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Chemkin 4.1.1 Chapter 3: Catalytic Processes The SURFACE KINETICS input file describes chemistry occurring on one material called 3WAYCATALYST that has two site types each representing one of the metals, called PLATINUM and RHODIUM. The PLATINUM site can be occupied by any of 18 surface species, of which Pt(S) is the open site, and one of which, the C3H6(S) species, occupies two sites. The RHODIUM site has simpler chemistry with only 5 surface species, of which Rh(S1) is the open site. Thermochemical data are provided for some of the surface species, but others have placeholder values. All surface reactions are irreversible, such that none of the thermochemical data are used to obtain rates for reverse reactions. The PLATINUM site in this reaction mechanism has a site density of 2.04E-9 moles • cm -2 , which is close to the number of 2.717E- 9moles•cm -2 that is obtained from the density and molecular weight of solid platinum. This indicates that the creators of this mechanism chose to describe some of the effects of the high-surface-area nature of the catalyst material by increasing the site density, rather than by increasing the effective surface area for chemistry. This aspect should be kept in mind in applying this mechanism to other systems. The oxidation of C 3 H 6 (or UHCs) on platinum is described in 47 irreversible reactions that include simple and dissociative adsorptions, simple and associative desorptions, plus reactions between adsorbed species. The latter include decomposition of adsorbed hydrocarbon species by hydrogen transfer to Pt(S) species, as well as various oxidation reactions for adsorbed hydrocarbon species ranging from a global description of C3H5(S) oxidation to more step-wise oxidation reactions for other hydrocarbon fragments. NO reduction on platinum is described in 5 reactions, while NO reduction and CO oxidation on rhodium are described in another 9 reactions, each involving adsorptions, desorptions and reactions among adsorbed species. Some reactions are described in terms of sticking coefficients, a few reactions have reaction-order overrides, and in several cases, the activation energy for a reaction varies quite strongly with the extent of coverage of the surface by one or more species. © 2007 Reaction Design 114 RD0411-C20-000-001
Chemkin 4.1.1 4 4 Materials Problems 4.1 Chemical Vapor Deposition A number of CHEMKIN Reactor Models can be used for CVD simulations. Thermal CVD processes generally involve furnaces or heated surfaces. Simulations of CVD processes, therefore, often treat the temperature of the reactor or deposition surfaces a fixed, experimentally determined input parameter, rather than a quantity that is obtained by solving an energy equation. The surface temperature is usually assumed to be equal to the temperature of the adjacent gas, except for processes performed at very low pressures. Many CVD processes also have fairly long run times compared with startup and end-of-process transients, such that a steady-state simulation is often a good representation of a CVD process. Generally, a low-dimensional simulation, such as an Equilibrium calculation or a PSR simulation, will be used to assess the chemistry, and possibly to simplify a reaction mechanism before using it in higher-dimensional simulations. A multiple-phase equilibrium calculation can be used to determine the maximum possible deposition rate for a certain gas mixture, pressure and temperature. In a PSR simulation of a CVD process, the assumption is made that the chemical kinetics are rate limiting rather than mass transport effects or the inlet reagent supply rate. Higher dimensional simulations allow evaluation of the relative effects of chemical kinetics and mass transport. The reactor models in the CHEMKIN software can be used to simulate a number of reactor geometries used for CVD. The Plug Flow Reactor and Cylindrical Shear Flow Reactor Models can be used to model tube-furnace CVD systems, while the Planar Shear Flow Reactor can be used to model horizontal flow CVD systems. The Stagnation Flow Reactor Model can simulate a vertical showerhead system, while the Rotating Disk Reactor Model is for reactors in which high-speed rotation of RD0411-C20-000-001 115 © 2007 Reaction Design
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Chemkin 4.1.1 Contents Table of Con
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Chemkin 4.1.1 Tables List of Tables
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Tutorials Manual 4.1.1 Figures List
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List of Figures Tutorials Manual 2-
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Chemkin 4.1.1 1 1 Introduction The
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Chemkin 4.1.1 2 2 Combustion in Gas
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Tutorials Manual 4.1.1 Index Index
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