Global Kinetic Models for SCR Over Cu-Zeolites

We developed a global kinetic model for ammonia SCR over Cu-ZSM-5 [10], consisting of seven reaction steps  

The first step is ammonia adsorption/desorption and this step was tuned to an ammonia TPD experiment described in an earlier section (Eq. 12.21, see Fig. 12.4). The second step is ammonia oxidation (Eq. 12.22) and the third step NO oxidation (Eq. 12.23). The parameters for these three steps were determined by separate experiments. In Eq. (12.24), the reaction for standard SCR is described and in Eq. (12.25), the rapid SCR with equimolar amounts of NO and NO2 is covered. NH3 also reacts with NO2 alone, which is described in Eq. (12.26). The final step (see Eq. 12.27) is N2O production from a reaction between adsorbed ammonia and NO2. The results from exposing the Cu-ZSM-5 catalyst to 500 ppm NO, 500 ppm NH3, 8 % O2 and 5 % H2O, while increasing the temperature stepwise are shown in Fig. 12.14 [10]. Initially, there is a total uptake of ammonia for an extended time period, but due to minor NO storage, NO breaks through immediately. The temperature is then increased to about 150 °C, while loosely bound ammonia is released. At this temperature, the standard SCR reaction is active and equimolar consumption of NO and NH3 is seen. At 200 °C, the conversion of NOx is close to 100 %. At high temperature, the NOx conversion again decreases due to the ammonia oxidation. Also, some NO2 is produced, since the back part of the catalyst is not exposed to ammonia, NO oxidation may occur. The global model described 

Nova et al. [37] presented a global kinetic model for ammonia SCR over a Cu-zeolite catalyst. They used the same seven reactions as presented above (Eqs. 12.21-12.27). In addition, they added two more reactions. In the first reaction (Eq. 12.28), N2O is formed from a reaction between ammonia on the surface (denoted NH3) and NO and in the second reaction, ammonium nitrate is formed 

8 % O2, 5 % H2O [37], Reprinted with permission from Nova et al. [37]. Copyright (2011) American Chemical Society 

Pant and Schmieg [7] used the model developed by Olsson et al. [10] (see Eqs. 12.21-12.27) with one exception. Instead of ammonia oxidation to N2, as described in Eq. (12.22), ammonia oxidation to NO was used  

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