A. Grossale et al. / Journal of Catalysis 265 (2009) 141–147
147
NH4NO3 þ NO ! NO2 þ N2 þ 2H2O
and its partial new formation from NO2 and ammonia, until its glo-
bal depletion.
ð12Þ
gas-phase ammonia concentration. As the blocking effect is strictly
related to the acid properties of the formed nitrates, another pos-
sibility to moderate its negative impact on the Fast SCR reactivity
at low T would be to modify the catalyst acid/base properties in or-
der to favor the interaction between ammonia and the catalyst
sites.
In any case, it is worth noticing that reaction (12), not occurring
at steady state in the presence of ammonia–NO/NO2 mixtures, is
restored upon ammonia removal from the feed flow. This is in line
with the previously discussed ammonia blocking effect. Indeed,
shut off of NH3 shifts the equilibrium of the ammonium nitrate dis-
sociation reaction (6) to the RHS.
Similar results were obtained at 170 °C (Fig. 5B). At this temper-
ature both the Fast SCR reaction and the ammonium nitrate forma-
tion reaction take place in the presence of ammonia, NO, and NO2.
In this case in fact the Fast SCR reaction is more active than at
150 °C, as quantified by the NO consumption at the end of the tran-
sient phase (50 ppm at 170 °C vs. 0 ppm at 150 °C).
Acknowledgments
The authors gratefully acknowledge financial support from
Daimler AG, useful discussions with Dr. Daniel Chatterjee, Dr. Mi-
chel Weibel, and Dr. Bernd Krutzsch, Daimler, and the assistance
of Dr. Luca Zampori for catalyst characterization work.
References
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Experiments showed that one way to partially avoid this unde-
sired effect is to modify the equilibrium of ammonium nitrate dis-
sociation, e.g. by increasing the temperature or by decreasing the