The UBI-QEP method: Mechanistic and kinetic studies of heterogeneous catalytic reactions

Applications of the unity bond index-quadratic exponential potential (UBI-QEP) method in mechanistic and kinetic studies of complex heterogeneous catalytic reactions are discussed. It is shown how UBI-QEP energetics helps to answer specific questions regarding the mechanisms of various reactions on metal surfaces. Examples of the following reactions are considered: elementary reactions of hydrogen transfer (from one carbon atom connected to the metal surface to another such atom in a different surface species), H-X bond breaking with the assistance of coadsorbed oxygen, methanol synthesis, Fischer-Tropsch synthesis, ammonia synthesis and decomposition, and partial methanol oxidation to formaldehyde. Then, examples of kinetic simulations using UBI-QEP energetics are considered for the following processes: ethane hydrogenolysis, watergas shift reaction, selective hydrogenation of acetylene in ethylene-rich mixtures, oxidative conversions of hydrogen and methane on Pt and Rh surfaces, ammonia decomposition, C₁–C₂ product formation in Fischer-Tropsch synthesis over cobalt, and steam reforming of methane. It is shown how the use of UBI-QEP and Monte Carlo methods makes it possible to calculate reaction parameters that depend on temperature in the equilibrium and kinetic regimes. To increase the accuracy of simulations, we added nonenergetic parameters affecting energetics at nonzero coverages: a spatial constraint on the distance between adsorbed atoms and the distance of hot atom traveling upon oxygen dissociation on metal surfaces. The UBI-QEP/Monte Carlo simulation method is illustrated by the study of molecular oxygen adsorption on single crystalline nickel surfaces. In most UBI-QEP-based mechanistic and kinetic studies of heterogeneous catalytic reactions, good agreement is observed with experimental observations, in many cases on a quantitative level. Taking into account diversity of reactions to which the method has been applied the agreement with experiments supports the efficiency of the method in solving mechanistic and kinetic problems.     

Bistable kinetics of simple reactions on solid surfaces: lateral interactions, chemical waves, and the equistability criterion

In heterogeneous reactions, the rate constants of desorption, diffusion and elementary reaction steps are usually strongly dependent on reactant coverages due to adsorbate-adsorbate lateral interactions. We analyze the effect of this factor on the bistable regime of the reaction kinetics. As an example, we consider CO oxidation on Pt(111). The equistability lines in the bistable region for this reaction are calculated by analyzing propagation of chemical waves and taking into account the coverage dependence of the CO diffusion coefficient. The results of simulations are compared with the available experimental data. We show that it is possible to obtain the relationship between various kinetic parameters, for example, between CO and oxygen sticking probabilities, by studying special features of the bistable kinetics.     

Hybrid Isothermal Model for the Ferrohydrodynamic Chemically Reactive Species

A hybrid isothermal model for the homogeneous-heterogeneous reactions in ferrohydrodynamic boundary layer flow is established. The characteristics of Newtonian heating and magnetic dipole in a ferrofluid due to a stretchable surface is analyzed for three chemical species. It is presumed that the isothermal cubic autocatalator kinetic gives the homogeneous reaction and the first order kinetics gives the heterogeneous (surface) reaction. The analysis is carried out for equal diffusion coefficients of all autocatalyst and reactions. Heat flux is examined by incorporating Fourier's law of heat conduction. Characteristics of materialized parameters on the magneto-thermomechanical coupling in the flow of a chemically reactive species are investigated. Further, the heat transfer rate and friction drag are depicted for the ferrohydrodynamic chemically reactive species. It is evident that the Schmidt number has increasing behavior on the rate of heat transfer in the boundary layer. Comparison with available results for specific cases is found an excellent agreement.     

Data-Driven Analysis of Nonlinear Heterogeneous Reactions through Sparse Modeling and Bayesian Statistical Approaches

Heterogeneous reactions are chemical reactions that occur at the interfaces of multiple phases, and often show a nonlinear dynamical behavior due to the effect of the time-variant surface area with complex reaction mechanisms. It is important to specify the kinetics of heterogeneous reactions in order to elucidate the microscopic elementary processes and predict the macroscopic future evolution of the system. In this study, we propose a data-driven method based on a sparse modeling algorithm and sequential Monte Carlo algorithm for simultaneously extracting substantial reaction terms and surface models from a number of candidates by using partial observation data. We introduce a sparse modeling approach with non-uniform sparsity levels in order to accurately estimate rate constants, and the sequential Monte Carlo algorithm is employed to estimate time courses of multi-dimensional hidden variables. The results estimated using the proposed method show that the rate constants of dissolution and precipitation reactions that are typical examples of surface heterogeneous reactions, necessary surface models, and reaction terms underlying observable data were successfully estimated from only observable temporal changes in the concentration of the dissolved intermediate products.     

A DFT-based microkinetic theory for Fe2O3 reduction by CO in chemical looping

Redox kinetics of oxygen carrier in chemical looping is an important component for material preparation, reactor design and process demonstration. How to bridge the gap between the microscale density functional theory (DFT) and the macroscale redox kinetics and develop a first-principle-based theoretical model is still a challenge in the field of chemical looping. This study addresses this challenge and proposes a DFT-based microkinetic rate equation theory to calculate the heterogeneous kinetics of Fe₂O₃ reduction by CO in chemical looping. Firstly, the DFT calculation is adopted to search the reaction pathways and to obtain the energy barriers of elementary reactions. Secondly, the DFT results are introduced into the transition state theory (TST) to calculate the reaction rate constants and build the rate equations of elementary surface reactions. Finally, by considering the bulk diffusion, a rate equation is developed to bridge the gap between the elementary surface reactions and the grain conversion. In the theory, the reaction mechanism obtained from DFT and kinetic rate constants obtained from TST are directly implemented into the rate equation to predict the reduction kinetics of oxygen carriers without fitting experimental data. The accuracy of the developed theory is validated by experimental data of two Fe₂O₃ oxygen carriers obtained from the thermogravimetric analyzer (TGA). The microkinetic rate equation theory is based on the first principles calculation and can predict directly the redox kinetics of oxygen carriers without depending on the experimental kinetic data, therefore, it provides a powerful theoretical tool to screen the oxygen carrier materials and optimize the microstructure of oxygen carriers.     

Insights into sticking of radicals on surfaces for smart plasma nano-processing

In reactive plasma processing, species produced in the plasma reach the surface of a substrate and cause etching, deposition and surface modification through surface reactions. These reactions are characterized by the densities and energies of species incident on the surfaces. In order to realize nano-scale plasma processing, important species for plasma processing have been identified and characterized, and their behavior, not only in the gas phase, but also on the surface, have been clarified and controlled. One of the most critical parameters for insights into surface reaction kinetics of radicals is sticking and surface loss probability. On the basis of radical densities measured by various methods, the sticking and surface reaction loss probabilities have been compiled, and they enable the quantitative understanding of the kinetics of radicals on the surface in the plasma. In this article, the sticking and surface reaction loss probabilities measured thus far are reviewed focusing on fluorocarbon gas, silane gas and methane gas based plasma processes. The establishment of a smart plasma process and the development of an autonomous production device with control of radicals on the basis of insights into the surface reactions for nano-scale plasma processing are presented.     

Modulated beam relaxation spectrometry: Its application to the study of heterogeneous kinetics

The results obtained to date using modulated beam relaxation spectrometry (MBRS) to study heterogeneous kinetics are reviewed. The method of MBRS is described with particular emphasis on judicious use of appropriate experimental variables to infer the kinetics and mechanism of surface reactions. Recent calculations are presented for simulated MBRS studies of nonlinear reaction sequences, including coverage dependent adsorption-desorption phenomena and second order reactions. Linearization methods are discussed which enable the study of more complex reactions. Finally, the general applicability of MBRS to heterogeneous kinetics is discussed.     

Monte Carlo simulations of oscillations, chaos and pattern formation in heterogeneous catalytic reactions

Experimental studies employing surface science methods indicate that kinetic oscillations, chaos, and pattern formation in heterogeneous catalytic reactions often result from the interplay of rapid chemical reaction steps and relatively slow complementary processes such as oxide formation or adsorbate-induced surface restructuring. In general, the latter processes should be analysed in terms of theory of phase transitions. Therefore, the conventional mean-field reaction–diffusion equations widely used to describe oscillations in homogeneous reactions are strictly speaking not applicable. Under such circumstances, application of the Monte Carlo method becomes almost inevitable. In this review, we discuss the advantages and limitations of employing this technique and show what can be achieved in this way. Attention is focused on Monte Carlo simulations of CO oxidation on (1 0 0) and (1 1 0) single-crystal Pt and polycrystal Pt, Pd and Ir surfaces and of NO reduction by CO and H₂ on Pt(1 0 0). CO oxidation on supported nanometre-sized catalyst particles and NO reduction on composite catalysts are also discussed. The results show that with current computer facilities the MC technique has become an effective tool for analysing temporal oscillations and pattern formation on the nanometre scale in catalytic reactions occurring on both single crystals and supported particles.     

T-FTIR研究大气颗粒物表面非均相反应

大气颗粒物表面发生的非均相反应已成为目前大气科学领域的研究热点.由于现有的研究手段存在很多不足,目前人们对大气中痕量气体物质在颗粒物表面非均相反应动力学和机理了解得还很有限.文章开发了基于透射傅里叶变换红外光谱(T-FTIR)的研究颗粒物表面反应的方法,可以从分子水平观测颗粒物表面的物理化学变化.通过对一个大气模型反应体系即常温常压下甲基丙烯醛(MAC)在二氧化硅颗粒物表面反应的表征,验证了该方法可以有效地定性和定量表征颗粒物上的非均相反应过程.研究结果显示,反应过程的红外谱图信号清晰,表征灵敏;红外光能量较低,不会损害样品,可以实现对反应的原位观测.经HPLC分析结果证明,T-FTIR表征结果的重现性和准确性好,即使在湿度存在条件下其定性和定量也是可靠的.该方法为实验室模拟研究实际大气条件下的非均相反应提供了一种有效手段.     

Role of adsorption complexes in catalytic acceleration of heterogeneous reactions

The universal equation that imposes restrictions on the kinetics of heterogeneous chemical reactions has been derived. A method of studying the participation of chemisorbed or physisorbed gas molecules in heterogeneous chemical reactions has been proposed. It has been shown that the heterogeneous reaction rate is independent of the degree of occupation of the catalyst surface by the reactants as long as the product is formed by chemisorbed complexes and physisorbed molecules. It has been found that the formation of CO₂ in the heterogeneous chemical reactions CO+ O → CO₂ and 2CO + O₂ → 2CO₂ is participated by physisorbed particles and chemisorbed complexes of oxygen atoms and CO molecules.     

Reduced modeling of Flame-Wall-Interactions of premixed isooctane-air systems including detailed transport and surface reactions

In this work, the Reaction-Diffusion Manifold (REDIM) method, a method for model reduction, is applied to a premixed isooctane-air system with Flame-Wall-Interactions (FWI). In order to provide a highly accurate reduced kinetic model, a detailed model for the diffusive processes is applied and complex boundary conditions that account for heterogeneous wall reactions are implemented.The REDIM is constructed and validated by comparing results of detailed and reduced kinetics in the system state space. The results of the reduced computations are compared with those of the detailed computations. It is shown that the reduced kinetics reproduce the results of the FWI very accurately. In particular, the difference between detailed kinetics with and without wall reactions is larger than the difference between detailed and reduced kinetics with heterogeneous wall reactions, which demonstrates the quality of the model reduction.     

A detailed chemical kinetic reaction mechanism for oxidation of four small alkyl esters in laminar premixed flames

A detailed chemical kinetic reaction mechanism has been developed for a group of four small alkyl ester fuels, consisting of methyl formate, methyl acetate, ethyl formate, and ethyl acetate. This mechanism is validated by comparisons between computed results and recently measured intermediate species mole fractions in fuel-rich, low-pressure, premixed laminar flames. The model development employs a principle of similarity of functional groups in constraining the H atom abstraction and unimolecular decomposition reactions for each of these fuels. As a result, the reaction mechanism and formalism for mechanism development are suitable for extension to larger oxygenated hydrocarbon fuels, together with an improved kinetic understanding of the structure and chemical kinetics of alkyl ester fuels that can be extended to biodiesel fuels. Variations in concentrations of intermediate species levels in these flames are traced to differences in the molecular structure of the fuel molecules.     

A kinetic study of the effect of ultrasound power on the sonohydrolysis of tetraethyl orthosilicate

A kinetic study of the ultrasound-stimulated and acid-catalyzed sonohydrolysis of tetraethyl orthosilicate (TEOS) in solventless TEOS–water heterogeneous mixtures was carried out by means of a calorimetric method as a function of the ultrasound power. The hydrolysis reaction starts in acidulated heterogeneous water–TEOS mixtures after an induction period under ultrasonic stimulation. The ultrasound power seems to play a role on the dynamical coupling of the system originating a continuum upward shifting of the base line during the induction period of sonication. The rate in which the base line is upward shifted diminishes with the power. The best coupling between the ultrasound and the reactant heterogeneous mixtures for this experimental setup was found to occur at 50 W, for which the gelation time was found to be a minimum. The kinetics of the heterogeneous TEOS sonohydrolysis was studied on the basis of a dissolution and reaction modeling. The heterogeneous reaction pathway as deduced from the kinetic study was drawn in a ternary diagram as a function of the ultrasound power.     

Chirality of molecular nanostructures on surfaces via molecular assembly and reaction: manifestation and control

The formation of chiral nanostructures via molecular assembly and reaction on solid surfaces is a ubiquitous surface process due to the symmetry-breaking at 2D surface. Studying chirality during the adsorption, assembly, and reaction of molecules on 2D solid surfaces at molecular level not only sheds deep insights into the enantioselective heterogeneous catalysis, chiral recognition, origin and evolution of chirality, and many important physical chemistry processes but also provides an important strategy to create chiral nanostructures. Here, we give a survey of recent advances in chiral expression and control in molecular assemblies and reactions on surfaces. We firstly give a brief introduction to the general concepts of chiral molecular nanostructures on surfaces. And then we focus on the induction and control of chirality expressed in molecular assemblies. The recent developments in the control strategies such as chiral co-adsorber, chiral auxiliary, chiral solvent, chiral templated surfaces, as well as the underlying mechanism to achieve the chiral induction and amplification, are reviewed. After that, we review the studies of chirality expressed in on-surface synthesis which has been proved to be a promising strategy to fabricate covalently bonded low-dimensional nanostructures and materials. In this respect, we introduce the chiral expression in the intramolecular and intermolecular coupling reactions on surfaces. In addition, we survey the methods to steer the stereoselectivity of on-surface reactions including the design of precursor structure, steric hindrance effect, substrate, kinetic parameters et al. Finally, the future outlook in this field is discussed.     

Surface studies of supported model catalysts

Metal particles grown by vapour deposition on clean and well-defined oxide surfaces are used as model catalysts. These new model catalysts allow, unlike metal single crystals, a study of size and support effects in heterogeneous catalysis. The structure, the electronic properties and the reactivity of these supported model catalysts have been studied, in situ, by a large number of surface science techniques. In order to get relevant information from those studies it is necessary to control the nucleation and growth in order to get uniform collections of metal particles. The preparation conditions and the characterisation methods will be reviewed. Particles with well-defined shapes are obtained by epitaxial growth at high temperature on clean ordered surfaces. The electronic properties of the small metal particles depend not only on their size but also on their shape. The chemisorption properties are strongly related to the surface structure of the particles. The interplay between the surface structure, the local electronic properties and the adsorption energy will be discussed for CO chemisorption. The presence of the support plays an important role in the control of the particle morphology. Furthermore, it can increase the adsorption rate. The intrinsic heterogeneity of the supported model catalysts has to be taken into account to understand in detail the catalytic reactions. The reaction rate cannot be considered as an average on the different crystalline facets present on the particle. Finally, we will discuss the possibility to study in situ and at the atomic level simple chemical reactions on supported catalysts.     

LCVD of copper: Deposition rates and deposit shapes

Laser chemical vapor deposition of copper has been performed under a variety of conditions. The results are interpreted using the kinetic model of Ehrlich and Tsao. We find that the kinetics of the process are limited by the rates of surface reactions and not by diffusion of reactant molecules to the surface. Furthermore, we find that deposit shapes are quite sensitive to laser intensity and scan rate.     

XPK程序包：扩展唯象动力学方法(XPK)与常规动力学蒙特卡罗(KMC)方法的比较

在常规的动力学蒙特卡罗方法(KMC)中,扩散过程的速率往往远大于化学反应,因而造成KMC方法在模拟表面化学体系演化时效率非常低下. 为了解决这一时间尺度分离问题,本文最近发展了扩展唯象动力学方法(XPK). 本文基于加氢反应体系模型,利用新发展的XPK程序包,对XPK方法与常规的KMC方法进行了细致的对比. 为了更全面地说明问题,测试中包含了两条不同的势能曲线,以及多种吸附物之间的相互作用. 对比的内容包括计算消耗、并行效率以及稳态的收敛行为等. 测试结果表明,相比于常规的KMC方法,XPK方法在兼顾精度的同时大大提高了模拟效率. 因而可以预期,XPK方法将成为多相催化理论研究的强有力工具. 特别是在表面吸附物种相互作用有决定性影响的情况下,XPK方法的优势尤其突出.     

The surface science of enzymes

One of the largest challenges to science in the coming years is to find the relation between enzyme structure and function. Can we predict which reactions an enzyme catalyzes from knowledge of its structure—or from its amino acid sequence? Can we use that knowledge to modify enzyme function? To solve these problems we must understand in some detail how enzymes interact with reactants from its surroundings. These interactions take place at the surface of the enzyme and the question of enzyme function can be viewed as the surface science of enzymes. In this article we discuss how to describe catalysis by enzymes, and in particular the analogies between enzyme catalyzed reactions and surface catalyzed reactions. We do this by discussing two concrete examples of reactions catalyzed both in nature (by enzymes) and in industrial reactors (by inorganic materials), and show that although analogies exist and the two kinds of catalyst can be described by similar tools, nature and human effort have come up with different solutions. This on the other hand implies that new and improved catalysts may be made by learning from nature.     

Global mechanism of methane autoignition: Approach and algorithm

An approach to constructing universal global mechanisms of methane autoignition is proposed. It is based on the requirement of obligatory reproduction of all the kinetic stages of the autoignition process and the types of their kinetics. It is shown that satisfying these requirements enables to construct global mechanisms of methane autoignition that are applicable in a wide range of initial conditions and adaptable to new problems. A global autoignition mechanism (10 species and reactions 9) is developed, as well as an extended global mechanism for describing plasma-induced methane autoignition (10 species, 10 reactions).