Catalytic Mechanisms monomolecular

Strictly speaking, mechanisms for heterogeneous catalytic reactions can never be monomolecular. Thus they always include adsorption steps in which the initial substances are a minimum of two in number, i.e. gas and catalyst. But if one considers conversions of only surface compounds (at a constant gas-phase composition), a catalytic reaction mechanism can also be treated as monomolecular. It is these mechanisms that Temkin designates as linear (see Chap. 2). 

In studies of catalytic reactions, linear (monomolecular) mechanisms are observed in the following two cases. 

The isomerization of open-chain alkanes with more than six carbon atoms gives isobutane as the main product, together with disproportionated materials, even though the reaction proceeds by the monomolecular pathway [144]. On the other hand, for cyclic alkanes the monomolecular process with preservation of the cyclic structure seems to be the most probable, judging from the results for cyclohexane. The absence of isobutane in the products indicates that the reaction path does not involve open-chain intermediate species. Therefore, it is of interest to try cycloalkanes larger than cyclohexane for clarification of the reaction mechanism along with the catalytic action of S04/Zr02. 

Molecular recognition between biomolecules is another strategy that has been used to deposit assemblies of redox enzyme on electrodes. It has led to the construction of true monomolecular layers and of spatially defined multimonomolecular structures. The dynamics of these systems has been fully characterized and the activity of the deposited enzyme tested quantitatively, which has been very seldom the case with the systems depicted in the preceding text. For this reason, the following sections focus on these structures. Besides describing the construction procedures, emphasis is laid on the detailed analysis of the catalysis responses and on the mechanisms it allows one to uncover. As a prelude to such kinetic analyses, the catalytic responses of a redox enzyme with a one-electron mediator in solution are first examined. 

---