SCR Mechanism

So far we have proven that not only nitrates are stored onto Fe- (and Cu-) zeolite catalysts in the presence of NO2, but also that they do participate effectively in the NH3-SCR catalytic chemistry, being indeed responsible for the very high DeNOx activity associated with the Fast SCR reaction. In the next paragraph we make use of transient reaction analysis to elucidate in more detail the reactivity of surface nitrates with NO and NH3, i.e., the SCR reactants in so doing, we will also explore the individual steps of the Fast SCR mechanism. 

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Another way to work in transient conditions is to stop suddenly (or conversely to instantaneously introduce) one of the reactants, in order to destabilize the system and to enhance the concentration of labile species. With this method, for example, Poignant et al. studied the DeNO. reaction mechanism on a H—Cu-ZSM-5 catalyst, using propane or propene as reducing agents. The introduction of 2000 ppm of hydrocarbon in a flow of NO (2000 ppm) + 5% 02 allowed to evidence the formation of acrylonitrile, which behaved as an intermediate. Its reactivity with NO+ species constituted a fundamental point to describe a detailed SCR mechanism for NO removal on zeolitic compounds [137],... 

Marban G, Valdes-Solis T, Fuertes A B (2004) Mechanism of low-temperature selective catalytic reduction of NO with NH3 over carbon-supported Mn304 Role of surface NH3 species SCR mechanism. J. Catal. 226 138-155. 

In this chapter, the reaction mechanisms of the standard SCR reaction are discussed from various perspectives including steady-state kinetics, the relations with NH3/NO oxidation ability and acid site amount, the effect of coexisting gases, and transient reaction behavior. Through these comprehensive analyses, some similarities and differences of the reaction mechanism among the conventional SCR catalysts could be extracted. Also, new perspectives on standard SCR mechanism could be suggested. 

To trace the surface-adsorbed species during the transient NO reduction, in situ FT-IR analysis was applied. Analyzing the transient change of these adsorbates would help in understanding the SCR mechanism and identifying the rate-determining step. 

For the SCR reaction, NO activation to form NO2 species would be the key step. For a deeper understanding of the SCR mechanism, further reaction kinetic analysis combined with tracing the reversible valence change of active metal is necessary. However, it can be said from the result of Sects. 8.3.1 and 8.3.2 that gaseous and/or high coverage NH3 inhibits the rate-determining step, and thus reaction rate can be... 

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. 

We have shown in Sect. 9.5.2 that the reduction of surface nitrates by NO, the key step in the Fast SCR mechanism over Me-exchanged zeolites and V-based catalysts, is active already at 50 °C. A dedicated study over an Fe-BEA catalyst [20] pointed out, however, that in the presence of ammonia the reaction between NO and nitrates is stopped and proceeds only on raising the temperature up to 140-160 °C, which thus represents an intrinsic lower bound to the Fast SCR activity. Similar results were reported for a BaNa-Y zeolite [5]. 

The role of nitrates as key intermediates in the Fast SCR mechanism has been fully confirmed in recent years by the discovery of the so-caUed Enhanced SCR reaction [3, 31],... 

It is worth noticing that both the Fast SCR and the Enhanced SCR chemistries can be formally represented according to a simple sequential scheme, that we originally proposed for the Fast SCR mechanism only. We showed by transient kinetic experiments [17] that at temperatures as low as 150-170 °C the Fast SCR reaction proceeds via a two-step sequence in which ammonium nitrate is first formed from NO2 and NH3, reaction (9.14), and then reacts with NO, reaction (9.15) ... 

The best compromise between activity and selectivity is obtained with the PGM/ SCR mechanical mixmre complete NH3 conversion was indeed obtained already at 275 °C, with N2 concentration above lOOppm over the whole 150-550 °C T-range. Correspondingly, lower NOx and N2O yields compared with the PGM-only catalyst and the double bed configuration were measured between 300 and 500 °C. 

To gather information for the role of O2 in the H2-SCR mechanism, similar SSITKA experiments with the use of 02 were conducted at 140 °C. Figure 26.8 presents the transient response curves of N2 0, N2 0, and Ar obtained on Pt/Lao.sCeo.sMnOs (Figure 26.8a) and Pt/Si02 (Figure 26.8b) catalysts after the switch N0/H2/ 02/Ar/He N0/H2/ 02/He was made at 140 °C. As seen in Figure 26.8, the concentration of N2 0 produced by both catalysts is reduced after the isotopic switch, whereas the continuous evolution of N2 0 is noticed. The sum of the steady-state concentrations of N2 0 and N2 0 formed under H2-SCR in the isotopic gas mixture is the same as the steady-state concentration... 

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