Gas-Solid Surface Reaction Processes

Gas-solid reactions are among the most common type of heterogeneous reaction processes. The platinum surface catalyzed oxidation of hydrogen, discussed in the previous example, is an excellent example of a heterogeneous gas-solid surface reaction process. In Chapter 5, we will smdy a number of different gas-solid kinetic processes in great detail. To prepare for those smdies, in this section we will discuss a few more simple gas-solid surface reaction processes. 

The next step is to write a rate equation (based on experimental evidence) that correctly captures the reaction order and hence the rate law for this reaction process. 

FIGURE 3.8 Schematic illustration of CO poisoning a Pt catalyst snrface. This is an example of a heterogeneous gas-solid surface reaction process and can be described nsing reaction kinetic principles very similar to those we developed for homogeneons gas-phase reactions. 

In this case, the rate law has been experimentally determined to be first order with respect to CO and also first order with respect to the Pt surface sites available for reaction (second order overall). Since we would like to know how fast the Pt surface is poisoned, we write the rate law in terms of the CO surface coverage, 3  

The concentration of available (unpoisoned) surface sites is therefore given by 

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In the sections that follow, we will delve deeply into the atomistic world of reaction kinetics and learn how to predict the rates of a number of fairly simple zero, first, and second-order reaction processes. While this chapter will focus mostly on simple gas-phase chemical reaction processes, the principles learned here will apply just as well to the solid-state materials kinetic examples that we will confront later in the textbook. This is because bond-breaking and bond-forming processes are remarkably similar at the atomistic level whether they happen between molecules in the gas phase or between atoms in a solid. Thus, most reaction processes can be described using a common set of approaches. Toward the end of the chapter, in preparation for later solid-state applications of reaction kinetic principles, we will examine how reaction rates can be affected by a catalyst or a surface, and we will learn how to model several gas-solid surface reaction processes relevant to materials science and engineering. 

ALD is a cyclic, self-limiting gas-solid surface reaction process that allows films to be grown one atomic layer at a time. A classical ALD reaction scheme for the deposition of AI2O3 is shown in Figure 5.12. The deposition process consists of two distinct reaction steps that are repeatedly cycled to build up the AI2O3 film layer by layer ... 

As many studies of gas-solid surface reactions involving rare earth inter-metallics are in some sense or other related to hydride formation, the interaction with hydrogen shall be of central interest in this section. The interaction of hydrogen with the intermetallic compounds under consideration can be represented schematically by the following sequence of processes ... 

A gas-solid reaction usually involves heat and mass transfer processes and chemical kinetics. One important factor which complicates the analysis of these processes is the variations in the pore structure of the solid during the reaction. Increase or decrease of porosity during the reaction and variations in pore sizes would effect the diffusion resistance and also change the active surface area. These facts indicate that the real mechanism of gas-solid noncatalytic reactions can be understood better by following the variations in pore structure during the reaction. 

This is a different kind of heterogeneous reaction—a gas-solid noncatalytic one. Let us examine the process at initial conditions (t 0), so that there has been no opportunity for a layer of UF4.(5) to be formed around the UO pellet The process is much like that for gas-solid catalytic reactions. Hydrogen fluoride gas is transferred from the bulk gas to the surface of the UO2 pellets and reacts at the pellet-gas interface, and H2O diffuses out into the bulk gas. If the pellet is nonporous, all the reaction occurs at the outer surface of the UO2 pellet, and only an external transport process is possible. Costa studied this system by suspending spherical pellets 2 cm in diameter in a stirred-tank reactor. In one run, at a bulk-gas temperature of 377°C, the surface temperature was 462°C and the observed rate was — Tuo = 6.9 x 10 mole U02/(sec) (cm reaction surface). At these conditions the concentrations of... 

In many noncatalytic types a solid product builds up around the reacting core [for example, Na2S04(j) is deposited around the NaCl particles in the last illustration]. This introduces the additional physical processes of heat and mass transfer through a product layer around the solid reactant. A somewhat different form of noncatalytic gas-solid reaction is the regeneration of catalysts which have been deactivated by the deposition of a substance on the interior surface. The most common is the burning of carbon (with air) which has been gradually deposited on catalyst particles used in hydrocarbon reactions. Many of the physical and chemical steps involved here are. the same as those for gas-solid catalytic reactions. The chief difference is the transient nature of the noncatalytic reaction. This type of heterogeneous reaction will be considered in Chap. 14. 

The global transformation rate of a gas-liquid reaction catalyzed by a solid catalyst is influenced by the mass transfer between the gas-liquid and the liquid-solid. The two mass transfer processes and the surface reaction are in series and for fast chemical reactions, mass transfer will influence the reactant concentration on the catalytic surface and, as a consequence, influence the reactor performance and the product selectivity. Compared to gas-solid catalytic reactions as discussed in Section 2.5, an additional resistance in the liquid must be considered (Figure 8.5). 

Heterogeneous gas-solid surface adsorption reaction processes can frequently be treated using the same reaction rate law approach used for homogeneous chemical reactions. In such cases, surface sites are often a key reactant, and their concentration is often represented in terms of a fractional occupancy or availability [e.g., O or (1 - O)]. Using these principles, as an example, the rate at which a Pt surface is poisoned by CO gas adsorption can be modeled as 4> = 1 - (1 - 3>o)c where O is the fraction of the Pt surface that is poisoned... 

Active gas corrosion is a gas-solid kinetic process involving etching (removal) of a solid surface by a corrosive gas species. The rate of this corrosion process depends on both the rate of transport of gases to/from the solid surface and the rate of the corrosion reaction on the solid surface. Depending on the temperature and pressure conditions, either the gas diffusion or the surface reaction process can limit the overall corrosion rate. An overall corrosion rate can be derived which takes into account both processes according to... 

The final step of the whole reaction process is the desorption of the products. This step is essential not only for the practical purpose of collecting and storing the desired output, but also for the regeneration of the catalytic active sites of the surface. Most reactions have at least one rate-hmiting step, which frequently makes the reaction prohibitively slow for practical purposes when, e.g., it is intended for homogeneous (gas or fluid) media. The role of a good solid-state catalyst is to obtain an acceptable... 

More recently, Rosen (R3), Spalding (S5), and Johnson and Nachbar (J4) have considered a simplified approach using the analysis of laminar-flame propagation velocities. According to these investigators, the principal exothermic reactions occur in the gas phase. Some of the heat liberated by these reactions is then transferred back to the solid surface to sustain the endothermic surface-gasification processes. Thus, the temperature profile within the reactive zone is quite similar to that of Rice and Crawford. However, gasification of the solid surface is assumed to be endothermic, while exothermic reactions were considered in the studies discussed previously. 

Powling (P7) recently reported on the results of an extensive study of the combustion characteristics of ammonium perchlorate-based composite propellants. The nature of the chemical processes taking place at the solid-gas interface and the possibility of heat release in the condensed phase were considered. Although the evidence is that some heat release is likely to occur within the solid surface, Powling found that the combustion in all pressure regions appears to be dominated by gas-phase reactions. 

Section 8 deals with reactions which occur at gas—solid and solid—solid interfaces, other than the degradation of solid polymers which has already been reviewed in Volume 14A. Reaction at the liquid—solid interface (and corrosion), involving electrochemical processes outside the coverage of this series, are not considered. With respect to chemical processes at gas-solid interfaces, it has been necessary to discuss surface structure and adsorption as a lead-in to the consideration of the kinetics and mechanism of catalytic reactions. 

Much of the pioneering work which led to the discovery of efficient catalysts for modern Industrial catalytic processes was performed at a time when advanced analytical Instrumentation was not available. Insights Into catalytic phenomena were achieved through gas adsorption, molecular reaction probes, and macroscopic kinetic measurements. Although Sabatier postulated the existence of unstable reaction Intermediates at the turn of this century. It was not until the 1950 s that such species were actually observed on solid surfaces by Elschens and co-workers (2.) using Infrared spectroscopy. Today, scientists have the luxury of using a multitude of sophisticated surface analytical techniques to study catalytic phenomena on a molecular level. Nevertheless, kinetic measurements using chemically specific probe molecules are still the... 

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