Catalytic Activity on Surfaces Modified by Promoters or Poisons

6 CATALYTIC ACTIVITY ON SURFACES MODIFIED BY PROMOTERS OR POISONS 

In section 2.5 we have examined the effect of promoters and poisons on the chemisorption of some key reactants on catalyst surfaces.We saw that despite the individual geometric and electronic complexities of each system there are some simple rules, presented at the beginning of section 2.5 which are always obeyed. These rules enable us to make some predictions on the effect of electropositive or electronegative promoters on the coverage of catalytic reactants during a catalytic reaction. 

Thus referring to the very simple promotional LHHW rate expression (2.17) we can already write it as  

It should be emphasized that is the actual, promoter modified, work function of the catalyst surface and not that of a clean metal surface for which we reserve the symbol o- It should also be clarified that the kinetic constant kR is also expected to vary with . Since, however, we have no rules on how it varies with we will attempt here to rationalize some classical promotional kinetics treating it as a constant. What is amazing is that this procedure works, which indicates that the promoter action effect on kD andkA, together with the 1-0P term, is dominant. 

Needless to remind that Equation (2.29) is a very approximate expression which can be expected to provide only a qualitative description and not a quantitative fit to actual promotional kinetic data. 

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