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.
The main and side reactions of the three selective catalytic reduction (SCR) reactions with ammonia over TiO 2 -WO 3 -V 2 O 5 catalysts have been investigated using synthetic gas mixtures matching the composition of diesel exhaust. The three SCR reactions are standard SCR with pure NO, fast SCR with an equimolar mixture of NO and NO 2 , and NO 2 SCR with pure NO 2 . At high temperatures the selective catalytic oxidation (SCO) of ammonia and the formation of nitrous oxide compete with the SCR reactions. Water strongly inhibits the SCO of ammonia and the formation of nitrous oxide, thus increasing the selectivity of the SCR reactions. However, water also inhibits SCR activity, most pronounced at low temperatures. NO 2 fractions exceeding 50% enhance the formation of nitrous oxide at low temperatures. If the feed of NO x consists of pure NO 2 , the formation of nitrous oxide may occur by two different reactions having different temperature regimes. The reaction responsible for N 2 O formation at low temperatures probably involves ammonium nitrate or nitroamine as an intermediate species.
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