Showing papers on "Reaction rate published in 1997"

Kinetics of transesterification of soybean oil

Abstract: Transesterification of soybean oil with methanol was investigated. Three stepwise and reversible reactions are believed to occur. The effect of variations in mixing intensity (Reynolds number=3,100 to 12,400) and temperature (30 to 70°C) on the rate of reaction were studied while the molar ratio of alcohol to triglycerol (6:1) and the concentration of catalyst (0.20 wt% based on soybean oil) were held constant. The variations in mixing intensity appear to effect the reaction parallel to the variations in temperature. A reaction mechanism consisting of an initial mass transfer-controlled region followed by a kinetically controlled region is proposed. The experimental data for the latter region appear to be a good fit into a second-order kinetic mechanism. The reaction rate constants and the activation energies were determined for all the forward and reverse reactions.

Atomic and Macroscopic Reaction Rates of a Surface-Catalyzed Reaction

Abstract: The catalytic oxidation of carbon monoxide (CO) on a platinum (111) surface was studied by scanning tunneling microscopy The adsorbed oxygen atoms and CO molecules were imaged with atomic resolution, and their reactions to carbon dioxide (CO 2 ) were monitored as functions of time The results allowed the formulation of a rate law that takes the distribution of the reactants in separate domains into account From temperature-dependent measurements, the kinetic parameters were obtained Their values agree well with data from macroscopic measurements In this way, a kinetic description of a chemical reaction was achieved that is based solely on the statistics of the underlying atomic processes

A Semi-Empirical Reaction Mechanism for n-Heptane Oxidation and Pyrolysis

Abstract: A new semi-empirical mechanism for n-heptane oxidation and pyrolysis has been developed and validated against several independent data sets, including new flow reactor experiments. Previous semi-empirical chemical kinetic mechanisms assumed that a generic n-alkyl radical, formed by abstraction of an H-atom from the parent fuel, thermally decomposes into a fixed ratio of methyl and propene. While such an approach has been reasonably successful in predicting premixed, laminar flame speeds, the mechanism lacks sufficient detail to quantitatively capture transient phenomena and intermediate species distributions. The new chemical kinetic mechanism retains significantly more detail, yet is sufficiently compact to be used in combined fluid-mechanical/chemical kinetic computational studies. The mechanistic approach is sufficiently general to be extended to a wide variety of large linear and branched alkane fuels.

Chemistry induced by hydrodynamic cavitation

Abstract: Cavitation (the formation, growth, and implosive collapse of gas or vapor-filled bubbles in liquids) can have substantial chemical and physical effects. While the chemical effects of acoustic cavitation (i.e., sonochemistry and sonoluminescence) have been extensively investigated during recent years,1-5 little is known about the chemical consequences of hydrodynamic cavitation created during turbulent flow of liquids. Hydrodynamic cavitation is observed when large pressure differentials are generated within a moving liquid and is accompanied by a number of physical effects, erosion being most notable from a technological viewpoint.6,7 In contrast, reports of hydrodynamically induced chemistry or luminescence and direct comparisons to sonochemistry or sonoluminescence have been extremely limited.8,9 In aqueous liquids, acoustic cavitation leads to the formation of reactive species such as OH•, H•, and H2O2. These shortlived species are capable of effecting secondary oxidation and reduction reactions. For example, iodide can be sonochemically oxidized to triiodide by OH• radicals or H2O2 produced during cavitation. From aqueous solutions containing chlorocarbons, Cl• and Cl2 are also liberated in high yields and this increases rates of iodide oxidation.10 The rate of triiodide formation is easily monitored spectrophotometrically. For many years, this so-called Weissler reaction has remained the standard dosimeter for sonochemical reactions. The recent advent of commercially available high-pressure jet fluidizers capable of pressure drops as high as 2 kbar and jet velocities approaching 200 m/s has led to numerous applications in the physical processing of liquids, for emulsification, cell disruption, etc. Chemical consequences of such processing, however, have received little examination. One important exception comes fromW. R. Moser and co-workers,11 who have shown that such a device can be utilized to prepare nanostructured catalytic materials. Moser speculated that the unusual properties of his catalysts resulted from hydrodynamic cavitation within the fluidizer.11 We describe here conclusive experimental evidence for chemical reactions caused by hydrodynamic cavitation within a jet fluidizer. In a typical run,12a 60 mL of 1.0 M KI in purified water saturated with carbon tetrachloride was introduced at a constant flow rate into the Microfluidizer with a liquid pressure of 1.24 kbar. The reaction solution temperature increased 10 to 12 °C within 90 s and stabilized at the temperatures reported herein. Aliquots (4 mL) of the processed solution were periodically extracted from the reaction system by airtight syringes, analyzed spectrophotometrically, and returned to the reservoir after analysis. The rate of I3 formation was calculated from the change in absorbance at 353 nm ( ) 26 400 M-1 cm-1)12b as a function of reaction time. Initial studies conducted with Arsparged water gave relatively low rates of I3 production; saturation of the Ar sparged H2O with CCl4 resulted in a 20fold increase in I3 production, as has been typically observed for ultrasonic cavitation.10 This is attributed to ready formation of Cl• and Cl2 from CCl4 under cavitation conditions. The effect of upstream liquid pressure on the rate of I3 production was investigated over the range 100-1500 bar. The reaction rate increases linearly with liquid pressure (Figure 1), but with a threshold pressure of 150 bar. Below 150 bar of hydrostatic pressure, no chemical reactions were observed; this probably represents the minimum jet velocity necessary to induce cavitation. The resistance of a turbulent flow to cavitation is given by its cavitation number σ, as defined in eq 1:6

High-pressure co oxidation on pt(111) monitored with infrared-visible sum frequency generation (sfg)

Abstract: Catalytic CO oxidation on Pt(111) to CO2 was studied under atmospheric pressures of CO and O2 and at various temperatures. Surface vibrational spectroscopy by sum frequency generation was used to probe the surface species, while the reaction rate and gas composition were simultaneously monitored by gas chromatography. Correlation between the turnover rates and the surface coverage of various CO species were utilized to identify the active CO species in the reaction. Ignition, above which the reaction becomes self-sustained, divides the reaction into two reactivity regimes. Below ignition, atop bonded CO appears as the major species on the surface, but the reaction rate is inversely proportional to the surface coverage of this species, indicating that it is not the active species but rather an inhibitor. The observed activation energy for the reaction in this regime suggests that desorption of atop CO is the rate-limiting step in the reaction. Above ignition, the atop CO becomes hardly detectable and the a...

Unique Effects of Iron(III) Ions on Photocatalytic and Photoelectrochemical Properties of Titanium Dioxide

Abstract: Photocatalytic oxidation of water on TiO2 (rutile) powder proceeded with a fairly high efficiency (about 9%) when iron(III) ions were used as the electron acceptor. The reaction continued until all iron(III) ions added to the solution were reduced into iron(II) ions. This behavior was in marked contrast to other reversible photocatalytic reactions, whose reaction rates decelerate as the result of the back reactions of the products on the photocatalysts. The efficient oxidation of water in the presence of iron(III) ions was attributed to preferential adsorption of iron(III) ions on TiO2 over iron(II) ions, which enabled efficient oxidation of water, although this reaction was thermodynamically less favorable than oxidation of iron(II) ions. Furthermore, from the measurements of photocurrents at crystalline TiO2 electrodes, iron(III) ions were concluded to have a catalytic function for the oxidation of water on photoirradiated TiO2.

Kinetic study of the reverse water-gas shift reaction over CuO/ZnO/Al2O3 catalysts

Abstract: The kinetics of the reverse water-gas shift (RWGS) reaction over CuO/ZnO/Al2O3 catalysts was studied by use of CO2H2 cycles, hydrogen chemisorption and catalytic tests performed in both differential and integral plug flow reactors. The effect of the reactant composition on the reaction rate was specifically studied by changing the PH20/PCO20 ratio between 9.0 and 0.3. It was found that different reagents become rate limiting depending upon pressure. While in a H2-rich region the rate increases strongly with CO2 partial pressure and is zero order in hydrogen, under low PH20/PCO20 ratios the reaction is less active and is strongly positive order in hydrogen and low order in carbon dioxide. The experimental data were modeled by considering that the reaction proceeds through a surface redox mechanism, copper being the active metal. A good agreement between experimental and calculated data was obtained by assuming that in the redox mechanism either the dissociative CO2 adsorption (H2-rich region) or both the CO2 dissociation and the water formation (H2-lean region) determine the rate of the overall reaction. Based on previous studies performed on copper crystal surfaces, such a change in kinetics may be explained by assuming that under H2-rich atmosphere a surface structural or phase transition occurs involving a change in reactivity with respect to CO2 dissociation.

Kinetics of soot—O2, soot—NO and soot—O2—NO reactions over spinel-type CuFe2O4 catalyst

Abstract: Kinetics of soot-O 2 , soot-NO and soot-O 2 -NO reactions over CuFe 2 O 4 were investigated by using temperature programmed reaction (TPR) technique in which a soot-catalyst mixture was loaded in a reactor and exposed to a fluent stream containing gaseous reactant(s) under a constant heating rate. Within the temperature range where a substantial amount of the charged soot remained and the reaction can be regarded to be the zero order in the amount of soot, the kinetic analysis of non-steady TPR results was possible. The half-order kinetics in the partial pressure of O 2 ( P O 2 ) was obtained for the soot-O 2 reaction. The soot-NO reaction was complicated in the temperature dependence of CO 2 and N 2 formation as well as in the reaction order with respect to P NO ; the reaction order was first at lower temperatures and increased with increasing temperature. In the soot-O 2 -NO reaction, i.e. the so-called simultaneous soot-NO x removal, the rate of CO 2 formation and those of both N 2 and N 2 O formation depended on partial pressures as P 0.6 NO P 0.6 O 2 and P 1.0 NO P 0.4 O 2 , respectively. Based on the kinetic results, possible reaction mechanisms were discussed.

Reaction of naphthalene and its derivatives with hydroxyl radicals in the gas phase

Abstract: Naphthalene is the most abundant polycyclic aromatic hydrocarbon (PAH) found in urban air. It is reactive in the atmosphere under ambient conditions, its chief reaction partner being the hydroxyl radical, OH•. In this work, the reactions of OH• with naphthalene, 1- and 2-naphthol, and 1- and 2-nitronaphthalene were studied in a 9.4 m3 smog chamber. Relative rates of reaction accorded well with previous studies and allowed estimates to be made of the atmospheric lifetimes of these compounds. Numerous oxidation products were identified, and mechanisms proposed for their formation were based on the further transformation of benzocyclohexadienyl radicals formed by addition of OH• to naphthalene. The naphthols and nitronaphthalenes were deduced not to be on the major reaction pathway to the more oxidized products. Because of the high reactivity of PAH in air, we suggest that priority be given to identifying and quantitating their reaction products, some of which may be relatively persistent air toxics.

Oxidation of Aqueous Pollutants Using Ultrasound: Salt-Induced Enhancement

Abstract: Ultrasound can be used to oxidize aqueous pollutants; however, due to economic reasons, higher oxidation/destruction rates are needed. This study reports enhancements of reaction rates by the addition of sodium chloride salt. Using 20 kHz ultrasound, large salt-induced enhancements are observed6-fold for chlorobenzene, 7-fold for p-ethylphenol, and 3-fold for phenol oxidation. The reaction rate enhancements are proportional to the diethyl ether−water partitioning coefficient of the pollutants. It appears that the majority of oxidation reactions occur in the bubble−bulk interface region. The addition of salt increases the ionic strength of the aqueous phase which drives the organic pollutants toward the bubble−bulk interface. A first order reaction rate equation is proposed which can represent the observed enhancement with a good accuracy. A new sonochemical-waste-oxidation process is proposed utilizing the salt-induced enhancement.

Reaction of Nitric Oxide with Amines

Abstract: Reactions of nitric oxide (NO) with amines in organic solvents were studied using Hantzsch dihydropyridines and aromatic primary amines as substrates. Hantzsch dihydropyridines are readily oxidized by nitric oxide to give the corresponding pyridines in quantitative yields. The addition of oxygen accelerates the reaction rate considerably. On the other hand, aromatic primary amines give deaminated products by the reaction with nitric oxide only in the presence of oxygen in ethereal solvents or chloroform.

Long-range electron-transfer reaction rates to cytochrome c across long- and short-chain alkanethiol self-assembled monolayers: Electroreflectance studies

Abstract: The kinetics of electron transfer (ET) between cytochrome c and a gold (111) electrode through self-assembled monolayers of alkanethiols with terminal carboxylic acid groups, COOH(CH 2 ) n SH, have been studied for n=2–11 using an ac potential-modulated UV–VIS reflectance spectroscopic technique (electroreflectance spectroscopy, ER). For 9⩽n⩽11, the standard ET rate constant, k app , depends exponentially on the chain lengths and the exponential decay factor is 1.09 per methylene group; for n<9, however, k app deviates from the exponential plot. The ET reaction through short-chain alkanethiol monolayers is controlled by the preceding chemical reaction. The rate-controlling step is very likely to be the reorganization of cytochrome c to the favourable conformation for the ET reaction. The ET reaction rate constant from cytochrome c in the favourable conformation to the electrode surface obeys Marcus theory for long-range ET. The ET reaction through long-chain alkanethiol monolayers is controlled by the ET rate through alkanethiols.

Polychlorinated Dibenzo-p-dioxins and Dibenzofurans: Gas-Phase Hydroxyl Radical Reactions and Related Atmospheric Removal

Abstract: Gas-phase reactions with the hydroxyl radical (OH) are expected to be an important removal pathway of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) in the atmosphere. Our laboratory recently developed a system to measure the rate constants of the gas-phase reactions of OH with semivolatile organic compounds using on-line mass spectrometry. We have now incorporated electron capture mass spectrometry (EC-MS) into this system to increase its sensitivity to PCDD/F, which tend to have low vapor pressures. OH reaction rate constants were determined in helium for 1,2,3,4-tetrachlorodibenzo-p-dioxin at 373−432 K using a heated quartz reaction chamber. The photolysis of O3 in the presence of H2O and the photolysis of H2O2 (both at λ = 254 nm) served as OH sources. An extrapolation using the Arrhenius equation gives a 1,2,3,4-tetrachlorodibenzo-p-dioxin−OH reaction rate constant of 8.5 × 10-13 cm3 s-1 at 298 K, which is in excellent agreement with the value predicted by a structure−activity method. T...

Photocatalytic Degradation of 2-Chlorophenol in TiO2 Aqueous Suspension: Modeling of Reaction Rate

Abstract: The photoassisted oxidation of a dilute aqueous solution of 2-chlorophenol (2-CP) was investigated, over a heterogeneous catalyst of TiO2 (anatase), in an annular photocatalytic reactor. The effects of some physical and chemical parameters such as 2-CP concentration, catalyst concentration, dissolved oxygen concentration, pH, temperature, and absorbed light intensity were studied in order to optimize the process. The experiment, carried out in the presence of electron scavengers such as metallic ions, shows that the reaction rate is significantly higher than that obtained when oxygen is used alone. The results obtained in this study have led us, on the basis of experimentally determined adsorption, to propose a kinetic approach in which the rate-determining step is the reaction of OH• radicals, identified by a spin trapping technique (EPR), with adsorbed 2-CP. A kinetic model proposed was based on a Langmuir type adsorption involving a competition between solvent and substrate with a supplementary assumpt...

Anomalous behavior of Ru for catalytic oxidation: A theoretical study of the catalytic reaction CO + 1/2 O_2 -> CO_2

Abstract: Recent experiments revealed an anomalous dependence of carbon monoxide oxidation at Ru(0001) on oxygen pressure and a particularly high reaction rate. Below we report density functional theory calculations of the energetics and reaction pathways of the speculated mechanism. We will show that the exceptionally high rate is actuated by a weakly but nevertheless well bound (1{times}1)-oxygen adsorbate layer. Furthermore, it is found that reactions via scattering of {ital gas-phase} CO at the oxygen covered surface may play an important role. Our analysis reveals, however, that reactions via {ital adsorbed} CO molecules (the so-called Langmuir-Hinshelwood mechanism) dominate. {copyright} {ital 1997} {ital The American Physical Society}