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Improving high-temperature NOx conversion in the combined NSR-SCR system with an SCR catalyst mixed with an NH3 adsorbent
Applied Catalysis A, General 582 (2019) 117105
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Improving high-temperature NOx conversion in the combined NSR-SCR system with an SCR catalyst mixed with an NH3 adsorbent
T
Masatoshi Sakai , Tsuyoshi Hamaguchi, Toshiyuki Tanaka ⁎
Toyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192, Japan
ARTICLE INFO
ABSTRACT
Keywords: NSR SCR NSR-SCR High temperature NOx reduction NH3 adsorbent
The growing worldwide interest in environmental protection and the strong demand to decrease the emission of pollutants from automotive exhaust require the development of catalysts and post-cleaning systems that are very efficient and stable. To that end, we investigated ways of improving the high-temperature NH3 selective catalytic reduction (SCR) activity in the NOx storage reduction (NSR) and SCR combined system, and found that NH3 oxidation is promoted under high-temperature NSR-SCR reaction conditions. To suppress NH3 oxidation and improve the NH3-SCR reaction in the NSR-SCR system, we examined mixing an NH3 adsorbent with the SCR catalyst (NSR-SCR + NH3 adsorbent system). The H-type mordenite zeolite (H-MOR) adsorbent demonstrated superior performance for retaining NH3 without oxidation under high-temperature transient conditions than the other NH3 adsorbents examined. NOx conversion under high-temperature transient conditions was improved in going from the single NSR system (71% at 450 °C) to NSR-SCR (75% at 450 °C), and NSR-SCR+H-MOR (85% at 450 °C). We confirmed that N2 formed over long times, from the commencement to the end of the lean phase, in the NSR-SCR+H-MOR system. This phenomenon, in which the NH3 adsorbed on H-MOR is gradually released to the SCR up to the end of the lean phase, enhances NOx reduction in the NSR-SCR system.
1. Introduction In response to the growing worldwide interest in environmentally sustainable transportation, both industry and government have been focusing on decreasing the emission of pollutants from automotive exhaust and developing zero emission technologies. In order to promote improved atmospheric quality in urban areas, and in particular, to suppress NOx emissions from diesel engine vehicles, a new testing program (the “Real-Driving Emissions” or RDE program) for measuring the emissions from on-road vehicles has been introduced in Europe [1]. In the RDE program, automobiles are tested under a wide range of driving conditions, including continuous stop/start driving in urban areas, and high-load and high-speed driving conditions in rural areas or on motorways. Therefore, automotive exhaust catalysts and postcleaning systems with high purification efficiencies are required, even under unsteady operating conditions in which temperature, gas concentrations, and gas velocities of exhaust gases fluctuate highly. Reducing the large amounts of NOx emissions from “lean-burn” combustion-engine vehicles is a major RDE-testing problem. NOx storage reduction (NSR) systems and selective catalytic reduction (SCR) systems are widely known methods for effectively removing NOx under lean conditions. The NSR system operates under cyclic fuel-lean and ⁎
fuel-rich conditions [2–4]. NSR catalysts contain precious metals (typically Pt or Rh), while and NOx storage materials typically contain BaO or K2O. NOx storage materials store NOx in the form of nitrate or nitrite for a few minutes in the lean phase, while stored NOx is reduced to N2, as the most desirable product, over a few seconds in the rich phase. The disadvantages of the NSR system include the relatively narrow temperature window of 250–400 °C required for high NOx removal efficiency, the production of undesirable NH3 and N2O under certain richphase conditions, and the dependence on the catalyst composition. The SCR system, which is the other lean NOx reduction system, uses NH3 to reduce NOx through the following reactions:
2NO + 2NH3 + 1/2O2
NO + NO2 + 2NH3
2N2 + 3H2 O
(standard
2N2 + 3H2 O (fast
SCR)
SCR)
(1) (2)
Basically, Cu or Fe ion-exchanged zeolites show excellent NH3-SCR performance. In the commercialized SCR system for Diesel vehicles, urea is injected into the fuel gases as the NH3 precursor; however, the requirements for a tank for the aqueous urea solution and a complex NH3 injection system are disadvantages of this system [5]. While the urea-SCR system is capable of achieving high NOx conversions in a higher temperature range than the NSR system, side reactions
Corresponding author. E-mail address: [email protected] (M. Sakai).
https://doi.org/10.1016/j.apcata.2019.06.003 Received 30 March 2019; Received in revised form 2 June 2019; Accepted 4 June 2019 Available online 05 June 2019 0926-860X/ © 2019 Elsevier B.V. All rights reserved.
Applied Catalysis A, General 582 (2019) 117105
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associated with NH3 oxidation cannot be neglected:
2NH3 + 3/2O2
2. Experimental (3)
N2 + 3H2 O
2.1. Catalysts and NH3 adsorbent materials
Combining the NSR and SCR catalysts has been proposed as a method of overcoming the disadvantages of both systems (i.e., NH3 formation from the NSR and the urea-injection system of the SCR) [6–15]. The NSR-SCR system uses the NH3 actively produced on the NSR catalyst during the rich phase to reduce NOx on the SCR catalyst during the subsequent lean phase. The catalytic performance and reaction mechanisms of various NSR-SCR system configurations, such as physical mixtures [7,8], sequential dual-beds [6–10], and dual-layer coated monoliths [11,12], have been discussed. Compared to the single NSR system, higher NOx reduction efficiency and higher N2 formation selectivity were confirmed in each NSR-SCR system at moderate temperatures (250–400 °C). De-La-Torre’s group has reported that high NO conversion was observed in a temperature range from 220 °C to 320 °C in monolith-type NSR and SCR catalysts, as in the real application form. They suggested that Cu-CHA, which Cu exchanges in chabazite-type zeolites, is the most promising SCR catalyst because of its high thermal durability and high N2 selectivity [10]. On the other hand, few reports have focused on the performance of NSR-SCR systems in the hightemperature region (> 400 °C). The NSR-SCR system would not suitable for NOx reduction under high-temperature conditions because of the lack of NH3 formation over the NSR catalyst and the enhanced NH3 oxidation over the SCR catalyst at high temperature. The group of Digiulio has suggested that the optimum operating temperature is around 250–400 °C; the formation of a sufficient amount of NH3 over the NSR catalyst may be difficult above 400 °C [13]. The effect of NH3 oxidation in the NSR-SCR system has mainly been discussed for non-zeolite SCR catalysts [14,15]; however, zeolite-type SCR catalysts would be also affected by NH3 oxidation, depending on the chemical/physical properties of active metal species [16]. Catalysis temperatures exceed 400 °C in some cases, depending on the weight of the car and the driving condition [9,17], and highly efficient NOx reduction at high temperatures is still required for lean-burn engines. With the aim of improving NOx reduction under high-temperature conditions (> 400 °C in this article), we focused particularly on suppressing NH3 oxidation on the SCR catalyst by mixing an NH3 adsorbent with the SCR catalyst in order to assist high-temperature NH3 adsorption. We considered that the selectivity of the NH3-SCR reaction on the SCR catalyst would be enhanced if the number of NH3 adsorption sites, which retain NH3 without oxidizing it under high-temperature conditions, could be increased in the SCR catalyst. In this article, we first discuss factors that enhance activity by examining the catalytic performance of an NSR-SCR system under high-temperature conditions, and then report the NOx-conversion performance of NSR-SCR systems with NH3 adsorbents mixed with the SCR catalyst. Finally, we discuss the N2 production mechanism of different catalytic configurations to obtain guidelines for high-performance catalyst design.
The NSR catalyst was synthesized by the wet impregnation method. The active component, namely Pt (0.74 wt%), and NOx storage materials of Ba (0.74 mmol/g), K (0.56 mmol/g), and Li (0.37 mmol/g) were supported on Al2O3. The NSR catalyst was calcined at 750 °C for 5 h in air to stabilize its catalytic performance. A Cu ion-exchanged zeolite with the Chabazite framework (Cu-CHA) was used as the SCR catalyst. High-silica-content zeolites provided by the Tosoh Corporation (Japan) were selected as NH3 adsorbents. The NH3 adsorbents are labeled according their framework and the SiO2/Al2O3 ratio as: H-[framework code of the International Zeolite Association](SiO2/Al2O3 ratio), for example “H-MOR(20)”. Each NH3 adsorbent was physically mixed with the SCR catalyst using a mortar and pestle. Pellet-type catalysts were produced by compressing each catalyst powder to a hydrostatic pressure of 1 t, and then sieving its particles to 0.5–1 mm in size. Monolithtype catalysts were prepared by continuously wash-coating with an aqueous slurry of the as-prepared catalyst and the Al2O3 binder onto φ 30 mm × 25 mm cordierite honeycomb monoliths to the desired catalyst loading (NSR: 200 g/L, SCR: 360 g/L). The catalytic configuration in which only the NSR catalyst is used is referred to as the single-NSR system. The configuration which the SCR catalyst was located downstream from the NSR catalyst is referred to as the NSR-SCR system. 2.2. Characterization by NH3-TPD NH3-Temperature programmed desorption (NH3-TPD) with a TP5000 apparatus (Ohkura Riken Co., Ltd.) was used to characterize the acidic properties of the NH3 adsorbents. A 0.2 g sample of each NH3 adsorbent was pretreated under 20%-O2/He-balance conditions for 30 min prior to any experiment. A mixed gas composed of 4000 ppm NH3 with a He balance was supplied to each NH3 adsorbent at a flow rate of 100 mL/min for 30 min at 100 °C until saturated with adsorbed NH3. After 1 h the sample was purged with He at 300 mL/min and the reactor temperature was increased from 100 °C to 800 °C at a rate of 10 °C/min. The amount of desorbed NH3 was determined by mass spectrometry by monitoring the m/z = 16 signal using a quadrupole mass spectrometer (Qulee BGM-202, ULVAC, Inc.). 2.3. Reactor testing of the catalysts and NH3 adsorbents Test 1 was conducted on single-NSR and NSR-SCR monolith-type catalysts using fix-bed flow reactor (BEX-8900, CATA-5000 Best Instruments Co, Ltd.). The total flow rate was 15 L/min (SV = 50,000 h−1 for each catalyst). The gas feed conditions are also summarized in Table 1. The lean component gas (Test 1 Lean) and the rich component gas (Test 1 Rich) were repeatedly fed with a 60-s/3-s lean/rich cycle.
Table 1 Gas feed conditions in each test. Test Test 1 Test 2 Test 3 Test 4 Test 5 Pretreat
Lean Rich Lean Rich Static Lean Rich Lean Rich Lean Rich Lean Rich
Duration (s)
CO (vol%)
H2 (vol%)
CO2 (vol%)
H2O (vol%)
O2 (vol%)
NO (ppm)
NH3 (ppm)
Balance gas
Flow rate (L/min)
60 3 56 4 – 180 180 56 4 60 3 300 300
– 4.5 – – – – – – – – – – 0.13
– 1.5 – – – – – – – – 1.5 – 0.04
10 10 10 10 10 10 10 10 10 – – 10 –
5 5 8 8 10 8 8 8 8 – – 8 8
5.5 – 10 – 10 10 – 10 – 5 – 10 –
400 – 100 – 440 – – 100 – 450 – – –
– – – 1500 550 – 1000 – 1500 – – – –
N2 N2 N2 N2 N2 N2 N2 N2 N2 He He N2 N2
15 15 10 10 10 10 10 10 10 10 10 15 15
2
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Fig. 1. (a) NOx conversions as function of temperature for the single-NSR and coupled NSR-SCR systems. Product selectivities from NO for the (b) single-NSR and (c) NSR-SCR systems. (d) Reaction selectivities from NH3 over the SCR catalyst.
Prior to any experiment, each catalyst was pretreated at 550 °C under reduction conditions (Pretreat Rich) and oxidation condition (Pretreat Lean). The NOx conversion (XNOx) and the product selectivity from NO (Sx) were calculated using the following equations: tc
[NOx ]0
0
XNOx (%) =
tc 0 tc 0 tc
SNO (slip) (%) =
tc 0 tc
SN2 O (%) =
2
tc 0 tc 0
SN2 (%) = 100
× 100 (5)
× 100
[NOx ]0 dt
Adsorbed NH3 = NH3in, rich
(7)
[N2 O] dt [NOx ]0 dt
SNOslip
(6)
× 100
[NOx ]0 dt
0
× 100 (4)
[NO] dt
[NH3 ] dt
0 tc
SNH3 (%) =
[NO] dt
[NOx ]
0
SNO2 (%) =
[NOx ]0 dt
[NOx ]0 dt
0 tc
[NOx ] dt
reduced from 550 °C to 250 °C at a rate of 10 °C/min. Under transient condition, 1 g of the pellet-type SCR catalyst was repeatedly exposed to a lean gas (Test 2 Lean) and a rich gas (Test 2 Rich) with a 56-s/4-s lean/rich cycle. The temperature was gradually decreased from 550 °C to 250 °C in 50 °C increments. NOx conversion was determined over 10lean/rich cycles at each temperature, using the same equations as those used in Test 1 (Eq. 4–9). Before each experiment, the catalyst was pretreated under the same conditions described for Test 1. Test 3 was used to determine the NH3-adsorption/desorption profiles of the NH3 adsorbents under cyclic lean/rich flow conditions. Each NH3 adsorbent was also pretreated as described in Test 1. The “Test 3 Rich” gas was fed to 1 g of a pellet-type NH3 adsorbent for 3 min at a total flow rate of 10 L/min at 450 °C to fully adsorb NH3 onto the NH3 adsorbent. The gas conditions were subsequently switched to “Test 3 Lean” for 3 min to examine NH3 desorption under an O2 atmosphere. After rich/lean cycles, the amounts of adsorbed NH3 and desorbed NH3 in each step were calculated using the following equations:
Desorbed × 100
SNO2
S N2 O
NH3 = NH3out, lean
NH3in, lean
(10) (11)
Test 4 was used to evaluating the transient NOx conversions of SCR catalysts under modeled NSR outlet-gas conditions. The reaction conditions (gas components, total flow rate, lean/rich duration times) were as described for the transient conditions in Test 2, but the catalyst weight was increased to 2 g. Test 5 was performed to determine the dynamics of N2 formation under the NSR-SCR system by mass spectrometry (JMS-GC-mate II, JEOL Ltd.) and an emissions analyzer (MEXA-7100, HORIBA, Ltd.). The mixture gas flowed to 1 g of a pellet-type NSR catalyst, or sequentially to 1 g of the NSR catalyst and 1 g of the SCR catalyst. The total gas flow rate was 10 L/min.
(8)
SNH3
NH3out, rich
(9)
Values within square brackets are the concentrations of each component, tc is the total time required for 10 rich/lean cycles at each temperature. Test 2 was conducted on the Cu-CHA catalyst under static conditions (standard SCR conditions) and cyclic lean/rich transient conditions to evaluate the difference in SCR catalytic performance according to the two reaction conditions. Under static conditions, 1 g of the pellettype SCR catalyst was exposed to the reactant gas indicated in Table 1 in which NH3 and NO coexist (Test 2 Static). The temperature was 3
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3. Results and discussion
under transient conditions, in which NH3 and NO feed the SCR catalyst separately. NH3 slip or NH3 oxidation over the SCR catalyst is possibly due to unreacted NH3 present with NOx. Fig. 2(b) shows the NH3 reaction selectivity under transient reaction conditions at 450 °C; half of the supplied NH3 was oxidized under these conditions, while the amount of slipped NH3 was low (less than 0.2%). This result indicates that preventing the NH3-O2 reaction on the SCR catalyst under transient conditions is the key to improving NOx conversion over the SCR catalyst in the NSR-SCR system. The reaction over the SCR catalyst under transient conditions (the NSR-SCR system) is discussed in terms of three phase, namely rich, lean I, and lean II. Fig. 3(a) shows the time dependence of the concentration of slipped NOx from the NSR catalyst. The NSR catalyst stores sufficient NOx at the beginning of the lean-I phase; hence the slipped NOx concentration is low during this phase. The NOx concentration gradually increases from the beginning of the lean-I phase to the end of the lean-II phase, because the NSR catalyst releases unadsorbed NOx. Fig. 3(b) depicts the reactions of adsorbed NH3 on the SCR catalyst under low-temperature conditions (≤ 400 °C) in the NSR-SCR system. NH3 is adsorbed onto the SCR catalyst during the rich phase. At sufficiently low temperatures under which NH3 oxidation reactivity is low, high NOx conversion is expected through the reaction of adsorbed NH3 and slipped NOx during the NOx-rich lean-II phase, because adsorbed NH3 remains present and is not oxidized during the lean-I phase. Fig. 3(c) depicts the reactions of adsorbed NH3 on the SCR catalyst under high-temperature conditions (> 400 °C) in the NSR-SCR system. Adsorbed NH3 is oxidized at several Cu sites in the Cu-CHA during the lean-I phase at high temperatures. Only unoxidized NH3 is able to react with slipped NOx during the lean-II phase. The difference in NH3- and NO-feed timings to the SCR catalyst highly influences NOx conversion under high-temperature conditions in the NSR-SCR system. Increasing the number of NH3 adsorption sites that have low NH3O2 selectivity is important for suppressing NH3 oxidation on the SCR catalyst. The H-type NH3-adsorbing zeolite is suitable for assisting NH3 adsorption by the SCR catalyst. Fig. 3(d) depicts a reaction mechanism for the NSR-SCR system using an NH3 adsorbent mixed with the SCR catalyst. The NH3 produced by the NSR catalyst is adsorbed at the acidic sites of the NH3 adsorbent and the SCR catalyst. During the lean-I phase, NH3 is oxidized on the SCR catalyst, while the NH3 on the adsorbent remains unoxidized. The NH3 adsorbent enhances NH3-SCR selectivity during the lean-II phase by supplying adsorbed NH3 to the SCR catalyst.
3.1. Reaction selectivity for NO and NH3 in the high-temperature NSR-SCR system Fig. 1(a) compares the NOx conversions of the single-NSR and NSRSCR systems at each of the temperatures under the Test-1 reaction conditions. The NOx conversion of the single-NSR system exhibits a volcano-like temperature dependency, with a maximum of 71% observed at 450 °C. The NSR-SCR system exhibited higher NOx conversions (79% at 350 °C, 83% at 400 °C) than the NSR system (54% at 350 °C, 64% at 400 °C). In contrast, no clear differences in NOx conversion between the single-NSR and NSR-SCR systems were observed at temperatures above 500 °C. Fig. 1(b) shows NO product selectivities from the single-NSR system. Effluent concentration profiles for each gas species from the single-NSR and NSR-SCR systems are shown in the Supplementary Information (Fig. S1). In the NSR system, NO is oxidized to NO2 in the lean phase and then reduced to N2, N2O, and NH3 in the rich phase. NO2 released from the NSR catalyst showed no clear temperature dependency, with 7%–10% NO2 observed under all temperature conditions. N2O selectivity was quite low in the single-NSR system, with a maximum selectivity of 0.3% observed at 350 °C; NH3 was preferably produced under low-temperature conditions. Maximum NH3 selectivity was observed to be 37% at 350 °C, and N2 selectivity was high under high-temperature conditions, with a maximum value of 34% observed at 550 °C. Fig. 1(c) shows NO product selectivities from the NSR-SCR system. Compared with the single-NSR system (Fig. 1(b)), product selectivity for NH3 and NO2 were remarkably lower, while that of N2 was higher; NH3 is converted to N2 on the SCR catalyst in the NH3-SCR reaction or through NH3 oxidation. Fig. 1(d) shows the NH3 reaction selectivities over the SCR catalyst in the NSR-SCR system, which were evaluated from the results shown in Figs. 1(b) and 1(c) using the following equations: t
SNH3 slip (%) =
[NH3 ]NSR
0
t 0 t
SNH3
NO (%)
=
0
[NH3 ]NSR dt [NOx ]NSR dt t 0
SNH3
O2
(%) = 100%
dt
SCR
SNH3 slip
× 100 (12) t 0
[NOx ]NSR
[NH3 ]NSR dt
SNH3
NO
SCR dt
× 100 (13) (14)
3.3. Evaluating NH3-storage performance of the adsorbents by NH3-TPD
Half of the NH3 contributed to NOx reduction over the SCR catalyst under low-temperature conditions (350 °C); on the other hand, only 7% of the NH3 contributed to NOx reduction under high-temperature conditions (450 °C). These results reveal that almost all of the NH3 was oxidized over the SCR catalyst under high-temperature conditions.
Fig. 4(a)–(f) display the NH3-TPD profiles of Cu-CHA and each NH3 adsorbent. Three NH3-desorption peaks are evident in the NH3-TPD profile of H-CHA (Fig. 4(a)). The low-temperature NH3-desorption peak observed at around 200 °C (peak I) is believed to be derived from weakly adsorbed NH3 on the insides/outsides of zeolite sieves. The high-temperature NH3-desorption peaks observed at around 300 °C (Peak II) and 400 °C (Peak III) are believed to be associated with strongly adsorbed NH3 at acidic sites of zeolitic inner cages [18,19]. The NH3-TPD profile of the Cu ion-exchanged Cu-CHA (Fig. 4(b)) shows a more-intense peak II and a less-intense peak III compared to those of H-CHA. We believe that peak III in the profile of Cu-CHA originates from H-type Brønsted-acidic sites, while peak II corresponds to Cu2+type Lewis-acidic sites. A broad NH3-desorption peak was observed for Cu-CHA at higher temperatures (> 500 °C). Similar features have been reported by other groups [18–20]. The strengths of the acidic sites in a zeolite depend on the framework structure of the zeolite [21]; NH3desorption observed for Cu-CHA at higher temperatures is possibly ascribable to several different acidic sites in the CHA cage induced by structural strain brought on by Cu ion-exchange. H-MOR(20), H-MFI(30), H-BEA(25), and H-FAU(30) were chosen to assist NH3 adsorption by Cu-CHA. The peak-III temperature, total
3.2. Influence of reaction conditions (transient/static) on NH3 reactivity The high NH3-O2 selectivities (SNH3 O2 ) displayed by the NSR-SCR system in Fig. 1(d) is strongly related to the unsteady reaction conditions of the NSR-SCR system, in which transient rich and lean atmospheres are cycled To clarify the influence of the gas-feed conditions on the NH3-O2 reaction over the SCR catalyst, the NOx conversions of CuCHA under two reaction conditions were evaluated (static and transient, Fig. 2(a)). Cu-CHA exhibited high NOx conversions of more than 98% under both static and transient conditions at less than 400 °C. On the other hand, the NOx-conversion performance of Cu-CHA under static and transient conditions was quite different; NOx conversion under static conditions decreased somewhat, from 99% at 400 °C to 60% at 550 °C. On the other hand, the NOx conversion under transient conditions decreased dramatically, from 98% at 400 °C to 16% at 550 °C, which indicates that the Cu-CHA catalyst performs more poorly 4
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Fig. 2. (a) NOx conversions as functions of temperature over the SCR catalyst under static and transient conditions. (b) NH3 reaction selectivity under transient conditions at 450 °C. Fig. 3. (a) Schematically illustration of the time dependence of the slipped NOx concentration from the NSR catalyst. Schematic illustration of the NH3-SCR reaction mechanisms on the SCR catalyst in the NSR-SCR system under (b) low-temperature (≤400 °C) and (c) high-temperature (> 400 °C) conditions. (d) Schematic illustration of the function of the NH3 adsorbent under high-temperature conditions.
amount of desorbed NH3, and the amount of desorbed NH3 associated with peak III for Cu-CHA with each of the NH3 adsorbents are listed in Table 2. The highest peak-III temperature was observed for H-MOR(20), followed in order by H-CHA and H-MFI(30); this order corresponds to the acid strength of each zeolite as reported by Katada et al. [22,23]. Cu-CHA exhibited the highest total amount of desorbed NH3, followed in order by H-CHA, H-MOR(20) and H-MFI(30). Peak III is most intense for Cu-CHA, followed by H-CHA and H-MOR(20).
3.4. NH3 adsorption/oxidation under high-temperature flow conditions NH3 adsorption, NH3 desorption, and NH3 oxidation under hightemperature rich/lean transient flow conditions (Test 4 in Table 1) were examined, the results of which are displayed in Fig. 5, which schematically illustrates effluent NH3 concentrations from the NH3 adsorbents in each adsorption (rich) and desorption (lean) step; an image showing the amount of adsorbed NH3 in each step is also shown in Fig. 5. Table 3 summarizes the amount of NH3 adsorbed and desorbed in each of the steps in Fig. 5, while the actual effluent NH3 concentration profile for 5
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Fig. 4. NH3-TPD profiles of (a) HA, (b) Cu-CHA, (c) H-MOR(20), (d) H-MFI(30), (e) H-BEA(25), and (f) H-FAU(30). Table 2 Peak-III temperatures, total amounts of adsorbed NH3, and NH3 desorption associated with peak III from the NH3-TPD profiles.
Peak-III Temperature (°C) Total amount of desorbed NH3 (mmol/g) NH3 desorption associated with peak III (mmol/g)
Cu-CHA
H-CHA
H-MOR(20)
H-BEA(25)
H-MFI(30)
H-FAU(30)
378 1.15 0.55
403 1.08 0.53
512 0.85 0.36
330 0.77 0.19
402 0.85 0.27
359 0.62 0.28
each step is shown in the Supplementary Information (Fig. S2). Step I corresponds to the rich phase, in which NH3 is supplied to the empty NH3 adsorbent. The difference between the total inlet NH3 concentration and the effluent NH3 concentration is equivalent to the saturated NH3 adsorption. Cu-CHA exhibited the highest amount of adsorbed NH3 in step I (adsorbed NH3, panel A), followed by H-MOR(20) and H-MFI (30). This order is the same as that of the NH3-desorption temperatures from the strongly acidic sites (peak III) in the NH3-TPD profiles shown in Fig. 5; however the highest amount of NH3 adsorption observed in step I (adsorbed NH3, panel A) was observed for Cu-CHA, despite the NH3-desorption peak in its NH3-TPD trace being less intense at temperatures less than 450 °C, which is ascribable to the influence of other gas species, such as CO2 and H2O. Hanna et al. has discussed kinetics
models for NH3 adsorption and desorption in the presence of H2O [24], and have suggested that H2O blocks NH3 adsorption; Brønsted acidic sites are much more influenced by adsorbed H2O than Cu-exchanged NH3-adsorption sites. The adsorbed NH3 was desorbed or oxidized in the lean phase during step II. The adsorbents were refilled with NH3 in the subsequent rich phase during step III. The difference between the amount of NH3 desorbed in step II (desorbed NH3, panel B) and the amount of adsorbed NH3 in step III (adsorbed NH3, panel C) is equivalent to the amount of oxidized NH3 in step II. Desorbed NH3 (B) was low and adsorbed NH3 (C) was high for Cu-CHA, which indicates that the amount of NH3 oxidized by Cu-CHA is high. Other H-type NH3 adsorbents did not appear to oxidize NH3. The difference between the amount of saturated 6
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catalyst under transient conditions at 450 °C as a function of temperature. The mixed H-MOR(20)/Cu-CHA catalyst exhibited high NOx conversions over a wide temperature range (150–550 °C); the NOx conversions of this catalyst are significantly higher than those of CuCHA at temperatures higher than 400 °C, but are slightly lower at temperatures below 300 °C. The amount of Cu in the H-MOR(20)/CuCHA catalyst is half that of Cu-CHA in this experiment; there was a slight insufficiency of the active component in the mixed catalyst to sustain reactivity at the low temperatures. We next examined the optimum H-MOR(20)/Cu-CHA ratio for activity over a wide temperature range. Fig. 9 shows NOx conversions at various mixing ratios at 250 °C and 450 °C under the transient conditions of Test 4, as detailed in Table 1. NOx conversion was observed to gradually decrease with increasing H-MOR/Cu-CHA ratio above 450 °C. The mixed catalyst with an H-MOR/Cu-CHA ratio of 10 exhibited higher NOx conversion than that of Cu-CHA, despite most of the mixed catalyst being composed of H-MOR(20), which indicates that the NH3 adsorbent at high temperature is able to overcome the effect of the lower amount of Cu-CHA. NOx conversion was observed to increase with increasing H-MOR/Cu-CHA ratio to 250 °C, which is believed to be due to a decrease in the total amount of Cu in the catalyst; SCR performance was highly influenced by the amount of Cu present at low temperatures.
Fig. 5. Schematic diagram depicting Test 3: NH3 adsorption and NH3 desorption/oxidation under high-temperature flow conditions.
NH3 adsorption (adsorbed NH3, panel A) and the amount of oxidized NH3 (B–C) is equivalent to the amount of unoxidized NH3 stored in the NH3 adsorbent or desorbed from the NH3 adsorbent even under O2-rich conditions. H-MOR(20) exhibited the highest amount of unoxidized NH3, followed by H-MFI(30) and then Cu-CHA. 3.5. Transient NOx reduction by the mixed NH3-adsorbent + Cu-CHA catalyst system
3.6. Performance of the NH3-adsorbent + Cu-CHA catalyst in the NSRSCR system
Fig. 6 shows NO conversions from the mixed NH3-adsorbent + CuCHA catalysts under transient conditions at 450 °C, which models the outlet of an NSR catalyst, as described in Test 4 and summarized in Table 1. Almost all of the NH3-adsorbent-mixed Cu-CHA catalysts showed higher NOx conversions than Cu-CHA alone. The mixed HMOR/Cu-CHA catalyst exhibited higher NOx conversions than all of the catalysts in this study. When the same zeolite structures are compared, a lower SiO2/Al2O3 ratio in the adsorbent results in a higher NOx conversion, which indicates that the number of acidic sites in the NH3 adsorbent is important for NOx conversion in the transient SCR reaction. We believe that increasing the number of NH3 adsorption sites capable of storing NH3 without its oxidation, even under high-temperature lean/rich transient conditions, is important for improving NH3-SCR reactivity under NSR-SCR conditions. Fig. 7 displays relationships between the NOx conversions shown Fig. 6 and the amount of unoxidized NH3 in each of the NH3 adsorbents listed in Table 3; the best correlation between unoxidized NH3 and transient NOx conversion was observed at 450 °C, with a correlation coefficient of 0.98. Other relationships between NOx conversions (Fig. 6) and the NH3-TPD profiles of each NH3 adsorbent listed in Table 2 are provided in the Supplementary Information (Fig. S3). A high correlation was observed between NOx conversion and the peak-III (NH3-desorption) temperature, with a correlation coefficient of 0.88, which indicates that the acid strength of the NH3 adsorbent is important for NOx reduction under high-temperature transient SCR-reaction conditions. H-MOR(20) was expected to be the best NH3 adsorbent for mixing with the SCR catalyst in order to improve NOx conversion in the NSRSCR system. Fig. 8 shows NOx conversion over the mixed H-MOR(20)
A mixed H-MOR(20)+Cu-CHA monolith catalyst with the preferred 2:1 ratio was prepared, and its catalytic performance when combined with an NSR catalyst composed of a mixture of H-MOR(20) and the SCR catalyst (which is referred to as the “NSR-SCR + NH3ad” system) was evaluated. Fig. 10(a) shows NOx conversions for the NSR-SCR + NH3ad system under the reaction conditions of Test 1, as detailed in Table 1. The NSR-SCR + NH3ad system exhibited higher NOx conversions than the NSR-SCR system in the high (350–550 °C) temperature range, while Fig. 10(b) shows the NO reaction selectivities of the NSR- SCR + NH3ad system, which reveals selectivities to N2 that were further improved over those of the NSR-SCR system, especially at 450 °C (from 74% to 84%) and 500 °C (from 62% to 69%). Fig. 10(c) displays the NH3 reaction selectivities. Compared to the NSR-SCR system, the reaction selectivities were further improved, from 5% to 38% at 450 °C, and from almost 0 to 9% at 500 °C, in the NSR-SCR + NH3ad system. The NOx effluent profiles of each system are shown in Fig. 10(d), while those of other gas species are shown in the Supplementary Information (Fig. S4). In the case of the NSR-SCR system, the NOx concentration was lower than that of the single-NSR system at the beginning of the lean phase, but it gradually increased and was almost the same as that of the singleNSR system at the end of the lean phase. On the other hand, the NOx effluent concentration was highly suppressed from the beginning to the end of lean phase in the case of NSR-SCR + NH3ad system. 3.7. Function of the NH3 adsorbent in the NSR-SCR system The N2 production profile during the lean phase was evaluated by
Table 3 Amounts of adsorbed, desorbed, and oxidized NH3 from Test 3. Step
Test 4
H-MOR(20) mmol/g
H-MFI(30) mmol/g
H-BEA(25) mmol/g
H-FAU(30) mmol/g
H-CHA mmol/ g
Cu-CHA mmol/g
I(Rich) II(Lean) III(Rich)
Adsorbed NH3 (A) Desorbed NH3 (B) Adsorbed NH3 (C)
0.47 0.29 0.27
0.16 0.16 0.15
0.05 0.05 0.05
0.06 0.09 0.06
0.12 0.07 0.12
0.75 0.03 0.69
Oxidized NH3 (C-B) Non-Oxidized NH3 (A-(C-B)) (= Desorbed NH3 + Stored NH3)
0 0.47
0 0.16
0 0.05
0 0.06
0.05 0.07
0.66 0.09
7
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Fig. 6. NOx conversions of several mixed Cu-CHA + NH3 adsorbent catalysts under transient reaction conditions at 450 °C.
Fig. 9. NOx conversions of the mixed Cu-CHA+H-MOR(20) catalyst as a function of the H-MOR/Cu-CHA ratio at 250 °C and 450 °C.
Fig. 7. NOx conversion under transient SCR conditions at 450 °C (from Fig. 6) as a function of unoxidized NH3 under the lean conditions detailed in Table 3.
intensity increased from the middle of the lean phase onwards. Both the NH3-SCR and NH3-O2 reactions produce N2 (Eqs. (1)–(3)). The N2 observed at the beginning of lean phase is believed to be mainly derived from the NH3-O2 reaction, because of the low slipped NOx concentration during this lean phase, as depicted in Fig. 3(a). The adsorbed NH3 can react with NOx from the middle to the end of the lean phase, during which the slipped NOx concentration is increased. Therefore, the N2 observed at the later lean phase is believed to be mainly derived from the NH3-SCR reaction. The characteristic N2 formation observed suggests that the NH3 adsorbent suppresses the NH3O2 reaction at the beginning of the lean phase, and enhances the NH3NO reaction during the later lean phase. The gradual increase of N2 production shown in Fig. S5 suggests a new reaction mechanism, in which a NH3 adsorbed on the NH3 adsorbent gradually diffuses to the SCR catalyst and reacts with slipped NOx, as illustrated on Fig. 11. The N2 formation properties of NSR-SCR + NH3ad system also suggest that the NH3 adsorbent is capable of stabilizing the NH3 concentration fluctuations from rich/lean operation. We only examined a physical mixture of the SCR catalyst and NH3 adsorbent in this report; we believe that a sequential triple-bed or tri-layer coated NSRNH3adsorbent-SCR configuration would be effective in extending the NOx reaction period of the lean phase. The NH3 diffusion rate of the internal NH3 adsorbent should be important if the contact area of the NH3 adsorbent and SCR catalyst is decreased. The properties of the NH3 adsorbent (acid strength, particle size, pore structure, and defects) should be adjusted according to catalytic configuration and operation conditions. Moreover, the thermal stability of the NH3 adsorbent is
Fig. 8. NOx conversions of the mixed Cu-CHA+H-MOR(20) and Cu-CHA catalysts as functions of temperature.
GC–MS using a He balance. Fig. 11 shows the formation of N2 as monitored by the intensity of the m/z = 28 signal during rich/lean cycling under Test-5 conditions at 450 °C for the single-NSR, NSR-SCR, and NSR-SCR + NH3ad systems; the results for 350 and 400 °C are provided in the Supplementary Information (Fig. S5). In the case of the single-NSR system, N2 production was observed only during the rich phase, while the NSR-SCR system exhibited additional N2 production at the beginning of the lean phase. In the case of the NSR-SCR + NH3ad system, N2 production was less intense at the beginning of the lean phase and was lower than that of the NSR-SCR system; however, the 8
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M. Sakai, et al.
Fig. 10. (a) NOx conversions as functions of temperature for the single-NSR, NSR-SCR, and NSR-SCR + NH3ad systems (Cu-CHA:HMOR = 1:2). (b) Product selectivities from NO for the NSR-SCR + NH3ad system. (c) NH3 reaction selectivities for the NSR-SCR + NH3ad system. (d) Outlet NO concentration profiles for the single-NSR, NSR-SCR, and NSRSCR + NH3ad systems.
Fig. 11. N2 formation from single-NSR, NSR-SCR, and NSR-SCR + NH3ad pellet catalysts under transient condition at 450 °C. Reaction conditions: 5% O2, 450 ppm NO, and He balance during the lean phase (60 s), and 1.5% H2 and He balance during the rich phase (3 s). GHSV = 120,000 h−1.
another important problem in this system that needs to be addressed. The results and discussion considering the thermal stability of H-MOR (20) and H-MOR(20) mixed Cu-CHA were included in the supplemental
information (Figs. S6–S8). The suppression of zeolite dealumination and development of thermally stable NH3 adsorbents can be studied in future research. 9
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4. Conclusion
References
We first investigated ways of improving the high-temperature NOx reduction performance for the NSR-SCR system by measuring its catalytic activity under several catalytic configuration and reaction conditions, and then attempted to modify the NH3 adsorption and reaction properties of the SCR catalyst for high-temperature reaction by adding the NH3 adsorbent. NH3 oxidation is highly enhanced under hightemperature transient NSR-SCR conditions, which is due to the time lag between the slipped NOx and NH3 fed to the SCR catalyst. Herein, we introduce a new approach that involves mixing an NH3 adsorbent with the SCR catalyst to suppress NH3 oxidation under transient NSR-SCR reaction condition. H-MOR was found to be the most suitable NH3 adsorbent, which can retain NH3 in its inner sites at high temperatures in the absence of highly oxidizing Cu reaction sites. N2 is formed in the mixed H-MOR/NSR-SCR system during the end of the lean phase. The reaction mechanism is believed to involve the gradual diffusion of the NH3 adsorbed on the H-MOR during the rich phase to the SCR catalyst, where it reacts with NOx until the end of the lean phase. The proposed strategy provides useful insight for future catalyst design. The approach that suppresses side reactions with promoters is more effective than increasing the number of active sites, especially when the catalysts are used under transient reaction conditions.
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Acknowledgement We would like to thank Editage (www.editage.jp) for English language editing. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcata.2019.06.003.
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Improving high-temperature NOx conversion in the combined NSR-SCR system with an SCR catalyst mixed with an NH3 adsorbent
T
Masatoshi Sakai , Tsuyoshi Hamaguchi, Toshiyuki Tanaka ⁎
Toyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192, Japan
ARTICLE INFO
ABSTRACT
Keywords: NSR SCR NSR-SCR High temperature NOx reduction NH3 adsorbent
The growing worldwide interest in environmental protection and the strong demand to decrease the emission of pollutants from automotive exhaust require the development of catalysts and post-cleaning systems that are very efficient and stable. To that end, we investigated ways of improving the high-temperature NH3 selective catalytic reduction (SCR) activity in the NOx storage reduction (NSR) and SCR combined system, and found that NH3 oxidation is promoted under high-temperature NSR-SCR reaction conditions. To suppress NH3 oxidation and improve the NH3-SCR reaction in the NSR-SCR system, we examined mixing an NH3 adsorbent with the SCR catalyst (NSR-SCR + NH3 adsorbent system). The H-type mordenite zeolite (H-MOR) adsorbent demonstrated superior performance for retaining NH3 without oxidation under high-temperature transient conditions than the other NH3 adsorbents examined. NOx conversion under high-temperature transient conditions was improved in going from the single NSR system (71% at 450 °C) to NSR-SCR (75% at 450 °C), and NSR-SCR+H-MOR (85% at 450 °C). We confirmed that N2 formed over long times, from the commencement to the end of the lean phase, in the NSR-SCR+H-MOR system. This phenomenon, in which the NH3 adsorbed on H-MOR is gradually released to the SCR up to the end of the lean phase, enhances NOx reduction in the NSR-SCR system.
1. Introduction In response to the growing worldwide interest in environmentally sustainable transportation, both industry and government have been focusing on decreasing the emission of pollutants from automotive exhaust and developing zero emission technologies. In order to promote improved atmospheric quality in urban areas, and in particular, to suppress NOx emissions from diesel engine vehicles, a new testing program (the “Real-Driving Emissions” or RDE program) for measuring the emissions from on-road vehicles has been introduced in Europe [1]. In the RDE program, automobiles are tested under a wide range of driving conditions, including continuous stop/start driving in urban areas, and high-load and high-speed driving conditions in rural areas or on motorways. Therefore, automotive exhaust catalysts and postcleaning systems with high purification efficiencies are required, even under unsteady operating conditions in which temperature, gas concentrations, and gas velocities of exhaust gases fluctuate highly. Reducing the large amounts of NOx emissions from “lean-burn” combustion-engine vehicles is a major RDE-testing problem. NOx storage reduction (NSR) systems and selective catalytic reduction (SCR) systems are widely known methods for effectively removing NOx under lean conditions. The NSR system operates under cyclic fuel-lean and ⁎
fuel-rich conditions [2–4]. NSR catalysts contain precious metals (typically Pt or Rh), while and NOx storage materials typically contain BaO or K2O. NOx storage materials store NOx in the form of nitrate or nitrite for a few minutes in the lean phase, while stored NOx is reduced to N2, as the most desirable product, over a few seconds in the rich phase. The disadvantages of the NSR system include the relatively narrow temperature window of 250–400 °C required for high NOx removal efficiency, the production of undesirable NH3 and N2O under certain richphase conditions, and the dependence on the catalyst composition. The SCR system, which is the other lean NOx reduction system, uses NH3 to reduce NOx through the following reactions:
2NO + 2NH3 + 1/2O2
NO + NO2 + 2NH3
2N2 + 3H2 O
(standard
2N2 + 3H2 O (fast
SCR)
SCR)
(1) (2)
Basically, Cu or Fe ion-exchanged zeolites show excellent NH3-SCR performance. In the commercialized SCR system for Diesel vehicles, urea is injected into the fuel gases as the NH3 precursor; however, the requirements for a tank for the aqueous urea solution and a complex NH3 injection system are disadvantages of this system [5]. While the urea-SCR system is capable of achieving high NOx conversions in a higher temperature range than the NSR system, side reactions
Corresponding author. E-mail address: [email protected] (M. Sakai).
https://doi.org/10.1016/j.apcata.2019.06.003 Received 30 March 2019; Received in revised form 2 June 2019; Accepted 4 June 2019 Available online 05 June 2019 0926-860X/ © 2019 Elsevier B.V. All rights reserved.
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associated with NH3 oxidation cannot be neglected:
2NH3 + 3/2O2
2. Experimental (3)
N2 + 3H2 O
2.1. Catalysts and NH3 adsorbent materials
Combining the NSR and SCR catalysts has been proposed as a method of overcoming the disadvantages of both systems (i.e., NH3 formation from the NSR and the urea-injection system of the SCR) [6–15]. The NSR-SCR system uses the NH3 actively produced on the NSR catalyst during the rich phase to reduce NOx on the SCR catalyst during the subsequent lean phase. The catalytic performance and reaction mechanisms of various NSR-SCR system configurations, such as physical mixtures [7,8], sequential dual-beds [6–10], and dual-layer coated monoliths [11,12], have been discussed. Compared to the single NSR system, higher NOx reduction efficiency and higher N2 formation selectivity were confirmed in each NSR-SCR system at moderate temperatures (250–400 °C). De-La-Torre’s group has reported that high NO conversion was observed in a temperature range from 220 °C to 320 °C in monolith-type NSR and SCR catalysts, as in the real application form. They suggested that Cu-CHA, which Cu exchanges in chabazite-type zeolites, is the most promising SCR catalyst because of its high thermal durability and high N2 selectivity [10]. On the other hand, few reports have focused on the performance of NSR-SCR systems in the hightemperature region (> 400 °C). The NSR-SCR system would not suitable for NOx reduction under high-temperature conditions because of the lack of NH3 formation over the NSR catalyst and the enhanced NH3 oxidation over the SCR catalyst at high temperature. The group of Digiulio has suggested that the optimum operating temperature is around 250–400 °C; the formation of a sufficient amount of NH3 over the NSR catalyst may be difficult above 400 °C [13]. The effect of NH3 oxidation in the NSR-SCR system has mainly been discussed for non-zeolite SCR catalysts [14,15]; however, zeolite-type SCR catalysts would be also affected by NH3 oxidation, depending on the chemical/physical properties of active metal species [16]. Catalysis temperatures exceed 400 °C in some cases, depending on the weight of the car and the driving condition [9,17], and highly efficient NOx reduction at high temperatures is still required for lean-burn engines. With the aim of improving NOx reduction under high-temperature conditions (> 400 °C in this article), we focused particularly on suppressing NH3 oxidation on the SCR catalyst by mixing an NH3 adsorbent with the SCR catalyst in order to assist high-temperature NH3 adsorption. We considered that the selectivity of the NH3-SCR reaction on the SCR catalyst would be enhanced if the number of NH3 adsorption sites, which retain NH3 without oxidizing it under high-temperature conditions, could be increased in the SCR catalyst. In this article, we first discuss factors that enhance activity by examining the catalytic performance of an NSR-SCR system under high-temperature conditions, and then report the NOx-conversion performance of NSR-SCR systems with NH3 adsorbents mixed with the SCR catalyst. Finally, we discuss the N2 production mechanism of different catalytic configurations to obtain guidelines for high-performance catalyst design.
The NSR catalyst was synthesized by the wet impregnation method. The active component, namely Pt (0.74 wt%), and NOx storage materials of Ba (0.74 mmol/g), K (0.56 mmol/g), and Li (0.37 mmol/g) were supported on Al2O3. The NSR catalyst was calcined at 750 °C for 5 h in air to stabilize its catalytic performance. A Cu ion-exchanged zeolite with the Chabazite framework (Cu-CHA) was used as the SCR catalyst. High-silica-content zeolites provided by the Tosoh Corporation (Japan) were selected as NH3 adsorbents. The NH3 adsorbents are labeled according their framework and the SiO2/Al2O3 ratio as: H-[framework code of the International Zeolite Association](SiO2/Al2O3 ratio), for example “H-MOR(20)”. Each NH3 adsorbent was physically mixed with the SCR catalyst using a mortar and pestle. Pellet-type catalysts were produced by compressing each catalyst powder to a hydrostatic pressure of 1 t, and then sieving its particles to 0.5–1 mm in size. Monolithtype catalysts were prepared by continuously wash-coating with an aqueous slurry of the as-prepared catalyst and the Al2O3 binder onto φ 30 mm × 25 mm cordierite honeycomb monoliths to the desired catalyst loading (NSR: 200 g/L, SCR: 360 g/L). The catalytic configuration in which only the NSR catalyst is used is referred to as the single-NSR system. The configuration which the SCR catalyst was located downstream from the NSR catalyst is referred to as the NSR-SCR system. 2.2. Characterization by NH3-TPD NH3-Temperature programmed desorption (NH3-TPD) with a TP5000 apparatus (Ohkura Riken Co., Ltd.) was used to characterize the acidic properties of the NH3 adsorbents. A 0.2 g sample of each NH3 adsorbent was pretreated under 20%-O2/He-balance conditions for 30 min prior to any experiment. A mixed gas composed of 4000 ppm NH3 with a He balance was supplied to each NH3 adsorbent at a flow rate of 100 mL/min for 30 min at 100 °C until saturated with adsorbed NH3. After 1 h the sample was purged with He at 300 mL/min and the reactor temperature was increased from 100 °C to 800 °C at a rate of 10 °C/min. The amount of desorbed NH3 was determined by mass spectrometry by monitoring the m/z = 16 signal using a quadrupole mass spectrometer (Qulee BGM-202, ULVAC, Inc.). 2.3. Reactor testing of the catalysts and NH3 adsorbents Test 1 was conducted on single-NSR and NSR-SCR monolith-type catalysts using fix-bed flow reactor (BEX-8900, CATA-5000 Best Instruments Co, Ltd.). The total flow rate was 15 L/min (SV = 50,000 h−1 for each catalyst). The gas feed conditions are also summarized in Table 1. The lean component gas (Test 1 Lean) and the rich component gas (Test 1 Rich) were repeatedly fed with a 60-s/3-s lean/rich cycle.
Table 1 Gas feed conditions in each test. Test Test 1 Test 2 Test 3 Test 4 Test 5 Pretreat
Lean Rich Lean Rich Static Lean Rich Lean Rich Lean Rich Lean Rich
Duration (s)
CO (vol%)
H2 (vol%)
CO2 (vol%)
H2O (vol%)
O2 (vol%)
NO (ppm)
NH3 (ppm)
Balance gas
Flow rate (L/min)
60 3 56 4 – 180 180 56 4 60 3 300 300
– 4.5 – – – – – – – – – – 0.13
– 1.5 – – – – – – – – 1.5 – 0.04
10 10 10 10 10 10 10 10 10 – – 10 –
5 5 8 8 10 8 8 8 8 – – 8 8
5.5 – 10 – 10 10 – 10 – 5 – 10 –
400 – 100 – 440 – – 100 – 450 – – –
– – – 1500 550 – 1000 – 1500 – – – –
N2 N2 N2 N2 N2 N2 N2 N2 N2 He He N2 N2
15 15 10 10 10 10 10 10 10 10 10 15 15
2
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Fig. 1. (a) NOx conversions as function of temperature for the single-NSR and coupled NSR-SCR systems. Product selectivities from NO for the (b) single-NSR and (c) NSR-SCR systems. (d) Reaction selectivities from NH3 over the SCR catalyst.
Prior to any experiment, each catalyst was pretreated at 550 °C under reduction conditions (Pretreat Rich) and oxidation condition (Pretreat Lean). The NOx conversion (XNOx) and the product selectivity from NO (Sx) were calculated using the following equations: tc
[NOx ]0
0
XNOx (%) =
tc 0 tc 0 tc
SNO (slip) (%) =
tc 0 tc
SN2 O (%) =
2
tc 0 tc 0
SN2 (%) = 100
× 100 (5)
× 100
[NOx ]0 dt
Adsorbed NH3 = NH3in, rich
(7)
[N2 O] dt [NOx ]0 dt
SNOslip
(6)
× 100
[NOx ]0 dt
0
× 100 (4)
[NO] dt
[NH3 ] dt
0 tc
SNH3 (%) =
[NO] dt
[NOx ]
0
SNO2 (%) =
[NOx ]0 dt
[NOx ]0 dt
0 tc
[NOx ] dt
reduced from 550 °C to 250 °C at a rate of 10 °C/min. Under transient condition, 1 g of the pellet-type SCR catalyst was repeatedly exposed to a lean gas (Test 2 Lean) and a rich gas (Test 2 Rich) with a 56-s/4-s lean/rich cycle. The temperature was gradually decreased from 550 °C to 250 °C in 50 °C increments. NOx conversion was determined over 10lean/rich cycles at each temperature, using the same equations as those used in Test 1 (Eq. 4–9). Before each experiment, the catalyst was pretreated under the same conditions described for Test 1. Test 3 was used to determine the NH3-adsorption/desorption profiles of the NH3 adsorbents under cyclic lean/rich flow conditions. Each NH3 adsorbent was also pretreated as described in Test 1. The “Test 3 Rich” gas was fed to 1 g of a pellet-type NH3 adsorbent for 3 min at a total flow rate of 10 L/min at 450 °C to fully adsorb NH3 onto the NH3 adsorbent. The gas conditions were subsequently switched to “Test 3 Lean” for 3 min to examine NH3 desorption under an O2 atmosphere. After rich/lean cycles, the amounts of adsorbed NH3 and desorbed NH3 in each step were calculated using the following equations:
Desorbed × 100
SNO2
S N2 O
NH3 = NH3out, lean
NH3in, lean
(10) (11)
Test 4 was used to evaluating the transient NOx conversions of SCR catalysts under modeled NSR outlet-gas conditions. The reaction conditions (gas components, total flow rate, lean/rich duration times) were as described for the transient conditions in Test 2, but the catalyst weight was increased to 2 g. Test 5 was performed to determine the dynamics of N2 formation under the NSR-SCR system by mass spectrometry (JMS-GC-mate II, JEOL Ltd.) and an emissions analyzer (MEXA-7100, HORIBA, Ltd.). The mixture gas flowed to 1 g of a pellet-type NSR catalyst, or sequentially to 1 g of the NSR catalyst and 1 g of the SCR catalyst. The total gas flow rate was 10 L/min.
(8)
SNH3
NH3out, rich
(9)
Values within square brackets are the concentrations of each component, tc is the total time required for 10 rich/lean cycles at each temperature. Test 2 was conducted on the Cu-CHA catalyst under static conditions (standard SCR conditions) and cyclic lean/rich transient conditions to evaluate the difference in SCR catalytic performance according to the two reaction conditions. Under static conditions, 1 g of the pellettype SCR catalyst was exposed to the reactant gas indicated in Table 1 in which NH3 and NO coexist (Test 2 Static). The temperature was 3
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3. Results and discussion
under transient conditions, in which NH3 and NO feed the SCR catalyst separately. NH3 slip or NH3 oxidation over the SCR catalyst is possibly due to unreacted NH3 present with NOx. Fig. 2(b) shows the NH3 reaction selectivity under transient reaction conditions at 450 °C; half of the supplied NH3 was oxidized under these conditions, while the amount of slipped NH3 was low (less than 0.2%). This result indicates that preventing the NH3-O2 reaction on the SCR catalyst under transient conditions is the key to improving NOx conversion over the SCR catalyst in the NSR-SCR system. The reaction over the SCR catalyst under transient conditions (the NSR-SCR system) is discussed in terms of three phase, namely rich, lean I, and lean II. Fig. 3(a) shows the time dependence of the concentration of slipped NOx from the NSR catalyst. The NSR catalyst stores sufficient NOx at the beginning of the lean-I phase; hence the slipped NOx concentration is low during this phase. The NOx concentration gradually increases from the beginning of the lean-I phase to the end of the lean-II phase, because the NSR catalyst releases unadsorbed NOx. Fig. 3(b) depicts the reactions of adsorbed NH3 on the SCR catalyst under low-temperature conditions (≤ 400 °C) in the NSR-SCR system. NH3 is adsorbed onto the SCR catalyst during the rich phase. At sufficiently low temperatures under which NH3 oxidation reactivity is low, high NOx conversion is expected through the reaction of adsorbed NH3 and slipped NOx during the NOx-rich lean-II phase, because adsorbed NH3 remains present and is not oxidized during the lean-I phase. Fig. 3(c) depicts the reactions of adsorbed NH3 on the SCR catalyst under high-temperature conditions (> 400 °C) in the NSR-SCR system. Adsorbed NH3 is oxidized at several Cu sites in the Cu-CHA during the lean-I phase at high temperatures. Only unoxidized NH3 is able to react with slipped NOx during the lean-II phase. The difference in NH3- and NO-feed timings to the SCR catalyst highly influences NOx conversion under high-temperature conditions in the NSR-SCR system. Increasing the number of NH3 adsorption sites that have low NH3O2 selectivity is important for suppressing NH3 oxidation on the SCR catalyst. The H-type NH3-adsorbing zeolite is suitable for assisting NH3 adsorption by the SCR catalyst. Fig. 3(d) depicts a reaction mechanism for the NSR-SCR system using an NH3 adsorbent mixed with the SCR catalyst. The NH3 produced by the NSR catalyst is adsorbed at the acidic sites of the NH3 adsorbent and the SCR catalyst. During the lean-I phase, NH3 is oxidized on the SCR catalyst, while the NH3 on the adsorbent remains unoxidized. The NH3 adsorbent enhances NH3-SCR selectivity during the lean-II phase by supplying adsorbed NH3 to the SCR catalyst.
3.1. Reaction selectivity for NO and NH3 in the high-temperature NSR-SCR system Fig. 1(a) compares the NOx conversions of the single-NSR and NSRSCR systems at each of the temperatures under the Test-1 reaction conditions. The NOx conversion of the single-NSR system exhibits a volcano-like temperature dependency, with a maximum of 71% observed at 450 °C. The NSR-SCR system exhibited higher NOx conversions (79% at 350 °C, 83% at 400 °C) than the NSR system (54% at 350 °C, 64% at 400 °C). In contrast, no clear differences in NOx conversion between the single-NSR and NSR-SCR systems were observed at temperatures above 500 °C. Fig. 1(b) shows NO product selectivities from the single-NSR system. Effluent concentration profiles for each gas species from the single-NSR and NSR-SCR systems are shown in the Supplementary Information (Fig. S1). In the NSR system, NO is oxidized to NO2 in the lean phase and then reduced to N2, N2O, and NH3 in the rich phase. NO2 released from the NSR catalyst showed no clear temperature dependency, with 7%–10% NO2 observed under all temperature conditions. N2O selectivity was quite low in the single-NSR system, with a maximum selectivity of 0.3% observed at 350 °C; NH3 was preferably produced under low-temperature conditions. Maximum NH3 selectivity was observed to be 37% at 350 °C, and N2 selectivity was high under high-temperature conditions, with a maximum value of 34% observed at 550 °C. Fig. 1(c) shows NO product selectivities from the NSR-SCR system. Compared with the single-NSR system (Fig. 1(b)), product selectivity for NH3 and NO2 were remarkably lower, while that of N2 was higher; NH3 is converted to N2 on the SCR catalyst in the NH3-SCR reaction or through NH3 oxidation. Fig. 1(d) shows the NH3 reaction selectivities over the SCR catalyst in the NSR-SCR system, which were evaluated from the results shown in Figs. 1(b) and 1(c) using the following equations: t
SNH3 slip (%) =
[NH3 ]NSR
0
t 0 t
SNH3
NO (%)
=
0
[NH3 ]NSR dt [NOx ]NSR dt t 0
SNH3
O2
(%) = 100%
dt
SCR
SNH3 slip
× 100 (12) t 0
[NOx ]NSR
[NH3 ]NSR dt
SNH3
NO
SCR dt
× 100 (13) (14)
3.3. Evaluating NH3-storage performance of the adsorbents by NH3-TPD
Half of the NH3 contributed to NOx reduction over the SCR catalyst under low-temperature conditions (350 °C); on the other hand, only 7% of the NH3 contributed to NOx reduction under high-temperature conditions (450 °C). These results reveal that almost all of the NH3 was oxidized over the SCR catalyst under high-temperature conditions.
Fig. 4(a)–(f) display the NH3-TPD profiles of Cu-CHA and each NH3 adsorbent. Three NH3-desorption peaks are evident in the NH3-TPD profile of H-CHA (Fig. 4(a)). The low-temperature NH3-desorption peak observed at around 200 °C (peak I) is believed to be derived from weakly adsorbed NH3 on the insides/outsides of zeolite sieves. The high-temperature NH3-desorption peaks observed at around 300 °C (Peak II) and 400 °C (Peak III) are believed to be associated with strongly adsorbed NH3 at acidic sites of zeolitic inner cages [18,19]. The NH3-TPD profile of the Cu ion-exchanged Cu-CHA (Fig. 4(b)) shows a more-intense peak II and a less-intense peak III compared to those of H-CHA. We believe that peak III in the profile of Cu-CHA originates from H-type Brønsted-acidic sites, while peak II corresponds to Cu2+type Lewis-acidic sites. A broad NH3-desorption peak was observed for Cu-CHA at higher temperatures (> 500 °C). Similar features have been reported by other groups [18–20]. The strengths of the acidic sites in a zeolite depend on the framework structure of the zeolite [21]; NH3desorption observed for Cu-CHA at higher temperatures is possibly ascribable to several different acidic sites in the CHA cage induced by structural strain brought on by Cu ion-exchange. H-MOR(20), H-MFI(30), H-BEA(25), and H-FAU(30) were chosen to assist NH3 adsorption by Cu-CHA. The peak-III temperature, total
3.2. Influence of reaction conditions (transient/static) on NH3 reactivity The high NH3-O2 selectivities (SNH3 O2 ) displayed by the NSR-SCR system in Fig. 1(d) is strongly related to the unsteady reaction conditions of the NSR-SCR system, in which transient rich and lean atmospheres are cycled To clarify the influence of the gas-feed conditions on the NH3-O2 reaction over the SCR catalyst, the NOx conversions of CuCHA under two reaction conditions were evaluated (static and transient, Fig. 2(a)). Cu-CHA exhibited high NOx conversions of more than 98% under both static and transient conditions at less than 400 °C. On the other hand, the NOx-conversion performance of Cu-CHA under static and transient conditions was quite different; NOx conversion under static conditions decreased somewhat, from 99% at 400 °C to 60% at 550 °C. On the other hand, the NOx conversion under transient conditions decreased dramatically, from 98% at 400 °C to 16% at 550 °C, which indicates that the Cu-CHA catalyst performs more poorly 4
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Fig. 2. (a) NOx conversions as functions of temperature over the SCR catalyst under static and transient conditions. (b) NH3 reaction selectivity under transient conditions at 450 °C. Fig. 3. (a) Schematically illustration of the time dependence of the slipped NOx concentration from the NSR catalyst. Schematic illustration of the NH3-SCR reaction mechanisms on the SCR catalyst in the NSR-SCR system under (b) low-temperature (≤400 °C) and (c) high-temperature (> 400 °C) conditions. (d) Schematic illustration of the function of the NH3 adsorbent under high-temperature conditions.
amount of desorbed NH3, and the amount of desorbed NH3 associated with peak III for Cu-CHA with each of the NH3 adsorbents are listed in Table 2. The highest peak-III temperature was observed for H-MOR(20), followed in order by H-CHA and H-MFI(30); this order corresponds to the acid strength of each zeolite as reported by Katada et al. [22,23]. Cu-CHA exhibited the highest total amount of desorbed NH3, followed in order by H-CHA, H-MOR(20) and H-MFI(30). Peak III is most intense for Cu-CHA, followed by H-CHA and H-MOR(20).
3.4. NH3 adsorption/oxidation under high-temperature flow conditions NH3 adsorption, NH3 desorption, and NH3 oxidation under hightemperature rich/lean transient flow conditions (Test 4 in Table 1) were examined, the results of which are displayed in Fig. 5, which schematically illustrates effluent NH3 concentrations from the NH3 adsorbents in each adsorption (rich) and desorption (lean) step; an image showing the amount of adsorbed NH3 in each step is also shown in Fig. 5. Table 3 summarizes the amount of NH3 adsorbed and desorbed in each of the steps in Fig. 5, while the actual effluent NH3 concentration profile for 5
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Fig. 4. NH3-TPD profiles of (a) HA, (b) Cu-CHA, (c) H-MOR(20), (d) H-MFI(30), (e) H-BEA(25), and (f) H-FAU(30). Table 2 Peak-III temperatures, total amounts of adsorbed NH3, and NH3 desorption associated with peak III from the NH3-TPD profiles.
Peak-III Temperature (°C) Total amount of desorbed NH3 (mmol/g) NH3 desorption associated with peak III (mmol/g)
Cu-CHA
H-CHA
H-MOR(20)
H-BEA(25)
H-MFI(30)
H-FAU(30)
378 1.15 0.55
403 1.08 0.53
512 0.85 0.36
330 0.77 0.19
402 0.85 0.27
359 0.62 0.28
each step is shown in the Supplementary Information (Fig. S2). Step I corresponds to the rich phase, in which NH3 is supplied to the empty NH3 adsorbent. The difference between the total inlet NH3 concentration and the effluent NH3 concentration is equivalent to the saturated NH3 adsorption. Cu-CHA exhibited the highest amount of adsorbed NH3 in step I (adsorbed NH3, panel A), followed by H-MOR(20) and H-MFI (30). This order is the same as that of the NH3-desorption temperatures from the strongly acidic sites (peak III) in the NH3-TPD profiles shown in Fig. 5; however the highest amount of NH3 adsorption observed in step I (adsorbed NH3, panel A) was observed for Cu-CHA, despite the NH3-desorption peak in its NH3-TPD trace being less intense at temperatures less than 450 °C, which is ascribable to the influence of other gas species, such as CO2 and H2O. Hanna et al. has discussed kinetics
models for NH3 adsorption and desorption in the presence of H2O [24], and have suggested that H2O blocks NH3 adsorption; Brønsted acidic sites are much more influenced by adsorbed H2O than Cu-exchanged NH3-adsorption sites. The adsorbed NH3 was desorbed or oxidized in the lean phase during step II. The adsorbents were refilled with NH3 in the subsequent rich phase during step III. The difference between the amount of NH3 desorbed in step II (desorbed NH3, panel B) and the amount of adsorbed NH3 in step III (adsorbed NH3, panel C) is equivalent to the amount of oxidized NH3 in step II. Desorbed NH3 (B) was low and adsorbed NH3 (C) was high for Cu-CHA, which indicates that the amount of NH3 oxidized by Cu-CHA is high. Other H-type NH3 adsorbents did not appear to oxidize NH3. The difference between the amount of saturated 6
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catalyst under transient conditions at 450 °C as a function of temperature. The mixed H-MOR(20)/Cu-CHA catalyst exhibited high NOx conversions over a wide temperature range (150–550 °C); the NOx conversions of this catalyst are significantly higher than those of CuCHA at temperatures higher than 400 °C, but are slightly lower at temperatures below 300 °C. The amount of Cu in the H-MOR(20)/CuCHA catalyst is half that of Cu-CHA in this experiment; there was a slight insufficiency of the active component in the mixed catalyst to sustain reactivity at the low temperatures. We next examined the optimum H-MOR(20)/Cu-CHA ratio for activity over a wide temperature range. Fig. 9 shows NOx conversions at various mixing ratios at 250 °C and 450 °C under the transient conditions of Test 4, as detailed in Table 1. NOx conversion was observed to gradually decrease with increasing H-MOR/Cu-CHA ratio above 450 °C. The mixed catalyst with an H-MOR/Cu-CHA ratio of 10 exhibited higher NOx conversion than that of Cu-CHA, despite most of the mixed catalyst being composed of H-MOR(20), which indicates that the NH3 adsorbent at high temperature is able to overcome the effect of the lower amount of Cu-CHA. NOx conversion was observed to increase with increasing H-MOR/Cu-CHA ratio to 250 °C, which is believed to be due to a decrease in the total amount of Cu in the catalyst; SCR performance was highly influenced by the amount of Cu present at low temperatures.
Fig. 5. Schematic diagram depicting Test 3: NH3 adsorption and NH3 desorption/oxidation under high-temperature flow conditions.
NH3 adsorption (adsorbed NH3, panel A) and the amount of oxidized NH3 (B–C) is equivalent to the amount of unoxidized NH3 stored in the NH3 adsorbent or desorbed from the NH3 adsorbent even under O2-rich conditions. H-MOR(20) exhibited the highest amount of unoxidized NH3, followed by H-MFI(30) and then Cu-CHA. 3.5. Transient NOx reduction by the mixed NH3-adsorbent + Cu-CHA catalyst system
3.6. Performance of the NH3-adsorbent + Cu-CHA catalyst in the NSRSCR system
Fig. 6 shows NO conversions from the mixed NH3-adsorbent + CuCHA catalysts under transient conditions at 450 °C, which models the outlet of an NSR catalyst, as described in Test 4 and summarized in Table 1. Almost all of the NH3-adsorbent-mixed Cu-CHA catalysts showed higher NOx conversions than Cu-CHA alone. The mixed HMOR/Cu-CHA catalyst exhibited higher NOx conversions than all of the catalysts in this study. When the same zeolite structures are compared, a lower SiO2/Al2O3 ratio in the adsorbent results in a higher NOx conversion, which indicates that the number of acidic sites in the NH3 adsorbent is important for NOx conversion in the transient SCR reaction. We believe that increasing the number of NH3 adsorption sites capable of storing NH3 without its oxidation, even under high-temperature lean/rich transient conditions, is important for improving NH3-SCR reactivity under NSR-SCR conditions. Fig. 7 displays relationships between the NOx conversions shown Fig. 6 and the amount of unoxidized NH3 in each of the NH3 adsorbents listed in Table 3; the best correlation between unoxidized NH3 and transient NOx conversion was observed at 450 °C, with a correlation coefficient of 0.98. Other relationships between NOx conversions (Fig. 6) and the NH3-TPD profiles of each NH3 adsorbent listed in Table 2 are provided in the Supplementary Information (Fig. S3). A high correlation was observed between NOx conversion and the peak-III (NH3-desorption) temperature, with a correlation coefficient of 0.88, which indicates that the acid strength of the NH3 adsorbent is important for NOx reduction under high-temperature transient SCR-reaction conditions. H-MOR(20) was expected to be the best NH3 adsorbent for mixing with the SCR catalyst in order to improve NOx conversion in the NSRSCR system. Fig. 8 shows NOx conversion over the mixed H-MOR(20)
A mixed H-MOR(20)+Cu-CHA monolith catalyst with the preferred 2:1 ratio was prepared, and its catalytic performance when combined with an NSR catalyst composed of a mixture of H-MOR(20) and the SCR catalyst (which is referred to as the “NSR-SCR + NH3ad” system) was evaluated. Fig. 10(a) shows NOx conversions for the NSR-SCR + NH3ad system under the reaction conditions of Test 1, as detailed in Table 1. The NSR-SCR + NH3ad system exhibited higher NOx conversions than the NSR-SCR system in the high (350–550 °C) temperature range, while Fig. 10(b) shows the NO reaction selectivities of the NSR- SCR + NH3ad system, which reveals selectivities to N2 that were further improved over those of the NSR-SCR system, especially at 450 °C (from 74% to 84%) and 500 °C (from 62% to 69%). Fig. 10(c) displays the NH3 reaction selectivities. Compared to the NSR-SCR system, the reaction selectivities were further improved, from 5% to 38% at 450 °C, and from almost 0 to 9% at 500 °C, in the NSR-SCR + NH3ad system. The NOx effluent profiles of each system are shown in Fig. 10(d), while those of other gas species are shown in the Supplementary Information (Fig. S4). In the case of the NSR-SCR system, the NOx concentration was lower than that of the single-NSR system at the beginning of the lean phase, but it gradually increased and was almost the same as that of the singleNSR system at the end of the lean phase. On the other hand, the NOx effluent concentration was highly suppressed from the beginning to the end of lean phase in the case of NSR-SCR + NH3ad system. 3.7. Function of the NH3 adsorbent in the NSR-SCR system The N2 production profile during the lean phase was evaluated by
Table 3 Amounts of adsorbed, desorbed, and oxidized NH3 from Test 3. Step
Test 4
H-MOR(20) mmol/g
H-MFI(30) mmol/g
H-BEA(25) mmol/g
H-FAU(30) mmol/g
H-CHA mmol/ g
Cu-CHA mmol/g
I(Rich) II(Lean) III(Rich)
Adsorbed NH3 (A) Desorbed NH3 (B) Adsorbed NH3 (C)
0.47 0.29 0.27
0.16 0.16 0.15
0.05 0.05 0.05
0.06 0.09 0.06
0.12 0.07 0.12
0.75 0.03 0.69
Oxidized NH3 (C-B) Non-Oxidized NH3 (A-(C-B)) (= Desorbed NH3 + Stored NH3)
0 0.47
0 0.16
0 0.05
0 0.06
0.05 0.07
0.66 0.09
7
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Fig. 6. NOx conversions of several mixed Cu-CHA + NH3 adsorbent catalysts under transient reaction conditions at 450 °C.
Fig. 9. NOx conversions of the mixed Cu-CHA+H-MOR(20) catalyst as a function of the H-MOR/Cu-CHA ratio at 250 °C and 450 °C.
Fig. 7. NOx conversion under transient SCR conditions at 450 °C (from Fig. 6) as a function of unoxidized NH3 under the lean conditions detailed in Table 3.
intensity increased from the middle of the lean phase onwards. Both the NH3-SCR and NH3-O2 reactions produce N2 (Eqs. (1)–(3)). The N2 observed at the beginning of lean phase is believed to be mainly derived from the NH3-O2 reaction, because of the low slipped NOx concentration during this lean phase, as depicted in Fig. 3(a). The adsorbed NH3 can react with NOx from the middle to the end of the lean phase, during which the slipped NOx concentration is increased. Therefore, the N2 observed at the later lean phase is believed to be mainly derived from the NH3-SCR reaction. The characteristic N2 formation observed suggests that the NH3 adsorbent suppresses the NH3O2 reaction at the beginning of the lean phase, and enhances the NH3NO reaction during the later lean phase. The gradual increase of N2 production shown in Fig. S5 suggests a new reaction mechanism, in which a NH3 adsorbed on the NH3 adsorbent gradually diffuses to the SCR catalyst and reacts with slipped NOx, as illustrated on Fig. 11. The N2 formation properties of NSR-SCR + NH3ad system also suggest that the NH3 adsorbent is capable of stabilizing the NH3 concentration fluctuations from rich/lean operation. We only examined a physical mixture of the SCR catalyst and NH3 adsorbent in this report; we believe that a sequential triple-bed or tri-layer coated NSRNH3adsorbent-SCR configuration would be effective in extending the NOx reaction period of the lean phase. The NH3 diffusion rate of the internal NH3 adsorbent should be important if the contact area of the NH3 adsorbent and SCR catalyst is decreased. The properties of the NH3 adsorbent (acid strength, particle size, pore structure, and defects) should be adjusted according to catalytic configuration and operation conditions. Moreover, the thermal stability of the NH3 adsorbent is
Fig. 8. NOx conversions of the mixed Cu-CHA+H-MOR(20) and Cu-CHA catalysts as functions of temperature.
GC–MS using a He balance. Fig. 11 shows the formation of N2 as monitored by the intensity of the m/z = 28 signal during rich/lean cycling under Test-5 conditions at 450 °C for the single-NSR, NSR-SCR, and NSR-SCR + NH3ad systems; the results for 350 and 400 °C are provided in the Supplementary Information (Fig. S5). In the case of the single-NSR system, N2 production was observed only during the rich phase, while the NSR-SCR system exhibited additional N2 production at the beginning of the lean phase. In the case of the NSR-SCR + NH3ad system, N2 production was less intense at the beginning of the lean phase and was lower than that of the NSR-SCR system; however, the 8
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Fig. 10. (a) NOx conversions as functions of temperature for the single-NSR, NSR-SCR, and NSR-SCR + NH3ad systems (Cu-CHA:HMOR = 1:2). (b) Product selectivities from NO for the NSR-SCR + NH3ad system. (c) NH3 reaction selectivities for the NSR-SCR + NH3ad system. (d) Outlet NO concentration profiles for the single-NSR, NSR-SCR, and NSRSCR + NH3ad systems.
Fig. 11. N2 formation from single-NSR, NSR-SCR, and NSR-SCR + NH3ad pellet catalysts under transient condition at 450 °C. Reaction conditions: 5% O2, 450 ppm NO, and He balance during the lean phase (60 s), and 1.5% H2 and He balance during the rich phase (3 s). GHSV = 120,000 h−1.
another important problem in this system that needs to be addressed. The results and discussion considering the thermal stability of H-MOR (20) and H-MOR(20) mixed Cu-CHA were included in the supplemental
information (Figs. S6–S8). The suppression of zeolite dealumination and development of thermally stable NH3 adsorbents can be studied in future research. 9
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4. Conclusion
References
We first investigated ways of improving the high-temperature NOx reduction performance for the NSR-SCR system by measuring its catalytic activity under several catalytic configuration and reaction conditions, and then attempted to modify the NH3 adsorption and reaction properties of the SCR catalyst for high-temperature reaction by adding the NH3 adsorbent. NH3 oxidation is highly enhanced under hightemperature transient NSR-SCR conditions, which is due to the time lag between the slipped NOx and NH3 fed to the SCR catalyst. Herein, we introduce a new approach that involves mixing an NH3 adsorbent with the SCR catalyst to suppress NH3 oxidation under transient NSR-SCR reaction condition. H-MOR was found to be the most suitable NH3 adsorbent, which can retain NH3 in its inner sites at high temperatures in the absence of highly oxidizing Cu reaction sites. N2 is formed in the mixed H-MOR/NSR-SCR system during the end of the lean phase. The reaction mechanism is believed to involve the gradual diffusion of the NH3 adsorbed on the H-MOR during the rich phase to the SCR catalyst, where it reacts with NOx until the end of the lean phase. The proposed strategy provides useful insight for future catalyst design. The approach that suppresses side reactions with promoters is more effective than increasing the number of active sites, especially when the catalysts are used under transient reaction conditions.
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Acknowledgement We would like to thank Editage (www.editage.jp) for English language editing. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcata.2019.06.003.
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