Showing papers on "Reaction rate published in 1988"

Pore evolution and channel formation during flow and reaction in porous media

Abstract: A theoretical and experimental study on the dissolution of porous media by flowing acid has been carried out. Dissolution of the media results in an evolution of the pore geometry and the formation of random flow channels. The goal is to predict the range of conditions under which channels will form, and the effects of various parameters on the structure of the channels and on their rate of propagation through the media. A random network model is used to describe the behavior of the stochastic, rootlike channels that form during flow and dissolution in carbonate rock and in other systems. The structure of the flow channels that form as a result of acid attack are characterized and studied using a Wood's metal casting technique. A comparison of model results shows that the rates of channel formation and growth are intimately related to the developing structure of the channels (size of branches and degree of branching), which in turn is controlled by factors such as the fluid velocity and the rate of reaction. Depending on the experimental conditions, the channels range from a single conduit with a minimum of branching to a highly branched, spongy network of channels. The dependence of permeability increase and channel branchedness on injection rates and acid diffusion rates is described by the Damkohler number for flow and reaction.

An analytical study of the hydrogen-air reaction mechanism with application to scramjet combustion

Abstract: A chemical kinetic mechanism for the combustion of hydrogen has been assembled and optimized by comparing the observed behavior as determined in shock tube and flame studies with that predicted by the mechanism. The reactions contained in the mechanism reflect the current state of knowledge of the chemistry of the hydrogen/air system, and the assigned rate coefficients are consistent with accepted values. It was determined that the mechanism is capable of satisfactorily reproducing the experimental results for a range of conditions relevant to scramjet combustion. Calculations made with the reaction mechanism for representative scramjet combustor conditions at Mach 8, 16, and 25 showed that chemical kinetic effects can be important and that combustor models which use nonequilibrium chemistry should be used in preference to models that assume equilibrium chemistry. For the conditions examined the results also showed the importance of including the HO2 chemistry in the mechanism. For Mach numbers less than 16, the studies suggest that an ignition source will most likely be required to overcome slow ignition chemistry. At Mach 25, the initial temperature and pressure was high enough that ignition was rapid and the presence of an ignition source did not significantly affect reaction rates.

Kinetics of CO oxidation on single-crystal Pd, Pt, and Ir

Abstract: The activity of Pt(100), Pd(110), Ir(111), and Ir(110) single-crystal catalysts for CO oxidation has been studied as a function of temperature and partial pressure of O/sub 2/ and CO in a high-pressure reactor-ultra-high-vacuum surface analysis apparatus over the temperature range 425-725 K and pressure range 0.1-600 Torr. The specific rates and the partial pressure dependencies determined for the single crystals are in excellent agreement with results obtained previously for high surface area supported catalysts, demonstrating the structure insensitivity of this reaction. The single-crystal catalysts exhibit simple Arrhenius behavior over most of the temperature range studied, and the observed activation energies lie between 22 and 33 kcal/mol, close to the desorption energy of CO from these surfaces. These results are consistent with the generally accepted model in which the surface is primarily covered with CO and the reaction rate is controlled by the desorption of CO. Deviation from Arrhenius behavior below 500 K for Pt is interpreted as a change in the reaction mechanism. Under highly oxidizing conditions surfaces of both Pd and Ir show negative-order dependence on O/sub 2/ partial pressure, indicating the presence of a strongly bound oxygen species. The oxygen species was similar to surface oxide formed bymore » deliberate oxidation and could be detected as CO/sub 2/ desorbing at high temperatures in postreaction temperature-programmed desorption. Oxide formed by oxidation of the Pd and Ir samples prior to high-pressure reaction was only stable at 475 K on Pd(110) in an 11:1 O/sub 2/:CO mixture and to 500 K on Ir(111) in an 80:1 O/sub 2/CO:CO mixture. Deliberate oxidation resulted in a rate decrease but did not affect the activation energy significantly, indicating that the oxide served merely as a simple site blocker.« less

Kinetics of carbon monoxide oxidation by oxygen or nitric oxide on rhodium(111) and rhodium(100) single crystals

Abstract: The oxidation of CO by O/sub 2/ or NO over Rh(111) and Rh(100) single crystals has been studied in a high-pressure reaction-high-vacuum surface analysis apparatus. Steady-state catalytic activity as a function of temperature and partial pressures of CO, O/sub 2/, and NO has been measured. The CO-O/sub 2/ reaction was found to be insensitive to the structure of the surface as evidenced by the identical rates, activation energies, and partial pressure dependencies measured on the two-single-crystal surfaces. Both surfaces deactivated at high O/sub 2/ partial pressures to the formation of a near-surface oxide (probably Rh/sub 2/O/sub 3/) which is catalytically inactive for this reaction. The deactivation occurred at a slightly lower O/sub 2/ partial pressure on Rh(100), likely due to the relative ease of oxygen diffusion through this surface leading to more rapid bulk oxidation. Unlike the CO-O/sub 2/ reaction, CO oxidation by NO was very sensitive to the geometric structure of the surface. This behavior was evident in all kinetic parameters measured on the two surfaces. The kinetic data obtained on Rh(111) agree quantitatively with a surface chemistry model previously developed for this reaction. The model predicts that the rate is limited by the formation of N/sub 2/ frommore » the recombination of adsorbed N atoms. and that the surface, under steady-state reaction conditions, is covered largely with adsorbed N atoms and smaller amounts of adsorbed NO. On Rh(100) the data are consistent with a reaction rate limited by the formation of N/sub 2/ from the surface reaction of adsorbed NO and N atoms.« less

Kinetics of bimolecular reactions in condensed media: critical phenomena and microscopic self-organisation

Abstract: Starting from the analysis of the hierarchy of equations for many-point reactant densities involved in three kinds of basic bimolecular reactions, A+ A + B, A+ B + C, A+ B + B, in condensed media, a review is given of a new class of self-organisation phenomena. Unlike the usual synergetic effects, these phenomena are characterised by the appearance of microscopic dynamical clustering of similar reactants, which, however, does not violate the macroscopic homogeneity of the system. The many-particle effects are described in terms of the correlation length and critical exponents in much the same way as is done in the theory of critical phenomena (phase transitions) developed in statistical physics. It is shown that microscopic self-organisation results in asymptotic decay laws for reactant densities which are unusual for standard physical and chemical kinetics. The corresponding reduction of reaction rate with time is due to the emergence, in the course of biomolecular reaction, of a non-Poisson fluctuation spectrum of reactant densities governing the time development of average quantities. The universal character of the newly discovered self-organisation phenomena has been demonstrated to occur not only in numerous kinds of diffusion-controlled reactions, but also for static reactions at low temperatures, including reactant accumulation, when there is a source creating them (e.g. irradiation) and a long-range (tunnelling) recombination of immobile donors and acceptors in crystals. The mathematical formalism developed is applied to the two-stage bimolecular processes using the Lotka and Lotka-Volterra models as examples. Their analysis has revealed that in this case the generally accepted viewpoint on the self-organisation phenomena fails.

Non-faradaic electrochemical modification of catalytic activity

Abstract: It was found that catalytic activity and selectivity of metal catalysts can be altered dramatically and in a reversible manner. This is accomplished by electrochemically supplying oxygen anions onto catalytic surfaces via polarized solid electrolyte cells. Oxygen anions, forced electrochemically to adsorb on the catalyst surface, alter the catalyst work function in a predictable way and lead to reaction rate increases of the order of 1500%. Changes in catalytic reaction rates typically exceed the rate of O/sup 2 -/ transport to or from the catalyst surface by 10/sup 2/-10/sup 4/. The case of several catalytic reactions in which this new phenomenon has been observed is discussed.

Kinetics of trace metal complexation: role of alkaline-earth metals

Abstract: The rate of reaction of inorganic copper with the model ligand ethylenediaminetetraacetic acid (EDTA) is significantly retarded in the presence of calcium at seawater concentrations. The half-life for inorganic copper reacting with EDTA in seawater is /similar to/2 h at 10/sup /minus/7/M EDTA. This kinetic hindrance to the formation of the thermodynamically favored CuEDTA species results from several factors: (1) the preponderance of the calcium complex in the speciation of EDTA, (2) the competition of calcium and copper for reaction with any free EDTA formed by the dissociation of CaEDTA, and (3) the slow kinetics of direct attack of copper on CaEDTA compared to reaction with free or protonated EDTA species. If metal-complexing agents in natural waters behave as discrete ligands, then the reaction of a metal at strong binding sites may also be kinetically hindered at high alkaline-earth concentrations. In contrast with the reaction of EDTA, however, the rate of completion of copper by humic acid is not observably affected by high calcium concentrations.

Three‐Dimensional Flow Effects in Silicon CVD in Horizontal Reactors

Abstract: Numerical modeling of Si homoepitaxial deposition from in the horizontal reactor has been undertaken employing the steady‐state, fully parabolic flow approximation for the heat, momentum, and mass‐transfer equations. Reactants are assumed to be dilute in their carrier gas. The resulting set of partial differential equations are discretized using finite elements and solved using the method of lines. The effect of a third dimension, the side temperature boundary conditions, and natural convection are explored. Given recent kinetics data on the silane decomposition and silylene insertion reactions, it is shown that a quasi‐thermodynamic equilibrium exists in the heated region above the surface, at least for hydrogen gas ambients. The combination of a fast silylene surface reaction and a slow silane surface reaction implies that the effective surface reaction rate is governed by the homogeneous thermodynamic equilibrium in the gas phase and diffusion through a thin mass‐transfer boundary layer near the surface. Examples of how tilting the susceptor contributes to growth uniformity are presented, and the effectiveness of similarity solution models at predicting this enhanced axial uniformity is tested.

Neutral radical deposition from silane discharges

Abstract: The fractional contributions of the various SiHn radicals (n=0–3) to deposition are calculated for low‐power, pure‐silane rf and dc discharges. This is done using a radical diffusion plus reaction equation, combined with current knowledge of SiH4 dissociation fractionation, of SiHn+SiH4 reactions, and of the distributed source of radicals. The conclusion reached is that more than 98% of neutral radical deposition is by SiH3 for typical deposition pressures (>100 mT at 240 °C). The effect of SiH3+SiH3 reactions at higher power is also evaluated using an estimated reaction rate coefficient (k3). The resulting loss in deposition rate is given as a function of film growth rate and of k3.

Dependence of metal cluster reaction kinetics on charge state. I. Reaction of neutral (Nbx) and ionic (Nb+x, Nb−x) niobium clusters with D2

Abstract: The effect of charge state on niobium cluster chemisorption kinetics is explored via measurement of the relative rates of D2 activation by Nb−x, Nbx, and Nb+x containing up to 28 atoms. The presence of the + or − charge is found to have only a minor effect on rate for the majority of the clusters, with the reactivity of the ions being generally within a factor of 2.5 of the corresponding neutrals. The excess charge does, however, have a profund influence on reaction rate for a number of clusters in the 7≤x≤16 size range, which may be indicative of the importance of cluster electronic structure in the chemisorption process. Kinetic data for Nb9, Nb12, and Nb+12 are found to deviate significantly from the expected pseudo‐first‐order behavior, suggesting the existence of structural isomers for these species. The anomalous behavior for Nb9 and Nb12 was not observed in previous neutral Nbx chemisorption studies. The maximum uptake of D2 by niobium clusters is found to be essentially independent of charge state...

Reactivity‐structure correlations in ion clusters: A study of the unimolecular fragmentation patterns of argon ion clusters, Ar+n, for n in the range 30–200

Abstract: A high resolution double focusing mass spectrometer in association with a pulsed supersonic nozzle has been used to study the unimolecular fragmentation of Ar+n clusters for n in the range 30–200. All ion clusters within this range are observed to undergo the reaction Ar+n→Ar+n−1+Ar; with some ions also exhibiting further fragmentation involving the loss of two and three argon atoms. A systematic study of the above reaction as a function of n reveals that the relative intensities of the fragment ions fluctuate considerably. In many cases, these fluctuations coincide with depleted or enhanced ion intensities in the normal ion cluster mass spectrum. Through the use of a model based on unimolecular reaction rate theory (RRKM), it has been possible to reproduce many of the features observed in both the fragmentation spectra and the normal mass spectrum. Of the two principle variables involved in the model, reaction symmetry number and dissociation or binding energy, we have been able to show that only the bin...

Products and relative reaction rates of the oxidation of tocopherols with singlet molecular oxygen.

Abstract: — The relative reactivity of singlet molecular oxygen, 02(1Δg), α-,β-,Γ-and δ with -tocopherol (vitamin E) was investigated using microwave discharge generation as a uniquely clean source of singlet oxygen and using a hydrocarbon solvent to approximate the membrane environment. The relative efficiencies of the tocopherols for O2(1Δg) were found to decrease in the order: D-α-tocopherol > D-β-tocopheroI > D–Γ-tocopherol > D-δ-tocopherol. The reaction products in all cases were found to be mixtures of quinone and quinone epoxides apparently resulting from decomposition of the primary product, the hydroperoxydienone.

Hydrogenolysis of ethane, propane, n-butane, and neopentane on the (111) and (110)-(1 times 2) surfaces of iridium

Abstract: The hydrogenolysis of ethane, propane, n-butane, and neopentane has been investigated on the (111) and (110)-(1{times}2) single-crystalline surfaces of iridium at reactant partial pressures between 0.2 and 5.0 Torr of hydrocarbon and between 20 and 500 Torr of hydrogen and for surface temperatures from 400 to 700 K. A coupled high-pressure reactor-ultrahigh vacuum analysis chamber was employed, which permitted both the measurement of the specific rates of reaction and in situ pre- and postreaction surface characterization. Both the apparent reaction kinetics (activation energies and preexponential factors) and the dependence of the rates of reaction on the reactant partial pressures (apparent reaction orders) were examined in detail. Postreaction surface analysis by Auger electron spectroscopy indicated the presence of a submonolayer carbonaceous residue, the coverage of which was nearly identical on both surfaces and essentially independent of reaction conditions, i.e. surface temperature and reactant partial pressures. Titration of these residues with hydrogen produced only methane in all cases, suggesting that the carbonaceous residue plays the role of a spectator. A mechanistic model involving a rate-limiting, irreversible, unimolecular C-C bond cleavage step has been employed to describe the variations in the specific activity and selectivity of hydrogenolysis with variations in both the temperaturemore » and the reactant partial pressures. The apparent kinetic parameters implicated by this model, which represent combinations of rate coefficients of several elementary reactions, have been found to be consistent with the expected values for the preexponential factors of the contributing elementary reactions. This model is applied to the data to describes the reaction for the different compounds. 71 refs., 17 figs., 5 tabs.« less

Carbon dioxide absorption into promoted carbonate solutions

Abstract: CO2 absorption rates into diethanolamine (DEA)-promoted carbonate solutions were measured over a wide range of carbonate conversions and temperatures. Analysis of the data showed that the CO2-DEA reaction rate was controlled by the rate of formation of zwitterion intermediate for carbonate conversions below 30%, and was controlled by the rate of abstraction of proton from the zwitterion intermediate for carbonate conversions greater than 30%. Hydroxyl ion, free amine, and carbonate ion were identified to act as bases to abstract the proton from the zwitterion. The modeling of the CO2 absorption rates and the effects of carbonate conversion and amine concentration on the CO2-DEA reaction kinetics were investigated.

Substrate specificity of the 3-methylaspartate ammonia-lyase reaction: observation of differential relative reaction rates for substrate-product pairs.

Abstract: A range of substituted fumaric and aspartic acid substrates for the enzyme 3-methylaspartate ammonia-lyase (EC 4.3.1.2) have been synthesized and used to study the kinetics of the catalyzed reaction in both the forward (deamination) and reverse (conjugative amination) reaction directions. The rates of amination for all of the alpha, beta-unsaturated substrates studied (bearing substituents the size of an ethyl group or smaller) were similar under [s] much greater than KM conditions although KM values for the substrates varied by a factor of 25. The rates of deamination for the corresponding 3-substituted amino acid substrates varied widely with structure under [s] much greater than KM conditions, and thus for substrate-product pairs the ratio for V(forward)/V(reverse) also varied. These differential reaction rates indicate that there is a step in the deamination direction that is especially sensitive to the size of the 3-substituent of the substrate and that a relatively large group (methyl to ethyl in size) is required for binding in order to reduce the activation energy for this step. Given that it is proposed that the enzyme operates via an E1cb-type mechanism where C-N bond cleavage is rate limiting, it is likely that binding of the C-3 substituent of aspartic acid substrates affects the alignment of the nascent carbanion with the C-N bond for elimination.