are adsorption equilibrium constant.

Then the adsorbed phase concentration can be written as

Substituting all these value in equation (1)

(2)

 

(3)

For the reaction , at equilibrium, the overall equilibrium constant is.

All concentrations corresponds to the equilibrium conditions in gas phase

(4)

Substituting (3) &(4) in equation (2),

(5)

The above rate expression can also be derived in terms of bulk partial pressure.