What is the chemical process by which a catalytic converter reduces NOx emissions?

A catalytic converter reduces NOx emissions through a chemical process called reduction. NOx, or nitrogen oxides, are formed during high-temperature combustion in an engine. Catalytic converters typically use a three-way catalyst, which simultaneously reduces NOx, oxidizes hydrocarbons (HC), and oxidizes carbon monoxide (CO). The NOx reduction process involves converting NOx into nitrogen gas (N2) and oxygen gas (O2). This occurs on the catalyst surface, which is typically made of platinum, palladium, and rhodium. Rhodium is particularly effective at catalyzing the reduction of NOx. The reaction requires a reducing agent, which is typically CO or unburnt hydrocarbons present in the exhaust gas. In a simplified example, the reaction can be represented as: 2NOx -> N2 + xO2, where x is typically 1 or 2 depending on the specific nitrogen oxide compound (NO or NO2). The catalyst provides a surface for this reaction to occur more efficiently at lower temperatures than would otherwise be possible. For the three-way catalyst to function effectively, the air-fuel ratio entering the engine must be tightly controlled near the stoichiometric point (the ideal air-fuel ratio for complete combustion). This ensures that there are sufficient reducing agents (CO and HC) available to reduce the NOx, while also allowing for oxidation of HC and CO. Oxygen sensors are used to monitor the exhaust gas composition and provide feedback to the engine control unit (ECU), which adjusts the air-fuel ratio accordingly. If the catalyst becomes contaminated or aged, its efficiency at reducing NOx decreases, leading to increased NOx emissions. Therefore, proper maintenance of the engine and catalytic converter is crucial for maintaining low NOx emissions.