Two-step Reaction Mechanism Research Articles

Transformation of Nanomaterials in Environment

Carbon nanotubes are new class of active environmental pollutants. The authors report interactions between chlorine used as disinfectant/oxidant during typical water treatment and single-wall carbon nanotubes (SWCNTs). Alterations in SWCNT morphology were determined by TEM, while FT-IR spectra helped to determine chemical changes before, during and after chlorination. The analysis of original FT-IR spectra, and their second order derivatives revealed the formation of new, oxygen-bearing groups, namely quinones, ethers, and additional carboxyl groups on the surface of SWCNTs. The two-step reaction mechanism: (1) initial adsorption of mixture of hypochlorous acid and hypochlorite anions on the surface of carbon nanotubes, and (2) oxidation of surface and formation of new functional groups, was suggested. It was found that standard disinfection has activated previously inert carbon nanotubes and turn them into active species able to participate in further cross-contamination reactions. Interestingly, the oxygen-bearing groups were formed as a product of reaction with chlorine.

Effect of inlet temperature and equivalence ratio on HCCI engine performance fuelled with ethanol: Numerical investigation

A numerical model is presented to investigate the performance of homogeneous charge compression ignition (HCCI) engines fueled with ethanol. Two approaches are studied. On one hand, two-step reaction mechanisms with Arrhenius reaction rates are implemented in combustion chemistry modeling. On the other hand, a reduced mechanism containing important reactions of ethanol involving heat release rate and reaction rates compatible with experimental data is employed. Since controls of combustion phenomenon and ignition timing are the main issues of these engines, the effects of inlet temperature and equivalence ratio as the controlling factors on the operating parameters such as ignition timing, burn duration, in-cylinder temperature and pressure of HCCI engines are explored. The results show that the maximum predicted pressures for thermodynamic model are about 71.3×105 Pa and 79.79×105 Pa, and for chemical kinetic model, they are about 71.48×105 Pa and 78.123×105 Pa, fairly comparable with corresponding experimental values of 72×105 Pa and 78.7×105 Pa. It is observed that increasing the initial temperature advances the ignition timing, decreases the burn duration and increases the peak temperature and pressure. Moreover, the maximum temperature and pressure are associated with richer mixtures.

Determination of equilibrium and rate constants for complex formation by fluorescence correlation spectroscopy supplemented by dynamic light scattering and Taylor dispersion analysis.

The equilibrium and rate constants of molecular complex formation are of great interest both in the field of chemistry and biology. Here, we use fluorescence correlation spectroscopy (FCS), supplemented by dynamic light scattering (DLS) and Taylor dispersion analysis (TDA), to study the complex formation in model systems of dye-micelle interactions. In our case, dyes rhodamine 110 and ATTO-488 interact with three differently charged surfactant micelles: octaethylene glycol monododecyl ether C12E8 (neutral), cetyltrimethylammonium chloride CTAC (positive) and sodium dodecyl sulfate SDS (negative). To determine the rate constants for the dye-micelle complex formation we fit the experimental data obtained by FCS with a new form of the autocorrelation function, derived in the accompanying paper. Our results show that the association rate constants for the model systems are roughly two orders of magnitude smaller than those in the case of the diffusion-controlled limit. Because the complex stability is determined by the dissociation rate constant, a two-step reaction mechanism, including the diffusion-controlled and reaction-controlled rates, is used to explain the dye-micelle interaction. In the limit of fast reaction, we apply FCS to determine the equilibrium constant from the effective diffusion coefficient of the fluorescent components. Depending on the value of the equilibrium constant, we distinguish three types of interaction in the studied systems: weak, intermediate and strong. The values of the equilibrium constant obtained from the FCS and TDA experiments are very close to each other, which supports the theoretical model used to interpret the FCS data.

Reaction waves in solid fuels for adiabatic competitive exothermic reactions

We investigate travelling premixed reaction waves in a diffusional-thermal model with a two-step competitive reaction mechanism where both reactions are exothermic. Travelling waves are assumed to propagate at constant speed. An approximation of the Arrhenius reaction rate is adopted to simplify the combustion model. Based on this assumption, an asymptotic theory is presented for solid fuels under adiabatic conditions. This approach provides a convenient way to analyse the system in the phase plane. The asymptotic speeds for the flame fronts are compared with numerical solutions by solving the governing partial differential equations. In addition, piecewise approximate solutions for the temperature and fuel mass fraction profiles are presented and compared with those obtained numerically. Our results can be applied to combustion synthesis in the production of advanced materials. References Martirosyan, N. A., Dolukhanyan, S. K., and Merzhanov, A. G., Experimental observation of the nonuniqueness of stationary combustion in systems with parallel reactions. Combust. Explos. Shock Waves 19(6):711–712, 1983. doi:10.1007/BF00750777 Martirosyan, N. A., Dolukhanyan, S. K., and Merzhanov, A. G., Nonuniqueness of stationary states in combustion of mixtures of zirconium and soot powders in hydrogen. Combust. Explos. Shock Waves 19(5):569–571, 1983. doi:10.1007/BF00750423 Sidhu, H. S., Towers, I. N., Gubernov, V. V., Kolobov, A. V., and Polezhaev, A. A., Investigation of flame propagation in a model with competing exothermic reactions. Chemeca 2013 , Brisbane, Australia, 29 September–2 October, 2013. http://www.conference.net.au/chemeca2013/papers/29350.pdf Towers, I. N., Gubernov, V. V., Kolobov, A. V., Polezhaev, A. A., and Sidhu, H. S., Bistability of flame propagation in a model with competing exothermic reactions. P. R. Soc. Lond. A Mat. 469:2158, 2013. doi:10.1098/rspa.2013.0315 Hmaidi, A., McIntosh, A. C., and Brindley, J., A mathematical model of hotspot condensed phase ignition in the presence of a competitive endothermic reaction. Combust. Theory Model. 14(6):893–920, 2010. doi:10.1080/13647830.2010.519050 Matkowsky, B. J., and Sivashinsky, G. I., Propagation of a pulsating reaction front in solid fuel combustion. SIAM J. Appl. Math. 35(3):465–478, 1978. doi:10.1137/0135038 Forbes, L. K., A note on travelling waves in competitive reaction systems. ANZIAM J. 55(1):1–13, 2013. doi:10.1017/S1446181113000333 Barenblatt, G., The Mathematical theory of combustion and explosions . Springer, 1985. http://www.springer.com/us/book/9781461294399 Jovanoski, Z., and Robinson, G., Piecewise linear approximation to Fisher's equation. ANZIAM J. 53:C465–C477, 2011. http://journal.austms.org.au/ojs/index.php/ANZIAMJ/article/view/5129 Schiesser, W. E., The numerical method of lines: integration of partial differential equations . Academic Press, 1991.

In-Situ FTIR Kinetic Study in the Silylation of Low-k Films with Hexamethyldisilazane Dissolved in Supercritical CO2

In-situ Fourier transform infrared spectroscopy measurements were obtained by using an innovative equipment to study the heterogeneous reaction between a hydrolyzed porous methylsilsesquioxane film and hexamethyldisilazane (HMDS) dissolved in CO2 at supercritical conditions. Gas and solid infrared signatures were separated to obtain kinetic information of the heterogeneous reaction. A two-step reaction mechanism was observed: a fast first step controlled by kinetics and a second step controlled by the diffusion of the HMDS inside the porous material. Infrared information was used to derive a rate law expression of the silylation reaction between HMDS and Si-OH. A first order of reaction relative to the concentration of hydrophilic sites was observed with activation energy of 51.85 ± 1.25 kJ/mol.

Electrochemical characteristics of pyrrhotine as anode material for lithium-ion batteries

Micron-sized pyrrhotine particles with a standard hexagonal structure were prepared by simple precipitation and subsequent heat treatment and were investigated as anode material for lithium-ion batteries. The as-prepared Fe7S8 electrode delivered a highly reversible capacity of 604.1 mAh/g with a voltage range of 2.5–0.05 V. X-ray diffraction, scanning electronic microscopy and X-ray photoelectron spectroscopy were employed to characterize the reaction products at different stages, and a possible two-step reaction mechanism was proposed. The electrochemical test results demonstrated that the discharge/charge voltage range had a remarkable influence on the capacity retention and coulombic efficiency, which could be associated with the decomposability of lithiation products and volume change. Additionally, the dissolution of Li2Sx (2 < x < 8) in electrolyte was found to cause severe capacity loss as well. Thus a proper coating layer and cycling voltage range were essential for this kind of electrode material to achieve practical application in lithium-ion batteries.

Theoretical study of the oxidation mechanisms of thiophene initiated by hydroxyl radicals.

The mechanisms for the oxidation of thiophene by OH radicals under inert conditions (Ar) have been studied using density functional theory in conjunction with various exchange-correlation functionals. These results were compared with benchmark CBS-QB3 theoretical results. Kinetic rate constants were estimated by means of variational transition state theory (VTST) and the statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Effective rate constants were calculated via a steady-state analysis based upon a two-step model reaction mechanism. In line with experimental results, the computed branching ratios indicate that the most kinetically efficient process involves OH addition to a carbon atom adjacent to the sulfur atom. Due to the presence of negative activation energies, pressures larger than 10(4)bar are required to reach the high-pressure limit. Nucleus-independent chemical shift indices and natural bond orbital analysis show that the computed activation energies are dictated by changes in aromaticity and charge-transfer effects due to the delocalization of lone pairs from sulfur to empty π(*) orbitals. Graphical Abstract CBS-QB3 energy profiles for the reaction pathways 1-3 characterizing the oxidation of thiophene by hydroxyl radicals into the related products.

The mechanism of carboxyl activated-transfer reactions and its applications in chemistry and biology

The activation and transfer reaction on carboxyl group is not only an important topic in organic chemistry, but also a vital issue in biochemical systems. Understanding the details of these reactions could help investigators develop new synthetic methods, syntheze functional proteins and design new drugs or inhibitors. In this article, potential energy surfaces based on general physical organic and biochemical princeples are employed to analyze the structures and properties the transition states or intermediates in these reactions, as well as catalysts involved situations. A tetrahedron intermediate mediated two-step addition-elimination reaction mechanism is propused to be the most possible reaction process and Hammond postulate is used to determine the relative energy value of different transition states and intermediates. During the reaction, those activating groups which can effectively stablized the tetrahedron intermediates and eliminating from the centra carbon atom easily are illustrated to be better carboxyl group activating reagents, as the similar situations in catalysts of carboxyl activated reactions. Different cases in chemistry and biology of carboxyl activated-transfer reactions are compared and proved to be follow the same rules. And the applications of carboxyl activated-transfer reactions in chemical synthesis of peptides and biochemical process are also discussed. The analysis and comments of this review might serve researchers as meaningful reference on devoloping new carboxyl activated-transfer reactions.

A characterization of the two-step reaction mechanism of phenol decomposition by a Fenton reaction

Phenol is one of the worst contaminants at date, and its degradation has been a crucial task over years. Here, the decomposition process of phenol, in a Fenton reaction, is described. Using scavengers, it was observed that decomposition of phenol was mainly influenced by production of hydroxyl radicals. Experimental and theoretical activation energies (Ea) for phenol oxidation intermediates were calculated. According to these Ea, phenol decomposition is a two-step reaction mechanism mediated predominantly by hydroxyl radicals, producing a decomposition yield order given as hydroquinone>catechol>resorcinol. Furthermore, traces of reaction derived acids were detected by HPLC and GS-MS.

Degradation kinetics of starch-g-poly(phenyl methacrylate) copolymers

Both the thermal behavior and kinetic decomposition of starch-g-poly(phenyl methacrylate) copolymers were studied by the TG/DSC/FTIR/QMS coupled technique under inert conditions. In order to compare the results, potato starch was used as a reference material. The results indicate that the thermal degradation takes place in two large stages: (i) destruction of the molecular polysaccharide blocks at temperatures ≤300°C and (ii) structural graft depolymerization from 320 to 440°C. The kinetic analysis in terms of E(α) on α indicates that during the initial stage, with activation energies ranging from 140 to 205kJmol−1, a two-step reaction mechanism involving two consecutive reactions takes place, an initial endothermic reversible reaction followed by an irreversible one, which is related to the cracking of the starch glycosidic bonds. Meanwhile the degradation analysis of the graft polymer reveals that the stage involves several overlapping steps with average activation energies between 120 and 180kJmol−1.

Numerical simulations of the radiative transfer in a 2D axisymmetric turbulent non-premixed methane–air flame using up-to-date WSGG and gray-gas models

This paper presents a study of the effect of thermal radiation in the simulation of a turbulent, non-premixed methane–air flame. In such a problem, modeling of radiative properties of the gaseous mixture is a fundamental aspect to be evaluated. In this work, such properties were modeled using three different models [a constant-ratio (CR-) and a non-constant-ratio (NCR-) weighted-sum-of-gray-gases (WSGG); and a gray-gas (GG) model], both based on newly obtained correlations from HITEMP 2010 database. The chemical reaction rates were considered as the minimum values between Arrhenius and Eddy Break-Up rates. A two-step global reaction mechanism was used, while the turbulence modeling was considered via standard k–e model. The source terms of the energy equation consisted of the heat generated in the chemical reaction rates as well as in the radiation exchanges. The discrete ordinates method (DOM) was employed to solve the radiative transfer equation (RTE), including the TRI. Comparisons of simulations with/without radiation demonstrated that the temperature, the radiative heat source, and the wall heat flux were importantly affected by thermal radiation, while the influence on species concentrations proved to be less important. The numerical results considering radiation in the analysis were closer to the experimental data from literature when compared to the case neglecting it; the computation of radiation with both the CR-WSGG and NCR-WSGG models provided better results than GG model, when compared with experimental data. For the combustion chamber and turbulence and combustion models employed in the present work, it was shown that the CR-WSGG is the recommended model, due to the acceptable agreement with experimental data and relatively low computational requirements in comparison to the NCR-WSGG model. The results show the importance of thermal radiation for an accurate prediction of the thermal behavior of a combustion chamber.

Synthesis and characterization of a novel mucoadhesive derivative of xyloglucan

A novel polymer in the form of a thiolated derivative of natural tamarind seed polysaccharide or xyloglucan was synthesized and its chacteristics as a mucoadhesive polymer were studied as a part of the study undertaken herein. The synthetic route followed involves a two-step reaction mechanism of firstly oxidizing xyloglucan and then further conjugating it with l-cysteine to form thiolated xyloglucan or thiomer via imine linkage. The thiomer thus formed was characterized using various analytical techniques as differential scanning calorimetry (DSC), X-ray diffraction analysis (XRD), and nuclear magnetic resonance (NMR). Ellman's method was used to determine the numbers of thiol groups/g of thiolated xyloglucan. Zeta potential measurements were carried out for thiolated xyloglucan. Viscosities of the formulated xyloglucan and thiolated xyloglucan gels were comparatively evaluated along with the evaluation of mucoadhesive properties of the gels using ex vivo bioadhesion study employing freshly excised sheep intestinal mucosa.

The effect of depletion of radicals on freely propagating hydrocarbon flames

In this work we investigate the properties and stability of deflagration waves in the model which include the two-step chain-branching reaction mechanism, first order reaction of conversion of radicals into products, and Newtonian cooling to the surroundings. The propagation velocity, the maximum temperature and concentration of radicals monotonically decay as the rate of the radical termination reaction is increased. The critical parameter values for the onset of instabilities and quenching of the travelling combustion wave are found. It is demonstrated that the depletion of radicals through the additional termination reaction enhances the onset of instabilities and extinction. The comparison of the efficiency of the radical and thermal quenching mechanisms is performed. The solutions emerging once the travelling deflagration wave becomes unstable are studied.

Reaction mechanism of diphenylborinic acid with d-fructose in aqueous solution

The reaction of d-fructose with diphenylborinic acid (Ph2B(OH)), to which d-fructose cannot coordinate tridentately, was kinetically investigated in order to solve the reaction mechanism, and to find a clue to resolve the two-step reaction mechanism of boronic acid (RB(OH)2) with d-fructose. The title reaction was a single-step reaction under the pseudo first-order conditions with large excess d-fructose over Ph2B(OH), which suggests that the reaction of RB(OH)2 with d-fructose consists of the fast formation of a two-coordinate complex followed by the slow formation of a three-coordinate complex. The kinetic reactivity of Ph2B(OH) was considerably higher than that of its conjugate base, diphenylborinate ion (Ph2B(OH)2−). It was shown that Ph2B(OH)2− interacted with CHES buffer to form an outer-sphere complex, whereas no interaction was observed between Ph2B(OH) and CHES buffer. Both the free borinate ion and the restrained borinate ion into the outer-sphere complex react with d-fructose to form the chelate complex via stable binary and ternary outer-sphere complexes, respectively.

A faradaic impedance study on the kinetic properties of water photosplitting at illuminated TiO2/solution interface

Photoelectrochemical (PEC) water oxidation is the key reaction for hydrogen generation from water photosplitting at illuminated n-semiconductor/solution interfaces. In this work, the kinetic property of water photooxidation at TiO2/solution interface, under illumination of simulated sunlight with constant incident light intensity (AM 1.5, 100 mW cm−2) and in pH 6–10 solutions (either or not containing carbonate additives), was investigated by using photocurrent measurement and electrochemical impedance spectroscopy (EIS) techniques. Tafel-type behavior was revealed for the potential dependence of photocurrents within a potential region above the photocurrent onset potential. The measured EIS spectra were first analyzed by equivalent electrical circuit (EEC) fitting. Then, by interpreting the Faradaic impedance for the photocharge transfer processes based on a proposed two-step reaction mechanism accounting for the water photooxidation at TiO2 surface, it was shown that more interested and more important kinetic parameters (rate constants and surface coverage of intermediates) can be obtained from the EEC element parameters. Based on the results of this work, novel features for the effects of solution pH and the carbonate addition on the kinetics of water photooxidation were revealed and discussed.

Production and optimization of biodiesel using mixed immobilized biocatalysts in packed bed reactor.

Vegetable oils are used as raw materials for biodiesel production using transesterification reaction. Several methods for the production of biodiesel were developed using chemical (alkali and acidic compounds) and biological catalysts (lipases). Biodiesel production catalyzed by lipases is energy and cost-saving processes and is carried out at normal temperature and pressure. The need for an efficient method for screening larger number of variables has led to the adoption of statistical experimental design. In the present study, packed bed reactor was designed to study with mixed immobilized biocatalysts to have higher productivity under optimum conditions. Contrary to the single-step acyl migration mechanism, a two-step stepwise reaction mechanism involving immobilized Candida rugosa lipase and immobilized Rhizopus oryzae cells was employed for the present work. This method was chosen because enzymatic hydrolysis followed by esterification can tolerate high free fatty acid containing oils. The effects of flow rate and bed height on biodiesel yield were studied using two factors five-level central composite design (CCD) and response surface methodology (RSM). Maximum biodiesel yield of 85 and 81% was obtained for jatropha oil and karanja oil with the optimum bed height and optimum flow rate of 32.6cm and 1.35L/h, and 32.6cm and 1.36L/h, respectively.

Reacting porous solids with phase segregation

In this work, a detailed single particle model was developed to describe the reaction of porous solids consisting of different solid species, with an initial composition that can be non-uniformly distributed along its radius, and multiple reactions. The continuous particle model is used to describe the rate of reduction of iron–titanium oxide particles for Chemical Looping Combustion processes, considering a two-step reaction mechanism involving two solid reagents: hematite and pseudobrookite. The experimentally observed non-uniform initial distribution of solid species, as a consequence of the solids activation procedure, was accounted for by assuming an initial core–shell structure with a diffused interface with different compositions of the two areas of the pellets. A detailed sensitivity analysis was carried out assuming different kinetics and initial concentration profiles of the different solid species involved. The results confirm that the initial distribution of hematite and pseudobrookite has a major influence on the predicted particle conversion rate. A comparison was carried out with experimental data obtained by thermogravimetric analysis for the reduction of small spherical ilmenite particles (200µm) with CO and H2 at 600–800°C and at atmospheric pressure. The study proves that the model can explain features of the experimental results that do not fit any shrinking core model.

Optimization of a log wood boiler through CFD simulation methods

This paper describes the steady-state modeling and simulation of a wood log fired boiler. It is designed and manufactured by THERMODYNAMIKI S.A. (KOMBI), a company based on Northern Greece. Its nominal fuel power is around 32kW and its efficiency is close to 75%. A specific operating case, selected and experimentally investigated by the manufacturer, is examined. Due to the highly transient and complex nature of wood log combustion, an ad-hoc model has been developed. Its main characteristic is the fuel division into three components: water vapor, volatiles and fixed char. The latter is considered to form a layer, which is treated as a porous medium thus forming a volumetric source zone adjacent to the firebed area. Volatiles and water vapor are modeled as volumetric mass sources, whereas a two-step reaction mechanism of the former is considered. The numerical results are compared against corresponding experimental data for the nominal load as far as flue gas characteristics (temperature and species concentration) at the boiler exit, are concerned. Based on the validated model, a proposed optimization pattern of the boiler is undertaken and examined by means of CFD.

Solvothermal Synthesis of LiMn1–xFexPO4Cathode Materials: A Study of Reaction Mechanisms by Time-Resolved in Situ Synchrotron X-ray Diffraction

We applied time-resolved in situ synchrotron X-ray diffraction (XRD) to study the reaction processes and pathways during the solvothermal synthesis of the olivine-structured LiFePO4, LiMnPO4, and LiFe0.4Mn0.6PO4 with ethylene glycol C2H6O2 (EG) as the solvent by following the evolution of the crystal structures of the Fe/Mn-containing phases. We identified a stable intermediate phase in the synthetic reaction process of LiFePO4, viz., a ferrous oxalate EG complex (FeC2O4·C2H6O2), and resolved its structure; thus, we established a two-step reaction mechanism involving dissolution–precipitation followed by interface-coupled dissolution–reprecipitation for the synthesis of LiFePO4. The synthetic reactions in an LiFe0.4Mn0.6PO4 solid solution also followed a two-step process via the formation of a metastable intermediate phase bearing a structural similarity to FeC2O4·C2H6O2 that, however, has a slightly larger unit-cell, indicating that the substitution of Fe by Mn occurred at the intermediate stage. In cont...

Simulation of Gaseous Oxygen/Hydroxyl-Terminated Polybutadiene Hybrid Rocket Flowfields and Comparison with Experiments

Numerical simulations of the flowfield in a gaseous oxygen/hydroxyl-terminated polybutadiene hybrid rocket engine are carried out with a Reynolds-averaged Navier–Stokes solver including detailed gas/surface interaction modeling based on surface mass and energy balances. Fuel pyrolysis is modeled via finite-rate Arrhenius kinetics. A simplified two-step global reaction mechanism is considered for the gas-phase chemistry to model the combustion of 1,3-butadiene in oxygen. Results are compared with the firing test data obtained from a laboratory-scale hybrid rocket in which gaseous oxygen is fed into axisymmetric hydroxyl-terminated polybutadiene cylindrical grains through an axial conical subsonic nozzle. With the oxidizer fed by this injector, which generates nonuniform conditions at the entrance of the fuel port, the fuel regression rate is shown to increase several times with respect to the case of homogeneous injection of the oxidizer through all the grain port area, in agreement with the experimental findings. The spatial distribution of the regression rate is substantially different from the one expected in a turbulent developing flow through a pipe because of the presence of a strong recirculation zone at the motor head end. Numerical simulations, although not capturing the absolute regression-rate values, are fairly able to predict the main ballistic features: that is, the regression rate’s weaker dependence on the mass flux, the influence of port diameter, and the pressure evolution over time, as well as the general trends of different firing tests.