Intrinsic Rate Constants Research Articles

Photon Reabsorption Masks Intrinsic Bimolecular Charge-Carrier Recombination in CH3NH3PbI3 Perovskite.

An understanding of charge-carrier recombination processes is essential for the development of hybrid metal halide perovskites for photovoltaic applications. We show that typical measurements of the radiative bimolecular recombination constant in CH3NH3PbI3 are strongly affected by photon reabsorption that masks a much larger intrinsic bimolecular recombination rate constant. By investigating a set of films whose thickness varies between 50 and 533 nm, we find that the bimolecular charge recombination rate appears to slow by an order of magnitude as the film thickness increases. However, by using a dynamical model that accounts for photon reabsorption and charge-carrier diffusion we determine that a single intrinsic bimolecular recombination coefficient of value 6.8 × 10-10 cm3s-1 is common to all samples irrespective of film thickness. Hence, we postulate that the wide range of literature values reported for such coefficients is partly to blame on differences in photon out-coupling between samples with crystal grains or mesoporous scaffolds of different sizes influencing light scattering, whereas thinner films or index-matched surrounding layers can reduce the possibility for photon reabsorption. We discuss the critical role of photon confinement on free charge-carrier retention in thin photovoltaic layers and highlight an approach to assess the success of such schemes from transient spectroscopic measurement.

External Mass Transfer Model for Hydrogen Peroxide Decomposition by Terminox Ultra Catalase in a Packed-Bed Reactor

Abstract It is known that external diffusional resistances are significant in immobilized enzyme packed-bed reactors, especially at large scales. Thus, the external mass transfer effects were analyzed for hydrogen peroxide decomposition by immobilized Terminox Ultra catalase in a packed-bed bioreactor. For this purpose the apparent reaction rate constants, kP, were determined by conducting experimental works at different superficial velocities, U, and temperatures. To develop an external mass transfer model the correlation between the Colburn factor, JD, and the Reynolds number, Re, of the type JD = K Re(n-1) was assessed and related to the mass transfer coefficient, kmL. The values of K and n were calculated from the dependence (am kp-1 - kR-1) vs. Re-1 making use of the intrinsic reaction rate constants, kR, determined before. Based on statistical analysis it was found that the mass transfer correlation JD = 0.972 Re-0.368 predicts experimental data accurately. The proposed model would be useful for the design and optimization of industrial-scale reactors.

Distance-Dependence of Interparticle Energy Transfer in the Near-Infrared within Electrostatic Assemblies of PbS Quantum Dots.

This paper describes control of the rate constant for near-infrared excitonic energy transfer (EnT) within soluble aqueous assemblies of PbS quantum dots, cross-linked by Zn2+, by changing the length of the mercapto-alkanoic acid (MAA) that serves as the cross-linking ligand. Sequestration of Zn2+ by a chelating agent or zinc hydroxide species results in deaggregation of the assemblies with EnT turned "off". Upon decreasing the number of methylene groups in MAAs from 16 to 3, the interparticle separation decreases from 5.8 nm to 3.7 nm and the average observed EnT rate increases from ∼(150 ns)-1 to ∼(2 ns)-1. A master equation translates intrinsic (single-donor-single-acceptor) EnT rate constants predicted for each ligand length using Förster theory to observed average rate constants. For interparticle distances greater than ∼4 nm, the point dipole approximation (PDA) implementation of Förster theory agrees with experimentally measured rates. At shorter interparticle distances, the PDA drastically underestimates the observed EnT rate. The prediction of the rates of these short-distance EnT processes is improved by ∼20% by replacing the PDA with a transition density cube calculation of the interparticle Coulombic coupling.

Generalized Temporal Acceleration Scheme for Kinetic Monte Carlo Simulations of Surface Catalytic Processes by Scaling the Rates of Fast Reactions

A novel algorithm is presented that achieves temporal acceleration during kinetic Monte Carlo (KMC) simulations of surface catalytic processes. This algorithm allows for the direct simulation of reaction networks containing kinetic processes occurring on vastly disparate time scales which computationally overburden standard KMC methods. Previously developed methods for temporal acceleration in KMC were designed for specific systems and often require a priori information from the user such as identifying the fast and slow processes. In the approach presented herein, quasi-equilibrated processes are identified automatically based on previous executions of the forward and reverse reactions. Temporal acceleration is achieved by automatically scaling the intrinsic rate constants of the quasi-equilibrated processes, bringing their rates closer to the time scales of the slow kinetically relevant nonequilibrated processes. All reactions are still simulated directly, although with modified rate constants. Abrupt changes in the underlying dynamics of the reaction network are identified during the simulation, and the reaction rate constants are rescaled accordingly. The algorithm was utilized here to model the Fischer-Tropsch synthesis reaction over ruthenium nanoparticles. This reaction network has multiple time-scale-disparate processes which would be intractable to simulate without the aid of temporal acceleration. The accelerated simulations are found to give reaction rates and selectivities indistinguishable from those calculated by an equivalent mean-field kinetic model. The computational savings of the algorithm can span many orders of magnitude in realistic systems, and the computational cost is not limited by the magnitude of the time scale disparity in the system processes. Furthermore, the algorithm has been designed in a generic fashion and can easily be applied to other surface catalytic processes of interest.

Extracting Kinetic Isotope Effects From a Global Analysis of Reaction Progress Curves.

Enzyme reaction progress curves, or time course datasets, are often rich in information, yet their analysis typically reduces their information content to a single parameter, the initial velocity. An alternative approach is described here, where the time course is described by a model constructed from rate equations. In combination with global nonlinear regression, intrinsic rate and/or equilibrium constants can be directly obtained by fitting these data. This method can be greatly enhanced when combined with the measurement of (usually deuterium) isotope effects, which selectively perturb individual step(s) within the reaction, allowing better separation of fitted parameters and more robust model testing. This chapter focuses on practical considerations when using analytical and/or numerically integrated rate equations to model enzyme reactions. The emphasis is on the underlying methodology, which is demonstrated with specific examples alongside recommendations of suitable software.

Transformation of aminopyrine in the presence of free available chlorine: Kinetics, products, and reaction pathways

Aminopyrine (AMP) has been frequently detected in the aquatic environment. In this study, the transformation mechanism of AMP by free available chlorine (FAC) oxidation was investigated. The results showed that FAC reacted with AMP rapidly, and a 74% elimination was achieved for 1.30 μM AMP after 2 min at 14.08 μM FAC dose. AMP chlorination was strongly pH-dependent, and its reaction included second- and third-order kinetic processes. Three active FAC species, including chlorine monoxide (Cl2O), molecular chlorine (Cl2), and hypochlorous acid (HOCl), were observed to contribute to AMP degradation. The intrinsic rate constants of each FAC species with neutral (AMP0) and cation (AMP+) species were obtained by kinetic fitting. Cl2O exhibited the highest reactivity with AMP0 (kAMP0, Cl2O = (4.33 ± 1.4) × 109 M−1s−1). In addition, Cl2 showed high reactivity (106–107 M−1s−1) in the presence of chloride, compared with HOCl (kAMP+, HOCl = (5.73 ± 0.23) × 102 M−1s−1, kAMP0, HOCl = (9.68 ± 0.96) × 102 M−1s−1). At pH 6.15 and 14.08 μM FAC dose without chloride addition, the contribution of Cl2O reached to the maximum (33.3%), but in the whole pH range, HOCl was the main contributor (>66.6%) for AMP degradation. The significance of Cl2 was noticeable in water containing chloride. Moreover, 11 transformation products were identified, and the main transformation pathways included pyrazole ring breakage, hydroxylation, dehydrogenation, and halogenation.

Insight into the kinetic mechanism of the first-step dehydrogenation of Mg(AlH4)2

The detailed mechanism of the first-step dehydrogenation of Mg(AlH4)2 is investigated by using a comprehensive kinetic analysis. The reaction mode (geometrical contraction or nucleation-growth-impingement), intrinsic rate constant, dimensionality, rate-controlling step and activation energies of nucleation and growth are obtained from the simultaneous fitting of isothermal and non-isothermal experimental curves. The interpretation and comparison of these kinetic parameters for the pristine, high-energy ball milled and TiF4 doped Mg(AlH4)2 help reveal the mechanism of the dehydrogenation of Mg(AlH4)2, including how Mg(AlH4)2 decomposes to MgH2, Al and H2, and why high-energy ball milling and TiF4 doping can significantly improve the dehydrogenation properties.

Kinetic Mechanism and Intrinsic Rate Constants for the Reaction of a Bacterial Phenylalanine Hydroxylase.

The pterin-dependent aromatic amino acid hydroxylases are non-heme iron enzymes that catalyze the hydroxylation of the aromatic side chain of their respective substrates using an FeIVO intermediate. While the eukaryotic enzymes are homotetramers with complex regulatory properties, bacterial phenylalanine hydroxylases are monomers that lack regulatory domains. As a result, the bacterial enzymes are more tractable for mechanistic studies. Using single turnover methods, the complete kinetic mechanism and intrinsic rate constants for Chromobacterium violaceum phenylalanine hydroxylase have been determined with both tetrahydrobiopterin and 6-methyltetrahyropterin as substrates. In addition the kinetics of formation of the enzyme-pterin complex have been determined with the unreactive 5-deaza, 6-methyltetrahydropterin. For all three pterins, binding of phenylalanine and pterin occurs in random order with binding of the pterin first the preferred pathway. The reaction of the ternary enzyme-phenylalanine-tetrahydropterin complex can be described by a mechanism involving reversible oxygen binding, formation of an early intermediate preceding formation of the FeIVO, and rate-limiting product release.

Kinetic analysis and CFD simulations of the photocatalytic production of hydrogen in silicone microreactors from water-ethanol mixtures

Silicone microreactors containing microchannels of 500μm width in a single or triple stack configuration have been manufactured, coated with an Au/TiO2 photocatalyst and tested for the photocatalytic production of hydrogen from water-ethanol gaseous mixtures under UV irradiation. Computational fluid dynamics (CFD) simulations have revealed that the design of the distributing headers allowed for a homogeneous distribution of the gaseous stream within the channels of the microreactors. A rate equation for the photocatalytic reaction has been developed from the experimental results obtained with the single stack operated under different ethanol partial pressures, light irradiation intensities and contact times. The hydrogen photoproduction rate has been expressed in terms of a Langmuir-Hinshelwood-type equation that accurately describes the process considering that hydrogen is produced through the dehydrogenation of ethanol to acetaldehyde. This equation incorporates an apparent rate constant (kapp) that has been found to be proportional to the intrinsic kinetic rate constant (k), and that depends on the light intensity (I) as follows: kapp=k·I0.65. A three-dimensional isothermal CFD model has been developed in which the previously obtained kinetic equation has been implemented. The model adequately describes the production of hydrogen of both the single and triple stacks. Moreover, the specific hydrogen productions (i.e. per gram of catalyst) are very close for both stacks thus suggesting that the scaling-up of the process could be accomplished by simply numbering-up. However, small deviations between the experimental and predicted hydrogen production suggest that a fraction of the radiation is absorbed by the microreactor components which should be taken into account for scaling-up purposes.

Reaction rate constant of CO2–Tetra‐n‐butylammounium bromide semi‐clathrate formation

Semi‐clathrate growth experiments were performed using a carbon dioxide‐water‐tetra‐n‐butyl ammonium bromide system. Experiments were conducted in a semi‐batch stirred tank crystallizer at temperatures of 287, 288, and 289 K with a 3‐K subcooling at each temperature. Gas consumption, temperature, hydrate former mole fraction, and particle size were measured throughout growth. Significant differences in gas consumption and heat effects were observed with these systems compared with pure clathrate systems. A kinetic growth model was applied to estimate the intrinsic rate constant. Rate constants did not show a clear trend with temperature at the conditions studied. Intrinsic reaction rate constant values ranged from 7.6 × 10−6 m/s to 13.8 × 10−6 m/s.

Two-dimensional electronic spectroscopy can fully characterize the population transfer in molecular systems.

Excitation energy transfer in complex systems often proceeds through series of intermediate states. One of the goals of time-resolved spectroscopy is to identify the spectral signatures of all of them in the acquired experimental data and to characterize the energy transfer scheme between them. It is well known that in the case of transient absorption spectra such decomposition is ambiguous even if many simplifying considerations are taken. In contrast to transient absorption, absorptive 2D spectra intuitively resemble population transfer matrices. Therefore, it seems possible to decompose the 2D spectra unambiguously. Here we show that all necessary information is encoded in the combination of absorptive 2D and linear absorption spectra. We set up a simple model describing a broad class of absorptive 2D spectra and prove analytically that they can be inverted uniquely towards physical parameters fully determining the species-associated spectra of individual constituents together with all connecting intrinsic rate constants. Due to the matrix formulation of the model, it is suitable for fast computer calculation necessary to efficiently perform the inversion numerically by fitting the combination of experimental 2D and absorption spectra. Moreover, the model allows for decomposition of the 2D spectrum into its stimulated emission, ground-state bleach, and excited-state absorption components almost unambiguously. The numerical procedure is illustrated exemplarily.

Acid strength and metal-acid proximity effects on methylcyclohexane ring contraction turnover rates and selectivities

Methylcyclohexane ring contraction is used here to assess the effects of acid strength and metal-acid site proximity on turnover rates and selectivities for bifunctional catalysts consisting of Keggin type polyoxometalates (POM) with different central atoms that act as Brønsted acids. Bifunctional catalysts with metal sites that fully equilibrate cycloalkanes and cycloalkenes give methylcyclohexene conversion rate constants that decreased exponentially with increasing deprotonation energy, a rigorous descriptor of acid strength, consistent with the ion-pair character of transition states that mediate the kinetically-relevant ring contraction of bound methylcyclohexoxide intermediates. The measured rates of formation of each alkylcyclopentane isomer, however, do not reflect their intrinsic formation kinetics, because of fast diffusion-enhanced interconversions within acid domains; thus, isomer selectivities cannot be used to infer, even indirectly, the strength of acid sites, as often proposed in previous studies. Selectivities reflect instead diffusional effects that become more severe as the number and strength of acid sites and the size and diffusive resistances increase within these acid domains, which shift, in turn, the products formed from relative abundances dictated by kinetics to those prescribed by thermodynamics. A rigorous accounting of these diffusional effects using a kinetic-transport model leads to ratios of intrinsic rate constants for the formation of the different alkylcyclopentane isomers that do not depend on acid strength, because all isomerization routes are mediated by bicyclo[3.1.0]hexyl cation transition states similar in the amount and location of charge and that therefore benefit to the same extent from the more stable conjugate anions characteristic of stronger acids.

Spouted bed reactor for kinetic measurements of reduction of Fe2O3 in a CO2/CO atmosphere Part I: Atmospheric pressure measurements and equipment commissioning

A high pressure and high temperature spouted bed reactor, operating in fluidisation mode, has been designed and validated at low pressure for the study of gas–solid reaction kinetics. Measurements suggested the bed exhibited a fast rate of gas interchange between the bubble and particulate phases. Pressurised injection of the particles to the bottom of the bed allowed the introduction of solid reactants in a simple and controlled manner. The suitability of the reactor for the purpose of kinetic studies was demonstrated by investigation of the intrinsic kinetics of the initial stage of the reduction of Fe2O3 with CO over multiple cycles for chemical looping.Changes of pore structure over the initial cycles were found to affect the observed kinetics of the reduction. The initial intrinsic rate constant of the reduction reaction (ki) was measured by using a kinetic model which incorporated an effectiveness factor. The uncertainty arising from the measurement of particle porosity in the model was compensated for by the tortuosity factor. The average activation energy obtained for cycles three to five was 61±8kJ/mol, which is comparable with previous studies using both fluidised beds and thermogravimetry.

Monomolecular Cracking Rates of Light Alkanes over Zeolites Determined by IR Operando Spectroscopy

The coverage of H-MFI zeolite acid sites by light alkanes (C3–C7) at monomolecular cracking reaction conditions was determined using infrared operando spectroscopy. Under such conditions, alkane adsorption through H-bonding leads to a fully reversible perturbation of the zeolite νOH band at 3600 cm–1. This was used to assess the coverage at various temperatures and pressures, allowing for the determination of the adsorption thermodynamic parameters at reaction conditions. The simultaneous determination of apparent monomolecular cracking rate constants allowed for the direct determination of the intrinsic cracking rate constants, activation energies, and activation entropies. These results show that while the coverage of the active sites increases with the alkane size, the differences tend to decrease at high temperature because of entropic effects. The intrinsic activation energy was constant for all alkanes investigated in this study (∼190 kJ mol–1), lying in the lower range of the values usually derived...

Kinetics of extraction of nitric acid into binary mixture of tri-n-butyl phosphate and normal paraffin hydrocarbon

The extraction kinetics of nitric acid with tri-n-butyl phosphate (TBP) dissolved in normal paraffin hydrocarbon (NPH) was investigated. Experiments were carried out in a constant area stirred cell. Influences of stirring speed, temperature, specific interfacial area and concentrations of extractant and nitric acid on the extraction rate of nitric acid were studied. The objective of the present study was to identify the reaction location, extraction reaction regime and to determine intrinsic rate constant of the reaction and mass transfer coefficients. A systematic approach was adopted for the determination of these rate parameters. Based on experimental results, it is concluded that the liquid-liquid reaction takes place at the interface and the overall rate is controlled by both the intrinsic kinetics as well as the mass transfer of the species involved.

Nanoclimbing-wall-like CoSe2/carbon composite film for the counter electrode of a highly efficient dye-sensitized solar cell: A study on the morphology control

Three types of composite films of CoSe2 and carbon (CoSe2/C), which have different morphologies, were obtained on fluorine-doped tin oxide (FTO) substrates via a simple two-step synthesis, i.e., first a pulse-potential electro-deposition and then an annealing process. A two-stage growth mechanism is proposed: stage 1 involves the formation of three types of CoSe2/C films consisting of nanowalls through an electro-deposition process, by using bathes with different pH values; stage 2 involves “morphological transformation” of the films in an annealing process. After the annealing, nanograin (NG)-like, nanorock (NR)-like, and nanoclimbing-wall (NCW)-like films of CoSe2/C were obtained on the FTO substrates. The film of CoSe2/C-NCW exhibits a great conductivity, a good intrinsic heterogeneous rate constant, a large effective surface area, and thereby an excellent electro-catalytic ability toward triiodide (I3−) reduction. All the annealed CoSe2/C films were used as the counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). The DSSC with the CoSe2/C-NCW exhibits the highest solar-to-electricity conversion efficiency (η) of 8.92±0.03%, which is found to be even higher than that of the cell with a platinum (Pt) CE (8.25±0.03%). The CoSe2/C-NCW was further electro-deposited onto two low-cost, highly porous, and flexible substrates, i.e., nickel foam (NF) and carbon cloth (CC). Each tiny part of the NF or the CC is observed to be covered with the CoSe2/C-NCW, as in a core shell structure, where NF or CC is the core and CoSe2/C-NCW is the shell. The DSSC with the CoSe2/C-NCW on NF exhibits a very high η of 10.46±0.23% at 1sun and an η of 7.90±0.05% at 0.2sun. The low-weight CC rendered for its DSSC a high η of 9.87±0.06% at 1sun and an η of 7.83±0.03% at 0.2sun. The low cost CoSe2/C-NCW is an attractive replacement for the expensive Pt for DSSCs, and is also suitable for outdoor, indoor, or wearable DSSCs.

Electrochemical behavior of copper species in the 1-buthyl-3-methyl-imidazolium chloride (BMIMCl) ionic liquid on a Pt electrode

The electrochemical behavior of Cu(I) and Cu(II) species in the ionic liquid 1-buthyl-3-methylimidazolium chloride, BMIMCl, on a platinum electrode in the interval of 333–363K, has been investigated. Experiments performed in a controlled N2 atmosphere cell show that: (i) Cu(I) can be oxidized to Cu(II) and reduced to Cu(0), and (ii) Cu(II) can be reduced to Cu(I) by copper metal according to a comproportionation reaction whose constant has been calculated.The diffusion coefficients of Cu(I) and Cu(II) have been obtained by different electrochemical techniques. The dimensionless Schmidt numbers (Sc=ν/D) were also calculated to characterize the solute global mass transport through its environment. The kinetic parameters for the quasi-reversible electrochemical exchange Cu(II)/Cu(I) (i.e. the intrinsic rate constant of charge transfer, k0, and the charge transfer coefficient, α), as well as the reversible half wave potential, Er1/2, have been obtained for the first time in the mentioned ionic liquid, by simulation of the cyclic voltammograms and logarithmic analysis of the convoluted curves.The apparent standard potential of the electrochemical system Cu(I)/Cu(0) has been determined by potentiometry. Founding that nucleation and crystal growth plays an important role in the electrodeposition of metallic Cu onto the Pt surface. Experimental current–time transients followed the theoretical models based on instantaneous nucleation with three-dimensional growth of the nuclei at the studied temperatures. Electrodeposits of copper were obtained by potentiostatic electrolysis and analyzed by scanning electron microscopy with EDAX.

Elución de iones de níquel desde esferas de alginato en un medio ácido

Elution of nickel ions from alginate beads was studied in a temperature range of 2 to 80 °C and a sulfuric acid concentration between 0.005 to 2.5 g L−1. The elution mechanism was established by ion exchange between nickel and ions protons, obtaining a value of 93% elution at a temperature of 80 °C and a H2SO4 concentration of 0.25 g L−1. The influence of other acids on nickel elution was also studied. The nickel elution rate was significantly influenced by temperature and concentration of H2SO4. Elution kinetics of nickel ions was studied, and the 1-(1-α)1/3=kappt model properly described the kinetics of this reaction. The dependence of nickel elution on the sulfuric acid concentration was of the order of 0.33. Moreover, the intrinsic rate constants were determined and an activation energy value of 54.5 kJ mol−1 was obtained for the temperature range studied. The results indicated that the nickel elution is a process controlled by chemical reactions.

Pyrite oxidation kinetics in an oxygen–nitrogen atmosphere at temperatures from 400 to 500°C

This study analyses pyrite (FeS2) roasting in a temperature range of 400–500°C, with partial oxygen pressure from 1·01 to 28·60 kPa and particle size between 12·3 and 33·0 μm, using thermogravimetric methods. The pyrite oxidation rate was significantly influenced by temperature, partial oxygen pressure and particle size. Pyrite oxidation kinetics was also analysed, and the model X=kt gave a good description of the reaction kinetics. The dependency of pyrite oxidation on partial oxygen pressure was in the order of 3/5. The intrinsic oxidation rate constants were also determined and an activation energy value of 60·5 kJ mol−1 was obtained for the temperature range used in the study. The results indicate that the rate of pyrite oxidation was controlled by chemical reactions under the conditions of the experiments.

The intrinsic rate constants in diffusion-influenced reactions.

Intrinsic rate constants play a dominant role in the theory of diffusion-influenced reactions, but usually as abstract quantities that are implicitly assumed to be known. However, recently it has become clear that modeling complex processes requires explicit knowledge of these intrinsic rates. In this paper we provide microscopic expressions for the intrinsic rate constants for association and dissociation processes of isotropically interacting particles and illustrate how these rates can be computed efficiently using rare event simulations techniques. In addition, we address the role of the orientational dynamics, for particles interacting via anisotropic potentials.