The NOx Adsorber or NOx Trap is a device that is use to reduce NOx (NO and NO2) emissions from a lean burn internal combustion engine.
Purpose and Function of a NOx Adsorber
A NOx Adsorber is designed to reduce oxides of nitrogen emitted in the exhaust gas of a lean burn internal combustion engine. Lean Burn (Diesel) engines present a special challenge to emission control system designers because of the relatively high levels of O2 (atmospheric Oxygen) in the exhaust gas stream. The 3-Way Catalytic Converter technology that has been successfully used on Rich Burn Internal Combustion Engines (typically fueled by gasoline or petrol but also sometimes fueled by LPG, CNG, or Ethanol) since the middle 1980s will not function at O2 levels in excess of 1.0%, and does not function well at levels above 0.5%. Because of the increasing need to limit NOx emissions from Diesel engines technologies such as EGR and SCR have been used, however EGR is limited in it's effectivness and SCR requires a reductant, and if the reductant tank is emptied the SCR system ceases to function.
The NOx Adsorber was designed to avoid the problems that EGR and SCR experienced as NOx reduction technologies. The theory is that the zeolite will trap the NO and NO2 molecules - in effect acting as a molecular sponge. Once the trap is full (like a sponge full of water) no more NOx can be adsorbed, and it is passed out of the exhaust system. Various schemes have been designed to "purge" or "regenerate" the adsorber. Injection of diesel fuel (or other reactant) before the adsorber can purge it - the NO2 in particular is unstable and will join with Hydrocarbons to produce H2O and N2. Use of Hydrogen has also been tried, with the same results, however Hydrogen is difficult to store. Some experimental engines have mounted Fuel Reformers for on board Hydrogen generation, however Fuel Reformers are not mature technology.
NOx Adsorbers are experimental technology as of early 2006, and thus extremely expensive. Whether or not this technology will be successfully commercialized is open to question - only time will tell.
The NOx Adsorber is based on a catalytic converter support that has been coated with a special washcoat containing zeolites.