Single Crystals as Model Catalysts

In this same series of reactions it was also found that C-C bond cleavage took place most readily over the 111 face catalyst (A in Fig. 3.2) and to a considerably less extent on the others. This reaction, which is stmcture sensitive, obviously teikes place over ensembles of face atoms. It was also found 8 that the dehydrocyclization of hexane to benzene, another structure sensitive reaction, took place four times faster on the 111 face of a platinum crystal than on one cleaved to expose the 100 face. The hydrogenolysis of ethane, on the other hand, was found to be significantly faster on Ni (100) than on Ni (111).22 Thus, it is not sufficient merely to define a site as an ensemble of face atoms the orientation of the atoms must also be specified. 

While these results show a relationship between single crystals and supported metals when used for structure sensitive reactions, similar relations for those reactions promoted by a single atom site may not be as straightforward since there are more different types of single atom sites possible on a metal surface than there are groups of face atoms. 

Over the past several years a number of instrumental techniques have been applied to the characterization of catalysts.12,33 Ultraviolet, infrared,25.36 a d raman22 spectroscopic data have provided information concerning the nature of substrates adsorbed on a catalyst surface as well as the composition and stmcture of many oxide catalysts. Extended X-ray absorption fine structure (EXAFS) is 

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