26.2.2—Heterogeneous catalyst action

On a solid catalyst, reactants adsorb onto active sites, bonds weaken or orient, the reaction occurs with a lower barrier, and products desorb to free the site.

Too strong adsorption blocks sites; too weak adsorption prevents activation. Iron in Haber synthesis and Pt/Pd/Rh in catalytic converters illustrate the surface mechanism.

N₂ and H₂ adsorb on iron, allowing N≡N activation before NH₃ forms and leaves. In a converter, NO and CO react on precious-metal sites.

The catalyst is not consumed into the final product, and surface adsorption is not the same as dissolving the reactant.