Module 3 : Pollution control techniques

Lecture 2 : Oxide of Nitrogen

Catalytic converters

  • The catalytic converter consists of several components:

  • The core, or substrate. In modern catalytic converters, this is most often a ceramic honeycomb, however stainless steel foil honeycombs are also used. The purpose of the core is to "support the catalyst“. The ceramic substrate was invented by Rodney Bagley, Irwin Lachman and Ronald Lewis at Corning Glass.

  • The washcoat. In an effort to make converters more efficient, a washcoat is utilized, most often a mixture of silica and alumina. The washcoat, when added to the core, forms a rough, irregular surface which has a far greater surface area than the flat core surface and therefore more places for active precious metal sites. The catalyst is added to the washcoat (in suspension) before application to the core.

  • The catalyst itself is most often a precious metal. Platinum is the most active catalyst and is widely used. However, it is not suitable for all applications because of unwanted additional reactions and/or cost. Palladium and rhodium are two other precious metals that are used. Platinum and rhodium are used as a reduction catalyst, while platinum and palladium are used as an oxidization catalyst. Cerium, iron, manganese and nickel are also used, though each has its own limitations. Nickel is not legal for use in the European Union (due to reaction with carbon monoxide). While copper can be used, its use is illegal in North America due to the formation of dioxin.

In the first chamber, unburned hydrocarbons and water from exhaust gases react to form elemental hydrogen (H2). The most common catalyst in this chamber of the converter is finely divided rhodium metal.

The hydrogen produced in this reaction then reacts with oxides of nitrogen in exhaust gases, reducing them to elemental nitrogen:

Carbon monoxide formed from the reduction of water to hydrogen is also available to reduce nitrogen oxides:

These gases then pass out of the first chamber of the converter and into the second chamber, which usually contains a platinum/palladium catalyst.

Oxidation reactions take place in the second chamber.

Carbon monoxide is converted to carbon dioxide and any remaining unburned hydrocarbons are converted to carbon dioxide and water:

Therefore, engineers are constantly working to devise new methods for improving the efficiency of the catalytic converter.

One problem occurs with lean-burning engines, engines that use a low fuel-to-air ratio.

With such engines, the relatively high concentration of oxygen in the exhaust makes it difficult for a traditional three-stage catalytic converter to reduce all of the NOx that passes through it.

One solution that has been devised makes use of barium carbonate to trap the NOx.

The trap contains a finely divided platinum catalyst embedded in barium carbonate. The carbonate reacts readily with oxides of nitrogen, forming barium nitrate:

When the fuel-to-air mixture temporarily becomes richer (contains more fuel and less air), as during acceleration, larger amounts of carbon monoxide are produced for a short period.

This carbon monoxide then reacts with the oxides of nitrogen “stored” in the barium nitrate, converting them to elemental nitrogen: