Showing papers on "Reaction rate published in 1986"

Dynamical effects in electron transfer reactions

Abstract: A theoretical treatment is given for the effect of intramolecular vibrational and diffusive solvent orientational motions on the rate of electron transfer reactions. Four limiting cases are considered for the two‐electronic state problem: slow reaction, wide and narrow reaction window, and nondiffusing limits. With the aid of a decoupling approximation, an expression is derived for the reaction rate which reduces to the appropriate expression for each limiting case when the latter is approached. Under certain conditions the time dependence of the survival probability is multiexponential rather than single exponential. Because of this behavior two average survival times are defined and expressions for each are obtained. Experimental data are considered with the present treatment in mind. One feature of the present work is a more general analysis for the case that both vibrational and solvent diffusive motion contribute to the activation process. The relation to previous works in the literature is described.

Mass-Transport Considerations Pertinent to Aqueous Phase Reactions of Gases in Liquid-Water Clouds

Abstract: Reactions of gases in liquid-water clouds are potentially important in the transformation of atmospheric pollutants affecting their transport in the atmosphere and subsequent removal and deposition to the surface. Such processes consist of the following sequence of steps: Mass-transport of the reagent gas or gases to the air-water interface; transfer across the interface and establishment of solubility equilibria locally at the interface; mass-transport of the dissolved gas or gases within the aqueous phase; aqueous-phase chemical reaction(s); mass-transport of reaction product(s) and possible subsequent evolution into the gas-phase. Description of the rate of the overall process requires identification of the rate-limiting step (or steps) and evaluation of the rate of such step(s). Identification of the rate-limiting step may be achieved by evaluation and comparison of the characteristic times pertinent to the several processes and may be readily carried out by methods outlined herein, for known or assumed reagent concentrations, drop size, and fundamental constants as follows: gas- and aqueous-phase diffusion coefficients; Henry’s law coefficient and other pertinent equilibrium constants; interfacial mass-transfer accommodation coefficient; aqueous-phase reaction rate constants(s). A graphical method is described whereby it may be ascertained whether a given reaction is controlled solely by reagent solubility and intrinsic chemical kinetic or is mass-transport limited by one or another of the above processes. In the absence of mass-transport limitation, reaction rates may be evaluated uniformly for the entire liquid-water content of the cloud using equilibrium reagent concentrations. In contrast, where appreciable mass-transport limitation is indicated, evaluation of the overall rate requires knowledge of and integration over the drop-size distribution characterizing the cloud.

Kinetics of carbon monoxide oxidation over ruthenium(0001)

Abstract: The oxidation of CO over a model Ru(0001) single-crystal catalyst has been studied in a high-pressure reaction-low pressure surface analysis apparatus. Steady-state catalytic activity as a function of temperature and partial pressure of CO and O/sub 2/ was measured. Both the specific rates and the relative activity (Ru > Pd, Rh) obtained in this study compare very favorably with the results on high area supported catalysts. Surface concentrations of oxygen were monitored following reaction and found to be dependent on the partial pressures of the reactants. Further, the highest rates of reaction corresponded to reaction on a ruthenium surface covered with a monolayer of oxygen as detected subsequent to reaction by AES. The kinetics measured at various reactant partial pressures (leading to various surface oxygen coverages following reaction) suggest that a chemisorbed atomic oxygen species present at high oxygen coverages may be a crucial reaction intermediate, largely responsible for the optimum reaction rates. 32 references, 7 figures, 1 tables.

Unimolecular reaction rate theory for transition states of any looseness. 3. Application to methyl radical recombination

Abstract: The theory for unimolecular reactions described in part 1 is applied to the recombination of methyl radicals in the high-pressure limit. The model potential energy surface and the methodology are briefly described. Results are presented for the recombination rate constant k_ ∞ at T = 300, 500, 1000, and 2000 K. Canonical and Boltzmann averaged microcanonical values of k_ ∞ are compared, and the influence of a potential energy interpolation parameter and a separation-dependent symmetry correction on k_ ∞ are examined. Earlier theoretical models and extensive experimental results are compared with the present results which are found to have a negative temperature dependence. The present results agree well with some of the available but presently incomplete experimental determinations of the high-pressure recombination rate constant for this reaction over the 300-2000 K temperature range. There is also agreement with a decomposition rate constant for a vibrationally excited ethane molecule produced by chemical activation.

Free radical polymerization models for simulating reactive processing

Abstract: The generally accepted kinetic mechanism for free radical copolymerization was simplified by various assumptions and restrictions to give several realistic and easily evaluated models for the simulation of industrial molding. Six assumptions, including conversion-dependent rate coefficients and constant comonomer concentration ratios, were used to obtain a simplified model. Special cases of this simplified model were obtained by the following additional constraints: (1) Restriction C, consecutive inhibition and radical generation reactions; (2) Restriction I, constant initiator decomposition rate; (3) Restriction Z, zero termination rate for free radicals; and (4) Restriction K or P, all rate coefficients independent of conversion or only polymerization rate coefficient dependent on conversion. For various combinations of these restrictions, the time and concentration variables in the simplified model are separated and solved; the separate solutions are then combined in various ways to give models capable of predicting a wide variety of behavior. Many of these models have analytical solutions that greatly facilitate the evaluation of rate constants. Models based on restrictions KCI, PCI, and PCZ are shown to give good agreement with isothermal DSC data plotted as reaction rate versus time and, in a more sensitive test, as reaction rate divided by monomer concentration versus fractional conversion. Because of their predictive ability and ease of evaluating constants. Models PCI and PCZ are recommended for simulating industrial processing; they are particularly well suited for simulating compression molding of sheet molding compound.

Acetaldehyde oxidation in the negative temperature coefficient regime: Experimental and modeling results

Abstract: Acetaldehyde oxidation at temperatures between 500 and 850/sup 0/K has been studied in experiments carried out in a low-pressure static reactor and in numerical modeling calculations using a detailed chemical kinetic reaction mechanism. Results of the experimental study were used to construct and validate the reaction mechanism, which was then used to examine acetaldehyde oxidation in the negative temperature coefficient regime. The overall rate of reaction and the properties of the negative temperature coefficient regime were found to be sensitive to the competition between radical decomposition reactions and the addition of molecular oxygen to acetyl and methyl radicals. Implications of the results for future kinetic modeling of engine knock are discussed.

Ammonia synthesis over iron single-crystal catalysts: the effects of alumina and potassium

Abstract: They have investigated the effects of potassium and aluminum oxide on the synthesis of ammonia from nitrogen and hydrogen over model iron single-crystal catalysts at 20-atm reactant pressure. Elemental potassium does not remain on a Fe(100) surface under reaction conditions but a small amount of potassium can be stabilized by coadsorption with oxygen. The presence of the stabilized K + O has no effect on the rate of ammonia synthesis. Substantially more potassium can be stabilized at higher temperatures on Fe(100), Fe(111), and Fe(110) surfaces by coadsorption with aluminum oxide. The cooperative interaction between the potassium and oxidized aluminum seems to be due to compound formation. There is an inverse relationship between the rate of reaction and amount of aluminum oxide and potassium present on all three iron surfaces investigated.

Carbon monoxide + oxygen reaction on rhodium(III): steady-state rates and adsorbate coverages

Abstract: Steady-state kinetics of CO oxidation on clean Rh(111) were measured over a wide range of CO and O/sub 2/ gas-phase compositions and surface temperatures for pressures between 10/sup -6/ and 10/sup -/( Torr. Coverages were measured by XPS during steady-state reaction. Below 425 K the reaction rate increases with temperature with an activation energy of 20 kcal/mol, while above 450 K the rate decreases with temperature with an activation energy of -7 kcal/mol. At low temperatures and in excess CO, the reaction rate is proportional to P/sub O/sub 2//P/sub CO//sup -1/, while in excess O/sub 2/ and at high temperatures the reaction rate is negative order in P/sub O/sub 2// and more than first order in P/sub CO/. Near stoichiometric reactant ratios XPS shows that CO is the dominant adsorbed species below 400 K, while from 425 to 900 K the surface is nearly saturated with oxygen. Experimental rates and coverages are fit qualitatively by using a simple Langmuir-Hinshelwood model assuming competitive adsorption, although adsorption parameters for CO and O/sub 2/ are not in agreement with clean surface values. Modified Langmuir-Hinshelwood models involving strong inhibition of oxygen adsorption by adsorbed CO, either through coverage-dependent sticking coefficients or a dependence ofmore » the heat of adsorption of oxygen on the CO coverage, give good agreement with UHV parameters, measured CO and oxygen coverages, and reaction rates.« less

Acetonitrile in the stratosphere and implications for positive ion composition

Abstract: The present paper describes a probable budget for acetonitrile and discusses the release of this gas through biomass burning and human activity. The different loss processes in the middle atmosphere are mainly due to the reaction with hydroxyl radicals and atomic oxygen. It is shown that the destruction of CH3CN by scavenging due to precipitation is probably not more efficient than the direct gas phase reactions. Losses due to ion chemistry are very difficult to estimate at present but are probably of secondary importance, except locally, where formation of multi-ion complexes is significant. A one-dimensional calculation shows that the vertical profiles of CH3CN, deduced from ion mass spectrometry data, can be reproduced satisfactorily if an annual global emission ranging from 1.5 × 1010 g to 5 × 1011 g is adopted, depending on the values of the reaction rate constants and eddy diffusion coefficient. The global atmospheric lifetime of CH3CN is estimated to be about 0.5 to 1.4 years. Finally, the calculated acetonitrile profiles are introduced in an ion model to calculate the abundances of the major positive stratospheric ions. The results are consistent with present observations.

Charge dependence of chemisorption patterns for transition metal clusters

Abstract: A method is presented for the measurement of the chemisorptive reactivity of transition metal cluster ions at near room temperature. Similar to a technique introduced previously for neutral clusters [Rev. Sci. Instrum. 56, 2123 (1985)], this cluster ion method utilizes a fast‐flow reactor attached to a supersonic, laser vaporization metal cluster source, followed by time‐of‐flight mass spectral analysis of the cluster ions as a function of reactant concentration. Results are presented for clusters of cobalt and niobium in the 1–22 atom size range for their chemisorptive reactions with CO, CO2, and N2. Both Nb+n and Co+n clusters displayed chemical reactivity that is remarkably similar to that of the corresponding neutral clusters. For both charge states of each metal, CO was found to chemisorb with a rate which varied in a slow, monotonically increasing fashion with cluster size. The chemisorption rate of N2 and CO2, on the other hand, was found to be significantly slower than that of CO and sharply dependent upon the cluster size, this dependency being roughly independent of whether the transition metal cluster had a net positive or neutral charge. Photodissociation measurements of the mass‐selected positive ion chemisorption products showed that the desorption energy of these products parallels the relative reaction rate.

Low-temperature oxidation

Abstract: Low-temperature oxidation is a reaction, occurring at or below room temperature, between a solid and a gas. It usually involves the combination of oxygen with metals, and it has the greatest commercial impact in the presence of moisture, as in corrosion. Cabrera and Mott put forward a theory of low-temperature oxidation, based on the assumption that cation migration occurs under the influence of a potential built up across the growing oxide film. Recent experimental results require that this theory be expanded to explain recent observations such as anion migration during oxide growth and the transition from the initial chemisorbed monolayer to a bulk, threedimensional oxide. The additional ideas put forward in the present paper may be summarized as follows. Low-temperature oxidation is controlled by the nature of the oxide; whether it is a network former or a modifier. A period of fast, linear oxidation is followed by a slow logarithmic reaction whose rate, in turn, can increase if the oxide film crystallizes to form grain boundaries. The initial fast oxidation is a continuation of the chemisorption process. Place exchange (anions and cations interchanging positions) occurs when the energy due to the image force of an oxygen ion is greater than the bond energy holding the ion in place. A stable film forms when this bond energy is greater than the image force energy. The oxygen ions formed on the oxide surface then set up a potential across the film. This potential provides the driving force for continued reaction. Oxide growth during this later stage is a slow, logarithmic process. A barrier to ion transport exists at the gas-oxide interface in the case of anion migration and at the metal-oxide interface in the case of cation migration. In both cases, the field built up across the oxide lowers the barrier sufficiently so that ion migration can occur. Network modifiers allow cation migration. The reaction rate is sensitive to crystallographic orientation of the metal, but not to oxygen pressure. A constant voltage is maintained across the film, so that the Cabrera-Mott theory explains the logarithmic kinetics. Network-forming oxides allow onion migration. The number of anions, and hence, the rate of reaction, is sensitive to oxygen pressure, but not crystallographic orientation of the metal substrate. Since the potential is a result of the mobile anions, the film tends to grow under constant field. The logarithmic kinetics then must be explained by an increasing activation energy for ion transport, as proposed by Eley and Wilkinson. The logarithmic growth rate can be increased by the presence of water vapor if the water introduces “dangling” bonds into an oxide network structure. Crystallization of the oxide film also increases its rate of growth and results in the formation of oxide islands.

A kinetic model for the thermal nitridation of SiO2/Si

Abstract: A kinetic model has been developed in order to understand the underlying reasons for observed nitrogen distributions in SiO2 films on Si which have been thermally nitrided in NH3 The calculations simulate the nitridation process, considering first‐order chemical kinetics and Arrhenius dependence of the diffusion and reaction rates on temperature The calculations show that as the substrate reacts with diffusing species, which initially consist primarily of nitrogen, a nitrogen‐rich oxynitride forms at the interface For nitridation temperature of 1000 °C and above, an oxygen‐rich oxynitride subsequently forms at the interface due to reaction of the substrate with an increasing concentration of diffusion oxygen which has been displaced by the slower nitridation of the SiO2 This sequence of events results in a nitrogen distribution in which the peak in the interfacial nitrogen concentration occurs away from the the interface The results of the calculations are compared with observed nitrogen distributions The calculations correctly predict that, (i) for a nitridation temperature of 800 °C, the peak of the interfacial nitrogen concentration remains at the interface, while for nitridation temperatures≥1000 °C it moves away from the interface, and (ii) for a nitridation temperature of 1150 °C, the peak interfacial nitrogen concentration is lower than that which occurs at 1000 °C, even though the position of the peak is essentially the same The effect of interfacial strain is included in the simulations, and is found to be necessary to account for the observed width of the interfacial nitrogen distribution

Oxidation of SiC powder by high-temperature, high-pressure H2O

Abstract: The reaction between SiC powder and H2O has been studied at 400°–800 °C under 10 and 100 MPa. Silicon carbide reacted with H2O to yield amorphous SiO2 and CH4 by the reaction SiC + 2H2O→SiO2 + CH4 above 500 °C. Cristobalite and tridymite crystallized from amorphous silica after the almost complete oxidation of SiC above 700 °C. The oxidation rate, as calculated from the weight gain, increased with temperature and pressure. The Arrhenius plotting of the reaction rate based on a Jander-type model gave apparent activation energies of 167–194 kJ/mol. Contrasted with oxidation in oxidative atmosphere, oxidation in H2O is characterized by the diffusion of H2O and CH4 in an amorphous silica layer where the diffusion seemed to be rate determining. Present results suggest that the oxidation of SiC includes the diffusion process of H2O in silica layers when atmospheres contain water vapor.

Studies of the reaction of O+2 with deuterated methanes

Abstract: In the gas phase O+2 reacts with methane at 300 K to produce a hydrogen atom and the CH3O+2 ion. The structure of this ion has recently been determined to be H2COOH+, methylene hydroperoxide ion. The reaction rate coefficients and product distributions have now been measured at 300 K for the CHnD4−n isotopes. The reaction shows both inter‐ and intramolecular isotope effects, e.g., CH2D2 reacts more slowly than methane and more rapidly than CD4, but loses hydrogen or deuterium with equal probability. The ion readily transfers HO+ to alkenes, CS2, and many other neutral molecules. The reaction with CS2 has been used to investigate the isotopic distribution within mixed isotope product ions. In addition, the reaction rate coefficients for both CH4 and CD4 have been measured as functions of temperature between 20 and 500 K; in both cases a clear minimum is observed in the reaction rate coefficient near room temperature. A mechanism for the reaction is proposed which allows us to model the temperature dependen...

Oxygen supply to immobilized cells: 5. Theoretical calculations and experimental data for the oxidation of glycerol by immobilized Gluconobacter oxydans cells with oxygen or p-benzoquinone as electron acceptor.

Abstract: Theoretical calculations of reaction kinetics were done for one-step reactions catalyzed by cells immobilized in spherical beads. The reactions catalyzed by free cells were assumed to obey Michaelis-Menten kinetics for a one-substrate reaction. Both external (outside the beads) and internal (inside the beads) mass transfer of the substrate were considered for the immobilized preparations. The theoretical calculations were compared with experimental data for the oxidation of glycerol to dihydroxyacetone by Gluconobacter oxydans cells immobilized in calcium alginate gel. Glycerol was present in excess so that the reaction rate was limited by oxygen. The correlation between experimental data and theoretical calculations was quite good. The calculations showed how the overall effectiveness factor was influenced by, for example, the particle size and the cell density in the beads. In most cases the reaction rate was mainly limited by internal mass transfer of the substrate (oxygen). As shown previously, p-benzoquinone can replace oxygen as the electron acceptor in this reaction. The same equations for reaction kinetics and mass transfer were used with p-benzoquinone as the rate-limiting substrate. Parameters such as diffusivity, maximal reaction rate, and K were, of course, different. In this case also, the correlation between the model and the experimental results was quite good. Much higher production rates were obtained with p-benzoquinone as the electron acceptor compared to when oxygen was used. The reasons for this fact were that p-benzoquinone gave a higher maximal reaction rate for free cells and the solubility of p-benzoquinone was higher than for oxygen. Different methods of increasing the rate of microbial oxidation reactions are discussed.

Gas-phase reactions of CF3 and CF2 with atomic and molecular fluorine: Their significance in plasma etching

Abstract: Reaction rate coefficients have been measured at 295 K for both CF3 and CF2 with atomic and molecular fluorine. The reaction between CF3 and F was studied over a gas number density range of (2.4–23)×1016 cm−3 with helium as the bath gas. The measured rate coefficient increased from (1.1–1.7)×10−11 cm3 s−1 as the gas number density increased over this range. In contrast to this relatively small change in rate coefficient with gas number density, the rate coefficient for CF2+F increased from (0.4–2.3)×10−12 cm3 s−1 as the helium gas number density increased from (3.4–28.4)×1016 cm−3. Even for the highest bath gas number density employed, the rate coefficient was still more than an order of magnitude lower than earlier measurements of this coefficient performed at comparable gas number densities.

Determination of electrochemical heterogeneous electron-transfer reaction rates from steady-state measurements at ultramicroelectrodes

Abstract: Donnees sur l'utilisation d'ultramicroelectrodes a anneau tres fin, dans la mesure des constantes de vitesse des reactions, tres rapides, de transfert d'electrons, dans des conditions de diffusion stationnaires