Showing papers on "Reaction rate constant published in 2000"

Critically evaluated rate coefficients for free-radical polymerization, 2.. Propagation rate coefficients for methyl methacrylate

Abstract: Propagation rate coefficients, k(P), for free-radical polymerization of butyl acrylate (BA) previously reported by several groups are critically evaluated. All data were determined by the combination of pulsed-laser polymerization (PLP) and subsequent polymer analysis by size exclusion (SEC) chromatography. The PLP-SEC technique has been recommended as the method of choice for the determination of k(P) by the IUPAC Working Party on Modeling of Polymerization Kinetics and Processes. Application of the technique to acrylates has proven to be very difficult and, along with other experimental evidence, has led to the conclusion that acrylate chain-growth kinetics are complicated by intramolecular transfer (backbiting) events to form a mid-chain radical structure of lower reactivity. These mechanisms have a significant effect on acrylate polymerization rate even at low temperatures, and have limited the PLP-SEC determination of k(P) of chain-end radicals to low temperatures (<20 degreesC) using high pulse repetition rates. Nonetheless, the values for BA from six different laboratories, determined at ambient pressure in the temperature range of -65 to 20 degreesC mostly for bulk monomer with few data in solution, fulfill consistency criteria and show excellent agreement, and are therefore combined together into a benchmark data set. The data are fitted well by an Arrhenius relation resulting in a pre-exponential factor of 2.21 x 10(7) L (.) mol(-1) (.) s(-1) and an activation energy of 17.9 kJ (.) mol(-1). It must be emphasized that these PLP-determined k(P) values are for monomer addition to a chain-end radical and that, even at low temperatures, it is necessary to consider the presence of two radical structures that have very different reactivity. Studies for other alkyl acrylates do not provide sufficient results to construct benchmark data sets, but indicate that the family behavior previously documented for alkyl methacrylates also holds true within the alkyl acrylate family of monomers. [GRAPHICS] Arrhenius plot of propagation rate coefficients, k(P), for BA as measured by PLP-SEC.

Equilibria, Kinetics, and Mechanism in the Bicarbonate Activation of Hydrogen Peroxide: Oxidation of Sulfides by Peroxymonocarbonate

Abstract: Bicarbonate ion is an effective activator for hydrogen peroxide in the oxidation of sulfides. Kinetic and spectroscopic results support the formation of peroxymonocarbonate ion (HCO4-) as the oxidant in the catalytic reactions. The reaction of hydrogen peroxide and bicarbonate to form HCO4- occurs rapidly at 25 °C (t1/2 ≈ 300 s) near neutral pH in aqueous solution and alcohol/water mixtures, and an equilibrium analysis of the reaction by 13C NMR leads to an estimate of the electrode potential for the HCO4-/HCO3- couple (1.8 V vs NHE). Solubility of the bicarbonate catalyst is enhanced by the use of NH4HCO3 rather than by the use of group 1 salts, which tend to have lower solubility in the mixed solvents and can lead to phase separation. Rate laws and mechanistic analyses are presented for the oxidation of ethylphenylsulfide and related sulfides. The second-order rate constants for sulfide oxidations by HCO4- are ∼300-fold greater than those for H2O2, and this increase is consistent with expectations based...

Mechanism and Kinetics of the OH-Radical Intervention during Fenton Oxidation in the Presence of a Significant Amount of Radical Scavenger (Cl-)

Abstract: Orange II, the 4-(2-hydroxy-1-naphthylazo)benzenesulfonic acid Na-salt, was taken as a model for the oxidation of organic compounds in photoassisted Fenton process in the presence of Cl--anion in solution. The HO• radicals seem to originate due to the photolysis of Fe(OH)2+ complexes in solution, whereas the Cl2-• radical was a product of the photolysis of the FeCl2+ complexes. The rate constants for the reaction of HO• and Cl2-• radicals with Or II were determined by laser kinetic spectroscopy: kOH•+OrII = 6.0 × 109(M s)-1 and kCl2•+OrII = 3.7 × 107 (M s)-1. A significant decrease in the rate of decoloration was observed for Orange II upon addition of Cl- (10 mM), but further addition of Cl- only marginally affected the latter reaction rate. Chlorinated hydrocarbons are observed as the products of Orange II oxidation in photoassisted and dark Fenton processes in the presence of Cl- anions. Light irradiation was observed to decrease the amount of chlorinated organic products (AOX) in solution as compared...

Kinetics and Mechanism of Oxidative Dehydrogenation of Propane on Vanadium, Molybdenum, and Tungsten Oxides

Abstract: The effect of cation identity on oxidative dehydrogenation (ODH) pathways was examined using two-dimensional VOx, MoOx, and WOx structures supported on ZrO2. The similar kinetic rate expressions obtained on MoOx and VOx catalysts confirmed that oxidative dehydrogenation of propane occurs via similar pathways, which involve rate-determining C−H bond activation steps using lattice oxygen atoms. The activation energies for propane dehydrogenation and for propene combustion increase in the sequence VOx/ZrO2 < MoOx/ZrO2 < WOx/ZrO2; the corresponding reaction rates decrease in this sequence, suggesting that turnover rates reflect C−H bond cleavage activation energies, which are in turn influenced by the reducibility of these metal oxides. Propane ODH activation energies are higher than for propene combustion. This leads to an increase in maximum alkene yields and in the ratio of rate constants for propane ODH and propene combustion as temperature increases. This difference in activation energy (48−61 kJ/mol) be...

Kinetics of Arsenate Reduction by Dissolved Sulfide

Abstract: Arsenic toxicity and mobility in soil and aquatic environments depends on its speciation, with reducing environments generally leading to more hazardous conditions with respect to this element. Aqueous sulfide (H2S or HS-) is a strong reductant and often occurs at appreciable concentrations in reduced systems. Consequently, it may play an integral part in arsenic redox chemistry. Therefore, reactions between arsenic and sulfide may strongly influence water quality in arsenic-contaminated systems. To evaluate this possibility, we investigated the kinetics and reaction pathways of arsenate with sulfide. Arsenate reduction by hydrogen sulfide is rapid and conforms to a second-order kinetic model, having a rate constant, k = 3.2 × 102 M-1 h-1, that is more than 300 times greater at pH 4 than at pH 7. However, arsenite is not the direct reaction product. Rather, arsenic−sulfide complexes develop, including the formation of a trimeric species (HxAs3S6x-3), that persist in solution for several days, ultimately d...

Proton-Coupled Electron Transfer from Tyrosine in a Tyrosine−Ruthenium−tris-Bipyridine Complex: Comparison with TyrosineZ Oxidation in Photosystem II

Abstract: The pH- and the temperature dependence of the rate constant for electron transfer from tyrosine to ruthenium in Ru(II)(bpy)2(4-Me-4‘CONH-l-tyrosine etyl ester-2,2‘-bpy) 2PF6 was investigated using flash photolysis. At a pH below the tyrosine pKa ≈ 10 the rate constant increased monotonically with pH. This increase was consistent with a concerted electron transfer/deprotonation mechanism. Also indicative of a concerted reaction was the unusually high reorganization energy, 2 eV, extracted from temperature-dependent measurements. Deprotonation of the tyrosine group, at pH > pKa, resulted in a 100-fold increase in rate constant due to a decreased reorganization energy, λ = 0.9 eV. Also, the rate constant became independent of pH. In Mn-depleted photosystem II a similar pH dependence has been found for electron transfer from tyrosineZ (TyrZ) to the oxidized primary donor P680+. On the basis of the kinetic similarities we propose that the mechanisms in the two systems are the same, that is, the electron transf...

Oxidation of Phenols by Triplet Aromatic Ketones in Aqueous Solution

Abstract: Aromatic ketones efficiently mediate the photo-oxidative degradation of phenols in aerated aqueous solution, a process likely to be relevant in sunlit natural waters. Absolute bimolecular rate constants for the quenching of three model ketone triplets by nine phenols bearing various substituents, from electron-donating alkyl and alkoxy groups to the electron-withdrawing cyano group, were measured by nanosecond laser flash photolysis. Triplet benzophenone (BP) is quenched at nearly diffusion-controlled rates (2.6−5.6 × 109 M-1 s-1). Triplet state quenching of 3‘-methoxyacetophenone (3‘-MAP) and 2-acetonaphthone (2-AN) by the same set of phenols occurs more selectively, with rate constants spanning a range of 1 and more than 2 orders of magnitude, respectively. Quenching rate constants obey a Rehm−Weller relationship to the free energy of electron transfer from the phenol to the ketone triplet. By comparison of the quenching constants with overall photo-oxidation rates obtained by stationary irradiation in ...

A Quantum Chemical and Classical Transition State Theory Explanation of Negative Activation Energies in OH Addition To Substituted Ethenes

Abstract: The OH addition to ethene has been modeled using ab initio quantum chemical calculations and classical transition state theory (CTST). The results agree with the hypothesis of Singleton and Cvetanovic (Singleton, D. L.; Cvetanovic, R. J. J. Am. Chem. Soc. 1976, 98, 6812) that the reaction is not elemental, and that it consists of a reversible first step involving the formation of a prereactive complex, followed by the irreversible formation of an addition adduct. The overall rate depends on the rates of two competitive reactions, i.e., the reverse of the first step and the second step, the former being more favored by an increase in temperature than the latter. Applying CTST to the proposed mechanism, we obtain an overall rate constant of 11.7 × 10-12 cm3 molecule-1 s-1, which agrees very well with the experimental results. New results for the activation energies of the OH addition to a series of substituted ethenes have also been obtained assuming that the above mechanism holds. The activation energies w...

Intrinsic kinetics of the gas-solid Fischer-Tropsch and water gas shift reactions over a precipitated iron catalyst

Abstract: The kinetics of the gas–solid Fischer–Tropsch synthesis over a commercial Fe–Cu–K–SiO 2 catalyst was studied in a continuous spinning basket reactor. Experimental conditions were varied as follows: reactor pressure of 0.8–4.0 MPa, H 2 /CO feed ratio of 0.25–4.0, and space velocity of 0.5–2.0×10 −3 Nm 3 kg cat −1 s −1 at a constant temperature of 523 K. A number of Langmuir–Hinshelwood–Hougen–Watson type rate equations were derived on the basis of a detailed set of possible reaction mechanisms originating from the carbide mechanism for the hydrocarbon formation and the formate mechanism for the water gas shift reaction, respectively. 14 models for the Fischer–Tropsch reaction rate and two water gas shift reaction rate models were fitted to the experimental reaction rates. Bartlett’s test was used to reduce the set of Fischer–Tropsch rate equations to 3 models, which were statistically indistinguishable. It could be concluded that the reaction rate of the Fischer–Tropsch synthesis is controlled by the formation of the monomer species (methylene) by hydrogenation of associatively adsorbed CO, whereas the carbon dioxide formation rate (water gas shift) is determined by the formation of a formate intermediate species from adsorbed CO and dissociated hydrogen. Simulations using the optimal kinetic models derived showed good agreement both with experimental data and with some kinetic models from literature.

Reactions of ethanol over metal oxides

Abstract: The reaction of ethanol over a series of oxides (Fe2O3, Fe2O3/CaO, Fe3O4, TiO2, CaO, and SiO2) has been investigated. The main reaction product in all cases is acetaldehyde, with secondary products acetone and ethyl acetate. At 473 K, the rate constant k decreases according to the series Fe2O3>Fe3O4>CaO>TiO2≫SiO2. Titration of basic sites by CO2 adsorption at room temperature shows that the reaction rate can be successfully normalised by basic site density for oxides that adsorb CO2 except for CaO. Acetone production, presumably via acetate ketonization, is highest over TiO2 while ethyl acetate formation, by Tishchenko reaction, is highest over Fe2O3.

Full dimensional quantum calculations of the CH4+H→CH3+H2 reaction rate

Abstract: Accurate full-dimensional quantum mechanical calculations are reported for the CH4+H→CH3+H2 reaction employing the Jordan–Gilbert potential energy surface. Benchmark results for the thermal rate constant and the cumulative reaction probability are presented and compared to classical transition state theory as well as reduced dimensionality quantum scattering calculations. The importance of quantum effects in this system is highlighted.

Hydroisomerization and hydrocracking of long chain n-alkanes on Pt/amorphous SiO2–Al2O3 catalyst

Abstract: The hydroisomerization and hydrocracking of n-hexadecane, n-octacosane and n-hexatriacontane on a 0.3% platinum/amorphous silica–alumina (MSA/E) catalyst was investigated in a stirred microautoclave at 345, 360 and 380°C and between 2 and 13.1 MPa hydrogen pressure. For each n-paraffin, the reaction pathway and the kinetic parameters were determined. The results were used to elucidate the effect of chain length and operating conditions on isomerization and cracking selectivity. The conversion of the n-paraffins lead to the formation of a mixture of the respective isomers, as the main product, together with cracking products. At every temperature, the iso-alkane/n-alkane ratio of cracking products increased considerably with increasing conversion degree. At the same conversion level, higher reaction temperatures lead to cracking products characterized by a lower iso-alkane/n-alkane ratio. The conversion rate constants showed a considerable increase between n-C16 and n-C28, whereas a slight decrease between n-C28 and n-C36 was observed. The hydroisomerization selectivities showed a decrease as a function of chain length and with increasing conversion levels. The increase in reaction temperature leads to a small decrease in the isomerization selectivities only at low-medium conversion degrees and at the highest temperature investigated, while the effect of this parameter on the maximum yields achievable in iso-C16, iso-C28 and iso-C36 was negligible. The results indicate that the conversion of the n-paraffins follows a first-order kinetic in hydrocarbon while the order in hydrogen pressure was −1.1 ± 0.21 for n-C16 and −0.66 ± 0.15 for n-C28. Furthermore, an increase in hydroisomerization selectivity at higher hydrogen pressure for n-C28 conversion was observed.

Continuous-mode photocatalytic degradation of chlorinated phenols and pesticides in water using a bench-scale TiO2 rotating disk reactor

Abstract: Photocatalytic degradation of phenol, chlorinated phenols, and lindane was evaluated in a continuous flow TiO 2 rotating disk photocatalytic reactor (RDPR). The RDPR operated at a hydraulic residence time of 0.25 day and at a disk angular velocity of 12 rpm. At low molar feed concentrations (0.038 mmol/l), the removal efficiencies for phenol and chlorinated phenols were in the order of 86% or higher, whereas the removal efficiency for lindane at a feed concentration of 0.016 mmol/l was in the order of 63%. For 2,4,6-trichlorophenol (TCP), an increase in the molar influent concentration resulted in a decrease in removal efficiency but in an increase in removal rate. The degradation rate of 2,4,6-TCP followed a saturation type dependancy with the effluent concentration, suggesting a Langmuir–Hinshelwood (L–H) reaction rate equation. A L–H equation was employed to determine the reaction rate constant and the adsorption coefficient for 2,4,6-TCP. The photonic efficiency increased from 0.68% at an influent concentration of 0.13 mmol/l to 2.06% at an influent concentration of 1.0 mmol/l.

Hydrolysis of Isocyanic Acid on SCR Catalysts

Abstract: The hydrolysis of isocyanic acid (HNCO) was investigated on various oxidic catalysts, including TiO2, V2O5/TiO2, and V2O5−WO3/TiO2. The kinetics were studied using powdered samples in a microreactor. The conversions of HNCO are high even at the high space velocities (106 h-1) and low temperatures (150 °C) used in the tests. The highest rate for the hydrolysis of HNCO was found on pure TiO2 powder, while the addition of vanadia and tungsta decreased the activity slightly. To compare the rate of HNCO hydrolysis with the rate of the SCR reaction, uncorrected first-order rate constants were calculated for both reactions using the integral equation for a plug flow reactor. For a catalyst sample containing both WO3 and V2O5, the rate constant for hydrolysis of HNCO at low temperatures is about 2 orders of magnitude higher than the respective rate constant for the SCR reaction, whereas at high temperatures, the rate constants are of comparable magnitude. The apparent activation energy of the hydrolysis reaction ...

Kinetics and Mechanisms for TCE Oxidation by Permanganate

Abstract: The oxidation of trichloroethylene (TCE) by permanganate proceeds in three sequential reaction steps. In the initial step, a cyclic hypomanganate ester is formed via an activated organometallic complex. The activation parameters for this step were determined to be Ea = 41.46 kJ/mol, ΔH⧧ = 39 kJ/mol, and ΔS⧧ = −14 J/mol. The initial reaction is a rate-limiting step (second-order rate constant k1p = 0.65−0.68 M-1 s-1 at 21 °C) and independent of pH. In the second step, the decomposition of the cyclic ester with complete chlorine liberation proceeds quickly via various reaction pathways to form four carboxylic acids. Approximately 77% of the TCE was transformed to formic acid at pH 4, while 95−97% of the TCE was transformed to oxalic and glyoxylic acids at pH values of 6−8. Kinetic data suggest that the decomposition rate of the cyclic ester is at least 100 times higher than its formation rate. In the final step, all carboxylic acids are oxidized by permanganate to the final product, CO2. Second-order rate c...

Chemical Activity of Cholesterol in Membranes

Abstract: Measurements are reported for the rate constants for the release of cholesterol (and dihydrocholesterol) to β-cyclodextrin from mixtures with phospholipids in homogeneous monolayers at constant pressure at the air−water interface. In each mixture, it is found that the release rate shows a sharp decrease as the cholesterol concentration in the monolayer decreases through a composition corresponding to the stoichiometry of a cholesterol−phospholipid complex. The stoichiometry of the complex was established previously by the position of a sharp cusp in the thermodynamic phase diagram of each mixture and also by a minimum in average molecular area versus composition measurements. A theoretical model used earlier to account for the phase diagrams predicts the chemical potential and chemical activity of cholesterol in these mixtures. The calculated chemical activity also shows a sharp change at the complex stoichiometry in homogeneous monolayers. The similarities in change of observed release rate and calculate...

Variation of Langmuir adsorption constant determined for TiO2-photocatalyzed degradation of acetophenone under different light intensity

Abstract: The Langmuir–Hinshelwood (L–H) kinetic model has been used to describe semiconductor photocatalysis. In this report, the L–H rate constant (kL–H)and the Langmuir adsorption constant (K) have been determined under different light intensity for the photocatalytic degradation of poorly adsorbed acetophenone over TiO2 of Degussa P25 in aqueous medium (pH 6.2). The result shows that K decreases when the irradiation is performed at higher light intensity, while kL–H increases expectedly. It is also demonstrated that the initial time interval selected for the initial rate calculation is quite critical to the final determination for the constants.

Mechanisms and kinetics of living radical polymerizations

Abstract: The polymerization rates and activation processes of several variants of living radical polymerization (LRP) are discussed on the basis of recent experimental and theoretical results. Because of bimolecular termination, which is inevitable in LRP as well as in conventional radical polymerization, the time-conversion curves of LRP have several characteristic features depending on the experimental conditions, such as the presence or absence of conventional initiation. Despite the presence of termination (and initiation, in some cases), polymers obtained by LRP can have a low polydispersity, provided that the number of terminated chains is small compared to the number of potentially active chains. A large rate constant of activation, kact, is another fundamental requisite for low polydispersities. Systematic experimental investigation into kact has clarified the exact mechanisms of activation in several LRP systems. The magnitudes of kact was found to largely differ from system to system.

The spin-conserved reaction CH+N2→H+NCN: A major pathway to prompt no studied by quantum/statistical theory calculations and kinetic modeling of rate constant

Abstract: A new spin-conserved path for the CH(2H)+N2 reaction at temperatures relevant to prompt NO formation has been theoretically investigated by means of ab initio MO calculations at the G2M level of theory. The result of the calculation reveals that the CH(2H)+N2 reaction takes place primarily via the ground electronic doublet potential energy surface, producing H+NCN instead of the commonly assumed, spin-forbidden HCN+N(4S) products. The overall rate constant for NCN production has been computed by a multichamel canonical variational Rice-Ramsperger-Kassel-Marcus theory calculation for the temperature range 1500–4000 K at 0.5–2 atm pressure: k3=2.22×107 T1.48 exp (−11760/T) cm3/(mol·s). The theoretically predicted rate constant was found to be in good agreement with high-temperature shock tube data kinetically modeled with the new mechahism that includes NCN reactions. In addition, k, was also found to be consistent with the apparent rate constants previously modeled for prompt NO formation in several flamer studies.

The reactions of ozone with tertiary amines including the complexing agents nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid (EDTA) in aqueous solution

Abstract: Using the stopped-flow technique, the rate constants of the reaction of ozone with a number of amines have been determined. While the protonated amines do not react with ozone, the free amines react with rate constants of around 106 dm3 mol−1 s−1 in the case of tertiary and secondary amines, while primary amines react more slowly. Mono-protonated EDTA reacts only with k=1.6×105 and mono-protonated 1,4-diazabicyclo[2.2.2]octane (DABCO) with k = 3.5 × 103 dm3 mol−1 s−1. In aqueous solution, tertiary amines react with ozone mainly by forming the aminoxide and singlet dioxygen [O2(1Δg)] and to a lesser extent the secondary amine and the corresponding aldehyde, a reaction which can be partially suppressed by tert-butyl alcohol. These data suggest that O-transfer [aminoxide plus O2(1Δg)] is in competition with an electron transfer which leads to the amine radical cation and an ozonide radical. In water, the latter gives rise to ˙OH which further reacts with the amine (and ozone). The amine radical cation deprotonates at a neighbouring carbon. The resulting radical adds dioxygen. Subsequent elimination of O2˙− and hydrolysis of the Schiff-base thus formed leads to the secondary amine and the corresponding aldehyde. In its reaction with ozone, O2˙− yields further ˙OH. Their reaction with the amines leads to the same intermediate as the free-radical pathway of ozone does, i.e. induces a chain reaction. This is interfered with by tert-butyl alcohol at the OH-radical stage. When complexed to Fe(III), EDTA reacts only very slowly with ozone (k = 330 dm3 mol−1 s−1). This explains why EDTA is not readily removed by ozonation in drinking-water processing.

Interaction of Urea with a Hydroxide-Bridged Dinuclear Nickel Center: An Alternative Model for the Mechanism of Urease

Abstract: A hydroxide-bridged dinuclear nickel complex with a urea molecule linking the two metal ions through its carbonyl oxygen atom has been prepared as a model for the metalloenzyme urease. This complex, [Ni2(μ-OH)(μ-urea)(bdptz)(urea)(CH3CN)](ClO4)3, where bdptz is the dinucleating ligand 1,4-bis(2,2‘-dipyridylmethyl)phthalazine, effects the hydrolysis of urea upon heating in a two-step reaction. In the first step, a molecule of ammonia is eliminated from urea with concomitant production of cyanate, the first-order rate constant in acetonitrile being (7.7 ± 0.5) × 10-4 h-1. This reaction is at least 500 times faster than the spontaneous decomposition of urea under the same conditions. When the cyanate-containing product is further heated in the presence of water, the cyanate is hydrolyzed with a second-order rate constant of (9.5 ± 1) × 10-4 M-1 h-1. Reaction of [Ni2(μ-OH)(μ-urea)(bdptz)(urea)(CH3CN)](ClO4)3 in 50% aqueous acetonitrile afforded ammonia with no appreciable buildup of the cyanate-containing spe...