Mathematical simulation of self-oscillations in methane oxidation on nickel: An isothermal model

The dynamics of methane oxidation on nickel was studied by mathematical modeling within the framework of an 18-step microkinetic scheme. The model examined predicts the appearance of self-oscillations caused by the periodic oxidation-reduction of nickel in the reaction proceeding under isothermal conditions.     

Isothermal waves of hydrogen oxidation

It is analytically shown that the chain ignition region is extended in the reaction of hydrogen oxidation due to the nonlinear interaction H + HO₂ = 2 OH. As a result, the reaction propagates isothermally under the isothermal conditions outside the ignition region, which is calculated by the scheme taking into account only linear reactions with respect to radicals, if the reaction is initiated by additives of hydrogen atoms. The ignition limits were calculated with allowance for the above interaction, and the mathematical modeling of propagation of the hydrogen oxidation reaction under the isothermal conditions was performed.     

Parametric analysis of kinetic models, 8. Two-center catalytic oscillators

The results of parametric analysis for a kinetic model of CO oxidation reaction on two types of active centers of Pt are given. The local bifurcation curves are constructed for every type of center. The regions with self-oscillations are singled out in a plane (p_CO, T). A sum of rate oscillations on every center can have a quasi-chaotic character.     

Inhibition of the reaction between hydrogen and oxygen by multiatomic gas admixtures behind the incident shock wave front

The inhibiting effect of multiatomic gases has been investigated by numerical simulation of hydrogen oxidation with account taken of the nonequilibrium state of the initial components, intermediates, and reaction products behind the shock wave in the framework of a vibrationally nonequilibrated model. The central feature of the model is successively taking into account the vibrational disequilibrium of the HO₂ radical as the most important intermediate in the chain branching process. The inhibiting effect can be explained by the influence of the multiatomic gases on the rate of the vibrational relaxation of the vibrationally excited HO₂ radical resulting from the reaction. Methane, tetrafluoromethane, fluoromethane, difluoromethane, chlorofluoromethane, formaldehyde, ethane, hexafluoroethane, ethylene, tetrafluoroethylene, and propane have been considered as inhibitory admixtures.     

Propane oxidation on nickel in a self-oscillation mode

The kinetics of the catalytic oxidation of propane with oxygen on nickel in a self-oscillation mode was studied. A comparative analysis of changes in reaction rate oscillations with time in the presence of nickel wire and foil was performed. It was found that the reaction medium influenced the morphology of the catalyst surface. With the use of X-ray photoelectron spectroscopy, it was demonstrated that a NiO layer no less than 100 nm in thickness was formed on the catalyst surface in the course of the reaction of propane oxidation. The mechanism of the appearance of self-oscillations in the test system is discussed.__________Translated from Kinetika i Kataliz, Vol. 46, No. 2, 2005, pp. 269–277.Original Russian Text Copyright © 2005 by Gladky, Kaichev, Bukhtiyarov, Parmon.Deceased.     

Reactions of methyl and ethyl halides with the complexes Ni[P(C2H5)3]4 and Ni(X)[P(C2H5)3]3

The rates of the thermal reaction of the nickel(0) complex Ni[P(C₂H₅)₃]₄ with the alkyl halides CH₃Br, CH₃I in toluene have been compared with those of the reactions of the nickel(I) complexes Ni(X)[P(C₂H₅)₃]₃ (X  Br,I). The organic products from CH₃X are methane and ethane, and those from C₂H₅I are ethane and ethylene. The reactivity of the nickel(I) complexes is 10–20 times less than that of the nickel(0) complex. The result suggest that the first step of the reaction of nickel(0) with CH₃I is the expected oxidative addition of the halide to the metal substrate. The intermediate thus formed decomposes to produce ethane (and small amounts of methane) without further reaction with the organic halide. This mechanism is supported by deuterium-labeling experiments.     

Oscillatory electrocatalytic oxidation of methanol on an Ni(OH)2 film electrode

A film of Ni(OH)₂ deposited cathodically on a roughened nickel substrate consists of even nanoparticles, which were characterized by atomic-force microscopy (AFM). The mechanism of potential oscillations in the electrocatalytic oxidation of methanol on this film electrode in alkaline medium was studied in situ by means of Raman spectroscopy in combination with electrochemical measurements. The redox change of the nickel hydroxide film, the concentration distribution of methanol in the diffusion layer, and the oxidation products of methanol were characterized in situ by time-resolved, spatial-resolved, and potential-dependent Raman spectroscopy, respectively. Electrochemical reactions, i.e. methanol oxidation and periodic oxygen evolution, coupling with alternately predominant diffusion and convection mass transfer of methanol, account for the potential oscillations that occur during oxidation of methanol above its limiting diffusion current. This mechanism is totally different from that of methanol oxidation on platinum electrodes, for which surface steps, i.e. formation and removal of CO_ad, are essential.This work is dedicated to Professor Gyorgy Horanyi on the occasion of his 70th birthday in recognition of his numerous contributions to field of electrochemical oscillations and electrocatalysis at Ni-hydroxide electrodes.     

The role of water in promoting the oxidative dehydrogenation of ethane

The addition of water vapor has a strong positive effect on the rate of ethane oxidation at 575°C. This effect is attributed to the role of H₂O as a third body in the decomposition of H₂O₂ to OH radicals. Carbon tetrafluoride likewise enhances the rate of ethane conversion, although not to the extent realized with H₂O. A kinetic model, based on known elementary reactions, adequately accounts for the conversions and selectivities observed as a function of H₂O pressure, temperature, contact time, and O₂ pressure. © 1994 John Wiley & Sons, Inc.     

Inelastic electron scattering in the adsorbed system

The study revealed additional channels of inelastic electron scattering, which accompany the threshold excitation of the substrate Pt4d level — ionization of the valent states of adsorbed particles chemically bonded to the excited atom, and excitation of the surface plasmon vibrations. The conjugate excitation of this type shows up as a series of typical satellites in the spectra of disappearance potentials, which reflects the structure of valent states of adsorbed particles. Analysis of the satellite structure revealed the intermediate formation of NH _x,ads particles in the reaction NO_gas + H_ads on the surface of Pt(100) single crystal and, taking into account the earlier data, made it possible to formulate a general mechanism of selfoscillations in the NO + H₂ reaction on platinum metals. Mathematical modeling of reaction kinetics on the Pt(100) surface within the suggested mechanism demonstrated the presence of regular self-oscillations of the reaction rate at invariable values of the step constants.     

Modeling of ammonia oxidation on a platinoid catalyst,taking into account the N<Subscript>2</Subscript>O formation

A mathematical model of ammonia oxidation on a platinoid catalyst, taking into account the N₂O formation, was developed. The possibilities of lowering the amount of N₂O, which is formed as by-product in high-temperature oxidation of ammonia in nitric acid production, are examined. The developed model allows calculation of the reactor for ammonia oxidation using platinoid catalysts of different geometric profiles.     

Synchronization of local oscillators in oxidation reactions of C1–C4 hydrocarbons over metal catalysts

The synchronization of reaction rate oscillations in the oxidation of C₁–C₄ hydrocarbons over polycrystalline nickel, cobalt, and palladium foils has been investigated. The synchronization of foil temperature oscillations during the reaction takes place via the diffusion of the reactants in the gas phase. For the nickel catalysts, the synchronization of the oscillators occurs in the same phase, while for the palladium catalysts, both in-phase and antiphase oscillations are observed. This distinction between the dynamic behaviors of the systems of two coupled oscillators is due to the fact that the mechanism of reaction rate oscillations varies from one metal to another.     

CH bond activation in the presence or absence of oxygen or nitrous oxide

The reaction of C₂H₆with lattice oxygen, O^2- (in the absence of gaseous oxygen), or “adsorbedℍ oxygen (in the presence of gaseous oxygen) over NiMoO₄ catalysts has been performed and compared to C₃H₈ activation. The results obtained indicate that adsorbed oxygen exhibits a higher reactivity to C₂H₆, while lattice oxygen is more reactive relative to C₃H₈. Kinetic studies of these two reactions in presence of molecular oxygen have indeed shown that the ethane oxidative dehydrogenation (ODH) is dependent on the oxygen partial pressure, whilst on the contrary propane ODH is not. In order to confirm the presence of “adsorbed” oxygen for ethane activation, ODH tests have been performed with N₂O. On increasing temperature, the O^- adsorbed species enhances the mild oxidation of ethane. The activation energy of ethane consumption E_C2H6, relative to propane (E_C3H8 = 133 kJ/mol) is 145 kJ/mol. A possible mechanism is proposed for the oxidative dehydrogenation of ethane. This revised version was published online in August 2006 with corrections to the Cover Date.     

Peculiarities of Catalytic Oxidation of Ethane by Nitrous Oxide on ZSM-5 Type Zeolites Containing Iron Ions in Different Localization Sites

The temperature dependence of the Fe-HZSM-5 activity and selectivity in the process of catalytic oxidation of ethane by the excess of N₂O at 250–350°C exhibits a pronounced hysteresis. The oxidized catalysts free from condensation products are active only in the complete oxidation of ethane. At low temperatures of the reaction of the C₂H₆ + N₂O mixture with the catalyst, coke formation takes place and the coordination state of iron ions differs from the initial sample. Under these conditions, the process of complete oxidation of ethane is essentially suppressed and the process of oxidative dehydrogenation dominates. The catalytic properties of iron-containing zeolites prepared either by direct synthesis or by introduction of iron ions into the cationic positions of H[Al]ZSM-5 are quite similar, because irreversible formation of new iron species considerably different from the initial species takes place during the catalytic reaction on both series of samples. The activity of HZSM-5 containing trace amounts of iron is much lower than that of iron-containing samples.     

Automated Generation of Microkinetics for Heterogeneously Catalyzed Reactions Considering Correlated Uncertainties**

The study presents an ab-initio based framework for the automated construction of microkinetic mechanisms considering correlated uncertainties in all energetic parameters and estimation routines. 2000 unique microkinetic models were generated within the uncertainty space of the BEEF-vdW functional for the oxidation reactions of representative exhaust gas emissions from stoichiometric combustion engines over Pt(111) and compared to experiments through multiscale modeling. The ensemble of simulations stresses the importance of considering uncertainties. Within this set of first-principles-based models, it is possible to identify a microkinetic mechanism that agrees with experimental data. This mechanism can be traced back to a single exchange-correlation functional, and it suggests that Pt(111) could be the active site for the oxidation of light hydrocarbons. The study provides a universal framework for the automated construction of reaction mechanisms with correlated uncertainty quantification, enabling a DFT-constrained microkinetic model optimization for other heterogeneously catalyzed systems.     

Thermodynamic Approach to the Calculation of the Rate Constants of Monomolecular Reactions in the Strong-Collision Limit

A thermodynamic approach to the calculation of the dissociation (recombination) rate constants of polyatomic molecules in the strong-collision limit is suggested. The approach is based on the density of states obtained by the inverse Laplace transform of the integral expression for the statistical sum of the internal degrees of freedom of the molecule. Although the mathematical apparatus of this model is rather simple, it enables one to take into account the real energy distribution of molecular states. The potential of this method is demonstrated for O₃, H₂O, H₂O₂, CH₄, and C₂H₆. The results obtained are compared with data calculated using other methods. The temperature dependence of the collision efficiency correction β_c is analyzed for water and ethane.     

Hierarchical multiscale mechanism development for methane partial oxidation and reforming and for thermal decomposition of oxygenates on Rh

A thermodynamically consistent C1 microkinetic model is developed for methane partial oxidation and reforming and for oxygenate (methanol and formaldehyde) decomposition on Rh via a hierarchical multiscale methodology. Sensitivity analysis is employed to identify the important parameters of the semiempirical unity bond index quadratic exponential potential (UBI-QEP) method and these parameters are refined using quantum mechanical density functional theory. With adjustment of only two pre-exponentials in the CH4 oxidation subset, the C1 mechanism captures a multitude of catalytic partial oxidation (CPOX) and reforming experimental data as well as thermal decomposition of methanol and formaldehyde. We validate the microkinetic model against high-pressure, spatially resolved CPOX experimental data. Distinct oxidation and reforming zones are predicted to exist, in agreement with experiments, suggesting that hydrogen is produced from reforming of methane by H2O formed in the oxidation zone. CO is produced catalytically by partial oxidation up to moderately high pressures, with water-gas shift taking place in the gas-phase at sufficiently high pressures resulting in reduction of CO selectivity.     

Partial Oxidation of Ethane to Syngas over Supported Metal Catalysts

The reaction performance for C₂H₆-O₂ to syngas over different supported metal catalysts was investigated in a flow-reactor. The activated behavior of ethane is different from that of methane over the supported nickel catalysts. Although there may exist a gas phase reaction at high temperatures, over a Ni (or Rh)/-Al₂O₃ catalyst, the partial oxidation of ethane to syngas is a heterogeneous process, while over a Pt (or Pd)/-Al₂O₃ catalyst, it may be a homo-heterogeneous process. The Ni/-Al₂O₃ and Rh/-Al₂O₃ catalysts are suitable for partial oxidation of ethane to syngas at high temperatures.     

Including lateral interactions into microkinetic models of catalytic reactions

In many catalytic reactions lateral interactions between adsorbates are believed to have a strong influence on the reaction rates. We apply a microkinetic model to explore the effect of lateral interactions and how to efficiently take them into account in a simple catalytic reaction. Three different approximations are investigated: site, mean-field, and quasichemical approximations. The obtained results are compared to accurate Monte Carlo numbers. In the end, we apply the approximations to a real catalytic reaction, namely, ammonia synthesis.     

Influences of ClH and BrH on the pyrolyses of neopentane and ethane at small extents of reaction

A symbolic mechanism “μH, YH” has been proposed to account for the homogeneous chain pyrolysis of an organic compound μH in the presence of a hydrogenated additive YH at small extents of reaction. An analysis of this mechanism leads to two limiting cases: the thermal decomposition of neopentane corresponds to the first one (A), that of ethane to the second one (B). Previous experimental work has shown that this mechanism seems to account for a number of experimental observations, especially the inhibition of alkane pyrolyses by alkenes. Experimental investigations were extended by examining the influences oftwo hydrogen halides (ClH and BrH) upon the pyrolyses of neopentane (at 480°C) and ethane (around 540°C). The experiments have been performed in a conventional static Pyrex apparatus and reaction products have been analyzed by gas-liquid chromatography. The study shows that ClH and BrH accelerate the pyrolysis of neopentane (into i-C₄H₈ + CH₄). The experimental results are interpreted by reaction schemes which appear as examples of the mechanism “μH, YH” in the first limiting case (A). The proposed schemes enable one to understand why the accelerating influence of ClH is lower or higher than that of BrH, depending on the concentration of the additive. An evaluation of the rate constant of the elementary steps neo-C₅H₁₁ · → i-C₄H₈ + CH₃ · is discussed. In the case of ethane pyrolysis, BrH inhibits the formation of the majorproducts (C₂H₄ + H₂) and, even more, that of n-butane traces. The experimental results are interpreted by a reaction scheme which appears as an example of the mechanism “μH, YH” in the second limiting case (B). On the contrary, ClH has no noticeable influence on the reaction kinetics. This result inessentially due to the fact that the bond dissociation energy of Cl? H(?103 kcal/mol) is higher than that of C₂H₅—H (?98 kcal/mol), whereas that of Br—H (?88 kcal/mol) is lower.