Kinetics Of Heterogeneous Reactions Research Articles

ChemInform Abstract: Kinetics of Heterogeneous Catalytic Reactions

AbstractReview: 19 refs.

Pore diffusion model to predict the kinetics of heterogeneous catalytic esterification of acetic acid and methanol

A novel pore diffusion model in combination with a homogeneous reaction in the bulk solution is proposed to model the kinetic behavior of conversion of acetic acid during its esterification with methanol using ion exchange resin catalyst. The model parameters are determined based on the experimental data of a batch reactor with reactant quantities in stoichiometric ratio. Experimental kinetic data of conversion of acetic acid is obtained for various ranges of parameters such as catalyst loading in g/cc of reactant mixture volume, temperature in the range of 323.15–353.15K, average diameter of catalyst particle varied from 425μm to 925μm and RPM of magnetic stirrer. A multi time scale model is adopted in updating the conversion of acetic acid in the bulk liquid where the measurement is carried out. Since the homogeneous part of catalytic esterification is slow reaction, a large time step is used for updating bulk concentration or conversion, whereas a small time step is used to find the concentration distribution and inward flux of acetic acid inside the catalyst particle in a quasi steady state mode. This multi time scale approach leads to utilizing only the initial time kinetics of reaction for predicting two reaction constants namely kf1, the forward reaction rate constant for homogeneous reaction in bulk solution and kf2, the forward reaction rate constant for reaction inside the catalyst particle. The equilibrium conversion data provides the reaction equilibrium constant. The model predicts the overall kinetics of heterogeneous esterification reaction accurately.

Photosensitised heterogeneous oxidation kinetics of biomass burning aerosol surrogates by ozone using an irradiated rectangular channel flow reactor

Abstract. Heterogeneous reaction kinetics involving organic aerosol and atmospheric oxidants such as ozone can be enhanced under visible or UV irradiation in the presence of a photosensitiser, with subsequent implications for the climate, cloud radiative properties, air quality, and source appointment. In this study we report the steady-state reactive uptake coefficient, γ, of O3 by levoglucosan and 5-nitroguaiacol acting as surrogates for biomass burning aerosol particles, with and without the presence of Pahokee peat acting as a photosensitiser. The reactive uptake has been determined in the dark and as a function of visible and UV-A irradiation and ozone concentration. In addition, γ was determined for 1 : 1, 1 : 10, and 1 : 100 by mass mixtures of Pahokee peat and 5-nitroguaiacol, and for a 10 : 1 : 3 mixture of levoglucosan, Pahokee peat, and 5-nitroguaiacol. We developed a novel irradiated rectangular channel flow reactor (I-RCFR) that was operated under low pressures of about 2–4 hPa, and allowed for uniform irradiation of the organic substrates. The I-RCFR was coupled to a chemical ionisation mass spectrometer and has been successfully validated by measuring the kinetics between various organic species and oxidants. γ of O3 and levoglucosan in the dark and under visible and UV-A irradiation was determined to be in the range of (2–11) × 10−6 and did not change in the presence of Pahokee peat. The determined γ of O3 and 5-nitroguaiacol in the dark was 5.7 × 10−6 and was only enhanced under UV-A irradiation, yielding a value of 3.6 × 10−5. γ of the 1 : 1 Pahokee peat/5-nitroguaiacol substrate was enhanced under visible and UV-A irradiation to 2.4 × 10−5 and 2.8 × 10−5, respectively. Decreasing the amount of Pahokee peat in the 5-nitroguaiacol/Pahokee peat substrate resulted in lower values of γ under visible irradiation, however, γ was consistent under UV-A irradiation regardless of the amount of Pahokee peat. The 10 : 1 : 3 mixture by mass of levoglucosan, Pahokee peat, and 5-nitroguaiacol, under both visible and UV-A irradiation yielded γ values of 2.8 × 10−5 and 1.4 × 10−5, respectively. γ was determined as a function of photon flux for O3 with the 1 : 1 Pahokee peat/5-nitroguaiacol substrate, yielding a linear relationship under both visible and UV-A irradiation. γ of O3 with the 1 : 1 Pahokee peat/5-nitroguaiacol substrate was determined as a function of ozone concentration and exhibited an inverse dependence of γ on ozone concentration, commonly interpreted as a Langmuir–Hinshelwood mechanism. The reactive uptake data have been represented by a Langmuir-type isotherm. From the O3 uptake data under visible irradiation, the following fit parameters have been derived: ks = (5.5 ± 2.7) × 10−19 cm2 s−1 molecule−1 and KO3 = (2.3 ± 2.0) × 10−12 cm3 molecule−1; and under UV-A irradiation: ks = (8.1 ± 2.0) × 10−19 cm2 s−1 molecule−1 and KO3 = (1.7 ± 0.7) × 10−12 cm3 molecule−1. The oxidative power, or the product of γ and [O3], was determined for O3 with the 1 : 1 Pahokee peat/5-nitroguaiacol substrate and was in the range of (1.2–26) × 106 molecule cm−3. Atmospheric particle lifetimes were estimated for a 0.4 μm 5-nitroguaiacol particle as a function of visible and UV-A irradiation and ozone concentration.

CFD modeling using heterogeneous reaction kinetics for catalytic dehydrogenation syngas reactions in a fixed-bed reactor

A comprehensive 2D computational fluid dynamics (CFD) model was developed to simulate the flow behavior and catalytic dehydrogenation reaction of syngas in a heterogenous fixed-bed reactor (FBR). The model combined the porous medium CFD model with a reaction kinetics model. To acquire an accurate reaction kinetics model, a comprehensive reaction mechanism was studied for the heterogeneous catalytic dehydrogenation reaction of syngas over a supported metal catalyst. Based on the reaction mechanism and a statistical test, a reliable kinetics model was proposed. The CFD model combined with the above kinetics model was validated with one set of experimental data. The CFD model was also used to predict key reaction variable distributions such as the temperature and the component concentrations in the reactor.

Review of Heat Transfer Research for Solar Thermochemical Applications

This article reviews the progress, challenges and opportunities in heat transfer research as applied to high-temperature thermochemical systems that use high-flux solar irradiation as the source of process heat. Selected pertinent areas such as radiative spectroscopy and tomography-based heat and mass characterization of heterogeneous media, kinetics of high-temperature heterogeneous reactions, heat and mass transfer modeling of solar thermochemical systems, and thermal measurements in high-temperature systems are presented, with brief discussions of their methods and example results from selected applications.

Discrimination of Inner- and Outer-Sphere Electrode Reactions by Cyclic Voltammetry Experiments

A laboratory experiment for undergraduate students who are studying homogeneous and heterogeneous electron-transfer reactions is described. Heterogeneous or electrode reaction kinetics can be examined by using the electrochemical reduction of three FeIII/FeII redox couples at platinum and glassy carbon disk electrodes. Cyclic voltammetric measurement is suitable for determining the electrode-reaction rate constants. The dependence of the rate constant on the electrode materials enables the students to discriminate and discuss the mechanism of the electrode reaction. The experiment is completed in a 5-h laboratory period for pairs of students in analytical chemistry lab class. This material is also suitable for an inorganic chemistry lab class.

Kinetics of Heterogeneous Reaction of Ozone with Linoleic Acid and its Dependence on Temperature, Physical State, RH, and Ozone Concentration

Heterogeneous reaction between ozone and linoleic acid (LA) thin film was investigated by a flow reactor coupled to attenuated total reflection infrared spectroscopy (FR-ATR-IR) over wide ranges of temperature, relative humidity (RH), and ozone concentration under atmospheric pressure condition. Pseudo-first-order rate constants kapp and overall reactive uptake coefficients γ were acquired on the basis of changes in absorbance from peaks located near 1743, 1710, 1172, and 1110 cm(-1), which can be assigned to C═O in ester, C═O in acid, and C-C and C-O stretching modes, respectively. Results showed that the kapp and γ increased nearly by a factor of 6 with increasing temperatures from 258 to 314 K. It was noted the temperature effect on the reaction kinetics was much more pronounced at lower temperatures. Such behavior can be explained by a change in the physical state of LA at lower temperatures. In addition, kapp and γ were enhanced by 2-fold as the RH increased from 0 to 80%. Moreover, the effect of ozone concentration on the reaction kinetics was reported for the first time. kapp was found to display a Langmuir-Hinshelwood dependence on ozone concentration with KO3 = (1.146 ± 0.017) × 10(-15) molecules cm(-3) and k[S] = 0.0522 ± 0.0004 s(-1), where KO3 is a parameter that describes the partitioning of ozone to the thin film surface, and k[S] is the maximum pseudo-first-order coefficient at high ozone concentration. Furthermore, yields and hygroscopic properties of reaction products were also investigated by FTIR spectroscopy. The intensity ratio of two C═O stretching bands, A1743/A1710, which was utilized as an indicator of the product yields, increased sharply with increasing temperatures in the lower temperature region (258-284 K), and then remained nearly constant in the higher temperature region (284-314 K). The product yields showed no significant variation with RH, for the intensity ratio of A1743/A1710 barely changed in the wide RH range 0-80%. Water uptake studies showed that the LA thin film absorbed water with an increasing RH, and the hygroscopicity of the thin film was enhanced after ozone exposure.

Catalytic oxidation of hydrogen on platinum

Using the thermochemical approach to interpret the kinetics of heterogeneous reactions and the mechanism of congruent dissociative decomposition of solids developed in the 1980s and (re)analyzing the experimental data available in the literature over the last 90 years, a novel mechanism for the catalytic oxidation of H2 by PtO2 is proposed. In place of the conventional Langmuir–Hinshelwood and Eley–Rideal adsorption reaction mechanisms, our model is based on the reactions: PtO2(s) + 2H2 ↔ Pt(g) + 2H2O and Pt(g) + O2 ↔ PtO2(g) → PtO2(s). The first reaction determines the kinetics of H2 oxidation and the second determines the kinetics of restoration of the PtO2 layer. Thermochemical consideration of kinetic features of this model enables (for first time in the history of this reaction) the enthalpy and equilibrium constants for H2 oxidation on platinum to be calculated. The results are in good agreement with experimental data. In addition, the proposed mechanism explains the origin of the surface-retexturing effect, the impact of autocatalysis, the influence of H2O vapor on oxidation rate, and the three-fold variation of the Arrhenius E parameter with temperature. This all convincingly demonstrates the value of the thermochemical approach in interpreting heterogeneous reactions.

Heterogeneous reaction kinetics of epoxide-functionalized regenerated cellulose membrane and aliphatic amine

The reaction kinetics of an epoxide-functionalized regenerated cellulose membrane with excess aliphatic amine are examined using differential scanning calorimetry. Both isothermal and dynamic experiments are analyzed. The reaction is found to follow first-order autocatalytic kinetics. The activation energy based on five different methods ranges from 77.0 to 79.7 kJ/mol, indicating that the apparent activation energy for the heterogeneous epoxy/aliphatic amine reaction is considerably higher than the values reported in the literature for the homogeneous reaction.

Modeling of the process of burning-out of coke–ash particles in a fluidized bed

A generalized statistical model of the process of burning out of coal particles in a fluidized-bed furnace is constructed. It takes into account the kinetics of heterogeneous chemical reactions, as well as radiant and convective–conductive heat transfer, which made it possible to considerably expand the region of application of the existing models. Different schemes of the behavior of ash in the process of burning of carbon particles are considered. Boundary conditions (conditions of matching) were formulated, and analytical solutions for the distribution function of particles in small intervals of carbon concentration for surface and bulk reaction have been obtained.

Heterogeneous photocatalytic removal and reaction kinetics of Rhodamine-B dye with Au loaded TiO2 nanohybrid catalysts

Heterogeneous photocatalytic removal and reaction kinetics of Rhodamine-B dye with Au loaded TiO2 nanohybrid catalysts Heterogeneous photocatalytic removal of Rhodamine-B (RhB) dye by metallic Au nanopatrticles deposited TiO2 photocatalyst was studied. For this study, a chemical reduction method by hydrazine hydrate for gold deposition was employed in order to synthesize Au/TiO2 nanocomposite system. For the characterization of the synthesized nanomaterials, X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (DRS), the Fourier transformation infrared spectroscopy (FTIR) and photoluminescence spectroscopy (PLS) techniques were performed. The obtained results show that the deposition of gold onto TiO2 surface could effectively inhibit the recombination of the photoinduced electron and holes, improving the absorption capability for the visible light source and leading to the increased surface OH group density. The degradation experiment reveals that the efficiency of color removal from RhB aqueous solution containing Au/TiO2 powders for the photocatalytic bleaching of RhB dye is more efficient than that of bare TiO2 sample upon UV-vis light activation. In addition, degradation kinetics of RhB dye in aqueous suspensions can be well simulated by the Langmuir-Hinshelwood model and obeys the pseudo-first order law, with a decolorization rate of 0.0252 min-1 to the photocatalytic removal of RhB dye.

On the kinetics of the Langmuir-type heterogeneous reactions

In this paper we investigate three two-dimensional in space mathematical models of the kinetics of unimolecular heterogeneous reactions proceeding onto planar surfaces. All models include the diffusion of the reactant from a bounded vessel towards an adsorbent, adsorption of the molecules of the reactant, their desorption, conversion (reaction) of the adsorbate into a product, instantaneous product desorption, and the diffusion of the product from the adsorbent into the same vessel. One of these models is based on the Langmuir-type kinetics of the surface reactions, the other one is based on the local steady-state value of the surface coverage, and the last one, in addition to the first model, involves the diffusion of the adsorbate along the adsorbent. Diffusivity of all species is assumed to be constant. Models were solved numerically by using the finite difference technique. By changing input parameters the effects of the rate constants of the reactant adsorption, desorption, and reaction and the influence of the surface diffusion of the adsorbate and approximation of the surface coverage by its steady-state value on the kinetics of surface reactions were studied numerically.

ChemInform Abstract: Role of Chemical Kinetics in the Heterogeneous Catalysis Studies

AbstractReview: 131 refs.

Measurement of Heterogeneous Reaction Rates: Three Strategies for Controlling Mass Transport and Their Application to Indium-Mediated Allylations

We describe three new strategies for determining heterogeneous reaction rates using photomicroscopy to measure the rate of retreat of metal surfaces: (i) spheres in a stirred solution, (ii) microscopic powder in an unstirred solution, and (iii) spheres on a rotating shaft. The strategies are applied to indium-mediated allylation (IMA), which is a powerful tool for synthetic chemists because of its stereoselectivity, broad applicability, and high yields. The rate-limiting step of IMA, reaction of allyl halides at indium metal surfaces, is shown to be fast, with a minimum value of the heterogeneous rate constant of 1 × 10(-2) cm/s, an order of magnitude faster than the previously determined minimum value. The strategies described here can be applied to any reaction in which the surface is retreating or advancing, thereby broadening the applicability of photomicroscopy to measuring heterogeneous reaction kinetics.

Numerical study of the kinetics of unimolecular heterogeneous reactions onto planar surfaces

In this paper we investigate two-dimensional in space mathematical models of the kinetics of unimolecular heterogeneous reactions proceeding onto planar surfaces. The models are based on Langmuir-type kinetics of the adsorption, desorption, and reaction including the surface diffusion of the adsorbate, surface diffusion of the product before its desorption, and slow desorption of the product from the adsorbent. It is also assumed that the reactant diffuses towards an adsorbent from a bounded vessel and the product diffuses from the adsorbent into the same vessel. Diffusivity of all species and kinetic coefficients are constants. The numerical simulation was carried out using the finite difference technique for four models: one model neglects the surface diffusion of the adsorbate and product, the second one includes the surface diffusion of the adsorbate and product, the third of them includes the surface diffusion of the adsorbate and neglects diffusion of the product along the surface, and the last one neglects the surface diffusion of the adsorbate and includes diffusion of the product along the adsorbent. By changing input parameters effects of the surface diffusion of the adsorbate and product and the slow desorption of the product are studied numerically.

Equations for the kinetics of liquid-phase reactions over solid catalysts and for activity coefficients

Equations are proposed for the kinetics of heterogeneous catalytic liquid-phase reactions that allow for the Langmuir-Hinshelwood sorption concept and the concept of local concentrations and equations for activity coefficients in homogeneous and separating systems based on the combination of the Wilson and Margules concepts.

Nonaffine Displacements in Flexible Polymer Networks

The validity of the affine assumption in model flexible polymer networks is explored. To this end, the displacements of fluorescent tracer beads embedded in polyacrylamide gels are quantified by confocal microscopy under shear deformation, and the deviations of these displacements from affine responses are recorded. Nonaffinity within the gels is quantified as a function of polymer chain density and cross-link concentration. Observations are compared with current theories of nonaffinity in random elastic media. We note that the mean-squared nonaffine deviation is proportional to the square of the applied strain in the linear elasticity regime, as per theoretical predictions. The measured degree of nonaffinity in the polyacrylamide gels suggests the presence of structural inhomogeneities which likely result from heterogeneous reaction kinetics during gel preparation. In addition, the macroscopic elasticity of the polyacrylamide gels is confirmed to behave in accordance with standard models of flexible polymer network elasticity.

Role of Chemical Kinetics in the Heterogeneous Catalysis Studies

The paper presents shortly some of the important elements of the theory and of the practical applications of the kinetics of heterogeneous catalytic reactions. Discussed are some of the most important concepts of the kinetics of complex heterogeneous catalytic reactions, methodology of building kinetic models and mathematical treatment of experimental data, influence of heat and mass transfer, types of laboratory reactors, kinetics and nanosized catalysts and others. Examples for use of the kinetic studies for the development and application of industrial catalysts and modeling of industrial reactors are presented.

Effects of heat and mass transfer on the kinetics of CO oxidation over RuO 2(1 1 0) catalyst

Combining first-principles kinetic Monte Carlo (KMC) simulation with a finite difference continuum model, a hybrid computational model was developed to study the effects of heat and mass transfer on the heterogeneous reaction kinetics. The integrated computational framework consists of a surface phase where catalytic surface reactions occur and a gas-phase boundary layer imposed on the catalyst surface where the fluctuating temperature and pressure gradients exist. The surface phase domain is modeled by the site-explicit first-principles KMC simulation. The gas-phase boundary layer domain is described using the second-order grid-based Crank–Nicolson method. To simplify the model, the flow and gas-phase reactions are excluded. The temperature and pressure gradients in the gas-phase boundary layer are the consequence of thermal and molecular diffusions of reactants and products under nominal reaction conditions. Different from previous hybrid multiscale models, the heat and mass fluxes between two domains are directly coupled by the varying boundary conditions at each simulation timestep from the unsteady state reaction regime to the steady state reaction regime in the present model. At the steady-state reaction regime, the activity, the surface coverages of reaction intermediates, along with the temperature and pressure gradient profiles in the gas-phase boundary layer are statistically constant with very small fluctuations. As an illustration example, we studied the effects of heat and mass transfer on the reaction kinetics of CO oxidation over the RuO 2(1 1 0) catalysts. We assume the heat from CO oxidation is exclusively dissipated into the gas-phase via thermal diffusion. By varying the thickness of RuO 2(1 1 0) catalysts, the surface temperature changes correspondingly with the heat produced by occurring surface reactions, resulting in the pronounced temperature and pressure gradients in the gas-phase boundary layer. Our simulation results indicate that the limitation of heat and mass transfer in the surrounding environment over the catalyst could dramatically affect the observed macroscopic reaction kinetics under presumed operating reaction conditions. To fully elucidate the complex heterogeneous catalytic system, proper physical description of fluid phase that imposed on the catalyst and its effect on the surface kinetics should be integrated with current surface computational models.

Numerical solving of coupled systems of parabolic and ordinary differential equations

Two coupled systems of parabolic and nonlinear ordinary differential equations arising in kinetics of heterogeneous reactions are studied numerically by using computer calculations. Some numerical results are discussed.