26.2 Homogeneous and heterogeneous catalysts

A homogeneous catalyst is in the same phase as the reactants; a heterogeneous catalyst is in a different phase, often a solid surface with gaseous or liquid reactants.

Both provide an alternative pathway with lower activation energy and are regenerated overall. Phase affects separation, surface access and how the mechanism is studied.

A dissolved Fe²⁺ catalyst in an aqueous redox reaction is homogeneous; iron in the Haber process is heterogeneous.

A catalyst is not defined by whether it is a metal, and “used then regenerated” applies to both categories.

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.

A dissolved catalyst participates in one elementary step to form a temporary intermediate or changed oxidation state, then is regenerated in a later step. It lowers the overall activation barrier without appearing in the net equation.

Trace the catalyst through both equations and cancel it only after adding the steps. Fe²⁺/Fe³⁺ and NO/NO₂ cycles are useful examples.

Fe²⁺ can reduce S₂O₈²⁻ to SO₄²⁻ while becoming Fe³⁺; Fe³⁺ then oxidises I⁻ and returns to Fe²⁺.

A catalyst being regenerated does not mean it is unchanged in every intermediate step; its temporary chemical form is part of the mechanism.