NOX storage

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

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. 

Figure 10.10. Principle of operation of NOx storage catalyst. During lean combustion, NO is oxidized to NO2 and stored by BaO as barium nitrates. Once the getter is saturated, a short rich excursion of the air-fuel mixture...

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. 

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. 

Mosqueda-Jimenez, B.I., Lahougue, A., Bazin, P. et al. (2007) Operando systems for the evaluation of the catalytic performance of NOx storage and reduction materials, Catal. 

Lesage, T., Saussey, J., Malo, S. et al. (2006) Operando FTIR study of NOx storage over a Pt/K/Mn/Al203-Ce02 catalyst, Appl. Catal. B Environ., 72, 166. 

Abstract The reaction network of the NOx storage and reduction over Pt—Ba/Al203 lean NOt trap catalysts is presented in this paper. 

The NOx storage was investigated at first. The collected results showed that a dual pathway is operating when starting from N0/02 mixtures the first route implies the well-known oxidation of NO to N02, and its subsequent adsorption via disproportionation to form nitrates (nitrate route), whereas the second novel route consists of a stepwise oxidation of NO in the presence of oxygen to form nitrite ad-species, which are progressively oxidized to nitrates (nitrite route). 

Then the reduction of stored NOx with hydrogen is addressed. The bulk of data points out that the reduction of stored nitrates occurs under near isothermal conditions through a Pt-catalysed surface reaction that does not involve the thermal desorption of the stored nitrates as a preliminary step. A specific role of a Pt—Ba interaction was suggested, which plays a role in the NOx storage phase as well. 

An extensive investigation on the analysis of LNT systems has been carried out during the last few years in our labs, and the respective results are presented hereafter. In particular, the mechanisms of the NOx storage and of their subsequent reduction have been addressed, along with the effect of the operating conditions on both the storage and the reduction phases. 

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

Gobel, U., Hoehne, J., Lox, E.S. et al. (1999) Durability Aspects of NOx Storage Catalysts for Direct Injection Gasoline Vehicles, SAE Technical Paper 99FL-103. 

Westerberg, B. and Fridell, E. (2001) A Transient FTIR Study of Species Formed During NOx Storage in the PtBa/Al203 System, J. Mol. Catal. A Chem., 165, 249. 

Anderson, J.A., Bachiller-Baeza, B. and Fernandez-Garcia, M. (2003) Role of Pt in Pt/Ba/Al203 NOx Storage and Reduction Traps, Phys. Chem. Chem. Phys., 5, 4418. 

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