Autopoisoning reactions over rough surface: A multifractal scaling analysis

The effect of surface roughness and number of reaction event on the decay type diffusion limited reaction (DLR) over rough surface of random deposition model was studied to examine the surface morphological effect on the surface reaction. Effect of decay profiles on the reaction probability distribution (RPD) of the reaction was then analyzed with multifractal scaling techniques. The dynamics of these autopoisoning reactions is controlled by the two parameters, namely, the initial sticking probability ( P _ini) of every site and the decay rate (m). More the rough surface, less are the number of active sites and wider is the distribution of reaction probability. More the number of reaction events, more are the number of active sites and more is the homogeneity in the RPD. The q‐τ(q) multifractal curves are found to be nonlinear for all the cases which give wide range of α values in α‐f(α) multifractal spectrum. Smaller the decay rate, narrower is the range of α values. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 175–182, 2005     

Computer Simulation of Eley‐Rideal Reactions Over Rough Surface

The effect of surface roughness on an Eley‐Rideal reaction mechanism is studied. The rough surfaces are generated over a 2‐dimensional square lattice using the random deposition model. In an Eley‐Rideal reaction mechanism, a reacting particle approaches from the bulk and reacts with an active site upon collision with it. Three different rough surfaces are considered for the study. Multifractal scaling analysis is performed where the complex distribution of reaction probabilities is analyzed. Two cases of reactions are considered. 1) The reacting particle reacts at its first contact to the surface and 2) the reacting particle diffuses till it finds the lowest‐height position and reacts. The results obtained from the above two cases are compared. Significant differences between τ(q) and f(α) multifractal spectra for these two cases are found. The larger deviation from linearity in the τ(q) curves for the latter than the former gives wider f(α) spectra, indicating greater heterogeneity in the reaction probability distribution. Dynamic scaling theory is also applied on the Eley‐Rideal reaction mechanism over the surface with different surface roughnesses to obtain the two scaling parameters α_d and β_d. The values of α_d and β_d are found to be negative.     

A new approach to response surface development for detailed gas‐phase and surface reaction kinetic model optimization

We propose a new method for constructing kinetic response surfaces used in the development and optimization of gas‐phase and surface reaction kinetic models. The method, termed as the sensitivity analysis based (SAB) method, is based on a multivariate Taylor expansion of model response with respect to model parameters, neglecting terms higher than the second order. The expansion coefficients are obtained by a first‐order local sensitivity analysis. Tests are made for gas‐phase combustion reaction models. The results show that the response surface obtained with the SAB method is as accurate as the factorial design method traditionally used in reaction model optimization. The SAB method, however, presents significant computational savings compared to factorial design. The effect of including the partial and full third order terms was also examined and discussed. The SAB method is applied to optimization of a relatively complex surface reaction mechanism where large uncertainty in rate parameters exists. The example chosen is laser‐induced fluorescence signal of OH desorption from a platinum foil in the water/oxygen reaction at low pressures. We introduce an iterative solution mapping and optimization approach for improved accuracy. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 94–106, 2004     

Polymer translocation in solid-state nanopores: dependence of scaling behavior on pore dimensions and applied voltage

We investigate unforced and forced translocation of a Rouse polymer (in the absence of hydrodynamic interactions) through a silicon nitride nanopore by three-dimensional Langevin dynamics simulations, as a function of pore dimensions and applied voltage. Our nanopore model consists of an atomistically detailed nanopore constructed using the crystal structure of β-Si(3)N(4). We also use realistic parameters in our simulation models rather than traditional dimensionless quantities. When the polymer length is much larger than the pore length, we find the translocation time versus chain length scales as τ ～ N(2+ν) for the unforced case and as τ ～ N((1+2ν)/(1+ν)) for the forced case. Our results agree with theoretical predictions which indicate that memory effects and tension on the polymer chain play an important role during the translocation process. We also find that the scaling exponents are highly dependent on the applied voltage (force). When the length of the polymer is on the order of the length of the pore, we do not find a continuous scaling law, but rather scaling exponents that increase as the length of the polymer increases. Finally, we investigate the scaling behavior of translocation time versus applied voltage for different polymer and pore lengths. For long pores, we obtain the theoretical scaling law of τ ～ 1/V(α), where α ? 1 for all voltages and polymer lengths. For short pores, we find that α decreases for very large voltages and/or small polymer lengths, indicating that the value of α = 1 is not universal. The results of our simulations are discussed in the context of experimental measurements made under different conditions and with differing pore geometries.     

Dynamic scaling method and interface growth

The dynamic scaling approach is an important tool for describing the time evolution of rough interfaces. It computes dynamic scaling parameters alpha and beta, which characterize surface and time correlations. Applications of dynamic scaling to random deposition and ballistic deposition are discussed. A model of random adsorption on fractal substrates is presented. Then the influence of surface diffusion on adsorption is analyzed. The dependence of alpha and beta on the substrate fractal dimension, together with the dependence of the fractal dimension of the gas-solid interface on adsorption coverage are computed.     

Density functional approach to thermodynamics of self-assembly

A local density functional approximation for predicting the surface crystallization of a thermodynamically small system under gravity is described and tested. Using the model of the classical soft-sphere fluid, the state parameters for such systems are identified. A generalized phase diagram based upon the scaling variables is obtained; systems with the same reduced-state parameters exhibit identical profiles of thermodynamic properties such as density, pressure, and intrinsic chemical potential, measured in the direction of the applied field. The point-thermodynamic approximation of Rowlinson and the local density approximation of the density functional formalism are found to be remarkably accurate. A configurational temperature is defined and shown to agree with the corresponding kinetic temperature for inhomogeneous systems at equilibrium. The structural profiles at the crystal-fluid interface are indicative of a mesolayer of lower density crystal, not seen in the field-free isobaric crystal-liquid interface.     

Some methodological problems concerning nonisothermal kinetic analysis of heterogeneous solid–gas reactions

Isoconversional methods, those using only one curve α = α(T) (α is the conversion degree and T is the temperature), and invariant kinetic parameter method were applied to estimate the kinetic parameters from the following nonisothermal data: (1) simulated TG curves for a single reaction; (2) TG curves for thermal degradation of PVC; and (3) TG curves for the dehydration of CaC₂O₄·H₂O. The results obtained by applying various methods for the same system are compared and discussed. Finally, a procedure of kinetic analysis is suggested. Its application could lead to kinetic parameter values that can be used to predict either α = α(t) curves for other heating rates or α = α(T) curves for isothermal conditions. © 2001 John Wiley & Sons, Inc. Int J chem Kinet 33: 564–573, 2001     

Molecular dynamics simulations of vapor/liquid coexistence using the nonpolarizable water models

The surface tension, vapor-liquid equilibrium densities, and equilibrium pressure for common water models were calculated using molecular dynamics simulations over temperatures ranging from the melting to the critical points. The TIP4P/2005 and TIP4P-i models produced better values for the surface tension than the other water models. We also examined the correlation of the data to scaling temperatures based on the critical and melting temperatures. The reduced temperature (T/T(c)) gives consistent equilibrium densities and pressure, and the shifted temperature T + (T(c, exp) - T(c, sim)) gives consistent surface tension among all models considered in this study. The modified fixed charge model which has the same Lennard-Jones parameters as the TIP4P-FQ model but uses an adjustable molecular dipole moment is also simulated to find the differences in the vapor-liquid coexistence properties between fixed and fluctuating charge models. The TIP4P-FQ model (2.72 Debye) gives the best estimate of the experimental surface tension. The equilibrium vapor density and pressure are unaffected by changes in the dipole moment as well as the surface tension and liquid density.     

Modeling of Current Distribution on Smooth and Columnar Platinum Structures

Studying the growth and stability of anisotropic or isotropic disordered surfaces in electrodeposition is of importance in catalytic electrochemistry. In some cases, the metallic nature of the electrode defines the topography and roughness, which are also controlled by the experimental time and applied external potential. Because of the experimental restrictions in conventional electrochemical techniques and ex situ electron microscopies, a theoretical model of the surface geometry could aid in understanding the electrodeposition process and current distributions. In spite of applying a complex theory such as dynamic scaling method or perturbation theories, the resolution of mixed mass‐/charge‐transfer equations (tertiary distribution) for the electrodeposition process would give reliable information. One of the main problems with this type of distribution is the mathematics when solving the spatial n‐dimensional differential equations. Use of a primary current distribution is proposed here to simplify the differential equations; however it limits wide application of the first assumption. Distributions of concentration profile, current density, and electrode potential are presented here as a function of the distance normal to the surface for the cases of smooth and rough platinum growth. In the particular case of columnar surfaces, cycloid curves are used to model the electrode, from which the concentration profile is presented in a parameterized form after solving a first‐type curvilinear integral. The concentration contour results in a combination of a trigonometric inverse function and a linear distribution leading to a negative concavity curve. The calculation of the current density and electrode potential contours also show trigonometric shapes exhibiting forbidden imaginary values only at the minimal values of the trochoid curve.     

ANN-MATOPT hybrid algorithm: determination of kinetic and non-kinetic parameters in different reaction mechanisms

In this work we have applied the computational methodology based on Artificial Neural Networks (ANN) to the kinetic study of distinct reaction mechanisms to determine different types of parameters. Moreover, the problems of ambiguity or equivalence are analyzed in the set of parameters to determine in different kinetic systems when these parameters are from different natures. The ambiguity in the set of parameters show the possibility of existence of two possible set of parameter values that fit the experimental data. The deterministic analysis is applied to know beforehand if this problem occurs when rate constants of the different stages of the mechanism and the molar absorption coefficients of the species participating in the reaction are obtained together. Through the deterministic analysis we will analyze if a system is identifiable (unique solution or finite number of solutions) or if it is non-identifiable if it possesses infinite solutions. The determination of parameters of different nature can also present problems due to the different magnitude order, so we must analyze in each case the necessity to apply a second method to improve the values obtained through ANN. If necessary, an optimization mathematical method for improving the values of the parameters obtained with ANN will be used. The complete process, ANN and mathematical optimizations constitutes a hybrid algorithm ANN-MATOPT. The procedure will be applied first for the treatment of synthetic data with the purpose of checking the applicability of the method and after, it will be used in the case of experimental kinetic data.

     

Thermophysical properties of polypropylene/aluminum composites

This article is dedicated to the study of the thermal parameters of composite materials. A nonlinear least‐squares criterion is used on experimental transfer functions to identify the thermal conductivity and the diffusivity of aluminum‐polymer composite materials. The density measurements were achieved to deduce the specific heat and thereafter they were compared to values given by differential scanning calorimetry measurement. The thermal parameters of the composite material polypropylene/aluminum were investigated for the two different types of aluminum filler sizes. The experimental data were compared with several theoretical thermal conductivity prediction models. It was found that both the Agari and Bruggeman models provide a good estimation for thermal conductivity. The experimental values of both thermal conductivity and diffusivity have shown a better heat transport for the composite filled with large particles. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 722–732, 2004     

DFT calculations on the retro-ene reactions, part II: allyl n-propyl sulfide pyrolysis in the gas phase

The mechanism and kinetic aspects of the retro-ene reaction of the Allyl n-propyl sulfide and its deuterated derivative were studied using four different types of density functional theory methods with eight different levels of the basis sets. The activation energies were determined at 550.65 K. As a consequence of our calculations, a transition state is concluded that consists of a polar six-center cyclic structure. We found that the combination B3PW91/6-311++G** produces activation energy values closer to the experimental ones, but the simpler combination B3LYP/6-31G* produces excellent values too in less time. Our calculations show that the activation parameters obtained from the B3 methods are better than those obtained using the BLYP method. The mechanistic studies on the reaction show that the reaction proceeds through an asynchronous concerted mechanism. Theoretical calculations indicate that the reaction displays a kinetic isotope effect of 2.86 at 550.65 K.     

Ozonolysis of oleic acid adsorbed to polar and nonpolar aerosol particles

Single-particle kinetic studies of the reaction between oleic acid and O 3 have been conducted on two different types of core particles: polystyrene latex (PSL) and silica. Oleic acid was found to adsorb to both particle types in multilayer islands that resulted in an adsorbed layer of a total volume estimated to be less than one monolayer. The rate of the surface reaction between surface-adsorbed oleic acid and O 3 has been shown for the first time to be influenced by the composition of the aerosol substrate in a mixed organic/inorganic particle. A Langmuir-Hinshelwood mechanism was applied to the observed dependence of the pseudo-first-order rate constant with [O 3], and the resulting fit parameters for the ozone partition coefficient ( K O 3 ) and maximum first order rate constant ( k 1,max ) suggest that the reaction proceeded faster on the less polar PSL core at lower [O 3] due to the increased residence time of O 3 on the PSL surface, but the reaction was ultimately more efficient on the silica surface at high [O 3]. Values for the uptake coefficient, gamma oleic , for reaction of oleic acid on PSL spheres decrease from 2.5 x 10 (-5) to 1 x 10 (-5) with increasing [O 3] from 4 to 25 ppm and overlap at high [O 3] with the estimated values for gamma oleic on silica, which decrease from 1.6 x 10 (-5) to 1.3 x 10 (-5). The relationship between gamma oleic and the more common expression for gamma O 3 is discussed.     

端基附壁模型聚合物环形链的构象统计理论

环形链定义为两个端基均附壁的线形聚合物链所得的环 .采用精确计数和蒙特卡罗模拟方法 ,研究了自回避行走 (SAW)模型表示的环形链的构象 ,求得构象数和均方回转半径随链长的变化 .对于二维和三维SAW环形链 ,精确计数的最大链长分别为N =2 9和 1 9.用标度理论处理了数值结果 ,所得的标度指数和其他参数与理论预示值进行了比较 .模拟结果表明 ,SAW环形链限制壁平行方向的尺寸大于垂直方向的尺寸 ,与SAW尾形链尺寸的变化正好相反 .     

Modeling of adsorption in pores with strongly heterogeneous walls: parametric lattice-site wall model

We present results of grand canonical Monte Carlo simulations of adsorption in cylindrical pores with rough surface modeled by a parametric lattice-site approach. The sites are randomly distributed over the pore walls. They could be attractive, neutral or repulsive with respect to the smooth pore model. Each site is characterized by two amplitudes (structural and energetic) which modify locally the structure and energetic properties of the surface. The results presented here show how different parameters of the model affect the mechanism of adsorption and, consequently, the form of the isotherm.     

Evaluation of kinetic parameters from thermogravimetric curves: Part I. Combined numerical method

A new method is proposed to evaluate kinetic parameters from thermogravimetric traces. The method consists of two steps, α, T and dα/dT values are first employed to estimate kinetic parameters by linear least squares fitting, using the five types of mechanism for solid phase reactions suggested by ?esták. From the different sets of parameters thus obtained, the most probable mechanism type is decided. The resultant parameters for the chosen type of mechanism may further be improved by differential correction method, if necessary, using the more accurate data of α and T. The proposed method was tested with artificial data and the data for the thermal dehydration of gypsum by ?esták et al. The results were very satisfactory.     

Chemomechanics of surface stresses induced by DNA hybridization

When biomolecular reactions occur on one surface of a microcantilever beam, changes in intermolecular forces create surface stresses that bend the cantilever. While this phenomenon has been exploited to create label-free biosensors and biomolecular actuators, the mechanisms through which chemical free energy is transduced to mechanical work in such hybrid systems are not fully understood. To gain insight into these mechanisms, we use DNA hybridization as a model reaction system. We first show that the surface grafting density of single-stranded probe DNA (sspDNA) on a surface is strongly correlated to its radius of gyration in solution, which in turn depends on its persistence length and the DNA chain length. Since the persistence length depends on ionic strength, the grafting density of sspDNA can be controlled by changing a solution's ionic strength. The surface stresses produced by the reaction of complementary single-stranded target DNA (sstDNA) to sspDNA depend on the length of DNA, the grafting density, and the hybridization efficiency. We, however, observe a remarkable trend: regardless of the length and grafting density of sspDNA, the surface stress follows an exponential scaling relation with the density of hybridized sspDNA.     

在HZSM-5分子筛上甲醇脱水反应动力学研究——在活塞流反应器及连续搅拌槽式反应器中稳态反应的分析

以连续流动稳态法研究了甲醇在HZSM-5分子筛上生成二甲醚的反应动力学。使用活塞流反应器及连续搅拌槽式反应器,在150～190℃内分别得到相应的动力学数据。依据分子态吸附的甲醇在表面反应的可逆性及二甲醚吸附能力极弱的特性建立了反应机理模型,认为吸附态的与气相中的甲醇进行分子间反应生成二甲醚是速控步骤的Rideal-Eley机理。     

Highlevel ab initio and hybrid density functional theory study of the energy profile for the CO+CO reaction

Two complete basis set and three hybrid density functional computational studies were applied in the exploration of the ¹CO+²CO⁺ reaction potential energy surface. One molecular carbon monoxide–carbon monoxide cation molecular associate was elucidated as the structure with the lowest energy on the potential energy surface. Ionization energies, bond dissociation energies, and enthalpies of formation for every di and tri-atomic molecule on the potential energy surface were estimated with the two complete basis sets and the three hybrid density functional theory methods. Six different endothermic channels for the ¹CO+²CO⁺ reaction were evaluated with ab initio and DFT methods. The computed energies and structural parameters are compared with experimental values where available. Some new energies for this reaction system were suggested.     

Role of molecular conjugation in the surface radical reaction of aldehydes with H-Si(111): first principles study

Within the current effort to understand and develop the organic functionalization of silicon surfaces, recent experiments have identified the radical chain reaction of unsaturated organic molecules with H-terminated silicon surfaces as a particularly promising route for controlled formation of such functionalized surfaces. Using periodic density functional theory calculations, we theoretically study and characterize the basic steps of the radical chain reaction mechanism for different aldehyde molecules (formaldehyde, benzaldehyde, propanaldehyde, propenaldehyde) reacting with the H-Si(111) surface, under the assumption that a Si dangling bond is initially present on the surface. Molecular conjugation is found to play a crucial role in the viability of the reaction, by controlling the delocalization of the spin density at the reaction intermediate. Interesting differences between our present results for aldehydes and our previous study for the reactions of alkene/alkyne molecules with H-Si(111) are observed and discussed (Takeuchi et al. J. Am. Chem. Soc. 2004, 126, 15890).