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Showing papers on "Reaction rate constant published in 2001"


Journal ArticleDOI
01 Apr 2001-Fuel
TL;DR: In this paper, a kinetic study in free catalyst transesterification of rapeseed oil was made in subcritical and supercritical methanol under different reaction conditions of temperatures and reaction times.

793 citations


Journal ArticleDOI
TL;DR: In this article, a diffusion equation was formulated by assuming that an inflammable gas (target gas) moves inside the film by Knudsen diffusion, while it reacts with the adsorbed oxygen following a first-order reaction kinetic.
Abstract: Influences of gas transport phenomena on the sensitivity of a thin film semiconductor gas sensor were investigated theoretically. A diffusion equation was formulated by assuming that an inflammable gas (target gas) moves inside the film by Knudsen diffusion, while it reacts with the adsorbed oxygen following a first-order reaction kinetic. By solving this equation under steady-state conditions, the target gas concentration inside the film was derived as a function of depth (x) from the film surface, Knudsen diffusion coefficient (DK), rate constant (k) and film thickness (L). The gas concentration profile thus obtained allowed to estimate the gas sensitivity (S) defined as the resistance ratio (Ra/Rg), under the assumption that the sheet conductance of the film at depth x is linear to the gas concentration there with a proportionality constant (sensitivity coefficient), a. The derived equation shows that S decreases sigmoidally down to unity with an increase in L k/D K . Further by assuming that the temperature dependence of rate constant (k) and sensitivity coefficient (a) follows Arrenius type ones with respective activation energies, it was possible to derive a general expression of S involving temperature (T). The expression shows that, when the activation energies are selected properly, the S versus T correlation results in a volcano-shaped one, its height increasing with decreasing L. The dependence of S on L at constant T as well as on T at constant L can thus be simulated fairly well based on the equation.

550 citations


Journal ArticleDOI
TL;DR: In this article, aqueous TiO2 dispersions under UV illumination was examined to assess the influence of temperature, pH, concentration of dissolved oxygen (DOC), initial concentration of MB, and light intensity on the kinetics of decomposition.
Abstract: Methylene blue (MB) is a representative of a class of dyestuffs resistant to biodegradation Its decomposition was examined in aqueous TiO2 dispersions under UV illumination to assess the influence of temperature, pH, concentration of dissolved oxygen (DOC), initial concentration of MB, and light intensity on the kinetics of decomposition Hypsochromic effects (ie blue shifts of spectral bands) resulting from N-demethylation of the dimethylamino group in MB occurs concomitantly with oxidative degradation The maximum quantity of MB adsorbed on TiO2, and the kinetics of degradation of MB and of total organic carbon (TOC) removal were also measured at constant pH 4 Photobleaching of MB solutions takes place at low DOCs and is caused by a reversible reductive process involving photogenerated electrons on TiO2 The rate of degradation of MB remains fairly constant regardless of whether the dispersion was purged with oxygen prior to irradiation or with air during the light irradiation period The photocatalytic process depends on light intensity, but not on the total light energy absorbed The photoreaction followed pseudo-first-order kinetics even at high MB concentrations (03 mM) The temperature dependence of the photodegradation kinetics was assessed (E a =89 kJ/mol ) , as well as the relative photonic efficiency, ξr, relative to phenol (048)

541 citations


Journal ArticleDOI
TL;DR: In this article, the mechanism of nanoparticle formation at supercritical conditions was discussed based on the metal oxide solubility and kinetics of the hydrothermal synthesis reaction using a flow type apparatus over the range of temperature from 523 to 673 K at 30 MPa.
Abstract: Hydrothermal synthesis of CeO2 and AlO(OH) were conducted using a flow type apparatus over the range of temperature from 523 to 673 K at 30 MPa. Nanosize crystals were formed at supercritical conditions. The mechanism of nanoparticle formation at supercritical conditions was discussed based on the metal oxide solubility and kinetics of the hydrothermal synthesis reaction. The reaction rate of Ce(NO3)3 and Al(NO3)3 was evaluated using a flow type reactor. The Arrhenius plot of the first order rate constant fell on a straight line in the subcritical region, while it deviated from the straight line to the higher values above the critical point. The solubility of Ce(OH)3 and AlO(OH) was estimated by using a modified HKF model in a wide range of pH and temperature. In acidic conditions, where hydrothermal synthesis reaction is concerned, solubility gradually decreased with increasing temperature and then drastically dropped above the critical point. The trend of the solubility and the kinetics around the critical point could be explained by taking account of the dielectric constant effect on the reactions. There are two reasons why nanoparticle are formed at supercritical conditions. Larger particles are produced at subcritical conditions due to Ostwald ripening; that could not be observed in supercritical water because of the extremely low solubility. Second reason is the faster nucleation rate in supercritical water because of the lower solubility and the extremely fast reaction rate.

537 citations


Journal ArticleDOI
Willem H. Koppenol1
TL;DR: The Haber-Weiss reaction was revived by Beauchamp and Fridovich in 1970 to explain the toxicity of superoxide, but this time the oxygen was believed to be in the singlet (1∆g) state, and the reaction was dropped from the scheme of oxygen toxicity, and superoxide became the source of hydrogen peroxide.
Abstract: The chain reactions HO• + H2O2 → H2O + O2•- + H+ and O2•- + H+ + H2O2 → O2 + HO• + H2O, commonly known as the Haber-Weiss cycle, were first mentioned by Haber and Willst?tter in 1931. George showed in 1947 that the second reaction is insignificant in comparison to the fast dismutation of superoxide, and this finding appears to have been accepted by Weiss in 1949. In 1970, the Haber-Weiss reaction was revived by Beauchamp and Fridovich to explain the toxicity of superoxide. During the 1970s various groups determined that the rate constant for this reaction is of the order of 1 M-1s-1 or less, which confirmed George's conclusion. The reaction of superoxide with hydrogen peroxide was dropped from the scheme of oxygen toxicity, and superoxide became the source of hydrogen peroxide, which yields hydroxyl radicals via the Fenton reaction, Fe 2+ + H2O2 → Fe3+ + HO- + HO•. In 1994, Kahn and Kasha resurrected the Haber-Weiss reaction again, but this time the oxygen was believed to be in the singlet (1∆g) s...

435 citations


Journal ArticleDOI
TL;DR: In this paper, transient absorption spectroscopy was employed to study charge transfer dynamics in dye-sensitized nanocrystalline solar cells (DSSC) containing a new one-electron redox mediator, cobalt(II)-bis[2,6-bis(1‘)-butylbenzimidazol-2‘-yl)pyridine] Photovoltaic cells incorporating this relay have yielded light-toelectricity power conversion efficiencies of up to 52%
Abstract: Transient absorption spectroscopy was employed to study charge-transfer dynamics in dye-sensitized nanocrystalline solar cells (DSSC) containing a new one-electron redox mediator, cobalt(II)-bis[2,6-bis(1‘-butylbenzimidazol-2‘-yl)pyridine] Photovoltaic cells incorporating this relay have yielded light-to-electricity power conversion efficiencies of up to 52% This rivals the performance of the tri-iodide/iodide couple that is currently almost exclusively used in DSSC Interception of the dye oxidized state by electron transfer from the Co(II) complex in diluted electrolyte was found to follow a first-order kinetics with a rate constant of km = 5 × 105 s-1 Above a threshold of 10-2 M, under which the cationic relay is essentially adsorbed onto the negatively charged particle surface, larger concentrations of the reduced mediator resulted in a linear increase of the apparent rate, yielding a second-order rate constant of km‘‘ = 29 × 106 M-1 s-1 Dynamics of the recombination reaction between injected co

404 citations


Journal ArticleDOI
TL;DR: In this paper, carbon prepared from peanut husks (PHC) has been used for the adsorption of Pb2+, Zn2+, Ni2+ and Cd2+, over a range of initial metal ion concentration (0.15 mM).

333 citations


Journal ArticleDOI
TL;DR: In this article, the polymerization rate of styrene and methyl methacrylate in the presence of model polymer−dithiocarbonate adducts as mediators and benzoyl peroxide (BPO) as a conventional initiator were kinetically studied.
Abstract: The bulk polymerizations of styrene and methyl methacrylate in the presence of model polymer−dithiocarbonate adducts as mediators and benzoyl peroxide (BPO) as a conventional initiator were kinetically studied. The polymerization rate, and hence the concentration of polymer radical P•, was proportional to [BPO]1/2. The pseudo-first-order activation rate constants kact were determined by the GPC peak-resolution and the polydispersity-analysis methods. The results showed that kact is directly proportional to [P•], indicating that reversible addition−fragmentation chain transfer (RAFT) is the only important mechanism of activation. The magnitude of the exchange rate constant kex (= kact/[P•]) was strongly dependent on both the structures of the dithiocarbonate group and the polymer. The kex values for the three RAFT systems examined in this work were all very large, which explains why these systems can provide low-polydispersity polymers from an early stage of polymerization. The activation energy of kex for...

310 citations


Journal ArticleDOI
TL;DR: In this article, the electron transfer dynamics in solar cells that utilize sensitized nanocrystalline titanium dioxide photoelectrodes and the iodide/triiodide redox couple have been studied on a nanosecond time scale.
Abstract: The electron transfer dynamics in solar cells that utilize sensitized nanocrystalline titanium dioxide photoelectrodes and the iodide/triiodide redox couple have been studied on a nanosecond time scale. The ruthenium and osmium bipyridyl complexes Ru(H2L‘)2(CN)2, Os(H2L‘)2(CN)2, Ru(H2L‘)2(NCS)2, and Os(H2L‘)2(NCS)2, where H2L‘ is 4,4‘-dicarboxylic acid 2,2‘-bipyridine, inject electrons into the semiconductor with a rate constant >108 s-1. The effects of excitation intensity, temperature, and applied potential on the recombination reaction were analyzed using a second-order kinetics model. The rates of charge recombination decrease with increasing driving force to the oxidized sensitizer, indicating that charge recombination occurs in the Marcus inverted region. The electronic coupling factors between the oxidized sensitizer and the injected electrons in TiO2 and the reorganization energies for the recombination reaction vary significantly for the different metal complexes. The charge recombination rates a...

285 citations


Journal ArticleDOI
TL;DR: In this paper, a full kinetic scheme for the free-radical reversible addition-fragmentation chain transfer (RAFT) process is presented and implemented into the program package PREDICI (R).
Abstract: A full kinetic scheme for the free-radical reversible addition-fragmentation chain transfer (RAFT) process is presented and implemented into the program package PREDICI (R). With the cumyl dithiobenzoate-mediated bulk polymerization of styrene at 60 degreesC as an example, the rate coefficients associated with the addition-fragmentation equilibrium are deduced by the careful modeling of the time-dependent evolution of experimental molecular weight distributions. The rate coefficient for the addition reaction of a free macroradical to a polymeric RAFT species (k(beta)) is approximately 5 . 10(5) L mol(-1) s(-1), whereas the fragmentation rate coefficient of the formed macroradical RAFT species is close to 3 . 10(-2) s(-1). These values give an equilibrium constant of K = k(beta)/k(-beta) = 1.6 . 10(7) L mol(-1). Conclusive evidence is given that the equilibrium lies well on the side of the macroradical RAFT species. The high value of k(beta) is comparable in size to the propagation rate coefficients reported for acrylates. The transfer rate coefficient to cumyl dithiobenzoate is close to 3.5 . 10(5) L mol(-1) s(-1). A careful sensitivity analysis was performed, which indicated that the reported rate coefficients are accurate to a factor of 2. (C) 2001 John Wiley & Sons, Inc.

276 citations


Journal ArticleDOI
TL;DR: The literature concerning the kinetics of the Maillard reaction has been critically discussed according to the initial, intermediate and advanced stages, as this is the way the MAillard reaction is traditionally analysed.
Abstract: The literature concerning the kinetics of the Maillard reaction was critically discussed according to the initial, intermediate and advanced stages, as this is the way the Maillard reaction is traditionally analysed. For each stage, a division is made between simple kinetics and complex kinetics. Simple kinetics means that the general rate law is used and results are reported as zero-, first- or second-order reactions (sometimes a fractional order). It is emphasized that this approach for a complex reaction as the Maillard reaction only results in a mathematical fit procedure, not in mechanistic insight. The rate constants and activation energies derived are in fact composed of several elementary rate constants. With complex kinetics, i.e. trying to establish the kinetics for individual reaction steps, more mechanistic information can be extracted. However, there are conflicting results in literature and the interpretation is not always correct, as is shown in several examples. A summary of activation energies reported for the various stages in the Maillard reaction reveals large discrepancies, probably reflecting strong effects of experimental conditions on results that are obtained. Careful control of experimental conditions and proper kinetic analysis of the various stages in the Maillard reaction should lead to more consistent results in the future.

Journal ArticleDOI
TL;DR: In this article, the role of Fenton's reactions in the pulsed corona reactor is analyzed both experimentally and with computer simulations in the present work Experimental data shows the existence of optimal iron concentrations for the degradation of phenol, and that the formation of hydrogen peroxide by the pulsing corona discharge is dependent upon both the applied electric field and the solution conductivity.

Journal ArticleDOI
TL;DR: Results show that ionic liquids are excellent enzyme-stabilizing agents and reaction media for clean biocatalysis in non-conventional conditions.
Abstract: Five different ionic liquids, based on dialkylimidazolium and quaternary ammonium cations associated with perfluorinated and bis (trifluoromethyl) sulfonyl amide anions, were used as reaction media to synthesize N-acetyl-L-tyrosine propyl ester by transesterification with alpha-chymotrypsin at 2% (v/v) water content at 50 degrees C. The synthetic activity was reduced by the increase in alkyl chains length of cations and by increases in anion size, which was related to the decrease in polarity. Incubation of the enzyme (with and without substrate) in ionic liquids exhibited first-order deactivation kinetics at 50 degrees C, allowing determination of deactivation rate constants and half-life times (1-3 h). Ionic liquids showed a clear relative stabilization effect on the enzyme, which was improved by increased chain length of the alkyl substituents on the imidazolium ring cations and the anion size. This effect was 10-times enhanced by the presence of substrate. For example, 1-butyl-3-methylimidazolium hexafluorophosphate increased the alpha-chymotrypsin half-life by 200 times in the presence of substrate with respect to the 1-propanol medium. These results show that ionic liquids are excellent enzyme-stabilizing agents and reaction media for clean biocatalysis in non-conventional conditions.

Journal ArticleDOI
TL;DR: In this paper, the kinetics of the gas phase reaction between atomic mercury and hydroxyl radical has been determined at room temperature and atmospheric pressure of air by relative rate technique, and the rate coefficient obtained was k(Hg 0 + · OH )=(8.7±2.8)×10 −14 cm 3 s −1 leading to natural lifetimes of mercury at global mean conditions of 4-7 month due to this reaction.

Journal ArticleDOI
TL;DR: Comparison of the ultraviolet spectra for 6 at various pH values with those for O- and N-alkylated diazeniumdiolates suggests that protonation at the R(2)N nitrogen initiates dissociation to NO at physiological pH, with a second protonations accounting for both the spectral change and the enhanced dissociation rate at pH <4.
Abstract: Diazeniumdiolate ions of structure R2N[N(O)NO]- (1) are of pharmacological interest because they spontaneously generate the natural bioregulatory species, nitric oxide (NO), when dissolved in aqueous media. Here we report the kinetic details for four representative reactivity patterns: (a) straightforward dissociation of the otherwise unfunctionalized diethylamine derivative 2 (anion 1, where R = Et) to diethylamine and NO; (b) results for the zwitterionic piperazin-1-yl analogue 4, for which the protonation state of the neighboring basic amine site is an important determinant of dissociation rate; (c) data for 5, a diazeniumdiolate derived from the polyamine spermine, whose complex rate equation can include terms for a variety of medium effects; and (d) the outcome for triamine 6 (R = CH2CH2NH3+), the most stable structure 1 ion identified to date. All of these dissociations are acid-catalyzed, with equilibrium protonation of the substrate preceding release of NO. Specific rate constants and pKa values ...

Journal ArticleDOI
TL;DR: The influence of pH and cadmium sulfide on the photocatalytic degradation of 2-chlorophenol in titanium dioxide suspensions was investigated and the feasibility of mixed semiconductors on the photodegradation of chlorinated organics in aqueous solution was evaluated.

Journal ArticleDOI
TL;DR: In this paper, the kinetics of Basic Red 18 and Acid Blue 9 sorption onto activated clay have been investigated and a 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 sorption capacity, and initial sorption rate with the effect of initial dye concentration, activated clay particle size, temperature, and pH value.
Abstract: The kinetics of Basic Red 18 and Acid Blue 9 sorption onto activated clay have been investigated. A 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 sorption capacity, and initial sorption rate with the effect of initial dye concentration, activated clay particle size, temperature, and pH value. In addition, an activation energy of sorption has also been determined based on the pseudo–second order rate constants.

Journal ArticleDOI
TL;DR: The results indicate that the manganese ion in Mn-SOD plays an important role in the decomposition kinetics of peroxynitrite and in peroxlynitrite-dependent nitration of self and remote tyrosine residues.

Journal ArticleDOI
TL;DR: In this article, a combination of classical steady-state voltammetry and impedance spectroscopy can help to elucidate dilemmas concerning the role of the Heyrovsky and Tafel steps in the mechanism of the hydrogen evolution reaction.

Journal ArticleDOI
TL;DR: In this paper, the effect of recombination kinetics on photoactivity was discussed, and a simple kinetic model to explain the overall rate of these photocatalytic reactions was proposed, and the relationship between kr and the rate of CO2 production by unplatinized M-TiO2 under aerated conditions was ambiguous; other factor(s) might control the latter.
Abstract: Photocatalytic reactions by transition-metal (V, Cr, Fe, Co, Cu, Mo, or W) loaded TiO2 (M-TiO2) powders suspended in aqueous solutions of methanol, (S)-lysine (Lys), or acetic acid were investigated. The photoactivities of various samples were compared with the rate constant (kr) of recombination of photoexcited electrons and positive holes determined by femtosecond pump–probe diffuse reflection spectroscopy (PP-DRS). As a general trend, increased loading decreased the rate of formation of the main products (H2 , pipecolinic acid (PCA), and CO2) under UV (>300 nm) irradiation, and the effect became more intense on increasing the loading. In PP-DRS, these M-TiO2 gave similar decays of absorption at 620 nm arising from excitation by a 310 nm pulse (<100 fs). The second-order rate constant (kr) markedly increased with loading, even at a low level (0.3%) and further increased with an increase in loading up to 5%. The photocatalytic activity of platinized M-TiO2 for H2 and PCA production under deaerated conditions depended strongly on kr, but the relation between kr and the rate of CO2 production by unplatinized M-TiO2 under aerated conditions was ambiguous; other factor(s) might control the rate of the latter. These different kr dependences of photoactivity on the reaction kinetics governed by e−–h+ recombination were attributed to the presence of O2 and Pt deposits. A simple kinetic model to explain the overall rate of these photocatalytic reactions is proposed, and the effect of recombination kinetics on photoactivity is discussed.

Journal ArticleDOI
TL;DR: The kinetics indicate formation of BrOOO(-) as a steady-state intermediate with an acid-assisted step to give BrOH and O(2) in acidic solutions, important in studies of annual ozone depletion in the Arctic troposphere at polar sunrise.
Abstract: Reactions of ozone with Br-, SO32-, HSO3-, I-, and NO2-, studied by stopped-flow and pulsed-accelerated-flow techniques, are first order in the concentration of O3(aq) and first order in the concentration of each anion. The rate constants increase by a factor of 5 × 106 as the nucleophilicities of the anions increase from Br- to SO32-. Ozone adducts with the nucleophiles are proposed as steady-state intermediates prior to oxygen atom transfer with release of O2. Ab initio calculations show possible structures for the intermediates. The reaction between Br- and O3 is accelerated by H+ but exhibits a kinetic saturation effect as the acidity increases. The kinetics indicate formation of BrOOO- as a steady-state intermediate with an acid-assisted step to give BrOH and O2. Temperature dependencies of the reactions of Br- and HSO3- with O3 in acidic solutions are determined from 1 to 25 °C. These kinetics are important in studies of annual ozone depletion in the Arctic troposphere at polar sunrise.

Journal ArticleDOI
TL;DR: It is concluded that at the Ag/SAM interface the energy barrier for the PT processes of the adsorbed Cyt-c is raised by the electric field, which may represent a possible mechanism for controlling biological redox reactions via changes of the transmembrane potential.
Abstract: Cytochrome c (Cyt-c) was electrostatically bound to self-assembled monolayers (SAM) on an Ag electrode, which are formed by ω-carboxyl alkanethiols of different chain lengths (Cx). The dynamics of the electron-transfer (ET) reaction of the adsorbed heme protein, initiated by a rapid potential jump to the redox potential, was monitored by time-resolved surface enhanced resonance Raman (SERR) spectroscopy. Under conditions of the present experiments, only the reduced and oxidized forms of the native protein state contribute to the SERR spectra. Thus, the data obtained from the spectra were described by a one-step relaxation process yielding the rate constants of the ET between the adsorbed Cyt-c and the electrode for a driving force of zero electronvolts. For C16- and C11-SAMs, the respective rate constants of 0.073 and 43 s-1 correspond to an exponential distance dependence of the ET (β = 1.28 A-1), very similar to that observed for long-range intramolecular ET of redox proteins. Upon further decreasing th...

Journal ArticleDOI
TL;DR: In this article, the surface reaction rate law for monocrystalline selenite was determined by using a rotating disc set-up, where the transport coefficients are well known.

Journal ArticleDOI
TL;DR: Absolute rate constants for the cross-coupling reaction of several carbon-centered radicals with various nitroxides and their temperature dependence have been determined in liquids by kinetic absorption spectroscopy.
Abstract: Absolute rate constants for the cross-coupling reaction of several carbon-centered radicals with various nitroxides and their temperature dependence have been determined in liquids by kinetic absorption spectroscopy. The rate constants range from <2 × 105 M-1 s-1 to 2.3 × 109 M-1 s-1 and depend strongly on the structure of the nitroxide and the carbon-centered radical. Grossly, they decrease with increasing rate constant of the cleavage of the corresponding alkoxyamine. In many cases, the temperature dependence shows a non-Arrhenius behavior. A model assuming a short-lived intermediate that is hindered to form the coupling product by an unfavorable activation entropy leads to a satisfactory analytic description. However, the behavior is more likely due to a barrierless single-step reaction with a low exothermicity where the free energy of activation is dominated by a large negative entropy term.

Journal ArticleDOI
TL;DR: The activation and deactivation rate constants in atom transfer radical polymerization (ATRP) were measured using model compounds and provided further quantitative insights into understanding the ATRP processes.
Abstract: The activation and deactivation rate constants in atom transfer radical polymerization (ATRP) were measured using model compounds. The activation rate constants were determined using HPLC or GC under the kinetic isolation condition achieved by trapping the generated radical with 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO). The deactivation rate constants were measured by trapping 1-phenylethyl radicals with TEMPO in a competitive reaction. The effects of several parameters in ATRP systems were examined, including alkyl groups, ligands, transferred groups, and solvents. The data obtained were consistent with ATRP kinetics and provided further quantitative insights into understanding the ATRP processes.

Journal ArticleDOI
TL;DR: A model was developed to predict the enhancement factor of ozone mass transfer from the following parameters: initial dye concentration, applied ozone dose, temperature and concentration of dissolved ozone in the organic-free water.

Journal ArticleDOI
TL;DR: In this paper, a modified shrinking-core model for the chemical reaction control was proposed, in which the effective surface area is proportional to the degree of hydrolyzed PET, x, affected by formation and growth of pore and crack on PET powder.
Abstract: Poly(ethylene terephthalate) (PET) powder from waste bottles was degradated at atmospheric pressure in 3−9 M sulfuric acid below 150−190 °C for 12 h to clarify the mechanism for a feedstock recycle process. Terephthalic acid (TPA) and ethylene glycol (EG) were produced by the acid-catalyzed heterogeneous hydrolysis of PET in sulfuric acid. The TPA yield agreed with the degree of hydrolysis of PET, but the EG yield decreased with increasing sulfuric acid concentration because of carbonization of EG. The kinetics of hydrolysis of PET in sulfuric acid could be explained by a modified shrinking-core model for the chemical reaction control, in which the effective surface area is proportional to the degree of hydrolyzed PET, x, affected by formation and growth of pore and crack on PET powder. The apparent rate constant was proportional to the reciprocal of the particle size and the activity of sulfuric acid, and the activation energy was 88.7 kJ/mol.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the kinetics of catalytic hydrogenation of nitrobenzene in acid medium to p-aminophenol in a batch slurry reactor in a temperature range of 323-353 K.

Journal ArticleDOI
TL;DR: The formation of beta-hematin closely parallels many mineralization processes, and this suggests that hemozoin formation may be a unique biomineralization process.
Abstract: Formation of beta-hematin in acidic acetate solution has been investigated using quantitative infrared spectroscopy, X-ray diffraction, and scanning and transmission electron microscopy. The process occurs via rapid precipitation of amorphous (or possibly nanocrystalline) hematin, followed by slow conversion to crystalline beta-hematin. Definitive evidence that the reaction occurs during incubation in acetate medium, rather than during the drying stage, is provided by X-ray diffraction and infrared spectroscopy of the wet material. The reaction follows a sigmoidal function indicative of a process of nucleation and growth and was modeled using the Avrami equation. Reaction rates and the dimensionality of growth (as indicated by the value of the Avrami constant) are strongly influenced by stirring rate. The reaction follows Arrhenius behavior, and there is a strong dependence of both the rate constant and the Avrami constant on acetate concentration. Acetate may act as a phase transfer catalyst, solubilizing hematin and facilitating its redeposition as beta-hematin. The pH dependence of the process indicates that only the monoprotonated species of hematin is active in forming beta-hematin. The formation of beta-hematin closely parallels many mineralization processes, and this suggests that hemozoin formation may be a unique biomineralization process. Inferences are drawn with respect to the formation of hemozoin in vivo.

Journal ArticleDOI
TL;DR: In this paper, experiments were carried out with liquid 1-butyl-3-methylimidazolium (BMI+) in solution under various conditions, and the reduced species exhibits an absorption peak at 323 nm (e = 5.9 × 103 L mol-1 cm-1).
Abstract: Radiolysis of liquid 1-butyl-3-methylimidazolium (BMI+) salts leads to formation of several species with similar absorption spectra. To identify and characterize these species, experiments were carried out with BMI+ in solution under various conditions. BMI+ is reduced by eaq- very rapidly (k = 1.9 × 1010 L mol-1 s-1) but not by (CH3)2ĊO- radicals (k ≤ 1 × 105 L mol-1 s-1). The reduced species exhibits an absorption peak at 323 nm (e = 5.9 × 103 L mol-1 cm-1). It decays via radical−radical reactions but does not react with O2 or with methyl viologen. BMI+ reacts rapidly also with OH radicals (k = 3.7 × 109 L mol-1 s-1) to produce a mixture of isomeric OH-adducts. The spectrum of these adducts is similar to that of the reduced species, it exhibits a peak at 322 nm (e = 2.5 × 103 L mol-1 cm-1) but with a pronounced shoulder at 380 to 420 nm. The adducts react with O2 and with OH-. In the latter reaction they lose H+ to form a hydroxylated analogue of the electron adduct. Irradiation of liquid BMI+ salts pro...