Effect of the NH4NO3 Addition on the Low-T NH3-SCR Performances of Individual and Combined Fe- and Cu-Zeolite Catalysts
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We have measured NOx conversions and N2O productions over Fe-BEA and Cu-SAPO catalysts and over their sequential arrangements under Enhanced SCR conditions, resulting from the addition of an aqueous solution of ammonium nitrate (AN) to the typical Standard SCR feed stream, and we have compared them to those observed under Standard and Fast SCR conditions. The expected strong enhancement of the poor low temperature activity of the Fe-BEA catalyst was confirmed: both NH3 and NOx conversions and N2O formations similar to those of the Fast SCR reaction were achieved when cofeeding ammonium nitrate. On the other hand, the Cu-SAPO efficiency was drastically decreased by the addition of AN at low temperatures, possibly due to trapping of the ammonium nitrate salt within the SAPO zeolite, characterized by smaller pores than those of the BEA zeolite. The Cu-SAPO performances were recovered only at T > 250 °C with a huge release of N2O due to the thermal decomposition of AN. The combined system with the Fe-zeolite sample placed upstream of the Cu-zeolite also exhibited outstanding low temperature deNOx performances, with even lower N2O production than over the Fe-zeolite only at the same Enhanced SCR (E-SCR) conditions.
KeywordsNH4NO3 Enhanced SCR Low-T deNOx activity NH3-SCR Cu-zeolite Fe-zeolite
The deNOx performance of the SCR systems is strictly connected to the temperature of the exhaust gases. One big challenge is the improvement of the efficiency when the temperatures are too low for the SCR catalysts to be active and for the urea to effectively decompose. Moreover, the development of more efficient Diesel engines is leading to operating conditions with high contents of oxygen. Thus, such increasingly efficient engines produce higher amounts of NOx as well as exhaust gases with increasingly lower temperatures, aggravating the task of the SCR converter. A way to maximize the NOx conversion to N2 in the mobile applications is by increasing the NO2/NO molar feed ratio in order to promote the Fast SCR reaction (2). Since the NOx gases, produced during the combustion, are composed mainly by NO (95%), the Standard SCR reaction (1) is the main reaction involved in the NOx control, which is unfortunately characterized by a significantly lower efficiency than the Fast SCR (2) in the low temperature range . The NO2/NO molar feed ratio can be modified in the catalytic oxidation step mainly dedicated to the abetment of unburned hydrocarbon and CO, the Diesel Oxidation Catalyst (DOC) or/and Methane Oxidation Catalyst (MOC). The presence of NO2 in the exhausts allows the improvement of the deNOx efficiency of the SCR systems in the low T range, especially when the NO2/NO molar ratio is 1/1 and the Fast SCR can take place (2). However, the oxidation activity of the DOC/MOC is strongly dependent on temperature and flow rate of the exhaust gases and, consequently, the optimal NO2/NO feed ratio cannot be guaranteed for all the possible engine operating conditions .
The produced NO2 is, then, able to react further with the remaining NO and with NH3 in accordance with the Fast SCR Reaction (5): the Enhanced SCR reaction (6), therefore, is the sequential combination of these two catalytic steps: (4) + (2) . The Enhanced SCR and the Fast SCR are characterized by the same reaction mechanism, which differs only in their first step, existing a direct equivalence between AN and NO2 .
The ammonium nitrate feed can be obtained by injecting or vaporizing an aqueous solution of ammonium nitrate upstream of the SCR converter, together with the urea solution: indeed, single aqueous solutions containing both the reducing agent (urea) and the oxidizing additive (ammonium nitrate) are commercialized and available for this purpose .
This work aims at brushing up this concept since it gives interesting perspectives for the enhancement of the critical low temperature deNOx activity without necessarily relying on the generation of NO2 in the DOC/MOC. Since it was demonstrated to be a concept applicable to two classes of commercial SCR catalysts (V-based systems and Fe-exchanged zeolites), here we complete the study by investigating also the deNOx performances of a Cu-promoted zeolite catalyst and of combined Fe- and Cu-zeolite systems under Enhanced SCR conditions, eventually comparing them with the Standard and Fast SCR data.
Steady-state Standard SCR, Fast SCR and Enhanced SCR catalytic activity tests were performed over two NH3-SCR catalysts, namely a thermally stable Cu-SAPO and a Fe-BEA, supplied in the shape of coated monoliths by Dinex Finland . The Fe-BEA catalyst was coated onto a cylindrical rolled metallic substrate made of flat and corrugated thin (50 μm) AlCr foils (cell density of 600 cpsi). The coating load was about 150 g/L (40 g/m2). The dimensions of the tested sample were L = 20 mm, D = 12.5 mm, V = 2453 mm3. The Cu-SAPO catalyst was tested in the form of a coated ceramic honeycomb (400 cpsi). The coating load was about 140 g/L. The dimensions of the tested sample were L = 41.8 mm, h = 7.7 mm, w = 7.7 mm, V = 2478 mm3. The combined systems were realized putting both catalysts in series, so that 50% of the total catalyst volume was composed by the Cu-sample and 50% by the Fe-sample. The molar Si/Al2 ratio in the zeolites was in the range of 25–40 to maintain a good hydrothermal stability. A small amount (< 15wt% of final coating) of binder was added in the coating process. The catalysts were hydrothermally (HT) aged at 700 °C for 20 h in air flow with 10% of water [2, 19]. Data on the sulphur resistance of both catalysts are also given in .
The experimental equipment used to carry out this work was described in our previous work . Steady-state runs were performed in order to investigate the catalytic activities in a wide T range (150–550 °C), using defined reactant feed concentrations and temperature steps. The feed composition was representative of real after treatment systems: NH3 = 500 ppm, NOx = 500 ppm (NO2/NOx = 0–0.5), O2 = 5% (v/v), H2O = 5% (v/v) and balance N2. Under Enhanced SCR conditions an aqueous solution of NH4NO3 was fed to the reactor: the ammonium nitrate feed concentration level of 200 ppm was achieved by properly diluting a master 2.5 M solution. Either liquid water or the AN + H2O solution was metered by a volumetric piston pump (Gilson model 305): the feed rate was around 0.025 ml/min +/− 0.0001 for GHSV = 75,000 h−1. Afterwards, the liquid feed was vaporized in a hot pipeline kept at 190 °C, and then mixed with the other gaseous species and fed to the reactor. All the gaseous species (except N2 and AN) were continuously monitored at the reactor outlet by a FT-IR gas analyser (Bruker MATRIX MG5).
3.1 Enhanced SCR on Fe-BEA
In our previous work , we reported the investigation of two commercial SCR catalysts, a Fe-BEA and Cu-SAPO, under typical exhaust aftertreatment conditions for natural gas or duel fuel vehicles. Moreover, we demonstrated that the sequential arrangement of the two Fe- and Cu- monolith catalysts allowed to overcome the issues connected to the use of only one of these metal-promoted zeolite catalysts under Standard SCR conditions, namely the poor low-T deNOx activity typical of Fe-zeolite catalysts and the lower selectivity in the high-T range of Cu-zeolites. Their combination (Fe followed by Cu), indeed, permitted to mitigate these drawbacks keeping only the advantages of the two catalytic systems, i.e. the outstanding low-T deNOx efficiency of the Cu-zeolite and the high-T selectivity and activity of the Fe-zeolite. The Cu-zeolite, in fact, placed after the Fe-zeolite, compensated the poor Fe- catalyst activity at low temperature, thus achieving greater overall deNOx performances under Standard SCR conditions.
Reactions (7) and (8) determine the production of NH3 and NO2. In our systems, we observed a negligible formation of NO2, which suggests that the nitrate ad-species possibly formed by reaction (7) directly participate in SCR reactions . On the other hand, traces of N2O were detected, which are shown in Fig. 2b and compared with the amount of N2O emitted under Standard SCR, Fast SCR conditions in the case of Fe-BEA. Figure 2c shows the N2 selectivities computed as [1–2*N2Oproduction/(NH3 + NO+NH4NO3)consumption], i.e. assuming that N2O is the only side product of the SCR process. Even though the Standard SCR reaction is characterized by the worst activity performance, the NO-NH3 reacting system shows the highest N2 selectivity at any investigated temperature. The Fast and the E-SCR reactions, instead, contribute to a significant N2O production (Fig. 2b). Moreover, they show also the same selectivity up to 450 °C, while at higher temperature the E-SCR reaction recovers the Standard SCR selectivity (Fig. 2c).
3.2 Enhanced SCR on Cu-SAPO
Accordingly, the N2 selectivity under E-SCR conditions remained lower than under Standard and Fast SCR conditions (Fig. 3c), confirming a negative impact of the NH4NO3 addition on the deNOx performance of the Cu-SAPO catalyst.
3.3 Enhanced SCR Fe-BEA + Cu-SAPO: Effect of the Sequential Arrangement
Such a negative impact of the AN dosing observed on Cu-SAPO in the previous section is in turn reflected on the sequential configuration in which the Cu-zeolite was placed before the Fe-zeolite (Fig. 5). Under Standard SCR conditions, the two configurations worked in a similar way in the low-T range : in both cases, in fact, most of the reactants were converted over the Cu-zeolite catalyst. Indeed, when the Fe-BEA sample was placed upstream, being its activity lower than the following Cu-SAPO, the gases reacted in the second monolith sample. In the case of the reverse configuration, instead, NO and NH3 were consumed in the upstream Cu-zeolite section while the following Fe-zeolite was hardly utilized. In the E-SCR reaction, however, the order of the two samples plays instead an important role. Indeed, being the Fe-BEA activity significantly promoted by the AN addition, when placed upstream the Fe-zeolite was able to convert most of the reactants, obtaining the good performances shown in Fig. 1. When instead Cu-SAPO, which is adversely affected by AN, as observed in Fig. 3, was placed upstream, ammonium nitrate blocked the active sites at low temperature and in turn the consumption of the reactants moved to the second section, i.e. the Fe-BEA catalyst, which at those temperatures and without all the available ammonium nitrate feed showed its poor deNOx efficiency.
Accordingly, the E-SCR reaction represents a promising and easy method to improve the activity of metal-promoted zeolite catalysts which are characterized by a poor activity in the low-T range, as seen for the Fe-BEA sample. However, it is important to pay attention to the zeolite structure: the smaller the pore size of the zeolite framework, the worse the performances.
In this work, the cofeed of ammonium nitrate according to the Enhanced SCR reaction was demonstrated to be an effective method to improve the typically poor low-T activity of NH3-SCR Fe-zeolite catalysts. The dosage of an aqueous solution containing ammonium nitrate to Standard SCR conditions allowed to boost the overall NOx conversion via the NO oxidation to NO2 enabled by the ammonium nitrate, reaction (4). Once NO2 is formed, the Fast SCR reaction can take place thanks to the co-presence of NO, NO2 and NH3. On the other hand, the Enhanced SCR conditions had a negative impact on the Cu-SAPO catalyst: the addition of ammonium nitrate to a catalyst based on a small-pore zeolite determined a dramatic loss of activity in the low temperature region and a huge release of N2O due to the unselective thermal decomposition of NH4NO3 salt deposited within the zeolite structure. On the other hand, the sequential arrangement of a Fe-BEA catalyst followed by the Cu-SAPO sample under the E-SCR conditions showed outstanding performances, close to those corresponding to Fast SCR conditions even in the absence of NO2 in the feed stream.
The research leading to these results has received funding from the European Community’s Horizon 2020 Programme under grant agreement No. 653391 (HDGAS).
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