Stoichiometric heterogeneous reaction

A heterogeneous stoichiometric reaction is a reaction in which a condensed substance (solid or liquid) or one of its constituents (in the case of solutions) is consumed in stoichiometric proportions compared to the heterogeneous catalysis, which is also an heterogeneous reaction but not stoichiometric. We have already discussed the heterogeneous stoichiometric reactions which do not produce a condensed phase in section 13.3. 

Figure 14.1. Generic shapes of the kinetics curves (fractional extent versus time) and rate curves (rate versus time) for heterogeneous stoichiometric reactions...

In this chapter we will discuss the results of the studies of the kinetics of some systems of consecutive, parallel or parallel-consecutive heterogeneous catalytic reactions performed in our laboratory. As the catalytic transformations of such types (and, in general, all the stoichiometrically not simple reactions) are frequently encountered in chemical practice, they were the subject of investigation from a variety of aspects. Many studies have not been aimed, however, at investigating the kinetics of these transformations at all, while a number of others present only the more or less accurately measured concentration-time or concentration-concentration curves, without any detailed analysis or quantitative kinetic interpretation. The major effort in the quantitative description of the kinetics of coupled catalytic reactions is associated with the pioneer work of Jungers and his school, based on their extensive experimental material 17-20, 87, 48, 59-61). At present, there are so many studies in the field of stoichiometrically not simple reactions that it is not possible, or even reasonable, to present their full account in this article. We will therefore mention only a limited number in order for the reader to obtain at least some brief information on the relevant literature. Some of these studies were already discussed in Section II from the point of view of the approach to kinetic analysis. Here we would like to present instead the types of reaction systems the kinetics of which were studied experimentally. 

The main results following from the investigation of stoichiometrically not simple heterogeneous catalytic reactions carried out in our laboratory can be summarized in this way ... 

When more than one reaction affects the mass of the ion i in solution (e.g., dissolution and/or precipitation of heterogeneous solid assemblages homogeneous reactions in solution), the overall change in mass of component i is given by the summation of individual absolute reaction velocities Vj, multiplied by their respective stoichiometric reaction coefficients ... 

This chapter begins, after this brief Introduction, by considering the different designs of HP IR cell, with particular emphasis on more recent developments. Applications of HP IR spectroscopy to mechanistic studies of catalytic reactions will then be discussed, illustrated by examples of both in situ catalytic investigations and model stoichiometric reactions. The chapter will concentrate on homogeneous catalytic processes. The reader is referred elsewhere for coverage of in situ IR spectroscopic methods in heterogeneous catalysis [1]. 

To exemplify a complex mechanism of a heterogeneous catalytic reaction, we will consider the interaction of methane with steam on a nickel surface. Stoichiometric equations for this reaction are... 

In addition to the chemical steps, which are the only steps involved in stoichiometric or in homogeneous catalysis reactions, heterogeneous catalysis reactions involve also physical steps, i.e. transport (transfer) of organic molecules (and heat) from the reaction mixture to the active sites of the solid catalyst and vice versa.113-151 Another difference deals with the chemical steps, which do not occur in the fluid phase, but for part of them involve both fluid and solid phases (chemisorption and desorption), the other part occurring at the surface of the catalyst.113-151... 

Stoichiometric reaction of the type shown by 7.4 also leads to the formation of ethylene oligomers. In the Ethyl Corporation process one step involves stoichiometric reaction of this type. Another variant of this is the Conco process, where such stoichiometric reactions are followed by oxidation and hydrolysis of the aluminum alkyls. This gives linear a-alcohols that are used in biodegradable detergents. The co-product is highly pure alumina, which has a variety of uses, including that of an acidic heterogeneous catalyst. 

Electrode reactions are heterogeneous chemical reactions in which stoichiometric transfer of electric charges takes place between the electrode and the electrolyte. The kinetics of such reactions depend not only on concentration and chemical structure, but also on the electrical conditions in and near the phase boundary. Semiconductor and metal electrodes present very different electrical conditions. Here we shall discuss current-potential curves which show the significance of these differences with respect to the mechanism of electrode reactions. 

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