The Chemical Reactions

The essential requirement for an effective three-way catalyst is high conversions of NO_x, CO, and hydrocarbons at and near the stoichiometrically balanced exhaust composition. In general, performance is limited by low conversions of CO and hydrocarbons as the A/F is changed in the reducing (rich) direction and decreased conversion of NO(NO_x) as the A/F is changed in the oxidizing (lean) direction. The narrow range of A/F around the stoichiometric point where conversions are high is commonly referred to as the operating “window”. A wide A/F range or window of high simultaneous NO_x, CO, and HC conversions is a desirable catalyst characteristic because it lessens the need for tight A/F control.