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ZSM-5
Applied Catalysis A: General 366 (2009) 84–92
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Transient reaction analysis and steady-state kinetic study of selective catalytic reduction of NO and NO + NO2 by NH3 over Fe/ZSM-5 Masaoki Iwasaki *, Kiyoshi Yamazaki, Hirofumi Shinjoh Toyota Central R&D Labs Inc., 41-1 Yokomichi Nagakute-cho, Aichi 480-1192, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 May 2009 Received in revised form 23 June 2009 Accepted 23 June 2009 Available online 30 June 2009
The reaction kinetics of selective catalytic reduction (SCR) by NH3 on NO (standard SCR) and on NO + NO2 (fast SCR) over Fe/ZSM-5 were investigated using transient and steady-state analyses. In the standard SCR, the N2 production rate was transiently promoted in the absence of gaseous NH3; this enhancement can be attributed to the negative reaction order of NH3 (between 0.21 and 0.11). The steady-state data for the standard SCR could be fit to a Langmuir–Hinshelwood-type reaction between NOad and Oad to form NO2. In the fast SCR, however, the promotion behavior in the absence of gaseous NH3 was not observed and the apparent NH3 order changed from positive to negative with NH3 concentration. The steady-state rate analysis combined with elementary reaction modeling suggested that competitive adsorption between NO2 and NH3 was occurring due to strong NO2 adsorption; this must be the main reason for the absence of the promotion effect. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Selective catalytic reduction Nitric oxide Zeolite Fe Reaction mechanism
1. Introduction Selective catalytic reduction (SCR) of NOx (NO + NO2) with NH3 as a reducing agent has been widely used for stationary sources. However, in recent years, there has been increased interest in using SCR to treat exhaust from diesel engines. For this application, urea has received considerable attention as an NH3 carrier because urea is easy to handle and can be safely transported. One of the most suitable catalysts for urea/NH3-SCR is Fe/ZSM-5 because of its high activity and durability [1,2]. Although many characterization studies have been performed over Fe/ZSM-5 [3–7], there have been far fewer reports on the kinetics of the NH3-SCR reaction under either transient or steady-state conditions. The typical SCR process is based on the following reaction between NH3 and NO, the so-called ‘‘standard SCR’’: 2NO þ 2NH3 þ 12O2 ! 2N2 þ 3H2 O
(1)
However, if NO2 is present in the exhaust gases, an equimolar NO– NO2 reaction, which is considerably faster than (1), occurs; this is the so-called ‘‘fast SCR’’: NO þ NO2 þ 2NH3 ! 2N2 þ 3H2 O
(2)
* Corresponding author at: Toyota Central R&D Labs Inc., Applied Catalysis, 41-1 Yokomichi Nagakute-cho, Aichi 480-1192, Japan. Tel.: +81 561 71 7810; fax: +81 561 63 6712. E-mail address: [email protected] (M. Iwasaki). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.06.036
The importance of the fast SCR reaction (2) has recently become better appreciated because a preoxidation catalyst located upstream of the SCR catalyst could convert a fraction of NO to NO2, leading to excellent DeNOx activity. Regarding kinetic studies over H-ZSM-5, there are several reports on transient operations and steady-state measurements [8–10]. Wallin et al. [8] have reported that NOx reduction activity increases in transient tests when NH3 is removed from the feed during the standard SCR. However, this transient increase was not observed during fast SCR conditions. Eng and Bartholomew [9] have observed the same behavior in transient tests on the standard SCR; they also performed steady-state kinetic experiments, which indicated that the standard SCR has positive order in NO and O2 (0.73 and 1.06, respectively) but negative order in NH3 (0.61). Stevenson et al. [10] observed similar trends in reaction orders (NO and O2 were first order, while NH3 was between 0.55 and 0.35); they concluded that NH3 inhibits the reaction by blocking sites necessary for NO oxidation step, this step appears to be the ratedetermining step. Concerning Fe/ZSM-5, Huang et al. [11] reported that the reaction orders of the standard SCR were 0.88–0.94 in NO, 0.36– 0.41 in O2, and between 0.15 and 0.11 in NH3, and that the ratedetermining step was probably the NO2 formation reaction between NO and O2. Kro¨cher et al. [12] reported that NOx conversion decreased when NH3 was present in excess because of the inhibiting effect of NH3. Recently, Grossale et al. [13,14] have carried out transient reaction analyses of the standard and fast SCRs; they found that high NH3 concentrations inhibit both
M. Iwasaki et al. / Applied Catalysis A: General 366 (2009) 84–92
reactions and that the rate-determining step in the fast SCR is a reduction of surface nitrates by NO. As noted above, few studies have been performed that include both transient and steady-state analyses for the standard and fast SCRs over Fe/ZSM-5. In particular, to our knowledge, the reaction orders of the fast SCR have not been reported. Additionally, few steady-state kinetic studies have been combined with modeling, despite the fact that this approach often gives clues about reaction mechanisms. In this work, we investigated the effects of a periodic NH3 supply under standard and fast SCR conditions. Then we performed NO pulse reaction tests under standard SCR conditions to study NH3 inhibition effects in further detail. Finally, we performed steady-state rate measurements under standard and fast SCR conditions and discussed them with a reaction modeling to support the transient results. By these systematic studies, we not only establish a direct link between the standard and fast SCR reactions, but we also gain a comprehensive understanding of the transient and steady-state reaction behaviors. 2. Experimental 2.1. Catalyst preparation The Fe/ZSM-5 catalyst was prepared by modifying the sublimation method developed in Sachtler’s group [15]. As a starting zeolite, we chose NH4-ZSM-5 (Tosoh HSZ-840NHA, Si/ Al2 = 40) which was transformed to H-ZSM-5 by calcining at 600 8C in air for 5 h. FeCl3 (Wako, >99%) mixed with dehydrated H-ZSM-5 was sublimed at 650 8C for 2.5 h. Then, the iron-loaded sample was washed with deionised water twice and dried at 110 8C. Finally, the sample was calcined at 650 8C in 20% O2/N2 for 5 h. The detailed procedure is described in our previous paper [7]. Elemental analysis by inductively coupled plasma (ICP) atomic emission showed the Fe amount was 4.5 wt%, which corresponds to an Fe/Al ratio of around 1. The catalyst was pelletized to grain sizes of 500– 900 mm (apparent density: 0.52 g/cm3). 2.2. Periodic NH3 supply tests under standard and fast SCR conditions Periodic NH3 supply tests were carried out in a fixed-bed flow reactor (id = 13 mm). A 0.5 g sample of Fe/ZSM-5 was placed on a small piece of quartz wool supported by the fixed-bed. The total gas flow was maintained at a predetermined temperature by mass flow controllers at 5 l/min (gas hourly space velocity (GHSV) = 3.1 105 h1). The gas composition of the standard SCR was 380 ppm NO, 0 or 400 ppm NH3, 8% O2, 10% CO2, 8% H2O, and the remainder N2; that of the fast SCR was 135 ppm NO, 135 ppm NO2, 0 or 300 ppm NH3, 8% O2, 10% CO2, 8% H2O, and the remainder N2. NH3 was periodically supplied every 500 s. The catalyst bed temperature and the inlet gas temperature were measured using thermocouples. The inlet gas thermocouple was positioned 10 mm above the catalyst bed and the temperature there was controlled by a programmable controller. The inlet temperatures of the standard SCR were 250 and 300 8C; that of the fast SCR were 180 and 200 8C. The inlet and outlet NOx concentrations were sampled every second by a chemiluminescent NOx analyzer. Almost no formation of N2O was observed by a nondispersive infrared (NDIR) N2O analyzer in either reaction. 2.3. NO pulse reaction tests under standard SCR condition Pulse reaction tests for NO in the presence and absence of NH3 were carried out in a fixed-bed flow reactor (id = 9 mm) equipped with a quadrupole mass spectrometer (QMS, ULVAC messmate200). A 0.25 g sample of Fe/ZSM-5 was placed on a porous quartz plate in the reactor. NO pulses were introduced to a continuous gas
85
flow through a six-way valve and a gas sampler. The NO pulse volume was 0.34 ml (13.6 mmol), the full width at half maximum (FWHM) of the pulse was approximately 8 s, and each pulse interval was 340 s. The continuous gas flow was maintained at 0.1 l/min (GHSV = 1.2 104 h1) and the gas composition was 0 or 400 ppm NH3, 10% O2, and the remainder He. The reaction gases (NO, NH3, and O2) and the product gases (N2 and H2O) were analyzed by QMS; neither N2O nor NO2 was detected. The N2 production rate was calculated according to 2 ðoutlet N2 amountÞ N2 production rate ¼ 100 1 inlet NO amount The outlet of N2 amount was calculated by introducing an N2 pulse instead of the NO pulse. Two types of NO pulse sequences were applied. In the first (type I), three pulses were introduced in the presence of NH3, and then seven pulses were introduced in the absence of NH3. The reaction temperatures were 120, 150, and 200 8C. In the second (type II), NH3 was cycled on or off at intervals of three pulses at 150 8C. That the temperature was lower than in the periodic NH3 supply tests (Section 2.2) was because this pulse reaction was conducted in the absence of CO2 and H2O, which would deactivate the SCR reaction, and because the NH3 inhibiting effect should be amplified at lower temperatures. 2.4. Steady-state rate measurements under standard and fast SCR conditions Steady-state rate measurements, which included concentration tests for reaction order analysis and temperature tests for Arrhenius plot, were carried out using a reactor (id = 15 mm) similar to that used in the periodic NH3 supply tests (Section 2.2). The reaction was induced by controlling total gas flow and catalyst weight to maintain pseudodifferential conditions. For the standard SCR, the total gas flow was 15 l/min and the catalyst weight was 0.45–1.0 g (GHSV between 1.7 and 3.8 106 h1). For the fast SCR, the total gas flow was 25 l/min and the catalyst weight was 0.3 g (GHSV = 9.6 106 h1). The total pressure upstream of the catalyst was measured using a pressure gauge and found to be 120– 137 kPa. Concentrations of NO/NOx, NH3, O2, and N2O were monitored using a chemiluminescent NO/NOx analyzer, a microwave NH3 analyzer, a magnetic system O2 analyzer, and an NDIR N2O analyzer. 3. Results 3.1. Periodic NH3 supply tests under standard and fast SCR conditions Fig. 1 shows the outlet NOx concentration (Fig. 1a) and catalyst temperature change (Fig. 1b) in the standard SCR condition for two inlet temperatures (250 and 300 8C). When NH3 was added to the feed (Fig. 1a), the NOx concentration quickly decreased due to the standard SCR reaction, went through a minimum, and then approached a steady-state level. The steady-state NOx conversion was about 25% at 250 8C and 50% at 300 8C. The catalyst temperature (Fig. 1b) increased sharply just after adding NH3; this increase was caused by the heat of NH3 adsorption plus the heat of the SCR reaction. When NH3 was removed from the feed, the NOx concentration went through a minimum and then began to increase, eventually reaching the inlet value. The catalyst temperature did not increase but gradually decreased with increasing NOx concentration and approached a constant value. Similar transient NOx removal behavior, just after NH3 addition and NH3 shutoff, has been reported over H-ZSM-5 [8,9] and Fe/ ZSM-5 [14,16]; this behavior is considered to be an inhibiting effect of gaseous NH3. In other words, the reaction could be accelerated
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Fig. 1. Periodic NH3 supply test under the standard SCR condition at 250 and 300 8C. Outlet NOx concentration (a) and catalyst temperature change (b). Feed = 380 ppm NO, 0 or 400 ppm NH3, 8% O2, 10% CO2, 8% H2O, the remainder N2. GHSV = 3.1 105 h1.
by decreasing the NH3 concentration. However, just after NH3 addition, the increasing catalyst temperature would also affect the reaction rate (Fig. 1b). Furthermore, we cannot rule out the possibility of NO adsorption because of the lack of N2 production data, which is verified in Section 3.2. Fig. 2 shows the outlet NOx concentration (Fig. 2a) and catalyst temperature change (Fig. 2b) in the fast SCR condition for two inlet temperatures (180 and 200 8C). The steady-state NOx conversion, was about 87% at 180 8C and 97% at 200 8C, was much higher than that in the standard SCR condition despite the lower temperatures. However, no transient NOx removal behavior was observed, indicating that the inhibiting effect by NH3 is probably small in the fast reaction. These different behaviors between standard and fast SCRs are probably related to the adsorption strength of each adsorbate and to their competitive adsorption onto active sites. This is discussed in detail in Section 4.2.
Fig. 2. Periodic NH3 supply test under the fast SCR condition at 180 and 200 8C. Outlet NOx concentration (a) and catalyst temperature change (b). Feed = 135 ppm NO, 135 ppm NO2, 0 or 300 ppm NH3, 8% O2, 10% CO2, 8% H2O, the remainder N2. GHSV = 3.1 105 h1.
Table 1 shows the total amounts of N2 produced from pulses 4– 10, i.e., the NH3-absent region. The N2 amount increased with decreasing temperature due to higher NH3 adsorption capacities at lower temperatures [12]. All the stored NH3 was considered to be consumed by the NO pulses, because in an FT-IR study at 200 8C pre-adsorbed NH3 bands were completely vanished by flowing NO + O2 mixture [17].
3.2. NO pulse reaction tests under the standard SCR condition To get a deeper understanding of transient NOx removal behavior in the standard SCR, we performed two types of NO pulse reaction tests. Fig. 3 shows N2 production rates at 120, 150, and 200 8C from the type I pulse sequence. In the NH3-present region (pulses 1–3), the N2 production rates were almost constant and the rates increased with increasing temperature. When NH3 was removed from the feed, the N2 amount increased (pulse 4) and then decreased (pulses 5–10), suggesting that the absence of gaseous NH3 promoted the standard reaction.
Fig. 3. N2 production rate in type I pulse sequence. T = 120, 150, and 200 8C.
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Table 1 Integrated N2 production amounts from pulses 4–10 in the type I sequence. See Fig. 3. 120 8C
150 8C
200 8C
154a
129a
112a
a
mmol g1.
Fig. 4 shows N2 production rates from the type II pulse sequence. The rates at pulses 4 and 10 (the first pulses in the NH3absent regions) were higher than those in the NH3-present regions; this suggests that the promotion phenomenon is repeatable. This agrees with the behavior of the periodic NH3 supply test shown in Fig. 1a. 3.3. Steady-state rate measurements in standard and fast SCR conditions To further understand the reaction kinetics, we carried out steady-state rate measurements at a predetermined temperature. In the standard SCR condition, the NH3 conversion amount was consistent with the NOx conversion amount, and N2O was not detected; this suggests that NH3 oxidation and N2O production reactions are negligible. Thus, only the standard SCR reaction (1) progressed and we estimated its rate from the NOx conversion amount. Fig. 5 shows the dependence of the standard SCR rate on NO, O2, and NH3 concentrations at 200, 250, and 300 8C. The typical
Fig. 4. N2 production rate in type II pulse sequence. T = 150 8C.
gas composition was 0.12% NO, 0.12% NH3, 5% O2, 10% CO2, 5% H2O, and the remainder N2. The concentrations of the reactants were varied systematically: 0.025–0.3% NO, 0.035–0.3% NH3, 0.4–19% O2. The SCR rate was increased with NO and O2 concentrations but slightly decreased with NH3 concentration. Fig. 6 shows log–log plots of the data in Fig. 5. Since all the data gave linear relationships, the reaction rate can be expressed simply as a power law: r s ¼ ka p p ½NOa ½O2 b ½NH3 g
Fig. 5. Standard SCR rate as a function of NO (a), O2 (b), and NH3 (c) concentrations. Typical gas composition = 0.12% NO, 0.12% NH3, 5% O2, 10% CO2, 5% H2O and remainder N2. GHSV = 1.7 106, 1.9 106, and 3.8 106 h1 at 200, 250, and 300 8C, respectively.
88
M. Iwasaki et al. / Applied Catalysis A: General 366 (2009) 84–92 Table 2 Apparent reaction orders for NO, O2, and NH3 from power law expressions for the standard SCR reaction. See Fig. 6. Temperature
aa
ba
ga
200 8C 250 8C 300 8C
0.81 0.88 0.86
0.34 0.31 0.29
0.21 0.18 0.11
a
Fig. 6. Log–log plots of standard SCR rate versus NO (*), O2 (~), and NH3 (&) concentrations. T = 200 8C for lowest line in each set; T = 250 8C for the middle line in each set; T = 300 8C for the upper line in each set. Reaction conditions are the same as those in Fig. 5.
where rs is the standard SCR rate, kapp is the apparent rate constant, and a, b, and g are apparent reaction orders for NO, O2, and NH3, respectively. We estimated the orders from the slopes in Fig. 6; the resulting values are given in Table 2. The NO orders (a) were 0.81– 0.88, slightly lower than first order. The O2 orders (b) were 0.29– 0.34, slightly lower than half order. The NH3 orders (g) were between 0.21 and 0.11, slightly lower than zeroth order. These
a
b
g
Rate = kapp[NO] [O2] [NH3] .
are close to the values reported by Huang et al. [11] (0.88–0.94 for NO, 0.36–0.41 for O2, and between 0.15 and 0.11 for NH3). Interestingly, in the case of H-ZSM-5, both NO and O2 are around first order [9,10]. The reason for the differences in O2 orders between Fe-ZSM-5 and H-ZSM-5 may be because different forms of O2 contribute to the rate-determining step: whereas dissociated atomic Oad should participate in the elementary reaction of the rate-determining step over Fe/ZSM-5 (see Section 4.1), undissociated molecular O2ad might be participating over H-ZSM-5. In fast SCR conditions, only a small amount of N2O was observed (roughly less than 5% of the total NOx conversion amount), indicating that the N2O production reaction contributes little to the conversion. Moreover, the NO conversion amount was almost twice as high as the NH3 conversion amount in all conditions. Thus, we can exclude contributions from the standard SCR (1) and from the NO2-SCR reaction, 3NO2 þ 4NH3 ! 72N2 þ 6H2 O
(3)
Fig. 7. Fast SCR rate as a function of NO (a), NO2 (b), NH3 (c) and O2 (d) concentrations. Typical gas composition = 0.075% NO, 0.053% NO2, 0.11% NH3, 5% O2, 10% CO2, 5% H2O, and remainder N2. GHSV = 9.6 106 h1.
M. Iwasaki et al. / Applied Catalysis A: General 366 (2009) 84–92
because both reactions are much slower than reaction (2) [18]. In addition, the emission of NH4NO3 from the catalyst surface, 2NO2 þ 2NH3 ! NH4 NO3 þ N2 þ H2 O
(4)
was not observed under these conditions. (Reaction (4) could not be ignored at lower temperatures (<180 8C) [13,18–20].) Hence, only the fast SCR reaction (2) progressed and we estimated its rate from doubling the NO conversion amount. Fig. 7 shows the dependence of the fast SCR rate on NO, NO2, NH3, and O2 concentrations at 195 and 250 8C. The typical gas composition was 0.075% NO, 0.053% NO2, 0.11% NH3, 5% O2, 10% CO2, 5% H2O, and the remainder N2. The concentrations of the following gases were varied systematically: 0.014–0.15% NO, 0.015–0.13% NO2, 0.023–0.28% NH3, and 0.4–19% O2. The SCR rate was increased with NO and NO2 concentration (Fig. 7a and b), suggesting that their apparent reaction orders are positive. However, the rate passed through a maximum with NH3 concentration (Fig. 7c), indicating that the apparent reaction order changed from positive to negative. The NH3 concentration dependence for the fast SCR (Fig. 7c) is much different from that for the standard SCR (Fig. 5c); this difference is considered to be the main reason for the different behavior in the periodic NH3 supply tests (Figs. 1a and 2a). Thus, because of the positive reaction order at low NH3 concentrations, the promotion effect cannot be expected to occur in fast SCR. Note that O2 concentration has little influence on the SCR rate (Fig. 7d), suggesting that O2 does not participate in competitive adsorption on active sites or in elementary steps in fast SCR reaction. A more detailed discussion of reaction mechanism is presented in Section 4.2. Fig. 8 shows Arrhenius plots for the standard and fast SCR reactions. The gas composition for the standard SCR was 0.125% NO, 0.125% NH3, 5% O2, 10% CO2, 5% H2O and the remainder N2; that for the fast SCR was 0.08% NO, 0.07% NO2, 0.13% NH3, 5% O2, 10% CO2, 5% H2O and the remainder N2. Fig. 8 shows that the standard SCR rate is much slower than the fast SCR rate. The difference between the two rates increases as temperature decreases. At 200 8C, the difference in rates is a factor of about 30. Apparent
89
activation energies at 200–280 8C were 35 kJ/mol for the standard SCR and 8 kJ/mol for the fast SCR. These are close to values reported elsewhere [21]: 39 kJ/mol for the standard SCR and 7 kJ/mol for the fast SCR. 4. Discussion 4.1. Standard SCR kinetics The results from the periodic NH3 supply test (Fig. 1) and the NO pulse reaction test (Figs. 3 and 4) show that the standard SCR was promoted when gaseous NH3 was shut off. This could be explained by the negative reaction order for NH3 (Table 2). To understand this, let us first consider the probable adsorption mechanisms involved. Analyses by extended X-ray absorption fine structure spectroscopy (EXAFS) [4,22,23] suggest that binuclear oxygenbridged Fe complexes occur on the catalyst. From Fourier transform infrared (FT-IR) studies, NO could adsorb on these Fe sites [24–26], as could NH3 [24,27,28]. However, oxygen could adsorb onto Fe–&–Fe bridged vacancy sites (& stands for a vacancy), which are observed by EXAFS [29]. Oxygen on such bridged sites plays an important role in many reactions [25,28,30– 32]. Combining these observations, we write the adsorption steps for NO, NH3 and O2 as K1
NO þ !NOad K2
NH3 þ !NH3ad K3
O2 þ 20 !2Oad
(5) (6) (7)
where * and *0 denote adsorption sites and Ki represent equilibrium constants. Consider now the rate-determining step in the standard SCR, which has been suggested to be NO oxidation between NOad and Oad [8–11]: k1
NOad þ Oad !NO2ad
(8)
where k1 is the rate constant. Once formed, NO2ad is probably consumed very quickly and the subsequent reaction should be much faster than reaction (8). Thus, any adsorbed species other than NOad, NH3ad, and Oad could be negligible. Assuming a Langmuir–Hinshelwood-type mechanism, one can describe the overall reaction rate of the standard SCR by the following rate expression: pffiffiffiffiffiffiffiffiffiffiffiffiffi k1 K 1 P NO K 3 P O2 pffiffiffiffiffiffiffiffiffiffiffiffiffi r 1 ¼ k1 uNO uO ¼ (9) ð1 þ K 1 PNO þ K 2 P NH3 Þ 1 þ K 3 P O2 where ui and Pi are the coverage of the adsorbate i and the partial pressure of gas i, respectively. Estimation of the kinetic parameters in (9) was obtained using the solver set up on a Microsoft Excel spreadsheet. The calculation minimizes the total sum of residual squares between calculated and experimental reaction rates for each set of isothermal data n X ðr i;exp: r i;calc: Þ2
(10)
i¼1
Fig. 8. Arrhenius plot of standard and fast SCRs. For standard SCR: feed = 0.125% NO, 0.125% NH3, 5% O2, 10% CO2, 5% H2O and remainder N2; GHSV = 1.7 106 h1. For fast SCR: feed = 0.08% NO, 0.07% NO2, 0.13% NH3, 5% O2, 10% CO2, 5% H2O, and remainder N2; GHSV = 9.6 106 h1.
The resulting parameter values are collected in Table 3, while the experimental and calculated values of the reaction rate are compared in Fig. 9. Since Fig. 9 shows good agreement, the model (9) is a reasonable and reliable one. Hence, our steady-state kinetic analysis demonstrates that the rate-determining step should be the formation of NO2 adsorbate from NOad and Oad.
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90
Table 3 Optimized kinetic parameters calculated for the standard SCR reaction.
Similarly, nitric acid reacts with NH3 to form NH4NO3:
Temperature
k1 a
K1 b
K2 b
K3 b
200 8C 250 8C 300 8C
38 217 799
106 33 15
238 145 44
3.2 5.7 7.4
a
mmol g1 s1.
b
atm1.
HNO3ad þ NH3ad $ NH4 NO3
(14)
NH4NO3 is relatively stable (200 8C) compared to ammonium nitrite [19]. Hence, when the reactants are only NO2 and NH3 without NO, NH4NO3 accumulates or desorbs at temperatures below 180 8C [13,14,20]; but at temperatures above 200 8C it decomposes to N2O [8,14,21]: NH4 NO3 ! N2 O þ 2H2 O
(15)
Table 3 shows the equilibrium constant of NH3 (K2) to be larger than that of NO (K1), suggesting that adsorption of NH3 was much stronger than that of NO. This is consistent with in situ FT-IR results [24,27]; NH3 is the main species adsorbed during the standard SCR reaction. This strong adsorption probably inhibits reaction (8). Here, K1 and K2 decreased with temperature because the adsorption reactions (5) and (6) are exothermic; i.e., the adsorption enthalpies are negative. In contrast, K3 increased with temperature, which is thought to be due to O2 dissociation processes that may be included in reaction (7). Finally, Table 3 shows that the rate constant k1 in (9) increased with temperature; the corresponding activation energy was estimated to be 68 kJ/mol, which is larger than the apparent activation energy (see Section 3.3). This is because the apparent rate constant kapp includes the adsorption terms.
But the resulting NO2 and nitrous acid are still consumed via reactions (11) and (13). Therefore, the sum of reactions (11)–(13) with (16) yields the fast SCR reaction (2). To fit our steady-state data, we simplify the above reactions as follows: first, we introduce the NO2 adsorption reaction (17) in addition to the NO and NH3 adsorption reactions (5) and (6):
4.2. Fast SCR kinetics
2NO2ad þ H2 O !HONOad þ HNO3ad
However, in the presence of NO, which is the fast SCR condition, NO can react with nitric acid to form NO2 and nitrous acid [13,19]: HNO3ad þ NOad ! NO2ad þ HONOad
K4
NO2 þ !NO2ad
(17)
Then we combine the NO2 reactions (11) and (12) to obtain K5
In the periodic NH3 supply test (Fig. 2), no definite NH3 inhibition was observed, and in the steady-state rate measurements, the reaction rates follow a parabolic curve with NH3 concentration (Fig. 7c). From these results, we infer that NH3-related competitive adsorption was occurring. In fact, it has been reported that NO2 strongly adsorbs onto active Fe sites as easily as NH3 [7]. In studies of the reaction mechanism, the first step of the fast SCR reaction is thought to be NO2 dimerization, followed by formation of nitrous and nitric acids [13,19,20]: 2NO2ad $ N2 O4ad
(11)
N2 O4ad þ H2 O $ HONOad þ HNO3ad
(12)
(16)
(18)
and we rewrite reaction (13) as k2
HONOad þ NH3ad !N2 þ 2H2 O
(19)
Nitric acid reduction by NO (16) is considered to be the ratedetermining step [13,19], which we now write as k3
HNO3ad þ NOad !NO2ad þ HONOad
0
(16 )
The contributions of NH4NO3 formation (14) and subsequent N2O formation (15) can be ignored under these conditions, because of the consistency between NH3 and double NO conversion amount and because of the vanishingly small N2O production observed. From the adsorption reactions (5), (6) and (17), Langmuir-type adsorption equations are obtained:
In the presence of NH3, nitrous acid forms ammonium nitrite which readily decomposes to N2:
uNO ¼ K 1 PNO uv
(20)
HONOad þ NH3ad $ ½NH4 NO2 ! N2 þ 2H2 O
uNH3 ¼ K 2 PNH3 uv
(21)
uNO2 ¼ K 4 PNO2 uv
(22)
(13)
Here uv is a coverage of vacancy sites, which can be expressed as
uv ¼ 1 ðuNO þ uNH3 þ uNO2 þ uHONO þ uHNO3 Þ
(23)
In (23) oxygen coverage is not included because the fast SCR reaction was not affected by O2 concentration (Fig. 7d) and because oxygen probably adsorbs onto the bridged vacancy sites, Fe–&–Fe as described in Section 4.1. The equilibrium constant in (18) can be written as K5 ¼
uHONO uHNO3 u2NO2
(24)
where the H2O contribution is neglected because an extremely large amount of H2O would exist over the catalyst. The reaction rates for (19) and (160 ) are described by (25) and (26), respectively: Fig. 9. Correlations between experimental and calculated values of the standard SCR rate. Reaction conditions are the same as those in Fig. 5 and fitted parameters are given in Table 3.
r 2 ¼ k2 uHONO u NH3
(25)
r 3 ¼ k3 uHNO3 uNO
(26)
M. Iwasaki et al. / Applied Catalysis A: General 366 (2009) 84–92
Conservation of mass relates these two rates: r 2 ¼ 2r 3
(27)
which is the fast SCR rate. From (20)–(27) we obtain the fast SCR rate rf as a function only of the partial pressures Pi:
rf ¼
1 þ K 1 P NO þ K 2 P NH3
Table 4 Optimized kinetic parameters calculated for the fast SCR reaction. Temperature
k2 a
k3 a
K1 b
K2 b
K4 b
K5 c
195 8C 250 8C
2.3 106 7.3 106
9.5 102 6.9 103
290 161
539 199
656 468
2.5 104 3.4 105
a
mmol g1 s1.
b
atm1. No unit.
c
NH3-related species have been detected during standard SCR, not only NH3 species but also NOx species have been detected during NO2-SCR. This indicates that NO2 can absorb onto Fe sites despite NH3 coexistence. It seems plausible that competitive adsorption between NO2 and NH3 was occurring, and this must be the main reason for the absence of NOx reduction promotion at NH3 shutoff.
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi K 4 P NO2 2K 1 K 2 K 5 k2 k3 PNO P NH3 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi þ K 4 P NO2 1 þ K 2 K 5 k2 P NH3 =2K 1 k3 PNO þ 2K 1 K 5 k3 P NO =K 2 k2 P NH3
We have carried out parameter estimations for kn and Kn. The results are collected in Table 4, and the experimental and calculated values of the reaction rates are compared in Fig. 10. The values of parameters K1 and K2 estimated for the standard SCR (Table 3), and which are relevant for the fast SCR fitting procedure, were used as starting values to facilitate the calculations. However, the parameter values calculated for the fast SCR differ from the values found for the standard SCR; this could be because the fast SCR is much more complex due to the larger number of species present and the possibility that the rate constants might depend on coverage and lateral interactions of adsorbates. In our studies, all surface sites were considered identical and interactions between adsorbed species were neglected. From Table 4, the equilibrium constant of NO2 (K4) was large compared to that of NH3 (K2), which indicates that adsorption of NO2 should be relatively strong. This assumption is supported in our previous TPD studies [7]; NO2 desorption from active Fe sites has been observed around 350 8C which is higher than the NH3 desorption temperature from Fe sites (300 8C) in NH3-TPD [7,28]. Furthermore, from in situ FT-IR studies [21], whereas only
91
(28)
Table 4 shows that k2 is much larger than k3. This implies that nitric acid reduction by NO (160 ) is the rate-determining step. Grossale et al. [13] conducted fast SCR transient analysis at low temperatures (<190 8C) and also concluded that this is the ratedetermining step. Thus, we come to the same conclusion from a different approach, i.e., steady-state kinetic analysis. However, Grossale et al. [13] also proposed that high NH3 concentrations have a detrimental effect on the fast SCR. Actually, they have observed that reaction (160 ) was promoted in the absence of gaseous NH3 at 150 8C. In contrast, Schwidder et al. [16] conducted periodic NH3 supply tests at 300 8C, and they did not observe the promotion effect. Likewise, in our tests at 180 and 200 8C, it was not observed (Fig. 2a). The reason for this discrepancy is probably due to the degree of NH4NO3 formation (14). In fact, Grossale et al. [13] reported that although NH4NO3 production was vanishingly small during fast SCR at 190 8C, it grew during NO2-SCR and/or at lower temperature condition. In addition, their recent report [33] shows that a threshold temperature of NH4NO3 decomposition was at 140–160 8C which was quite close to its melting point of 170 8C. In the presence of NH4NO3, decreasing NH3 concentration would shift the equilibrium (14) toward higher nitric acid concentration, thereby promoting reaction (160 ). Grossale et al. [33] have pointed out that NH4NO3 formation or a strong interaction between NH3 and nitrate species affects the reactivity at low temperature. This expectation is consistent with our result of Arrhenius plot in Fig. 8 that the fast SCR rate began to decrease sharply at an inlet temperature of 180 8C. Thus, the influence of the NH4NO3 formation would appear around the melting temperature which seems to be the critical point for the emergence of NH3 inhibition. The reaction mechanisms in this low temperature region have been investigated in greater detail elsewhere [13,17,18,33]. 5. Conclusions
Fig. 10. Correlations between experimental and calculated values of the fast SCR rate. Reaction conditions are the same as those in Fig. 7 and fitted parameters are given in Table 4.
This work reports the reaction kinetics of SCRs by NH3 on NO (standard SCR) and NO + NO2 (fast SCR) over Fe/ZSM-5 using periodic NH3 supply tests, NO pulse reaction tests, and steady-state rate analyses. In the standard SCR reaction, the NOx reduction rate improved just after NH3 shutoff. This improvement could be attributed to increasing N2 production in the absence of gaseous NH3. The steady-state rate analysis suggested that NO2 formation between NOad and Oad was the rate-determining step, and that NH3 inhibited this reaction due to strong adsorption onto active Fe sites. In the fast SCR reaction, the promotion effect was not observed in periodic NH3 supply tests under 180 and 200 8C. Steady-state measurements revealed that the reaction rate passed through a maximum with NH3 concentration, indicating that the apparent reaction order changed from positive to negative. Fitting analysis combined with modeling predicted that competitive adsorption between NO2 and NH3 was occurring due to strong NO2 adsorption. This must be the main reason for the lack of the promotion effect.
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Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Transient reaction analysis and steady-state kinetic study of selective catalytic reduction of NO and NO + NO2 by NH3 over Fe/ZSM-5 Masaoki Iwasaki *, Kiyoshi Yamazaki, Hirofumi Shinjoh Toyota Central R&D Labs Inc., 41-1 Yokomichi Nagakute-cho, Aichi 480-1192, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 May 2009 Received in revised form 23 June 2009 Accepted 23 June 2009 Available online 30 June 2009
The reaction kinetics of selective catalytic reduction (SCR) by NH3 on NO (standard SCR) and on NO + NO2 (fast SCR) over Fe/ZSM-5 were investigated using transient and steady-state analyses. In the standard SCR, the N2 production rate was transiently promoted in the absence of gaseous NH3; this enhancement can be attributed to the negative reaction order of NH3 (between 0.21 and 0.11). The steady-state data for the standard SCR could be fit to a Langmuir–Hinshelwood-type reaction between NOad and Oad to form NO2. In the fast SCR, however, the promotion behavior in the absence of gaseous NH3 was not observed and the apparent NH3 order changed from positive to negative with NH3 concentration. The steady-state rate analysis combined with elementary reaction modeling suggested that competitive adsorption between NO2 and NH3 was occurring due to strong NO2 adsorption; this must be the main reason for the absence of the promotion effect. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Selective catalytic reduction Nitric oxide Zeolite Fe Reaction mechanism
1. Introduction Selective catalytic reduction (SCR) of NOx (NO + NO2) with NH3 as a reducing agent has been widely used for stationary sources. However, in recent years, there has been increased interest in using SCR to treat exhaust from diesel engines. For this application, urea has received considerable attention as an NH3 carrier because urea is easy to handle and can be safely transported. One of the most suitable catalysts for urea/NH3-SCR is Fe/ZSM-5 because of its high activity and durability [1,2]. Although many characterization studies have been performed over Fe/ZSM-5 [3–7], there have been far fewer reports on the kinetics of the NH3-SCR reaction under either transient or steady-state conditions. The typical SCR process is based on the following reaction between NH3 and NO, the so-called ‘‘standard SCR’’: 2NO þ 2NH3 þ 12O2 ! 2N2 þ 3H2 O
(1)
However, if NO2 is present in the exhaust gases, an equimolar NO– NO2 reaction, which is considerably faster than (1), occurs; this is the so-called ‘‘fast SCR’’: NO þ NO2 þ 2NH3 ! 2N2 þ 3H2 O
(2)
* Corresponding author at: Toyota Central R&D Labs Inc., Applied Catalysis, 41-1 Yokomichi Nagakute-cho, Aichi 480-1192, Japan. Tel.: +81 561 71 7810; fax: +81 561 63 6712. E-mail address: [email protected] (M. Iwasaki). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.06.036
The importance of the fast SCR reaction (2) has recently become better appreciated because a preoxidation catalyst located upstream of the SCR catalyst could convert a fraction of NO to NO2, leading to excellent DeNOx activity. Regarding kinetic studies over H-ZSM-5, there are several reports on transient operations and steady-state measurements [8–10]. Wallin et al. [8] have reported that NOx reduction activity increases in transient tests when NH3 is removed from the feed during the standard SCR. However, this transient increase was not observed during fast SCR conditions. Eng and Bartholomew [9] have observed the same behavior in transient tests on the standard SCR; they also performed steady-state kinetic experiments, which indicated that the standard SCR has positive order in NO and O2 (0.73 and 1.06, respectively) but negative order in NH3 (0.61). Stevenson et al. [10] observed similar trends in reaction orders (NO and O2 were first order, while NH3 was between 0.55 and 0.35); they concluded that NH3 inhibits the reaction by blocking sites necessary for NO oxidation step, this step appears to be the ratedetermining step. Concerning Fe/ZSM-5, Huang et al. [11] reported that the reaction orders of the standard SCR were 0.88–0.94 in NO, 0.36– 0.41 in O2, and between 0.15 and 0.11 in NH3, and that the ratedetermining step was probably the NO2 formation reaction between NO and O2. Kro¨cher et al. [12] reported that NOx conversion decreased when NH3 was present in excess because of the inhibiting effect of NH3. Recently, Grossale et al. [13,14] have carried out transient reaction analyses of the standard and fast SCRs; they found that high NH3 concentrations inhibit both
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reactions and that the rate-determining step in the fast SCR is a reduction of surface nitrates by NO. As noted above, few studies have been performed that include both transient and steady-state analyses for the standard and fast SCRs over Fe/ZSM-5. In particular, to our knowledge, the reaction orders of the fast SCR have not been reported. Additionally, few steady-state kinetic studies have been combined with modeling, despite the fact that this approach often gives clues about reaction mechanisms. In this work, we investigated the effects of a periodic NH3 supply under standard and fast SCR conditions. Then we performed NO pulse reaction tests under standard SCR conditions to study NH3 inhibition effects in further detail. Finally, we performed steady-state rate measurements under standard and fast SCR conditions and discussed them with a reaction modeling to support the transient results. By these systematic studies, we not only establish a direct link between the standard and fast SCR reactions, but we also gain a comprehensive understanding of the transient and steady-state reaction behaviors. 2. Experimental 2.1. Catalyst preparation The Fe/ZSM-5 catalyst was prepared by modifying the sublimation method developed in Sachtler’s group [15]. As a starting zeolite, we chose NH4-ZSM-5 (Tosoh HSZ-840NHA, Si/ Al2 = 40) which was transformed to H-ZSM-5 by calcining at 600 8C in air for 5 h. FeCl3 (Wako, >99%) mixed with dehydrated H-ZSM-5 was sublimed at 650 8C for 2.5 h. Then, the iron-loaded sample was washed with deionised water twice and dried at 110 8C. Finally, the sample was calcined at 650 8C in 20% O2/N2 for 5 h. The detailed procedure is described in our previous paper [7]. Elemental analysis by inductively coupled plasma (ICP) atomic emission showed the Fe amount was 4.5 wt%, which corresponds to an Fe/Al ratio of around 1. The catalyst was pelletized to grain sizes of 500– 900 mm (apparent density: 0.52 g/cm3). 2.2. Periodic NH3 supply tests under standard and fast SCR conditions Periodic NH3 supply tests were carried out in a fixed-bed flow reactor (id = 13 mm). A 0.5 g sample of Fe/ZSM-5 was placed on a small piece of quartz wool supported by the fixed-bed. The total gas flow was maintained at a predetermined temperature by mass flow controllers at 5 l/min (gas hourly space velocity (GHSV) = 3.1 105 h1). The gas composition of the standard SCR was 380 ppm NO, 0 or 400 ppm NH3, 8% O2, 10% CO2, 8% H2O, and the remainder N2; that of the fast SCR was 135 ppm NO, 135 ppm NO2, 0 or 300 ppm NH3, 8% O2, 10% CO2, 8% H2O, and the remainder N2. NH3 was periodically supplied every 500 s. The catalyst bed temperature and the inlet gas temperature were measured using thermocouples. The inlet gas thermocouple was positioned 10 mm above the catalyst bed and the temperature there was controlled by a programmable controller. The inlet temperatures of the standard SCR were 250 and 300 8C; that of the fast SCR were 180 and 200 8C. The inlet and outlet NOx concentrations were sampled every second by a chemiluminescent NOx analyzer. Almost no formation of N2O was observed by a nondispersive infrared (NDIR) N2O analyzer in either reaction. 2.3. NO pulse reaction tests under standard SCR condition Pulse reaction tests for NO in the presence and absence of NH3 were carried out in a fixed-bed flow reactor (id = 9 mm) equipped with a quadrupole mass spectrometer (QMS, ULVAC messmate200). A 0.25 g sample of Fe/ZSM-5 was placed on a porous quartz plate in the reactor. NO pulses were introduced to a continuous gas
85
flow through a six-way valve and a gas sampler. The NO pulse volume was 0.34 ml (13.6 mmol), the full width at half maximum (FWHM) of the pulse was approximately 8 s, and each pulse interval was 340 s. The continuous gas flow was maintained at 0.1 l/min (GHSV = 1.2 104 h1) and the gas composition was 0 or 400 ppm NH3, 10% O2, and the remainder He. The reaction gases (NO, NH3, and O2) and the product gases (N2 and H2O) were analyzed by QMS; neither N2O nor NO2 was detected. The N2 production rate was calculated according to 2 ðoutlet N2 amountÞ N2 production rate ¼ 100 1 inlet NO amount The outlet of N2 amount was calculated by introducing an N2 pulse instead of the NO pulse. Two types of NO pulse sequences were applied. In the first (type I), three pulses were introduced in the presence of NH3, and then seven pulses were introduced in the absence of NH3. The reaction temperatures were 120, 150, and 200 8C. In the second (type II), NH3 was cycled on or off at intervals of three pulses at 150 8C. That the temperature was lower than in the periodic NH3 supply tests (Section 2.2) was because this pulse reaction was conducted in the absence of CO2 and H2O, which would deactivate the SCR reaction, and because the NH3 inhibiting effect should be amplified at lower temperatures. 2.4. Steady-state rate measurements under standard and fast SCR conditions Steady-state rate measurements, which included concentration tests for reaction order analysis and temperature tests for Arrhenius plot, were carried out using a reactor (id = 15 mm) similar to that used in the periodic NH3 supply tests (Section 2.2). The reaction was induced by controlling total gas flow and catalyst weight to maintain pseudodifferential conditions. For the standard SCR, the total gas flow was 15 l/min and the catalyst weight was 0.45–1.0 g (GHSV between 1.7 and 3.8 106 h1). For the fast SCR, the total gas flow was 25 l/min and the catalyst weight was 0.3 g (GHSV = 9.6 106 h1). The total pressure upstream of the catalyst was measured using a pressure gauge and found to be 120– 137 kPa. Concentrations of NO/NOx, NH3, O2, and N2O were monitored using a chemiluminescent NO/NOx analyzer, a microwave NH3 analyzer, a magnetic system O2 analyzer, and an NDIR N2O analyzer. 3. Results 3.1. Periodic NH3 supply tests under standard and fast SCR conditions Fig. 1 shows the outlet NOx concentration (Fig. 1a) and catalyst temperature change (Fig. 1b) in the standard SCR condition for two inlet temperatures (250 and 300 8C). When NH3 was added to the feed (Fig. 1a), the NOx concentration quickly decreased due to the standard SCR reaction, went through a minimum, and then approached a steady-state level. The steady-state NOx conversion was about 25% at 250 8C and 50% at 300 8C. The catalyst temperature (Fig. 1b) increased sharply just after adding NH3; this increase was caused by the heat of NH3 adsorption plus the heat of the SCR reaction. When NH3 was removed from the feed, the NOx concentration went through a minimum and then began to increase, eventually reaching the inlet value. The catalyst temperature did not increase but gradually decreased with increasing NOx concentration and approached a constant value. Similar transient NOx removal behavior, just after NH3 addition and NH3 shutoff, has been reported over H-ZSM-5 [8,9] and Fe/ ZSM-5 [14,16]; this behavior is considered to be an inhibiting effect of gaseous NH3. In other words, the reaction could be accelerated
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Fig. 1. Periodic NH3 supply test under the standard SCR condition at 250 and 300 8C. Outlet NOx concentration (a) and catalyst temperature change (b). Feed = 380 ppm NO, 0 or 400 ppm NH3, 8% O2, 10% CO2, 8% H2O, the remainder N2. GHSV = 3.1 105 h1.
by decreasing the NH3 concentration. However, just after NH3 addition, the increasing catalyst temperature would also affect the reaction rate (Fig. 1b). Furthermore, we cannot rule out the possibility of NO adsorption because of the lack of N2 production data, which is verified in Section 3.2. Fig. 2 shows the outlet NOx concentration (Fig. 2a) and catalyst temperature change (Fig. 2b) in the fast SCR condition for two inlet temperatures (180 and 200 8C). The steady-state NOx conversion, was about 87% at 180 8C and 97% at 200 8C, was much higher than that in the standard SCR condition despite the lower temperatures. However, no transient NOx removal behavior was observed, indicating that the inhibiting effect by NH3 is probably small in the fast reaction. These different behaviors between standard and fast SCRs are probably related to the adsorption strength of each adsorbate and to their competitive adsorption onto active sites. This is discussed in detail in Section 4.2.
Fig. 2. Periodic NH3 supply test under the fast SCR condition at 180 and 200 8C. Outlet NOx concentration (a) and catalyst temperature change (b). Feed = 135 ppm NO, 135 ppm NO2, 0 or 300 ppm NH3, 8% O2, 10% CO2, 8% H2O, the remainder N2. GHSV = 3.1 105 h1.
Table 1 shows the total amounts of N2 produced from pulses 4– 10, i.e., the NH3-absent region. The N2 amount increased with decreasing temperature due to higher NH3 adsorption capacities at lower temperatures [12]. All the stored NH3 was considered to be consumed by the NO pulses, because in an FT-IR study at 200 8C pre-adsorbed NH3 bands were completely vanished by flowing NO + O2 mixture [17].
3.2. NO pulse reaction tests under the standard SCR condition To get a deeper understanding of transient NOx removal behavior in the standard SCR, we performed two types of NO pulse reaction tests. Fig. 3 shows N2 production rates at 120, 150, and 200 8C from the type I pulse sequence. In the NH3-present region (pulses 1–3), the N2 production rates were almost constant and the rates increased with increasing temperature. When NH3 was removed from the feed, the N2 amount increased (pulse 4) and then decreased (pulses 5–10), suggesting that the absence of gaseous NH3 promoted the standard reaction.
Fig. 3. N2 production rate in type I pulse sequence. T = 120, 150, and 200 8C.
M. Iwasaki et al. / Applied Catalysis A: General 366 (2009) 84–92
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Table 1 Integrated N2 production amounts from pulses 4–10 in the type I sequence. See Fig. 3. 120 8C
150 8C
200 8C
154a
129a
112a
a
mmol g1.
Fig. 4 shows N2 production rates from the type II pulse sequence. The rates at pulses 4 and 10 (the first pulses in the NH3absent regions) were higher than those in the NH3-present regions; this suggests that the promotion phenomenon is repeatable. This agrees with the behavior of the periodic NH3 supply test shown in Fig. 1a. 3.3. Steady-state rate measurements in standard and fast SCR conditions To further understand the reaction kinetics, we carried out steady-state rate measurements at a predetermined temperature. In the standard SCR condition, the NH3 conversion amount was consistent with the NOx conversion amount, and N2O was not detected; this suggests that NH3 oxidation and N2O production reactions are negligible. Thus, only the standard SCR reaction (1) progressed and we estimated its rate from the NOx conversion amount. Fig. 5 shows the dependence of the standard SCR rate on NO, O2, and NH3 concentrations at 200, 250, and 300 8C. The typical
Fig. 4. N2 production rate in type II pulse sequence. T = 150 8C.
gas composition was 0.12% NO, 0.12% NH3, 5% O2, 10% CO2, 5% H2O, and the remainder N2. The concentrations of the reactants were varied systematically: 0.025–0.3% NO, 0.035–0.3% NH3, 0.4–19% O2. The SCR rate was increased with NO and O2 concentrations but slightly decreased with NH3 concentration. Fig. 6 shows log–log plots of the data in Fig. 5. Since all the data gave linear relationships, the reaction rate can be expressed simply as a power law: r s ¼ ka p p ½NOa ½O2 b ½NH3 g
Fig. 5. Standard SCR rate as a function of NO (a), O2 (b), and NH3 (c) concentrations. Typical gas composition = 0.12% NO, 0.12% NH3, 5% O2, 10% CO2, 5% H2O and remainder N2. GHSV = 1.7 106, 1.9 106, and 3.8 106 h1 at 200, 250, and 300 8C, respectively.
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M. Iwasaki et al. / Applied Catalysis A: General 366 (2009) 84–92 Table 2 Apparent reaction orders for NO, O2, and NH3 from power law expressions for the standard SCR reaction. See Fig. 6. Temperature
aa
ba
ga
200 8C 250 8C 300 8C
0.81 0.88 0.86
0.34 0.31 0.29
0.21 0.18 0.11
a
Fig. 6. Log–log plots of standard SCR rate versus NO (*), O2 (~), and NH3 (&) concentrations. T = 200 8C for lowest line in each set; T = 250 8C for the middle line in each set; T = 300 8C for the upper line in each set. Reaction conditions are the same as those in Fig. 5.
where rs is the standard SCR rate, kapp is the apparent rate constant, and a, b, and g are apparent reaction orders for NO, O2, and NH3, respectively. We estimated the orders from the slopes in Fig. 6; the resulting values are given in Table 2. The NO orders (a) were 0.81– 0.88, slightly lower than first order. The O2 orders (b) were 0.29– 0.34, slightly lower than half order. The NH3 orders (g) were between 0.21 and 0.11, slightly lower than zeroth order. These
a
b
g
Rate = kapp[NO] [O2] [NH3] .
are close to the values reported by Huang et al. [11] (0.88–0.94 for NO, 0.36–0.41 for O2, and between 0.15 and 0.11 for NH3). Interestingly, in the case of H-ZSM-5, both NO and O2 are around first order [9,10]. The reason for the differences in O2 orders between Fe-ZSM-5 and H-ZSM-5 may be because different forms of O2 contribute to the rate-determining step: whereas dissociated atomic Oad should participate in the elementary reaction of the rate-determining step over Fe/ZSM-5 (see Section 4.1), undissociated molecular O2ad might be participating over H-ZSM-5. In fast SCR conditions, only a small amount of N2O was observed (roughly less than 5% of the total NOx conversion amount), indicating that the N2O production reaction contributes little to the conversion. Moreover, the NO conversion amount was almost twice as high as the NH3 conversion amount in all conditions. Thus, we can exclude contributions from the standard SCR (1) and from the NO2-SCR reaction, 3NO2 þ 4NH3 ! 72N2 þ 6H2 O
(3)
Fig. 7. Fast SCR rate as a function of NO (a), NO2 (b), NH3 (c) and O2 (d) concentrations. Typical gas composition = 0.075% NO, 0.053% NO2, 0.11% NH3, 5% O2, 10% CO2, 5% H2O, and remainder N2. GHSV = 9.6 106 h1.
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because both reactions are much slower than reaction (2) [18]. In addition, the emission of NH4NO3 from the catalyst surface, 2NO2 þ 2NH3 ! NH4 NO3 þ N2 þ H2 O
(4)
was not observed under these conditions. (Reaction (4) could not be ignored at lower temperatures (<180 8C) [13,18–20].) Hence, only the fast SCR reaction (2) progressed and we estimated its rate from doubling the NO conversion amount. Fig. 7 shows the dependence of the fast SCR rate on NO, NO2, NH3, and O2 concentrations at 195 and 250 8C. The typical gas composition was 0.075% NO, 0.053% NO2, 0.11% NH3, 5% O2, 10% CO2, 5% H2O, and the remainder N2. The concentrations of the following gases were varied systematically: 0.014–0.15% NO, 0.015–0.13% NO2, 0.023–0.28% NH3, and 0.4–19% O2. The SCR rate was increased with NO and NO2 concentration (Fig. 7a and b), suggesting that their apparent reaction orders are positive. However, the rate passed through a maximum with NH3 concentration (Fig. 7c), indicating that the apparent reaction order changed from positive to negative. The NH3 concentration dependence for the fast SCR (Fig. 7c) is much different from that for the standard SCR (Fig. 5c); this difference is considered to be the main reason for the different behavior in the periodic NH3 supply tests (Figs. 1a and 2a). Thus, because of the positive reaction order at low NH3 concentrations, the promotion effect cannot be expected to occur in fast SCR. Note that O2 concentration has little influence on the SCR rate (Fig. 7d), suggesting that O2 does not participate in competitive adsorption on active sites or in elementary steps in fast SCR reaction. A more detailed discussion of reaction mechanism is presented in Section 4.2. Fig. 8 shows Arrhenius plots for the standard and fast SCR reactions. The gas composition for the standard SCR was 0.125% NO, 0.125% NH3, 5% O2, 10% CO2, 5% H2O and the remainder N2; that for the fast SCR was 0.08% NO, 0.07% NO2, 0.13% NH3, 5% O2, 10% CO2, 5% H2O and the remainder N2. Fig. 8 shows that the standard SCR rate is much slower than the fast SCR rate. The difference between the two rates increases as temperature decreases. At 200 8C, the difference in rates is a factor of about 30. Apparent
89
activation energies at 200–280 8C were 35 kJ/mol for the standard SCR and 8 kJ/mol for the fast SCR. These are close to values reported elsewhere [21]: 39 kJ/mol for the standard SCR and 7 kJ/mol for the fast SCR. 4. Discussion 4.1. Standard SCR kinetics The results from the periodic NH3 supply test (Fig. 1) and the NO pulse reaction test (Figs. 3 and 4) show that the standard SCR was promoted when gaseous NH3 was shut off. This could be explained by the negative reaction order for NH3 (Table 2). To understand this, let us first consider the probable adsorption mechanisms involved. Analyses by extended X-ray absorption fine structure spectroscopy (EXAFS) [4,22,23] suggest that binuclear oxygenbridged Fe complexes occur on the catalyst. From Fourier transform infrared (FT-IR) studies, NO could adsorb on these Fe sites [24–26], as could NH3 [24,27,28]. However, oxygen could adsorb onto Fe–&–Fe bridged vacancy sites (& stands for a vacancy), which are observed by EXAFS [29]. Oxygen on such bridged sites plays an important role in many reactions [25,28,30– 32]. Combining these observations, we write the adsorption steps for NO, NH3 and O2 as K1
NO þ !NOad K2
NH3 þ !NH3ad K3
O2 þ 20 !2Oad
(5) (6) (7)
where * and *0 denote adsorption sites and Ki represent equilibrium constants. Consider now the rate-determining step in the standard SCR, which has been suggested to be NO oxidation between NOad and Oad [8–11]: k1
NOad þ Oad !NO2ad
(8)
where k1 is the rate constant. Once formed, NO2ad is probably consumed very quickly and the subsequent reaction should be much faster than reaction (8). Thus, any adsorbed species other than NOad, NH3ad, and Oad could be negligible. Assuming a Langmuir–Hinshelwood-type mechanism, one can describe the overall reaction rate of the standard SCR by the following rate expression: pffiffiffiffiffiffiffiffiffiffiffiffiffi k1 K 1 P NO K 3 P O2 pffiffiffiffiffiffiffiffiffiffiffiffiffi r 1 ¼ k1 uNO uO ¼ (9) ð1 þ K 1 PNO þ K 2 P NH3 Þ 1 þ K 3 P O2 where ui and Pi are the coverage of the adsorbate i and the partial pressure of gas i, respectively. Estimation of the kinetic parameters in (9) was obtained using the solver set up on a Microsoft Excel spreadsheet. The calculation minimizes the total sum of residual squares between calculated and experimental reaction rates for each set of isothermal data n X ðr i;exp: r i;calc: Þ2
(10)
i¼1
Fig. 8. Arrhenius plot of standard and fast SCRs. For standard SCR: feed = 0.125% NO, 0.125% NH3, 5% O2, 10% CO2, 5% H2O and remainder N2; GHSV = 1.7 106 h1. For fast SCR: feed = 0.08% NO, 0.07% NO2, 0.13% NH3, 5% O2, 10% CO2, 5% H2O, and remainder N2; GHSV = 9.6 106 h1.
The resulting parameter values are collected in Table 3, while the experimental and calculated values of the reaction rate are compared in Fig. 9. Since Fig. 9 shows good agreement, the model (9) is a reasonable and reliable one. Hence, our steady-state kinetic analysis demonstrates that the rate-determining step should be the formation of NO2 adsorbate from NOad and Oad.
M. Iwasaki et al. / Applied Catalysis A: General 366 (2009) 84–92
90
Table 3 Optimized kinetic parameters calculated for the standard SCR reaction.
Similarly, nitric acid reacts with NH3 to form NH4NO3:
Temperature
k1 a
K1 b
K2 b
K3 b
200 8C 250 8C 300 8C
38 217 799
106 33 15
238 145 44
3.2 5.7 7.4
a
mmol g1 s1.
b
atm1.
HNO3ad þ NH3ad $ NH4 NO3
(14)
NH4NO3 is relatively stable (200 8C) compared to ammonium nitrite [19]. Hence, when the reactants are only NO2 and NH3 without NO, NH4NO3 accumulates or desorbs at temperatures below 180 8C [13,14,20]; but at temperatures above 200 8C it decomposes to N2O [8,14,21]: NH4 NO3 ! N2 O þ 2H2 O
(15)
Table 3 shows the equilibrium constant of NH3 (K2) to be larger than that of NO (K1), suggesting that adsorption of NH3 was much stronger than that of NO. This is consistent with in situ FT-IR results [24,27]; NH3 is the main species adsorbed during the standard SCR reaction. This strong adsorption probably inhibits reaction (8). Here, K1 and K2 decreased with temperature because the adsorption reactions (5) and (6) are exothermic; i.e., the adsorption enthalpies are negative. In contrast, K3 increased with temperature, which is thought to be due to O2 dissociation processes that may be included in reaction (7). Finally, Table 3 shows that the rate constant k1 in (9) increased with temperature; the corresponding activation energy was estimated to be 68 kJ/mol, which is larger than the apparent activation energy (see Section 3.3). This is because the apparent rate constant kapp includes the adsorption terms.
But the resulting NO2 and nitrous acid are still consumed via reactions (11) and (13). Therefore, the sum of reactions (11)–(13) with (16) yields the fast SCR reaction (2). To fit our steady-state data, we simplify the above reactions as follows: first, we introduce the NO2 adsorption reaction (17) in addition to the NO and NH3 adsorption reactions (5) and (6):
4.2. Fast SCR kinetics
2NO2ad þ H2 O !HONOad þ HNO3ad
However, in the presence of NO, which is the fast SCR condition, NO can react with nitric acid to form NO2 and nitrous acid [13,19]: HNO3ad þ NOad ! NO2ad þ HONOad
K4
NO2 þ !NO2ad
(17)
Then we combine the NO2 reactions (11) and (12) to obtain K5
In the periodic NH3 supply test (Fig. 2), no definite NH3 inhibition was observed, and in the steady-state rate measurements, the reaction rates follow a parabolic curve with NH3 concentration (Fig. 7c). From these results, we infer that NH3-related competitive adsorption was occurring. In fact, it has been reported that NO2 strongly adsorbs onto active Fe sites as easily as NH3 [7]. In studies of the reaction mechanism, the first step of the fast SCR reaction is thought to be NO2 dimerization, followed by formation of nitrous and nitric acids [13,19,20]: 2NO2ad $ N2 O4ad
(11)
N2 O4ad þ H2 O $ HONOad þ HNO3ad
(12)
(16)
(18)
and we rewrite reaction (13) as k2
HONOad þ NH3ad !N2 þ 2H2 O
(19)
Nitric acid reduction by NO (16) is considered to be the ratedetermining step [13,19], which we now write as k3
HNO3ad þ NOad !NO2ad þ HONOad
0
(16 )
The contributions of NH4NO3 formation (14) and subsequent N2O formation (15) can be ignored under these conditions, because of the consistency between NH3 and double NO conversion amount and because of the vanishingly small N2O production observed. From the adsorption reactions (5), (6) and (17), Langmuir-type adsorption equations are obtained:
In the presence of NH3, nitrous acid forms ammonium nitrite which readily decomposes to N2:
uNO ¼ K 1 PNO uv
(20)
HONOad þ NH3ad $ ½NH4 NO2 ! N2 þ 2H2 O
uNH3 ¼ K 2 PNH3 uv
(21)
uNO2 ¼ K 4 PNO2 uv
(22)
(13)
Here uv is a coverage of vacancy sites, which can be expressed as
uv ¼ 1 ðuNO þ uNH3 þ uNO2 þ uHONO þ uHNO3 Þ
(23)
In (23) oxygen coverage is not included because the fast SCR reaction was not affected by O2 concentration (Fig. 7d) and because oxygen probably adsorbs onto the bridged vacancy sites, Fe–&–Fe as described in Section 4.1. The equilibrium constant in (18) can be written as K5 ¼
uHONO uHNO3 u2NO2
(24)
where the H2O contribution is neglected because an extremely large amount of H2O would exist over the catalyst. The reaction rates for (19) and (160 ) are described by (25) and (26), respectively: Fig. 9. Correlations between experimental and calculated values of the standard SCR rate. Reaction conditions are the same as those in Fig. 5 and fitted parameters are given in Table 3.
r 2 ¼ k2 uHONO u NH3
(25)
r 3 ¼ k3 uHNO3 uNO
(26)
M. Iwasaki et al. / Applied Catalysis A: General 366 (2009) 84–92
Conservation of mass relates these two rates: r 2 ¼ 2r 3
(27)
which is the fast SCR rate. From (20)–(27) we obtain the fast SCR rate rf as a function only of the partial pressures Pi:
rf ¼
1 þ K 1 P NO þ K 2 P NH3
Table 4 Optimized kinetic parameters calculated for the fast SCR reaction. Temperature
k2 a
k3 a
K1 b
K2 b
K4 b
K5 c
195 8C 250 8C
2.3 106 7.3 106
9.5 102 6.9 103
290 161
539 199
656 468
2.5 104 3.4 105
a
mmol g1 s1.
b
atm1. No unit.
c
NH3-related species have been detected during standard SCR, not only NH3 species but also NOx species have been detected during NO2-SCR. This indicates that NO2 can absorb onto Fe sites despite NH3 coexistence. It seems plausible that competitive adsorption between NO2 and NH3 was occurring, and this must be the main reason for the absence of NOx reduction promotion at NH3 shutoff.
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi K 4 P NO2 2K 1 K 2 K 5 k2 k3 PNO P NH3 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi þ K 4 P NO2 1 þ K 2 K 5 k2 P NH3 =2K 1 k3 PNO þ 2K 1 K 5 k3 P NO =K 2 k2 P NH3
We have carried out parameter estimations for kn and Kn. The results are collected in Table 4, and the experimental and calculated values of the reaction rates are compared in Fig. 10. The values of parameters K1 and K2 estimated for the standard SCR (Table 3), and which are relevant for the fast SCR fitting procedure, were used as starting values to facilitate the calculations. However, the parameter values calculated for the fast SCR differ from the values found for the standard SCR; this could be because the fast SCR is much more complex due to the larger number of species present and the possibility that the rate constants might depend on coverage and lateral interactions of adsorbates. In our studies, all surface sites were considered identical and interactions between adsorbed species were neglected. From Table 4, the equilibrium constant of NO2 (K4) was large compared to that of NH3 (K2), which indicates that adsorption of NO2 should be relatively strong. This assumption is supported in our previous TPD studies [7]; NO2 desorption from active Fe sites has been observed around 350 8C which is higher than the NH3 desorption temperature from Fe sites (300 8C) in NH3-TPD [7,28]. Furthermore, from in situ FT-IR studies [21], whereas only
91
(28)
Table 4 shows that k2 is much larger than k3. This implies that nitric acid reduction by NO (160 ) is the rate-determining step. Grossale et al. [13] conducted fast SCR transient analysis at low temperatures (<190 8C) and also concluded that this is the ratedetermining step. Thus, we come to the same conclusion from a different approach, i.e., steady-state kinetic analysis. However, Grossale et al. [13] also proposed that high NH3 concentrations have a detrimental effect on the fast SCR. Actually, they have observed that reaction (160 ) was promoted in the absence of gaseous NH3 at 150 8C. In contrast, Schwidder et al. [16] conducted periodic NH3 supply tests at 300 8C, and they did not observe the promotion effect. Likewise, in our tests at 180 and 200 8C, it was not observed (Fig. 2a). The reason for this discrepancy is probably due to the degree of NH4NO3 formation (14). In fact, Grossale et al. [13] reported that although NH4NO3 production was vanishingly small during fast SCR at 190 8C, it grew during NO2-SCR and/or at lower temperature condition. In addition, their recent report [33] shows that a threshold temperature of NH4NO3 decomposition was at 140–160 8C which was quite close to its melting point of 170 8C. In the presence of NH4NO3, decreasing NH3 concentration would shift the equilibrium (14) toward higher nitric acid concentration, thereby promoting reaction (160 ). Grossale et al. [33] have pointed out that NH4NO3 formation or a strong interaction between NH3 and nitrate species affects the reactivity at low temperature. This expectation is consistent with our result of Arrhenius plot in Fig. 8 that the fast SCR rate began to decrease sharply at an inlet temperature of 180 8C. Thus, the influence of the NH4NO3 formation would appear around the melting temperature which seems to be the critical point for the emergence of NH3 inhibition. The reaction mechanisms in this low temperature region have been investigated in greater detail elsewhere [13,17,18,33]. 5. Conclusions
Fig. 10. Correlations between experimental and calculated values of the fast SCR rate. Reaction conditions are the same as those in Fig. 7 and fitted parameters are given in Table 4.
This work reports the reaction kinetics of SCRs by NH3 on NO (standard SCR) and NO + NO2 (fast SCR) over Fe/ZSM-5 using periodic NH3 supply tests, NO pulse reaction tests, and steady-state rate analyses. In the standard SCR reaction, the NOx reduction rate improved just after NH3 shutoff. This improvement could be attributed to increasing N2 production in the absence of gaseous NH3. The steady-state rate analysis suggested that NO2 formation between NOad and Oad was the rate-determining step, and that NH3 inhibited this reaction due to strong adsorption onto active Fe sites. In the fast SCR reaction, the promotion effect was not observed in periodic NH3 supply tests under 180 and 200 8C. Steady-state measurements revealed that the reaction rate passed through a maximum with NH3 concentration, indicating that the apparent reaction order changed from positive to negative. Fitting analysis combined with modeling predicted that competitive adsorption between NO2 and NH3 was occurring due to strong NO2 adsorption. This must be the main reason for the lack of the promotion effect.
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