Role of Gas Phase Reactions

Chemical Reaction Mechanisms and Kinetics. CVD chemistry is complex, involving both gas-phase and surface reactions. The role of gas-phase reactions expands with increasing temperature and partial pressure of the reactants. At high reactant concentrations, gas-phase reactions may eventually lead to gas-phase nucleation that is detrimental to thin-film growth. The initial steps of gas-phase nucleation are not understood for CVD systems, not even for the nucleation of Si from silane, which has a potential application in bulk Si production (97). In addition to producing film precursors, gas-phase reactions can have adverse effects by forming species that are potential impurity sources. 

The role of gas-phase reactions during heterogeneous methane activation ... 

The Role of Gas-Phase Reactions During Heteropaieous Activation of Methane... 

Krogh, O. Wicker, T. Chapman, B. The role of gas phase reactions, electron impact, and colli-sional energy transfer processes relevant to plasma etching of polysilicon with H2 and CI2. J. Vac. Sci. Technol. 1986, A4 (3), 1796. 

Though conhrming the important role of gas-phase reactions in governing the production of olefins, the authors succeeded in avoiding the use of the noble metal, and largely increased the catalyst resistance to coking, which may become a major issue at high alkane concentrations. 

Mackie JC. Partial oxidation of methane the role of gas phase reaction. Catal Rev Sci Eng 1991 33 169. Edwards JH, Foster NR. The potential for methanol production from natural gas by direct catalytic partial oxidation. Fuel Sci Technol Int 1986 4 365—90. 

Amama PB, Itoh K, Murabayashi M (2004) Photocatalytic degradation of trichloroethylene in dry and humid atmospheres role of gas-phase reactions. J Mol Catal A 217 109-115... 

Gas-phase reactions play a fundamental role in nature, for example atmospheric chemistry [1, 2, 3, 4 and 5] and interstellar chemistry [6], as well as in many teclmical processes, for example combustion and exliaust fiime cleansing [7, 8 and 9], Apart from such practical aspects the study of gas-phase reactions has provided the basis for our understanding of chemical reaction mechanisms on a microscopic level. The typically small particle densities in the gas phase mean that reactions occur in well defined elementary steps, usually not involving more than three particles. 

The present study was undertaken to determine the role of gas phase CH, radicals, formed on the surface of a Sr/La O, catalyst, during the reduction of NO by CH4. This material was selected because it is one of the most active catalysts in the reduction reaction, and, under suitable conditions, it is v 7 effective in producing CH, radicals [10]. 

The role of gas phase initiation processes was further explored by Tibbitt et al. . These authors proposed that the polymerization of unsaturated hydrocarbons in a 13.56 MHz plasma is initiated by free radicals formed in the gas by electron-monomer collisions, the initiation reactions listed in Table 6. Moreover, it was assumed that the formation of free radicals on the polymer surface due to the impact of charged particles could be neglected. This assumption is supported by the fact that at 13.56 MHz and pressures near one torr the discharge frequency is significantly greater than either f, or f and that as a result the fluxes of charged particles to the electrode surfaces are quite small. 

This discussion will concentrate on two aspects of MEK oxidation over VPO catalysts, namely, the establishment of a reaction network and the respective roles of gas phase or adsorbed 02 and lattice oxygen in this network. 

The next chapter reviews the reactions of free atoms and radicals which play an important role in the modeling of complex processes occurring in the polluted atmosphere and in combustion chemistry. J. Jodkowski discusses the computational models of the reaction rate theory most frequently used in the theoretical analysis of gas-phase reaction kinetics and presents examples of the reactions of reactive components of the polluted atmosphere, such as 02, NOx, OH, NH2, alkyl radicals, and halogen atoms. Kinetic parameters of the reactions under investigation are provided in an analytical form convenient for kinetic modeling studies. The presented expressions allow for a successful description of the kinetics of the reaction systems in a wide temperature range and could be used in kinetic studies of related species. 

The H+H2 reaction has played a central role in the theory of gas phase reactions. Very recently, Schatz reported CS calculations on H+H2 on the LSTH surface for total energies up to 1.2 eV (also see his chapter in this book). A number of features, identified as resonances were observed. We decided to perform reduced dimensionality CEQB calculations on this system with special attention to these resonant features. 

According to the collision theory proposed by Trautz and Lewis in 1916 and 1918 [2], the pre-exponential factor of the empirical Arrhenius law can be interpreted as product of the theoretically predictable collision frequency and a steric factor. Furthermore, the collision frequency depends on the number of molecules of the reactants per volume, that is, the reactants concentrations, which is the reason for the rate equation being formulated with concentrations instead of, for example, molar fractions. While the reactant concentrations within incompressible phases are mainly defined by the composition, temperature and pressure play important roles for gas-phase reactions. As it can be seen from the ideal gas law. 

Catalytic gas-phase reactions play an important role in many bulk chemical processes, such as in the production of methanol, ammonia, sulfuric acid, and nitric acid. In most processes, the effective area of the catalyst is critically important. Since these reactions take place at surfaces through processes of adsorption and desorption, any alteration of surface area naturally causes a change in the rate of reaction. Industrial catalysts are usually supported on porous materials, since this results in a much larger active area per unit of reactor volume. 

Because of the expanded scale and need to describe additional physical and chemical processes, the development of acid deposition and regional oxidant models has lagged behind that of urban-scale photochemical models. An additional step up in scale and complexity, the development of analytical models of pollutant dynamics in the stratosphere is also behind that of ground-level oxidant models, in part because of the central role of heterogeneous chemistry in the stratospheric ozone depletion problem. In general, atmospheric Hquid-phase chemistry and especially heterogeneous chemistry are less well understood than gas-phase reactions such as those that dorninate the formation of ozone in urban areas. Development of three-dimensional models that treat both the dynamics and chemistry of the stratosphere in detail is an ongoing research problem. 

InEq. (8-17), M represents a so-called third body. In gas phase reactions of atoms, M plays an essential role in conserving energy. The bulk molecules (reactant, products, added inert gases) play this role. (No third body need be involved in solution reactions, however, owing to the presence of the solvent.)... 

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