By: Dr. Jeff Rieck
Senior Technology Manager, Johnson Matthey
It takes a precise combination of precious metals to help make catalytic converters an effective method for emission control.
Here’s how they work:
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 exhuast 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.
Palladium, which is currently the cheapest of the three metals, has excellent activity for the oxidation of hydrocarbons as well as very good thermal durability. In addition, with a well-designed washcoat, palladium can have very good activity for the removal of NOx. Drawbacks to palladium include its sensitivity to poisons.
Rhodium, currently the most expensive of the three, has by far the highest activity for the removal of NOx from the exhaust. In addition, it has significant activity for the oxidation of HC and CO and very good resistance to the poisons present in the exhaust stream. Its primary drawback is its high cost.
Most catalytic converters today consist of some combination of palladium and rhodium. With current precious metal prices, this gives a good trade-off between cost and performance. While efforts continue to find cheaper alternatives to the precious metals, the tightening aftermarket and OEM emission standards make it likely that they will remain the key components of catalytic converters in the future.