Johnson Matthey - Model TWC - Three-Way Catalysts
Three-way catalysts (TWC) are designed to simultaneously convert three pollutants to harmless emissions: Carbon Monoxide (CO) → Carbon Dioxide (CO2). Hydrocarbons (HC) → Water (H2O). Oxides of Nitrogen (NOx) → Nitrogen (N2). A three-way catalyst can cut CO, HC and NOx by over 99% if the air to fuel ratio is accurately controlled.
Three-way catalysts will only give full conversion of each of these three pollutants in a gas stream containing a stoichiometric ratio of CO and NOx as shown in the chart below.
Rich/Lean Catalyst Conversions
There are many reaction pathways in a three-way catalyst system, including those shown for the oxidation.
The most important reactions for removing NOx can be summarised as:
2CO + 2NO → 2CO2 + N2[HC] + NO →N2 + CO2 + H2