NOx can be removed from a lean gas stream by chemical adsorption onto a catalyst, hence the term NOx adsorber catalyst (NAC). The process of adsorption releases CO2.

NOx Adsorber Catalyst (NAC) - lean conditions

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The NAC has a finite capacity for NO2 but it can be regenerated by changing to a rich gas stream. Under these conditions, two reactions happen. First, the catalyst releases the NOx and is thereby regenerated. Then, the NOx is reduced to nitrogen.

NOx Adsorber Catalyst (NAC) - rich conditions

NAC - rich conditions content image.jpg