Reactions gas-surface

The oxygen atoms just combine with 02, most of the time, to reform ozone  

In most regions of the atmosphere, the interconversion between O and O3 is so rapid that the two species are often considered together as the chemical family, Ox = O + O3, which is denoted odd oxygen.  

In summary, a chemical family refers to two compounds that interchange with each other sufficiendy rapidly such that they tend to behave chemically as a group. As noted, we will encounter a number of such systems in the chapters to come. 

A number of important chemical reactions in the atmosphere involve a gas molecule striking the surface of an airborne particle. For gas molecules A in three-dimensional random motion, the number of molecules of A striking a unit area per unit time is 

Then the number of collisions per second with a single spherical particle of radius Rp is ( ava)(4ti/ p). Usually one is interested in reaction occurring with an ensemble of particles, all of different sizes. If the particle population has a total surface area per unit volume of air of Ap(cm2cm-3), then the total number of collisions of A molecules with particles is ( nAvA)Ap. 

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Rettner C T and Auerbach D J 1994 Distinguishing the direct and indirect products of a gas-surface reaction Science 263 365... 

The principal applications of REELS are thin-film growth studies and gas-surface reactions in the few-monolayer regime when chemical state information is required. In its high spatial resolution mode it has been used to detect submicron metal hydride phases and to characterize surface segregation and difRision as a function of grain boundary orientation. REELS is not nearly as commonly used as AES orXPS. 

Heterogeneous uptake on surfaces has also been documented for various free radicals (DeMore et al., 1994). Table 3 shows values of the gas/surface reaction probabilities (y) of the species assumed to undergo loss to aerosol surface in the model. Only the species where a reaction probability has been measured at a reasonable boundary layer temperature (i.e. >273 K) and on a suitable surface for the marine boundary layer (NaCl(s) or liquid water) have been included. Unless stated otherwise, values for uptake onto NaCl(s), the most likely aerosol surface in the MBL (Gras and Ayers, 1983), have been used. Where reaction probabilities are unavailable mass accommodation coefficients (a) have been used instead. The experimental values of the reaction probability are expected to be smaller than or equal to the mass accommodation coefficients because a is just the probability that a molecule is taken up on the particle surface, while y takes into account the uptake, the gas phase diffusion and the reaction with other species in the particle (Ravishankara, 1997). 

GAS-SURFACE REACTIONS MOLECULAR DYNAMICS SIMULATIONS OF REAL SYSTEMS ... 

The future ofmolecular dynamics for modeling gas-surface reactions 325... 

Central to the understanding of surface-related phenomena has been the study of gas-surface reactions. A comprehensive understanding of these reactions has proven challenging because of the intrinsic many-body nature of surface dynamics. In terms of theoretical methods, this complexity often forces us either to treat complex realistic systems using approximate approaches, or to treat simple systems with realistic approaches. When one is interested in studying processes of technological importance, the latter route is often the most fruitful. One theoretical technique which embodies the many-body aspect of the dynamics of surface chemistry (albeit in a very approximate manner) is molecular dynamics computer simulation. 

In section 3.1, reactions of diatomic molecules with metal surfaces are discussed. These studies, although perhaps not sufficiently complicated to directly address processes of technological interest, have produced considerable insight into the dynamics of gas-surface reactions. Simulations of metal surfaces where more i istic interactions are required than are used in the gas-surface studies are presented in section 3.2. This is followed in section 3.3 by a discussion of simulations of reactions on the surfaces of covalently bonded solids. These final studies are particularly suited for addressing technologically relevant processes due to the importance of semiconductor technology. 

The exchange between the gas-phase and chemisorbed states of small molecules plays a vital role in such technologically important fields as heterogeneous catalysis and corrosion. The dynamics involved in these processes, however, are not currently well understood. Molecular-beam studies combined with classical trajectory calculations have proven to be a successful tool for understanding the underlying features of atomic-scale motion in the gas phase. The extension of these techniques to surfaces has also helped in elucidating the details of gas-surface reactions. 

Studies of H2 have proven the feasibility of using the LEPS formalism to study gas-surface reactions, and have indicated that relationships between the potential surface and chemical dynamics derived from gas-phase studies can be generalized to reactions with surfaces. Reactions of H2, however, represent simple systems compared even to other diatomic molecules, and extensions to other more complicated reactions are rare. A few studies of other diatomic... 

The modification of theoretical gas-phase reaction techniques to study gas-surface reactions continues to hold promise. In particular, the LEPS formalism appears to capture a sufficient amount of realistic bonding characteristics that it will continue to be used to model gas-surface reactions. One computational drawback of the LEPS-style potentials is the need to diagonalize a matrix at each timestep in the numerical integration of the classical equations of motion. The size of the matrix increases dramatically as the number of atoms increases. Many reactions of more direct practical interest, such as the decomposition of hydrocarbons on metal surfaces, are still too complicated to be realistically modeled at the present time. This situation will certainly change in the near future as advances in both dynamics techniques and potential energy surfaces continue. 

The past few years have been an exciting time for modeling gas-surface reactions. Computational techniques as well as potential energy functions have become sufficiently advanced that dynamics simulations can now described many realistic situations without introducing severe approximations. As computer resources continue to grow, the impact which computer modeling has on science and enginwring will also continue to increase. 

In this article we have tried to present a general, although somewhat limited overview of molecular dynamics simulations of gas-surface reactions as they pertain to technologically important processes. In the course of this review we have undoubtedly left out a great deal of very important work. We hope. 

C, when the gas surface reactions can be expected to occur at a faster rate. Now it is seen that the response has reached a steady-state value after exposure to the ammonia atmosphere. The extra dip in the response curve seen in the oxygen environment might be due to the slow diffusion of ammonia. Some gas molecules might still be left under the sensor surface in this experiment when hit by the oxygen gas outlet. 

Wu, C.-H., and H. Niki, Fluorescence Spectroscopic Study of Kinetics of Gas-Surface Reactions between Nitrogen Dioxide and Adsorbed Pyrene, Environ. Sci. Technol, 19, 1089-1094 (1985). 

This chapter focuses on the dynamics of gas-surface chemistry as defined above. Both the theoretical and experimental methodology inherent in such an approach borrow much from an older sibling, i.e., the study of the dynamics of atom-molecule chemical reactions in the gas phase [1]. However, gas-surface reactions are more... 

Certainly most surface chemistry occurs as adsorbates come together as a result of thermal diffusion on the surface. When both reagents are in thermal equilibrium with the surface before reacting, the surface chemistry is described as a Langmuir-Hinschelwood (LH) mechanism. Even most gas-surface reactions occur via this mechanism. However, when the product of the reaction also remains on the surface, no dynamic information is available. Therefore, the only LH reactions discussed in this chapter are when the product of the reaction is a gas phase species. One example already discussed extensively is associative desorption. Here, another well-studied example is considered. 

Gas-Surface Reactions Proceeding via a Strongly Adsorbed Precursor. 471... 

In more detail, our approach can be briefly summarized as follows gas-phase reactions, surface structures, and gas-surface reactions are treated at an ab initio level, using either cluster or periodic (plane-wave) calculations for surface structures, when appropriate. The results of these calculations are used to calculate reaction rate constants within the transition state (TS) or Rice-Ramsperger-Kassel-Marcus (RRKM) theory for bimolecular gas-phase reactions or unimolecular and surface reactions, respectively. The structure and energy characteristics of various surface groups can also be extracted from the results of ab initio calculations. Based on these results, a chemical mechanism can be constructed for both gas-phase reactions and surface growth. The film growth process is modeled within the kinetic Monte Carlo (KMC) approach, which provides an effective separation of fast and slow processes on an atomistic scale. The results of Monte Carlo (MC) simulations can be used in kinetic modeling based on formal chemical kinetics. 

Once the thermodynamic parameters of stable structures and TSs are determined from quantum-chemical calculations, the next step is to find theoretically the rate constants of all elementary reactions or elementary physical processes (say, diffusion) relevant to a particular overall process (film growth, deposition, etc.). Processes that proceed at a surface active site are most important for modeling various epitaxial processes. Quantum-chemical calculations show that many gas-surface reactions proceed via a surface complex (precursor) between an incident gas-phase molecule and a surface active site. Such precursors mostly have a substantial adsorption energy and play an important role in the processes of dielectric film growth. They give rise to competition among subsequent processes of desorption, stabilization, surface diffusion, and chemical transformations of the surface complex. 

Generally speaking, gas-surface reactions via a precursor (with the reaction path profiles shown in Figures 9.1 and 9.2) cannot be treated in the... 

Fig. 9.1. Reaction path profile of an exothermic gas-surface reaction without activation energy via a precursor.

Thus, in the framework of the model under consideration, the most important effect of the bulk on the gas-surface reaction via the precursor is the relaxation of the internal energy of AB/s/. This relaxation is governed by the bulk thermal conductivity and the relaxation time trei can be estimated from... 

Molecular beams are limited to reactions that are carried out in vacuum, where well-defined beams of reactant molecules can be prepared. This limits their application to gas-phase reactions and to reactions of gaseous molecules with solid surfaces. Molecular beam methods cannot be used to study kinetics in liquid solvents. The detailed information they provide for gas-gas and gas-surface reactions allows precise testing of models and theories for the dynamics of these classes of reactions. 

Although many similarities exist between gas-solid catalytic and gas-solid noncatalytic reactions, the noncatalytic systems, particularly when a porous reactant is converted to a porous product, are more complex. Both occur as the result of a number of series-parallel steps. Mass transfer of reacting gas from the bulk gas to the exterior of the solid and that of gas product from the solid to the bulk gas are involved in each. Diffusion of the reacting gas from the exterior surface into a porous catalyst or porous solid reactant and that of gas product from the pores to the exterior surface are also common to the two types of reactions. Adsorption of reacting gas, surface reaction, and... 

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