Theoretical study of pattern formation during the catalytic oxidation of CO on Pt{100} at low pressures

Theoretical studies have thus far been unable to model pattern formation during the reaction in this system on physically feasible length and time scales. In this paper, we derive a computational reaction-diffusion model for this system in which most of the input parameters have been determined experimentally. We model the surface on a mesoscopic scale intermediate between the microscopic size of CO islands and the macroscopic length scale of pattern formation. In agreement with experimental investigations [M. Eiswirth et al., Z. Phys. Chem., Neue Folge 144, 59 (1985)], the results from our model divide the CO and O(2) partial pressure parameter space into three regions defined by the level of CO coverage or the presence of sustained oscillations. We see CO fronts moving into oxygen-covered regions, with the 1 x 1 to hex phase change occurring at the leading edge. There are also traveling waves consisting of successive oxygen and CO fronts that move into areas of relatively high CO coverage, and in this case, the phase change is more gradual and of lower amplitude. The propagation speed of these reaction waves is similar to those observed experimentally for CO and oxygen fronts [H. H. Rotermund et al., J. Chem. Phys. 91, 4942 (1989); H. H. Rotermund et al., Nature (London) 343, 355 (1990); J. Lauterbach and H. H. Rotermund, Surf. Sci. 311, 231 (1994)]. In the two-dimensional version of our model, the traveling waves take the form of target patterns emitted from surface inhomogeneities.     

Pattern formation and self-organization in a simple precipitation system

Various types of pattern formation and self-organization phenomena can be observed in biological, chemical, and geochemical systems due to the interaction of reaction with diffusion. The appearance of static precipitation patterns was reported first by Liesegang in 1896. Traveling waves and dynamically changing patterns can also exist in reaction-diffusion systems: the Belousov-Zhabotinsky reaction provides a classical example for these phenomena. Until now, no experimental evidence had been found for the presence of such dynamical patterns in precipitation systems. Pattern formation phenomena, as a result of precipitation front coupling with traveling waves, are investigated in a new simple reaction-diffusion system that is based on the precipitation and complex formation of aluminum hydroxide. A unique kind of self-organization, the spontaneous appearance of traveling waves, and spiral formation inside a precipitation front is reported. The newly designed system is a simple one (we need just two inorganic reactants, and the experimental setup is simple), in which dynamically changing pattern formation can be observed. This work could show a new perspective in precipitation pattern formation and geochemical self-organization.     

Iodine oxidation by hydrogen peroxide and Bray-Liebhafsky oscillating reaction: effect of the temperature

This work presents a new experimental kinetic study at 39° and 50° of the iodine oxidation by hydrogen peroxide. The results allow us to obtain the temperature effect on the rate constants previously proposed at 25° for our model of the Bray-Liebhafsky oscillating reaction (G. Schmitz, Phys. Chem. Chem. Phys. 2010, 12, 6605.). The values calculated with the model are in good agreement with many experimental results obtained under very different experimental conditions. Numerical simulations of the oscillations observed formerly by different authors are presented, including the evolutions of the iodine, hydrogen peroxide, iodide ions and oxygen concentrations. Special attention is paid to the perturbing effects of oxygen and of the iodine loss to the gas phase.     

Modeling the oxidative coupling of methane： Heterogeneous chemistry coupled with 3D flow field simulation

The oxidative coupling of methane (OCM) over titanate perovskite catalyst has been developed by three-dimensional numerical simulations of flow field coupled with heat transfer as well as heterogeneous kinetic model. The reaction was assumed to take place both in the gas phase and on the catalytic surface. Kinetic rate constants were experimentally obtained using a ten step kinetic model. The simulation results agree quite well with the data of OCM experiments, which were used to investigate the effect of temperature on the selectivity and conversion obtained in the methane oxidative coupling process. The conversion of methane linearly increased with temperature and the selectivity of C2 was practically constant in the temperature range of 973–1073 K. The study shows that CFD tools make it possible to implement the heterogeneous kinetic model even for high exothermic reaction such as OCM.     

The influence of drift gas composition on the separation mechanism in traveling wave ion mobility spectrometry: insight from electrodynamic simulations

The influence of three different drift gases (helium, nitrogen, and argon) on the separation mechanism in traveling wave ion mobility spectrometry is explored through ion trajectory simulations which include considerations for ion diffusion based on kinetic theory and the electrodynamic traveling wave potential. The model developed for this work is an accurate depiction of a second-generation commercial traveling wave instrument. Three ion systems (cocaine, MDMA, and amphetamine) whose reduced mobility values have previously been measured in different drift gases are represented in the simulation model. The simulation results presented here provide a fundamental understanding of the separation mechanism in traveling wave, which is characterized by three regions of ion motion: (1) ions surfing on a single wave, (2) ions exhibiting intermittent roll-over onto subsequent waves, and (3) ions experiencing a steady state roll-over which repeats every few wave cycles. These regions of ion motion are accessed through changes in the gas pressure, wave amplitude, and wave velocity. Resolving power values extracted from simulated arrival times suggest that momentum transfer in helium gas is generally insufficient to access regions (2) and (3) where ion mobility separations occur. Ion mobility separations by traveling wave are predicted to be effectual for both nitrogen and argon, with slightly lower resolving power values observed for argon as a result of band-broadening due to collisional scattering. For the simulation conditions studied here, the resolving power in traveling wave plateaus between regions (2) and (3), with further increases in wave velocity contributing only minor improvements in separations.     

Frequency- and time-domain femtosecond vibrational sum frequency generation from CO adsorbed on Pt111

We have studied the effects of intermolecular and intramolecular coupling on the C-O stretching vibration of CO adsorbed on Platinum (111) by means of femtosecond broadband vibrational sum frequency generation (VSFG). Resonant intermolecular coupling is investigated through the coverage dependence of the VSFG signal. The experimental observations can be accurately modeled as lateral coupling of the molecular transition dipole moments; this coupling is invoked in the nonlinear optical response model as a local field correction. The linear polarizability, which appears in this model, is modified by both the dipole-dipole coupling and the population of bridged adsorption sites. By extending the formalism to include these effects, we deduce a vibrational polarizability of 0.32 A(3) from the data. Intramolecular coupling to the frustrated translational mode is observed as temperature dependence of the C-O stretch. The present data can be described either by pertubative or nonpertubative lineshape models from the literature. Measurements of the temperature dependence of the vibrational free induction decay indicate a population relaxation time T(1) of (0.8+/-0.1) ps, in agreement with the observed low-temperature linewidth. Moreover, the ability of this time-domain method to discriminate spectral inhomogeneity yields clear evidence of the order-disorder transition near 275 K. Above this temperature an inhomogeneous linewidth component of (12+/-3) cm(-1) is observed. This value allows us to estimate the structural heterogeneity of the disordered phase, which result agrees with published Monte Carlo simulations.     

明胶层中的扩散和反应的动力学研究——(Ⅴ)油溶性成色剂的偶合反应的动力学

研究了明胶层中油溶性成色剂油珠的偶合反应动力学。通过实验和数值模拟的比较证明:在明胶层中油溶性成色剂(C-1)油珠与彩色显影剂CD-4的氧化产物的偶合反应主要是发生在油珠-明胶水溶液的界面上。偶合反应速度与油珠大小、比速率常数、成色剂在油相中的浓度以及QDI的扩散系数有关。并且得到C-1成色剂油珠的比速率常数、CD-4氧化产物的扩散系数以及QDI和生成的染料在明胶层中的时空分布。同时观察到温度对偶合反应动力学的影响。     

Solvent-dependent transition states for decarboxylations

The rate constants and kinetic isotope effects for decarboxylation of 4-pyridylacetic acid depend strongly on whether the solvent is water or dioxane, and the present paper interprets this finding. We calculate the solvent dependence of the free energy barrier and of the (13)C and (18)O kinetic isotope effects using a quantum mechanical solvation model based on class IV charges and semiempirical atomic surface tensions. The calculations provide a consistent interpretation of the experimental results, which provides a striking confirmation of the soundness of the solvation modeling. Even more significantly, the agreement of theory and experiment gives us confidence in the physical picture of the reaction provided by the model. This indicates that the location of the transition state, as measured by the length of the breaking C--C bond, is 0.24 A later than the gas phase in dioxane and 0.37 A later than the gas phase in water. Charge development at the transition state also depends strongly on the solvent; in particular the CO(2) moiety is 0.07 electronic charge units more negative at the transition state in dioxane than in water.     

硫化物在铂电极上电氧化时空动力学的模型模拟和分析(英文)

根据铂电极上硫化物电催化氧化的反应机理,本文提取动力学模型并利用数值模拟研究了N型负微分阻抗(N-NDR)振荡区域的电极表面时空反应动力学.在均相体系模拟中观察到电流简单振荡和复杂振荡,其来源于双电层电势自催化与传质限制和毒化物种吸附负反馈的相互耦合.为了更接近于真实体系,在模型中考虑了平行和垂直于电极表面两个方向的传质过程.模拟结果发现了与实验现象具有相同演化行为的复杂斑图,如行波和闪烁波;同时在传质耦合体系模拟中观察到双电层电势双臂螺旋波.本研究工作促进对电化学体系时空斑图的理解和预测.     

Capacitive-discharge-heated anisotropic pyrolytic graphite furnace used for atomic absorption spectrometry Part 2. Experimental Verification of Temperature Distribution of the Furnace Surface and of the Gas Phase

The predictions of a theoretical model, embodied in a computer program, describing the heating characteristics of the furnace surface and the gas phase of an anisotropic pyrolytic graphite furnace heated by the capacitive discharge technique are compared with the experimental results obtained by optical pyrometry and by two-line atomic absorption spectrometry, respectively. The surface temperature gradient around the circumference of the type 1 furnace and along the optical axis of the type 3 furnace are calculated and compared with the measured temperatures. The weighted-average of the theoretically predicted gas temperature is in reasonable agreement with the effective vapour temperature measured by two-line atomic absorption method. The heating rate of the furnace does not have a significant effect on the temperature distribution of either the furnace surface or the gas phase. The effect of the difference in the temperature distribution of the type 1 and type 3 furnaces on the atomic absorption signals is also discussed.     

Kinetics of gas oil catalytic cracking

Summary In the present work catalytic cracking of gas oils has been studied in a standard MAT reactor. The cracking product distribution was measured at four different temperatures. Based on the experimental results, a 4-lump kinetic model was applied. Kinetic constants were estimated using the sequential step optimization method. Simulation results have shown to be in a good agreement with experimental data. An isothermal pseudo steady-state model for fixed bed MAT reactor is proposed.     

Computational study on the partial dechlorination of the pesticide chloropicrin by sulfur species

Density functional and MP2 calculations with extended basis sets were performed on the species participating in both the previously suggested and a newly proposed mechanisms of partial dechlorination of chloropicrin by simple sulfur species, both in gas phase and in a simulated water environment. Thermochemistry of both mechanisms in the gas phase was also studied using the chemical models G3 and G4. It is shown that the previously proposed reductive dehalogenation is not thermodynamically feasible at room temperature, as it should be according to the experimental evidence. Although inclusion of the solvent improves the results with respect to gas phase, the thermodynamics of the proposed mechanism by Zheng et al. is still unfavorable for obtaining the experimental products. An alternative mechanism is then proposed, involving the formation of HSCl, which is the intermediate that then undergoes redox reactions. Such a mechanism is exothermic and spontaneous, according to the computational results, and produces elementary sulfur in agreement with the experimental facts.     

The water exchange process of tetraaquaplatinum(II): density-functional theory and ab initio computational study

The water exchange process of square planar tetraaquaplatinum(II) was computationally investigated at the Hartree-Fock, density-functional theory (B3LYP, PW91PW91, and mPW1PW91) and the second-order Moller-Plesset perturbation theory of levels of calculation. The stationary points on the gas phase and on the reaction field potential-energy surface were fully optimized and characterized. The self-consistent reaction field (SCRF) methods were also applied on the same system. The structures and energetic processes of all the species involved were investigated. The kinetic and thermodynamic properties for the gas phase and SCRF model data were compared with the available experimental data. The rate constants in MP2 level of theory and B3LYP isodensity polarized continuum model reaction field are in good agreement with the experimental data.     

Convective rolls and hydrothermal waves in evaporating sessile drops

Recent experiments on the evaporation of sessile droplets have revealed the spontaneous formation of various patterns including the presence of hydrothermal waves. These waves had previously been observed, in the absence of evaporation, in thin liquid layers subjected to an imposed, uniform temperature gradient. This is in contrast to the evaporating droplet case wherein these gradients arise naturally due to evaporation and are spatially and temporally varying. In the present paper, we present a theory of evaporating sessile droplets deposited on a heated surface and propose a candidate mechanism for the observed pattern formation using a linear stability analysis in the quasi-steady-state approximation. A qualitative agreement with experimental trends is observed.     

Comparative study of kinetic modeling for the oxidative coupling of methane by genetic and marquardt algorithms

Overall kinetic studies on the oxidative coupling of methane, OCM, have been conducted in a tubular fixed bed reactor, using perovskite titanate as the reaction catalyst. The appropriate operating conditions were found to be: temperature 750-775 ℃, total feed flow rate of 160 ml/min, CH4/O2 ratio of 2 and GHSV of 100 min-1. Under these conditions, C2 yield of 28% was achieved. Correlations of the kinetic data have been performed with lumped rate equations for C2 and COx formation as functions of temperature, O2 and CH4 partial pressures. Six models have been selected among the common lumped kinetic models. The selected models have been regressed with the experimental data which were obtained from the Catatest system by genetic algorithm in order to obtain optimized parameters. The kinetic coefficients in the overall reactions were optimized by different numerical optimization methods such as: the Levenberg-Marquardt and genetic algorithms and the results were compared with one another. It has been found that the Santamaria model is in good agreement with the experimental data. The Arrhenius parameters of this model have been obtained by linear regression. It should be noted that the Marquardt algorithm is sensitive to the first guesses and there is possibility to trap in the relative minimum.     

用CA模型研究SO2在锰盐表面上的催化氧化反应

本文用CA模型借助部分模拟法求得SO2在锰盐表面上催化氧化反应的动力学相图; 若反应按Langmuir-Hinshelwood机制进行, 且H2O以气态出现, 则模拟结果表明, 相图中有四个双组分中毒相和一个反应相, 反应相在一个线性的浓度区域上出现; 如在催化反应过程中, 尚需考虑反应物种的脱附, 则反应相将随脱附几率的增加而扩大。     

Non-isothermal Modelling of Hydrocarbon Adsorption on A Granulated Active Carbon

The adsorption of n -butane on extruded cylindrical activated carbon grains is studied providing two kinds of information: the influence of the temperature and the hydrocarbon partial pressure on the adsorption dynamics (kinetic study) and on the adsorption capacities (thermodynamic study). The thermodynamic aspect could be interpreted by a Langmuir model. From a kinetic point of view, we have experimentally proved that strong temperature variations occur inside the particles during the adsorption. In this paper, a kinetic model including both mass and heat transfer phenomena is proposed. Good agreement is found between the kinetic model predictions and the experimental mass and temperature variations inside the grain during the hydrocarbon adsorption. This revised version was published online in August 2006 with corrections to the Cover Date.     

Evaluation of the protonation thermochemistry obtained by the extended kinetic method

An evaluation of the results obtained by the extended kinetic method for a series of representative bases is presented here. Analysis of the original experimental data is conducted using the orthogonal distance regression (ODR) statistical treatment. A comparison with the proton affinities and protonation entropies obtained from variable temperature equilibrium constant measurements demonstrate deviations, which may be ascribed to random and systematic errors. Considerable random errors are associated with the extended kinetic method if the number of reference bases and the range of effective temperatures are too low. It is also confirmed that large systematic errors on proton affinities and protonation entropies are obtained when large protonation entropy is associated with the considered system. It is, however, encouraging to note that the gas phase basicities obtained by the extended kinetic method are generally comparable to that obtained by other methods within a few kJ/mol.     

Ni－海泡石催化剂催化苯气相加氢动力学

Ｎｉ－海泡石催化剂催化苯气相加氢动力学曹声春，杨礼嫦，彭峰，黄孟光（湖南大学化工系，长沙４１００８２）关键词镍，海泡石，负载型催化剂，苯，加氢，反应动力学Ｎｉ一海泡石催化剂通过制备工艺的优化，可使其苯加氢催化活性、选择性、热稳定性和抗毒性都优于现工业...     

Linearized non-equilibrium and non-isothermal two-dimensional model of liquid chromatography for studying thermal effects in cylindrical columns

A non-equilibrium and non-isothermal two-dimensional lumped kinetic model (2?D-LKM) is formulated and analytically solved to study the influence of temperature variations along the axial and radial coordinates of a liquid chromatographic column. The model includes convection-diffusion partial differential equations for mass and energy balances in the mobile phase coupled with differential equations for mass and energy in the stationary phase. The solutions are derived analytically through sequential implementation of finite Hankel and Laplace transformations using the Dirichlet inlet boundary conditions. The coupling between the thermal waves and concentration fronts is demonstrated through numerical simulations and important parameters are recognized that influence the column performance. For a more comprehensive study of the considered model, numerical temporal moments are obtained from the derived solutions. Several case studies are conducted and validity ranges of the derived analytical solutions are identified. The current analytical results will play a major role in the improvements of non-equilibrium and non-isothermal liquid chromatographic processes.