Critical radius for hot-jet ignition of hydrogen-air mixtures

This study addresses deflagration initiation of lean and stoichiometric hydrogen–air mixtures by the sudden discharge of a hot jet of their adiabatic combustion products. The objective is to compute the minimum jet radius required for ignition, a relevant quantity of interest for safety and technological applications. For sufficiently small discharge velocities, the numerical solution of the problem requires integration of the axisymmetric Navier–Stokes equations for chemically reacting ideal-gas mixtures, supplemented by standard descriptions of the molecular transport terms and a suitably reduced chemical-kinetic mechanism for the chemistry description. The computations provide the variation of the critical radius for hot-jet ignition with both the jet velocity and the equivalence ratio of the mixture, giving values that vary between a few tens microns to a few hundred microns in the range of conditions explored. For a given equivalence ratio, the critical radius is found to increase with increasing injection velocities, although the increase is only moderately large. On the other hand, for a given injection velocity, the smallest critical radius is found at stoichiometric conditions.

Investigação do mecanismo cinético da reação de redução de oxigênio em solventes não aquosos

O aumento no consumo energético e a crescente preocupação ambiental frente à emissão de gases poluentes criam um apelo mundial favorável para pesquisas de novas tecnologias não poluentes de fontes de energia. Baterias recarregáveis de lítio-ar em solventes não aquosos possuem uma alta densidade de energia teórica (5200 Wh kg-1), o que as tornam promissoras para aplicação em dispositivos estacionários e em veículos elétricos. Entretanto, muitos problemas relacionados ao cátodo necessitam ser contornados para permitir a aplicação desta tecnologia, por exemplo, a baixa reversibilidade das reações, baixa potência e instabilidades dos materiais empregados nos eletrodos e dos solventes eletrolíticos. Assim, neste trabalho um modelo cinético foi empregado para os dados experimentais de espectroscopia de impedância eletroquímica, para a obtenção das constantes cinéticas das etapas elementares do mecanismo da reação de redução de oxigênio (RRO), o que permitiu investigar a influência de parâmetros como o tipo e tamanho de partícula do eletrocatalisador, o papel do solvente utilizado na RRO e compreender melhor as reações ocorridas no cátodo dessa bateria. A investigação inicial se deu com a utilização de sistemas menos complexos como uma folha de platina ou eletrodo de carbono vítreo como eletrodos de trabalho em 1,2-dimetoxietano (DME)/perclorato de lítio (LiClO4). A seguir, sistemas complexos com a presença de nanopartículas de carbono favoreceu o processo de adsorção das moléculas de oxigênio e aumentou ligeiramente (uma ordem de magnitude) a etapa de formação de superóxido de lítio (etapa determinante de reação) quando comparada com os eletrodos de platina e carbono vítreo, atribuída à presença dos grupos laterais mediando à transferência eletrônica para as moléculas de oxigênio. No entanto, foi observada uma rápida passivação da superfície eletrocatalítica através da formação de filmes finos de Li2O2 e Li2CO3 aumentando o sobrepotencial da bateria durante a carga (diferença de potencial entre a carga e descarga > 1 V). Adicionalmente, a incorporação das nanopartículas de platina (Ptnp), ao invés da folha de platina, resultou no aumento da constante cinética da etapa determinante da reação em duas ordens de magnitude, o qual pode ser atribuído a uma mudança das propriedades eletrônicas na banda d metálica em função do tamanho nanométrico das partículas, e estas modificações contribuíram para uma melhor eficiência energética quando comparado ao sistema sem a presença de eletrocatalisador. Entretanto, as Ptnp se mostraram não específicas para a RRO, catalisando as reações de degradação do solvente eletrolítico e diminuindo rapidamente a eficiência energética do dispositivo prático, devido ao acúmulo de material no eletrodo. O emprego de líquido iônico como solvente eletrolítico, ao invés de DME, promoveu uma maior estabilização do intermediário superóxido formado na primeira etapa de transferência eletrônica, devido à interação com os cátions do líquido iônico em solução, o qual resultou em um valor de constante cinética da formação do superóxido de três ordens de magnitude maior que o obtido com o mesmo eletrodo de carbono vítreo em DME, além de diminuir as reações de degradação do solvente. Estes fatores podem contribuir para uma maior potência e ciclabilidade da bateria de lítio-ar operando com líquidos iônicos.

Interpretation of the temperature dependence of rate constants in biosensor studies

A comparison is made between Arrhenius and transition-state analyses of the temperature dependence of rate constants reported in four published biosensor studies. Although the Eyring transition-state theory seemingly affords a more definitive solution to the problem of characterizing the activation energetics, the analysis is equivocal because of inherent assumptions about reaction mechanism and the magnitude of the transmission coefficient. In view of those uncertainties it is suggested that a preferable course of action entails reversion to the empirical Arrhenius analysis with regard to the energy of activation and a preexponential factor. The former is essentially equivalent to the enthalpy of activation, whereas the magnitude of the latter indicates directly the extent of disparity between the frequency of product formation and the universal frequency factor (temperature multiplied by the ratio of the Boltzmann and Planck constants) and hence the likelihood of a more complicated kinetic mechanism than that encompassed by the Eyring transition-state theory. (C) 2004 Elsevier Inc. All rights reserved.

Lead compounds for antimalarial chemotherapy: Purine base analogs discriminate between human and P-falciparum 6-oxopurine phosphoribosyltransferases

The malarial parasite Plasmodium falciparum depends on the purine salvage enzyme hypoxanthine-guanine-xanthine phosphoribosyltransferase (HGXPRT) to convert purine bases from the host to nucleotides needed for DNA and RNA synthesis. An approach to developing antimalarial drugs is to use HGXPRT to convert introduced purine base analogs to nucleotides that are toxic to the parasite. This strategy requires that these compounds be good substrates for the parasite enzyme but poor substrates for the human counterpart, HGPRT. Bases with a chlorine atom in the 6-position or a nitrogen in the 8-position exhibited strong discrimination between P. falciparum HGXPRT and human HGPRT. The k(cat)/K-m values for the Plasmodium enzyme using 6-chloroguanine and 8-azaguanine as substrates were 50-80-fold and 336-fold higher than for the human enzyme, respectively. These and other bases were effective in inhibiting the growth of the parasite in vitro, giving IC50 values as low as 1 mu M.

Multiscale modelling of heterogeneously catalysed transesterification reaction process:an overview

Biodiesel is fast becoming one of the key transport fuels as the world endeavours to reduce its carbon footprint and find viable alternatives to oil derived fuels. Research in the field is currently focusing on more efficient ways to produce biodiesel, with the most promising avenue of research looking into the use of heterogeneous catalysis. This article presents a framework for kinetic reaction and diffusive transport modelling of the heterogeneously catalysed transesterification of triglycerides into fatty acid methyl esters (FAMEs), unveiled by a model system of tributyrin transesterification in the presence of MgO catalysts. In particular, the paper makes recommendations on multicomponent diffusion calculations such as the diffusion coefficients and molar fluxes from infinite dilution diffusion coefficients using the Wilke and Chang correlation, intrinsic reaction kinetic studies using the Eley-Rideal kinetic mechanism with methanol adsorption as the rate determining steps and multiscale reaction-diffusion process simulation between catalytic porous and bulk reactor scales. © 2013 The Royal Society of Chemistry.

Thermodynamic and kinetic studies on the mechanism of binding of methylumbelliferyl galactosides to the basic agglutinin from winged bean (Psophocarpus tetragonolobus)

The binding of winged bean basic agglutinin (WBA I) to 4-methylumbelliferyl (MeUmb) galactosides was examined by extrinsic fluorescence titration and stopped-flow spectrofluorimetry. Upon binding to WBA I, MeUmb alpha-galactosides show quenching in fluorescence intensity, decrease in UV absorbance with a concomitant blue shift, and decrease in fluorescence excited-state lifetimes. However, their beta-analogues show enhancement in fluorescence intensity, increase in UV absorbance with a red shift, and an increase in fluorescence excited-state lifetimes. This implies that the umbelliferyl groups of alpha- and beta-galactosides experience non-polar and polar microenvironments, respectively, upon binding to WBA I. Replacement of the anomeric hydroxyl group of galactose by 4-methylumbelliferyl moiety increases the affinity of resulting saccharides. Substitution of C-2 hydroxyl of galactose by an acetamido group leads to increased affinity due to a favorable entropy change. This suggests that acetamido group of MeUmb-alpha/beta-GalNAc binds to a relatively non-polar subsite of WBA I. Most interestingly, this substitution also reduces the association rate constants dramatically. Inspection of the activation parameters reveals that the enthalpy of activation is the limiting factor for the differences in the forward rate constants for these saccharides and the entropic contribution to the activation energy is small

Thermodynamic and kinetic studies on the mechanism of binding of methylumbelliferyl glycosides to jacalin

The binding of Artocarpus integrifolia lectin (jacalin) to 4-methylumbelliferyl (Meumb)-glycosides, Gal alpha Meumb, Gal beta Meumb, GalNAc alpha Meumb, GalNAc beta-Meumb, and Gal beta 3GalNAc beta Meumb was examined by extrinsic fluorescence quenching titration and stopped flow spectrofluorimetry. The binding was characterized by 100% quenching of fluorescence of Meumb-glycosides. Their association constants range from 2.0 x 10(4) to 1.58 x 10(6) M-1 at 15 degrees C. Entropic contribution is the major stabilizing force for avid binding of Meumb-glycosides indicating the existence of a hydrophobic site that is complementary to their methylumbelliferyl group. The second order association rate constants for interaction of these sugars with lectin at 15 degrees C vary from 8.8 x 10(5) to 3.24 x 10(6) M-1 S-1, at pH 7.2. The first order dissociation rate constants range from 2.30 to 43.0 S-1 at 15 degrees C. Despite the differences in their association rate constants, the overall values of association constants for these saccharides are determined by their dissociation rate constants. The second order rate constant for the association of Meumb-glycosides follows a pattern consistent with the magnitude of the activation energies involved therin. Activation parameters for association of all ligands illustrate that the origin of the barrier between binding of jacalin to Meumb-glycosides is entropic, and the enthalpic contribution is small. A correlation between these parameters and the structure of the ligands on the association rates underscores the importance of steric factors in determining protein saccharide recognitions.

Mechanism of lectin-cell interactions: thermodynamic and kinetic analysis of the binding of winged bean acidic lectin (WBA II) to human erythrocytes

In order to identify the forces involved in the binding and to understand the mechanism involved, equilibrium and kinetic studies were performed on the binding of the winged bean acidic lectin to human erythrocytes. The magnitudes of delta S and delta H were positive and negative respectively, an observation differing markedly from the lectin-simple sugar interactions where delta S and delta H are generally negative. Analysis of the sign and magnitudes of these values indicate that ionic and hydrogen bonded interactions prevail over hydrophobic interactions resulting in net -ve delta H (-37.12 kJ.mol-1) and +ve delta S (14.4 J.mole-1 K-1 at 20 degrees C), thereby suggesting that this entropy driven reaction also reflects conformational changes in the lectin and/or the receptor. Presence of two kinds of receptors for WBA II on erythrocytes, as observed by equilibrium studies, is consistent with the biexponential dissociation rate constants (at 20 degrees C K1 = 1.67 x 10(-3) M-1 sec-1 and K2 = 11.1 x 10(-3) M-1 sec-1). These two rate constants differed by an order of magnitude accounting for the difference in the association constants of the two receptors of WBA II. However, the association process remains monoexponential suggesting no observable difference in the association rates of the lectin molecule with both the receptors, under the experimental conditions studied. The thermodynamic parameters calculated from kinetic data correlate well with those observed by equilibrium. A two-step binding mechanism is proposed based on the kinetic parameters for WBA II-receptor interaction

Detailed mechanism and kinetic study of CO oxidation on cobalt oxide surfaces

Co3O4 catalysts were prepared by combustion synthesis using different fuels glycine (G), ODH (O) and urea (U). Morphological changes of the materials were observed by using different fuels. The prepared catalysts were characterized by XRD, XPS, SEM, TEM, BET and DRIFTS analysis. All compounds showed 100% conversion of CO below 175C. The prepared catalysts exhibited very high stability and conversions did not decrease even after 50 h of continuous operation. The oxygen storage capacity (OSC) of materials was measured by H-2-TPR analysis. Co3O4-O is having high OSC among the synthesized catalysts. The activation energies of these catalysts were found to be in the range of 42.3-64.8 kJ mol(-1). With DRIFTS analysis, the surface carbonates, superoxide anions, adsorbed CO, O-2 species on the catalyst surface were found and this information was used to develop a detailed reaction pathway. A kinetic model was developed with the help of proposed mechanism and used to fit the data. (C) 2014 Elsevier B.V. All rights reserved.

The use of short time-on-stream in situ spectroscopic transient kinetic isotope techniques to investigate the mechanism of hydrocarbon selective catalytic reduction (HC-SCR) of NOx at low temperatures

The problem of differentiating between active and spectator species that have similar infrared spectra has been addressed by developing short time-on-stream in situ spectroscopic transient isotope experimental techniques (STOS-SSITKA). The techniques have been used to investigate the reaction mechanism for the reduction of nitrogen oxides (NOx) by hydrocarbons under lean-burn (excess oxygen) conditions on a silver catalyst. Although a nitrate-type species tracks the formation of isotopically labeled dinitrogen, the results show that this is misleading because a nitrate-type species has the same response to an isotopic switch even under conditions where no dinitrogen is produced. In the case of cyanide and isocyanate species, the results show that it is possible to differentiate between slowly reacting spectator isocyanate species, probably adsorbed on the oxide support, and reactive isocyanate species, possibly on or close to the active silver phase. The reactive isocyanate species responds to an isotope switch at a rate that matches that of the rate of formation of the main product, dinitrogen. It is concluded that these reactive isocyanates could potentially be involved in the reduction of NOx whereas there is no evidence to support the involvement of nitrate-type species that are observable by infrared spectroscopy.

Chemistry and Mechanism of Alizarin Red S and Methylene Blue Biosorption onto Olive Stone: Equilibrium and Kinetic

The biosorption process of anionic dye Alizarin Red S (ARS) and cationic dye methylene blue (MB) as a function of solution pH, initial concentration and contact time onto olive stone (OS) biomass has been investigated. The main objectives of the current study are to: (i) study the chemistry and the mechanism of ARS and MB biosorption onto olive stone and the type of OS–ARS, MB interactions occurring, (ii) study the biosorption equilibrium and kinetic experimental data required for the design and operation of column reactors. Equilibrium biosorption isotherms and kinetics were also examined. Experimental equilibrium data were fitted to four different isotherms by non-linear regression method, however, the biosorption experimental data for ARS and MB dyes were well interpreted by the Temkin and Langmuir isotherms, respectively. The maximum monolayer adsorption capacity for ARS and MB dyes were 109.0 and 102.6 mg/g, respectively. The kinetic data of the two dyes could be better described by the pseudo second-order model. The data showed that olive stone can be effectively used for removing dyes from wastewater.

DNA hybridization mechanism in an interfacial environment: What hides beneath first order k (s(-1)) kinetic constant?

The scientific question addressed in this work is: what hides beneath first order kinetic constant k (s(-1)) measured for hybridization of a DNA target on a biosensor surface. Kinetics hybridization curves were established with a 27 MHz quartz microbalance (9 MHz, third harmonic) biosensor, constituted of a 20-base probe monolayer deposited on a gold covered quartz surface. Kinetics analysis, by a known two-step adsorption-hybridization mechanism, is well appropriate to fit properly hybridization kinetics curves, for complementary 20-base to 40-base targets over two concentration decades. It was found that the K-1 (M-1) adsorption constant, relevant to the first step, concerns an equilibrium between non hybridized targets and hybridized pre-complex and increases with DNA target length. It was established that k(2) (s(-1)), relevant to irreversible formation of a stable duplex, varies in an opposite way to K-1 with DNA target length. (C) 2012 Published by Elsevier B.V.

Shvo's catalyst in chemoenzymatic dynamic kinetic resolution of amines-inner or outer sphere mechanism?

Evidence is provided for the inner-sphere mechanism with actual metal coordination of the racemic amine in the crucial hydrogen transfer step promoted by Shvo's catalyst of the chemoenzymatic dynamic kinetic resolution (DKR) of amines. Key intermediates involved in this H-transfer step were intercepted and continuously monitored by electrospray ionization mass spectrometry (ESI-MS) and characterized by their dissociation chemistries via ESI-MS/MS. © 2013 The Royal Society of Chemistry.

Kinetic stability as a mechanism for protease longevity

The folding of the extracellular serine protease, α-lytic protease (αLP; EC 3.4.21.12) reveals a novel mechanism for stability that appears to lead to a longer functional lifetime for the protease. For αLP, stability is based not on thermodynamics, but on kinetics. Whereas this has required the coevolution of a pro region to facilitate folding, the result has been the optimization of native-state properties independent of their consequences on thermodynamic stability. Structural and mutational data lead to a model for catalysis of folding in which the pro region binds to a conserved β-hairpin in the αLP C-terminal domain, stabilizing the folding transition state and the native state. The pro region is then proteolytically degraded, leaving the active αLP trapped in a metastable conformation. This metastability appears to be a consequence of pressure to evolve properties of the native state, including a large, highly cooperative barrier to unfolding, and extreme rigidity, that reduce susceptibility to proteolytic degradation. In a test of survival under highly proteolytic conditions, homologous mammalian proteases that have not evolved kinetic stability are much more rapidly degraded than αLP. Kinetic stability as a means to longevity is likely to be a mechanism conserved among the majority of extracellular bacterial pro-proteases and may emerge as a general strategy for intracellular eukaryotic proteases subject to harsh conditions as well.