Lean NOX trap

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. 

NSR is very attractive method for NOx removal by storing NOx under lean conditions and then reducing the stored NOx to N2 under rich excursions by engine operation this technology is also referred as a lean NOx trap. In this chapter, NSR catalyst and the mechanism for NOx reduction is described. 

Lean NOx trap (LNT), in which NOx is stored on the catalyst and then the catalyst regenerated intermittently using engine control. 

Clearly, when modelling an LNT it is important to include the most important processes occurring in this relatively complex catalyst system. Kinetic and experimental studies of lean NOx trap catalysts, including those describing chemical principles, have been published previously (Brogan et al., 1995 Dou and Bailey, 1998 Fekete et al, 1997 Miyoshi et al., 1995 Takami et al., 1995). These processes can be summarised as follows ... 

From the reaction-kinetic modeling point of view, the NSRC, sometimes called lean NOx trap (LNT) or NOx adsorber, is the most complex of the currently used automobile exhaust converters. A variety of different physical and chemical processes and the number of gas and surface components participating in typical periodic lean/rich operation form a large and closely linked system. 

Sharma et al. (2005) developed a ID two-phase model for the analysis of periodic NOx storage and reduction by C3H6 in a catalytic monolith, based on a simplified kinetic scheme. They focused on the evaluation of temperature and reaction fronts along the monolith and their effect on NOx conversion. Kim et al. (2003) proposed a phenomenological control-oriented lean NOx trap model. 

A lean NOx trap (LNT) (or NOx adsorber) is similar to a three-way catalyst. However, part of the catalyst contains some sorbent components which can store NOx. Unlike catalysts, which involve continuous conversion, a trap stores NO and (primarily) N02 under lean exhaust conditions and releases and catalytically reduces them to nitrogen under rich conditions. The shift from lean to rich combustion, and vice versa, is achieved by a dedicated fuel control strategy. Typical sorbents include barium and rare earth metals (e.g. yttrium). An LNT does not require a separate reagent (urea) for NOx reduction and hence has an advantage over SCR. However, the urea infrastructure has now developed in Europe and USA, and SCR has become the system of choice for diesel vehicles because of its easier control and better long-term performance compared with LNT. NOx adsorbers have, however, found application in GDI engines where lower NOx-reduction efficiencies are required, and the switch between the lean and rich modes for regeneration is easier to achieve. 

DPF= Diesel Particulate Filter EG R = Exhaust Gas Recirculation SCR = Selective Catalytic Reduction LNT = Lean NOx Trap... 

P. (2013) Spectrokinetic analysis of the NOx storage over a Pt-Ba/Al 2O3 lean NOx trap catalyst Top. Catal, 56,... 

Another approach to the problem of NOx emissions from diesel vehicles is the use of NOx traps. NO2 is acidic and is adsorbed by basic oxide, such as BaO, to form Ba(N03)2. The NO2 is released and reduced to N2 during the periodic engine rich excursion. Basic oxides will, of course, bind sulfur oxides as well, and high temperature intervals with rich exhaust gas must be included in the engine control strategy to desulfate the catalyst. The lean-NOx-trapping concept is currently not in wide commercial use while development efforts are continuing. 

Various Sets of experiments were carried out to look at the effects of NOx trapping materials on the activity of supported nanoparticulate Pt with N0(g)/02(g) on the C(s) combustion reaction, and also to look at how the presence of C(s) affected the Pt/BaO system designed for lean NOx trapping. 

Although the first commercial lean deNOx system was a lean NOx trap (LNT) on the European Toyota Avensis in the early 2000s, and then on the US Dodge Ram truck (Cummins engine) in 2007, the first wide-scale use of deNOx was the implementation of SCR for heavy-duty (HD) tmck applications in Europe in 2005. The US Tier 2 and Cahfornia Low Emission Vehicle (LEVII) regulations were the first to force SCR on light-duty (LD) applications in 2007. SCR did not make its way into NR applications until 2011 in both the US and Europe. 

Onboard Generation of Ammonia Using Lean NOx Traps... 

Toops, T. J., Parks 11, J. E., PUil, J. A., DiGiulio, C. D., Amiridis, M. D. (2012) Lean Gasoline Emissions Control NH3 Generation Over Commercial Three-Way Catalysts and Lean-NOx Traps, presentation at US Department of Energy, Directions in Engine Efficiency and Emissions Research (DEER) Conference, October 2012, Detroit. 

Thus, it seems now well established that the presence of NO2 in the NH3-SCR reacting system involves the formation of nitrates adspecies in significant amounts over both Fe- and Cu-promoted zeolites catalysts. The similarity with the chemistry of NOx storage onto Pt-Ba/Al203 Lean NOx Traps (LNT) has been noted in this respect [16]. At this point, the question is whether such species are just spectators or rather participate actively in the NH3-SCR mechanism. We address this important issue in the next paragraph. 

Beside the NHs/urea SCR process, the NOx reduction from lean bum exhaust gas can be achieved using the cycled NOx-Storage Reduction or NSR system (also called Lean NOx-trap (LNT) system). In Europe, the NH3-SCR technology could be quickly implemented on heavier cars, as it is already the case for tmcks, while the NSR system is rather envisaged to be implemented in light passenger car. 

Kwak JH, Kim DH, Szanyi J, Peden CHF (2008) Excellent sulfur resistance cf Pt/BaO/ Ce02 lean NOx trap catalysts. Applied Catal. B 84 545-551... 

Szailer T, Kwak JH, Kim DH, Hanson JC, Peden CHF, Szanyi J (2006) Reduction of stored NOx on Pt/Al203 and Pt/BaO/Al20s catalysts with H2 and CO. J. Catal. 239 51-64 Nova I, Lietti L, Forzatti P, Prinetto F, Ghiotti G (2010) Experimental investigation of the reduction of NOx species by CO and H2 over Pt-Ba/AliOs lean NOx trap systems. Catal. Today 151 330-337... 

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... 

Pereda-Ayo B, Gonzalez-Velasco JR, Burch R, Hardacre C, Chansai S (2012) Regeneration mechanism of a Lean NOx Trap (LNT) catalyst in the presence of NO investigated using isotope labelling techniques. J. Catal., 285 177-186... 

Bhatia D, Harold MP, Balakotaiah V (2010) Modeling the effect of Pt dispersion and temperature during anaerobic regeneration of a lean NOx trap catalyst, Catal. Today 151 314-329... 

Choi JS, Partridge WP, Pihl JA, Daw CS (2008) Sulfur and temperature effects on the spatial distribution of reactions inside a lean NOx trap and resulting changes in global performance. Catal. Today 136 173-182... 

Nova I, Lietti L, Forzatti P (2008) Mechanistic aspects of the reduction of stored NOx over Pt-Ba/Al203 lean NOx trap systems. Catal. Today 136 128-135... 

Larson RS, Pihl JA, Chakravarthy VK, Toops TJ, Daw CS (2008) Microkinetic modeling of lean NOx trap chemistry under reducing conditions. Catal. Today 136 104—120... 

Olsson L, Blint RJ, Fridell E (2005) Global kinetic model for lean NOx traps. Industrial Engineering Chemistry Research, 44(9), 3021-3032... 

Currently, the major deNOx after-treatment technologies under consideration include the so-called Lean-NOx Traps (LNT), which are used with direct injection gasoline and Diesel engines, and the Selective Catalytic Reduction (urea-SCR) process. 

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