Models accounting for diffusional mass transfer

Diffusional mass transfer processes can be essential in complex catalytic reactions. The role of diffusion inside a porous catalyst pellet, its effect on the observed reaction rate, activation energy, etc. (see, for example, ref. 123 and the fundamental work of Aris [124]) have been studied in detail, but so far several studies report only on models accounting for the diffusion of material on the catalyst surface and the surface-to-bulk material exchange. We will describe only some macroscopic models accounting for diffusion (without claiming a thorough analysis of every such model described in the available literature). 

This description shows first, that due to the independent character of elementary diffusion acts between atoms of various substances, their diffusion mobility is controlled by the different partial diffusion coefficients second, that the diffusion of atoms and molecules adsorbed on the surface takes place due to their overjumps to neighbouring unoccupied sites (vacancies). 

for the oxidation of CO on a metal wire the model accounting for the surface diffusion is of the form [128] 

This model will primarily account for the resolving of local activity spots during the reaction. But in models of such kind, periodic spatial structures ( dissipative structures ) can also be formed and these have recently become of great interest. 

The effect of surface diffusion on the selectivity of the catalytic reaction A - B -y C has been examined [129]. The authors suggest that the sites of a-phase (assumed to be round) are uniformly distributed over the jS-phase. Reaction A - B takes place only on the a-phase, whereas B — C occurs only on the /f-phase. Substance B formed on the a-phase is transferred to the /i-phase due either to surface diffusion or adsorption-desorption processes. 

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