Effect of the NH4NO3 Addition on the Low-T NH3-SCR Performances of Individual and Combined Fe- and Cu-Zeolite Catalysts

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.


Introduction
The current well-established solution for the NOx emission control on Diesel and/or natural gas lean-burn engines is represented by the NH 3 selective catalytic reduction (NH 3 SCR) [1,2]. The NOx abatement occurs over state-of-the-art catalysts, such as Fe-, Cu-zeolite and Vbased catalysts [2][3][4][5][6][7][8][9][10][11][12][13], by means of the injection of urea, which, once decomposed to NH 3 , can react with NOx producing harmless N 2 and H 2 O, according to the main SCR reactions 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 NO x as well as exhaust gases with increasingly lower temperatures, aggravating the task of the SCR converter. A way to maximize the NOx conversion to N 2 in the mobile applications is by increasing the NO 2 /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 [2]. The NO 2 /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 NO 2 in the exhausts allows the improvement of the deNOx efficiency of the SCR systems in the low T range, especially when the NO 2 /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 NO 2 /NO feed ratio cannot be guaranteed for all the possible engine operating conditions [14].
In the last years, another catalytic route was proposed to overcome the issues connected to the presence of a proper amount of NO 2 in the gas mixture necessary to boost the typical poor deNOx activity of Fe-zeolite or V-based catalysts in the low T range. This involves the addition of an aqueous solution of ammonium nitrate (NH 4 NO 3 ) to a NO-NH 3 containing gas mixture, which determines the occurrence of the so-called Enhanced SCR reaction (3) The E-SCR reaction was previously studied on Fe-zeolite and V-based catalysts [14][15][16][17][18] and it was demonstrated to be associated with superior NO reduction efficiencies in the 200-350°C T range with respect to the Standard SCR reaction, allowing to approach the desired higher Fast SCR activity. It is worth emphasizing that this solution does not require the oxidation of NO to NO 2 upstream of the SCR converter since this oxidation is directly entrusted to the ammonium nitrate itself according to the reaction (4) The produced NO 2 is, then, able to react further with the remaining NO and with NH 3 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) [17]. 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 NO 2 [17].
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 [14].
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 NO 2 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.

Experimental
Steady-state Standard SCR, Fast SCR and Enhanced SCR catalytic activity tests were performed over two NH 3 -SCR catalysts, namely a thermally stable Cu-SAPO and a Fe-BEA, supplied in the shape of coated monoliths by Dinex Finland [19]. 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/m 2 ). The dimensions of the tested sample were L = 20 mm, D = 12.5 mm, V = 2453 mm 3 . 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 mm 3 . 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 Fesample. The molar Si/Al 2 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 [19].
The experimental equipment used to carry out this work was described in our previous work [2]. 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: NH 3 = 500 ppm, NO x = 500 ppm (NO 2 /NO x = 0-0.5), O 2 = 5% (v/v), H 2 O = 5% (v/v) and balance N 2 . Under Enhanced SCR conditions an aqueous solution of NH 4 NO 3 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 + H 2 O 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 N 2 and AN) were continuously monitored at the reactor outlet by a FT-IR gas analyser (Bruker MATRIX MG5).

Enhanced SCR on Fe-BEA
In our previous work [2], 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 Cuzeolite 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.
An alternative way to improve the low-T deNOx efficiency of Fe-zeolite systems, however, was already proposed and discussed in the literature [14][15][16][17][18]: the addition of AN to the feed stream, indeed, was demonstrated to increase significantly the low-T Standard SCR NOx conversions over Fe-zeolite catalysts. Figure 1 compares the NOx steady-state conversions collected under Standard SCR conditions (GHSV = 75,000 h −1 , NH 3 = 500 ppm, NO = 500 ppm, H 2 O = 5% v/v, O 2 = 5% v/v) over the Fe-BEA and Cu-SAPO samples and their sequential combination (Fe + Cu) (already shown in [2]) with the NOx conversions achieved on the Fe-BEA sample under the same Standard SCR conditions when 200 ppm of NH 4 NO 3 were added to the gas feed mixture (Enhanced SCR). Figure 1 confirms that the AN dramatically boosts the NOx conversion over the Fe-zeolite at temperatures below 300°C, matching the well-known outstanding Standard SCR activity of the Cu-zeolite catalyst, or even overcoming it in the intermediate temperature range. Moreover, the Enhanced SCR activity of the Fe-zeolite sample outperformed the Standard SCR activity of the combined Fe-+ Cu-zeolite system (Fig. 1). Figure 2 directly demonstrates the ammonium nitrate addition effect on the Fe-zeolite sample by comparing the steady-state NOx conversions and N 2 O productions measured under Fast SCR, Standard SCR and Enhanced SCR conditions. As previously noted, the Enhanced SCR reaction resulted in a significantly higher deNO x activity at low temperatures with respect to the Standard SCR reaction, approaching the activity of Fast SCR (Fig. 2a). In these tests, the adopted AN feed concentration was 200 ppm. As demonstrated by Marchitti et al. [17], the ammonium nitrate is able to oxidize NO to NO 2 , according to reaction (4). Then the co-presence of NO, NO 2 and NH 3 allows the occurrence of the Fast SCR reaction. Considering that all the ammonium nitrate injected in our system oxidized NO to NO 2 , the resulting NO 2 /NO x ratio was 2/5, a little lower than the ratio used under Fast SCR conditions (1/2), consistent with our E-SCR experiments remaining below the Fast SCR activity.
Even if the AN dosing is able to improve the NOx conversions, the N 2 O formation is a delicate aspect that needs to be controlled, being a strongly undesired by-product due to its high greenhouse potential. In fact, ammonium nitrate could be involved in other side reactions which are not advantageous for the deNOx performances but, on the contrary, can contribute to the emission of more pollutants, losing that benefit obtained by injecting nitrate species for the reduction of NOx. The ammonium nitrate, in fact, can decompose according to the following reactions [18]: Reactions (7) and (8) determine the production of NH 3 and NO 2 . In our systems, we observed a negligible formation of NO 2 , which suggests that the nitrate ad-species possibly formed by reaction (7) directly participate in SCR reactions [18]. On the other hand, traces of N 2 O were detected, which are shown in Fig. 2b and compared with the amount of N 2 O emitted under Standard SCR, Fast SCR conditions in the case of Fe-BEA. Figure 2c shows the N 2 selectivities computed as [1-2*N 2 Oproduction/(NH 3 + NO+NH 4 NO 3 )consumption], i.e. assuming that N 2 O is the only side product of the SCR process. Even though the Standard SCR reaction is characterized by the worst activity performance, the NO-NH 3 reacting system shows the highest N 2 selectivity at any investigated temperature. The Fast and the E-SCR reactions, instead, contribute to a significant N 2 O 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).

Enhanced SCR on Cu-SAPO
The Enhanced SCR reaction was carried out for the first time on a Cu-zeolite catalyst, specifically a Cu-SAPO sample, and compared with the data collected under Standard and Fast SCR conditions. The steady-state NOx conversions and N 2 O productions observed for the three reactions are compared in Fig. 3. For the Cu-SAPO catalyst, the situation was reversed with respect to Fe-BEA: at the lowest investigated temperature, the AN dosing worsened the good deNOx efficiency achieved both under Standard and Fast SCR conditions. The negative impact of the addition of the AN aqueous solution could be ascribed to the porous structure of the zeolite itself. It is well known that the SAPO chabazite framework is associated with pores with a smaller size compared, for example, to those of the BEA zeolite [20]. At low temperatures (≤ about 200°C), the ammonium nitrate salt can be trapped within the small pores of the zeolite, covering the active sites and thus inhibiting the SCR activity. Indeed, on increasing the temperature the activity was recovered, consistent with the fact that the ammonium nitrate was thermally decomposed according to reaction (9) releasing large amounts of N 2 O, as shown in Fig. 3b. Accordingly, the N 2 selectivity under E-SCR conditions remained lower than under Standard and Fast SCR conditions (Fig. 3c), confirming a negative impact of the NH 4 NO 3 addition on the deNOx performance of the Cu-SAPO catalyst.

Enhanced SCR Fe-BEA + Cu-SAPO: Effect of the Sequential Arrangement
We verified and shown in Fig. 1 that the addition of a Cu-SAPO catalyst after the Fe-BEA monolith can improve the NOx removal efficiency over that of the Fe-zeolite alone. However, since in the low-T range, the combined system exhibited lower NOx conversions under Std SCR conditions with respect to those achieved on the only Fe-zeolite catalyst under Enhanced SCR conditions; the very good activity of the combined system could be, theoretically, further improved by dosing ammonium nitrate and thus running the Enhanced SCR reaction over it. For the Fe-+Cu-zeolite sequential system, Fig. 4a compares the Standard SCR NOx conversions, already seen in Fig. 1, with those collected upon addition of 200 ppm of NH 4 NO 3 to the Std SCR conditions (E-SCR) and those obtained under the Fast SCR conditions. This plot proves that the combination of these two strategies, namely the AN boosting effect and the combination of Fe and Cu catalysts, can enable outstanding deNOx performance, already at 200°C, even better than those obtained on the only Fe catalyst (at 200°C the NOx conversions is about 80% against the 86% of Fe-+Cu). In addition, this combined catalytic system is beneficial also in terms of N 2 O production: Fig. 4b illustrates, as seen for the Fe-zeolite catalyst, that E-SCR and Fast SCR reactions are characterized by similar N 2 O formations and, consequently, similar N 2 selectivities (Fig. 4c). However, the measured N 2 O concentration was always lower with respect to those detected on the only Fe-zeolite samples, confirming, also for this aspect, that the sequential arrangement is the best choice towards this application.
In our previous work [2], we checked also that the correct positioning of the Fe and Cu samples in the sequential arrangement is crucial in order to generate a synergy between the two catalysts. Under Standard SCR conditions, we observed a loss of selectivity when the Cu catalyst was placed prior to the Fe-sample, while in the low-T region, we observed similar behaviours. Consequently, our investigation was focused on the Fe-+ Cu-zeolite combined arrangement. In this work, we have studied the effect of the catalyst order on the Enhanced SCR performances, too. Figure 5 illustrates the sequential arrangement effect on the deNOx performances under the Enhanced SCR conditions. Clearly, when the Cuzeolite was placed upstream the beneficial effect of the ammonium nitrate addition was lost, the NOx conversions (Fig. 5a) being even worse than those observed under Standard SCR conditions (no AN) on the Cu catalyst alone (Fig. 1), on the Fe-+ Cu samples (Fig. 1) and on Cu-+Fesamples ( Fig. 1 in [2]). Moreover, even if in the low-T range the N 2 O production on the Cu + Fe-sample is lower than that on the dual sample with reverse order (Fig. 5b), the two sequential combinations are similar in terms of N 2 selectivity (99% vs. 98%) (Fig. 5c). In the high-T region, instead, the Fe-+Cu-sample overcomes the other in terms of N 2 selectivity and minimum N 2 O formation. 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 [2]: 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 NH 3 were consumed in the upstream Cuzeolite 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.

Conclusions
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 NH 3 -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 NO 2 enabled by the ammonium nitrate, reaction (4). Once NO 2 is formed, the Fast SCR reaction can take place thanks to the co-presence of NO, NO 2 and NH 3. 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 N 2 O due to the unselective thermal decomposition of NH 4 NO 3 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 NO 2 in the feed stream.
Funding Information 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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