A Generalized Model for Heterogeneous Reactions

One can see that all the reactions discussed in 4.2 depend in a general way on the same series of processes. It is possible, therefore, to discuss the reaction steps in a general way, although detailed description is, of course, only possible for specific classes of reactions. Roberts and Anderson (10) have recently discussed a generalized model of this type. Here we are preoccupied with the formation or decomposition reactions of ionic solids and will be concerned at some stage with electronic transitions necessary to create or destroy ions. In the simplest reactions, such as alloy formation, we do not need to consider this step. 

Many of these reactions are complicated by a preliminary step, which also takes place on the surface, namely, the 

This primary step has created a defect in the surface of the metal in the fonn of an interstitial X and a more or less iocahzed positive charge in the immediate neighbourhood. 

Similar, though more complicated, schemes may be established for the primary step in other reaction types, but the essential feature of the stq is the creation or annihilation of defects. 

The defect aggregates themselves, depending on the nature of the defect, can acquire a net charge on the surface by trapping either electrons or positive holes. This allows them to trap further defects to increase the aggregate size. The step in this reaction may be sym-bohzed as follows, using the simple case of aggregation of F-centre defects to produce metal M from an ionic solid MX  

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