EFFECT OF SURFACE DIFFUSION ON THE SELECTIVITY OF CATALYTIC REACTIONS.

The analysis presented indicates that surface diffusion can greatly affect the observed selectivity of a reaction. The presence of surface diffusion reduces the dependence of selectivity on the flow rate. In fact, if surface diffusion of B is much faster than its desorption, a selectivity of unity, independent of flow rate, can be obtained. Conditions used in ultra high vacuum studies (like molecular beam experiments) are equivalent to the extreme of very high flow rate. This analysis shows that depending on the degree of surface diffusion, results from such studies may or may not differ significantly from the conventional studies. Another interesting result due to surface diffusion is the dependence of selectivity on the crystallite size. No such dependence would be expected without surface diffusion. Surface diffusion also results in the increase in selectivity with decreasing loading. The selectivity approaches unity at very low loading but would approach zero in the absence of surface diffusion.