The Effect of CH4 on NH3-SCR Over Metal-Promoted Zeolite Catalysts for Lean-Burn Natural Gas Vehicles
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We present a systematic investigation of the deNOx activity of two commercial metal exchanged zeolite NH3-SCR catalysts, a Cu-SAPO and a Fe-BEA, in view of their application to the exhaust after-treatment systems of lean-burn natural gas vehicles. The catalytic activity data collected under realistic operating conditions, representative of the after-treatment system of lean-burn vehicles, were compared to those obtained adding methane to the gas feed stream in order to assess the impact of this hydrocarbon, which is usually emitted from natural gas engines, on the NH3-SCR catalytic chemistry. Our results indicate a negligible impact of methane on the SCR activity at all conditions, but in the presence of a large excess of NO2 at T > 400 °C due to methane oxidation by NO2. The data collected over the two individual metal-promoted zeolites were also compared with those obtained combining both catalysts in sequential arrangements, in order to take advantage of their complementary high activities in different temperature ranges. The Fe-zeolite + Cu-zeolite sequence outperformed the two individual components in terms of both overall deNOx efficiency and N2O selectivity, and was equally insensitive to methane.
KeywordsNH3 SCR Natural gas vehicles Metal zeolite catalysts Methane oxidation
Currently, the abatement of gaseous polluting emissions from combustion processes has become one of the main challenges in the automotive field. The main pollutants in exhaust gases from vehicle engines include carbon dioxide, carbon monoxide, hydrocarbons, nitrogen oxides (NOx), sulfur dioxide and particulates. The use of natural gas (NG), in engines operating in lean conditions, or its blend with diesel can be a good way to produce lower emission compared to those produced by using regular diesel fuel. Indeed, lean operating natural gas fuelled engines have the potential to deliver low CO2 transportation solutions compared to diesel and dual fuel applications. Operating solely on natural gas provides the emissions control solution with its own unique set of challenges and opportunities. Simultaneous control of methane and oxides of nitrogen is required. In order to comply with the stricter and stricter emission standard limitations (e.g. Euro VI), the development of more advanced exhaust after-treatment technologies plays a key role. In order to purify the exhaust gases emitted by lean-burn natural gas vehicles and meet the current emission limitations, the typical after-treatment system has to comprise two main sections: the first one is dedicated to the oxidation of unburned methane over a dedicated catalyst (MOC—methane oxidation catalyst). The second one is devoted to the abatement of NOx emissions (SCR—selective catalytic reduction)  followed by an ammonia slip catalyst (ASC) which prevents the NH3 release caused by the limited SCR activity at low temperatures and during rapid changes in engine operation. Understanding the synergies and interactions of these catalyst systems is key to delivering highly efficient and durable after-treatment systems. Moreover, due to the nature of methane combustion and the fuel itself not containing carbon–carbon bonds, an exhaust particle filter is not required.
The MOC upstream of the SCR converter in the exhaust line ensures the presence of NO2 in the feed stream to the SCR converter, thus enabling improved deNOx efficiency in the low temperature region where the Fast SCR reaction (2) is by far more active [2, 11, 12, 13] than the Standard SCR reaction (1).
As mentioned before, one of the main issues related to the typical after-treatment system for lean-burn NG vehicles concerns the MOC. The long-term use of natural gas (< 10 ppm S content) as fuel in lean-burn engines can cause sulfur poisoning of the after-treatment system and the excess oxygen in the combustion chamber under lean operations leads to lower exhaust gas temperatures . The result is an incomplete conversion of methane in the exhaust, which would thus be present in the exhaust gases downstream of the MOC unit. In general terms, it is well known that the hydrocarbon slip can affect negatively the activity of the SCR catalysts [15, 16, 17]. Focusing on short chain hydrocarbons, only C3H6 seems to be able to modify the NH3-SCR catalytic activity. Heo et al.  showed how this hydrocarbon affects the SCR-deNOx activity of V2O5/TiO2, Cu-ZSM-5 and Fe-ZSM-5 based catalysts: at low temperatures, NH3 and C3H6 compete in the adsorption over catalyst surface, while at high temperatures NH3 is consumed by side reactions, which involve the hydrocarbon. On the other hand, it also known that light hydrocarbons can behave as reducing agents like ammonia and thus contribute to the NOx removal (Hydrocarbon SCR) [18, 19, 20, 21, 22, 23].
Very limited specific information is available about the two possible mechanisms through which CH4 can interact with NH3-SCR catalysts at the typical conditions of the after-treatment systems of lean-burn natural gas vehicles. To fill this gap, therefore, the present work aims to investigate the effect of CH4 on the activity of state-of-the-art NH3-SCR catalysts. To this end, a systematic catalytic activity study under typical NH3-SCR conditions both in presence and in absence of methane was performed over state-of-the-art Fe- and Cu-zeolite catalysts, focusing on the main reactions of the SCR system (Standard SCR, Fast SCR and NO2-SCR).
SCR runs were performed over two different NH3-SCR monolith catalysts (thermally stable Cu-SAPO and Fe-BEA), supplied by Dinex Ecocat . A small amount (< 15 wt% of the coating) of binder was mixed to the zeolites in coated catalysts. The Fe-BEA catalyst consisted of a cylindrical rolled metallic substrate whose flat and corrugated metal foils were coated with the catalyst layer (length = 20 mm, diameter = 12.5 mm, volume = 2453 mm3, coating = 0.42 g). Instead, the Cu-SAPO catalyst was tested in the form of a coated ceramic honeycomb (length: 41.8 mm; height: 7.7 mm; width: 7.7 mm, volume = 2478 mm3, coating = 0.283 g). The combined systems were realized putting both catalysts in series, so that the total catalyst volume was composed by 50% of the Cu-sample and 50% of the Fe-sample. The catalysts were hydrothermally (HT) aged at 700 °C for 20 h in air flow with 10% of water .
Before being loaded in a stainless-steel reactor tube, the samples were wrapped with a tape of inert quartz in order to avoid by-pass of gases. For the same reason, the hole in the center of the cylindrical metallic Fe-BEA catalyst was also plugged with inert quartz wool. The catalysts were topped with quartz spheres, and by a quartz wool layer, in order to increase the turbulence and thus ensure a good mixing of the reactants. The reactor tube (405 mm in length, 15 mm i.d.), containing the catalyst sample, was inserted in a cylindrical electric oven, whose temperature (up to 550 °C) was remotely controlled by a PID controller (Eurotherm model 2132). The reactor was equipped with three K-type thermocouples: one was used to monitor the inlet gas temperature and two were placed in contact with the top and the bottom of the catalyst.
Before starting the tests, the catalysts were subjected to the conditioning pre-treatment by heating them up to 500 °C with a ramp of 15 °C min−1 and holding the maximum temperature for 1 h in a continuous flow of 5% (v/v) O2 with nitrogen balance (75,000 h−1 GHSV). Steady state runs were carried out in order to investigate the catalytic activities, using defined reactant feed concentrations and temperature steps. The feed concentrations were chosen as similar as possible to those of real after treatment system: NH3 = 500 ppm, NOx= 500 ppm (NO2/NOx = 0–1), O2 = 5% (v/v), H2O = 5% (v/v) and balance N2. In the specific experiments that aim to the investigation of the hydrocarbon effect on the deNOx activity 1000 ppm of CH4 were also fed.
H2O was metered by a volumetric piston pump (Gilson model 305): the feed rate was around 0.025 mL min−1 ± 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.
A wide range of temperature (150–550 °C) was investigated for each adopted experimental condition and all the experimental runs were realized using the GHSV of 75,000 h−1. The GHSV was calculated as the flow rate divided by the overall volume of the monolith catalysts.
All the gaseous species (except N2) were continuously monitored at the reactor outlet by a FT-IR gas analyzer (Bruker MATRIX MG5).
3 Results and Discussion
3.1 Standard SCR
At temperature higher than 350 °C the Fe-zeolite catalyst showed better deNOx efficiency, reaching 90% of NO conversion at 450–500 °C. Also in this case, however, NO conversion never reached 100% because of the NH3 oxidation side reactions (4) and (5), which consumes NH3 (Fig. S1). Literature studies confirm that Cu-zeolites suffer from a greater NH3 oxidation activity than Fe-zeolite catalysts [13, 25]. Moreover, the copper-based catalyst exhibited worse performance in terms of N2O production compared to the Fe-zeolite, over which this undesired reaction can be considered negligible.
Hence, the collected data over these two different catalysts indicate that Cu-zeolites are more active at low temperature (< 350 °C) (Fig. 1), while Fe-zeolites are more selective at higher temperatures (> 400 °C) (Fig. S2), in agreement with the literature [25, 26, 27]. Since these two catalysts showed their best performance in different temperature regions, a catalytic system comprising both Fe- and Cu-zeolites was tested to exploit potential synergies, as already proposed in previous works [3, 6, 27, 28].
Two sequential arrangements were examined, namely the series with the Cu-zeolite monolith followed by a Fe-zeolite monolith and the reverse configuration. In fact, Fig. 1a contains not only the comparison among the NO conversions obtained at Standard SCR conditions over Cu-zeolite, over Fe-zeolite but also those reached over these two sequential catalyst combinations. The first sequential configuration (Cu-zeolite followed by Fe-zeolite) exhibited a behaviour similar to that of the Cu-zeolite catalyst: a very high NOx conversion at temperatures below 350 °C, with a maximum NO conversion of about 85% at that temperature, and a decreasing conversion at high temperature, very close to those reached over the Cu-zeolite catalyst only. These data indicate that in the whole investigated temperature region most of the reactants were consumed in the upstream Cu-zeolite section, while the Fe-zeolite was hardly utilized. This is clearly visible at high temperatures: although the Standard SCR reaction over the Fe-zeolite results in higher conversions, NO conversions are equal to those reached over the individual Cu-catalyst which confirms that the NO reduction occurs almost totally over the first section, namely the Cu-zeolite. When the catalyst sequence was reversed, the overall deNOx performance was improved. At low temperatures, NOx conversions measured over the two configurations are very similar and they approached those obtained over the copper-zeolite, but at high temperatures the configuration with the Fe-zeolite positioned in front of Cu-zeolite enabled NOx conversions equal to those of the Fe-zeolite, that again means that the second section of the combined system was not involved in the NO reduction. Since the Fe-zeolite catalyst outperformed the Cu-zeolite in this temperature range, the sequential arrangement in which the Fe-zeolite comes first can be considered the best configuration in order to obtain an overall NOx conversion at a high level at all the investigated temperatures, in agreement with literature results [27, 28]. Concerning N2O formation, both sequential configurations allowed to produce about the same amount of N2O produced over the individual catalysts but lower than that produced over the Cu-zeolite catalyst both in the low and in the high temperature ranges (Fig. 1b). In terms of N2O selectivity, the two sequential configurations grant better overall performances compared to those of the individual catalysts (Fig. S2). Anyway, due to all the considerations made, we focused the investigation of the other reacting systems and the effect of methane only on the two individual Me-zeolite catalysts and on the sequential arrangement in which the Fe-zeolite is placed before the Cu-one.
As mentioned in Sect. 1 , the main goal of this work is the evaluation of how methane, which is usually contained into exhausts of lean-burn natural gas or duel-fuel engines, interacts with the deNOx activity of the tested SCR catalysts. Therefore, the SCR catalytic activity runs were replicated with the addition of a fixed concentration of methane to the gaseous feed mixture. Specifically, the Standard SCR reaction was repeated including 1000 ppm of CH4 in the previous reacting system, varying the temperature between 150 and 550 °C and adopting the same space velocity (75,000 h−1) to the end to compare NO conversions and N2O productions of the two cases. Hence, Fig. 2 compares NO conversions and N2O productions in Standard SCR conditions in absence of methane with those obtained including CH4 in the reacting system (the comparison between the corresponding NH3 conversions is shown in Fig. S3). It is clearly apparent that, within experimental error (~ ± 5%), the Standard SCR activity in presence of methane shows exactly the same behavior observed without CH4. Moreover, CH4 started to be converted at temperatures above 450 °C with the corresponding formation of COx only (not shown), as confirmed by the carbon balance which remains equal to 1000 ppm in all the investigated temperature range.
3.2 Fast SCR
NOx reduction is promoted by the presence of NO2 in the gas stream, especially at low temperatures, both over Cu- and Fe-zeolite catalysts, although the activity increment is much more dramatic for Fe-zeolites [25, 27, 29]. The effect of NO2 on the deNOx activity of the tested Cu-zeolite, the Fe-zeolite and Fe- + Cu-zeolite sequential system was evaluated varying the NO2/NOx feed ratio in the NH3–NO–NO2 reacting system. Specifically, three levels of NO2/NOx were adopted: 0 (which corresponds to the Standard SCR conditions previously presented), 0.5 and 1.
Using a NO2/NOx feed ratio equal to 0.5 the deNOx efficiency was studied under Fast SCR conditions. Hence, the runs were firstly performed feeding to the reactor 500 ppm of NH3, 250 ppm of NO2 and 250 ppm of NO (NO2/NO = 1), 5% (v/v) of O2 and 5% (v/v) of H2O, with balance nitrogen, and then they were replicated with the addition of 1000 ppm of CH4. Figure 3a, b shows the results of the tests in terms of comparison between NOx conversions and N2O concentrations when the hydrocarbon is present and absent in the gaseous mixture, measured at steady state in the 200–550 °C T-range over all the tested catalysts. Looking at dotted lines, representative of the results of the reference conditions, namely Fast SCR in absence of CH4, it is clearly visible that through the occurrence of this reaction boosted deNOx performances can be obtained in the whole temperature range and over all the investigated catalytic systems with respect to those reached only with the Standard SCR reaction. Specifically, The highest deNOx activity was reached over Fe-zeolite, with NOx conversions always above the 85% (Fig. 3a). Cu-zeolite showed a similar trend but with conversions slightly lower than the Fe-zeolite in the whole temperature range. Concerning the NH3 emissions, Fast SCR conditions enabled to achieve 100% NH3 conversion almost over the whole investigated temperature range (Fig. S4). As seen for the Standard SCR, also for the Fast SCR the sequence of Fe- and Cu-zeolite catalysts enabled NOx conversion curves placed between those of the individual catalyst components. At high temperature NOx conversions decreased over all catalysts due to the occurrence of NH3 oxidation reaction, which became important above 400 °C reducing the ammonia available for the Fast SCR reaction. However, our data confirm that the Fast SCR reaction is associated with the highest deNOx activity in the 200–300 °C T-range for all the catalytic systems, as shown in the literature [30, 31].
Concerning N2O formation (Fig. 3b), again, Fe-zeolite produced the smallest amount of this undesired species in the whole temperature range. In fact, higher N2O concentrations were detected over the other two systems, in particular the Cu-zeolite showed a maximum of about 10 ppm at 500 °C while the Fe- + Cu-zeolite combination was associated with a maximum of 12 ppm at 200 °C. The corresponding N2O selectivity data are shown in Fig. S5.
Concerning the impact of methane, negligible changes of the NOx removal efficiency in the NO–NO2–NH3/O2 reacting system were observed upon addition of 1000 ppm of CH4. Again, under these operating conditions the only reaction which involved methane was its oxidation to COx above 450 °C, confirmed by the closure of carbon balance to 1000 ppm (not shown).
3.3 NO2 SCR
To complete the investigation of the possible impact that methane can have on the NH3-SCR application, we had examined the case of NO2/NOx = 1, i.e. at conditions corresponding to the NO2-SCR reaction. Steady state results were collected in the 200–550 °C T-range both feeding to the reactor 500 ppm of NH3, 500 ppm of NO2, O2 (5%, v/v), H2O (5%, v/v), and adding 1000 ppm of CH4 to the same gaseous mixture. In Fig. 4a, c, e the NO2 and NH3 conversions were plotted, comparing the base case (0 ppm of methane) and that with the addition of the hydrocarbon over all tested catalysts; instead, in Fig. 4b, d, f the NO and N2O produced under these operating conditions are shown, again as comparison between the two cases.
In their study of NH3-SCR over parent and Cu-promoted zeolites with different frameworks (BEA, CHA, SAPO), Ruggeri et al.  proposed that the zeolite pore size is determining for the NH4NO3 formation and its subsequent decomposition to N2O. Our data show in fact a lower production of N2O over the Cu-SAPO catalyst, which is characterized by a zeolite structure with smaller pores than Fe-BEA .
Concerning the sequence of Fe- and Cu-zeolite catalysts, NOx conversions were in between those measured over the individual Fe- and Cu-zeolite catalysts, thus enabling better deNOx performances than over the Cu-zeolite, and very similar to the optimal Fe-zeolite catalyst. Unfortunately, however, the N2O production over the Fe-zeolite + Cu-zeolite sequence was also in between the two individual catalysts below 300 °C, where the Fe-zeolite plays a more important role. Negligible differences were noted instead at higher temperatures (the direct comparison of the three catalytic systems in terms of N2O selectivity is shown in Fig. S6).
Contrary to Standard and Fast SCR, the NO2 SCR runs replicated with the addition of 1000 ppm of CH4 exhibited some differences from those without methane, as apparent from the inspection of Fig. 4. Looking at NO2 and NH3 (Fig. 4a, c, e), we can conclude that the presence of CH4 did not affect the reactivity of all systems up to 400 °C, as the light-off curves are more or less overlapped. Above this temperature, where the ammonia conversion was already complete, the presence of the hydrocarbon resulted in converting more NO2, specifically in the case of the Cu-zeolite catalyst. At the same time, the production of N2O remained more or less the same despite the presence of methane, while the NO production above 400 °C was higher when methane was present in the reacting mixture. Moreover, CH4 was consumed (up about 20% conversion) by oxidation reactions, which produced COx species (Fig. 5). These changes in the NO2 SCR reactivity when methane is present can be rationalized assuming that NO2 was involved in methane oxidation reactions above 400 °C. The participation of NO2 to the methane oxidation was supported by a greater conversion of NO2 at high temperatures when CH4 was contained in the gaseous mixture, with a corresponding greater production of NO. This strongly suggests the onset of a reactivity between CH4 and NO2 at high temperatures over all the investigated catalyst systems.
This last important result needs to be further investigated in more details in a dedicated study in order to clarify the potential of NO2 in the oxidation of CH4 and the related effect of the reaction conditions.
We have first systematically studied the NH3-SCR deNOx activity of Fe-BEA, of Cu-SAPO and of a sequential arrangement of these two commercial metal-exchanged zeolite catalysts at typical operating conditions of after treatment systems for lean-burn engines. Under Standard SCR conditions, the Cu-zeolite is associated with the highest deNOx efficiency below 350 °C, while in the high temperature range the highest NOx conversions and N2 selectivities are achieved over the Fe-zeolite catalyst. Accordingly, the sequential arrangement of the two zeolites (Fe-BEA followed by Cu-SAPO) has demonstrated a good synergy between the two systems, with high deNOx efficiencies across the whole temperature range, while ensuring also a low N2O production.
The main goal of this study was to assess the impact of the presence of methane in the exhausts (as typical of ATS for lean-burn NG engines) on the deNOx activity of the tested SCR catalyst systems. Our data clearly indicate that methane does not interfere with the chemistry and the kinetics of the Standard SCR and Fast SCR reactions. Under NO2-SCR conditions, on the other hand, we have observed an effective oxidation of methane to COx above 400 °C over all the tested catalyst systems. While such oxidation reactions did not affect the overall deNOx process to a large extent in our conditions, they suggest however a significant reactivity between NO2 and CH4 already at relatively low temperatures over metal-exchanged zeolite catalysts. We will further study such a reactivity, so far unreported, in future dedicated work.
The research leading to these results has received funding from the European Community’s Horizon 2020 Programme (H2020 Transport) under grant agreement No. 653391 (HDGAS).
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