Principles of Polystep Catalysis

If a catalyst mass contains only one type of catalytic site we shall call it a monofunctional catalyst. By one type is meant that every catalytic site or surface exhibits the same qualitative catalytic property as to what reaction or reaction steps it can catalyze. We shall concern ourselves only, of course, with reaction steps which are thought to be relevant to the reaction examined. For example, we normally assume that platinum/charcoal is a monofunctional catalyst in the hydrogenation of olefins. (For the present purpose we need not be concerned about the quantitative equivalence of every Pt-surface site, i.e., whether or not there is uniformity or a spectrum of catalytic effectiveness for the same reaction among different platinum sites.) 

The term monofunctional does not exclude the possibility that the intrinsic activity of a catalytic material may be influenced by chemical contact with a second material. Thus the intrinsic hydrogenation activity of a metal may differ depending on the nature of the support. Such effects may be due to varying degrees of metal dispersion, or due to more profound effects of electronic interaction which modify the electronic properties of the metal. In such cases, although the activity of the metal may depend on the nature of the support, the locus of activity is still at the metal constituent, and we still have a case of monofunctionality. 

In contrast, we shall see that in a paraflhi isomerization system a platinum on silica-alumina catalyst is a multifunctional, specifically, a hifunc-tional catalyst the platinum sites catalyze distinctly different reactions and reaction steps than do the silica-alumina sites neither catalyze the reactions of the other component furthermore, both tj ies of reactions are relevant to accomplish the over-all reactions of the desired conversion system. 

In a polyfunctional catalytic solid, we shall refer to the materials or sites responsible for distinctly different reactions or reaction steps as catalyst components.  

It is important to recognize the specific meaning of the term intermediate in this context. The use of the term will not relate to the concept of surface complex, or activated complex for, in this case, at least in heterogeneous catalysis, the catalyst, or a part of it, is structurally combined or, by specific force-fields, is interacting with a reaction-participating molecule. In contrast to this meaning, the term intermediate here will refer to a chemical species that is produced by the catalyst as a desorbed, normal chemical species, i.e., one that has its own name, structure, and thermodynamic properties normally associated with indei endent chemical compounds. 

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