NOX storage materials

Finally, unconventional OSC supports have also been investigated. Machida et al., for instance, studied a Pd/MnOx-Ce02 as NOx-storage materials [118]. This catalyst was tested in lean/rich conditions using H2 as reducer with, however, an unusual period (9 min. lean/3min. rich). The catalyst showed both excellent activity and selectivity (almost 100% to N2). 

The NSR technology is regarded as the most reliable and attractive de-NOx method. However, catalyst degradation by SOx poisoning is a big problem. Sulfate was detected on the NSR catalyst after the aging test. S02 is oxidized and reacts with the NOx storage materials to form sulfates, which means the... 

BaO within a few nanometers around the Pt can work as NOx storage material in the reaction containing S02. Also, the smaller the sulfate particle size in the aged catalyst is, the easier its decomposition. 

Several different models were proposed for the slow NOx storage process, while only few details and approximate models are available for the highly transient NOx reduction within the rich phase, lasting only several seconds. The models can be divided into two groups, depending on whether the internal diffusion in the particles of the NOx storage material is considered explicitly, or this effect is included implicitly into the evaluated kinetic parameters. The models can be further differentiated by the level of complexity for the reaction kinetics description, i.e., either (simplified) microkinetic scheme or the global kinetics. 

This was an encouraging first series of experiments which confirmed the beneficial effect of the operation of a NOx trap (rather than the presence of a NOx storage material per se) promoted PM combustion. 

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. 

An NSR catalyst consists of precious metal as active site, basic material as NOx storage site, and support for highly dispersing these sites. As active site, Pt and Rh are much effective elements for NSR catalysts. One important role of the active site is to oxidize NO under oxidizing condition. Our early catalyst test showed that, precious metals and Mn oxide loaded on alumina are indicated a high NO oxidation activity. Particularly, Pt showed the highest turn over... 

One of the big problems for NSR catalyst is SOx poisoning. The storage material forms stable sulfates in lean conditions and loses NOx adsorption... 

Understanding the storage capacity of the metal oxide material and the N02 adsorption rate is therefore also very important. Furthermore, oxides of sulphur in the exhaust gas, formed from combustion of sulphur compounds in the fuel (or lubricant oil), are stored in a similar manner. Since sulphates are thermodynamically more stable than the corresponding nitrate, this results in a reduction in the NOx storage capacity of the LNT. 

For the fresh and the specifically aged catalyst materials, the dependence of the normalized NOx storage capacity on temperature could be kept the same (Giithenke et al, 2007b). This minimized the number of parameters to be re-adapted for two catalysts with different ageing level. Thus, only the maximum NOx storage capacity and the pre-exponential factors for the reactions R1-R22 had to be re-evaluated, cf. Table III and Eq. (36). 

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