NOx Storage Catalyst (NSC)
In the NSC, nitrous oxides are stored as they react with basic components forming nitrates.
The first step of the storage mechanism is the oxidation of NO to NO2 on the precious metals of the catalyst, mainly Platinum (Pt). This NO2 is subsequently stored on the corresponding storage materials (e.g. Barium Carbonate BaCO3) incorporated in the catalyst formulation until the desired storage capacity is reached.
The NSC is operated non-continuously, i.e. it needs to be regenerated periodically by means of rich exhaust gas (λ<1) in order to decompose the nitrates and reduce them. The main reducing agents are carbon monoxide (CO) and hydrogen (H2), which are formed during rich combustion in the cylinder or directly on the catalyst.
Besides its ability to reduce NOx the NSC is also an excellent oxidation catalyst that can reduce CO and HC, even at exhaust temperatures below 100°C.
Sulfurization of the NSC needs to be considered as it is an undesired but unavoidable side reaction. With sulfur from the fuel and lubricant, barium sulfate can be formed. Hence NOx storage sites are blocked by the sulfur and the NSC is deactivated slowly with growing sulfur accumulation. In general the thermal resistance of the sulfate is higher than that of the nitrate, which is also the case if treated with rich exhaust gas. Therefore, the desulfurization procedure requires higher temperature than the NSC regeneration.