Adsorption Theory of Heterogeneous Catalysis

From old theory to modern view

The adsorption theory of catalysis holds that gaseous or dissolved reactants are adsorbed on the solid catalyst's surface. Concentrating the reactants there speeds the reaction, and the exothermic heat of adsorption further boosts the rate. A separate intermediate-compound-formation theory explains catalysis differently. The modern adsorption theory combines both: catalytic action is localised on the surface and proceeds in five steps:

1. Diffusion of reactants to the catalyst surface.
2. Adsorption of reactant molecules on the surface.
3. Reaction on the surface via an intermediate.
4. Desorption of the products, freeing the surface again.
5. Diffusion of products away from the surface.

As Fig. 5.3 depicts, the catalyst surface carries free valencies that loosely bind incoming molecules; adjacent adsorbed molecules react, and the product then evaporates, leaving the surface ready for fresh reactant. This neatly explains why the catalyst is unchanged in mass and composition and works even in small amounts — though it does not account for promoters or poisons.

Important features of solid catalysts

(a) Activity — the ability to accelerate a reaction, tied largely to the strength of chemisorption. Reactants must adsorb strongly enough to become active, but not so strongly that they are immobilised and block the surface. For hydrogenation, activity rises across Groups 5–11, peaking at Groups 7–9:

$2H_2(g) + O_2(g) \xrightarrow{\;Pt\;} 2H_2O(l)$ …