Phenomenological model of branched-chain reactions on a catalyst surface

6 PHENOMENOLOGICAL MODEL OF BRANCHED-CHAIN REACTIONS ON A CATALYST SURFACE 

In the 1950s, Semenov and Voevodskii [148] made an attempt to apply the concepts of the branching-chain reaction theory to the kinetics of heterogeneous catalysts. They applied the concept of free valencies migrating over the catalyst surface and of semi-chemisorbed radicals. But their attempt was criticized (see, for example, ref. 149 where Temkin, using hydrogenation of ethylene on palladium as an example, proved experimentally the inapplicability of the chain theory concepts). 

Recently, Barelko et al. [150-155] put forward a new version of this theory. They suggested a branching-chain process mechanism based on the concepts implying the existence of a two-dimensional gas of adsorbed atoms (adatoms) on solid surfaces which are in equilibrium with their crystal lattice. According to the suggested hypothesis, the active centre is the adatom. The energy evolved in the course of a reaction on the adatom can be applied to break out another atom from the lattice, i.e. to form a new active centre. This process is a step of branching. But the decay of the active centre takes place due to the return of the adatom back into the lattice. 

At present the literature lacks strong experimental proof for this hypothesis which was put forward to interpret the critical effects seen in catalytic oxidation reactions. As far as the model itself is concerned, it is of the form of ref. 150, which is similar to the chain reaction equation 

In conclusion let us present some simple considerations that are, to our mind, of importance. Heterogeneous catalytic reaction is a complex process 

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