Showing papers on "Reaction rate constant published in 2006"
TL;DR: A rate equation for the degradation based on Langmuir-Hinshelwood (L-H) model has been proposed and results show that the adsorption constant and rate constant in L-H model are dependent to the light intensity, and increase with increasing the lightintensity.
784 citations
TL;DR: In this paper, the effect of initial nitrogen and phosphorus concentrations on nutrient removal performance of microalgae Chlorella vulgaris and to determine biokinetic coefficients such as k ; reaction rate constant, K m, half saturation constant, and Y, yield coefficient by using Michaelis-Menten rate expression.
696 citations
TL;DR: The most common thiol catalysts used to date have been either a mixture of thiophenol/benzyl mercaptan, or the alkanethiol MESNA.
Abstract: Native chemical ligation of unprotected peptide segments involves reaction between a peptide-α-thioester and a cysteine-peptide, to yield a product with a native amide bond at the ligation site. Peptide-α-thioalkyl esters are commonly used because of their ease of preparation. These thioalkyl esters are rather unreactive so the ligation reaction is catalyzed by in situ transthioesterification with thiol additives. The most common thiol catalysts used to date have been either a mixture of thiophenol/benzyl mercaptan, or the alkanethiol MESNA. Despite the use of these thiol catalysts, ligation reactions typically take 24−48 h. To gain insight into the mechanism of native chemical ligaton and in order to find a better catalyst, we investigated the use of a number of thiol compounds. Substituted thiophenols with pKa > 6 were found to best combine the ability to exchange rapidly and completely with thioalkyl esters, and to then act as effective leaving groups in reaction of the peptide-thioester with the thiol...
542 citations
TL;DR: Results indicate that many antibacterial compounds will be oxidized in wastewater via moiety-specific reactions with ozone, including fluoroquinolones and lincomycin via slower reactions.
Abstract: Ozone and hydroxyl radical (•OH) reaction kinetics were measured for 14 antibacterial compounds from nine structural families, to determine whether municipal wastewater ozonation is likely to result in selective oxidation of these compounds' biochemically essential moieties. Each substrate is oxidized by ozone with an apparent second-order rate constant, > 1 × 103 M-1 s-1, at pH 7, with the exception of N(4)-acetylsulfamethoxazole ( is 2.5 × 102 M-1 s-1). values (pH 7) for macrolides, sulfamethoxazole, trimethoprim, tetracycline, vancomycin, and amikacin appear to correspond directly to oxidation of biochemically essential moieties. Initial reactions of ozone with N(4)-acetylsulfamethoxazole, fluoroquinolones, lincomycin, and β-lactams do not lead to appreciable oxidation of biochemically essential moieties. However, ozone oxidizes these moieties within fluoroquinolones and lincomycin via slower reactions. Measured values and second-order •OH rate constants, , were utilized to characterize pollutant losse...
436 citations
TL;DR: The TiO2/AC was shown high photoactivity for the photodegradation of methyl orange (MO) dyestuff in aqueous solution under UV irradiation and the kinetics of photocatalytic MO dyestUFF degradation was found to follow a pseudo-first-order rate law.
432 citations
TL;DR: Electrochemical measurements have been used to construct well-characterized, covalently modified monolayers that can be employed as functional electrode surfaces that react rapidly and quantitatively with terminal acetylenes forming 1,2,3-triazoles, via "click" chemistry.
Abstract: We have prepared and characterized mixed self-assembled monolayers (SAM) on gold electrodes from azido alkane thiols and various ω-functionalized alkane thiols In the presence of copper(I) catalysts, these azide-modified surfaces are shown to react rapidly and quantitatively with terminal acetylenes forming 1,2,3-triazoles, via “click” chemistry The initial azide substituents can be identified and monitored using both grazing-angle infrared (IR) and X-ray photoelectron spectrosopies Acetylenes possessing redox-active ferrocene substituents react with the azide-terminated mixed SAMs and electrochemical measurements of the ferrocene-modified SAM electrodes have been used to quantify the redox centers attached to these platforms Time-resolved electrochemical measurements have enabled us to follow the formation of these ferrocene centers and thus to measure the rate of the surface “click” reaction Under optimal conditions this well-behaved second-order reaction takes place with a rate constant of 1 × 103
344 citations
TL;DR: A time-gated luminescence imaging technique that can fully eliminate the short-lived background fluorescence from TMPyP and cell components has been successfully developed for monitoring the time-dependent generation of (1)O2 in living cells.
Abstract: A new europium(III) complex, [4‘-(10-methyl-9-anthryl)-2,2‘:6‘,2‘ ‘-terpyridine-6,6‘ ‘-diyl]bis(methylenenitrilo) tetrakis(acetate)−Eu3+, was designed and synthesized as a highly sensitive and selective time-gated luminescence probe for singlet oxygen (1O2). The new probe is highly water soluble with a large stability constant of ∼1021 and a wide pH available range (pH 3−10), and can specifically react with 1O2 to form its endoperoxide (EP-MTTA−Eu3+) with a high reaction rate constant at 1010 M-1 s-1, accompanied by the remarkable increases of luminescence quantum yield from 0.90% to 13.8% and lifetime from 0.80 to 1.29 ms, respectively. The wide applicability of the probe was demonstrated by detection of 1O2 generated from a MoO42-/H2O2 system, a photosensitization system of 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (TMPyP), and a horseradish peroxidase catalyzed aerobic oxidation system of indole-3-acetic acid (IAA). In addition, it was found that the new probe could b...
335 citations
TL;DR: In this article, the authors investigated solid-state self-quenching processes of highly efficient Ir(III) phosphorescent emitters by measuring thin film photoluminescence quantum efficiency and transient lifetime as a function of doping concentration in host matrix.
Abstract: Solid-state self-quenching processes of highly efficient Ir(III) phosphorescent emitters are investigated by the measurement of thin film photoluminescence quantum efficiency and transient lifetime as a function of doping concentration in a host matrix. The radiative decay rate constant is found to be independent from the average distance between dopant molecules (R), and the concentration-quenching rate constant is shown to be dependent on R(-6). The quenching dependence on R strongly suggests that luminescent concentration quenching in a phosphorescent Ir(III) complex:host film is controlled by dipole-dipole deactivating interactions as described by the Forster energy transfer model.
331 citations
TL;DR: The kinetics of several fast heterogeneous electron-transfer reactions were investigated by steady-state voltammetry at nanoelectrodes and scanning electrochemical microscopy (SECM), and the correlation between the heterogeneous and self-exchange rate constants is discussed.
Abstract: The kinetics of several fast heterogeneous electron-transfer reactions were investigated by steady-state voltammetry at nanoelectrodes and scanning electrochemical microscopy (SECM). The disk-type, polished Pt nanoelectrodes (3.7−400-nm radius) were characterized by a combination of voltammetry, scanning electron microscopy, and SECM. A number of experimental curves were obtained at the same nanoelectrode to attain the accuracy and reproducibility similar to those reported previously for micrometer-sized probes. A new analytical approximation was developed and used for analysis of steady-state tip voltammograms. The self-consistent kinetic parameter values with the uncertainty margin of ∼10% were obtained for electrodes of different radii and for a wide range of the SECM tip/substrate separation distances. The determined standard rate constants are compared to those previously measured at the electrodes of different dimensions, and the correlation between the heterogeneous and self-exchange rate constants...
330 citations
TL;DR: In this article, the decomposition of LiPF6 electrolytes with water was studied by measuring the water content after storage, and the experimental results were in good agreement with −d[H2O]/dt −=k[H 2O]2[LiPF6].
325 citations
TL;DR: The kinetics of oxidation of 24 contaminants-many for which data were not previously available were measured, finding the kinetics to be first-order with respect to both contaminant and MnO4- concentrations.
Abstract: To provide a more complete understanding of the kinetics of in situ chemical oxidation (ISCO) with permanganate (MnO4-), we measured the kinetics of oxidation of 24 contaminants-many for which data were not previously available. The new data reported here were determined using an efficient method based on continuous measurement of the MnO4- concentration by absorbance spectrometry. Under these conditions, the kinetics were found to be first-order with respect to both contaminant and MnO4- concentrations, from which second-order rate constants (k") were readily obtained. Emerging contaminants forwhich k" was determined (at 25 degrees C and pH 7) include 1,4-dioxane (4.2 x 10(-5) M(-1) s(-1)), methyl t-butyl ether (MTBE) (1.0 x 10(-4) M(-1) s(-1)), and methyl ethyl ketone (MEK) (9.1 x 10(-5) M(-1) s(-1)). Contaminants such as 2,4,6-trinitrotoluene (TNT), the pesticides aldicarb and dichlorvos, and many substituted phenols are oxidized with rate constants comparable to tetrachloroethene (PCE) and trichloroethene (TCE) (i.e., 0.03-1 M(-1) s(-1)) and therefore are good candidates for remediation with MnO4- in the field. There are several--sometimes competing--mechanisms by which MnO4- oxidizes contaminants, including addition to double bonds, abstraction of hydrogen or hydride, and electron transfer.
TL;DR: To investigate the primary process of photocatalytic oxidation of TiO2, interfacial charge-transfer reaction of trapped holes formed in nanocrystallineTiO2 films by UV irradiation was directly measured by highly sensitive femtosecond and nanosecond transient absorption spectroscopy under low intensity excitation condition to avoid fast electron-hole recombination.
Abstract: To investigate the primary process of photocatalytic oxidation of TiO2, interfacial charge-transfer reaction of trapped holes formed in nanocrystalline TiO2 films by UV irradiation was directly measured by highly sensitive femtosecond and nanosecond transient absorption spectroscopy under low intensity excitation condition to avoid fast electron−hole recombination. Accordingly, the rates and yields of photocatalytic oxidation of several alcohols adsorbed on TiO2 were evaluated successfully.
TL;DR: Simple modeling exercises suggest that reaction with OH radicals will dominate removal of perfluoroalkanesulfonamides from the gas phase (wet and dry deposition will not be important) and that the atmospheric lifetime of NEtFBSA in theGas phase will be 20-50 days, thus allowing substantial long-range atmospheric transport.
Abstract: Perfluorooctanesulfonamides [C8F17SO2N(R1)(R2)] are present in the atmosphere and may, via atmospheric transport and oxidation, contribute to perfluorocarboxylates (PFCA) and perfluorooctanesulfonate (PFOS) pollution in remote locations. Smog chamber experiments with the perfluorobutanesulfonyl analogue N-ethyl perfluorobutanesulfonamide [NEtFBSA; C4F9SO2N(H)CH2CH3] were performed to assess this possibility. By use of relative rate methods, rate constants for reactions of NEtFBSA with chlorine atoms (296 K) and OH radicals (301 K) were determined to be kCL) = (8.37 +/- 1.44) x 10(-12) and kOH = (3.74 +/- 0.77) x 10(-13) cm3 molecule(-1) s(-1), indicating OH reactions will be dominant in the troposphere. Simple modeling exercises suggestthat reaction with OH radicals will dominate removal of perfluoroalkanesulfonamides from the gas phase (wet and dry deposition will not be important) and that the atmospheric lifetime of NEtFBSA in the gas phase will be 20-50 days, thus allowing substantial long-range atmospheric transport. Liquid chromatography/tandem mass spectrometry (LC/MS/MS) analysis showed that the primary products of chlorine atom initiated oxidation were the ketone C4F9SO2N(H)COCH3; aldehyde 1, C4F9SO2N(H)CH2CHO; and a product identified as C4F9SO2N(C2H5O)- by high-resolution MS but whose structure remains tentative. Another reaction product, aldehyde 2, C4F9SO2N(H)CHO, was also observed and was presumed to be a secondary oxidation product of aldehyde 1. Perfluorobutanesulfonate was not detected above the level of the blank in any sample; however, three perfluoroalkanecarboxylates (C3F7CO2-, C2F5CO2-, and CF3CO2-) were detected in all samples. Taken together, results suggest a plausible route by which perfluorooctanesulfonamides may serve as atmospheric sources of PFCAs, including perfluorooctanoic acid.
TL;DR: In this paper, the effect of milling time and Nb 2 O 5 content on the magnesium hydrogen sorption reaction was investigated and the rate-limiting step of the absorption and desorption reactions was determined by fitting the respective kinetic equations.
TL;DR: The results indicate that in the palladium-catalyzed process the rate constants of the exchange processes are higher than those of the corresponding hydrolysis reactions.
Abstract: To gain insight on the mechanistic aspects of the palladium-catalyzed hydrolysis of NaBH4 in alkaline media, the kinetics of the reaction has been investigated by 11B NMR (nuclear magnetic resonance) measurements taken at different times during the reaction course. Working with BH4- concentration in the range 0.05−0.1 M and with a [substrate]/[catalyst] molar ratio of 0.03−0.11, hydrolysis has been found to follow a first-order kinetic dependence from concentration of both the substrate and the catalyst (Pd/C 10 wt %). We followed the reaction of NaBH4 and its perdeuterated analogue NaBD4 in H2O, in D2O and H2O/D2O mixtures. When the process was carried out in D2O, deuterium incorporation in BH4- afforded BH4-nDn- (n = 1, 2, 3, 4) species, and a competition between hydrolysis and hydrogen/deuterium exchange processes was observed. By fitting the kinetics NMR data by nonlinear least-squares regression techniques, the rate constants of the elementary steps involved in the palladium-catalyzed borohydride hyd...
TL;DR: In this article, the effect of initial concentration, temperature, shaking rate and pH on the adsorption of reactive yellow 2 (RY2) and reactive black 5 (RB5) by chitin (Sigma C 9213) was investigated.
TL;DR: In this article, the effects of chloride concentration on the rates of decomposition of H 2 O 2 by ferric ion and on the rate of oxidation of an organic solute in homogeneous aqueous solution have been investigated.
Abstract: The effects of chloride concentration on the rates of decomposition of H 2 O 2 by ferric ion and on the rate of oxidation of an organic solute in homogeneous aqueous solution have been investigated. Experiments were carried out in a batch reactor, in the dark, at pH ≤ 3, 25.0 ± 0.5 °C and at controlled ionic strength (≤1 M). The concentrations of chloride ranged from 0 to 1 M ([Fe(III)] 0 = 0.2 or 1 mM, [H 2 O 2 ] 0 = 1, 10 or 50 mM). The spectrophotometric study shows that chloride ions compete with hydrogen peroxide for the complexation of Fe(III) and that H 2 O 2 does not form complexes with iron(III)-chlorocomplexes. The kinetic study showed that the rates of decomposition of H 2 O 2 decreased in the presence of chloride. The measured rates were accurately predicted by a kinetic model which incorporates the formation of iron(II) and iron(III)-chlorocomplexes and reactions involving Cl 2 − radicals. At a fixed pH, the pseudo-first-order rate constants were found to decrease linearly with the molar fraction of Fe(III) complexed with chloride. The kinetic model was also able to predict the rate of oxidation of a probe compound (atrazine) by Fe(III)/H 2 O 2 in the presence of chloride. Computer simulations indicate that Cl 2 − which represents the predominant radical contributes to the oxidation of atrazine.
TL;DR: The chlorophenols photo-degradation kinetics and mechanism in the aquatic environment under UV-vis in the presence of hydroxyl radicals and singlet oxygen is presented and pathways for intermediate reactions e.g. dechlorination, oxidation, dimerization are presented.
TL;DR: Three phenols with pendant, hydrogen-bonded bases (HOAr-B) have been oxidized in MeCN with various one-electron oxidants, among the first detailed analyses of CPET reactions where the proton and electron move to different sites.
Abstract: Three phenols with pendant, hydrogen-bonded bases (HOAr-B) have been oxidized in MeCN with various one-electron oxidants. The bases are a primary amine (-CPh(2)NH(2)), an imidazole, and a pyridine. The product of chemical and quasi-reversible electrochemical oxidations in each case is the phenoxyl radical in which the phenolic proton has transferred to the base, (*)OAr-BH(+), a proton-coupled electron transfer (PCET) process. The redox potentials for these oxidations are lower than for other phenols, predominately from the driving force for proton movement. One-electron oxidation of the phenols occurs by a concerted proton-electron transfer (CPET) mechanism, based on thermochemical arguments, isotope effects, and DeltaDeltaG(++)/DeltaDeltaG degrees . The data rule out stepwise paths involving initial electron transfer to form the phenol radical cations [(*)(+)HOAr-B] or initial proton transfer to give the zwitterions [(-)OAr-BH(+)]. The rate constant for heterogeneous electron transfer from HOAr-NH(2) to a platinum electrode has been derived from electrochemical measurements. For oxidations of HOAr-NH(2), the dependence of the solution rate constants on driving force, on temperature, and on the nature of the oxidant, and the correspondence between the homogeneous and heterogeneous rate constants, are all consistent with the application of adiabatic Marcus theory. The CPET reorganization energies, lambda = 23-56 kcal mol(-)(1), are large in comparison with those for electron transfer reactions of aromatic compounds. The reactions are not highly non-adiabatic, based on minimum values of H(rp) derived from the temperature dependence of the rate constants. These are among the first detailed analyses of CPET reactions where the proton and electron move to different sites.
TL;DR: In this article, rate constant expressions for C1C4 alkyl and alkoxy radicals decomposition via β-scission are recommended based on the reverse exothermic reaction, the addition of a hydrogen atom or an alkyal radical to an olefin or carbonyl species with the decomposition reaction calculated using microscopic reversibility.
Abstract: Rate coefficients for alkyl and alkoxy radical decomposition are important in combustion, biological, and atmospheric processes. In this paper, rate constant expressions for C1C4 alkyl and alkoxy radicals decomposition via β-scission are recommended based on the reverse, exothermic reaction, the addition of a hydrogen atom or an alkyl radical to an olefin or carbonyl species with the decomposition reaction calculated using microscopic reversibility. The rate expressions have been estimated based on a wide-range study of available experimental data. Rate coefficients for hydrogen atom and alkyl radical addition to an olefin show a strong temperature curvature. In addition, it is found that there is a correlation between the activation energy for addition and (i) the type of atom undergoing addition and (ii) whether this radical adds to the internal or terminal carbon atom of the olefin. Rate coefficients for alkoxy radical decomposition show a strong correlation to the ionization potential of the alkyl radical leaving group and on the enthalpy of reaction. It is shown that the activation energy for alkyl radical addition to a carbonyl species can be estimated as a function of the alkyl radical ionization potential and enthalpy of reaction. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 250–275, 2006
TL;DR: It is found that Pd(II)-mediated alcohol oxidation is the turnover-limiting step of the catalytic reaction, and at elevated catalyst loading, the rate of catalytic turnover is limited by the dissolution of oxygen gas into solution.
Abstract: Pd(OAc)2 in DMSO is an effective catalyst for the aerobic oxidation of alcohols and numerous other organic substrates Kinetic studies of the catalytic oxidation of primary and secondary benzylic alcohol substrates provide fundamental insights into the catalytic mechanism In contrast to the conclusion reached in our earlier study (J Am Chem Soc 2002, 124, 766−767), we find that Pd(II)-mediated alcohol oxidation is the turnover-limiting step of the catalytic reaction At elevated catalyst loading, however, the rate of catalytic turnover is limited by the dissolution of oxygen gas into solution This mass-transfer rate is measured directly by using gas-uptake methods, and it correlates with the maximum rate observed during catalysis Initial-rate studies were complemented by kinetic analysis of the full-reaction timecourses at different catalyst concentrations Kinetic fits of these traces reveal the presence of unimolecular and bimolecular catalyst decomposition pathways that compete with productive c
TL;DR: A reaction mechanism is proposed for each pathway, and it is suggested that the dependencies of adsorption and reaction rates upon methyl-group substitution are a result of the substituents' effects on the electrostatic potential and orbitals rather than geometric (steric) effects.
Abstract: The hydrodeoxygenation of methyl-substituted phenols was carried out in a flow microreactor at 300 degrees C and 2.85 MPa hydrogen pressure over a sulfided CoMo/Al(2)O(3) catalyst. The primary reaction products were methyl-substituted benzene, cyclohexene, cyclohexane, and H(2)O. Analysis of the results suggests that two independent reaction paths are operative, one leading to aromatics and the other to partially or completely hydrogenated cyclohexanes. The reaction data were analyzed using Langmuir-Hinshelwood kinetics to extract the values of the reactant-to-catalyst adsorption constant and of the rate constants characterizing the two reaction paths. The adsorption constant was found to be the same for both reactions, suggesting that a single catalytic site center is operative in both reactions. Ab initio electronic structure calculations were used to evaluate the electrostatic potentials and valence orbital ionization potentials for all of the substituted phenol reactants. Correlations were observed between (a) the adsorption constant and the two reaction rate constants measured for various methyl-substitutions and (b) certain moments of the electrostatic potentials and certain orbitals' ionization potentials of the isolated phenol molecules. On the basis of these correlations to intrinsic reactant-molecule properties, a reaction mechanism is proposed for each pathway, and it is suggested that the dependencies of adsorption and reaction rates upon methyl-group substitution are a result of the substituents' effects on the electrostatic potential and orbitals rather than geometric (steric) effects.
TL;DR: In this paper, the second-order kinetic rate constant was found to be 70m3 mol−1 s−1 at a temperature of 298.15K, with an activation temperature of 4.1 × 103 K.
TL;DR: In this article, the adsorption characteristics of hexavalent chromium were studied with an adsorbent developed from waste tamarind hull, which was found to follow a pseudo-first-order rate mechanism and the rate constant was evaluated at 30 ÂC.
TL;DR: It was found that the reaction was spontaneous and exothermic in nature and a model was developed to predict the removal of Pb(II) from an aqueous solution.
TL;DR: In this article, the absorption rate of CO2 was measured in a wetted-wall column in 0.45−3.6 m piperazine (PZ) and 0.1 m potassium carbonate (K2CO3) at 25−110 °C.
Abstract: The absorption rate of CO2 was measured in a wetted-wall column in 0.45−3.6 m piperazine (PZ) and 0.0−3.1 m potassium carbonate (K2CO3) at 25−110 °C. A rigorous kinetic model was used to model the data and interpret diffusivities and rate constants. The rate approaches second-order behavior with PZ and is highly dependent on other strong bases. In 1 M PZ, the overall rate constant is 102 000 s-1, 20 times higher than in monoethanolamine. The activation energy is 35 kJ/mol, similar to other amine−CO2 reactions. Rate constants for contributions of carbonate, PZ carbamate, and water to the rate were determined according to base catalysis theory. The addition of neutral salts to aqueous PZ increases the apparent rate constant. In 2.7 M NaCl/0.6 M PZ, the overall rate constant is increased by a factor of 7. Ionic strength effects were accounted for within the rigorous model of K+/PZ mixtures. The absorption rate in concentrated K+/PZ mixtures is up to 3 times faster than in 30 wt % monoethanolamine. At low tem...
TL;DR: The most important deactivation paths of fluorescence quenching by O2 are again directed by excess energies and charge-transfer interactions similar to triplet-state quench by O3, which leads to lower efficiencies to singlet oxygen generation.
Abstract: This work gives an overview of what is currently known about the mechanisms of the photosensitized production of singlet oxygen. Quenching of pi pi* excited triplet states by O2 proceeds via internal conversion of excited encounter complexes and exciplexes of sensitizer and O2. Both deactivation channels lead with different efficiencies to singlet oxygen generation. The balance between the deactivation channels depends on the triplet-state energy and oxidation potential of the sensitizer, and on the solvent polarity. A model has been developed that reproduces rate constants and efficiencies of the competing processes quantitatively. Sensitization by excited singlet states is much more complex and hence only qualitative rules could be elaborated, despite serious efforts of many groups. However, the most important deactivation paths of fluorescence quenching by O2 are again directed by excess energies and charge-transfer interactions similar to triplet-state quenching by O2. Finally, two recent developments in photosensitization of singlet oxygen are reviewed: Two-photon sensitizers with particular application potential for photodynamic therapy and fluorescence imaging of biological samples and singlet oxygen sensitization by nanocrystalline porous silicon, a material with very different photophysics compared to molecular sensitizers.
TL;DR: In this paper, the authors quantified the rate of ferrihydrite conversion to goethite via the Fe(II) pathway using synchrotron radiation-based energy dispersive X-ray diffraction (ED-XRD).
Abstract: In this study, we quantified the rate of ferrihydrite conversion to goethite via the Fe(II) pathway using synchrotron radiation-based energy dispersive X-ray diffraction (ED-XRD). Ferrihydrite transformation experiments were conducted in oxygen-free solutions at neutral pH with synthetic 2-line ferrihydrite reacting with 100 mM Fe(II). The kinetics of goethite crystallization was measured in situ at temperatures ranging from 21 to 90 ??C. The results showed that in the presence of ferrous iron, the transformation of poorly ordered ferrihydrite into crystalline goethite is rapid and highly dependent on temperature. The time-resolved peak area data fitted using a Johnson-Mehl-Avrami-Kolmogorov (JMAK) kinetic model yielded rate constants of 4.0 ?? 10 -5, 1.3 ?? 10 -4, 3.3 ?? 10 -4, 2.27 ?? 10 -3, and 3.14 ?? 10 -3 l/s at reaction temperatures of 21, 45, 60, 85, and 90 ??C respectively. The activation energy for the transformation was determined to be 56 ?? 4 kJ/mol. Comparison with the activation energy predicted for the phase conversion in the absence of ferrous iron indicates that Fe(II) acts as a catalyst that decreases the activation energy barrier by approximately 38 kJ/mol. The kinetic parameters derived from the experimental data suggest that goethite crystallization is controlled by a 1-D phase boundary growth mechanism with a constant nucleation rate occurring during the reaction.
TL;DR: In this article, the authors investigated the reaction of polycyclic aromatic hydrocarbons (PAHs) with NO2 and OH radicals in diesel particulate exhaust using a fast flow reactor.
TL;DR: It is concluded that there is a negligible difference in the surface electronic properties of these Pt/CB catalysts due to size variations and therefore, the ORR activities are not affected by the differences in the particle size.
Abstract: Oxygen reduction reaction (ORR) measurements and 195Pt electrochemical nuclear magnetic resonance (EC-NMR) spectroscopy were combined to study a series of carbon-supported platinum nanoparticle electrocatalysts (Pt/CB) with average diameters in the range of roughly 1–5 nm. ORR rate constants and H2O2 yields evaluated from hydrodynamic voltammograms did not show any particle size dependency. The apparent activation energy of 37 kJ mol−1, obtained for the ORR rate constant, was identical to that obtained for bulk platinum electrodes. Pt/CB catalysts on Nafion produced only 0.7–1% of H2O2, confirming that the direct four-electron reduction of O2 to H2O is the predominant reaction. NMR spectral features showed characteristic size dependence, and the line shapes were reproduced by using the layer-deconvolution model. Namely, the variations in the NMR spectra with particle size can be explained as due to the combined effect of the layer-by-layer variation of the s-type and d-type local density of states. However, the surface peak position of 195Pt NMR spectra and the spin–lattice relaxation time of surface platinum atoms showed practically no change with the particle size variation. We conclude that there is a negligible difference in the surface electronic properties of these Pt/CB catalysts due to size variations and therefore, the ORR activities are not affected by the differences in the particle size.