Modeling reactions on uniform surfaces

Week 10 week 13

Langmuir- Hinshelwood (L-H)

• Numerous kinetic studies have had their experimental data satisfactorily correlated by a experimental data satisfactorily correlated by a rate expression derived from a reaction rate expression derived from a reaction sequence invoking a Rate Determining Step sequence invoking a Rate Determining Step (RDS) on an ideal surface(RDS) on an ideal surface, and the approaches associated with this assumption are typically referred to as Langmuir-Hinshelwood (L-H)type.

Reaction Models with a RDS – Uni-molecular Surface Reactions• The example describes a

reversible isomerization reaction of A to B, where S is an active site.

• This reaction model represents a series of elementary steps comprising a closed sequence and a catalytic cycle.

If irreversible ( >> ), irreversible ( >> ), the second term in the numerator disappears and the rate expression can be written as :

The form of the above two equation is also suitable for uni-molecular decom-position reactions provided that additional sites are not required for the decomposition products;Further, if [B-S] is very low because P, if [B-S] is very low because PBB is very low is very low (as in a differential reactor) or Kor KBB is is very small very small and the last term in equation can be neglected and r will be as follows:

k

k�

2) Another examples where one of the product is not adsorbed where one of the product is not adsorbed to any sites to any sites (the product C is not adsorbed).

3) if in a unimolecular decomposition reaction at least one unimolecular decomposition reaction at least one additional active site is required in the RDS additional active site is required in the RDS (or any step preceding it), then a different rate expression is obtained. If the product C is adsorbed.

Reaction Models with a RDS – BimolecularSurface Reactions

• One other possibility exists for an ideal surface in that two different types of active sites might exist, with one set of sites adsorbing only one of the reactants and the other set of sites adsorbing another reactant with or without competing adsorption from the first reactant. Thus, for example,one could have: