Simulation of oxygen atom adsorption on an Al<Subscript>2</Subscript>O<Subscript>3</Subscript> surface by the density functional method

Several cluster models of oxygen atom adsorption on an Al₂O₃ surface are constructed on the basis of the density functional method. The performed quantum mechanical computations allow one to reveal a number of important features of the potential energy surface to describe the heterogeneous catalytic processes with the use of molecular dynamics methods. The heterogeneous recombination of oxygen atoms is simulated according to the Eley-Rideal mechanism. It is shown that the potential energy surface should be used with consideration of the internal relaxation of surface monolayers to correctly describe the process under study.     

Investigation of the Influence of Different Heterogeneous Recombination Mechanisms on the Heat Fluxes to a Catalytic Surface in Dissociated Carbon Dioxide

A kinetic model of heterogeneous recombination in dissociated carbon dioxide on high-temperature heat-shield coatings is developed; the model takes into account the nonequilibrium adsorption-desorption reactions of oxygen atoms and their recombination in the Eley-Rideal and Langmuir-Hinshelwood reactions. On the basis of a comparison of the calculated heat fluxes in dissociated carbon dioxide with those measured in the VGU-3 plasma generator of the Institute for Problems in Mechanics of the Russian Academy of Sciences (IPM RAS) and the available literature data, the parameters of the catalysis model are chosen for the glassy coating of the Buran orbiter tile heat shield based on the SiO₂–B₂O₃–SiB₄ system. The effects of heterogeneous recombination proceeding in accordance with the Langmuir-Hinshelwood mechanism, as well as the processes involving carbon atoms and those involving physically adsorbed oxygen atoms, on the heat fluxes to the glassy coating are analyzed on the surface temperature range from 300 to 2000 K.     

Analysis of the heterogeneous recombination of oxygen atoms on aluminum oxide by methods of quantum mechanics and classical dynamics

On the basis of the density-functional theory, cluster models of the adsorption of oxygen atoms on aluminum oxide are constructed and the corresponding potential-energy surface is calculated. Quantum-mechanical calculations showed that it is necessary to take into account the angular dependence of the potential-energy surface and the relaxation of the surface monolayers. Using this surface in molecular dynamics calculations made it possible to obtain the probabilities of the heterogeneous recombination of oxygen atoms on the α-Al₂O₃ surface, which are in good agreement with experimental data. The calculations performed substantially decrease the amount of experimental investigations necessary reliably to describe the heterogeneous catalysis on promising reusable heat shield coatings for analyzing heat transfer during spacecraft entry into the atmosphere.     

Catalytic properties of a copper transducer for determining flow parameters in a high-frequency induction plasmatron

The rate coefficients of the elementary stages of the complete system of heterogeneous catalytic recombination of dissociated oxygen on a copper oxide surface are determined on the basis of quantum-mechanics calculations within the framework of cluster models. The coefficients are used to calculate the dependence of the effective coefficient of heterogeneous catalytic recombination of oxygen atoms on the temperature and the partial pressure on a wide range of surface conditions. It is established that it can considerably vary depending on these conditions.     

On the recombination mechanism of atomic nitrogen near a catalytic surface in a stream of dissociated air

It is shown that the heterogeneous recombination of nitrogen atoms on a catalytically active surface in a stream of dissociated air is accompanied by intense gas-phase recombination of the nitrogen in exchange reactions whose rate is determined by the rate of heterogeneous recombination of atomic oxygen.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 156–158, May–June, 1980.     

Methods and Results of an Experimental Determination of the Catalytic Activity of Materials at High Temperatures

Methods of experimental determination of the rate constants of heterogeneous catalytic recombination of nitrogen and oxygen atoms in dissociated subsonic nitrogen and air flows generated by a high-frequency inductive plasmatron are discussed. Together with the values of the probability of heterogeneous recombination used in the literature, the rate constant of the elementary process of nitrogen atomic recombination is determined in accordance with the Langmuir-Hinshelwood mechanism.     

Analysis of catalytic properties of siliconized heat-shielding coatings

The catalytic properties of heat-shielding coatings (β-cristobalite and SiC) used on space vehicles are analyzed on the basis of the microscopic approach with consideration of the molecular structure of the near-surface layer. The heterogeneous recombination coefficient of oxygen atoms and the recombination energy accommodation coefficient are determined. The energy distribution by internal degrees of freedom is calculated. In particular, it is found that, when the energy of collision of atoms with the surface is small, the oxygen atom heterogeneous recombination is more efficient for SiC coatings, whereas this recombination is more efficient in the case of β-cristobalite if the collision energy is large. Nevertheless, the heat-transfer coefficient is greater for SiC coatings in the studied range of collision energy variations, since the recombination energy accommodation is larger.     

Simulation of heterogeneous atom recombination on spacecraft heat shield coatings using the methods of molecular dynamics

An efficient method of investigating the processes of interaction between gas mixtures and catalytic surfaces is developed within the framework of classical molecular dynamics. The recombination and chemical-energy accommodation coefficients on the catalytic surface can be determined with fewer computational resources than in the quantum-mechanical and semiclassical approaches. Oxygen atom recombination on a β-cristobalite surface of the type frequently used in spacecraft heat shield systems is investigated. The probability of atom recombination and the recombination energy accommodation coefficient obtained are in satisfactory agreement with the available experimental data and calculations made by means of the semiclassical method. The hypothesis that the probability of the Eley-Rideal reaction decreases and the probability of atom adsorption increases with increase in the atom collision energy is confirmed. It is attributable to the tendency of atoms to be trapped in the potential well and be desorbed in the atomic state when the surface collision energy is high instead of entering into a recombination reaction and then being desorbed in the molecular state.     

Experimental and Theoretical Simulation of Heterogeneous Catalysis in Aerothermochemistry (a Review)

The design of aerospace vehicles has required the solution of radically new scientific and technological problems. One of the important problems has been to create reusable heat shield materials. In [1, 2] information concerning the methods and results of solving these problems, including the development of composites from ultrathin quartz fibers and carbon-carbon materials for the “Buran” orbital vehicle heat shield, was presented. The basic thermophysical characteristics of these materials include both the rate or probability coefficients of heterogeneous nitrogen and oxygen atom recombination and the accommodation coefficients of energy recombination at high surface temperatures. In the present paper the experimental and computational aspects of determining these parameters, which are also of interest for new heat shield materials for future space transport systems, are discussed.     

Liquid phase heterogeneous photocatalytic ozonation of phenol in liquid–solid fluidized bed: Simplified kinetic modeling

The isotherms of original AC (activated carbon) and photocatalysts (TiO₂-AC) calcined at 500 °C for phenol were measured. The results showed a reversible adsorption of phenol onto both kinds of particles at 25 °C, and could be fitted well to the Freundlich adsorption equation for the dilute solution. Five oxidation processes, namely O₃, O₃/UV, O₃/UV/AC, O₂/UV/TiO₂ and O₃/UV/TiO₂, for phenol degradation in fluidized bed were evaluated and compared, and the photocatalytic ozonation was found to give the highest phenol conversion because of the combined actions of homogenous ozonation in the liquid phase, heterogeneous ozonation on the surface of the catalyst support, i.e. activated carbon, and heterogeneous photocatalytic oxidation on the TiO₂ catalyst surface. With the simplified kinetic model, photolytic ozonation was confirmed to predominantly take place on the particle surface in comparison with the heterogeneous and homogeneous photolytic ozonation. Additionally, the heterogeneous photocatalytic oxidation constant was found to be enhanced by 3.73 times in photocatlaytic ozonation process with ozone as the scavenger compared to the photocatalytic oxidation process with oxygen as the scavenger.     

Determination of the catalytic activity of materials by solving the equations of a nonequilibrium multicomponent boundary layer on a flat plate

A method is presented for determining the dependence of the probability of heterogeneous recombination γw from results of measurements of the heat flux Q_w to the surface of a catalytic sensor exposed to a pulsed supersonic flow of gas dissociated by an incident shock wave propagating in a shock tube. It is shown that the accuracy of the determination of γw depends not only on the accuracy of the measurements in the experiment, but also on the results of mathematical modeling of the flow of the dissociated gas over the surface of the body. Results from an analysis of an experiment are presented. Institute of Applied Mathematics and Mechanics at Tomsk University, Tomsk 634050. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 39, No. 4, pp. 110–117, July–August, 1998.     

Multiscale modeling for ferroelectric materials: a transition from the atomic level to phase-field modeling

To link the atomic level and the mesoscale within a knowledge-based multiscale modeling approach for ferroelectric materials, a method is suggested to transfer results from first-principles calculations into a phase-field model. DFT calculations and atomistic simulations are applied and provide a set of intrinsic and extrinsic material properties for PbTiO₃ and tetragonal Pb(Zr_0.5Ti_0.5)O₃. The Helmholtz free energy of the phase-field model that contains all crystallographic and domain wall information is discussed in detail, and a sensitivity analysis is performed to identify the coefficients of the energy function. Then, a method is developed to adjust the coefficients of the Helmholtz free energy solely based on results from first-principles calculations. Full sets of adjusted energy coefficients for PbTiO₃ and Pb(Zr_0.5Ti_0.5)O₃ are presented and discussed, as well the limits of the suggested adjustment method.     

Ignition of hydrogen in a turbulent boundary layer on a plate

The ignition of hydrogen blown into a turbulent supersonic boundary layer on a flat plate is investigated numerically. It is assumed that the mixture consists of six chemically active components H, O, OH, H₂O, O₂, H₂ and inert nitrogen N₂. The boundary layer is divided into outer and inner regions, for which different expressions for the coefficients of turbulent transport are used. The influence of pulsations on the rates of the chemical reactions, and also the back reaction of the chemical processes on the mechanism of turbulent transfer are not taken into account. The surface of the plate is assumed to be absolutely catalytic with respect to the recombination reactions of the H and O atoms. The influence of the blowing intensity, the Mach number in the outer flow, and the pressure on the ignition delay is analyzed. The possibility of effective porous cooling of the surface when there is combustion in the boundary layer is demonstrated.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 33–40, November–December, 1979.I thank V. G. Gromov and V. A. Levin for their interest in the work.     

Effect of dopants and interlayers on the growth of thin insulating films

Surface morphologies of thin dielectric films deposited on gallium arsenide substrates are studied by atomic force microscopy (AFM). The quasi-periodic mesostructure with a corrugated configuration is found to form during the deposition process. A special dopant and thin interlayer at the film–substrate interface are used to decrease the surface roughness. The corrugated Si_xN_yO_z–SiO₂ film surface disappears by introducing Se atoms into the subsurface layer of the semiconductor. The root-mean-square roughness and the fractal dimension techniques are used for the numerical characterization of the surface morphologies of thin insulator films.     

Mechanism for the promotional formation of NH4+ by SO2 on different mineral dust surfaces

Ammonium is an important atmospheric particulate component that dictates many environmental processes. The promotion of the heterogeneous conversion of NH₃ to NH₄⁺ by SO₂ on different mineral dust surfaces displays remarkable discrepancies, especially on MgO and α-Fe₂O₃ surfaces, however, the underlying mechanisms are not well known. Here, using periodic density functional theory (DFT) calculation and Born-Oppenheimer molecular dynamics (BOMD) simulation, we explored the heterogeneous adsorption of NH₃ on MgO (110) and α-Fe₂O₃ (001) surfaces in the presence and absence of SO₂. The results show that on MgO (110) surface, hydrogen-bonding interactions of NH₃ on both adsorbed hydroxyl or bisulfite/bisulfate sites are observed no matter whether SO₂ is present or not. While, on the α-Fe₂O₃ (001) surface, significant conversion of NH₃ to NH₄⁺ occurs with the coexistence of SO₂, which is due to the hydrogen transfer reaction from surface HSO₄ to N in NH₃. The fundamental reason may be that the stronger electron affinity of Fe³⁺ than Mg²⁺ results in adsorbed bisulfate and/or bisulfite with greater acidity on α-Fe₂O₃ surface than MgO surface. Our results give a molecular-level explanation for the heterogeneous conversion of NH₃ to NH₄⁺ on different mineral dust surfaces under complex air pollution conditions. Considering the fact that ammonium is abundant in secondary particulates, this work would help in understanding the rapid conversion of ammonia to ammonium and in developing classification governance policies for the key precursor pollutants in China.     

Numerical estimation of heterogeneous reaction constants in the flow of rarefied gas through a cylindrical channel

The Direct Simulation Monte Carlo method is used to study the influence of the coefficients of heterogeneous dissociation and recombination reactions on the rarefied gas flow through a cylindrical channel. It is established that the degree of dissociation of the flow coming out of the channel is significantly dependent on the relationship between the dissociation and recombination coefficients. The technique for determining the dissociation and recombination coefficients on the basis of the experimental data is proposed.     

Diffusion separation of chemical elements on a catalytic surface

An asymptotic expansion of the solution, for large Schmidt numbers, of the system of equations of a chemically nonequilibrium multicomponent boundary layer on the catalytic surface of a blunt body [1] is used to obtain expressions for the diffusion fluxes of the reaction products and chemical elements and the heat flux as functions of the gradients of the reaction product concentrations, chemical element concentrations and enthalpy across the boundary layer. It is shown that when the body is exposed to a supersonic air flow, the diffusion separation of the chemical element oxygen depends importantly on the atom concentration at the outer edge of the boundary layer and the nature of the homogeneous and heterogeneous catalytic reactions. If the surface promotes the rapid recombination of oxygen atoms and is chemically neutral with respect to nitrogen atoms, then an excess of the chemical element oxygen is formed on the body. Otherwise we get an enhanced concentration of the element nitrogen. As distinct from the case of an ideally catalytic wall [2–4], on a surface possessing the property of catalytic selectivity the diffusion separation of chemical elements takes place even when only atoms are present at the outer edge of the boundary layer. On a chemically neutral surface diffusion separation may be caused by homogeneous recombination reactions between oxygen and nitrogen atoms if their rate constants are essentially different.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 115–121, July–August, 1988.     

Effect of the incomplete accommodation of the heterogeneous recombination energy on heat fluxes to a quartz surface

Taking both the heterogeneous catalytic processes, including the surface formation of particles with excited internal degrees of freedom, and the processes of multicomponent diffusion and heat transfer in the MESOX apparatus fully into account makes it possible to obtain a recombination coefficient and an accommodation coefficient of the oxygen-atoms-on-quartz recombination energy which are in good agreement with the experimental data. The heterogeneous catalysis model constructed can be used effectively for predicting the heat fluxes to the surface of reentry vehicles on their entry into the Earth’s atmosphere.     

偏压及测试环境对新型MoS2-Ti复合膜摩擦学性能的影响

采用新型高功率脉冲复合磁控溅射技术制备MoS2-Ti复合膜,并研究基体偏压和测试环境对复合膜摩擦学性能的影响.结果表明:制备的MoS2-Ti复合膜表面呈现颗粒状结构,Ti在薄膜表层与O反应形成氧化物有效抑制MoS2的氧化.随着基体负偏压从OV增大到-400 V,复合膜的S/Mo原子比逐渐减小.在-300 V偏压下,颗粒堆积最为紧密,薄膜硬度和弹性模量达到最大值,分别为9.7和137.1GPa,并具有最低的平均摩擦系数值(0.04)和磨损率[(10-7mm3/(N·m)].多种测试环境下的摩擦研究显示:在室温大气环境下复合膜的摩擦学性能与其结构的致密性紧密相关,而在N2以及不同湿度环境下薄膜表现出的优异摩擦学性能则归因于在摩擦过程中有效形成的转移膜贡献.