Catalysis on a Real Surface

A real semiconductor surface, in contrast to an ideal plane surface, contains different kinds of imperfections perturbing the strictly periodic structure of the surface. Such a surface is distinguished by a number of peculiarities in its adsorptive and catalytic properties (95). 

One must distinguish between macroscopic and microscopic imperfections existing on a real surface. Macroscopic imperfections are perturbations of the periodic structure covering a region of dimensions considerably greater than the lattice constant. They include cracks on the surface of the crystal, pores, and various macroscopic inclusions. We shall not deal with such imperfections here. Microscopic imperfections are perturbations of dimensions of the order of a crystallc raphic cell. Microscopic imperfections include vacancies in the surface layer of the crystal, foreign atoms or lattice atoms on the surface, different groups of such atoms (ensembles), etc. We shall limit ourselves to a consideration of this kind of imperfection. 

Surface imperfections may affect the adsorptive and catalytic properties of the surface in two ways  

First of all, they may displace the Fermi level, the position of the latter, as we have seen (Sec. VII,B) depending on the nature and concentration of the imperfections. On the other hand, this position determines the 

Secondly, the imperfections may directly participate in the act of adsorption, inasmuch as they may play the part of adsorption centers. This is the second mechanism. 

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