NOx Storage-Reduction Catalyst for Lean-burning Engines

NOx Storage-Reduction Catalyst for Lean-Burning Engines 

The NOx abatement in Diesel and lean-bum Otto engine exhaust gases is of special interest. The direct decomposition of NO into N2 and O2 (Eq. 10-10) is a dream reaction for catalyst researchers  

The reaction is strongly exothermic, hence the equilibrium constant favors the reaction at low temperatures. It is well-known that Cu-zeolites can decompose NO directly to molecular oxygen and nitrogen, but unfortunately the zeolite is not stable under humid conditions. The main features of the Cu-ZSM-5 catalyst are  

Toyota has developed 1994 a NOx-storage-reduction (NSR) catalyst based on a two step process. The engine switches periodically between a long lean-bum stage and a very short fuel-rich stage. The NSR catalyst used in this process consists of two compoimds the active oxidation catalyst Pt and the NOx storage compound based on BaO. 

Note that BaO is not a catalyst but reacts only in a stoichiometric manner with NO2. A major difficulty limiting the general applications of the NSR catalyst is the sulfur sensitivity. Therefore, up to now this concept is only applicable in markets where low-sulfur fuels ( 30 ppm S) are available, such as in lapan and Sweden. The NSR technology claims to meet future standards and will find wider apphcation aU over the world. 

One of the most straightforward methods to reduce carbon dioxide emissions is to enhance the fuel efficiency of engines. The three-way catalyst, although very successful at cleaning up automotive exhaust, dictates that engines operate at air-to-fuel ratios of around 14.7 1. Unfortunately, this is not the optimum ratio with respect to fuel efficiency, which is substantially higher under lean-burn conditions at A/F ratios of about 20 1, where the exhaust becomes rich in oxygen and NOx reduction is extremely difficult (Fig. 10.1). 

In the lean-burn stage all exhaust components are oxidized by the platinum particles in the catalyst. In particular, NO is oxidized to NO2. The latter reacts with BaO getter to form Ba(N03)2- In the rich mode, which only lasts for seconds, the exhaust stream is deficient in oxygen, and reducing components such as CO, H2 and hydro- 

Barium oxide is not a catalyst all reactions involving this component are entirely stoichiometric. Nevertheless, as Fig. 10.10 illustrates, even when the barium storage function is saturated, the NOx content in the outlet gas from the catalyst is lower than in the inlet, owing to the capability of platinum to reduce NOx by hydrocarbons in oxygen-rich exhausts. 

At present the NSR concept is only applicable in markets where low-sulfur fuels (30 ppm S or less) are available, such as in Japan and Sweden. The first NSR catalyst was applied by Toyota in 1994 and currently (end of 2000) about 300.000 cars in Japan have been equipped with it. As the sulfur specifications of fuels is to be tightened, NSR technology will find wider application, as it allows gasoline-fueled engines to operate under conditions of increased fuel efficiency, implying that CO2 emissions will be lower. 

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