26.2.1—Catalysts can be homogeneous or heterogeneous

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.