Heterogeneous chemical reactions defined

D Me-S surface alloy and/or 3D Me-S bulk alloy formation and dissolution (eq. (3.83)) is considered as either a heterogeneous chemical reaction (site exchange) or a mass transport process (solid state mutual diffusion of Me and S). In site exchange models, the usual rate equations for the kinetics of heterogeneous reactions of first order (with respect to the species Me in Meads and Me t-S>>) are applied. In solid state diffusion models, Pick s second law and defined boundary conditions must be solved using Laplace transformation. 

Thus, (jj) is the formation rate of species j expressed in terms of moles of species j formed per unit time per unit volume. For heterogeneous chemical reactions (reactions that take place on the surface of a solid catalyst or at the interface of the two phases), the surface-based formation rate of species j, rj)s, is defined by... 

Electrochemical reactions at an electrode snrface differ from normal heterogeneous chemical reactions in that they involve the participation of one or more electrons that are either added to (reduction) or removed from (oxidation) the reactant species. The explicit inclusion of electrons as reactants or products means that the reaction rate depends on the electric potential. Electron transfer processes occur within a small portion of the double layer immediately adjacent to the electrode surface (10 to 50 mn in thickness) where solution-phase electroneutrality does not hold and where very strong electric fields (on the order of 10 V/cm) exist during a charge transfer reaction. We begin the analysis of electrochemical kinetics by defining a generic electrode reaction ... 

As a prelude to the development of kinetic rate expressions for heterogeneous chemical reactions, if A reacts with B, for example, then the next step in the mechanism is ha + Ba, forming an activated complex on the snrface. Each reversible step in the seqnence above is characterized by a forward rate constant adsoiption for adsoiption, with units of mol/area time atm, and a backward rate constant A ,desoiption for desorption, with units of mol/area time. The ratio of these rate constants adsorption/ h, desoiption defines the adsorption/desorption equi-... 

Consider a straight tube of radius R with circular cross section and expensive metal catalyst coated in the inner wall. Reactant A is converted to products via first-order irreversible chemical reaction on the catalytic surface at r = R. Hence, diffusion of reactant A in the radial direction, toward the catalytic surface, is balanced by the rate of consumption of A due to heterogeneous chemical reaction. The boundary condition at the mathematically well-defined catalytic surface (i.e., r = R) is... 

In terms of a production engineering process (Allen 2004), etching is better defined as a material removal process by accelerated, controlled corrosion, comprising a heterogeneous chemical reaction in which a liquid (or, more rarely, a gas) reacts with a solid material and oxidizes it to produce a soluble (or volatile gaseous) reaction product. 

CVD is a synthesis process in which the chemical constituents react in the vapor phase near or on a heated substrate to form a solid deposit (Pierson 1999). The reactions happened in the CVD system can be divided into homogeneous gas phase reactions and heterogeneous substrate surface reactions. Normally, CVD technique is utilized to make thin films. CVD is also defined as a process whereby a thin solid film is synthesized from the gaseous phase by a chemical reaction (Hitchman and Jensen 1993). The CVD apparatus arrangement is dependant on the particular application. The apparatus is made up with three major components precursors and... 

The experiments with the inert tracer may only show that the time, necessary for the fluid in the reactor to be well mixed, is much smaller than the average residence time. When a chemical reaction takes place, an additional time-scale, the time constant of the chemical reaction, appears. This time characterizes the reaction rate and can be defined as the time in which the reaction proceeds to a certain conversion, say 50%. For many practical heterogeneous catalytic reactions, the reaction time is so short that reactants entering the reactor may be converted without being mixed, for example, during the first cycle. For such fast reactions, of course, the reactor cannot be considered as gradient-free, whatever the recirculation ratio is. 

Heterogeneous catalysts are defined as solids or mixture of solids that are used to accelerate a chemical reaction without undergoing change themselves. The types of solids used in industry as heterogeneous catalysts include simple oxides, mixed oxides, metal salts, solid acids and bases, metals, and dispersed metals. Catalysts are used in a wide variety of chemical and environmental processes worldwide. The global value of fuels and chemicals produced by catalytic routes is about US 2.4-3 trillion per year. About 20% of all products produced in the United States are derived from a catalytic process of some form. As important as catalysis is to the world economy, the number of various chemicals used as a catalyst as well as the form and shape of the material vary as much as the number of processes that use catalysts. Fig. 1 is a picture of a number of various types of catalysts and illustrates the numerous possibilities of shapes and sizes. Naturally, the preparation processes of such a wide variety of products is also numerous. 

Heterogeneous catalysis is an important field for the application of these techniques. Because of the use of these nanoscopies, advances have been made in the knowledge of the geometry and effective area of solid catalysts, the sintering process that decreases their performance and hfetime, the adsorbate film structure on crystallographically well-defined surfaces, and the influence of surface defects on the dynamic behavior of these films during adsorption, desorption, and chemical reaction stages. 

This chapter covers the second fundamental concept used in chemical reaction engineering—chemical kinetics. The kinetic relationships used in the analysis and design of chemical reactors are derived and discussed. In Section 3.1, we discuss the various definitions of the species formation rates. In Section 3.2, we define the rates of chemical reactions and discuss how they relate to the formation (or depletion) rates of individual species. In Section 3.3, we discuss the rate expression that provides the relationship between the reaction rate, the temperature, and species concentrations. Without going into the theory of chemical kinetics, we review the common forms of the rate expressions for homogeneous and heterogeneous reactions. In the last section, we introduce and define a measure of die reaction rate—the characteristic reaction time. In Chapter 4 we use the characteristic reaction time to reduce the reactor design equations to dimensionless forms. 

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