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.
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.