fractional selectivity in catalysis

The term selectivity ( $S$ ) is used to describe the relative rates of two or more competing reactions on a catalyst. Such competition includes cases of different reactants undergoing simultaneous reactions or of a single reactant taking part in two or more reactions. For the latter case, $S$ may be defined in two ways. The first of these defines a fractional selectivity, $S F$ , for each product by the equation

$S F = ξ i ∑ ξ i$

The second defines relative selectivities, $S R$ , for each pair of products by

$S R = ξ i ξ j$

In each case, $ξ i$ and $ξ j$ are the rates of increase of the extent of reactions i and j respectively, i.e.

$ξ i = d ξ i d t$

and

$ξ j = d ξ j d t$ ,

where $ξ i$ and $ξ j$ are the extents of reactions $i$ and $j$ respectively.

Source:

PAC, 1976, 46, 71 (Manual of Symbols and Terminology for Physicochemical Quantities and Units - Appendix II. Definitions, Terminology and Symbols in Colloid and Surface Chemistry. Part II: Heterogeneous Catalysis) on page 81