Promoters and Poisons in Catalysis

Relation between Catalytic Activity and Electrical Conductivity of a Semiconductor 

The third important consequence of the theory is the relation between the catalytic activity and the electrical conductivity of a semiconductor. 

The catalytic activity of the semiconductor is determined by the position of the Fermi level on the surface of the crystal ,+. Here, as we have seen (Sec. V,B), it is necessary to differentiate between two classes of reactions those which are accelerated and those which are retarded as the Fermi level rises (i.e., as e,+ increases see Fig. 22). We have called these reactions n-type and p-type reactions, respectively. 

We have also seen (Sec. VI,A) that the position of the Fermi level on the surface of the crystal ,+ is determined, other conditions being equal, by its position inside the crystal e +. An increase of e + entails an increase of ,+, and a decrease of a decrease of e.+. We are thus led to the conclusion that the factors which shift the Fermi level in the bulk of the crystal, i.e., which affect the electrical conductivity, will also shift the Fermi level on the surface (in the same direction) i.e., they will affect the catalytic activity. Furthermore we come to the conclusion that the factors directly displacing the Fermi level on the surface without affecting its position in the bulk (i.e., the factors affecting the degree of bending of the bands) will also affect both the electrical conductivity and the catalytic activity simultaneously. 

there must exist a certain parallelism between the changes in the electrical conductivity and in the catalytic activity. The physical origin of this parallelism is clear the electrical conductivity is determined by the concentration of free charge carriers in the semiconductor on the other hand, these take part in the reaction (as its components) and thus determine its rate. 

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