Gas-solid kinetic 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 first half of this textbook introduced the basic tools needed to understand most kinetic processes. Specifically, we learned how to calculate the main thermodynamic driving forces behind kinetic transformations (Chapter 2), we learned how to calculate the rates of reaction processes (Chapter 3), and we learned how to calculate the rates of transport processes (Chapter 4). In the second half of this textbook, we will use these tools to model and understand a number of real-world kinetic processes involving gas-solid, solid-liquid, and solid-solid transformations. In this chapter, we begin with gas-solid kinetic processes. 

Gas-solid kinetic processes are fundamentally heterogeneous as they involve both a gas phase and a solid phase. As a gas-solid kinetic process proceeds, atoms must pass from the gas phase to the solid phase or vice-versa. Thus, one question of fundamental importance in the study of gas-solid kinetic processes is How fast can atoms from the gas phase impinge upon a solid surface, or conversely, how fast can atoms from the solid surface evaporate into the gas phase In other words, we wish to know, in the absence of other limiting kinetic factors, what is the maximum rate at which atoms can move from the gas to the solid phase or vice-versa. 

In the previous section, we considered one of the most basic gas-solid kinetic processes the simple adsorption or desorption of atoms to/from a surface under the assumption that the rate is limited by the impingement of atoms from the gas phase to the surface. In this section, we consider a more complex situation in which a gas species actively etches or corrodes a solid surface via a chemical reaction process, thereby continuously removing material from the surface over time. Consider, for example, the corrosion of a Ti metal surface with HCl acid vapor ... 

This chapter examined gas-solid kinetic processes. We saw how to apply the basic tools we learned in calculating thermodynamic driving forces (Chapter 2), reaction rates (Chapter 3), and mass diffusion (Chapter 4) to understand and model a number of important gas-solid kinetic processes including adsorption/desorption, active gas corrosion, chemical vapor deposition, and passive oxidation. The main points introduced in this chapter include ... 

Gas-solid kinetic processes are fundamentally heterogeneous as they involve both a gas phase and a solid phase. 

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... 

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