Solid kinetics

To date, the use of computational methods to investigate iminium ion catalysis has been limited. The focus has been on rationalising the diastereo- and enantiose-lectivities observed in the laboratory, but this has largely been retrospective and the clear potential of these models as predictive tools for the design of improved catalysts or even entirely new scaffolds has yet to be realised. There are few examples of solid kinetic data in the literature making evaluation of the models difficult. 

Straathof, A.J.J., Wolff, A. and Heijnen, J.J. (1998) Sohd-to-solid kinetic resolution— determination of the enantiomeric ratio. Journal of Molecular Catalysis B-Enzymatic, 5, 55-61. 

The common problems with those metallomicelles may be summarized as follows (1) Most of these complexes were prepared in situ and often were not isolated. Hence, the intended structures of the metallomicelles in solution or in the solid state were not verified. (2) The metal complexes in solution were not identified or characterized in rigorous thermodynamic senses by potentiometric pH titration, etc. The complexation constants and possible species distribution at various pH s were totally unknown. (3) Possible catalytically active species L-Mn+—OH were not identified by means of the thermodynamic pvalues. Those described were all obtained merely in kinetics. (4) The product (phosphate anion) inhibition was not determined. Accordingly, it often was not clear whether it was catalytic or not. (5) Often, the substrates studied were limited. (6) The kinetics was complex, probably as a result of the existence of various species in solution. Thus, in most of the cases only pseudo-first-order rates (e.g., with excess metal complexes) were given. No solid kinetic studies combined with thermodynamic studies have been presented. It is thus impossible to compare the catalytic efficiency of these metallomicelles with that of the natural system. Besides, different... 

It may be of interest to note that summation of the normal components of the velocity correlations represents the solids kinetic energy, or granular temperature, which is a primary parameter in the kinetic theory of granular particles. 

Figure 4.2-3 Fractional conversion of solid reactant as a function of dimensionless time for homogeneous model with zero-order solid kinetics sphere). 

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... 

The solid kinetics of HMX is very complicated with an initial slow reaction succeeded by acceleratory and deceleratory periods. The initial slow reaction has been studied by Maksimov. He reported an activation energy of 37.9 kcal/mole and a frequency factor of 1.58 x 10 fisec. Rogers and Janney found an initial activation energy of 33.9 kcal/mole and a frequency factor of 4.8 x 10 /j,sec. ... 

Segregation-induced emichment and related electric fields in the interface layer may have substantial effects on the heterogeneous gas/solid kinetics, even at high temperature. Thus a correct understanding of the gas/solid phenomena and resultant diffusion data require that the picture of segregation be well defined. Many diffusion data available in the literature were determined assuming that the bulk transport is rate controlling. In mar r cases this assumption is not valid. Thus diffusion data determined in such a way should be considered to be apparent... 

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