Evaluation of NOx Storage Catalysts as an Effective System for NOx Removal from the Exhaust Gas of Leanburn Gasoline Engines 952490

One possibility to improve the fuel economy of SI-engines is to run the engine with a lean air-fuel-ratio (AFR). Hydrocarbon and carbon monoxide after-treatment has been proven under lean operation, but NOx-control remains a challenge to catalyst and car manufacturers. One strategy that is being considered is to run the engine lean with occasional operation at stoichiometry. This would be in conjunction with a three-way-catalyst (TWC) to achieve stoichiometric conversion of the three main pollutants in the normal way and a NOx trap. The NOx trap stores NOx under lean operation to be released and reduced under rich conditions. The trap also functions as a TWC and has good HC and CO conversion at both lean and stoichiometric AFR's.
Under lean conditions NO is oxidised to NO2 on Pt which is then adsorbed on an oxide surface. Typical adsorbent materials include oxides of potassium, calcium, zirconium, strontium, lanthanum, cerium and barium. Under reducing conditions the nitrate formed is unstable at temperatures greater than 100°C causing decomposition to release O2 and NO. These are subsequently reduced to N2 on Pt and Rh. In this paper we will discuss the parameters which control NOx adsorption efficiency. Of concern has been the durability of these adsorbers with regard to real exhaust-gas relevant to European and US markets, and the progress made in this area will be reported.
On a fresh catalyst 90,5 % NOx conversion was attained on a bench leanburn engine operating at λ = 1,35 for 60 s with rich spikes at λ = 0,70 for 0,3 s. With this regeneration period the fuel economy penalty due to the rich excursion is negligible. The deactivation of the NOx trap is, as with standard TWC's, attributable both to sulphur poisoning and thermal ageing. It appears that under operating conditions the accumulation of sulphur is the predominant cause of deactivation. However after exposure to temperatures in excess of 800°C thermal ageing is predominant. It has been observed that sulphur poisoning, resulting from competitive adsorption of SO2 with NO2 on the adsorbent is largely reversible after exposure to rich conditions at 600°C.


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