A Descriptive Model of Hydrocarbon Catalysis on Platinum Surfaces

A Descriptive Model of Hydrocarbon Catalysis on Platinum Surfaces 

Studies to correlate the reactivity and the surface structure and composition of platinum surfaces indicate that the active platinum crystal surface must be heterogeneous. The heterogeneity involves the presence of various atomic sites that are distinguishable by their number of nearest neighbors (atoms in terraces, steps, and kinks), and also variation in surface chemical composition. A model that depicts the active platinum surface is shown schematically in Fig. 28. Part of the surface is covered with a partially de- 

It should be noted that the presence of excess hydrogen is always necessary during hydrocarbon reactions, even in those circumstances when the reaction is hydrogen producing (dehydrogenation, dehydrocyclization, etc.). It appears that the main role of excess hydrogen is to keep the step and kink sites 

The discovery that kink sites in steps are effective in breaking C C bonds in addition to C-H and H H bonds, thereby initiating hydrogenolysis reactions, may also explain the effect of trace impurities or second component metals that introduce selectivity. Since these kink sites have fewer nearest neighbors than step or terrace sites, they are likely to bind impurities or other metal atoms with stronger chemical bonds. Thus, these sites are readily blocked by impurities. As a result selective poisoning of hydrogenolysis may be obtained by minute concentrations o veil-chosen impurities or another metal component. 

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IX. A Descriptive Model of Hydrocarbon Catalysis on Platinum Surfaces. . 58... 

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