Showing papers on "Reaction rate published in 1989"

A review of reaction rates and thermodynamic and transport properties for the 11-species air model for chemical and thermal nonequilibrium calculations to 30000 K

Abstract: Reaction rate coefficients and thermodynamic and transport properties are provided for the 11-species air model which can be used for analyzing flows in chemical and thermal nonequilibrium. Such flows will likely occur around currently planned and future hypersonic vehicles. Guidelines for determining the state of the surrounding environment are provided. Approximate and more exact formulas are provided for computing the properties of partially ionized air mixtures in such environments.

Polar Solvent Dynamics and Electron-Transfer Reactions

Abstract: Polar solvents often exert a dramatic influence on reactions in solution. Equilibrium aspects of this influence involve differential solvation of reactants compared to the transition state that lead to alteration of the free-energy barrier to reaction. Such effects are well known, and often give rise changes in reaction rates of many orders of magnitude. Less well understood are effects arising from non-equilibrium, dynamical aspects of solvation. During the course of reaction, charge is rapidly redistributed among reactants. How the reaction couples to its solvent environment depends critically on how fast the solvent can respond to these changes in reactant charge distribution. In this article the dynamics of solvation in polar liquids and the influence of this dynamics on electron-transfer reactions are discussed. A molecular picture suggests that polar solvation occurs on multiple time scales as a result of the involvement of different types of solvent motion. A hierarchy of models from a homogeneous continuum model to one incorporating molecular aspects of solvation, combined with computer simulations, gives insight into the underlying dynamics. Experimental measures of solvation dynamics from picosecond and subpicosecond time-dependent Stokes shift studies are compared with the predictions of theoretical models. The implication of these results for electron-transfer reactions in solution are then briefly considered.

Understanding complex chemical kinetics with computational singular perturbation

Abstract: A moderately complex hydrogen-oxygen reaction system is used to demonstrate the application of the Computational Singular Perturbation (CSP) technique Most of the “insights” normally clained by theoreticians after a successful convenient singular perturbation analysis on “tractable” problems can be obtained for highly complex problems using data generated by CSP, including simplified multi-step reaction models The basic idea of CSP is that the large number of physically meaningful elementary reactions in a complex reaciion system can be grouped into separate reaction groups each identified with a single characteristic time scale CSP theory provides an exact algorithm for the determination of this grouping and the reaction rates of earch of the groups, requiring no experience or intuition about the reaction system from the invetigator terms representing fast reaction groups can simply be discarded when they are exhausted to yield simplified models of the reaction system as a function of time Examples are given to show how physically meaningful information about the reaction system can be derived when relevant CSP data are available Sample calculations are performed for the hydrogen-oxygen reaction system at 16]]° K and low pressure CSP Examination of the CSP data suggests that, when [H], [O], [OH], [H2O], and [HO2] are present initially only in trace amounts, the reaction system can be represented reasonably accurately by a single set of 5 reaction groups (two extremely fast, two moderately fast, and one extremely slow) over an extended time period Depending on the interest of the invetigator, different simplified multi-step reaction models can be derived to represent the full kinetics formulation

Ab initio reaction paths and direct dynamics calculations

Abstract: A detailed study of methods for generating the minimum energy path of a chemical reaction using ab initio electronic structure calculations is presented; the convergence with respect to step size of the geometry and energy along this path is studied with several algorithms. The investigations are extended to the calculation of chemical reaction rate coefficients by interfacing the polyrate code for variational transition-state theory and semiclassical tunneling calculations with a locally modified Gaussian 82 electronic structure package that now contains reaction path following capabilities at both the Hartree-Fock and perturbation theory levels. This combined package is used to study the kinetics of the abstraction reaction CH{sub 3} + H{sub 2} {yields} CH{sub 4} + H, which is considered as a prototype organic reaction. They report calculations of reaction rates based on electronic structure theory and generalized transition-state theory, including a multidimensional tunneling correction, without performing an analytic fit to the potential surface. The calculation of dynamical processes directly from ab initio electronic structure input without the intermediary of a potential surface fit is called direct dynamics, and this paper demonstrates the feasibility of this approach for bimolecular reactions.

Steady and nonsteady rates of reaction in a heterogeneously catalyzed reaction: Oxidation of CO on platinum, experiments and simulations

Abstract: The rate of reaction for oxidation of CO over (210) and (111) single‐crystal surfaces of platinum has been studied as a function of reactant pressures (PO2,PCO) and sample temperature (T), both experimentally and by computer simulation Experimental results on both surfaces show regions with a steady high rate of reaction followed by a nonsteady transition region and, at high CO pressures, a region with low reactivity caused by CO poisoning of the surface At constant sample temperature, the transition region can be narrow and depends critically on the ratio of the gas phase concentration of reactants (PCO/PO2) The temperature dependences of the experimental data indicate that the critical ratio and the details for the occurrence of CO poisoning are strongly affected by surface processes such as adsorption, desorption, and diffusion ordering and reconstruction phenomena A computer simulation model of the Langmuir–Hinshelwood surface reaction as developed by Ziff et al was used for the simulation of the reaction under flow conditions The initial fair agreement between this model and the experiment can be significantly improved if processes such as adsorption, desorption, and diffusion are taken into account in an extended simulation model which in turn provides an insight into the kinetics of adsorbate poisoning and the effect of adsorbate‐induced processes on the reaction

The role of hydrogen in vapor deposition of diamond

Abstract: The effect of hydrogen addition on growth of diamond films under the conditions of chemical vapor deposition was investigated computationally. A detailed chemical kinetic mechanism was composed to describe the evolution of reaction species in pyrolysis of hydrogen‐ and argon‐diluted methane mixtures with imposed temperature profiles, simulating the gas‐phase conditions of diamond film growth in an idealized hot‐filament reactor. The reaction mechanism was comprised of two basic parts: decomposition of methane, and formation and growth of polycyclic aromatic hydrocarbons; it contained a total of 120 elementary reactions and 45 chemical species. The reaction rate coefficients included temperature and pressure dependencies. The computations were performed for a variety of initial conditions, elucidating the effects of critical parameters on the product composition in the regime of diamond deposition. Analysis of the computational results indicated that the key role of the hydrogen addition in the diamond deposition process is to suppress the formation of aromatic species by H2 in the gas phase and thereby to prevent the formation and growth of nondiamond, graphitic phases on the deposition surface.

Mechanisms of spatial self‐organization in isothermal kinetic oscillations during the catalytic CO oxidation on Pt single crystal surfaces

Abstract: The rate of catalytic CO oxidation on Pt(100) and (110) surfaces at low pressures (≤10−4 Torr) and under isothermal conditions may exhibit sustained temporal oscillations which are coupled with periodic transformations of the surface structures between reconstructed and nonreconstructed phases, the latter exhibiting higher oxygen sticking coefficients and hence higher reactivity. With Pt(100) the two surface phases exhibit a much larger difference in reactivity (=oxygen sticking coefficient) than with Pt(110), which effect accounts for the qualitative differences in the oscillatory behavior: if two of the control parameters (say pO2, T) are kept fixed, the third (pCO) may be varied with Pt(100) over a fairly wide range without leaving the oscillatory region. Minor (<1%) fluctuations of the partial pressures associated with the varying reaction rate are hence without any noticeable effect. Coupling between surface reaction and diffusion causes wave propagation of the surface phase transformations and therefore spatial self‐organization, as demonstrated by scanning LEED experiments. With Pt(110), on the other hand, the oscillatory region is very narrow. In this case mass transport through the gas phase as caused by the small pressure variations associated with the reaction lead to synchronization between different parts of the surface. Computer simulations with the cellular automaton technique confirm qualitatively the experimental findings and support the conclusions reached.

Reaction kinetics of the adsorption and desorption of nickel, zinc and cadmium by goethite. II Modelling the extent and rate of reaction

Abstract: SUMMARY A model of the reaction of metal ions with a variable charge surface was modified and applied to goethite that had been formed in the presence of silicate. The data included the effects of initial concentrations of Ni, Zn and Cd ranging from 1 to 100 μm, of periods from 2 h to 42 d, of temperatures from 5 to 35°C, of pH from about 4 to about 8, and of background electrolyte from 0.01 to 1.0 M. The observed effects of concentration and pH were explained by assuming that the reacting surface sites were heterogeneous and that they reacted with the MOH+ ions in solution. The heterogeneity was described by assuming that the affinity of the sites for MOH+ ions decreased logarithmically as the log of the amount adsorbed increased. This gave a Freundlich relation between sorption and surface activity. The effects of increasing the concentration of the background electrolyte were explained as being caused by decreased activity of MOH+ ions in solution counteracted by decreases in the electric potential of the surface. To explain the effects of time and temperature, it was assumed that an initial rapid adsorption reaction was followed by slow diffusion of metal ions into the goethite. Using these assumptions, a comprehensive description of the data was obtained.

Diffusion limitations in fischer‐tropsch catalysts

Abstract: The extent of diffusion limitations in the catalytic conversion of synthesis gas to hydrocarbons by the Fischer-Tropsch reaction has been established for a number of iron- and cobalt-based catalysts. The studies were performed in a fixed-bed microreactor system at temperatures in the range 473-523 {Kappa}. Variation of catalyst particle size in the range 0.2.-2.6 mm shows that the conversion of synthesis gas decreases considerably when the average particle size is increased. The effects of variation of particle size and pore diameter have been quantified with the Thiele model for diffusion limitations. Evidence has accumulated that the limited mobility of reactant molecules in the liquid-filled pores of Fischer-Tropsch catalysts is the main cause of retardation of the reaction rates. The experimentally determined reaction rates with various catalysts operated under different conditions show an excellent fit with the theoretical model.

Advanced oxidation processes. Test of a kinetic model for the oxidation of organic compounds with ozone and hydrogen peroxide in a semibatch reactor

Abstract: Experimental data are presented to test a kinetic model of the OE/H{sub 2}O{sub 2} process in a semibatch reactor. The effect of bicarbonate and carbonate ions is measured and found to be in concurrence with model predictions. The effect of pH in the ozone mass-transfer-limited region was examined in bicarbonate-spiked distilled water. Since the reaction is mass transfer limited, the primary effect above pH 7 is the result of changes in the distribution of inorganic carbon species which are OH-radical scavengers. Below pH 7, there is a lag period during which ozone and peroxide increase until the chain reaction begins. The effects of chloride ion and the concentration of radical scavengers other than carbonate species in ground waters are also measured. The mass-transfer/reaction rate model has been used to estimate rate constants for the reaction of hydroxyl radicals with trichloroethylene, 1,2-dibromoethane, 1,2-dibromo-3-chloropropane, carbon tetrachloride, and two bicyclic alcohols, 2-methylisoborneol and geosmin. While the model developed for the distilled water system was successful in predicting the rate of tetrachloroethylene (PCE) oxidation and the concentration of residual ozone and peroxide in regions I and III, respectively, there are several features of the model that remain unresolved when the matrix is changed tomore » a real surface or ground water. This and subsequent papers will investigate these effects.« less

Reaction rate analysis of complex kinetic systems

Abstract: Using the elementary sensitivity densities, a reaction rate sensitivity gradient is obtained which is the derivative of the rate of species concentration change with respect to the rate coefficient. The method is used to analyse the mechanism of high-temperature formaldehyde oxidation and high-temperature propane pyrolysis

A review of reaction rates in high temperature air

Abstract: The existing experimental data on the rate coefficients for the chemical reactions in nonequilibrium high temperature air are reviewed and collated, and a selected set of such values is recommended for use in hypersonic flow calculations. For the reactions of neutral species, the recommended values are chosen from the experimental data that existed mostly prior to 1970, and are slightly different from those used previously. For the reactions involving ions, the recommended rate coefficients are newly chosen from the experimental data obtained more recently. The reacting environment is assumed to lack thermal equilibrium, and the rate coefficients are expressed as a function of the controlling temperature, incorporating the recent multitemperature reaction concept.

Experimental evidence for nucleation during thin‐film reactions

Abstract: The reaction between solid layers to form a product phase has been studied using scanning calorimetry of multilayer Nb/Al and Ni/amorphous‐Si thin films. The most striking feature for both materials systems is the occurrence of two maxima in the reaction rate during the formation of a single product phase, suggesting a two step growth process. A model has been developed in which the first step is taken to be the nucleation and two‐dimensional growth to coalescence of the product phase, in the plane of the initial interface. The second step is taken to be the thickening of the product layer by growth perpendicular to the interface plane. The success of this simple model in describing the principal features of the experimental results on two different materials systems suggests that nucleation is an important aspect of phase formation and selection in these thin‐film reactions.

Time-resolved IR spectroscopy in liquid rare gases: direct rate measurement of an intermolecular alkane carbon-hydrogen oxidative addition reaction

Abstract: Since the first demonstration of the intermolecular oxidative addition of alkane C-H bonds to transition-metal centers, there have been many studies of the mechanism of this reaction. While these studies have illuminated many aspects of the C-H activation process, they do not provide direct information about the reactive intermediates or the potential energy surface for the elementary insertion reaction. Flash photolysis studies have been thwarted by extremely fast insertion rates in neat alkane solution and by the lack of a suitable inert and transparent solvent for dilution of the alkane. The authors have overcome these difficulties with the use of liquid rare gases as solvents. Using a novel combination of low-temperature and IR laser flash kinetic techniques, we are able to detect the C-H activating transient intermediate formed from Cp*Rh(CO){sub 2} (Cp* = ({eta}{sup 5}-C{sub 5}Me{sub 5})) and measure its rate of reaction with cyclohexane over a wide range of concentrations and temperatures.

Some New Observations on Methanol Oxidation Chemistry

Abstract: New experiments conducted in a turbulent flow reactor show kinetic characteristics of methanol oxidation that have not been observed previously. Data are presented for methanol oxidation at equivalence ratios in the range of 0.6−1.6 and initial temperatures or 1025-1090 K at atmospheric pressure. At intermediate extents of reaction, a marked deceleration in chemical reaction rate is observed to cause a “plateau” in the energy release and species concentration profiles. This effect becomes considerably more pronounced with increasing equivalence ratio and is not predicted by the earlier comprehensive kinetic mechanism of Westbrook and Dryer. A revised mechanism incorporating numerous recent reaction rate and thermochemical data provides both a significant improvement in agreement with flow reactor data and an improved understanding of the important reactions involved in methanol oxidation. Reaction path flux analyses reveal the importance of HO2 chemistry, the decreasing role of chain-branching re...

Imaging of Spatial Pattern-Formation in an Oscillatory Surface-Reaction by Scanning Photoemission Microscopy

Abstract: The rate of catalytic carbon monoxide oxidation on a Pt(100) single crystal surface under isothermal, low‐pressure conditions exhibits for certain ranges of parameters (O2 and CO partial pressures, temperature) sustained temporal oscillations whose mechanism had been explored in previous work. Coupling between reaction and diffusion leads to spatial pattern formation as manifested by patches with different work function on the intrinsically homogeneous surface. Imaging is performed by means of the novel technique of scanning photoemission microscopy. Typically, nuclei with dimensions of a few microns, as determined by the instrumental resolution, are formed spontaneously and expand with sharp fronts and velocities of about 0.5 mm/min (at 480 K) up to sizes ≥1 mm. Waves with even more extended fronts propagating with somewhat higher velocities across the sample surface are responsible for the occurrence of large amplitude temporal oscillations of the integral reaction rate.

NMR study of kinetic HH/HD/DD isotope, solvent and solid-state effects on the double proton transfer in azophenine

Abstract: Azophenine (AP, N,N'-diphenyl-3,6-bis(phenylimino)- 1,4-cyclohexadiene-l,4-diamine) is subject in liquid solution to a fast intramolecular double proton transfer involving two degenerate tautomers. Rate constants of this reaction have been measured as a function of temperature by applying different methods of dynamic NMR spectroscopy to various isotopically labeled AP species dissolved in different organic solvents. The rate constants do not depend on the dielectric constant of the solvent, which was varied between 2 (toluene) and 25 (benzonitrile). For C2D2CI4 as solvent, the full kinetic HH/HD/DD isotope effects were obtained at different temperatures. The observed kinetic isotope effects of kHH/kHD = 4.1 and kHD/kDD = 1.4 at 298 K indicate a breakdown of the rule of the geometric mean. I5N CPMAS NMR experiments on crystalline azophenine showed that the reaction also takes place in the solid state. However, the degeneracy of the tautomerism is lifted in this phase because of intermolecular interactions. The mechanism of this reaction is discussed in detail, especially with respect to the questions of whether tunneling is involved and whether one or two protons are transferred in the rate-limiting step. The kinetic isotope effects can best be explained in terms of a stepwise consecutive single proton transfer mechanism involving either a highly polar zwitterion or an apolar singlet-biradical as intermediate. The observation that solvent effects on the reaction rates are absent and that the activation entropy of the reaction almost vanishes excludes the formation of a strongly solvated zwitterionic intermediate. Static medium effects on the double minimum potential of the proton transfer are discussed, taking into account previous results of IR experiments on AP and of solid-state NMR experiments on double proton transfers in organic glasses.

Intramolecular esterification by lipase powder in microaqueous benzene: effect of moisture content.

Abstract: In view of the biochemical reaction catalyzed by enzyme powder suspended in a water-insoluble organic solvent, an equation was derived to estimate the amount of water bound to the enzyme powder. With this equation, an apparent adsorption isotherm between free water (water freely dissolved in benzene) and bound water (water bound to crude lipase powder of Pseudomonas fluorescens) was obtained. A direct lactonization reaction (synthesis of cyclopentadenolide from 15-hydroxypen-tadecanoic acid) catalyzed by crude lipase powder of Pseudomonas fluorescens was carried out batchwise in microaqueous benzene at 40oC. A kinetic model of the enzymatic reversible lactonization reaction was derived, from which the effect of moisture content on the initial reaction rate with a fully hydrated enzyme was mathematically expressed. The observed initial reaction rate first increased, then decreased with increasing moisture content, giving rise to the maximum rate at a certain level of the moisture content. The drop in the reaction rate at lower moisture content was due to a lesser hydration of the enzyme molecule (hydration-limited) and the decrease in the reaction rate at higher moisture content was attributed to the dependence of the true initial rate of the reversible reaction on the moisture content (true reversible reaction limited), and could be simulated by the kinetic model. The equilibrium yield approached 100% at a lower moisture content.