Showing papers on "Reaction rate published in 2003"

Low temperature water–gas shift: in situ DRIFTS-reaction study of ceria surface area on the evolution of formates on Pt/CeO2 fuel processing catalysts for fuel cell applications

Abstract: Steady state infrared (IR) measurements for adsorption of only CO and under water–gas shift (WGS) reaction conditions indicate that formates are present on the surface of reduced ceria, and that their concentrations vary with surface area of partially reduced ceria. Under steady state WGS, the concentrations of surface formates are strongly limited at high CO conversions. However, at low temperatures and conversions, the formates are close to the equilibrium adsorption/desorption coverages obtained from only CO adsorption. Comparisons at constant temperature indicate that formate bands from IR may provide an indication of the number of active sites present on the catalyst surface, as the rates varied accordingly. The IR results favor a formate intermediate mechanism to explain WGS. However, more kinetic studies are required, and over a broad range of temperatures, to verify this conclusion. Previous low temperature kinetic studies at a relatively high CO/H2O ratios have produced a zero-order dependency for CO and the authors related this to a mechanistic scheme involving reaction of Pt-CO with CeO2 to yield CO2, followed by reoxidation of Ce2O3 by H2O, with liberation of H2. The zero-order was suggested to be due to saturation of noble metal surface with CO during WGS. Saturation of ceria with carbonates was also reported. In this study, a high H2O/CO ratio was used where the CO rate dependency was first-order. This criteria requires that the surface coverage of the adsorbed CO intermediate should be reaction rate limited. Therefore, the formates are suggested to be the intermediates.

Insights into the Origin of High Activity and Stability of Catalysts Derived from Bulky, Electron-Rich Monophosphinobiaryl Ligands in the Pd-Catalyzed C-N Bond Formation

Abstract: A comparative kinetic examination of catalyst systems based on several monophosphinobiaryl ligands is reported. The bulk of the phosphine ligand controls the catalytic activity and the rate of catalyst activation with the catalyst based on 2-dicyclohexylphosphino-2‘,4‘,6‘-triisopropylbiphenyl providing the greatest activity and fastest activation. In the case where catalyst activation is slow (i.e., use of the smaller ligands such as 2-dicyclohexylphosphino-2‘-methylbiphenyl in combination with Pd(OAc)2) stirring the amine with the catalyst/base mixture prior to the commencement of the reaction increases the reaction rate along with the rate of catalyst activation. Kinetic isotope effects established that the catalyst activation process occurs through a β-hydride elimination pathway.

Heterogeneous Aspects of Acid Hydrolysis of α-Cellulose

Abstract: Hydrolysis of α-cellulose by H2SO4 is a heterogeneous reaction. As such the reaction is influenced by physical factors. The hydrolysis reaction is therefore controlled not only by the reaction conditions (acid concentration and temperature) but also by the physical state of the cellulose. As evidence of this, the reaction rates measured at the high-temperature region (above 200°C) exhibited a sudden change in apparent activation energy at a certain temperature, deviating from Arrhenius law. Furthermore, α-cellulose, once it was dissolved into concentrated H2SO4 and reprecipitated, showed a reaction rate two orders of magnitude higher than that of untreated cellulose, about the same magnitude as cornstarch. The α-cellulose when treated with a varying level of H2SO4 underwent an abrupt change in physical structure (fibrous form to gelatinous form) at about 65% H2SO4. The sudden shift of physical structure and reaction pattern in response to acid concentration and temperature indicates that the main factor causing the change in cellulose structure is disruption of hydrogen bonding. Finding effective means of disrupting hydrogen bonding before or during the hydrolysis reaction may lead to a novel biomass saccharification process.

A low water methanol carbonylation process for high acetic acid production and for water balance control

Abstract: The invention relates to a process for the production of acetic acid by carbonylation of methanol, and reactive derivatives thereof, in a reaction mixture using a rhodium-based catalyst in low water conditions. The process is used to achieve reaction rates of at least 15 g mol/l/hr. The high rate reactions proceed at water concentrations of less than 2.0 wt. %. Under certain conditions, the water concentration in the reaction mixture of the process is maintained at a desired concentration by at least one process step including adding a compound such as methyl acetate, dimethyl ether, acetic anhydride, or mixtures of these compounds to the reaction system. The process step of adding the components to the reaction mixture may be combined with other process steps for controlling water concentrations in reaction mixtures for the carbonylation of methanol.

Carbon Tunneling from a Single Quantum State

Abstract: We observed ring expansion of 1-methylcyclobutylfluorocarbene at 8 kelvin, a reaction that involves carbon tunneling. The measured rate constants were 4.0 × 10 −6 per second in nitrogen and 4 × 10 −5 per second in argon. Calculations indicated that at this temperature the reaction proceeds from a single quantum state of the reactant so that the computed rate constant has achieved a temperature-independent limit. According to calculations, the tunneling contribution to the rate is 152 orders of magnitude greater than the contribution from passage over the barrier. We discuss environmental effects of the solid-state inert-gas matrix on the reaction rate.

Gold nanoparticles in SBA-15 showing catalytic activity in CO oxidation

Abstract: The preparation of an active gold catalyst for CO oxidation, supported on silica, made by a novel solution technique is reported. The surface of SBA-15 was functionalized with positively charged groups, and [AuCl4]−-species were subsequently incorporated into the channel system via ion exchange. Upon reduction with NaBH4, highly dispersed nanoparticles of gold were formed in the channels of the mesoporous host. A reaction rate of 2.7×10 −4 mmol g cat − 1 s −1 in CO oxidation was found for this composite material. Compared with a previously reported material obtained by a chemical vapor deposition (CVD) method, this reaction rate is still lower by about one order of magnitude. However, the straightforward preparation method reported here still yields more active catalysts than any other Au/SiO2 system made by a solution technique, which show almost no activity at room temperature. Since the gold particles are essentially isolated from the support by the organic coating of the channels, a support–metal interaction is highly improbable as the source of the catalytic activity of the gold. TEM images of Au/SBA-15 reveal that gold particles start sintering at temperatures higher than 100 °C. Due to this effect, a significant drop in catalytic activity is observed.

Kinetics study of esterification of acetic acid with isobutanol in the presence of amberlite catalyst

Abstract: Kinetic data on the esterification of acetic acid with isobutanol have been obtained from both the uncatalyzed and heterogeneously catalyzed reactions using a stirred batch reactor in dioxane. The equilibrium constant, which is independent of temperature ranging from 318 to 368 K, was found to be 4. The uncatalyzed reaction was proved to be second-order reversible. In the presence of the catalyst, on the other hand, the reaction has been found to occur between an adsorbed alcohol molecule and a molecule of acid in the bulk fluid (Eley–Rideal model). It has also been observed that the initial reaction rate decreases with alcohol and water concentrations and it linearly increases with that of acid. The temperature dependency of the constants appearing in the rate expression were also determined.

Three-phase hydrogenation of d-glucose over a carbon supported ruthenium catalyst—mass transfer and kinetics

Abstract: The catalytic hydrogenation of d -glucose to d -sorbitol over a 5% Ru/C catalyst was studied in a semi-batch slurry autoclave operating at 373–403 K and 4.0–7.5 MPa hydrogen pressure. The d -glucose concentration was varied between 0.56 and 1.39 mol/l. The kinetic experiments were carried out in the absence of mass transport limitations, which was verified by using measured gas–liquid mass transfer coefficients and estimated diffusion and liquid–solid mass transfer coefficients. Many literature reports suffer from transport limitations. In the operating regime studied the reaction rate showed a first order dependency with respect to hydrogen. A shift in the order of d -glucose was observed. At low d -glucose concentrations (up to ca. 0.3 mol/l) the reaction showed a first order dependency, while at higher concentrations this changed to zero order behavior. No inhibition by sorbitol or mannitol was observed. The kinetic data were modeled using three plausible rate models based on Langmuir–Hinshelwood–Hougen–Watson (LHHW) kinetics assuming that the surface reaction is rate-determining. Model 1 involves non-competitive adsorption of hydrogen and d -glucose. Hydrogen adsorption is either molecular or dissociative, but due to the weak adsorption it results in both cases in a linear hydrogen pressure dependency, i.e. the same rate expression; Model 2 is based on competitive adsorption of molecular hydrogen and d -glucose; and Model 3 assumes competitive adsorption of dissociatively chemisorbed hydrogen and d -glucose. All three models described the data satisfactorily and further statistic discrimination between these models was not possible.

Homogeneous hydrogenation of carbon dioxide and bicarbonate in aqueous solution catalyzed by water-soluble ruthenium(II) phosphine complexes

Abstract: The water-soluble Ru(II)-phosphine complex, [{RuCl2(mTPPMS)(2)}(2)] was found a suitable catalyst for the hydrogenation of NaHCO3 to NaHCO2 in aqueous solution under mild conditions with catalyst turnover frequencies (TOFs) in the range of 35-50 h(-1) at 50degreesC and 10 bar total pressure. The suggested reaction mechanism involves the formation of Ru(II)-hydrides of the general formula [RuHX(mTPPMS)(4)] where X = H-, HCO3- or HCO2-. At 80degreesC and 95 bar total pressure, the reduction of NaHCO3 proceeded with high reaction rate (9600 h(-1)) hitherto unobserved in purely aqueous solutions. The reactions do not require the presence of organic amine additives, however, the addition of quinoline increased the rate considerably. Aqueous suspensions of calcium carbonate could also be hydrogenated with CO2/H-2 gas mixtures. (C) 2003 Elsevier B.V. All rights reserved.

Fractional reaction-diffusion equation

Abstract: A fractional reaction-diffusion equation is derived from a continuous time random walk model when the transport is dispersive. The exit from the encounter distance, which is described by the algebraic waiting time distribution of jump motion, interferes with the reaction at the encounter distance. Therefore, the reaction term has a memory effect. The derived equation is applied to the geminate recombination problem. The recombination is shown to depend on the intrinsic reaction rate, in contrast with the results of Sung et al. [J. Chem. Phys. 116, 2338 (2002)], which were obtained from the fractional reaction-diffusion equation where the diffusion term has a memory effect but the reaction term does not. The reactivity dependence of the recombination probability is confirmed by numerical simulations.

Organic reactions in microemulsions

Abstract: The use of microemulsions as media for organic reactions is a way to overcome the reagent incompatibility problems that are frequently encountered in organic synthesis. In this sense, microemulsions can be regarded as an alternative to phase transfer catalysis. The microemulsion approach and the phase transfer approaches can also be combined, i.e. the reaction can be carried out in a microemulsion in the presence of a small amount of phase transfer agent. A very high reaction rate may then be obtained. The reaction rate in a microemulsion is often influenced by the charge at the interface and this charge depends on the type of surfactant used. For instance, reactions involving anionic reactants may be accelerated by cationic surfactants. The surfactant counterion also plays a major role for the reaction rate. The highest reactivity is obtained with small counterions, such as acetate, that are only weakly polarizable. Large polarizable anions, such as iodide, bind strongly to the interface and may prevent other anionic species to reach the reaction zone.

Kinetics and mechanism of the heterogeneous catalyzed oxidative degradation of indigo carmine

Abstract: The kinetics of the oxidative degradation of the indigo carmine (IC) dye (disodium salt of 3,3-dioxobi-indolin-2,2-ylidine-5,5-disulfonate) with hydrogen peroxide catalyzed with the supported metal complexes have been investigated. The complexes used are [Cu(amm)4]2+, [Co(amm)6]2+, [Ni(amm)6]2+, [Cu(en)2]2+, and [Cu(ma)4]2+ (amm=ammonia, en=ethylenediamine, and ma=methylamine). Silica, alumina, silica-alumina (25% Al2O3), and cation-exchange resins (Dowex-50W, 2 and 8% DVB) are used as supports. The reaction is first order with respect to [IC] while the order with respect to [H2O2] was dependent on the initial concentration and the type of the catalyst used. At lower [H2O2]0 the order was first, which then decreases with increasing [H2O2]0, finally reaching zero. This aspect is consistent with the formation of a colored peroxo-complex on the catalysts surface. The reactivity of catalysts is dependent on the redox potential of the metal ions, the amount of complex loaded per gram of dry catalyst, the type of ligand, and the support. Moreover, the reaction rate was strongly dependent on the pH of the medium, the cationic and anionic surfactants, and the irradiation with UV-light. The reaction is enthalpy controlled as confirmed from the isokinetic relationship. A reaction mechanism was proposed with the formation of free radicals as reactive intermediates.

Rate and extent of aqueous perchlorate removal by iron surfaces.

Abstract: The rate and extent of perchlorate reduction on several types of iron metal was studied in batch and column reactors. Mass balances performed on the batch experiments indicate that perchlorate is initially sorbed to the iron surface, followed by a reduction to chloride. Perchlorate removal was proportional to the iron dosage in the batch reactors, with up to 66% removal in 336 h in the highest dosage system (1.25 g mL(-1)). Surface-normalized reaction rates among three commercial sources of iron filings were similar for acid-washed samples. The most significant perchlorate removal occurred in solutions with slightly acidic or near-neutral initial pH values. Surface mediation of the reaction is supported by the absence of reduction in batch experiments with soluble Fe2+ and also by the similarity in specific reaction rate constants (kSA) determined for three different iron types. Elevated soluble chloride concentrations significantly inhibited perchlorate reduction, and lower removal rates were observed for iron samples with higher amounts of background chloride contamination. Perchlorate reduction was not observed on electrolytic sources of iron or on a mixed-phase oxide (Fe3O4), suggesting that the reactive iron phase is neither pure zerovalent iron nor the mixed oxide alone. A mixed valence iron hydr(oxide) coating or a sorbed Fe2+ surface complex represent the most likely sites for the reaction. The observed reaction rates are too slow for immediate use in remediation system design, but the findings may provide a basis for future development of cost-effective abiotic perchlorate removal techniques.

Detailed Kinetics of Fischer−Tropsch Synthesis on an Industrial Fe−Mn Catalyst

Abstract: The detailed kinetics of the Fischer−Tropsch synthesis over an industrial Fe−Mn catalyst was studied in a continuous integral fixed-bed reactor under the conditions relevant to industrial operations [temperature, 540−600 K; pressure, 1.0−3.0 MPa; H2/CO feed ratio, 1.0−3.0; space velocity, (1.6−4.2) × 10-3 Nm3 kg of catalyst-1 s-1]. Reaction rate equations were derived on the basis of the Langmuir−Hinshelwood−Hougen−Watson type models for the Fischer−Tropsch reactions and the water-gas-shift reaction. Kinetic model candidates were evaluated by the global optimization of kinetic parameters, which were realized by first minimization of multiresponse objective functions with a genetic algorithm approach and second optimization with the conventional Levenberg−Marquardt method. It was found that an alkylidene mechanism based model could produce a good fit of the experimental data. This model shows that the desorption of the products and the insertion of methylene into the metal−alkylidene bond are the rate-dete...

Propane dehydrogenation over a Cr2O3/Al2O3 catalyst: transient kinetic modeling of propene and coke formation

Abstract: The kinetics of propane dehydrogenation to produce propene over a Cr 2 O 3 /Al 2 O 3 catalyst has been investigated over the temperature range of 525–575 °C at atmospheric pressure. The reaction rate of coke formation and its influence over catalyst deactivation has been studied. A Langmuir–Hinshelwood mechanism provides the best fit for the reaction, while a monolayer–multilayer mechanism is proposed to model the coke growth. Furthermore, this model was able to predict coke formation under conditions far from those employed in the experiments used to obtain the kinetics.

Hypochlorous Acid-Mediated Oxidation of Lipid Components and Antioxidants Present in Low-Density Lipoproteins: Absolute Rate Constants, Product Analysis, and Computational Modeling

Abstract: Oxidation of low-density lipoproteins (LDL) is believed to contribute to the increased uptake of LDL by macrophages, which is an early event in atherosclerosis. Hypochlorous acid (HOCl) has been implicated as one of the major oxidants involved in these processes. In a previous study, the rates of reaction of HOCl with the reactive sites in proteins were investigated (Pattison, D. I., and Davies, M. J. (2001) Chem. Res. Toxicol. 14, 1453−1464). The work presented here expands on those studies to determine absolute second-order rate constants for the reactions of HOCl with various lipid components and antioxidants in aqueous solution (pH 7.4). The reactions of HOCl with phosphoryl-serine and phosphoryl-ethanolamine are rapid (k ∼ 105 M-1 s-1) and of comparable reactivity to many of the protein sites. The major products formed in these reactions are chloramines, which decay to give both nitrogen- and carbon-centered radicals. Subsequent reactions of these species may induce oxidation of the LDL lipid compone...