Catalytic Converter
A well-tuned engine still produces many pollutants caused by 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 air to combine with oxygen). Carbon monoxide is a poison for any air-breathing animal. Nitrogen oxides lead to smog and acid rain, and hydrocarbons produce smog.
A catalytic converter is a device that uses a catalyst to convert the three harmful compounds into harmless ones. The catalyst formulation comprises three key constituents: precious metals (platinum, palladium, rhodium), alumina, and rare-earth-based materials which enhance catalytic activity of the metals. Ceria is typically used in this capacity. It is white when pure, but is usually pale yellow on account of the nonstoichiometric phases between Ce2O3 and CeO2. The nonstoichiometric ceria is responsible for the following functions to remove the pollutants from the motor vehicle engine exhaust:
- a
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Promotion of the water-gas shift reaction: CO + H2O ⇒ CO2 + H2.
- b
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Enhancement of the NOx reduction capability of rhodium.
- c
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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 this by going nonstoichiometric to CeO2−x. In leaner (fuel-deficient, air-rich) conditions, it re-oxides to CeO2, i.e., it stores oxygen during the air-rich periods.
Catalytic converters are an integral part of all automobile exhaust systems manufactured since the mid-1970s (in the United States as a result of the 1970 Clean Air Act)