Transition metals are excellent catalysts due to: (1) Variable oxidation states — allow electron transfer between reactant and catalyst, (2) Ability to form transient complexes with reactants, lowering activation energy, (3) Availability of d-orbitals for bonding with substrate molecules, (4) Large surface area when finely divided. Key examples: Fe in Haber process (N2 + H2 → NH3), V2O5 in Contact process (2SO2 + O2 → 2SO3), Ni in Margarine production (hydrogenation of oils), MnO2 in KClO3 decomposition, Pd/Pt in catalytic converters.