THE INFLUENCE OF MOLECULAR-SIZE ON THE KINETICS OF SURFACE-CATALYZED REACTIONS

In the description of surface-catalysed reaction two different concept s are mainly discussed: the Eley-Riedel mechanism and the Langmuir-Hin shelwood mechanism. Experience has shown that the latter is the most i mportant one. According to this concept, the educt species can only re act with one another if both types of molecules are simultaneously ads orbed on the catalyst surface. In order to describe this adsorption st ate, frequently, the adsorption model according to Langmuir is used. T his concept assumes, in particular, that the coadsorbed molecules have nearly the same molecular size. In practical problems, this assumptio n only seldom holds. In order to investigate the influence of the size of the adsorbed molecules on the reaction rate of a given surface-cat alysed reaction, an adsorption concept, recently proposed as the so-ca lled CS-model, can be taken. Using a simple example, the results obtai ned with these different concepts are discussed in detail.