The Nature of Active Sites
Catalysts function by formation of intermediates from reactants which subsequently decompose to yield products. Usually, catalysts have some degree of valence unsaturation so that they can form bonds to the intermediates, but these bonds must also be readily ruptured so that intermediates can readily form products. For most heterogeneous catalysts, reactions are limited to the atoms on the surface, and the valence unsaturation stems from the ‘dangling bonds’ of the surface atoms, which possess a lower coordination number than the sub-surface atoms. If, however, the solid catalyst consists of well-formed crystallites, not all surface atoms have the same coordination number, and, hence, the degree of unsaturation may be different for different atoms. For example, the coordination number of a Pt atom in a 111 surface plane is 9 compared to 12 for the bulk, but the coordination number of a Pt atom in the 110 surface plane is 7. If one includes atoms along edges of intersecting planes or at steps, even smaller coordination numbers are found. In general, the lower the coordination number, the higher the valence unsaturation attributed to the surface atoms. Thus, these different atoms could well be expected to exhibit different activity in the overall catalysis effected by a single crystal. Langmuir1 reached the same conclusion fifty years ago. In his discussion of activation of Pt catalysts, he concluded: ‘The surface thus becomes a composite, and there is then a relatively small fraction of the surface at which reaction occurs with extreme rapidity while over the larger part of the surface, it takes place at a very slow rate’.
KeywordsZinc Migration Hydroxyl Petroleum Platinum
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