Periodic NH3 Supply

As the composition and the concentration of diesel exhaust gases vary significantly depending on the engine operation, it is quite important to understand the transient behavior of NOx reduction following NH3 supply and shutoff. In this section, the effect of the periodical supply of NH3 on the NOx reduction behavior is presented. 

The same observation holds true for the Cu/ZSM-5 catalyst (Fig. 8.14b) below 250 °C. However, at 400 °C the periodic conversion is less than the steady-state one. The reason for this lower activity for the periodic condition is probably due to the decrease of the reaction selectivity between NO and NH3 [34, 35]. As shown in Fig. 8.1b, Cu/ZSM-5 exhibits higher activity for NH3 oxidation than Fe/ZSM-5 and V-W/Ti02. Thus, it can be assumed that a part of the adsorbed NH3 is consumed by O2 under NH3 off condition, leading to the lower activity for the periodic measurement. 

Meanwhile, the periodic conversions with V-W/Ti02 were roughly in accordance with the steady-state conversions at the low temperatures ( 250 °C). However, in the high temperature region ( 300 °C), the periodic conversions greatly decreased despite the fact that the steady-state conversions monotonicaUy increased with the temperature. This is because the NH3 adsorption capacity 

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Figure 8.16b shows the N2 production rates when the presence and absence of NH3 is repeated at the interval of three pulses. The N2 production rates in the first NH3 absent region (pulses 4 and 10) were higher than those in the NH3 present regions this suggests that the promotional effect of the absence of NH3 is repeatable. This is in conformity with the behavior observed during the periodical NH3 supply test as shown in Fig. 8.13a. 

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