Zeolites SCR catalysts

A similar reaction scheme has been recently proposed on the basis of an IR study of the N0/N02-NH3 reactivity over a BaNa-Y zeolite SCR catalyst (Yeom et al., 2005). 

Sulfur tolerances of the zeolite SCR catalysts (20/50 mesh size) were determined in a fixed-bed flow reactor system with a reaction mixture containing 500 ppm of NO, 1,000 ppm of C2H4 or 2,000 ppm of C3HJ, 4.2% of Oj, 1,000 ppm SO2 and He (balance). To observe the simultaneous effect of both SOj and HjO on the removal reaction, 7.3% H2O was also fed to the reaction system in addition to die reactants described above. H2O was injected to the feed gas stream by bubbling He into a water saturator with a small-pore frit immers in deionized water. To avoid condensation of H2O vapor after the bubbler, reactor lines were heated to a temperature higher than the saturation temperature of the feed gas stream including H2O. A gas flow rate of 300 cmVmin was employed for the present study, corresponding to a space velocity of 13,200 h . 

This paper describes the activity and the stability of several Ce exchanged zeolite SCR catalysts. NH3 is used as the reducing agent. CeNa-MOR is very active and reaches NOx conversions up to 100%, at a GHSV of 43000 h and temperatures between 300 and 500°C. The stability however, especially when SO2 is added, appeares to be poor. CeH-ZSM-5 on the contrary is less active but shows SO2 resistance, at least for the relatively short time it is investigated (37 hours) with SO2 concentrations up to 450 ppmv. CeH-ZSM-5 extruded with 50 wt% alumina suffers from irreversible deactivation when the catalyst is exposed to SO2 concentrations higher than 300 ppmv. 

Regarding the composition of diesel exhaust gases (containing amongst others water and SO2), developing a stable, zeolite based diesel exhaust deNOx catalyst is a challenging task. Zeolites can show dealumination under hydrothermal conditions accompanied by a loss of active material furthermore SO2 can also cause deactivation. Many authors already have reported on the hydrothermal stability of zeolite SCR catalysts [e.g. 7-9] and also some papers exist on the stabilization with respect to hydrothermal deactivation of zeolite SCR catalysts by the choice of proper cations [10-13]. A small number of articles describes the influence of SO2 on zeolite SCR catalysts [14-17]. The current paper gives the results of measurements on both the short term hydrothermal stability and the influence of SO2 on CeNa-MOR and CeH-ZSM-5 zeolite catalysts. 

For application of zeolite SCR catalysts a monolith t3T>e reactor can be used, which is however... 

Kamasamudram et al. [33] show that increasing copper loading in the Cu-zeolite SCR catalyst can decrease N2O formation by the first two mechanisms, and better DOC design and control can prevent the third mechanism. The investigators show that it is possible to reduce N2O to nitrogen, but these reactions occur at much higher temperatures than those at which they are formed. 

Another approach to managing the stored ammonia for improved low-temperature performance is described by Yasui et al. [53] and illustrated in Fig. 1.15. They use two Fe-zeolite SCR catalysts placed downstream from the DPF system. An ammonia sensor is placed between the two SCR catalysts, and ammonia is generously injected to keep the first catalyst loaded at all times, as conditions allow. This accomplishes two goals. First, the efficiency of the SCR system is improved as there is plentiful ammonia present in the system. More importantly, the strategy helps cold start management. In traditional cold start thermal management, the SCR catalyst is heated as fast as possible to get it. Here, the catalyst is always loaded with ammonia, and the catalyst is heated slowly to prevent rapid release of ammonia during this period. 

The sequence of inlet gas compositions and how these changes in feed affect the effluent concentrations are shown in Fig. 4.1. Further analysis of these traces allows the calculation of steady-state conversions, NH3 storage under three different conditions, as well as the stability of the stored NH3. Data generated from selected portions of the protocol using Fe-zeolite SCR catalysts will be discussed in the following section for the following conditions 150-550 °C, 60-120 k hr GHSV, and a total NO feed of 150—450 ppm. 

Fig. 4.12 NO,t conversion of the accelerated engine-aged Fe-zeolite SCR catalysts a front and b rear sections evaluated with 5 % CO2, 5 % H2O, 14 % O2, 350 ppm NO, 350 ppm NH3, N2 balance, GHSV = 30,000 h ...

Fig. 4.13 BET surface area measurements of fresh and accelerated engine-aged Fe-zeolite SCR catalysts...

Fig. 4.16 Two-dimensional 3QMAS A1-NMR spectra of a fresh Fe-zeolite SCR catalysts and samples aged at b 650, c 750, and d 850 °C, and e A cordierite spectrum was also recorded since it was present in each of the samples...

As is illustrated in this chapter, Fe-zeolites have a specific role in NHa-based SCR of NOx, provide reaction characteristics that are different from other SCR-based systems. Through application of an appropriate experimental protocol, it is possible to gain deep insight into the detailed workings of these catalysts. Although the chemistry is largely similar to that of Cu-zeolite SCR catalysts, there are key differences that differentiate the two systems ... 

Cu/Zeolite SCR Catalysts for Automotive Diesel NOx Emission Control... 

In this chapter, the chemistry and functionalities of Cu/zeolite SCR catalysts are discussed. Results on the investigation of the deactivation mechanisms of the previous generation of Cu/zeolite SCR catalysts, especially under hydrothermal aging conditions, are presented. Next, the development of small-pore zeolite supported Cu SCR catalysts will be reviewed. Finally, recent studies reported in the literature to understand the hydrothermal stability and performance of small-pore zeolite supported Cu SCR catalysts will be summarized. 

Deactivation Mechanisms of Cu/Zeolite SCR Catalysts 5.3.1 Hydrothermal Deactivation... 

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