Kinetics Of Heterogeneous Reactions Research Articles

Influence of relative humidity on heterogeneous kinetics of NO2 on kaolin and hematite.

In order to obtain reliable kinetic parameters, it is required to measure the reaction kinetics of important heterogeneous reactions at ambient relative humidity (RH). In this study, the uptake coefficients and HONO yields for the heterogeneous reaction of NO2 on kaolin and hematite were measured at RH from 7% to 74% and at ambient pressure in the dark using a coated-wall flow tube reactor. The initial true uptake coefficient (γt,ini) of NO2 at RH 7% was measured to be (1.44 ± 0.10) × 10(-7) and (1.58 ± 0.13) × 10(-6) on kaolin and hematite, respectively, while it decreased notably on both minerals, accompanied by an increase of HONO yields, as RH increased. The average γt,ini at 32-74% RH was (4.42 ± 1.17) × 10(-8) and (2.83 ± 0.84) × 10(-7) on kaolin and hematite, respectively. The corresponding mean HONO yield was (36.0 ± 16.1)% and (75.9 ± 3.32)%, respectively.

Simulation of the polymerization process on a silicon surface under plasma-chemical etching in CF4/H2

The hydrodynamic approach is used to simulate silicon passivation by unsaturated CFx radicals under plasma-chemical etching in CF4/H2. The plasma-chemical kinetics model contains 28 determinative gas-phase reactions with the participation of F, F2, CF2, CF3, CF4, C2F6, H, H2, HF, CHF3, and CH2F2. The extended kinetics of heterogeneous reactions on a silicon surface comprises the competing processes of the adsorption of CF2 and CF3 radicals. It is demonstrated that a major portion of atomic fluorine is actively involved in the formation of hydrogen fluoride, reducing the silicon etching rate to a significant extent. It is established that CF2 adsorption is predominant among the competing processes of the passivation of a silicon surface and that the CF2 layer covers the entire silicon surface at a 40% H2 content, thereby causing its etching to cease.

Bioinspired Complex-Nanoparticle Hybrid Catalyst System for Aqueous Perchlorate Reduction: Rhenium Speciation and Its Influence on Catalyst Activity

A highly active catalyst for reduction of the inert water contaminant perchlorate (ClO4–) to Cl– with 1 atm H2 at 25 °C is prepared by noncovalently immobilizing the rhenium complex ReV(O)(hoz)2Cl (hoz = 2-(2′-hydroxyphenyl)-2-oxazoline) together with Pd0 nanoparticles on a porous carbon support. Like the Mo complex centers in biological oxyanion reductases, the immobilized Re complex serves as a single site for oxygen atom transfer from ClO4– and ClOx– intermediates, whereas Pd0 nanoparticles provide atomic hydrogen reducing equivalents to sustain redox cycling of the immobilized Re sites, replacing the more complex chain of electron transfer steps that sustain Mo centers within oxyanion reductases. An in situ aqueous adsorption method of immobilization was used to preserve the active ReV(O)(hoz)2 structure during bimetallic catalyst preparation and enable study of Re redox cycling and reactions with ClO4–. Heterogeneous reaction kinetics, X-ray photoelectron spectroscopy, and experiments with homogeneous model Re complexes are combined to obtain insights into the catalytic reaction mechanisms and the influence of Re speciation on catalyst reactivity with ClO4–. Redox cycling between hoz-coordinated ReV and ReVII species serves as the main catalytic cycle for ClO4– reduction. Under reducing conditions, approximately half of the immobilized hoz-coordinated ReV is further reduced to ReIII, which is not directly reactive with ClO4–. A small fraction of the hoz-coordinated ReVII species can dissociate to ReO4– and free hoz, which are then reductively reimmobilized as a less reactive mixture of ReV, ReIII, and ReI species. This study provides an example wherein highly active metal complexes that were originally developed for homogeneous organic phase catalysis can be incorporated into heterogeneous catalysts for practical environmental applications. Findings suggest a general blueprint for developing hybrid catalysts combining single-site transition metal complexes with hydrogen-activating metal nanoparticles.

Theoretical investigation on the role of mineral dust aerosol in atmospheric reaction: A case of the heterogeneous reaction of formaldehyde with NO2 onto SiO2 dust surface

The role of mineral dusts on the heterogeneous reaction of formaldehyde with NO2 was investigated using a quantum chemical approach. SiO2 was selected as a model of mineral dust to investigate the heterogeneous reaction mechanism and kinetics because the mineral dust is comprised of ∼60% of SiO2. Compared with NO2, formaldehyde was easily adsorbed onto SiO2 model, indicating SiO2 was a sink for formaldehyde. Further data showed that the presence of SiO2 reduced the reaction barrier, but did not change reaction mechanism. The kinetics calculation using the canonical variational transition state theory plus small curvature tunneling correction showed that the presence of SiO2 could accelerate the atmospheric reaction rate of formaldehyde with NO2 to produce HONO. The effect of the altitudes was also considered, and the heterogeneous reaction rate decreased with increasing the altitude due to the atmospheric temperature decrease, suggesting that the degradation of atmospheric HCHO onto available mineral dusts might be competitive with the corresponding homogeneous reaction, especially in dusty urban and desert environments. This work can lay the foundation on the atmospheric heterogeneous reaction of VOCs, probe the role of mineral dust and establish the atmospheric transformation models, to better understand pollution mechanism.

Single char particle model for naphthalene reduction in a biomass gasification system

Tar removal in biomass gasification systems is one of the key challenges to overcome for the successful commercialization of this technology. This study focused on tar conversion over the active char particle besides the simultaneous carbon conversion of the char particle. In the presented model, the char particle was assumed a sphere surrounded by a gas film in a bulk gas composition of naphthalene, nitrogen, steam, hydrogen, carbon monoxide, carbon dioxide and methane. The model treated the heterogeneous gasification reaction kinetics, external and internal mass transfer and changing particle properties during gasification. In addition, the effect of different parameters on naphthalene and carbon conversion was investigated. The investigated parameters were particle size, temperature and time on stream. The gas and solid mass balance equations besides the energy balance equation were solved in the radial direction using a set of initial and boundary conditions. The model results show that the naphthalene and carbon conversion reactions were kinetically limited at the reference conditions of 850 °C bulk gas temperature and 600 μm char particle size. Further, the model calculations show that isothermal char particle can be assumed up to a bulk temperature of about 1160 °C. The variation of the physical properties and pore structure of the char particle with carbon conversion affects the active surface area of the particle. Finally, the results of the single particle model will be extended to a fixed bed reactor model and will be validated with experiments.

On Predicting Char Burnout in Pulverized Coal Combustion

Abstract The objective of this paper is twofold: to summarise the basic knowledge on kinetics of heterogeneous reactions and to clarify several misconceptions concerning both derivation and usage of expressions for calculating char oxidation rates. This paper is concerned with char reactions only; coal devolatilization is not considered although it may affect char oxidation rates.

Prediction of formation of gas-phase bubbles correlated by vortices in the fuel reactor of chemical looping combustion

Chemical looping combustion (CLC) as a potential CO2 capture technology has been considered as a promising and likely alternative to traditional combustion technology to mitigate the CO2 emission due to its prosecution of CO2 sequestration at a low cost. Although a number of studies on the hydrodynamic behaviours of the CLC process in fuel reactor have been documented in the open literature, there have been rare studies on the correlation between the bubble formation and the local particulate volume fraction. This paper aims to investigate the CLC process in a fuel reactor using the CFD modelling, coupled with the heterogeneous reactions, the hydrodynamics and reaction kinetics occurring in the fuel reactor. A parameter correlating the occurrence of bubble and dynamic properties is proposed. The parameter may be acted as an indicator of time-dependent bubble evolution with a potential to be adopted in the CLC for controlling the bubbling phenomena since the occurrence of the bubbles at specific positions is highly correlated with the local large eddies embedded in the flow. The results obtained clearly indicate that the CFD model developed in the current study reasonably forecasts the hydrodynamic behaviours and important phenomena observed in the fuel reactor.

Technical Note: Application of positive matrix factor analysis in heterogeneous kinetics studies utilizing the mixed-phase relative rates technique

Abstract. The mixed-phase relative rates approach for determining aerosol particle organic heterogeneous reaction kinetics is often performed utilizing mass spectral tracers as a proxy for particle-phase reactant concentration. However, this approach may be influenced by signal contamination from oxidation products during the experiment. In the current study, the mixed-phase relative rates technique has been improved by combining a positive matrix factor (PMF) analysis with electron ionization aerosol mass spectrometry (unit-mass resolution), thereby removing the influence of m / z fragments from reaction products on the reactant signals. To demonstrate the advantages of this approach, the heterogeneous reaction between OH radicals and citric acid (CA) was investigated using a photochemical flow tube coupled to a compact time-of-flight aerosol mass spectrometer (C-ToF-AMS). The measured heterogeneous rate constant (k2) of citric acid toward OH was (3.31 ± 0.29) × 10−12 cm3 molecule−1 s−1 at 298 K and (30 ± 3)% relative humidity (RH) and was several times greater than the results utilizing individual m / z fragments. This phenomenon was further evaluated for particulate-phase organophosphates (triphenyl phosphate (TPhP), tris-1,3-dichloro-2-propyl phosphate (TDCPP) and tris-2-ethylhexyl phosphate (TEHP)), leading to k2 values significantly larger than previously reported. The results suggest that heterogeneous kinetics can be significantly underestimated when the structure of the products is highly similar to the reactant and when a non-molecular tracer is measured with a unit-mass resolution aerosol mass spectrometer. The results also suggest that the heterogeneous lifetime of organic aerosol in models can be overestimated due to underestimated OH uptake coefficients. Finally, a comparison of reported rate constants implies that the heterogeneous oxidation of aerosols will be dependent upon a number of factors related to the reaction system, and that a single rate constant for one system cannot be universally applied under all conditions.

Oxidation of coal in industrial conditions. 1. Kinetics of natural oxidation

In the light of research on the artificial oxidation of coal in the laboratory, earlier results on the natural oxidation of coal in open storage at different coke Ukrainian plants are analyzed. On the basis of the kinetics of coal oxidation in natural conditions, the need to correct existing standards for open storage is evaluated. Since the dependence of the degree of oxidation do on the storage time is sigmoid, the results are analyzed on the basis of the Avrami-Erofeev equation for the kinetics of heterogeneous reactions. The constants of that equation and the rate constants of oxidation are calculated for enriched coal of different ranks (G, Zh, K, and OS) in summer and winter. The influence of the degree of metamorphism on the rate constant of natural oxidation in summer and winter is mathematically described. On the basis of the Avrami-Erofeev equation, limiting open-storage times for the coal are established. The earlier limiting storage times must be corrected.

ChemInform Abstract: Thermochemical Model in Kinetics of Heterogeneous Reactions: 100‐Year Jubilee

AbstractReview: 33 refs.

On the Role of Particle Inorganic Mixing State in the Reactive Uptake of N2O5to Ambient Aerosol Particles

The rates of heterogeneous reactions of trace gases with aerosol particles are complex functions of particle chemical composition, morphology, and phase state. Currently, the majority of model parametrizations of heterogeneous reaction kinetics focus on the population average of aerosol particle mass, assuming that individual particles have the same chemical composition as the average state. Here we assess the impact of particle mixing state on heterogeneous reaction kinetics using the N2O5 reactive uptake coefficient, γ(N2O5), and dependence on the particulate chloride-to-nitrate ratio (nCl(-)/nNO3(-)). We describe the first simultaneous ambient observations of single particle chemical composition and in situ determinations of γ(N2O5). When accounting for particulate nCl(-)/nNO3(-) mixing state, model parametrizations of γ(N2O5) continue to overpredict γ(N2O5) by more than a factor of 2 in polluted coastal regions, suggesting that chemical composition and physical phase state of particulate organics likely control γ(N2O5) in these air masses. In contrast, direct measurement of γ(N2O5) in air masses of marine origin are well captured by model parametrizations and reveal limited suppression of γ(N2O5), indicating that the organic mass fraction of fresh sea spray aerosol at this location does not suppress γ(N2O5). We provide an observation-based framework for assessing the impact of particle mixing state on gas-particle interactions.

Scalable inference of heterogeneous reaction kinetics from pooled single-cell recordings

Mathematical methods combined with measurements of single-cell dynamics provide a means to reconstruct intracellular processes that are only partly or indirectly accessible experimentally. To obtain reliable reconstructions, the pooling of measurements from several cells of a clonal population is mandatory. However, cell-to-cell variability originating from diverse sources poses computational challenges for such process reconstruction. We introduce a scalable Bayesian inference framework that properly accounts for population heterogeneity. The method allows inference of inaccessible molecular states and kinetic parameters; computation of Bayes factors for model selection; and dissection of intrinsic, extrinsic and technical noise. We show how additional single-cell readouts such as morphological features can be included in the analysis. We use the method to reconstruct the expression dynamics of a gene under an inducible promoter in yeast from time-lapse microscopy data.

Model of the Plate Oxygen Cutting Taking into Account the Kinetics of Heterogeneous Oxidation Reaction

This paper presents a mathematical model of oxygen cutting of the plate. It takes into account not only possible heat losses from the treatment zone, but also the heat release doe to the oxidation reaction. In turn, the oxidation leads to plate thickness change. The model allows analyzing the temperature distribution during the cutting process for different modes. The following cutting modes are revealed in the model: surface cutting, detail splitting, kinetic and diffusion regime.

An insight into the kinetics and interface sensitivity for catalytic ozonation: the case of nano-sized NiFe2O4

In this paper, the kinetics and interface sensitivity of nano-sized magnetic NiFe2O4 in incomplete catalytic ozonation were investigated in detail. By analyzing the kinetics of heterogeneous reactions, it was found that both the degradation of phenol and the decomposition of molecular ozone appeared to follow a first order kinetics model and the presence of nano-NiFe2O4 significantly enhanced these processes. The kinetics equations and reaction rate constants were determined according to experimental results. In light of the IR spectra, the Lewis acid sites were further confirmed as reactive centers for catalytic ozonation in the aqueous phase, and the reason for deactivation of the NiFe2O4 nanocatalyst during incomplete ozonation of organic compounds was determined. The catalytic activity of the NiFe2O4 nanocatalyst could be completely recovered by calcination and ozonation methods, which makes it an attractive nanomaterial with prospective applications in water treatment. According to the interface sensitivity of ozone in water, a strong interaction between ozone and NiFe2O4 was observed and the role of water was revisited. Water molecules were clarified as two different types: one is chemisorbed water on the catalyst surface and the other is bulk water in aqueous solution. Both bulk and chemisorbed water molecules should be weak competitors compared with ozone, and the chemisorbed water on the catalyst surface was proposed not as an inhibitor but an accelerator for catalytic activity.

Kinetics of heterogeneous reactions of ozone with representative PAHs and an alkene at the air–ice interface at 258 and 188 K

The kinetics of the reaction of an alkene (E-stilbene) and three polycyclic aromatic hydrocarbons (perylene, anthracene and fluoranthene), as examples of environmental pollutants, with ozone on the surface of ice grains (also called "artificial snow"), produced by shock-freezing of aqueous solutions, was studied at submonolayer pollutant coverages (c = 1.5 × 10(-8) to 3 × 10(-10) mol kg(-1)) and two different temperatures (258 and 188 K). This work supports and extends our previous discovery of a remarkable increase in the apparent ozonation rates with decreasing temperature. The ozonation kinetic results were evaluated using the Langmuir-Hinshelwood model and, in one case, the Eley-Rideal kinetic model. It is shown that the apparent rate enhancement is related to the specific nature of the ice surface at different temperatures, which influences the availability of contaminants to gaseous ozone, and to inherent reactivities of the contaminants. The maximum pseudofirst-order rate constants and the lifetimes of the studied compounds are provided. At a typical atmospheric ozone concentration in polar areas (50 ppbv), the lifetimes were estimated to be on the order of hours (258 K) or tens of minutes (188 K) for alkenes, and hundreds (258 K) or tens (188 K) of days for PAHs, thus approximately of the same magnitude or longer than those found for the gas-phase reactions. We imply that this rate enhancement at lower temperatures is a general phenomenon, and we provide data to implement heterogeneous reactions in snow in models that predict the extent of chemical reactions occurring in cold environments.

Computational Fluid Dynamics Modeling of Biomass Gasification in Circulating Fluidized-Bed Reactor Using the Eulerian–Eulerian Approach

A three-dimensional CFD (computational fluid dynamics) steady-state model was established to simulate biomass gasification in a circulating fluidized-bed (CFB) reactor. The standard k–ε turbulence model was coupled with the kinetic theory of granular flow to simulate the hydrodynamics in the gasifier. The kinetics of homogeneous and heterogeneous reactions were studied and integrated with the equations of continuity, motion, and energy to describe the distributions of velocity, temperature, and concentration. The simulation results were compared to experimental data. The impacts of turbulence models, radiation model, water–gas shift (WGS) reaction, and equivalence ratio (ER) were investigated to present a reliable understanding of biomass gasification in a CFB reactor.

Thermochemical model in kinetics of heterogeneous reactions: 100-year jubilee

The milestones in the development of the thermochemical model (TM) in kinetics of heterogeneous reactions over 100 years after publication in 1913 of the Langmuir equation, \( J\; = \;MP/\sqrt{2\pi MRT} \), were considered. However, the systematic development of TM was begun only in the early 1980s. Among the milestones of this latter period, which passed with the participation of the author, the most important were: the extension of Langmuir equation to dissociative vaporization of compounds and identification of two modes of vaporization (equimolar and isobaric); the discovery of the mechanism of congruent dissociative vaporization; the application of the third-law method to determination of the E parameters (“activation energies” in the Arrhenius equation) and formulation of theoretical and experimental evidence for the absence of activation effect in decomposition reactions. During this period, the new theory was applied to the interpretation of many unsolved problems in heterogeneous kinetics. In the majority of cases, the interpretation is reliably supported by agreement between theory and experiment. The traditional kinetic theories based on the activation effect (the classical Arrhenius theory, as well as the collision and transition state theories) appeared fruitless in this respect.

Interpretation of the kinetic compensation effect in heterogeneous reactions: thermochemical approach

The kinetic compensation effect (KCE), unexplained by the Arrhenius activation model, is interpreted in the framework of the novel thermochemical approach to kinetics of heterogeneous reactions. Unlike the activation model, this theory includes the process of congruent dissociative vaporisation of solids described by the Langmuir quasi-equilibrium equations and consideration of the two different reaction regimes: equimolar and isobaric (with the external pressure of gaseous product, respectively, below and above its equilibrium pressure). Change of regimes is shown to be the reason for the mutual variations of the Arrhenius parameters in the equation: ln A = a + bE. This theoretical interpretation of the KCE is quantitatively supported by experimental observations for the following different types of heterogeneous reactions: high temperature atomisation of metal oxides, low-temperature solid-state decompositions, NiO reduction by H2 and catalytic oxidations of CO and of H2 on PtO2. Fundamental reasons for the shortcomings of the activation model in the interpretation of mechanisms and kinetics of heterogeneous reactions are identified.

ChemInform Abstract: Interpretation of the Kinetic Compensation Effect in Heterogeneous Reactions: Thermochemical Approach

AbstractReview: 192 refs.

Heterogeneous reactions of particulate benzo[b]fluoranthene and benzo[k]fluoranthene with NO3 radicals

Benzo[b]fluoranthene (B[b]F) and benzo[k]fluoranthene (B[k]F) are widespread priority pollutants of polycyclic aromatic hydrocarbons (PAHs), which can react with atmospheric oxidants during transport in the troposphere and lead to the formation of more toxic compounds. At present, the rates of heterogeneous reactions of B[b]F and B[k]F aerosols with NO3 radicals, an important atmospheric oxidant, are not fully understood. Thus, this study investigated the products and kinetics of heterogeneous reactions of suspended B[b]F and B[k]F particles with NO3 radicals in an aerosol reaction chamber at room temperature (293±2K) under atmospheric pressure. The reactions are monitored online using a vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer (VUV-ATOFMS) and an atmospheric gas analysis mass spectrometer. The mono-nitro-, di-nitro-, tri-nitro-products, and those products containing both nitro and ketone groups are observed with VUV-ATOFMS. The effective rate constants for heterogeneous reactions of particulate B[b]F and B[k]F with NO3 radicals under the experimental conditions are respectively estimated to (1.2±0.1)×10−12cm3molecule−1s−1 and (5.8±0.3)×10−13cm3molecule−1s−1 using a mixed-phase relative rate method, and the corresponding effective uptake coefficients are respectively estimated to 0.22 and 0.65. The lifetimes of particulate B[b]F and B[k]F at a typical concentration of NO3 radicals (5×108moleculecm−3) in the lower troposphere during the night are estimated to 3.0 and 3.9h, respectively. The experimental results of these heterogeneous reactions in the aerosol state provide supplementary knowledge for kinetic behaviors of airborne PAHs particles.