Side reactions in the selective catalytic reduction of NOx with various NO2 fractions

The main and side reactions of the three selective catalytic reduction (SCR) reactions with ammonia over TiO2-WO3-V2O5 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 NO2, and NO2 SCR with pure NO2- 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. NO2 fractions exceeding 50% enhance the formation of nitrous oxide at low temperatures. If the feed of NO, consists of pure NO2, the formation of nitrous oxide may occur by two different reactions having different temperature regimes. The reaction responsible for N2O formation at low temperatures probably involves ammonium nitrate or nitroamine as an intermediate species.