on the enumeration of all possible mechanisms for a complex chemical reaction system based on the assumption of given elementary reaction steps and species. The procedure presented for such identification is been directly applied to a number of examples in the field of heterogeneous catalysis. Application to other areas is clearly indicated. These would include complex homogeneous reaction systems, many of which are characterized by the presence of intermediates acting as catalysts or free radicals. Enzyme catalysis should also be amenable to this approach. The subject of reaction mechanism also has a bearing on other fundamental problems of physical chemistry. In carrying out the procedure for determining mechanisms that is presented in the chapter, one obtains a set of independent chemical reactions among the terminal species in addition to the set of reaction mechanism. Consideration of a chemical system in terms of unique direct reaction mechanisms required to produce observable rates of change of terminal species has distinct advantages, especially when multiple overall reactions are involved. The required necessary assumptions regarding possible elementary reaction steps are becoming increasingly accessible through modern tools for surface spectroscopy and fundamental theories of chemical kinetics of elementary reaction steps.

Preparation of Silicon Nitride Powder from Silica

Abstract: Although thermodynamic data predict the difficulty in advancing the reaction, 3SiO2(S) + 6C(S) + 2N2(G) = Si3N4(S) + 6CO(G), in a closed system, it has been proved that heating fine powder mixtures of silica and carbon in nitrogen gas flow results in sufficient formation of silicon nitride powder, without detectable coexisting silicon carbide. Apparent activation energy (≃163 kcal/mol) on the reaction approximates to the value of ∆Hr (≃l60 kcal/mol at 1700 K) calculated on the elementary reaction, SiO2(S) + C(S) = SiO(G) + CO(G), which therefore seems to be a rate-determining one. The observation of fibrous silicon nitride growing out into gaps among aggregated powder lumps and that of weight losses of samples larger than expected suggest that SiO gas should be generated as an intermediate product. Silicon nitride powder prepared from silica through the improved reaction process possesses excellent characteristics such as high α-phase content, homogeneous shape and size, and very low content of metallic impurities.

Mechanism and thermochemistry of oxidation of HCl and HBr at high temperatures. The heat of formation of HO2

Abstract: Using currently available thermochemical and kinetic data and estimation methods to analyze the thermochemistry and the kinetic parameters of the elementary reactions involved in the oxidation of HCl and HBr, reaction mechanisms are proposed which account for the previously reported reaction products, the rate law, and the kinetic data. For oxidation of HCl, two competitive pathways, the radical initiation by hydrogen abstraction and the fourcenter reaction pathway, were invoked to account for the observations. In the oxidation of HBr one must invoke a fast surface reaction of the type to account for the reaction.

Reactions for chemical systems far from equilibrium

Abstract: Reactions formulated for systems operating far from equilibrium may generate misleading conclusions if they contain an implied assumption of steady state. This assumption is introduced most commonly by eliminating an intermediate product from a pair of elementary reactions, condensing them into a single summary reaction. The consequences of using such inadequate reactions are illustrated through two well known model systems, the Lotka oscillator and the Brusselator. The Brusselator includes a termolecular reaction which could be generated by summarizing any of several sets of elementary reactions. When an elementary set is used in place of the termolecular reaction, the revised Brusselator fails to show its characteristic limit cycle. Both the Brusselator and the Lotka scheme have reactions in which the same substance appears both as reactant and as product, formed by eliminating some intermediate in a catalytic cycle. Including even short-lived intermediates alters the stability shown by the Lotka scheme, so that behaviour in the phase plane changes from a conservative cycle into an expanding spiral.

Carbonic Anhydrase: Structure, Kinetics, and Mechanism

Abstract: The carbonic anhydrase-catalyzed hydration of carbon dioxide must involve the following elementary reactions. 1. The binding of C02. 2. The binding of H2O. 3. Breaking of an 0-H bond in H20. 4. Formation of an 0-C bond. 5. Dissociation of \( HCO_{3}^{ - } \). 6. Dissociation of H+. Kinetic evidence pertaining to the rates and relative order in time of these chemical events is discussed. A kinetic reaction scheme is presented, and this is related to the structure of the active site of the enzyme as known from X-ray diffraction studies.

Modeling studies of the homogeneous formation of aromatic compounds in the thermal decomposition of n-hexane

Abstract: The pyrolysis of n-hexane – as well as the thermal decomposition of n-hexane with addition of propene and 1 -butene – was studied at 819 K up to very high conversions. A reaction model, consisting of elementary reactions, was developed. With this model the experimental results including the formation of benzene and toluene can be described. Inhibition effects, the rates of formation of various products and the effects of products on the reacting system are discussed.

Addition of free radicals to unsaturated systems. Part 24. Kinetics and mechanism of the gas-phase thermal reactions of trifluoroiodomethane with propene

Abstract: The gas-thermal addition of trifluoroiodomethane to propene, at temperatures of 533–583 K, reactant ratios from 1 : 1 to 4 : 1, and total pressures of 100–300 mmHg, is shown to be a free-radical chain reaction with the kinetic equation as shown below. It is concluded that initiation is by the reaction of d[adduct]//dt=k8[I2]0.25[CF3I]1.09[C3H6]0.41 iodine atoms, in thermal equilibrium with molecular iodine, with trifluoroiodomethane, and termination is predominantly by reactions involving trifluorobutyl radicals. The Arrhenius parameters found for k8 are discussed in terms of those for the elementary reactions involved on the basis of the proposed mechanism.

Rate coefficients for unimolecular chemical reactions

Abstract: In this paper unimolecular reaction theory is formulated by introducing unstable molecules which are sufficiently long-lived to persist through several collisions. This is done by using the Fock-Tani representation, thus permitting the treatment of decay processes in the same way as bimolecular ones. Finally, a master equation is derived and the rate coefficients for decay processes are identified.

Evaluation of the Reaction Mechanism of the Sodium Pump by Steady-State Kinetics

Abstract: Publisher Summary Extensive investigation of the partial biochemical reactions and of the ion exchanges carried out by the Na pump has supported the reaction mechanism for the overall transport cycle originally proposed by Post and Albers. Although the partial reactions support the model, it is still possible that they are not, in fact, part of the main reaction pathway but are side reactions. For a partial reaction to be considered part of the overall reaction mechanism, it must in the first place be shown to be kinetically competent. The partial reaction occurs while the overall reaction is proceeding. The utility of the steady­ state kinetic approach to the evaluation of a reaction mechanism lies in its ability to distinguish between plausible mechanisms that are consistent with the known partial reactions. In particular, by a steady-state approach, it is frequently possible to determine the order in which substrates add and products are released while the overall reaction is proceeding.

Untangling the water gas shift from Fischer-Tropsch: a Gordian knot. [185 references]

Abstract: The water gas shift reaction is an integral part of the Fischer-Tropsch synthesis Although it may appear convenient to consider the water gas shift a separate reaction, in some cases, a detailed examination of the mechanism indicates theat the water gas shift and other synthesis gas reactions share several elementary reactions Experimental support for the relevant elementary reactions for the water gas shift on metals, metal oxides, and in homogeneous solution is examined, from both surface and complex chemistry Multiple paths leading to a net water gas shift reaction may be available; oxygen transfer and reaction through C-H-O intermediates may take place 185 references, 6 tables

The thermal decomposition of n-hexane

Abstract: The kinetics of the pyrolysis of n-hexane was studied in a conventional static reactor over a temperature range of 650–840 K. The overall reaction is essentially first order with the kinetic parameters A = 1013.92 s−1 and EA = 260.3 kJ/mol. The distributions of the main products were analyzed by gas chromatography. A reaction model involving 240 elementary reactions was developed to describe the experimental rate data. The agreement of the model with experimental data was surprisingly good over a wide range of temperatures and pressures and up to medium extents of conversion. Methods for sensitivity studies based upon the quasi-stationary-state assumption (QSSA) were developed, and for a number of more detailed effects, such as self-inhibition, explanations could be given. It was also shown that the hexyl isomerization reactions influence strongly the product distribution. The outstanding capability of kinetic modeling with computer simulations in handling complex kinetic systems is demonstrated.