The low-temperature behavior of the selective catalytic reduction (SCR) process with feed gases
containing both NO and NO2 was investigated. The two main reactions are 4NH3 + 2NO +
2NO2 → 4N2 + 6H2O and 2NH3 + 2NO2 → NH4NO3 + N2 + H2O. The “fast SCR reaction” exhibits
a reaction rate at least 10 times higher than that of the well-known standard SCR reaction
with pure NO and dominates at temperatures above 200 °C. At lower temperatures, the
“ammonium nitrate route” becomes increasingly important. Under extreme conditions, e.g., a
powder catalyst at T ≈ 140 °C, the ammonium nitrate route may be responsible for the whole
NO
x
conversion observed. This reaction leads to the formation of ammonium nitrate within the
pores of the catalyst and a temporary deactivation. For a typical monolithic sample, the lower
threshold temperature at which no degradation of catalyst activity with time is observed is around
180 °C. The ammonium nitrate route is interesting from a standpoint of general DeNO
x
mechanisms: This reaction combines the features typical to the SCR catalyst with the features
of the NO
x
storage−reduction catalyst, i.e., NO
x
adsorption to a basic site.
An energetic analysis of the thermal decomposition of solid urea and urea solutions is presented,
and the results are discussed in view of urea selective catalytic reduction (SCR) for automotive
DeNOx systems. Various types of decomposition reactors are possible which differ in their
effectiveness to produce ammonia from urea. For reasons of simplicity, the decomposition is
usually performed by atomizing urea solutions directly into the hot exhaust. However, this
technique suffers from short residence times, leading to incomplete decomposition into ammonia
and isocyanic acid and causing a significant performance loss of the SCR catalyst. The thermal
decomposition out of the main exhaust stream allows much increased residence times for the
process of urea decomposition. A reactor utilizing a partial stream of the exhaust seems
particularly promising, especially if such a reactor includes a hydrolyzing catalyst, leading to
ammonia practically free from isocyanic acid.
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