what is the role of platinum in a catalytuc converter
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Automotive exhaust contains three harmful pollutants, which are formed due to inefficiencies in the fuel combustion process. Hydrocarbons (HC) and carbon monoxide (CO) are formed as a result of the incomplete combustion of gasoline. Oxides of nitrogen (NOx) are created from the burning of the nitrogen present in the intake air at the high temperatures and pressures encountered in the cylinders during ignition. HC and NOx are major contributors to smog formation, and CO reduces the ability of the blood to pick up and transport oxygen through the body. As a result, catalytic converters were developed as an after-treatment to reduce these harmful emissions. Platinum, palladium, and rhodium have historically been the key active components used in these catalytic converters.
These precious metals are unique in their ability to facilitate the reactions of HC and CO with oxygen to produce water and carbon dioxide and to promote the reaction of CO with NOx to convert the NOx to harmless nitrogen gas. With the combination of a properly tuned engine and a properly designed catalytic converter, it is theoretically possible to have complete removal of these pollutants. The precious metals are typically dispersed in a washcoat, which is then coated on a flow-through ceramic or metallic substrate which supports them in the exhaust stream. The washcoat contains various components and additives to promote the activity and durability of the precious metals. Exhaust gasses pass through the catalytic converter substrate, which is coated with a washcoat containing platinum (Pt), palladium (Pd), or rhodium (Rh). Hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx) in the exhust are converted to carbon dioxide (CO2), nitrogen gas (NOx) and water vapor (H2O).
These three precious metals each have their own unique properties that come into play in determining which ones must be used for a particular application. Platinum is a very good oxidation catalyst and has good resistance to poisons such as sulfur, phosphorus, or lead, which may be present in the exhaust. Two drawbacks to platinum are its low activity for the conversion of NOx and its high price relative to palladium. In addition, platinum is sensitive to the high temperatures which may occur in the catalytic converter during high engine loads.
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