Influence of NO2 on the selective catalytic reduction of NO with ammonia over Fe-ZSM5
Introduction
During the last years, Fe-ZSM5 has been discovered to be an efficient catalyst for the selective catalytic reduction of nitrogen oxides in the exhaust of diesel vehicles by urea (urea-SCR) besides the established V2O5–WO3/TiO2 catalysts [1], [2], [3]. For both types of catalysts the reaction obeys the “standard-SCR” stoichiometry, involving equal amounts of NO and ammonia—the latter being the actual reducing agent, which is released from urea under the hydrothermal conditions in the exhaust pipe:4NH3 + 4NO + O2 → 4N2 + 6H2O
The main challenge for the application of Fe-ZSM5 in the urea-SCR process on board of diesel vehicles is to achieve high SCR activity and selectivity over a broad temperature range (150–700 °C). One possibility is the addition of NO2 to the feed, whose promoting effect on the SCR reaction was already observed in the 1980s [4], [5]. Koebel et al., who investigated the influence of NO2 on the SCR reaction over V2O5–WiO3/TiO2 in detail, pointed out the importance of having a 1:1 mixture of NO2 and NO for reaching the maximum SCR activity [6], [7]. The high rates at this NO2 to NO ratio are explained by SCR reaction (2), which is much faster than reaction (1), therefore called “fast-SCR”:4NH3 + 2NO + 2NO2 → 4N2 + 6H2O
If the NO2:NO ratio is larger than 1:1 NO2 reacts also via the “NO2-SCR” route (3), which is even slower than the “standard-SCR” reaction over vanadia-based catalysts:4NH3 + 3NO2 → 3.5N2 + 6H2O
Very recently, the promoting effect of NO2 was also reported for zeolite-based SCR catalysts by Tolonen et al., who performed a screening of metal-exchanged zeolites [8]. They added NO2 and NO to the feed in the ratio 0.4:0.6, which is nearly ideal for the “fast-SCR” stoichiometry, and found an increase in NOx removal efficiency at lower temperatures compared to 100% NO in feed.
The addition of NO2 to the feed implies also some disadvantages. The formation of N2O was observed over Fe-ZSM5 in the presence of NO2 [8] and over Cu-ZSM5 N2O is formed even without NO2 in the feed. On the basis of first performance tests the formation of N2O over Fe-ZSM5 was generally attributed to ammonia oxidation [8]. Based on a more detailed investigation, Long and Yang tried to explain the elevated formation of N2O by the reaction of NO2 with the postulated intermediate species NO2(NH4+)2 [9]. Despite this plausible explanation, many more reactions are conceivable to produce N2O that have mainly been discussed for vanadia-based SCR catalysts [10], but cannot be ruled out to take part on Fe-ZSM5 and other metal-exchanged catalysts. In the following, these reactions are summarized as a basis on which the observations in the present work are made.
The most important source of N2O over vanadia-based SCR catalyst is the selective catalytic oxidation (SCO) of ammonia by oxygen (reaction (4)):4NH3 + 4O2 → 2N2O + 6H2O
At lower temperatures up to 350 °C, NO2 which is a much stronger oxidant than O2 is expected to contribute to the oxidation of ammonia to N2O also by the following reactions:6NH3 + 8NO2 → 7N2O + 9H2O4NH3 + 4NO2 + O2 → 4N2O + 6H2O
Due to the thermodynamic equilibrium:NO + 1/2O2 ⇔ NO2considerable amounts of NO are formed at T > 350 °C and low space velocities, facilitating the formation of N2O by reaction (8), even if only NO2 is present in the feed gas:4NH3 + 4NO + 3O2 → 4N2O + 6H2O
In the very low temperature region below 200 °C the formation of N2O is explained by the decomposition of NH4NO3 as an intermediate complex:NH4NO3 → N2O + 2H2O
NH4NO3 itself is formed from NH3 and N2O4, which is the stable dimerization product of NO2:N2O4 + 2NH3 + H2O → NH4NO3 + NH4NO2
The simultaneously formed ammonium nitrite does not play an important role, as it is readily decomposed to nitrogen and water at T > 60 °C (reaction (11)):NH4NO2 → N2 + 2H2O
The aim of the present paper is to describe in detail the influence of NO2 on the SCR reaction over Fe-ZSM5 coated on a cordierite monolith, also comprising investigations on the formation of N2O at lower and intermediate temperatures and its depletion at higher temperatures due to the N2O decomposition and N2O-SCR reaction.
Section snippets
Experimental
Fe-ZSM5 coated on a cordierite monolith was provided by Umicore automotive catalysts, Germany. The catalytic tests were carried out in the laboratory test apparatus described in [10]. The flow rates were controlled by mass flow controllers (Brooks 5850S). Liquid water was dosed by means of a liquid mass flow controller (Brooks 5881) through a capillary tube into an electrically heated evaporator. All lines of the experimental apparatus were heated to 150 °C by heating tapes. The concentrations
NH3-SCR
Fig. 1a shows the NOx removal efficiency (DeNOx) of the Fe-ZSM5 monolith catalyst for varying NO:NO2 ratios in the feed at 10 ppm ammonia slip. Details about this way to represent the data are given in [11], [12]. DeNOx increased drastically with the addition of NO2 to the feed over the entire temperature range, most pronounced at lower and intermediate temperatures. Regarding the entire temperature range, NO2/NOx = 50% resulted in the best DeNOx performance. However, it has to be noticed that the
NH3-SCR
From the DeNOx measurements it is obvious that NO2 is involved in the mechanism of the SCR reaction. In the case of “standard-SCR” (reaction (1)), where no NO2 is dosed, the reaction is able to proceed due to the capability of Fe-ZSM5 to oxidize NO to NO2 [12]. When NO was dosed over Fe-ZSM5, first the NO2 fraction increased with temperature but decreased again above ∼350 °C due to the thermodynamic equilibrium between NO and NO2, which shifts to the NO side for higher temperatures [12]. The
Conclusions
By the present study it was shown that the SCR activity of Fe-ZSM5 is tremendously improved by the addition of NO2 to the feed gas. The maximum DeNOx was found for NO2/NOx = 50%, but, surprisingly, higher NO2 fractions resulted in similar high DeNOx values. This behaviour is explained by the assumption of [NH4+]2NO2 as intermediate complex species, which is formed fast from NH3, NO and NO2 in the optimum ratio 2:1:1 and reacts to the final product nitrogen. Higher NO2 fractions are also easily
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