The Effect of CH4 on NH3-SCR Over Metal-Promoted Zeolite Catalysts for Lean-Burn Natural Gas Vehicles

The Effect of CH4 on NH3-SCR Over Metal-Promoted Zeolite Catalysts for Lean-Burn Natural Gas... We present a systematic investigation of the de NO activity of two commercial metal exchanged zeolite NH -SCR catalysts, x 3 a Cu-SAPO and a Fe-BEA, in view of their application to the exhaust after-treatment systems of lean-burn natural gas vehi- cles. 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 NH -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 NO at T > 400 °C due to methane oxidation by N O . 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 N O selectivity, and was equally insensitive to methane. Keywords NH SCR · Natural gas vehicles · Metal zeolite catalysts · Methane oxidation 1 Introduction diesel fuel. Indeed, lean operating natural gas fuelled engines have the potential to deliver low CO transporta- Currently, the abatement of gaseous polluting emissions tion solutions compared to diesel and dual fuel applica- from combustion processes has become one of the main tions. Operating solely on natural gas provides the emis- challenges in the automotive field. The main pollutants sions control solution with its own unique set of challenges in exhaust gases from vehicle engines include carbon and opportunities. Simultaneous control of methane and dioxide, carbon monoxide, hydrocarbons, nitrogen oxides oxides of nitrogen is required. In order to comply with (NO ), sulfur dioxide and particulates. The use of natural the stricter and stricter emission standard limitations (e.g. gas (NG), in engines operating in lean conditions, or its Euro VI), the development of more advanced exhaust blend with diesel can be a good way to produce lower after-treatment technologies plays a key role. In order emission compared to those produced by using regular 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 Electronic supplementary material The online version of this main sections: the first one is dedicated to the oxidation article (https ://doi.org/10.1007/s1124 4-018-1004-4) contains supplementary material, which is available to authorized users. of unburned methane over a dedicated catalyst (MOC— methane oxidation catalyst). The second one is devoted to * Enrico Tronconi the abatement of NO emissions (SCR—selective catalytic enrico.tronconi@polimi.it reduction) [1] followed by an ammonia slip catalyst (ASC) Laboratory of Catalysis and Catalytic Processes, Department which prevents the NH release caused by the limited SCR of Energy, Politecnico di Milano, Via La Masa 34, activity at low temperatures and during rapid changes in 20156 Milan, Italy engine operation. Understanding the synergies and interac- Dinex Ecocat Oy, Global Catalyst Competence Centre, tions of these catalyst systems is key to delivering highly P.O. Box 20, 41331 Vihtavuori, Finland efficient and durable after-treatment systems. Moreover, Ricardo UK Ltd, Shoreham Technical Centre, due to the nature of methane combustion and the fuel itself Shoreham-by-Sea BN43 5FG, UK Vol.:(0123456789) 1 3 Topics in Catalysis not containing carbon–carbon bonds, an exhaust particle activity of the SCR catalysts [15–17]. Focusing on short filter is not required. chain hydrocarbons, only C H seems to be able to modify 3 6 The main challenges are controlling methane, in a the NH -SCR catalytic activity. Heo et al. [17] showed lean exhaust which is generally cool and determining how this hydrocarbon affects the SCR-deNO activity of the impact of any methane slip on the downstream SCR V O /TiO , Cu-ZSM-5 and Fe-ZSM-5 based catalysts: at 2 5 2 system. The lean operating natural gas engine operates at low temperatures, NH and C H compete in the adsorp- 3 3 6 exhaust temperatures higher than a diesel but lower than tion over catalyst surface, while at high temperatures NH a stoichiometric gasoline. Hence, selection of the most is consumed by side reactions, which involve the hydro- appropriate SCR catalyst is key to meeting the emissions carbon. On the other hand, it also known that light hydro- requirements, durability targets and minimising N O emis- carbons can behave as reducing agents like ammonia and sions, which are known to occur over SCR systems. Previ- thus contribute to the NO removal (Hydrocarbon SCR) ous investigations have not studied the impact of methane [18–23]. on zeolite and vanadia based SCR systems as currently Very limited specific information is available about the there are no dedicated natural gas lean burn engines on the two possible mechanisms through which CH can interact market. Current, natural gas engines operate under stoichi-with NH -SCR catalysts at the typical conditions of the ometric conditions. Hence, the focus of this paper is N O after-treatment systems of lean-burn natural gas vehicles. To abatement for a dedicated natural gas engine operating fill this gap, therefore, the present work aims to investigate under lean conditions. NH /urea SCR is worldwide recog- the effect of CH on the activity of state-of-the-art NH -SCR 3 4 3 nized as the most effective technology for the abatement catalysts. To this end, a systematic catalytic activity study of NO emission form heavy-duty diesel vehicles, with under typical NH -SCR conditions both in presence and in x 3 the growing introduction of the same technology for light absence of methane was performed over state-of-the-art Fe- duty applications [2]. In the SCR technology applied to and Cu-zeolite catalysts, focusing on the main reactions of the after-treatment system of lean-burn engines, nitrogen the SCR system (Standard SCR, Fast SCR and NO -SCR). oxides present in the flue gases can be reduced to harm- less N and H O through the injection of NH /urea over 2 2 3 iron- or copper-exchanged zeolite catalysts on monolithic 2 Experimental substrates [3–8] or on Vanadia-based catalysts [3, 9, 10]. The SCR process is based on the following three main SCR runs were performed over two different NH -SCR mon- reactions: olith catalysts (thermally stable Cu-SAPO and Fe-BEA), supplied by Dinex Ecocat [24]. A small amount (< 15 wt% 4NH + 4NO + O → 4N + 6H O Standard SCR 3 2 2 2 of the coating) of binder was mixed to the zeolites in coated (1) catalysts. The Fe-BEA catalyst consisted of a cylindrical rolled metallic substrate whose flat and corrugated metal 2NH + NO + NO → 2N + 3H O Fast SCR 3 2 2 2 (2) foils were coated with the catalyst layer (length = 20 mm, 8NH + 6NO → 7N + 12H O NO SCR (3) 3 2 2 2 2 diameter = 12.5 mm, volume = 2453 mm , coating = 0.42 g). Instead, the Cu-SAPO catalyst was tested in the form of The MOC upstream of the SCR converter in the exhaust a coated ceramic honeycomb (length: 41.8  mm; height: line ensures the presence of N O in the feed stream to the 7.7  mm; width: 7.7  mm, volume = 2478  mm , coat- SCR converter, thus enabling improved deNO efficiency ing = 0.283 g). The combined systems were realized put- in the low temperature region where the Fast SCR reac- ting both catalysts in series, so that the total catalyst volume tion (2) is by far more active [2, 11–13] than the Standard was composed by 50% of the Cu-sample and 50% of the SCR reaction (1). Fe-sample. The catalysts were hydrothermally (HT) aged at As mentioned before, one of the main issues related 700 °C for 20 h in air flow with 10% of water [24]. to the typical after-treatment system for lean-burn NG Before being loaded in a stainless-steel reactor tube, the vehicles concerns the MOC. The long-term use of natural samples were wrapped with a tape of inert quartz in order to gas (< 10 ppm S content) as fuel in lean-burn engines can avoid by-pass of gases. For the same reason, the hole in the cause sulfur poisoning of the after-treatment system and center of the cylindrical metallic Fe-BEA catalyst was also the excess oxygen in the combustion chamber under lean plugged with inert quartz wool. The catalysts were topped operations leads to lower exhaust gas temperatures [14]. with quartz spheres, and by a quartz wool layer, in order to The result is an incomplete conversion of methane in the increase the turbulence and thus ensure a good mixing of exhaust, which would thus be present in the exhaust gases the reactants. The reactor tube (405 mm in length, 15 mm downstream of the MOC unit. In general terms, it is well i.d.), containing the catalyst sample, was inserted in a cylin- known that the hydrocarbon slip can affect negatively the drical electric oven, whose temperature (up to 550 °C) was 1 3 Topics in Catalysis remotely controlled by a PID controller (Eurotherm model 100 (a) 2132). The reactor was equipped with three K-type thermo- couples: 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 50 −1 Cu-SCR with a ramp of 15 °C min and holding the maximum tem- Fe-SCR perature for 1 h in a continuous flow of 5% (v/v) O with Cu-SCR+Fe-SCR −1 nitrogen balance (75,000 h GHSV). Steady state runs were Fe-SCR+Cu-SCR 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 pos- 150200 250300 350 400 450 500 550 sible to those of real after treatment system: NH = 500 ppm, Temperature, °C NO = 500 ppm (N O /NO = 0–1), O = 5% (v/v), H O = 5% x 2 x 2 2 (v/v) and balance N . In the specific experiments that aim (b) to the investigation of the hydrocarbon effect on the deNO activity 1000 ppm of CH were also fed. Cu-SCR H O was metered by a volumetric piston pump (Gil- 8 Fe-SCR −1 Cu-SCR+Fe-SCR son model 305): the feed rate was around 0.025 mL min −1 Fe-SCR+Cu-SCR ± 0.0001 for GHSV = 75,000 h . 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 inves- tigated for each adopted experimental condition and all the experimental runs were realized using the GHSV of −1 75,000  h . The GHSV was calculated as the flow rate divided by the overall volume of the monolith catalysts. All the gaseous species (except N ) were continuously 2 150200 250300 350400 450500 550 monitored at the reactor outlet by a FT-IR gas analyzer Temperature, °C (Bruker MATRIX MG5). Fig. 1 Steady-state NO conversions (a) and N O concentrations (b) obtained in the Standard SCR reaction over Cu-zeolite, Fe- zeolite, Fe- + Cu-zeolite and Cu- + Fe-zeolite sequential systems. 3 Results and Discussion −1 GHSV = 75,000  h, NH = 500  ppm, NO = 500  ppm, O = 5% (v/v), 3 2 H O = 5% (v/v), T = 150–550 °C 2 range 3.1 Standard SCR temperature with respect to the Fe-zeolite, as well known SCR reactivity of the NH –NO–O mixture was studied at 3 2 steady state conditions over the 150–550 °C temperature [13]. Beyond 250 °C, where the maximum deNO efficiency was achieved, NO conversion started to decrease over the range for all tested catalysts. For this purpose, 500 ppm of NO and 500 ppm of NH were continuously fed to the reac- copper catalyst. This slight decrease was attributed to the oxidation of the reductant NH which approached complete tor in presence of 5% (v/v) of O and 5% (v/v) of H O and 2 2 3, balance nitrogen. In order to do compare the deNO activi- conversion already at 250 °C (Fig. S1), according to the fol- lowing reactions ties of the investigated catalysts, we show NO conversions and N O productions in Fig. 1. In the supporting information 4NH + 3O → 2N + 6H O 3 2 2 2 (4) we provide as well NH conversions and N O selectivities, 3 2 computed according to [2 × N O production/(NH + NO) 2 3 4NH + 5O → 4NO + 6H O (5) 3 2 2 consumption]. Since N O is the only side product of the SCR process, the N selectivity is then just the complement At temperature higher than 350 °C the Fe-zeolite catalyst of the N O selectivity. Looking at NO conversions and N O 2 2 showed better deN O efficiency, reaching 90% of NO con- productions obtained over the Fe- and the Cu-zeolite cata- version at 450–500 °C. Also in this case, however, NO con- lysts, shown in Fig. 1a, b, some considerations can be done. version never reached 100% because of the NH oxidation Cu-zeolite data showed a much greater d eNO activity at low 1 3 NO conversion, % N O concentration, ppm 2 Topics in Catalysis side reactions (4) and (5), which consumes NH (Fig. S1). N O produced over the individual catalysts but lower than 3 2 Literature studies confirm that Cu-zeolites suffer from a that produced over the Cu-zeolite catalyst both in the low greater NH oxidation activity than Fe-zeolite catalysts [13, and in the high temperature ranges (Fig. 1b). In terms of 25]. Moreover, the copper-based catalyst exhibited worse N O selectivity, the two sequential configurations grant bet- performance in terms of N O production compared to the ter overall performances compared to those of the individual Fe-zeolite, over which this undesired reaction can be con- catalysts (Fig. S2). Anyway, due to all the considerations sidered negligible. made, we focused the investigation of the other reacting sys- Hence, the collected data over these two die ff rent catalysts tems and the effect of methane only on the two individual indicate that Cu-zeolites are more active at low temperature Me-zeolite catalysts and on the sequential arrangement in (< 350 °C) (Fig. 1), while Fe-zeolites are more selective at which the Fe-zeolite is placed before the Cu-one. higher temperatures (> 400 °C) (Fig. S2), in agreement with As mentioned in Sect. 1 , the main goal of this work is the literature [25–27]. Since these two catalysts showed their the evaluation of how methane, which is usually contained best performance in different temperature regions, a catalytic into exhausts of lean-burn natural gas or duel-fuel engines, system comprising both Fe- and Cu-zeolites was tested to interacts with the deNO activity of the tested SCR catalysts. exploit potential synergies, as already proposed in previous Therefore, the SCR catalytic activity runs were replicated works [3, 6, 27, 28]. with the addition of a fixed concentration of methane to the Two sequential arrangements were examined, namely the gaseous feed mixture. Specifically, the Standard SCR reac- series with the Cu-zeolite monolith followed by a Fe-zeolite tion was repeated including 1000 ppm of CH in the previous monolith and the reverse configuration. In fact, Fig.  1a con- reacting system, varying the temperature between 150 and −1 tains not only the comparison among the NO conversions 550 °C and adopting the same space velocity (75,000 h ) obtained at Standard SCR conditions over Cu-zeolite, over to the end to compare NO conversions and N O productions Fe-zeolite but also those reached over these two sequential of the two cases. Hence, Fig. 2 compares NO conversions catalyst combinations. The first sequential configuration and N O productions in Standard SCR conditions in absence (Cu-zeolite followed by Fe-zeolite) exhibited a behaviour of methane with those obtained including CH in the react- similar to that of the Cu-zeolite catalyst: a very high N O ing system (the comparison between the corresponding NH x 3 conversion at temperatures below 350  °C, with a maxi- conversions is shown in Fig. S3). It is clearly apparent that, mum NO conversion of about 85% at that temperature, and within experimental error (~ ± 5%), the Standard SCR activ- a decreasing conversion at high temperature, very close to ity in presence of methane shows exactly the same behavior those reached over the Cu-zeolite catalyst only. These data observed without CH . Moreover, CH started to be con- 4 4 indicate that in the whole investigated temperature region verted at temperatures above 450 °C with the corresponding most of the reactants were consumed in the upstream Cu- formation of CO only (not shown), as confirmed by the zeolite section, while the Fe-zeolite was hardly utilized. This carbon balance which remains equal to 1000 ppm in all the is clearly visible at high temperatures: although the Standard investigated temperature range. SCR reaction over the Fe-zeolite results in higher conver- sions, NO conversions are equal to those reached over the 3.2 Fast SCR individual Cu-catalyst which confirms that the NO reduction occurs almost totally over the first section, namely the Cu- NO reduction is promoted by the presence of N O in the gas x 2 zeolite. When the catalyst sequence was reversed, the overall stream, especially at low temperatures, both over Cu- and deNO performance was improved. At low temperatures, Fe-zeolite catalysts, although the activity increment is much NO conversions measured over the two configurations are more dramatic for Fe-zeolites [25, 27, 29]. The effect of NO x 2 very similar and they approached those obtained over the on the deN O activity of the tested Cu-zeolite, the Fe-zeolite copper-zeolite, but at high temperatures the configuration and Fe- + Cu-zeolite sequential system was evaluated vary- with the Fe-zeolite positioned in front of Cu-zeolite ena- ing the N O /NO feed ratio in the NH –NO–NO reacting 2 x 3 2 bled NO conversions equal to those of the Fe-zeolite, that system. Specifically, three levels of NO /NO were adopted: x 2 x again means that the second section of the combined system 0 (which corresponds to the Standard SCR conditions previ- was not involved in the NO reduction. Since the Fe-zeolite ously presented), 0.5 and 1. catalyst outperformed the Cu-zeolite in this temperature Using a NO /NO feed ratio equal to 0.5 the deN O effi- 2 x x range, the sequential arrangement in which the Fe-zeolite ciency was studied under Fast SCR conditions. Hence, the comes first can be considered the best configuration in order runs were firstly performed feeding to the reactor 500 ppm to obtain an overall NO conversion at a high level at all of NH , 250 ppm of N O and 250 ppm of NO (N O /NO = 1), x 3 2 2 the investigated temperatures, in agreement with literature 5% (v/v) of O and 5% (v/v) of H O, with balance nitro- 2 2 results [27, 28]. Concerning N O formation, both sequential gen, and then they were replicated with the addition of configurations allowed to produce about the same amount of 1000  ppm of CH . Figure  3a, b shows the results of the 1 3 Topics in Catalysis (a) 100 (a) Fe-SCR Fe-SCR+Cu-SCR Cu-SCR Fe-SCR+Cu-SCR Cu-SCR Fe-SCR Fast SCR w CH Std SCR w CH 4 10 10 Fast SCR w/o CH Std SCR w/o CH 200 250 300 350 400450 500550 150 200 250 300 350 400450 500550 Temperature, °C Temperature, °C (b) (b) Fast SCR w CH Std SCR w CH 9 4 Fast SCR w/o CH Std SCR w/o CH 4 Cu-SCR Fe-SCR+Cu-SCR Cu-SCR Fe-SCR+Cu-SCR Fe-SCR Fe-SCR 200 250 300 350400 450500 550 150 200 250 300 350 400450 500550 Temperature, °C Temperature, °C Fig. 2 CH effect on Standard SCR over Cu-zeolite, Fe-zeolite and 4 Fig. 3 CH effect on Fast SCR over Cu-zeolite, Fe-zeolite and Fe- + Cu-zeolites sequential system: steady-state NO conversions (a) Fe- + Cu-zeolites sequential system: steady-state NO conversions (a) −1 −1 and N O concentrations (b). GHSV = 75,000  h, NH = 500  ppm, 2 3and N O concentrations (b). GHSV = 75,000  h, NH = 500  ppm, 2 3 NO = 500  ppm, CH = 0–1000  ppm, O = 5% (v/v), H O = 5% (v/v), 4 2 2 NO = 250  ppm, NO = 250  ppm, CH = 0–1000  ppm, O = 5% (v/v), 2 4 2 T = 150–550 °C range H O = 5% (v/v), T = 200–550 °C 2 range tests in terms of comparison between NO conversions and conversion almost over the whole investigated temperature N O concentrations when the hydrocarbon is present and range (Fig. S4). As seen for the Standard SCR, also for the absent in the gaseous mixture, measured at steady state in Fast SCR the sequence of Fe- and Cu-zeolite catalysts ena- the 200–550 °C T-range over all the tested catalysts. Look-bled NO conversion curves placed between those of the ing at dotted lines, representative of the results of the refer- individual catalyst components. At high temperature N O ence conditions, namely Fast SCR in absence of CH , it is conversions decreased over all catalysts due to the occur- clearly visible that through the occurrence of this reaction rence of NH oxidation reaction, which became important boosted deNO performances can be obtained in the whole above 400 °C reducing the ammonia available for the Fast temperature range and over all the investigated catalytic SCR reaction. However, our data confirm that the Fast SCR systems with respect to those reached only with the Stand- reaction is associated with the highest deN O activity in the ard SCR reaction. Specifically, The highest deNO activity 200–300 °C T-range for all the catalytic systems, as shown was reached over Fe-zeolite, with NO conversions always in the literature [30, 31]. above the 85% (Fig. 3a). Cu-zeolite showed a similar trend Concerning N O formation (Fig. 3b), again, Fe-zeolite but with conversions slightly lower than the Fe-zeolite in produced the smallest amount of this undesired species in the whole temperature range. Concerning the NH emis- the whole temperature range. In fact, higher N O concentra- 3 2 sions, Fast SCR conditions enabled to achieve 100% NH tions were detected over the other two systems, in particular 1 3 NO conversion, % N O concentration, ppm NO conversion, % N O concentration, ppm 2 Topics in Catalysis the Cu-zeolite showed a maximum of about 10  ppm at characterized by a zeolite structure with smaller pores than 500 °C while the Fe- + Cu-zeolite combination was associ- Fe-BEA [24]. ated with a maximum of 12 ppm at 200 °C. The correspond- Concerning the sequence of Fe- and Cu-zeolite catalysts, ing N O selectivity data are shown in Fig. S5. NO conversions were in between those measured over the 2 x Concerning the impact of methane, negligible changes individual Fe- and Cu-zeolite catalysts, thus enabling better of the N O removal efficiency in the NO–NO –NH /O deNO performances than over the Cu-zeolite, and very sim- x 2 3 2 x reacting system were observed upon addition of 1000 ppm ilar to the optimal Fe-zeolite catalyst. Unfortunately, how- of CH . Again, under these operating conditions the only ever, the N O production over the Fe-zeolite + Cu-zeolite 4 2 reaction which involved methane was its oxidation to CO sequence was also in between the two individual catalysts above 450 °C, confirmed by the closure of carbon balance below 300 °C, where the Fe-zeolite plays a more important to 1000 ppm (not shown). role. Negligible differences were noted instead at higher temperatures (the direct comparison of the three catalytic systems in terms of N O selectivity is shown in Fig. S6). 3.3 NO SCR Contrary to Standard and Fast SCR, the NO SCR runs 2 2 replicated with the addition of 1000 ppm of CH exhibited To complete the investigation of the possible impact that some differences from those without methane, as appar - methane can have on the NH -SCR application, we had ent from the inspection of Fig. 4. Looking at N O and NH 3 2 3 examined the case of NO /NO = 1, i.e. at conditions cor- (Fig. 4a, c, e), we can conclude that the presence of CH 2 x 4 responding to the N O -SCR reaction. Steady state results did not affect the reactivity of all systems up to 400 °C, as were collected in the 200–550 °C T-range both feeding to the light-off curves are more or less overlapped. Above this the reactor 500 ppm of NH , 500 ppm of N O, O (5%, v/v), temperature, where the ammonia conversion was already 3 2 2 H O (5%, v/v), and adding 1000 ppm of CH to the same complete, the presence of the hydrocarbon resulted in con- 2 4 gaseous mixture. In Fig.  4a, c, e the NO and NH con- verting more NO , specifically in the case of the Cu-zeolite 2 3 2 versions were plotted, comparing the base case (0 ppm of catalyst. At the same time, the production of N O remained methane) and that with the addition of the hydrocarbon over more or less the same despite the presence of methane, while all tested catalysts; instead, in Fig. 4b, d, f the NO and N O the NO production above 400 °C was higher when methane produced under these operating conditions are shown, again was present in the reacting mixture. Moreover, CH was con- as comparison between the two cases. sumed (up about 20% conversion) by oxidation reactions, Starting from the base case, under NO -SCR conditions which produced CO species (Fig. 5). These changes in the 2 x a catalytic activity was already visible below 250 °C cor- NO SCR reactivity when methane is present can be ration- responding to a 1/1 N O /NH molar consumption ratio, in alized assuming that N O was involved in methane oxida- 2 3 2 line with the stoichiometry of ammonium nitrate formation, tion reactions above 400 °C. The participation of N O to the reaction (6) [9] methane oxidation was supported by a greater conversion of NO at high temperatures when CH was contained in the 2 4 2NH + 2NO → N + NH NO + H O (6) 3 2 2 4 3 2 gaseous mixture, with a corresponding greater production of The Fe-zeolite catalyst showed a higher low-temperature NO. This strongly suggests the onset of a reactivity between activity with respect to the Cu-zeolite. Such a behavior could CH and N O at high temperatures over all the investigated 4 2 be ascribed to the different zeolite structures of the two cata- catalyst systems. lysts, being the BEA zeolite characterized by higher forma- This last important result needs to be further investigated tion of ammonium nitrate due to its larger pores [32]. This in more details in a dedicated study in order to clarify the can also explain the greater production of N O observed potential of N O in the oxidation of CH and the related 2 4 below 250 °C, over the Fe-zeolite catalyst, associated with effect of the reaction conditions. the thermal decomposition of ammonium nitrate according to reaction (7) 4 Conclusions NH NO → N O + 2H O (7) 4 3 2 2 In their study of NH -SCR over parent and Cu-promoted We have first systematically studied the NH-SCR deNO 3 x zeolites with different frameworks (BEA, CHA, SAPO), activity of Fe-BEA, of Cu-SAPO and of a sequential Ruggeri et al. [32] proposed that the zeolite pore size is arrangement of these two commercial metal-exchanged determining for the NH NO formation and its subsequent 4 3 zeolite catalysts at typical operating conditions of after decomposition to N O. Our data show in fact a lower treatment systems for lean-burn engines. Under Stand- production of N O over the Cu-SAPO catalyst, which is ard SCR conditions, the Cu-zeolite is associated with the highest deN O efficiency below 350 °C, while in the 1 3 Topics in Catalysis (a) (b) NH 3 175 NO SCR w CH 2 4 80 NO 2 NO SCR w/o CH 150 2 4 50 100 N O 75 NO NO SCR w CH 2 4 NO SCR w/o CH 2 4 200250 300 350 400 450 500 550 200 250 300 350 400 450 500 550 Temperature, °C Temperature, °C (c) (d) 200 NO SCR w CH 90 NH 2 4 3 NO NO SCR w/o CH 2 4 NO 50 100 N O NO SCR w CH 2 4 NO SCR w/o CH 2 4 200250 300 350 400 450 500 550 200 250 300 350 400 450 500550 Temperature, °C Temperature, °C (e) (f) NH NO SCR w CH 90 3 2 4 NO SCR w/o CH 2 4 NO 70 2 100 N O NO NO SCR w CH 2 4 10 NO SCR w/oCH 2 4 200 250 300350 400 450 500 550 200250 300350 400450 500550 Temperature, °C Temperature, °C −1 Fig. 4 CH effect on NO SCR: NO and NH steady state con- system (f). GHSV = 75,000  h, NH = 500  ppm, NO = 500  ppm, 4 2 2 3 3 2 versions over Fe-zeolite (a), Cu-zeolite (c) and Fe- + Cu-zeolite CH = 0/1000  ppm, O = 5% (v/v), H O = 5% (v/v), T = 200– 4 2 2 range sequential system (e); N O and NO steady-state concentrations 550 °C over Fe-zeolite (b), Cu-zeolite (d) and Fe- + Cu-zeolite sequential high temperature range the highest NO conversions and The main goal of this study was to assess the impact N selectivities are achieved over the Fe-zeolite catalyst. of the presence of methane in the exhausts (as typical of Accordingly, the sequential arrangement of the two zeolites ATS for lean-burn NG engines) on the deNO activity of (Fe-BEA followed by Cu-SAPO) has demonstrated a good the tested SCR catalyst systems. Our data clearly indicate synergy between the two systems, with high deN O efficien- that methane does not interfere with the chemistry and the cies across the whole temperature range, while ensuring also kinetics of the Standard SCR and Fast SCR reactions. Under a low N O production. NO -SCR conditions, on the other hand, we have observed 2 2 1 3 Conversion, % Conversion, % Conversion, % Concentration, ppm Concentration, ppm Concentration, ppm Topics in Catalysis (a) (b) C balance C balance CH CH 800 800 600 600 400 400 200 200 CO CO CO CO 0 0 0 0 200250 300350 400450 500550 200 250 300 350 400 450 500 550 Temperature, °C Temperature, °C (c) C balance CH CO CO 0 0 200250 300350 400450 500550 Temperature, °C Fig. 5 CH effect on NO SCR: CH , CO, CO steady state con- NH = 500  ppm, NO = 500  ppm, CH = 1000  ppm, O = 5% (v/v), 4 2 4 2 3 2 4 2 centrations and carbon balances over Fe-zeolite (a), Cu-zeolite (b) H O = 5% (v/v), T = 200–550 °C 2 range −1 and Fe- + Cu-zeolite sequential system (c). GHSV = 75,000  h , an effective oxidation of methane to CO above 400  °C References over all the tested catalyst systems. While such oxidation 1. Maunula T (2013) SAE technical paper 2013-01-0530 reactions did not affect the overall deNO process to a large 2. Colombo M, Koltsakis G, Nova I, Tronconi E (2012) Catal Today extent in our conditions, they suggest however a significant 188:42–52 reactivity between N O and CH already at relatively low 2 4 3. Shakya BM, Harold MP, Balakotaiah V (2015) Chem Eng J temperatures over metal-exchanged zeolite catalysts. We will 278:374–384 4. Fickel DW, D’Addio E, Lauterbach JA, Lobo RF (2011) Appl further study such a reactivity, so far unreported, in future Catal B 102:441–448 dedicated work. 5. Kamasamudram K, Currier NW, Chen X, Yezerets A (2010) Catal Today 151:212–222 Acknowledgements The research leading to these results has received 6. Metkar PS, Harold MP, Balakotaiah V (2013) Chem Eng Sci funding from the European Community’s Horizon 2020 Programme 87:51–65 (H2020 Transport) under grant agreement No. 653391 (HDGAS). 7. Metkar PS, Salazar N, Muncrief R, Balakotaiah V, Harold MP (2011) Appl Catal B 104:110–126 Open Access This article is distributed under the terms of the Crea- 8. Grossale A, Nova I, Tronconi E, Chatterjee D, Weibel M (2008) tive Commons Attribution 4.0 International License (http://creat iveco J Catal 256:312–322 mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- 9. Nova I, Ciardelli C, Tronconi E, Chatterjee D, Bandl-Konrad B tion, and reproduction in any medium, provided you give appropriate (2006) Catal Today 114:3–12 credit to the original author(s) and the source, provide a link to the 10. Tronconi E, Nova I, Ciarderlli C, Chatterjee D, Weibel M (2007) Creative Commons license, and indicate if changes were made. J Catal 245:1–10 11. Devedas M, KrÓ§cher O, Elsener M, Wokaun A, SÓ§ger N, Pfeifer M, Demel Y, Mussmann L (2006) Appl Catal B 67:187–196 1 3 Concentration, ppm Concentration, ppm C balance, ppm Concentration, ppm C balance, ppm C balance, ppm Topics in Catalysis 12. Madia G, Koebel M, Elsener M, Wokaun A (2002) Ind Eng Chem 22. Iwamoto M, Mizuno N (1993) Proc Inst Mech Eng D 207:23 Res 41:3512–3517 23. 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Houel V, James D, Millington P, Pollington S, Poulston S, Raja- 30. Kato A, Matsuda S, Kamo T, Nakajima F, Kuroda H, Narita T ram R, Torbati R (2005) J Catal 230:150–157 (1981) J Phys Chem 85:4099–4102 19. Burch R, Scire S (1994) Appl Catal B 3:295–318 31. Koebel M, Elsener M, Kleemann M (2000) Catal Today 20. Li L, Chen J, Zhang S, Guan N, Richter M, Eckelt R, Fricke R 59:335–345 (2004) J Catal 228:12–22 32. Ruggeri MP, Nova I, Tronconi E, Collier JE, York APE (2016) 21. Iwamoto M, Zengyo T, Hernandez AM, Araki H (1998) Appl Top Catal 59:875–881 Catal B 17:259 1 3 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Topics in Catalysis Springer Journals

The Effect of CH4 on NH3-SCR Over Metal-Promoted Zeolite Catalysts for Lean-Burn Natural Gas Vehicles

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Chemistry; Catalysis; Physical Chemistry; Pharmacy; Industrial Chemistry/Chemical Engineering; Characterization and Evaluation of Materials
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Abstract

We present a systematic investigation of the de NO activity of two commercial metal exchanged zeolite NH -SCR catalysts, x 3 a Cu-SAPO and a Fe-BEA, in view of their application to the exhaust after-treatment systems of lean-burn natural gas vehi- cles. 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 NH -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 NO at T > 400 °C due to methane oxidation by N O . 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 N O selectivity, and was equally insensitive to methane. Keywords NH SCR · Natural gas vehicles · Metal zeolite catalysts · Methane oxidation 1 Introduction diesel fuel. Indeed, lean operating natural gas fuelled engines have the potential to deliver low CO transporta- Currently, the abatement of gaseous polluting emissions tion solutions compared to diesel and dual fuel applica- from combustion processes has become one of the main tions. Operating solely on natural gas provides the emis- challenges in the automotive field. The main pollutants sions control solution with its own unique set of challenges in exhaust gases from vehicle engines include carbon and opportunities. Simultaneous control of methane and dioxide, carbon monoxide, hydrocarbons, nitrogen oxides oxides of nitrogen is required. In order to comply with (NO ), sulfur dioxide and particulates. The use of natural the stricter and stricter emission standard limitations (e.g. gas (NG), in engines operating in lean conditions, or its Euro VI), the development of more advanced exhaust blend with diesel can be a good way to produce lower after-treatment technologies plays a key role. In order emission compared to those produced by using regular 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 Electronic supplementary material The online version of this main sections: the first one is dedicated to the oxidation article (https ://doi.org/10.1007/s1124 4-018-1004-4) contains supplementary material, which is available to authorized users. of unburned methane over a dedicated catalyst (MOC— methane oxidation catalyst). The second one is devoted to * Enrico Tronconi the abatement of NO emissions (SCR—selective catalytic enrico.tronconi@polimi.it reduction) [1] followed by an ammonia slip catalyst (ASC) Laboratory of Catalysis and Catalytic Processes, Department which prevents the NH release caused by the limited SCR of Energy, Politecnico di Milano, Via La Masa 34, activity at low temperatures and during rapid changes in 20156 Milan, Italy engine operation. Understanding the synergies and interac- Dinex Ecocat Oy, Global Catalyst Competence Centre, tions of these catalyst systems is key to delivering highly P.O. Box 20, 41331 Vihtavuori, Finland efficient and durable after-treatment systems. Moreover, Ricardo UK Ltd, Shoreham Technical Centre, due to the nature of methane combustion and the fuel itself Shoreham-by-Sea BN43 5FG, UK Vol.:(0123456789) 1 3 Topics in Catalysis not containing carbon–carbon bonds, an exhaust particle activity of the SCR catalysts [15–17]. Focusing on short filter is not required. chain hydrocarbons, only C H seems to be able to modify 3 6 The main challenges are controlling methane, in a the NH -SCR catalytic activity. Heo et al. [17] showed lean exhaust which is generally cool and determining how this hydrocarbon affects the SCR-deNO activity of the impact of any methane slip on the downstream SCR V O /TiO , Cu-ZSM-5 and Fe-ZSM-5 based catalysts: at 2 5 2 system. The lean operating natural gas engine operates at low temperatures, NH and C H compete in the adsorp- 3 3 6 exhaust temperatures higher than a diesel but lower than tion over catalyst surface, while at high temperatures NH a stoichiometric gasoline. Hence, selection of the most is consumed by side reactions, which involve the hydro- appropriate SCR catalyst is key to meeting the emissions carbon. On the other hand, it also known that light hydro- requirements, durability targets and minimising N O emis- carbons can behave as reducing agents like ammonia and sions, which are known to occur over SCR systems. Previ- thus contribute to the NO removal (Hydrocarbon SCR) ous investigations have not studied the impact of methane [18–23]. on zeolite and vanadia based SCR systems as currently Very limited specific information is available about the there are no dedicated natural gas lean burn engines on the two possible mechanisms through which CH can interact market. Current, natural gas engines operate under stoichi-with NH -SCR catalysts at the typical conditions of the ometric conditions. Hence, the focus of this paper is N O after-treatment systems of lean-burn natural gas vehicles. To abatement for a dedicated natural gas engine operating fill this gap, therefore, the present work aims to investigate under lean conditions. NH /urea SCR is worldwide recog- the effect of CH on the activity of state-of-the-art NH -SCR 3 4 3 nized as the most effective technology for the abatement catalysts. To this end, a systematic catalytic activity study of NO emission form heavy-duty diesel vehicles, with under typical NH -SCR conditions both in presence and in x 3 the growing introduction of the same technology for light absence of methane was performed over state-of-the-art Fe- duty applications [2]. In the SCR technology applied to and Cu-zeolite catalysts, focusing on the main reactions of the after-treatment system of lean-burn engines, nitrogen the SCR system (Standard SCR, Fast SCR and NO -SCR). oxides present in the flue gases can be reduced to harm- less N and H O through the injection of NH /urea over 2 2 3 iron- or copper-exchanged zeolite catalysts on monolithic 2 Experimental substrates [3–8] or on Vanadia-based catalysts [3, 9, 10]. The SCR process is based on the following three main SCR runs were performed over two different NH -SCR mon- reactions: olith catalysts (thermally stable Cu-SAPO and Fe-BEA), supplied by Dinex Ecocat [24]. A small amount (< 15 wt% 4NH + 4NO + O → 4N + 6H O Standard SCR 3 2 2 2 of the coating) of binder was mixed to the zeolites in coated (1) catalysts. The Fe-BEA catalyst consisted of a cylindrical rolled metallic substrate whose flat and corrugated metal 2NH + NO + NO → 2N + 3H O Fast SCR 3 2 2 2 (2) foils were coated with the catalyst layer (length = 20 mm, 8NH + 6NO → 7N + 12H O NO SCR (3) 3 2 2 2 2 diameter = 12.5 mm, volume = 2453 mm , coating = 0.42 g). Instead, the Cu-SAPO catalyst was tested in the form of The MOC upstream of the SCR converter in the exhaust a coated ceramic honeycomb (length: 41.8  mm; height: line ensures the presence of N O in the feed stream to the 7.7  mm; width: 7.7  mm, volume = 2478  mm , coat- SCR converter, thus enabling improved deNO efficiency ing = 0.283 g). The combined systems were realized put- in the low temperature region where the Fast SCR reac- ting both catalysts in series, so that the total catalyst volume tion (2) is by far more active [2, 11–13] than the Standard was composed by 50% of the Cu-sample and 50% of the SCR reaction (1). Fe-sample. The catalysts were hydrothermally (HT) aged at As mentioned before, one of the main issues related 700 °C for 20 h in air flow with 10% of water [24]. to the typical after-treatment system for lean-burn NG Before being loaded in a stainless-steel reactor tube, the vehicles concerns the MOC. The long-term use of natural samples were wrapped with a tape of inert quartz in order to gas (< 10 ppm S content) as fuel in lean-burn engines can avoid by-pass of gases. For the same reason, the hole in the cause sulfur poisoning of the after-treatment system and center of the cylindrical metallic Fe-BEA catalyst was also the excess oxygen in the combustion chamber under lean plugged with inert quartz wool. The catalysts were topped operations leads to lower exhaust gas temperatures [14]. with quartz spheres, and by a quartz wool layer, in order to The result is an incomplete conversion of methane in the increase the turbulence and thus ensure a good mixing of exhaust, which would thus be present in the exhaust gases the reactants. The reactor tube (405 mm in length, 15 mm downstream of the MOC unit. In general terms, it is well i.d.), containing the catalyst sample, was inserted in a cylin- known that the hydrocarbon slip can affect negatively the drical electric oven, whose temperature (up to 550 °C) was 1 3 Topics in Catalysis remotely controlled by a PID controller (Eurotherm model 100 (a) 2132). The reactor was equipped with three K-type thermo- couples: 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 50 −1 Cu-SCR with a ramp of 15 °C min and holding the maximum tem- Fe-SCR perature for 1 h in a continuous flow of 5% (v/v) O with Cu-SCR+Fe-SCR −1 nitrogen balance (75,000 h GHSV). Steady state runs were Fe-SCR+Cu-SCR 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 pos- 150200 250300 350 400 450 500 550 sible to those of real after treatment system: NH = 500 ppm, Temperature, °C NO = 500 ppm (N O /NO = 0–1), O = 5% (v/v), H O = 5% x 2 x 2 2 (v/v) and balance N . In the specific experiments that aim (b) to the investigation of the hydrocarbon effect on the deNO activity 1000 ppm of CH were also fed. Cu-SCR H O was metered by a volumetric piston pump (Gil- 8 Fe-SCR −1 Cu-SCR+Fe-SCR son model 305): the feed rate was around 0.025 mL min −1 Fe-SCR+Cu-SCR ± 0.0001 for GHSV = 75,000 h . 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 inves- tigated for each adopted experimental condition and all the experimental runs were realized using the GHSV of −1 75,000  h . The GHSV was calculated as the flow rate divided by the overall volume of the monolith catalysts. All the gaseous species (except N ) were continuously 2 150200 250300 350400 450500 550 monitored at the reactor outlet by a FT-IR gas analyzer Temperature, °C (Bruker MATRIX MG5). Fig. 1 Steady-state NO conversions (a) and N O concentrations (b) obtained in the Standard SCR reaction over Cu-zeolite, Fe- zeolite, Fe- + Cu-zeolite and Cu- + Fe-zeolite sequential systems. 3 Results and Discussion −1 GHSV = 75,000  h, NH = 500  ppm, NO = 500  ppm, O = 5% (v/v), 3 2 H O = 5% (v/v), T = 150–550 °C 2 range 3.1 Standard SCR temperature with respect to the Fe-zeolite, as well known SCR reactivity of the NH –NO–O mixture was studied at 3 2 steady state conditions over the 150–550 °C temperature [13]. Beyond 250 °C, where the maximum deNO efficiency was achieved, NO conversion started to decrease over the range for all tested catalysts. For this purpose, 500 ppm of NO and 500 ppm of NH were continuously fed to the reac- copper catalyst. This slight decrease was attributed to the oxidation of the reductant NH which approached complete tor in presence of 5% (v/v) of O and 5% (v/v) of H O and 2 2 3, balance nitrogen. In order to do compare the deNO activi- conversion already at 250 °C (Fig. S1), according to the fol- lowing reactions ties of the investigated catalysts, we show NO conversions and N O productions in Fig. 1. In the supporting information 4NH + 3O → 2N + 6H O 3 2 2 2 (4) we provide as well NH conversions and N O selectivities, 3 2 computed according to [2 × N O production/(NH + NO) 2 3 4NH + 5O → 4NO + 6H O (5) 3 2 2 consumption]. Since N O is the only side product of the SCR process, the N selectivity is then just the complement At temperature higher than 350 °C the Fe-zeolite catalyst of the N O selectivity. Looking at NO conversions and N O 2 2 showed better deN O efficiency, reaching 90% of NO con- productions obtained over the Fe- and the Cu-zeolite cata- version at 450–500 °C. Also in this case, however, NO con- lysts, shown in Fig. 1a, b, some considerations can be done. version never reached 100% because of the NH oxidation Cu-zeolite data showed a much greater d eNO activity at low 1 3 NO conversion, % N O concentration, ppm 2 Topics in Catalysis side reactions (4) and (5), which consumes NH (Fig. S1). N O produced over the individual catalysts but lower than 3 2 Literature studies confirm that Cu-zeolites suffer from a that produced over the Cu-zeolite catalyst both in the low greater NH oxidation activity than Fe-zeolite catalysts [13, and in the high temperature ranges (Fig. 1b). In terms of 25]. Moreover, the copper-based catalyst exhibited worse N O selectivity, the two sequential configurations grant bet- performance in terms of N O production compared to the ter overall performances compared to those of the individual Fe-zeolite, over which this undesired reaction can be con- catalysts (Fig. S2). Anyway, due to all the considerations sidered negligible. made, we focused the investigation of the other reacting sys- Hence, the collected data over these two die ff rent catalysts tems and the effect of methane only on the two individual indicate that Cu-zeolites are more active at low temperature Me-zeolite catalysts and on the sequential arrangement in (< 350 °C) (Fig. 1), while Fe-zeolites are more selective at which the Fe-zeolite is placed before the Cu-one. higher temperatures (> 400 °C) (Fig. S2), in agreement with As mentioned in Sect. 1 , the main goal of this work is the literature [25–27]. Since these two catalysts showed their the evaluation of how methane, which is usually contained best performance in different temperature regions, a catalytic into exhausts of lean-burn natural gas or duel-fuel engines, system comprising both Fe- and Cu-zeolites was tested to interacts with the deNO activity of the tested SCR catalysts. exploit potential synergies, as already proposed in previous Therefore, the SCR catalytic activity runs were replicated works [3, 6, 27, 28]. with the addition of a fixed concentration of methane to the Two sequential arrangements were examined, namely the gaseous feed mixture. Specifically, the Standard SCR reac- series with the Cu-zeolite monolith followed by a Fe-zeolite tion was repeated including 1000 ppm of CH in the previous monolith and the reverse configuration. In fact, Fig.  1a con- reacting system, varying the temperature between 150 and −1 tains not only the comparison among the NO conversions 550 °C and adopting the same space velocity (75,000 h ) obtained at Standard SCR conditions over Cu-zeolite, over to the end to compare NO conversions and N O productions Fe-zeolite but also those reached over these two sequential of the two cases. Hence, Fig. 2 compares NO conversions catalyst combinations. The first sequential configuration and N O productions in Standard SCR conditions in absence (Cu-zeolite followed by Fe-zeolite) exhibited a behaviour of methane with those obtained including CH in the react- similar to that of the Cu-zeolite catalyst: a very high N O ing system (the comparison between the corresponding NH x 3 conversion at temperatures below 350  °C, with a maxi- conversions is shown in Fig. S3). It is clearly apparent that, mum NO conversion of about 85% at that temperature, and within experimental error (~ ± 5%), the Standard SCR activ- a decreasing conversion at high temperature, very close to ity in presence of methane shows exactly the same behavior those reached over the Cu-zeolite catalyst only. These data observed without CH . Moreover, CH started to be con- 4 4 indicate that in the whole investigated temperature region verted at temperatures above 450 °C with the corresponding most of the reactants were consumed in the upstream Cu- formation of CO only (not shown), as confirmed by the zeolite section, while the Fe-zeolite was hardly utilized. This carbon balance which remains equal to 1000 ppm in all the is clearly visible at high temperatures: although the Standard investigated temperature range. SCR reaction over the Fe-zeolite results in higher conver- sions, NO conversions are equal to those reached over the 3.2 Fast SCR individual Cu-catalyst which confirms that the NO reduction occurs almost totally over the first section, namely the Cu- NO reduction is promoted by the presence of N O in the gas x 2 zeolite. When the catalyst sequence was reversed, the overall stream, especially at low temperatures, both over Cu- and deNO performance was improved. At low temperatures, Fe-zeolite catalysts, although the activity increment is much NO conversions measured over the two configurations are more dramatic for Fe-zeolites [25, 27, 29]. The effect of NO x 2 very similar and they approached those obtained over the on the deN O activity of the tested Cu-zeolite, the Fe-zeolite copper-zeolite, but at high temperatures the configuration and Fe- + Cu-zeolite sequential system was evaluated vary- with the Fe-zeolite positioned in front of Cu-zeolite ena- ing the N O /NO feed ratio in the NH –NO–NO reacting 2 x 3 2 bled NO conversions equal to those of the Fe-zeolite, that system. Specifically, three levels of NO /NO were adopted: x 2 x again means that the second section of the combined system 0 (which corresponds to the Standard SCR conditions previ- was not involved in the NO reduction. Since the Fe-zeolite ously presented), 0.5 and 1. catalyst outperformed the Cu-zeolite in this temperature Using a NO /NO feed ratio equal to 0.5 the deN O effi- 2 x x range, the sequential arrangement in which the Fe-zeolite ciency was studied under Fast SCR conditions. Hence, the comes first can be considered the best configuration in order runs were firstly performed feeding to the reactor 500 ppm to obtain an overall NO conversion at a high level at all of NH , 250 ppm of N O and 250 ppm of NO (N O /NO = 1), x 3 2 2 the investigated temperatures, in agreement with literature 5% (v/v) of O and 5% (v/v) of H O, with balance nitro- 2 2 results [27, 28]. Concerning N O formation, both sequential gen, and then they were replicated with the addition of configurations allowed to produce about the same amount of 1000  ppm of CH . Figure  3a, b shows the results of the 1 3 Topics in Catalysis (a) 100 (a) Fe-SCR Fe-SCR+Cu-SCR Cu-SCR Fe-SCR+Cu-SCR Cu-SCR Fe-SCR Fast SCR w CH Std SCR w CH 4 10 10 Fast SCR w/o CH Std SCR w/o CH 200 250 300 350 400450 500550 150 200 250 300 350 400450 500550 Temperature, °C Temperature, °C (b) (b) Fast SCR w CH Std SCR w CH 9 4 Fast SCR w/o CH Std SCR w/o CH 4 Cu-SCR Fe-SCR+Cu-SCR Cu-SCR Fe-SCR+Cu-SCR Fe-SCR Fe-SCR 200 250 300 350400 450500 550 150 200 250 300 350 400450 500550 Temperature, °C Temperature, °C Fig. 2 CH effect on Standard SCR over Cu-zeolite, Fe-zeolite and 4 Fig. 3 CH effect on Fast SCR over Cu-zeolite, Fe-zeolite and Fe- + Cu-zeolites sequential system: steady-state NO conversions (a) Fe- + Cu-zeolites sequential system: steady-state NO conversions (a) −1 −1 and N O concentrations (b). GHSV = 75,000  h, NH = 500  ppm, 2 3and N O concentrations (b). GHSV = 75,000  h, NH = 500  ppm, 2 3 NO = 500  ppm, CH = 0–1000  ppm, O = 5% (v/v), H O = 5% (v/v), 4 2 2 NO = 250  ppm, NO = 250  ppm, CH = 0–1000  ppm, O = 5% (v/v), 2 4 2 T = 150–550 °C range H O = 5% (v/v), T = 200–550 °C 2 range tests in terms of comparison between NO conversions and conversion almost over the whole investigated temperature N O concentrations when the hydrocarbon is present and range (Fig. S4). As seen for the Standard SCR, also for the absent in the gaseous mixture, measured at steady state in Fast SCR the sequence of Fe- and Cu-zeolite catalysts ena- the 200–550 °C T-range over all the tested catalysts. Look-bled NO conversion curves placed between those of the ing at dotted lines, representative of the results of the refer- individual catalyst components. At high temperature N O ence conditions, namely Fast SCR in absence of CH , it is conversions decreased over all catalysts due to the occur- clearly visible that through the occurrence of this reaction rence of NH oxidation reaction, which became important boosted deNO performances can be obtained in the whole above 400 °C reducing the ammonia available for the Fast temperature range and over all the investigated catalytic SCR reaction. However, our data confirm that the Fast SCR systems with respect to those reached only with the Stand- reaction is associated with the highest deN O activity in the ard SCR reaction. Specifically, The highest deNO activity 200–300 °C T-range for all the catalytic systems, as shown was reached over Fe-zeolite, with NO conversions always in the literature [30, 31]. above the 85% (Fig. 3a). Cu-zeolite showed a similar trend Concerning N O formation (Fig. 3b), again, Fe-zeolite but with conversions slightly lower than the Fe-zeolite in produced the smallest amount of this undesired species in the whole temperature range. Concerning the NH emis- the whole temperature range. In fact, higher N O concentra- 3 2 sions, Fast SCR conditions enabled to achieve 100% NH tions were detected over the other two systems, in particular 1 3 NO conversion, % N O concentration, ppm NO conversion, % N O concentration, ppm 2 Topics in Catalysis the Cu-zeolite showed a maximum of about 10  ppm at characterized by a zeolite structure with smaller pores than 500 °C while the Fe- + Cu-zeolite combination was associ- Fe-BEA [24]. ated with a maximum of 12 ppm at 200 °C. The correspond- Concerning the sequence of Fe- and Cu-zeolite catalysts, ing N O selectivity data are shown in Fig. S5. NO conversions were in between those measured over the 2 x Concerning the impact of methane, negligible changes individual Fe- and Cu-zeolite catalysts, thus enabling better of the N O removal efficiency in the NO–NO –NH /O deNO performances than over the Cu-zeolite, and very sim- x 2 3 2 x reacting system were observed upon addition of 1000 ppm ilar to the optimal Fe-zeolite catalyst. Unfortunately, how- of CH . Again, under these operating conditions the only ever, the N O production over the Fe-zeolite + Cu-zeolite 4 2 reaction which involved methane was its oxidation to CO sequence was also in between the two individual catalysts above 450 °C, confirmed by the closure of carbon balance below 300 °C, where the Fe-zeolite plays a more important to 1000 ppm (not shown). role. Negligible differences were noted instead at higher temperatures (the direct comparison of the three catalytic systems in terms of N O selectivity is shown in Fig. S6). 3.3 NO SCR Contrary to Standard and Fast SCR, the NO SCR runs 2 2 replicated with the addition of 1000 ppm of CH exhibited To complete the investigation of the possible impact that some differences from those without methane, as appar - methane can have on the NH -SCR application, we had ent from the inspection of Fig. 4. Looking at N O and NH 3 2 3 examined the case of NO /NO = 1, i.e. at conditions cor- (Fig. 4a, c, e), we can conclude that the presence of CH 2 x 4 responding to the N O -SCR reaction. Steady state results did not affect the reactivity of all systems up to 400 °C, as were collected in the 200–550 °C T-range both feeding to the light-off curves are more or less overlapped. Above this the reactor 500 ppm of NH , 500 ppm of N O, O (5%, v/v), temperature, where the ammonia conversion was already 3 2 2 H O (5%, v/v), and adding 1000 ppm of CH to the same complete, the presence of the hydrocarbon resulted in con- 2 4 gaseous mixture. In Fig.  4a, c, e the NO and NH con- verting more NO , specifically in the case of the Cu-zeolite 2 3 2 versions were plotted, comparing the base case (0 ppm of catalyst. At the same time, the production of N O remained methane) and that with the addition of the hydrocarbon over more or less the same despite the presence of methane, while all tested catalysts; instead, in Fig. 4b, d, f the NO and N O the NO production above 400 °C was higher when methane produced under these operating conditions are shown, again was present in the reacting mixture. Moreover, CH was con- as comparison between the two cases. sumed (up about 20% conversion) by oxidation reactions, Starting from the base case, under NO -SCR conditions which produced CO species (Fig. 5). These changes in the 2 x a catalytic activity was already visible below 250 °C cor- NO SCR reactivity when methane is present can be ration- responding to a 1/1 N O /NH molar consumption ratio, in alized assuming that N O was involved in methane oxida- 2 3 2 line with the stoichiometry of ammonium nitrate formation, tion reactions above 400 °C. The participation of N O to the reaction (6) [9] methane oxidation was supported by a greater conversion of NO at high temperatures when CH was contained in the 2 4 2NH + 2NO → N + NH NO + H O (6) 3 2 2 4 3 2 gaseous mixture, with a corresponding greater production of The Fe-zeolite catalyst showed a higher low-temperature NO. This strongly suggests the onset of a reactivity between activity with respect to the Cu-zeolite. Such a behavior could CH and N O at high temperatures over all the investigated 4 2 be ascribed to the different zeolite structures of the two cata- catalyst systems. lysts, being the BEA zeolite characterized by higher forma- This last important result needs to be further investigated tion of ammonium nitrate due to its larger pores [32]. This in more details in a dedicated study in order to clarify the can also explain the greater production of N O observed potential of N O in the oxidation of CH and the related 2 4 below 250 °C, over the Fe-zeolite catalyst, associated with effect of the reaction conditions. the thermal decomposition of ammonium nitrate according to reaction (7) 4 Conclusions NH NO → N O + 2H O (7) 4 3 2 2 In their study of NH -SCR over parent and Cu-promoted We have first systematically studied the NH-SCR deNO 3 x zeolites with different frameworks (BEA, CHA, SAPO), activity of Fe-BEA, of Cu-SAPO and of a sequential Ruggeri et al. [32] proposed that the zeolite pore size is arrangement of these two commercial metal-exchanged determining for the NH NO formation and its subsequent 4 3 zeolite catalysts at typical operating conditions of after decomposition to N O. Our data show in fact a lower treatment systems for lean-burn engines. Under Stand- production of N O over the Cu-SAPO catalyst, which is ard SCR conditions, the Cu-zeolite is associated with the highest deN O efficiency below 350 °C, while in the 1 3 Topics in Catalysis (a) (b) NH 3 175 NO SCR w CH 2 4 80 NO 2 NO SCR w/o CH 150 2 4 50 100 N O 75 NO NO SCR w CH 2 4 NO SCR w/o CH 2 4 200250 300 350 400 450 500 550 200 250 300 350 400 450 500 550 Temperature, °C Temperature, °C (c) (d) 200 NO SCR w CH 90 NH 2 4 3 NO NO SCR w/o CH 2 4 NO 50 100 N O NO SCR w CH 2 4 NO SCR w/o CH 2 4 200250 300 350 400 450 500 550 200 250 300 350 400 450 500550 Temperature, °C Temperature, °C (e) (f) NH NO SCR w CH 90 3 2 4 NO SCR w/o CH 2 4 NO 70 2 100 N O NO NO SCR w CH 2 4 10 NO SCR w/oCH 2 4 200 250 300350 400 450 500 550 200250 300350 400450 500550 Temperature, °C Temperature, °C −1 Fig. 4 CH effect on NO SCR: NO and NH steady state con- system (f). GHSV = 75,000  h, NH = 500  ppm, NO = 500  ppm, 4 2 2 3 3 2 versions over Fe-zeolite (a), Cu-zeolite (c) and Fe- + Cu-zeolite CH = 0/1000  ppm, O = 5% (v/v), H O = 5% (v/v), T = 200– 4 2 2 range sequential system (e); N O and NO steady-state concentrations 550 °C over Fe-zeolite (b), Cu-zeolite (d) and Fe- + Cu-zeolite sequential high temperature range the highest NO conversions and The main goal of this study was to assess the impact N selectivities are achieved over the Fe-zeolite catalyst. of the presence of methane in the exhausts (as typical of Accordingly, the sequential arrangement of the two zeolites ATS for lean-burn NG engines) on the deNO activity of (Fe-BEA followed by Cu-SAPO) has demonstrated a good the tested SCR catalyst systems. Our data clearly indicate synergy between the two systems, with high deN O efficien- that methane does not interfere with the chemistry and the cies across the whole temperature range, while ensuring also kinetics of the Standard SCR and Fast SCR reactions. Under a low N O production. NO -SCR conditions, on the other hand, we have observed 2 2 1 3 Conversion, % Conversion, % Conversion, % Concentration, ppm Concentration, ppm Concentration, ppm Topics in Catalysis (a) (b) C balance C balance CH CH 800 800 600 600 400 400 200 200 CO CO CO CO 0 0 0 0 200250 300350 400450 500550 200 250 300 350 400 450 500 550 Temperature, °C Temperature, °C (c) C balance CH CO CO 0 0 200250 300350 400450 500550 Temperature, °C Fig. 5 CH effect on NO SCR: CH , CO, CO steady state con- NH = 500  ppm, NO = 500  ppm, CH = 1000  ppm, O = 5% (v/v), 4 2 4 2 3 2 4 2 centrations and carbon balances over Fe-zeolite (a), Cu-zeolite (b) H O = 5% (v/v), T = 200–550 °C 2 range −1 and Fe- + Cu-zeolite sequential system (c). GHSV = 75,000  h , an effective oxidation of methane to CO above 400  °C References over all the tested catalyst systems. While such oxidation 1. Maunula T (2013) SAE technical paper 2013-01-0530 reactions did not affect the overall deNO process to a large 2. Colombo M, Koltsakis G, Nova I, Tronconi E (2012) Catal Today extent in our conditions, they suggest however a significant 188:42–52 reactivity between N O and CH already at relatively low 2 4 3. Shakya BM, Harold MP, Balakotaiah V (2015) Chem Eng J temperatures over metal-exchanged zeolite catalysts. We will 278:374–384 4. Fickel DW, D’Addio E, Lauterbach JA, Lobo RF (2011) Appl further study such a reactivity, so far unreported, in future Catal B 102:441–448 dedicated work. 5. Kamasamudram K, Currier NW, Chen X, Yezerets A (2010) Catal Today 151:212–222 Acknowledgements The research leading to these results has received 6. Metkar PS, Harold MP, Balakotaiah V (2013) Chem Eng Sci funding from the European Community’s Horizon 2020 Programme 87:51–65 (H2020 Transport) under grant agreement No. 653391 (HDGAS). 7. Metkar PS, Salazar N, Muncrief R, Balakotaiah V, Harold MP (2011) Appl Catal B 104:110–126 Open Access This article is distributed under the terms of the Crea- 8. Grossale A, Nova I, Tronconi E, Chatterjee D, Weibel M (2008) tive Commons Attribution 4.0 International License (http://creat iveco J Catal 256:312–322 mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- 9. Nova I, Ciardelli C, Tronconi E, Chatterjee D, Bandl-Konrad B tion, and reproduction in any medium, provided you give appropriate (2006) Catal Today 114:3–12 credit to the original author(s) and the source, provide a link to the 10. Tronconi E, Nova I, Ciarderlli C, Chatterjee D, Weibel M (2007) Creative Commons license, and indicate if changes were made. J Catal 245:1–10 11. Devedas M, KrÓ§cher O, Elsener M, Wokaun A, SÓ§ger N, Pfeifer M, Demel Y, Mussmann L (2006) Appl Catal B 67:187–196 1 3 Concentration, ppm Concentration, ppm C balance, ppm Concentration, ppm C balance, ppm C balance, ppm Topics in Catalysis 12. Madia G, Koebel M, Elsener M, Wokaun A (2002) Ind Eng Chem 22. Iwamoto M, Mizuno N (1993) Proc Inst Mech Eng D 207:23 Res 41:3512–3517 23. 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Topics in CatalysisSpringer Journals

Published: Jun 4, 2018

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