Showing papers on "Reaction rate published in 2011"

Gas uptake and chemical aging of semisolid organic aerosol particles

Abstract: Organic substances can adopt an amorphous solid or semisolid state, influencing the rate of heterogeneous reactions and multiphase processes in atmospheric aerosols. Here we demonstrate how molecular diffusion in the condensed phase affects the gas uptake and chemical transformation of semisolid organic particles. Flow tube experiments show that the ozone uptake and oxidative aging of amorphous protein is kinetically limited by bulk diffusion. The reactive gas uptake exhibits a pronounced increase with relative humidity, which can be explained by a decrease of viscosity and increase of diffusivity due to hygroscopic water uptake transforming the amorphous organic matrix from a glassy to a semisolid state (moisture-induced phase transition). The reaction rate depends on the condensed phase diffusion coefficients of both the oxidant and the organic reactant molecules, which can be described by a kinetic multilayer flux model but not by the traditional resistor model approach of multiphase chemistry. The chemical lifetime of reactive compounds in atmospheric particles can increase from seconds to days as the rate of diffusion in semisolid phases can decrease by multiple orders of magnitude in response to low temperature or low relative humidity. The findings demonstrate that the occurrence and properties of amorphous semisolid phases challenge traditional views and require advanced formalisms for the description of organic particle formation and transformation in atmospheric models of aerosol effects on air quality, public health, and climate.

Catalytic Activity of Faceted Gold Nanoparticles Studied by a Model Reaction: Evidence for Substrate-Induced Surface Restructuring

Abstract: We present the analysis of the catalytic activity of gold nanoparticles in aqueous solution as a function of temperature. As a model reaction, the reduction of p-nitrophenol (Nip) by sodium borohydride (BH4–) is used. The gold nanoparticles are immobilized on cationic spherical polyelectrolyte brushes that ensure their stability against aggregation. High-resolution transmission electron microscopy shows that the Au nanoparticles are faceted nanocrystals. The average size of the nanoparticles is 2.2 nm, and the total surface area of all nanoparticles could be determined precisely and was used in the subsequent kinetic analysis. Kinetic data have been obtained between 10 and 30 °C by monitoring the concentrations of Nip and BH4– by UV–vis spectroscopy. The reaction starts after an induction time t0, and the subsequent stationary phase yields the apparent reaction rate, kapp. All kinetic data could be modeled in terms of the Langmuir–Hinshelwood model; that is, both reactants must be adsorbed onto the surfac...

Accelerated bimolecular reactions in microdroplets studied by desorption electrospray ionization mass spectrometry

Abstract: Functional group derivatization reactions occur in the course of microdroplet/surface collisions in the ambient ionization process of desorption electrospray ionization (DESI). The unique environment in the microdroplet causes rate enhancements of as much as several orders of magnitude in typical bimolecular reactions that proceed through either cationic or anionic intermediates. The environment in the evaporating charged microdroplet differs from that of the bulk: (i) the pH of the solution moves towards the extremes, (ii) the concentrations of the reagents increase, (iii) the relative surface area increases and (iv) collision frequencies increase. The rates of acid-catalyzed reactions, such as the reaction of Girard T reagent with ketosteroids, increase with decreasing pH in positively-charged microdroplets compared to the bulk solution rates. Similarly, the increased pH in evaporating negatively-charged microdroplets contributes to an increase in the rates of base-catalyzed Michael reactions over those recorded under bulk solution conditions. The amount of product formed depends on the reaction time and the droplet size. Nanoelectrospray ionization generates larger droplets than the secondary droplets of DESI so it does not show significant product formation in the analysis period and can be used to analyze products of the DESI experiments. When secondary microdroplets (ca. 1 micron diameter) are generated either by spraying a homogeneous solution of both reagents against an inert surface (reactive DESI) or when a solution of Girard T reagent is sprayed against a solid surface bearing the ketosteroid significant amounts of product are generated. In the case of the Michael reaction with cinnamic acid an alternative dehydrogenated reaction product is formed under microdroplet conditions. Some parallels between the phenomenon reported here and the rate acceleration seen in sonochemistry are noted. The potential value of mass spectrometry in establishing conditions that enhance reaction rates is also indicated. It is possible that these observations will assist in the selection of reaction conditions involving the use of charged microdroplets to enhance the rates of ordinary bulk chemical reactions, especially those involving strong steric hindrance.

Diffuse charge and Faradaic reactions in porous electrodes

Abstract: Porous electrodes instead of flat electrodes are widely used in electrochemical systems to boost storage capacities for ions and electrons, to improve the transport of mass and charge, and to enhance reaction rates. Existing porous electrode theories make a number of simplifying assumptions: (i) The charge-transfer rate is assumed to depend only on the local electrostatic potential difference between the electrode matrix and the pore solution, without considering the structure of the double layer (DL) formed in between; (ii) the charge-transfer rate is generally equated with the salt-transfer rate not only at the nanoscale of the matrix-pore interface, but also at the macroscopic scale of transport through the electrode pores. In this paper, we extend porous electrode theory by including the generalized Frumkin-Butler-Volmer model of Faradaic reaction kinetics, which postulates charge transfer across the molecular Stern layer located in between the electron-conducting matrix phase and the plane of closest approach for the ions in the diffuse part of the DL. This is an elegant and purely local description of the charge-transfer rate, which self-consistently determines the surface charge and does not require consideration of reference electrodes or comparison with a global equilibrium. For the description of the DLs, we consider the two natural limits: (i) the classical Gouy-Chapman-Stern model for thin DLs compared to the macroscopic pore dimensions, e.g., for high-porosity metallic foams (macropores g50 nm) and (ii) a modified Donnan model for strongly overlapping DLs, e.g., for porous activated carbon particles (micropores 2 nm). Our theory is valid for electrolytes where both ions are mobile, and it accounts for voltage and concentration differences not only on the macroscopic scale of the full electrode, but also on the local scale of the DL. The model is simple enough to allow us to derive analytical approximations for the steady-state and early transients. We also present numerical solutions to validate the analysis and to illustrate the evolution of ion densities, pore potential, surface charge, and reaction rates in response to an applied voltage.

Kinetics and mechanisms of nanosilver oxysulfidation.

Abstract: Among the many new engineered nanomaterials, nanosilver is one of the highest priority cases for environmental risk assessment. Recent analysis of field samples from water treatment facilities suggests that silver is converted to silver sulfide, whose very low solubility may limit the bioavailability and adverse impact of silver in the environment. The present study demonstrates that silver nanoparticles react with dissolved sulfide species (H(2)S, HS(-)) under relevant but controlled laboratory conditions to produce silver sulfide nanostructures similar to those observed in the field. The reaction is tracked by time-resolved sulfide depletion measurements to yield quantitative reaction rates and stoichiometries. The reaction requires dissolved oxygen, and it is sensitive to pH and natural organic matter. Focused-ion-beam analysis of surface films reveals an irregular coarse-grained sulfide phase that allows deep (>1 μm) conversion of silver surfaces without passivation. At high sulfide concentrations, nanosilver oxysulfidation occurs by a direct particle-fluid reaction. At low sulfide concentration, quantitative kinetic analysis suggests a mechanistic switch to an oxidative dissolution/precipitation mechanism, in which the biologically active Ag(+) ion is generated as an intermediate. The environmental transformation pathways for nanosilver will vary depending on the media-specific competing rates of oxidative dissolution and direct oxysulfidation.

Enhancement of the catalytic activity of supported gold nanoparticles for the Fenton reaction by light.

Abstract: Laser flash photolysis of supported gold nanopar-ticles exciting at the surface plasmon band (532 nm) has allowed in the case of Au/CeO2 and Au/OH-npD (OH-npD: Fenton-treated diamond nanoparticles) detection of transients decaying in the microsecond time scale that have been attributed as indicating photoinduced electron ejection from gold based on N2O quenching and the observation of the generation of methyl viologen radical cations. This photochemical behavior has led us to hypothesize that there could be assistance to the catalytic activity of these materials by irradiation in those cases wherein the mechanism involves electron transfer to or from a substrate to the gold. This hypothesis has been confirmed by observing that the catalytic activity of Au/OH-npD for the Fenton degradation of phenol with hydrogen peroxide can be increased over 1 order of magnitude by irradiation at 532 nm. Moreover, there is a linear relationship between the initial reaction rate and the incident photon flux. This photoenh...

Asymmetric catalytic reactions by NbO-type chiral metal–organic frameworks

Abstract: Chiral metal–organic frameworks (MOFs) constitute a unique class of multifunctional hybrid materials and are envisioned as a versatile tool for various enantioselective applications, including the separation of optical isomers and the promotion of catalytic enantioselective reactions Despite some pioneering works on catalytic enantioselective reactions promoted by chiral MOFs, there is still a need for practical catalysts and many fundamental issues must be answered; such as pin-pointing the site of the reaction and expedition of the reaction rate to the level of that in homogeneous media We have designed and synthesized a chiral metal–organic framework, (S)-KUMOF-1 (Cu2(S)-1)2(H2O)2, 1 = 2,2′-dihydroxy-6,6′-dimethyl(1,1′-biphenyl)-4,4′-dicarboxylate) of which a non-interpenetrating NbO type framework provides a spacious pore (2 × 2 × 2 nm3) and is equipped with potential catalytic sites exposed into the pore Since the functional group on the organic links, biphenols in this MOF, can be modified further on demand, this MOF can serve as a platform for new heterogeneous catalysis Two reactions, the carbonyl-ene reaction with modified MOF after replacement of the protons on biphenol on the organic links with Zn(II) and the hetero-Diels–Alder reaction with Ti(IV), respectively, were studied In this manoeuver, we observed that the reaction occurs entirely inside the pores and the reaction rate of the heterogeneous reaction by this specific MOF is comparable to that of its homogeneous counterpart In addition, it is also observed that the enantioselectivities are significantly improved by extra steric bias provided from the frames of the MOF These observations reinforce the legitimacy of the strategy of using a chiral MOF as a highly enantioselective heterogeneous catalyst

Catalytic asymmetric intermolecular Stetter reaction of enals with nitroalkenes: enhancement of catalytic efficiency through bifunctional additives.

Abstract: An asymmetric intermolecular Stetter reaction of enals with nitroalkenes catalyzed by chiral N-heterocyclic carbenes has been developed. The reaction rate and efficiency are profoundly impacted by the presence of catechol. The reaction proceeds with high selectivities and affords good yields of the Stetter product. Internal redox products were not observed despite of the protic conditions. The impact of catechol has been found to be general, facilitating far lower catalyst loadings than were previously achievable.

Bimolecular reaction rates from ring polymer molecular dynamics: Application to H + CH4→ H2 + CH3

Abstract: In a recent paper, we have developed an efficient implementation of the ring polymer molecular dynamics (RPMD) method for calculating bimolecular chemical reaction rates in the gas phase, and illustrated it with applications to some benchmark atom‐diatom reactions. In this paper, we show that the same methodology can readily be used to treat more complex polyatomic reactions in their full dimensionality, such as the hydrogen abstraction reaction from methane, H + CH4 → H2 + CH3. The present calculations were carried out using a modified and recalibrated version of the Jordan‐ Gilbert potential energy surface. The thermal rate coefficients obtained between 200 and 2000 K are presented and compared with previous results for the same potential energy surface. Throughout the temperature range that is available for comparison, the RPMD approximation gives better agreement with accurate quantum mechanical (multiconfigurational time-dependent Hartree) calculations than do either the centroid density version of quantum transition state theory (QTST) or the quantum instanton (QI) model. The RPMD rate coefficients are within a factor of 2 of the exact quantum mechanical rate coefficients at temperatures in the deep tunneling regime. These results indicate that our previous assessment of the accuracy of the RPMD approximation for atom‐diatom reactions remains valid for more complex polyatomic reactions. They also suggest that the sensitivity of the QTST and QI rate coefficients to the choice of the transition state dividing surface becomes more of an issue as the dimensionality of the reaction increases. © 2011 American Institute of Physics. [doi:10.1063/1.3533275]

Catalytic Mechanism of RNA Backbone Cleavage by Ribonuclease H from Quantum Mechanics/Molecular Mechanics Simulations

Abstract: We use quantum mechanics/molecular mechanics (QM/MM) simulations to study the cleavage of the ribonucleic acid (RNA) backbone catalyzed by ribonuclease H. This protein is a prototypical member of a large family of enzymes that use two-metal catalysis to process nucleic acids. By combining Hamiltonian replica exchange with a finite-temperature string method, we calculate the free energy surface underlying the RNA cleavage reaction and characterize its mechanism. We find that the reaction proceeds in two steps. In a first step, catalyzed primarily by magnesium ion A and its ligands, a water molecule attacks the scissile phosphate. Consistent with thiol-substitution experiments, a water proton is transferred to the downstream phosphate group. The transient phosphorane formed as a result of this nucleophilic attack decays by breaking the bond between the phosphate and the ribose oxygen. In the resulting intermediate, the dissociated but unprotonated leaving group forms an alkoxide coordinated to magnesium ion B. In a second step, the reaction is completed by protonation of the leaving group, with a neutral Asp132 as a likely proton donor. The overall reaction barrier of ~15 kcal mol−1, encountered in the first step, together with the cost of protonating Asp132, is consistent with the slow measured rate of ~1–100/min. The two-step mechanism is also consistent with the bell-shaped pH dependence of the reaction rate. The non-monotonic relative motion of the magnesium ions along the reaction pathway agrees with X-ray crystal structures. Proton transfer reactions and changes in the metal ion coordination emerge as central factors in the RNA cleavage reaction.

Effect of CO2 gasification reaction on oxy-combustion of pulverized coal char

Abstract: For oxy-combustion with flue gas recirculation, as is commonly employed, it is recognized that elevated CO 2 levels affect radiant transport, the heat capacity of the gas, and other gas transport properties. A topic of widespread speculation has concerned the effect of the CO 2 gasification reaction with coal char on the char burning rate. To give clarity to the likely impact of this reaction on the oxy-fuel combustion of pulverized coal char, the Surface Kinetics in Porous Particles (SKIPPY) code was employed for a range of potential CO 2 reaction rates for a high-volatile bituminous coal char particle (130 μm diameter) reacting in several O 2 concentration environments. The effects of boundary layer chemistry are also examined in this analysis. Under oxygen-enriched conditions, boundary layer reactions (converting CO to CO 2 , with concomitant heat release) are shown to increase the char particle temperature and burning rate, while decreasing the O 2 concentration at the particle surface. The CO 2 gasification reaction acts to reduce the char particle temperature (because of the reaction endothermicity) and thereby reduces the rate of char oxidation. Interestingly, the presence of the CO 2 gasification reaction increases the char conversion rate for combustion at low O 2 concentrations, but decreases char conversion for combustion at high O 2 concentrations. These calculations give new insight into the complexity of the effects from the CO 2 gasification reaction and should help improve the understanding of experimentally measured oxy-fuel char combustion and burnout trends in the literature.

Near-surface ion distribution and buffer effects during electrochemical reactions.

Abstract: The near-surface ion distribution at the solid–liquid interface during the Hydrogen Oxidation Reaction (HOR)/Hydrogen Evolution Reaction (HER) on a rotating platinum disc electrode is demonstrated in this work. The relation between reaction rate, mass transport and the resulting surface pH-value is used to theoretically predict cyclic voltammetry behaviour using only thermodynamic and diffusion data obtained from the literature, which were confirmed by experimental measurements. The effect of buffer addition on the current signal, the surface pH and the ion distribution is quantitatively described by analytical solutions and the fragility of the surface pH during reactions that form or consume H+ in near-neutral unbuffered solutions or poorly buffered media is highlighted. While the ideal conditions utilized in this fundamental study cannot be directly applied to real scenarios, they do provide a basic understanding of the surface pH concept for more complex heterogeneous reactions.

Pozzolanic behavior of bamboo leaf ash: Characterization and determination of the kinetic parameters

Abstract: The paper presents a characterization and study of the pozzolanic behavior between calcium hydroxide (CH) and bamboo leaf ash (BLAsh), which was obtained by calcining bamboo leaves at 600 °C for 2 h in a laboratory electric furnace. To evaluate the pozzolanic behavior the conductometric method was used, which is based on the measurement of the electrical conductivity in a BLAsh/CH solution with the reaction time. Later, the kinetic parameters are quantified by applying a kinetic-diffusive model. The kinetic parameters that characterize the process (in particular, the reaction rate constant and free energy of activation) were determined with relative accuracy in the fitting process of the model. The pozzolanic activity is quantitatively evaluated according to the values obtained of the kinetic parameters. Other experimental techniques, such as X-ray diffraction (XRD) and scanning electron microscopy (SEM), were also employed. The results show that this kind of ash is formed by silica with a completely amorphous nature and a high pozzolanic activity. The correlation between the values of free energy of activation (Δ G # ) and the reaction rate constants ( K ) are in correspondence with the theoretical studies about the rate processes reported in the literature.

Reduction of highly concentrated nitrate using nanoscale zero-valent iron: Effects of aggregation and catalyst on reactivity

Abstract: Nanoscale zero-valent iron (NZVI) has been studied as an effective nitrate reduction material. Here, the effect of NZVI aggregation on the reduction reaction order and rate was investigated by comparing the nitrate reduction performances of freshly synthesized NZVI (F-NZVI), dried NZVI (D-NZVI), and dried-sonicated NZVI (DS-NZVI). Also, the effects of a catalyst were evaluated by coating 0.2 wt% Ni on previous series of NZVIs: F-Ni-NZVI, D-Ni-NZVI, and DS-Ni-NZVI. Different types of NZVIs could effectively reduce highly concentrated nitrate without requiring pH control. F-NZVI and F-Ni-NZVI showed extremely fast reactions, reducing 10–10,000 ppm within 1 min, and thus their reaction kinetics could not be evaluated under these experimental conditions. In the case of 10,000 ppm of nitrate, NZVI was almost completely consumed after reducing about 5000 ppm within 1 min. In contrast, nitrate reduction using D-NZVI and D-Ni-NZVI were pseudo-first-order reactions and DS-NZVI and DS-Ni-NZVI were 1.37 and 1.71 order reactions, respectively. D-Ni-NZVI and DS-Ni-NZVI obtained a higher reduction rate than D-NZVI and DS-NZVI due to the existence of the Ni catalyst. These experimental results suggest that the aggregate size and catalyst prominently affect the nitrate reduction rate and that the aggregation effect is more important than the catalyst effect as the aggregate size becomes smaller. Furthermore, the importance of the NZVI structure, the branch of chain-like structures and its edges exposed to the aqueous phase in nanoscale, is proposed in this study in order to explain the ultra fast reaction of F-NZVI and F-Ni-NZVI, which have yet to be reported. The final product of the reaction was ammonium, with nitrite being produced as a byproduct; NZVI changed into different shapes of magnetite (Fe 3 O 4 ) after the reaction, depending on the reaction conditions.

A new shock tube study of the H + O2 → OH + O reaction rate using tunable diode laser absorption of H2O near 2.5 μm

Abstract: The rate coefficient of the reaction H + O2 → OH + O was determined using tunable diode laser absorption of H2O near 2.5 μm behind reflected shock waves over the temperature range 1100–1530 K, at approximately 2 atm. Detailed kinetic analysis of the recorded H2O temporal profiles yielded the rate coefficient expression: k = (1.12 ± 0.08) × 1014 exp [(−7805 ± 90)/T] cm3 mol−1 s−1, with estimated uncertainties of ±4.6% at 1500 K and ±8.8% at 1100 K. Excellent agreement between this study and that of Masten et al. (1990) was found in the overlapping temperature range. By combining the results of these two studies, the reaction rate coefficient over the range 1100–3370 K was found to be described well by: k = ( 1.04 ± 0.03 ) × 10 14 exp [ ( - 7705 ± 40 ) / T ] cm 3 mol - 1 s - 1 .

Highly efficient and straightforward functionalization of cellulose films with thiol-ene click chemistry

Abstract: Three efficient and straightforward chemical pathways have been studied to functionalize solid cellulose substrates, involving for the first time alkoxysilane chemistry coupled with the photochemical version of the thiol-ene reaction. The success of the reactions was confirmed using FTIR-ATR spectroscopy and XPS analysis, but different grafting efficiencies were observed depending on the combination used. In a first route, ene-functionalized cellulose films were synthesized using vinyltrimethoxysilane as coupling agent, and were photochemically coupled with methylthioglycolate (MeGlySH). A very fast reaction rate was observed for this reaction during the first 5 min. In a second route, the opposite reaction was envisaged by clicking allylbutyrate on a thiol-functionalized cellulose surface, previously synthesized using 3-mercaptopryltrimethoxysilane as coupling agent. The success of the reaction was highlighted, but lower modification rates were observed. In a third route, a novel approach was successfully proposed for the grafting of thiol molecules on cellulose, based on the click derivatization of the molecule with alkoxysilane functions. Through this study, we expand the modular and versatile character of click chemistry to natural cellulosic substrates. But most importantly, these modification routes can be envisaged for the functionalization of other surfaces (i.e., metal alkoxides for instance) where alkoxysilane chemistry can be employed.

Measurement of a Large Chemical Reaction Rate between Ultracold Closed-Shell Ca 40 Atoms and Open-Shell Yb + 174 Ions Held in a Hybrid Atom-Ion Trap

Abstract: Ultracold $^{174}\mathrm{Yb}^{+}$ ions and $^{40}\mathrm{Ca}$ atoms are confined in a hybrid trap. The charge exchange chemical reaction rate constant between these two species is measured and found to be 4 orders of magnitude larger than recent measurements in other heteronuclear systems. The structure of the ${\mathrm{CaYb}}^{+}$ molecule is determined and used in a calculation that explains the fast chemical reaction as a consequence of strong radiative charge transfer. A possible explanation is offered for the apparent contradiction between typical theoretical predictions and measurements of the radiative association process in this and other recent experiments.

Multi-structural variational transition state theory. Kinetics of the 1,4-hydrogen shift isomerization of the pentyl radical with torsional anharmonicity

Abstract: We present a new formulation of variational transition state theory (VTST) called multi-structural VTST (MS-VTST) and the use of this to calculate the rate constant for the 1,4-hydrogen shift isomerization reaction of 1-pentyl radical and that for the reverse reaction MS-VTST uses a multi-faceted dividing surface and provides a convenient way to include the contributions of many structures (typically conformers) of the reactant and the transition state in rate constant calculations In this particular application, we also account for the torsional anharmonicity We used the multi-configuration Shepard interpolation method to efficiently generate a semi-global portion of the potential energy surface from a small number of high-level electronic structure calculations using the M06 density functional in order to compute the energies and Hessians of Shepard points along a reaction path The M06-2X density functional was used to calculate the multi-structural anharmonicity effect, including all of the structures of the reactant, product and transition state To predict the thermal rate constant, VTST calculations were performed to obtain the canonical variational rate constant over the temperature range 200–2000 K A transmission coefficient is calculated by the multidimensional small-curvature tunneling (SCT) approximation The final MS-CVT/SCT thermal rate constant was determined by combining a reaction rate calculation in the single-structural harmonic oscillator approximation (including tunneling) with the multi-structural anharmonicity torsional factor The calculated forward rate constant agrees very well with experimentally-based evaluations of the high-pressure limit for the temperature range 300–1300 K, although it is a factor of 25–30 lower than the single-structural harmonic oscillator approximation over this temperature range We anticipate that MS-VTST will be generally useful for calculating the reaction rates of complex molecules with multiple torsions