Showing papers on "Reaction rate constant published in 2012"
TL;DR: In this review, an overview is provided of the photochemical reaction parameters (quantum yield, molar absorption, OH radical reaction rate constant) of more than 100 organic micropollutants that allow for a prediction of organic contaminant removal by UV advanced oxidation systems.
483 citations
TL;DR: The composite photocatalyst g-C 3 N 4 /ZnO was synthesized by heat treatment of the precursor obtained via the deposition-precipitation method as mentioned in this paper.
329 citations
TL;DR: Gold nanoparticles deposited on nanocrystalline magnesium oxide is a very efficient catalyst for the reduction of nitro-arenes in aqueous medium at room temperature as discussed by the authors, and the reaction kinetics of reduction of 4-nitrophenol to 4-aminophenol has been studied by UV-visible spectrophotometry, and its apparent rate constant has been determined and compared with those of other supported gold catalysts.
313 citations
TL;DR: In this article, the authors show that Fe(IV) is a less efficient oxidant for DMSO at pH 6-7 than is (H2O)5FeO2+, generated by ozone oxidation of Fe(H2 O)62+, in acidic solutions.
Abstract: A major pathway in the reaction between Fe(II) and H2O2 at pH 6–7 in non-coordinating buffers exhibits inverse kinetic dependence on [H+] and leads to oxidation of dimethyl sulfoxide (DMSO) to dimethyl sulfone (DMSO2). This step regenerates Fe(II) and makes the oxidation of DMSO catalytic, a finding that strongly supports Fe(IV) as a Fenton intermediate at near-neutral pH. This Fe(IV) is a less efficient oxidant for DMSO at pH 6–7 than is (H2O)5FeO2+, generated by ozone oxidation of Fe(H2O)62+, in acidic solutions. Large concentrations of DMSO are needed to achieve significant turnover numbers at pH ≥ 6 owing to the rapid competing reaction between Fe(II) and Fe(IV) that leads to irreversible loss of the catalyst. At pH 6 and ≤0.02 mM Fe(II), the ratio of apparent rate constants for the reactions of Fe(IV) with DMSO and with Fe(II) is ∼104. The results at pH 6–7 stand in stark contrast with those reported previously in acidic solutions where the Fenton reaction generates hydroxyl radicals. Under those conditions, DMSO is oxidized stoichiometrically to methylsulfinic acid and ethane. This path still plays a role (1–10%) at pH 6–7.
307 citations
TL;DR: In this paper, heat-activated persulfate oxidation of diuron, a commonly found herbicide in groundwater, was evaluated in the presence of three common groundwater solutes (CO 3 2, HCO 3, and Cl − ), to different degrees, influenced the degradation.
271 citations
TL;DR: The use of the exponential model or similar empirical alternative would eliminate the confusing temperature axis inversion, the unnecessary compression of the temperature scale, and the need for kinetic assumptions that are hard to affirm in food systems.
Abstract: The Arrhenius equation has been widely used as a model of the temperature effect on the rate of chemical reactions and biological processes in foods. Since the model requires that the rate increase monotonically with temperature, its applicability to enzymatic reactions and microbial growth, which have optimal temperature, is obviously limited. This is also true for microbial inactivation and chemical reactions that only start at an elevated temperature, and for complex processes and reactions that do not follow fixed order kinetics, that is, where the isothermal rate constant, however defined, is a function of both temperature and time. The linearity of the Arrhenius plot, that is, Ln[k(T)] vs. 1/T where T is in °K has been traditionally considered evidence of the model's validity. Consequently, the slope of the plot has been used to calculate the reaction or processes’ “energy of activation,” usually without independent verification. Many experimental and simulated rate constant vs. temperature relation...
229 citations
TL;DR: It is shown that trans-resveratrol always reacts with (•)OH radicals at a rate that is diffusion-controlled, independent of the reaction pathway, which explains why trans- Resver atrol is an excellent but very unselective (•]OH radical scavenger that provides antioxidant protection to the cell.
Abstract: In this work, we have carried out a systematic study of the antioxidant activity of trans-resveratrol toward hydroxyl ((•)OH) and hydroperoxyl ((•)OOH) radicals in aqueous simulated media using density functional quantum chemistry and computational kinetics methods. All possible mechanisms have been considered: hydrogen atom transfer (HAT), proton-coupled electron transfer (PCET), sequential electron proton transfer (SEPT), and radical adduct formation (RAF). Rate constants have been calculated using conventional transition state theory in conjunction with the Collins-Kimball theory. Branching ratios for the different paths contributing to the overall reaction, at 298 K, are reported. For the global reactivity of trans-resveratrol toward (•)OH radicals, in water at physiological pH, the main mechanism of reaction is proposed to be the sequential electron proton transfer (SEPT). However, we show that trans-resveratrol always reacts with (•)OH radicals at a rate that is diffusion-controlled, independent of the reaction pathway. This explains why trans-resveratrol is an excellent but very unselective (•)OH radical scavenger that provides antioxidant protection to the cell. Reaction between trans-resveratrol and the hydroperoxyl radical occurs only by phenolic hydrogen abstraction. The total rate coefficient is predicted to be 1.42 × 10(5) M(-1) s(-1), which is much smaller than the ones for reactions of trans-resveratrol with (•)OH radicals, but still important. Since the (•)OOH half-life time is several orders larger than the one of the (•)OH radical, it should contribute significantly to trans-resveratrol oxidation in aqueous biological media. Thus, trans-resveratrol may act as an efficient (•)OOH, and also presumably (•)OOR, radical scavenger.
224 citations
TL;DR: The reaction path for a complex-forming reaction is often barrierless, which results in weak and sometimes negative temperature dependence for its rate constant, and the product angular and internal distributions of such reactions also bear clear signatures as mentioned in this paper.
Abstract: Many gas-phase chemical reactions proceed via reaction intermediates, supported by potential wells. The characteristics of such complex-forming reactions differ drastically from those for direct reactions that involve barriers. For example, the reaction path for a complex-forming reaction is often barrierless, which results in weak and sometimes negative temperature dependence for its rate constant. The product angular and internal distributions of such reactions also bear clear signatures. Specifically, the angular distribution (i.e. differential cross-section) of a complex-forming reaction is often dominated by scattering in the forward and backward directions, and the product rotational state distribution usually peaks near the highest accessible rotational state, while vibrational state distribution often decays monotonically. While the quantum dynamics of direct reactions is well established, our understanding of complex-forming reactions is still far from complete. Given the importance of such react...
207 citations
TL;DR: Rate constants for recombination and hole transfer during oxygen evolution at illuminated α-Fe(2)O(3) electrodes were measured by intensity-modulated photocurrent spectroscopy and found to be remarkably low.
202 citations
TL;DR: In this article, cubic, octahedral, and rhombic dodecahedral gold nanocrystals were employed as catalysts for the examination of facet-dependent catalytic activity toward NaBH4 reduction of p-nitroaniline to p-phenylenediamine at different temperatures.
Abstract: In this study, cubic, octahedral, and rhombic dodecahedral gold nanocrystals synthesized by a seed-mediated growth method were employed as catalysts for the examination of facet-dependent catalytic activity toward NaBH4 reduction of p-nitroaniline to p-phenylenediamine at different temperatures. Different amounts of the nanocrystal solutions were used so that all samples contain particles with the same total surface area. UV–vis absorption spectra monitored the reaction progress. Rhombic dodecahedra showed the best catalytic efficiency at all the temperatures examined. Nanocubes have higher reaction rates than those of octahedra from 25 to 36 °C, so the catalytic activity for the reduction reaction follows the order of {110} > {100} > {111}. However, the reaction rates for octahedra increase rapidly with rising temperature; their reaction rate surpasses that for the nanocubes at 40 °C. Rate constants and activation energies were determined, again showing that the activation energy is lowest for rhombic do...
192 citations
TL;DR: Differences in translocation rates among various shapes may have implications on the structural evolution of pathogens from spherical to rodlike morphologies for enhanced efficacy.
Abstract: Nanotoxicity is becoming a major concern as the use of nanoparticles in imaging, therapeutics, diagnostics, catalysis, sensing, and energy harvesting continues to grow dramatically. The tunable functionalities of the nanoparticles offer unique chemical interactions in the translocation process through cell membranes. The overall translocation rate of the nanoparticle can vary immensely on the basis of the charge of the surface functionalization along with shape and size. Using advanced molecular dynamics simulation techniques, we compute translocation rate constants of functionalized cone-, cube-, rod-, rice-, pyramid-, and sphere-shaped nanoparticles through lipid membranes. The computed results indicate that depending on the nanoparticle shape and surface functionalization charge, the translocation rates can span 60 orders of magnitude. Unlike isotropic nanoparticles, positively charged, faceted, rice-shaped nanoparticles undergo electrostatics-driven reorientation in the vicinity of the membrane to max...
TL;DR: In this paper, a systematic study of the adsorption process of graphene was performed by varying pH, ionic strength and temperature, and the results indicated that the second order kinetics model was well-suited to model the kinetic adaption of phosphate.
Abstract: Graphene was prepared by a facile liquid phase exfoliation and characterized by Raman spectroscopy, Fourier transform infrared spectroscopy, powder X-ray diffraction, scanning electron microscopy and zeta potential measurements. A systematic study of the adsorption process was performed by varying pH, ionic strength and temperature. The experimental results showed that graphene is an excellent phosphate adsorbent with an adsorption capacity of up to 89.37 mg g−1 at an initial phosphate concentration of 100 mg L−1 and temperature of 303 K. The adsorption kinetics was modeled by first and second order rate, Elovich and Weber and Morris intraparticle diffusion models. The rate constants for all of these kinetic models were calculated and the results indicate that the second order kinetics model was well-suited to model the kinetic adsorption of phosphate. The Langmuir, Freundlich and D–R isotherm models were applied to describe the equilibrium isotherms and the isotherm constants were determined. Equilibrium data were well-described by the typical Langmuir adsorption isotherm. Thermodynamic studies revealed that the adsorption reaction was a spontaneous and endothermic process.
TL;DR: In this article, a simple experiment using a single crystal and its polycrystalline counterpart was conducted to demonstrate the sensitivity of dissolution rate to grain size, results that undermine the use of “classical” rate constants.
TL;DR: An analysis of the available evidence suggests the operation of polarity reversal catalysis in the reduction of alkyl and aryl halides by N-heterocyclic carbene boranes is suggested.
Abstract: Otherwise sluggish or completely ineffective radical reductions of alkyl and aryl halides by N-heterocyclic carbene boranes (NHC-boranes) are catalyzed by thiols. Reductions and reductive cyclizations with readily available 1,3-dimethylimidazol-2-ylidene borane and a water-soluble triazole relative are catalyzed by thiophenol and tert-dodecanethiol [C(9)H(19)C(CH(3))(2)SH]. Rate constants for reaction of the phenylthiyl (PhS•) radical with two NHC-boranes have been measured to be ~10(8) M(-1) s(-1) by laser flash photolysis experiments. An analysis of the available evidence suggests the operation of polarity reversal catalysis.
TL;DR: Simulations reveal that under most conditions relevant to combustion/ignition problems, the high-pressure rate rules can be used directly to describe the reactions of RO(2) and QOOH and indicate that equilibrium is established between the alkyl,RO(2), and γ- and δ-QOOH radicals.
Abstract: The unimolecular reactions of hydroperoxy alkyl radicals (QOOH) play a central role in the low-temperature oxidation of hydrocarbons as they compete with the addition of a second O2 molecule, which is known to provide chain-branching. In this work we present high-pressure rate estimation rules for the most important unimolecular reactions of the β-, γ-, and δ-QOOH radicals: isomerization to RO2, cyclic ether formation, and selected β-scission reactions. These rate rules are derived from high-pressure rate constants for a series of reactions of a given reaction class. The individual rate expressions are determined from CBS-QB3 electronic structure calculations combined with canonical transition state theory calculations. Next we use the rate rules, along with previously published rate estimation rules for the reactions of alkyl peroxy radicals (RO2), to investigate the potential impact of falloff effects in combustion/ignition kinetic modeling. Pressure effects are examined for the reaction of n-butyl radi...
TL;DR: In this paper, the authors describe a systematic investigation of the factors controlling step-by-step growth of the metal-organic framework (MOF) [Cu3(btc)2(H2O)3]·x H2O (also known as HKUST-1), using quartz crystal microbalance (QCM) electrodes as an in situ probe of the reaction kinetics and mechanism.
Abstract: We describe a systematic investigation of the factors controlling step-by-step growth of the metal–organic framework (MOF) [Cu3(btc)2(H2O)3]·xH2O (also known as HKUST-1), using quartz crystal microbalance (QCM) electrodes as an in situ probe of the reaction kinetics and mechanism. Electrodes coated with silica, alumina and gold functionalized with OH– and COOH–terminated self-assembled monolayers (SAMs) were employed to determine the effects of surface properties on nucleation. Deposition rates were measured using the high sensitivity available from QCM-D (D = dissipation) techniques to determine rate constants in the early stage of the process. Films were characterized using grazing incidence XRD, SEM, AFM, profilometry and reflection–absorption IR spectroscopy. The effects of reaction time, concentration, temperature and substrate on the deposition rates, film crystallinity and surface morphology were evaluated. The initial growth step, in which the surface is exposed to copper ions (in the form of an ethanolic solution of copper(II) acetate) is fast and independent of temperature, after which all subsequent steps are thermally activated over the temperature range 22–62 °C. Using these data, we propose a kinetic model for the Cu3(btc)2 growth on surfaces that includes rate constants for the individual steps. The magnitude of the activation energies, in particular the large entropy decrease, suggests an associative reaction with a tight transition state. The measured activation energies for the step-by-step MOF growth are an order of magnitude lower than the value previously reported for bulk Cu3(btc)2 crystals. Finally, the results of this investigation demonstrate that the QCM method is a powerful tool for quantitative, in situ monitoring of MOF growth in real time.
TL;DR: In this article, the authors used rotating ring disc electrode (RRDE) voltammetry to study the reaction of Li-air batteries to superoxide radical (O2•−) produced upon discharge at the battery's cathode.
Abstract: Despite the promising high specific energy density of lithium–air batteries, their commercialization remains hindered by numerous issues, including the poor stability of the electrolyte due to its reaction with the superoxide radical (O2•–) produced upon discharge at the battery’s cathode. In this work, we have used rotating ring disc electrode (RRDE) voltammetry to study this reaction and to quantify the stability of the electrolyte against O2•– by its pseudo-first-order reaction constant, k. Our results confirm the recently reported reactivity of propylene carbonate (PC, which was used in many of the initial works on Li–air batteries), while unveiling the enhanced stability of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr14TFSI), with a k value at least 3 orders of magnitude lower than that estimated in PC. Moreover, our RRDE-transient measurements indicate that the diffusion of O2•– in this ionic liquid is ≈70 times slower than that in PC, which could partially explain the poor d...
30 Oct 2012-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the interfacial reaction between Al, AA6111, and Mg AZ31 alloys has been studied as a function of welding energy, and it was shown that the reaction layer thickness was already ∼5μm thick.
Abstract: High power ultrasonic spot welding (USW) is a low heat input solid-state joining process that may offer a solution for welding difficult dissimilar-material couples, like magnesium (Mg) to aluminium (Al) for automotive body applications. However, the high strain rate dynamic deformation in USW has been claimed to accelerate inter-diffusion rates in dissimilar joints. The interfacial reaction between Al, AA6111, and Mg AZ31 alloys has been studied as a function of welding energy. For the optimum welding condition of 600 J (0.4 s) the reaction layer thickness was already ∼5 μm thick. Intermetallic reaction centres were found to nucleate within microwelds at the interface at very short welding times and spread and grow rapidly to form a continuous layer, composed of two sub-layers of Al 12 Mg 17 and Al 3 Mg 2 . Interface liquation was also found for longer welding times at temperatures below the recognised lowest eutectic reaction temperature in the Al–Mg binary system. Modelling has been used to show that the solid state reaction kinetics were over twice the rate expected from parabolic growth predictions made using rate constants obtained under static test conditions. The reasons for this discrepancy and the depressed melting reaction are discussed.
TL;DR: In this article, the authors investigated the global adsorption rate of tetracycline on adsorbents obtained from treatment sludge and found that the diffusion in the pore volume represents > 80% of total intraparticle diffusion.
TL;DR: In this article, the speed constants associated with the ET and PT processes of the hangman system were compared to those of corresponding values measured for porphyrins that lack an internal proton relay.
Abstract: Cobalt hangman porphyrins catalyze the hydrogen evolution reaction (HER). The hangman group is observed to facilitate HER by mediating a proton-coupled electron transfer (PCET) reaction. The details of the PCET pathway have been determined by comparing rate constants associated with the ET and PT processes of the hangman system to those of the corresponding values measured for porphyrins that lack an internal proton relay. A rapid intramolecular proton transfer from the carboxylic acid hanging group to the reduced cobalt centre of 8.5 × 106 s−1 provides a facile pathway for the formation of Co(II)H, which leads directly to H2 generation.
TL;DR: The incomplete loss of sensitization following borohydride reduction, as well as the inverse dependence of R(TMP) on [O(2)] for these samples, suggests that there remains another class of oxidizing triplet sensitizer, perhaps quinones.
Abstract: To probe the mechanism of the photosensitized loss of phenols by humic substances (HS), the dependence of the initial rate of 2,4,6-trimethylphenol (TMP) loss (RTMP) on dioxygen concentration was examined both for a variety of untreated as well as borohydride-reduced HS and C18 extracts from the Delaware Bay and Mid-Atlantic Bight. RTMP was inversely proportional to dioxygen concentration at [O2] > 50 μM, a dependence consistent with reaction with triplet excited states, but not with 1O2 or RO2. Modeling the dependence of RTMP on [O2] provided rate constants for TMP reaction, O2 quenching, and lifetimes compatible with a triplet intermediate. Borohydride reduction significantly reduced TMP loss, supporting the role of aromatic ketone triplets in this process. However, for most samples, the incomplete loss of sensitization following borohydride reduction, as well as the inverse dependence of RTMP on [O2] for these samples, suggests that there remains another class of oxidizing triplet sensitizer, perhaps q...
TL;DR: Nonstatistical dynamics results in nonstatistical partitioning of the available energy to XCH(3) +Y(-) reaction products, and the relative translational energy and temperature dependencies of the S(N)2 rate constants are not accurately given by statistical theory.
Abstract: Extensive classical chemical dynamics simulations of gas-phase X(-) + CH(3)Y → XCH(3) + Y(-) S(N)2 nucleophilic substitution reactions are reviewed and discussed and compared with experimental measurements and predictions of theoretical models. The primary emphasis is on reactions for which X and Y are halogen atoms. Both reactions with the traditional potential energy surface (PES), which include pre- and postreaction potential energy minima and a central barrier, and reactions with nontraditional PESs are considered. These S(N)2 reactions exhibit important nonstatistical atomic-level dynamics. The X(-) + CH(3)Y → X(-)---CH(3)Y association rate constant is less than the capture model as a result of inefficient energy transfer from X(-)+ CH(3)Y relative translation to CH(3)Y rotation and vibration. There is weak coupling between the low-frequency intermolecular modes of the X(-)---CH(3)Y complex and higher frequency CH(3)Y intramolecular modes, resulting in non-RRKM kinetics for X(-)---CH(3)Y unimolecular decomposition. Recrossings of the [X--CH(3)--Y](-) central barrier is important. As a result of the above dynamics, the relative translational energy and temperature dependencies of the S(N)2 rate constants are not accurately given by statistical theory. The nonstatistical dynamics results in nonstatistical partitioning of the available energy to XCH(3) +Y(-) reaction products. Besides the indirect, complex forming atomic-level mechanism for the S(N)2 reaction, direct mechanisms promoted by X(-) + CH(3)Y relative translational or CH(3)Y vibrational excitation are possible, e.g., the roundabout mechanism.
TL;DR: In this paper, the reaction kinetics and mass transfer performance of CO2 absorption into aqueous solutions of blended MDEA-MEA solutions were comprehensively studied using a laminar jet absorber in terms of a second order reaction rate constant and enhancement factor.
TL;DR: In this article, a circulatory airtight reactor system was developed to improve the energy efficiency in dye degradation, and the degradation mechanism of methyl orange (MO) was proposed based on bond dissociation energies (BDEs).
TL;DR: A new C5H9NO·SnCl2 coordinated ionic liquid (IL) was prepared by reacting N-methyl-pyrrolidone with anhydrous SnCl2 and could be recycled six times without a significant decrease in activity.
TL;DR: In this article, the authors investigated the influence of the catalyst topology on the reaction rate of benzene methylation by methanol on the acid zeolites H-ZSM-5 and H-beta.
TL;DR: In this paper, a combination of surface complexation models and transition state theory was used to estimate the rate of precipitation of aqueous CO 3 2 -activity and pH in mixed-flow reactors.
TL;DR: The kinetics of the fructose-to-HMF conversion is studied and the rate constants, reaction orders, and activation energies for the systems with and without bi-functional MSN catalysts are compared.
Abstract: Mesoporous silica nanoparticles functionalized with both sulfonic acid (HSO3) and ionic liquid (ILs) were synthesized and applied as effective and recyclable catalysts for generating 5-hydroxymethylfurfural (HMF) from fructose. For the first time a high HMF yield of 72.5% was achieved in DMSO systems under mild conditions (90 °C and 3 h). We further studied the kinetics of the fructose-to-HMF conversion and compared the rate constants, reaction orders, and activation energies for the systems with and without bi-functional MSN catalysts.
TL;DR: The recycling and sedimentation tests justified that the Vis/C-TiO(2) process is a cost-effective and feasible way for wastewater treatment.
TL;DR: In this article, the stability of BrHg with abundant radicals such as NO, NO2, HO2, ClO, or BrO has been investigated and shown to be stable with all of these radicals except NO.
Abstract: . Quantum calculations are used to determine the stability of reactive gaseous mercury (Hg(II)) compounds likely to be formed in the Br-initiated oxidation of gaseous elemental mercury (Hg(0)). Due to the absence of any evidence, current models neglect the possible reaction of BrHg with abundant radicals such as NO, NO2, HO2, ClO, or BrO. The present work demonstrates that BrHg forms stable compounds, BrHgY, with all of these radicals except NO. Additional calculations on the analogous ClHgY compounds reveal that the strength of the XHg-Y bond (for X = Cl, Br) varies little with the identity of the halogen. Calculations further suggest that HO2 and NO3 do not form strong bonds with Hg(0), and cannot initiate Hg(0) oxidation in the gas phase. The theoretical approach is validated by comparison to published data on HgX2 compounds, both from experiment and highly refined quantum chemical calculations. Quantum calculations on the stability of the anions of XHgY are carried out in order to aid future laboratory studies aimed at molecular-level characterization of gaseous Hg(II) compounds. Spectroscopic data on BrHg is analyzed to determine the equilibrium constant for its formation, and BrHg is determined to be much less stable than previously estimated. An expression is presented for the rate constant for BrHg dissociation.