Catalytic cycles, surfaces and enzymes

NO and NO₂ cycle between forms and cancel from the overall equation. This gas-phase scheme illustrates homogeneous catalysis of atmospheric SO₂ oxidation; real atmospheric sulfur chemistry also includes other oxidants and aqueous pathways. Formation and hydration of sulfur oxides contributes to acidic deposition.

The uncatalysed reaction requires encounters between two negative reactants, I^- and S₂O₈^2-. The iron redox cycle offers a different pathway involving positive iron ions, with a lower effective activation barrier. Fe²⁺ is oxidised and then regenerated; its concentration may be small compared with the reactants.

The solid catalyst is in a different phase from the reacting gases. A larger exposed surface gives more accessible active sites, while a poison that binds strongly can block them. Binding must be strong enough to activate reactants but not so strong that products cannot leave. Catalysis permits a useful rate at a lower temperature than the uncatalysed route; it does not independently increase the equilibrium yield.

In a vehicle catalytic converter, gases contact solid precious-metal surfaces such as platinum and rhodium. Adsorption and surface reaction help remove nitrogen oxides, with carbon monoxide acting as a reducing agent. A representative reaction is 2NO(g) + 2CO(g) → N₂(g) + 2CO₂(g). Other catalytic reactions oxidise CO and unburnt hydrocarbons. The converter requires a suitable operating temperature; it is less effective immediately after a cold start.

Specificity has two aspects: which substrate binds effectively and which reaction is catalysed. A substrate that does not fit or interact appropriately is not efficiently converted. The lock-and-key model captures this selectivity; it is a simplified model rather than a claim that proteins are perfectly rigid.