Fe- or Cu-Exchanged Zeolite Catalysts

For the SCR mechanism, Delahay et al. [13, 75] proposed a Fe /Fe (or Cu / Cu ) redox cycle as a part of the SCR scheme the extra-framework oxygen on Fe (Cu ) oxo/hydroxo species reacts with NO to form a surface nitrogen oxide intermediate (Fe -N iOy or Cu -N cOy). Then, this species reacts with NH3 to form N2 and H2O with concomitant reduction of Fe (Cu ) to Fe (Cu ) species. The Fe (Cu ) species is reoxidized back to Fe (Cu ) oxo/hydroxo species by O2 [13, 75]. 

By comparing the SCR scheme (Fig. 8.21) with NO oxidation scheme (Fig. 8.7), one can find similarities and differences. First of all, both reactions seem to progress via similar redox cycles of the active metal. However, judging from the reaction kinetic analysis discussed in Sects. 8.2.3 and 8.2.4, the ratedetermining step of NO oxidation would be NO2 desorption process. As for the SCR reaction, on the other hand, the process prior to the formation of NO2 related adspecies (e.g., nitrite or nitrate intermediate) would be the rate-determining step, as is discussed in Sect. 8.3.3. Therefore, the slowest step in the redox cycles would be different in the two reactions. 

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 

Komatsu T, Nunokawa M, Moon IS, Takahara T, Namba S, Yashima T (1994) J Catal 148 427 37. 

Brandenberger S, Krocher O, Tissler A, Althoff R (2010) Appl Catal B Environ 95 348-357. 

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