What is the role of adsorption in heterogeneous catalysis?

The mechanism of heterogeneous catalysis involves the following steps:
$1)$ Diffusion of the reactants to the surface of the catalyst.
$2)$ Adsorption of reactant molecules on the surface of the catalyst.
$3)$ Occurrence of the chemical reaction on the catalyst surface through the formation of an intermediate.
$4)$ Desorption of the reaction products from the catalyst surface,thereby making the surface available again for more reaction to occur.
$5)$ Diffusion of reaction products away from the catalyst's surface.
This is known as the Modern Adsorption Theory. This theory explains why the catalyst remains unchanged in mass and chemical composition at the end of the reaction and is effective even in small quantities. However,it failed to explain the action of catalytic promoters and inhibitors.