NOx adsorber

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A NO_x adsorber or NO_x trap (also called Lean NO_x trap, abbr. LNT)^[1] is a device that is used to reduce oxides of nitrogen (NO and NO₂) emissions from a lean burn internal combustion engine by means of adsorption.

A NO_x adsorber is designed to reduce oxides of nitrogen emitted in the exhaust gas of a lean burn internal combustion engine. Lean burn engines, particularly diesels, present a special challenge to emission control system designers because of the relatively high levels of O₂ (atmospheric oxygen) in the exhaust gas. The 3-way catalytic converter that has been successfully used since the 1980s on stoichiometric engines (such as fueled by petrol, LPG, CNG, or ethanol) 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 NO_x emissions from diesel engines, technologies such as exhaust gas recirculation (EGR) and selective catalytic reduction (SCR) have been used, however EGR is of limited effectiveness and SCR requires a continuous supply of reductant to the exhaust.

The NO_x adsorber was designed to avoid the problems that EGR and SCR experienced. An adsorbent such as zeolite traps the NO and NO₂ molecules — acting as a molecular sponge. Once the trap is full (like a sponge full of water) no more NO_x can be absorbed. Various schemes have been designed to "purge" or "regenerate" the trap. Normally the engine is operated for a short time in rich mode and produces CO as reactant.^[2] Another possible reactants used to this aim is diesel fuel. Injection of diesel fuel before the adsorber can purge it — the NO_x is made to desorb and react with hydrocarbons under rich conditions to produce water and nitrogen. Also hydrogen is a good reductant, but is dangerous and difficult to store. Some experimental systems have used fuel reformers for on-board hydrogen generation.

A NO_x trap is used on the Volkswagen Jetta TDI and the Volkswagen Tiguan concepts. Both are projected to be introduced into the American market by 2008.^[3] They were to be marketed as part of the BlueTec program from Audi, Daimler-Chrysler, and Volkswagen.

In 2015 an ADAC study (ordered by ICCT) of 32 Euro6 cars showed that few complied with on-road emission limits, and LNT cars had the highest emissions.^[4]

The NO_x adsorber is based on a monolithic catalyst support that has been coated with a NO_x adsorbing washcoat such as one containing zeolites. Alkali/alkaline oxide (carbonate) can also be used as the adsorbant.

Traps are gradually poisoned by SO_x which adsorbs more strongly than NO_x. It necessitates a periodic high temperature regeneration that tends to reduce the adsorber's operating life.^[5]