There are several catalytic converters types available, all of which serve the same purpose: to make the air we breathe cleaner. The methods used by these converters differ slightly. The two-way catalytic converter and the three-way catalytic converter are frequently compared. 

Two-way catalytic converter type: 

These were the first catalytic converters, and they helped to steer innovation efforts in the right direction. These catalytic converters are the least effective for gasoline-powered vehicles today. They contain platinum and palladium and are designed to convert carbon monoxide to carbon dioxide and hydrocarbons to carbon dioxide and water. 

Two-way (or “oxidation”) catalytic converter performs two tasks at the same time: 

1. Oxidation of carbon monoxide to carbon dioxide 
2. Oxidation of hydrocarbons (unburnt and partially burnt fuel) to carbon dioxide and water 

Prior to 1981, the two-way catalytic converter was primarily used, and due to its inability to control nitrogen oxides, the three-way converter was born. This is not to say that two-way catalytic converters are obsolete. Two-way converters are still used in lean-burn engines today. 

Three-way catalytic converter type:

These are the most common type and are far more efficient than two-way converters. Rhodium, which is added to the mix of precious metals such as platinum and palladium, is supported by three-way catalytic converters, which aid in the removal of carbon monoxide, hydrocarbons, and nitrogen oxides from exhaust. Nitrogen oxide has been blamed for the cause of acid rain which can poison ground water and lakes.  In general, engines fitted with three-way catalytic converters have a computerized closed-loop feedback fuel injection system with one or more oxygen sensors. Carburetors with feedback mixture control were used early in the deployment of three-way converters. 

Three-way (or “oxidation-reduction”) catalytic converter performs three tasks at the same time: 

1. Reduction of nitrogen oxides to nitrogen and oxygen 
2. Oxidation of carbon monoxide to carbon dioxide 
3. Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water 

Three-way catalytic converters have been used in vehicle emission control systems in the United States and Canada since 1981; plenty other nations have also adopted stringent vehicle emissions requirements that, in effect, require three-way converters on gasoline-powered vehicles.