Nox storage-reduction catalyst

NOx Storage-Reduction Catalyst for Lean-burning Engines... 

The three-way catalyst and the NOx storage-reduction catalyst represent remarkably successful catalytic technology. The catalysts are unique in that they have to operate under a wide range of conditions, depending on type of use, personal driving style, local climate, etc. This in contrast to the usual situation in industry, where conditions are optimized and kept constant. 

Matsumoto, S. (2000) Catalytic Reduction of Nitrogen Oxides in Automotive Exhaust Containing Excess Oxygen by NOx Storage-Reduction Catalyst, Cat. Tech., 4, 102. 

Another technology under consideration for NOx abatement in diesel vehicles is the LNT, which is also commonly called the NOx adsorber catalyst (NAC) or NOx storage/reduction catalyst (NSRC). 

Matsumoto S, (2000) Catalytic reduction of nitrogen oxides in automotive exhaust containing excess oxygen by NOx storage-reduction catalyst. Cattech 4 102-109... 

Pereda-Ayo B, Duraiswami D, Gonzalez-Marcos JA, GonzSlez-Velasco JR (2011) Performance of NOx storage-reduction catalyst in the temperature-reductant concentration domain by response surface methodology. Chem. Eng. J. 169 58-67... 

H. Shinjoh, N. Takahashi, K. Yokota (2007) Synergic effect of Pd/gamma-alumina and Cu/ZSM-5 on the performance of NOx storage reduction catalyst. Topics Catal., 42-43 215-219... 

The NOx storage-reduction (NSR) catalyst, developed by Toyota and other companies, offers a solution based on a two step process, in which the engine switches periodically between a long lean-burn stage and a very short fuel-rich stage. The NSR catalyst combines the oxidation activity of platinum with a NOx storage compound based on barium oxide. Figure 10.10 illustrates the principle of operation. 

Examples of multi-disciplinary innovation can also be found in the field of environmental catalysis such as a newly developed catalyst system for exhaust emission control in lean burn automobiles. Japanese workers [17] have successfully merged the disciplines of catalysis, adsorption and process control to develop a so-called NOx-Storage-Reduction (NSR) lean burn emission control system. This NSR catalyst employs barium oxide as an adsorbent which stores NOx as a nitrate under lean burn conditions. The adsorbent is regenerated in a very short fuel rich cycle during which the released NOx is reduced to nitrogen over a conventional three-way catalyst. A process control system ensures for the correct cycle times and minimizes the effect on motor performance. 

Figure 11.15. Promotional effect of different transition metals on initial NOx storage/reduction activity for 0.5Pt/7.5Ba/2.5 Promoter catalyst [90].

Figure 11.16. Initial NOx storage/reduction for different catalysts under fuel lean conditions [90].

Yamazaki, K., Suzuki, T., Takahashi, N. et al. (2001) Effect of the addition of transition metals to Pt/Ba/Al203 catalyst on the NOx storage-reduction cataysis under oxidizing conditions in the presence of S02. Appl. Catal. B Environ., 30, 459. 

Another important catalytic technology for removal of NOx from lean-burn engine exhausts involves NOx storage reduction catalysis, or the lean-NOx trap . In the lean-NOx trap, the formation of N02 by NO oxidation is followed by the formation of a nitrate when the N02 is adsorbed onto the catalyst surface. Thus, the N02 is stored on the catalyst surface in the nitrate form and subsequently decomposed to N2. Lean NOx trap catalysts have shown serious deactivation in the presence of SOx because, under oxygen-rich conditions, SO, adsorbs more strongly on N02 adsorption sites than N02, and the adsorbed SOx does not desorb altogether even under fuel-rich conditions. The presence of S03 leads to the formation of sulfuric acid and sulfates that increase the particulates in the exhaust and poison the active sites on the catalyst. Furthermore, catalytic oxidation of NO to N02 can be operated in a limited temperature range. Oxidation of NO to N02 by a conventional Pt-based catalyst has a maximum at about 250°C and loses its efficiency below about 100°C and above about 400°C. 

The complementary use of CO and NO as probe molecules allowed us to obtain information on the nature and dispersion of the Pt and/or Cu supported phases and on the effect of the pre-calcination temperature of the hydrotalcite support. The influence of the sample pre-treatment was also taken into account in view of the catalytic applications of these materials in the NOx storage-reduction process, which involves an alternation of oxidisit or reducing steps. We would just underline that the formation of a bimetallic phase evidenced in the case of the catalysts containing Pt and Cu can be particularly interesting from a catalytic point of view. Indeed, the modification observed in the metal properties due to the alloying can induce favourable changes in the specific activity and selectivity of the catalysts. 

Castoldi L, Lietti L, Forzatti P, Morandi S, Ghiotti G, Vindigni F (2010) The NOx storage-reduction on PtK/Al203 Lean NOx Trap catalyst. J. Catal. 276 335-350... 

Le Phuc N, Courtois X, Can F, Royer S, Marecot P, Duprez D (2011) NOx removal efficiency and ammonia selectivity during the NOx storage-reduction process over Pt/ BaO(Fe, Mn, Ce)/Al203 model catalysts. Part I Influence of Fe and Mn addition. Appl. Catal. B 102 353-361... 

Seldmair, C, Seshan, K, Jentys, A, Lercher, J A, 2003, Elementary steps of NOx adsorption and surface reaction on a commercial storage-reduction catalyst . Journal of Catalysis, v. 214, n.2, pp. 308-316. 

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