Steps in the mechanism of surface reactions

For a reaction to occur on a surface the following sequence of steps is required. 

Any one or a combination of these steps may be slow and therefore be rate determining. 

In gaseous reactions the diffusion steps (1) and (5) are very fast and are rarely, if ever, rate determining. For very fast reactions in solution the rate may be limited by diffusion to or from the surface of the catalyst. If diffusion is the slow step, then the concentration c of the diffusing species at the surface will differ from the concentration c in the bulk. In Fig. 34.1, the concentration is plotted as a function of the distance from the surface. This curve is conveniently approximated by the two dashed lines. The distance 3 is the thickness of the diffusion layer. This approximation was introduced by Nernst, and the layer in which the concentration differs appreciably from that in the bulk is called the Nernst diffusion layer. The concentration gradient across the diffusion layer is given by (c — c )/3, so that the rate of transport per square metre of the surface is 

It is more commonly observed that the rate of reaction is determined by step (2), or by a combination of steps (3) and (4). We consider these cases in order. 

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