Showing papers on "Reaction rate constant published in 1998"

Catalytic Decomposition of Hydrogen Peroxide on Iron Oxide: Kinetics, Mechanism, and Implications

Abstract: This research describes the heterogeneous catalytic reactions of H2O2 with granular size goethite (α-FeOOH) particles in aqueous solution under various experimental conditions. This is an important reaction for the environment since both H2O2 and iron oxides are common constituents of natural and atmospheric waters. Furthermore, iron oxides function as catalysts in chemical oxidation processes used for treatment of contaminated waters with H2O2. The results of this study demonstrated that the decomposition rate of H2O2 over goethite surface can be described by the second-order kinetic expression −d[H2O2]/dt = k[FeOOH][H2O2], where k = 0.031 M-1 s-1, at pH 7 in the absence of any inorganic or organic chemical species. The apparent reaction rate was dominated by the intrinsic reaction rates on the oxide surfaces rather than the mass transfer rate of H2O2 to the surface. The activation energy of the reaction of H2O2 with the iron oxide surface was determined to be 32.8 kJ/M. The reaction mechanism for the de...

Real-time enzyme kinetics monitored by dual-color fluorescence cross-correlation spectroscopy.

Abstract: A method for sensitively monitoring enzyme kinetics and activities by using dual-color fluorescence cross-correlation spectroscopy is described. This universal method enables the development of highly sensitive and precise assays for real-time kinetic analyses of any catalyzed cleavage or addition reaction, where a chemical linkage is formed or cleaved through an enzyme's action between two fluorophores that can be discriminated spectrally. In this work, a homogeneous assay with restriction endonuclease EcoRI and a 66-bp double-stranded DNA containing the GAATTC recognition site and fluorophores at each 5' end is described. The enzyme activity can be quantified down to the low picomolar range (>1.6 pM) where the rate constants are linearly dependent on the enzyme concentrations over two orders of magnitude. Furthermore, the reactions were monitored on-line at various initial substrate concentrations in the nanomolar range, and the reaction rates were clearly represented by the Michaelis-Menten equation with a KM of 14 +/- 1 nM and a kcat of 4.6 +/- 0.2 min-1. In addition to kinetic studies and activity determinations, it is proposed that enzyme assays based on the dual-color fluorescence cross-correlation spectroscopy will be very useful for high-throughput screening and evolutionary biotechnology.

Photocatalytic Degradation of Gaseous Formaldehyde Using TiO2 Film

Abstract: The photocatalytic degradation of gaseous formaldehydea major cause of sick building syndromewas studied using a TiO2 thin film. TiO2 thin films have many unique photoinduced properties, for example, self-cleaning, anti-fouling, and anti-bacterial functions. UV illumination of the TiO2 thin film placed in a gaseous formaldehyde/air environment resulted in the total mineralization of formaldehyde to CO2 and H2O. We invoked a Langmuir−Hinshelwood kinetic model to analyze the dependence of reaction rates on the concentration of formaldehyde. In addition, the overall decomposition rate constant for formaldehyde was comparable to that of acetaldehyde (a standard test reactant) for initial concentrations of up to 1000 ppmv. However, the apparent adsorption constant Kapp of formaldehyde onto TiO2 was ca. 2.5 times larger than that of acetaldehyde. Thus in the low concentration regime, the reactivity of formaldehyde appeared to be greater than that of acetaldehyde. In like manner, a dark adsorption experiment als...

Influence of Organic Ligands on Chromium(VI) Reduction by Iron(II)

Abstract: Iron(II) is one of the most important reductants that transforms toxic chromium(VI) to essentially nontoxic chromium(III), but the effect of iron speciation on this redox reaction is not well-understood. We determined rate constants for Cr(VI) reduction by a series of Fe(II)−organic complexes, using UV−vis spectroscopy and kinetic fitting. The experiments with 1−20 μM Cr(VI), 1−60 μM Fe(II), and 5−1000 μM organic ligand at pH 4.0−5.5 can be described with the following rate law: −d[Cr(VI)]/dt = ∑LkL[Fe(II)L][Cr(VI)], where kL is pH-dependent. Fe(III)-stabilizing ligands such as bi- and multidentate carboxylates and phenolates generally accelerate the reaction, whereas Fe(II)-stabilizing ligands such as phenanthroline essentially stop the reaction. The rate coefficients increase with decreasing electron reduction potential of the Fe(III)L−Fe(II)L redox couples. The relationship of log kL versus EH°(Fe(III)L) is quite linear over 10 orders of magnitude. Dissolved organic matter extracted from the organic h...

Kinetics and mechanism of the enhanced reductive degradation of nitrobenzene by elemental iron in the presence of ultrasound

Abstract: Enhanced rates of sonolytic degradation of CCl_4 in the presence of Fe^0 are demonstrated. In Ar-saturated solutions, the first-order rate constant for CCl_4 degradation is k_(US) = 0.107 min^(-1), whereas in the presence of Ar and Fe^0, the apparent first-order rate constant is found to depend on the total surface area of elemental iron in the following fashion: k_(obs) = (k_(US) + k_(Fe)^0A_(Fe)^0) min^(-1), where k^(US) = 0.107 min^(-1), k_(Fe)^0 = 0.105 L m_(-2) min^(-1), and A_(Fe)^0) = reactive surface area of Fe^0 in units of m^2 L^(-1). In the coupled ultrasound and iron system, the contribution to the overall degradation rate by direct reaction with Fe^0 results in an overall rate enhancement by a factor of 40. These enhancements are attributed (1) to the continuous cleaning and chemical activation of the Fe^0 surface by the combined chemical and physical effects of acoustic cavitation and (2) to accelerated mass transport rates of reactants to the Fe^0 surfaces. Additional kinetic enhancements are due to the production of H^+ during the course of the reaction. Furthermore, the concentrations of the principal reaction intermediates, C_2Cl_6 and C_2Cl_4, are influenced substantially by the total available surface area of Fe^0.

Kinetic Model for Lead(II) Sorption on to Peat

Abstract: The kinetics of lead sorption on to peat have been investigated. The batch sorption model, based on the assumption of a pseudo-second order mechanism, has been developed to predict the rate constant of sorption, the equilibrium capacity and initial sorption rate with the effect of initial lead(II) concentration, peat particle size and temperature. An equilibrium capacity of sorption has been evaluated with the pseudo-second order rate equation. In addition, an activation energy of sorption has also been determined based on the pseudo-second order rate constants.

Kinetics and Mechanism of the Sonolytic Destruction of Methyl tert-Butyl Ether by Ultrasonic Irradiation in the Presence of Ozone

Abstract: The kinetics and mechanism of the sonolytic degradation of methyl tert-butyl ether (MTBE) have been investigated at an ultrasonic frequency of 205 kHz and power of 200 W L-1. The observed first-order degradation rate constant for the loss of MTBE increased from 4.1 × 10-4 s-1 to 8.5 × 10-4 s-1 as the concentration of MTBE decreased from 1.0 to 0.01 mM. In the presence of O3, the sonolytic rate of destruction of MTBE was accelerated substantially. The rate of MTBE sonolysis with ozone was enhanced by a factor of 1.5−3.9 depending on the initial concentration of MTBE. tert-Butyl formate, tert-butyl alcohol, methyl acetate, and acetone were found to be the primary intermediates and byproducts of the degradation reaction with yields of 8, 5, 3, and 12%, respectively. A reaction mechanism involving three parallel pathways that include the direct pyrolytic decomposition of MTBE, the direct reaction of MTBE with ozone, and the reaction of MTBE with hydroxyl radical is proposed.

OH Reaction Kinetics of Polycyclic Aromatic Hydrocarbons and Polychlorinated Dibenzo-p-dioxins and Dibenzofurans

Abstract: Rate constants were measured for gas-phase reactions of the hydroxyl radical (OH) with six polycyclic aromatic hydrocarbons (PAH) and four polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F). These OH reaction rate constants were determined in helium at about 1 atm over various temperature ranges between 306 and 405 K. The experiments were carried out in a small, heated quartz reaction chamber sampled by on-line mass spectrometry, and OH was produced by the photolysis of O3 in the presence of H2O. Arrhenius regressions were performed with the rate constants of each compound, and the temperature dependencies were found to be slight to nonexistent. The OH reaction rate constants of PCDD/F were in agreement with those predicted by a structure−reactivity method. The resulting rate constants at 298 K (in units of 10-12 cm3 s-1) were as follows: naphthalene, 23; acenaphthene, 58; fluorene, 13; phenanthrene, 27; anthracene, 190; fluoranthene, 11; dibenzo-p-dioxin, 12; 2,7-dichlorodibenzo-p-dioxin, 4.4;...

Reaction of Ozone with Ethene and Its Methyl- and Chlorine-Substituted Derivatives in Aqueous Solution

Abstract: Byproduct formation in ozonation processes is of interest, especially in the area of drinking water, similar to the concerns regarding the health effects of disinfection byproducts generated by chlorine and other oxidants used for that purpose. Chlorinated olefins increasingly contaminate our drinking water resources, and the byproducts formed by ozone were determined here. For a better understanding of the underlying mechanisms, some other olefins were included in this study. The rate constants of the reaction of ozone with ethene and some of its methyl- and chlorine-substituted derivatives in aqueous solution have been measured by stopped-flow (rate constants in units of dm3 mol-1 s-1): tetramethylethene, >106; propene, 8 × 105; ethene, 1.8 × 105; buten-3-ol, 7.9 × 104; vinyl chloride, 1.4 × 104; trans-1,2-dichloroethene, 6.5 × 103; 1,1-dichloropropene, 2.6 × 103; cis-1,2-dichloroethene, 540; 1,1-dichloroethene, 110; trichloroethene, 14. From these data, it is concluded that the rate of reaction is mai...

Measurement of Residue Curve Maps and Heterogeneous Kinetics in Methyl Acetate Synthesis

Abstract: Kinetically controlled reactive distillation is a bridge between the two limiting cases of nonreactive and equilibrium reactive distillation. In this paper, we study the influence of heterogeneous catalysis on the transition from the nonreactive to the equilibrium reactive limits for the esterification of acetic acid with methanol. A Langmuir−Hinshelwood/Hougen−Watson rate model was developed to represent the reaction kinetics. Since reactive distillation is carried out at the boiling temperature of the liquid, the pressure of the system plays an important role in determining the product composition. Higher operating pressures imply higher temperatures and faster rates of reaction but could also lower equilibrium conversions (for exothermic systems) and trigger unwanted side reactions. To study the importance of side reactions and the effect of pressure on product selectivity, we also include the reaction kinetics for methanol dehydration. Isothermal batch kinetic experiments were performed using a hetero...

Laboratory and theoretical study of the oxy radicals in the OH- and Cl-initiated oxidation of ethene

Abstract: The products of the OH-initiated oxidation mechanism of ethene have been studied as a function of temperature (between 250 and 325 K) in an environmental chamber, using Fourier transform infrared spectroscopy for end product analysis. The oxidation proceeds via formation of a peroxy radical, HOCH2CH2O2. Reaction of this peroxy radical with NO is exothermic and produces chemically activated HOCH2CH2O radicals, of which about 25% decompose to CH2OH and CH2O on a time scale that is rapid compared to collisions, independent of temperature. The remainder of the HOCH2CH2O radicals are thermalized and undergo competition between decomposition, HOCH2CH2O → CH2OH + CH2O (6), and reaction with O2, HOCH2CH2O + O2 → HOCH2CHO + HO2 (7). The rate constant ratio, k6/k7, for the thermalized radicals was found to be (2.0 ± 0.2) × 1025 exp[−(4200 ± 600)/T] molecule cm-3 over the temperature range 250−325 K. With the assumption of an activation energy of 1−2 kcal mol-1 for reaction 7, the barrier to decomposition of the HOC...

Investigation of the Reactive Oxygen Intermediate in an Arene Hydroxylation Reaction Performed by Xylyl-Bridged Binuclear Copper Complexes

Abstract: The kinetics of the reaction, via an oxygen-bound intermediate, of [CuI2(NO2-XYL)][ClO4]2·CH3CN to [Cu2(NO2-XYL-O-)(OH)]2+, where the bridging arene is hydroxylated, have been examined with use of resonance Raman spectroscopy. A resonance Raman peak characteristic of peroxide bound in a side-on, μ-η2:η2 geometry is observed upon oxygenation of [CuI2(NO2-XYL)] for both intramolecularly and intermolecularly bridged complexes. The decay of the intramolecularly bridged peroxide stretch at ∼750 cm-1 and the growth of the phenolate stretch of the product at 1320 cm-1 were monitored over time with use of an excitation wavelength of 406.9 nm. Both the decay of the peroxide stretch and the growth of the phenolate stretch were found to be first order, and the rate constants are consistent, within experimental error, with the peroxide intermediate reacting directly to form the hydroxylated product. The possibility of an unobservable amount of a bis-μ-oxo isomer which is in rapid equilibrium with the side-on peroxide...

Accurate quantum calculations of thermal rate constants employing MCTDH: H2+OH→H+H2O and D2+OH→D+DOH

Abstract: An improved approach for the direct calculation of thermal rate constants, employing the multi-configurational time-dependent Hartree (MCTDH) approach and the flux–flux correlation function, is presented. In this approach, the thermal flux operator is diagonalized and its eigenstates are propagated. The cumulative reaction probability is obtained for all energies simultaneously by a simple Fourier transform of the matrix elements between the propagated flux eigenstates. Calculations for the H2+OH→H+H2O reaction, which include all six internal degrees of freedom accurately, demonstrate the accuracy and efficiency of the approach. Previous results for the rate constant are reproduced (for temperatures between 300 K and 600 K) and extended towards lower temperatures (200 K to 300 K). Also, thermal rate constants of the D2+OH→D+DOH reaction have been computed. All six internal degrees of freedom are included accurately in this calculation. The kinetic isotope effect is obtained and compared to experiment.

Reactivity of chlorine atoms in aqueous solution Part 1The equilibrium ClMNsbd+Cl-Cl2-

Abstract: The equilibrium constant for reaction (1) has been determined in neutral solution to be (1.4±0.1)×105 d mol-1, with forward and reverse rate constants, k1 and k-1, of (8.5±0.7)×109 d mol-1 s-1 and (6.0±0.5)×104 s-1, respectively. At 25°C the rate constants for the reactions of ClNsbd and Cl2- were measured to be (2.5±0.3)×105 s-1 and (1.3±0.1)×103 s-1 with water and (6.5±0.6)×108 d mol-1 s-1 and ca. 0, within experimental error, with 2-methyl-propan-2-ol. Deviation from the equilibrium values of [ClNsbd] and [Cl2-] in the early stages of the reactions was investigated and shown to account for the discrepancy between the value of K1 determined here and a previous estimate from our laboratory.

Formation of Peroxynitrate from the Reaction of Peroxynitrite with CO2: Evidence for Carbonate Radical Production

Abstract: The decomposition of peroxynitrite in the presence of bicarbonate and various substrates was studied using the stopped-flow technique. We have shown that peroxynitrite (ONOOH/ONOO-) reacts with excess of CO2 to form peroxynitrate (O2NOOH/O2NOO-) in aerated solutions containing HCO2-, H2O2, CH3OH, or (CH3)2CHOH. The yield of peroxynitrate increased with the increase in these substrate concentrations approaching 30−33% of added peroxynitrite. Competition kinetics study yields rate constants which are similar to those determined directly for the reactions of these substrates with the carbonate radical anion. We therefore suggest that the formation of peroxynitrate takes place via the following steps: (i) the rapid reaction of peroxynitrite with CO2 to form ONOOCO2-; (ii) the homolytic cleavage of 30−33% of ONOOCO2- into •NO2 and CO3•-; (iii) the reaction of CO3•- with the various substrates, yielding ultimately superoxide in aerated solutions; and (iv) the fast reaction of superoxide with •NO2 to form O2NOO-.

Neutral–neutral reactions at the temperatures of interstellar clouds Rate coefficients for reactions of C2H radicals with O2, C2H2, C2H4 and C3H6 down to 15 K

Abstract: A CRESU (Cine′tique de Re′action en Ecoulement Supersonique Uniforme) apparatus has been used to measure rate constants for the reactions of the ethynyl radical (C2H) with O2, C2H2, C2H4 and C3H6 at temperatures from 295 down to 15 K. C2H radicals are generated by photolysis of C2H2 at 193 nm using an ArF excimer laser and reaction rates are determined by observing the chemiluminescence from CH(A2Δ) which is generated in a minor channel of the reaction between C2H radicals and O2. The rate constants for all four reactions increase as the temperature is lowered, and those for reactions with the unsaturated hydrocarbons exceed 10-10 cm3 molecule-1 s-1 at all temperatures below 100 K. The results confirm that C2H radicals, like CN radicals, react rapidly with unsaturated hydrocarbons at very low temperatures and, in both cases, the radical replaces an H atom in the unsaturated molecule. It therefore seems likely that combinations of these reactions play a major role in forming species with long C chains and in synthesising the cyanopolyynes in dense interstellar clouds.

Effects of the Hydrophobicity of the Reactants on Diels-Alder Reactions in Water

Abstract: To assess the importance of the hydrophobicity of different parts of diene and dienophile on the aqueous acceleration of Diels-Alder reactions, second-order rate constants have been determined for the reactions of cyclopentadiene (1), 2,3-dimethyl-1,3-butadiene (4), and 1,3-cyclohexadiene (6) with N-methyl-, N-ethyl-, N-propyl-, and N-butylmaleimide (2a-d) in different solvents. All these reactions are accelerated in water relative to organic solvents as a result of enhanced hydrogen bonding and enforced hydrophobic interactions during the activation process. The beneficial influence of water as compared to 1-propanol on the rate of the Diels-Alder reaction of 4 with 2a-d increases linearly with the length of the alkyl chain of 2. In contrast, for the reaction of both 1 and 6 with 2a-d, no such effect was observed. This difference can be explained by a hydrophobic interaction between the methyl groups of 4 and the N-alkyl group of 2 during the activation process. In the reactions of 1 and 6, lacking the methyl substituents, this interaction is not possible and elongation of the alkyl chain from ethyl onward does not result in an additional acceleration by water. The enhanced hydrophobicity near the reaction center of dienes 4 and 6 compared to 1 results in an increased aqueous acceleration of the Diels-Alder reactions of the former dienes with 2a. These data indicate that an increase in the hydrophobicity close to the reaction center in the diene has a much more pronounced effect on the rate acceleration in water than a comparable increase in hydrophobicity in the dienophile further away from the reaction center. The Gibbs energies of transfer of initial state and activated complex of the Diels-Alder reactions under study have been determined. As expected, for all reactions the initial state in water is destabilized compared to that in 1-propanol. This destabilization becomes more pronounced when the nonpolar character of diene (close to the reaction center) or dienophile (distant from the reaction center) is increased. Likewise, an increase in the nonpolar character of 2 results in a destabilization of the activated complex. In contrast, addition of methyl or methylene units to the diene is not accompanied by a significant destabilization of the activated complex in water as compared to 1-propanol. We conclude that hydrophobic groups near the reaction center seem to lose their hydrophobic character completely in the activated complex of the Diels-Alder reaction, whereas more distant groups retain their nonpolar character throughout the reaction.

Reaction mechanism of direct decomposition of nitric oxide over Co- and Mn-based perovskite-type oxides

Abstract: Kinetics of NO decomposition have been investigated over La0.8Sr0.2CoO3 and La0.4Sr0.6Mn0.8Ni0.2O3 perovskite-type oxides. The NO decomposition reaction was practically first-order for NO in the range 0.1–0.5 vol.% NO. The activity was decreased by the presence of gaseous oxygen up to 9 vol.% with the reaction order between −0.16 and −0.82 depending on temperature, contact time and catalyst. A reaction mechanism has been proposed in which a pair of adjacent oxide ion vacancies serves as the active site, the reaction starts with the successive adsorption of two NO molecules at the active site, and the release of oxygen is equilibrated with the gas phase. The derived rate equation satisfactorily explains the experimental results.

OH reaction kinetics of gas-phase α- and γ-hexachlorocyclohexane and hexachlorobenzene

Abstract: Rate constants for the gas-phase reactions of the hydroxyl radical (OH) with α- and γ-hexachlorocyclohexane (α- and γ-HCH) and hexachlorobenzene (HCB) were measured over the temperature range 346−386 K. The experiments were carried out in He diluent gas at approximately 1 atm, in a 160-mL quartz chamber. OH was produced by the photolysis of ozone in the presence of H2O, and reactants in the chamber were monitored by online mass spectrometry. The rate constants measured at elevated tem peratures were extrapolated by the Arrhenius equation, and OH reaction rate constants at 298 K were estimated for each compound. These rate constants at 298 K (in units of 10-13 cm3 s-1) were α-HCH, 1.4; γ-HCH, 1.9; and HCB, 0.27. Atmospheric lifetimes based on OH reactions (τOH) were also estimated (in units of days): α-HCH, 120; γ-HCH, 96; and HCB, 940. These relatively high atmospheric lifetimes indicate that these compounds can be transported great distances through the atmosphere before removal processes associated wit...

Kinetics of hydrolysis of PET powder in nitric acid by a modified shrinking-core model

Abstract: Poly(ethylene terephthalate) (PET) powder from waste bottles was degraded at atmospheric pressure in 7−13 M nitric acid at 70−100 °C for 72 h, to clarify the mechanism of a feed stock recycling process. Terephthalic acid (TPA) and ethylene glycol (EG) were produced by the acid-catalyzed heterogeneous hydrolysis of PET in nitric acid, and the resulting EG was simultaneously oxidized to oxalic acid. The kinetics of the hydrolysis of PET in nitric acid could be explained by a modified shrinking core model of chemical reaction control, in which the effective surface area is proportional to the degree of unreacted PET, (1 − X), affected by the deposition of the product TPA. The apparent rate constant was inversely proportional to particle size and to the concentration of the nitric acid. The activation energy of the reaction was 101.3 kJ/mol.

Mechanism and Kinetics of Iodide-Mediated Polymerization of Styrene

Abstract: The kinetics of the bulk polymerization of styrene in the presence of a model -polystyryl iodide (Mn 2000, and Mw/Mn 1.26) as mediator and benzoyl peroxide as initiator was studied. The rate of polymerization, Rp, was found to be independent of the iodide concentration, showing that the stationary concentration of polymer radicals, [P*], is determined by the balance of initiation and termination rates, as in the conventional (iodide-free) system. The pseudo-first-order activation rate constant kact of the model iodide was determined as a function of BPO concentration and temperature (5080 C) by both the GPC curve-resolution and polydispersity-analysis methods. The results showed that kact is directly proportional to [P*], which means that degenerative transfer (active species-exchanging transfer) is the only important mechanism of activation in this system. The activation energy for the transfer rate constant kex was found to be 27.8 kJ mol-1, somewhat smaller than the known activation energy for the styr...