Manifestation of a Complex Reaction Mechanism

To find out the mechanism of a chemical reaction and the nature of its elementary processes it is very important to know whether the reaction occurs in the homogeneous (gas) phase only, or whether heterogeneous factors are also involved. With gas reactions such a factor usually is the reactor wall. The importance of the reactor wall, in particular of its nature and relative surface area in the kinetics of chemical gas reactions was first noticed by van t Hoff [499]. 

In subsequent years, the effect of the wall surface area and nature (of the reactor material and pretreatment) has been ascertained for many gas reactions. It appeared that sometimes the wall hinders, and sometimes accelerates the reaction. The effect can also be of a dual nature, the wall both favouring and hindering the same reaction in its course. The dual effects are connected with the generation and decay of active centers at the wall (see Chapter XI). 

One of the methods of estimating the wall effect on a chemical reaction is to study the reaction in vessels with a different ratio S/V of the wall surface area to the reactor volume. If the reaction rate or composition of products depend on S/V, this is evidence that the reaction involves heterogeneous steps, i.e. such that occur at the reactor wall. 

However, it has been found that, as a result of the dual wall effect, the reaction rate might sometimes be independent of S/V in spite of the occurence of heterogeneous reaction steps [230]. Therefore, the above method will give unambiguous indication that the heterogeneous factor is involved in the reaction only when the reaction rate depends on S/V. 

The method of differential calorimetry [50] is much more satisfactory both with regard to the unambiguity of interpretation of the results obtained and the quantitative estimation of the heterogeneous factor effect on the reaction rate. 

---