What is a catalytic converter?

A well tuned engine still producesmany pollutants causedby the incomplete combustion of the fuel. The three regulated harmful compounds from exhaust gas mixtures are hydrocarbons (in the form of unburned gasoline), carbon monoxide (formed by the combustion of gasoline), and nitrogen oxides (created when the heat in the engine forces nitrogen in the airto combine with oxygen). Carbon monoxide is a poison for any air-breathinganimal. Nitrogen oxides lead to smog and acid rain, and hydrocarbons producesmog.

A catalytic converter is a device that uses a catalyst to convert the threeharmful compounds into harmless ones. The catalyst formulation comprises threekey constituents: precious metals (platinum, palladium, rhodium), alumina, and rare-earth-basedmaterials which enhance catalytic activity of the metals. Ceria istypically used in this capacity. It is white when pure, but is usually paleyellow on account of the nonstoichiometric phasesbetween Ce2O3 and CeO2. The nonstoichiometric ceria isresponsible for the following functions to remove the pollutants from the motorvehicle engineexhaust:

A   Promotion of the water-gas shiftreaction: CO + H2O ⇒ CO2 + H2.

B   Enhancement of the NOx reductioncapability of rhodium.

C   Oxygen storage. In this role, ceria provides elemental oxygen in fuel-rich but air-poor conditions, to ensure oxidation of unburnt hydrocarbons and the removal of CO. It accomplishes thisby going nonstoichiometric to CeO2−x. In leaner (fuel-deficient,air-rich) conditions, it re-oxides to CeO2, i.e., it stores oxygen during theair-rich periods.

Catalytic converters are an integralpart of all automobile exhaust systems manufactured since the mid-1970s (in theUnited States as a result of the 1970 Clean Air Act)