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NOx removal from exhaust gas from lean burn internal combustion engines through adsorption on FAU type zeolites cation exchanged with alkali metals and alkaline earth metals
Applied Catalysis B: Environmental 21 (1999) 215–220
NOx removal from exhaust gas from lean burn internal combustion engines through adsorption on FAU type zeolites cation exchanged with alkali metals and alkaline earth metals Orietta Monticelli, Raf Loenders, Pierre A. Jacobs, Johan A. Martens ∗ Centrum voor Oppervlaktechemie en Katalyse, K.U. Leuven, Kard. Mercierlaan 92, B-3001 Heverlee, Belgium Received 13 December 1998; received in revised form 3 March 1999; accepted 5 March 1999
Abstract The adsorption of NOx from gas mixtures containing 10% of oxygen, 5% of carbon dioxide, 5% of water and 1000 ppm of NOx of different compositions (500 ppm NO and 500 ppm NO2 ; 100 ppm NO and 900 ppm NO2 ; 400 ppm NO and 600 ppm NO2 ) is investigated in the temperature range 373–833 K on Y zeolites, cation exchanged with barium, caesium, sodium or lithium. In a typical experiment the gas mixture is conducted at a VHSV of 30 000 h−1 over the adsorbent bed at 373 K, the temperature of which is increased at 4 K/min. A period of NOx adsorption at the lower temperatures is followed by NOx desorption at higher temperatures. The temperature window for NOx adsorption and desorption and the NOx adsorption capacity are dependent on the nature of the cation exchanged into Y zeolite. The NOx adsorption capacity decreases in the order: Ba-Y > Cs-Y > Na-Y > Li-Y. With the present gas mixtures and at temperatures of 373–450 K, nitrogen dioxide is selectively adsorbed, while nitric oxide is inert to these adsorbents. Nitrogen dioxide contained in the gas stream reacts quantitatively with water to nitric acid and nitric oxide, the former being transformed into ‘zeolitic’ nitrate species, giving rise to an infrared band at 1384 cm−1 . The NO is released from the zeolite. This chemistry is observed independently of the NO/NO2 composition and of the nature of the cations in the Y zeolite. Accordingly, the maximum NOx uptake level is limited to two-third of the NO2 present in the gas mixture. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Adsorption; Nitrogen monoxide; Nitrogen dioxide; Y zeolites; Nitrate formation
1. Introduction There is a large interest in developing systems to abate NOx emissions from stationary and mobile sources [1]. Whereas the use of three-way catalyst is an established technology for the catalytic reduction of NOx produced by gasoline engines operating at stoichiometry, there is still no appropriate catalytic ∗
Corresponding author. Tel.: +32-16-321637; fax: +32-16321998; e-mail: [email protected]
technology for NOx emission abatement in vehicles with diesel and lean burn gasoline engines [2,3]. The main difficulties with potential catalysts is their narrow temperature window of operation and their poisoning by sulphur oxides [4,5]. An alternative to the catalytic approach is based on NOx adsorption. In the NOx Storage and Reduction Systems (NSR), the adsorbent is a basic oxide combined with an oxidation catalyst such as platinum, favouring the oxidation of NO into NO2 , which is adsorbed. The NOx trap is regenerated by periodically
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running the engine rich, whereby the adsorbed nitrate is chemically reduced into nitrogen [6]. In the Selective NOx Recirculation technique (SNR), the NOx is temporarily stored on an adsorbent and periodically circulated to the combustion chamber where it is decomposed in the combustion process. The NOx storage concept is also applicable in other de-NOx processes in which NOx enrichment is required [7]. The success of adsorbent systems is connected with the development of a NOx adsorbent able to temporarily store high quantities of NOx at suitable temperatures and in the presence of other components, including water, oxygen, carbon oxides, traces of sulphur oxides and hydrocarbons. The peculiar chemical reactivity of NOx molecules in zeolites has been known for a long time [8]. In a variety of zeolites including Y zeolite, exchanged with sodium and calcium (Na-Y and Ca-Y, [8]) barium and zinc (Ba-Y, Zn-Y [9]), chabasite exchanged with calcium (Ca-CHA, [8]), and sodium mordenite partially exchanged with cerium and lanthanum (CeNa-MOR and LaNa-MOR [10]), nitric oxide undergoes a disproportionation reaction (1): 4NO → N2 O + N2 O3
2. Experimental 2.1. Adsorbents NaY zeolite powder with Si/Al atomic ratio of 2.7 from Zeocat was cation exchanged by slurring 5 g of zeolite in 1 dm3 of a 0.05 M aqueous solution of the respective chloride salts for 16 h at room temperature. The zeolites, recovered by filtration, were washed with deionised water and dried at 333 K. In this way 70% of the cation exchange capacity (CEC) of Na-Y was exchanged with Cs+ or Ba2+ . To prepare Li-Y, the ion exchange operation was repeated three times to achieve almost complete exchange of lithium for sodium. 2.2. NOx adsorption–desorption procedure A flow chart of the experimental set-up is shown in Fig. 1. NO is monitored with a non-dispersive infrared detector (NDIR, Maihak, Multor 610). The same in-
(1)
While the nitrous oxide is easily desorbed, the N2 O3 species appearing as an NO+ –NO2 − ion pair is much stronger retained in the zeolite. The N2 O3 species have also been generated in Na-Y, H-ZSM-5, Na-ZSM-5, CeNa-MOR and LaNa-MOR by contacting the zeolite with mixtures of NO and NO2 , or NO and O2 [10–12]. On CeNa-MOR and LaNa-MOR zeolites, NO2 adsorption leads to the formation of N2 O4 and ionization into NO+ + NO3 − [10]. All these studies involve NOx adsorptions at room temperature or below, using low pressures of pure NO or mixtures of NO and O2 diluted with helium. The presence of residual intracrystalline water strongly interferes with the NOx chemistry, giving rise to the formation of nitrous and nitric acid among other compounds [8,13], leading to a significant reduction of the NOx adsorption capacity [14]. In this study, the NOx adsorption–desorption behavior of Y zeolites has been studied using realistic temperatures of technological significance and synthetic exhaust gas mixtures mimicking oxidised exhaust from a lean burn internal combustion engine.
Fig. 1. Experimental setup for performing NOx adsorption and desorption experiments (MFC: mass flow controller).
O. Monticelli et al. / Applied Catalysis B: Environmental 21 (1999) 215–220
strument in combination with a molybdenum based catalytic converter for NO2 is used to determine continuously the total NOx concentration. Simultaneous monitoring of NO and NO2 concentrations is done with a chemiluminescence detector (Eco Physics, CLD 700 EL ht). N2 O was analysed by a NDIR (Maihak, Unor 610). An amount of 0.6 ml of zeolite particles (0.25– 0.50 mm) consisting of compressed, crushed and sieved powder, is mounted in a quartz tube fitted in a tubular furnace. The gases are heated to the adsorbent temperature in the first section of the quartz tube filled with quartz particles of the same size as the zeolite pellets. He, O2 , CO2 and NO are fed from gas cylinders using mass flow controllers. A stream of water saturated helium is added after mixing of all other gases. All gas tubes beyond the water saturator are heated at 373 K to avoid water condensation. The oxidised synthetic exhaust gas is composed of 5% of carbon dioxide, 5% of water, 10% of oxygen and 1000 ppm of NOx and helium as balance. To achieve a NOx composition of 900 ppm NO2 –100 ppm NO, the NO stream is premixed with oxygen and combined with the other components down-stream. In experiments with pure NO, the oxidation of NO in the tubes is avoided by adding it after dilution of the other components with oxygen. Other NOx compositions are achieved by combining the two ways of NO introduction. The different NOx compositions used were verified with the chemiluminescence detector. The VHSV (volume hourly space velocity) is 30 000 h−1 . In an adsorption–desorption experiment, the adsorber is first by-passed for 5 min. to verify the NOx concentration and composition. After admitting the gas mixture to the adsorber at 373 K, the temperature is raised to 833 K at a rate of 4 K/min. Cooling to the initial temperature is performed under helium containing 5.8% of water vapour. The temperature cycle is repeated until the NOx concentration profile in the outlet of the adsorber does no longer change (typically after two cycles). The NOx adsorption–desorption profiles are reproducible within 5%.
2.3. Physico-chemical characterisation of adsorbents FTIR spectra of zeolite samples saturated with NOx at 408 K and after NOx desorption were recorded with
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Fig. 2. NOx concentration in the outlet of BaY (a), CsY (b), NaY (c) and LiY (d) adsorbent bed contacted with synthetic exhaust gas at 373 K and heated at 4 K/min. Inlet gas composition: 500 ppm NO, 500 ppm NO2 , 5% CO2 , 5% H2 O, 10% O2 , balanced with helium. VHSV: 30 000 h−1 .
a Nicolet 730 FTIR spectrometer using the KBr technique. The spectra were recorded in the transmission mode with a resolution of 2 cm−1 and using an accumulation of 128 scans. The crystallinity of the powders before and after the adsorption experiments were verified using powder XRD (Siemens D5000 matic).
3. Results and discussion NOx concentration profiles at the outlet of the adsorber filled with Li-Y, Na-Y, Ba-Y or Cs-Y samples and recorded during a temperature programmed experiment using a gas mixture with 500 ppm NO and 500 ppm NO2 are shown in Fig. 2. On the four samples, the initial NOx concentration is around 670 ppm, indicating that ca. 330 ppm NOx is being adsorbed. At a certain moment upon heating, the NOx concentration raises above 1000 ppm, indicating that NOx is desorbing. NOx desorption occurs over a wide temperature range. On Li-Y and Na-Y, two desorption steps can be distinguished, a first one following rapidly the adsorption phase, a second one being spread over a broad range of higher temperatures. The origin of these two desorption steps is presently unknown. As the NOx concentration profiles in Fig. 2 are reproducible in consecutive cycles, it is concluded that zeolite Y, cation exchanged with Na+ , Li+ , Ba2+ and Cs+ is capable of performing reversible NOx adsorption/desorption in temperature swing cycles. The adsorbed and desorbed amounts of NOx estimated by
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Table 1 Adsorption capacities of zeolite Y adsorbent traps (a) at 408 K (Fig. 2); (b) starting at 373 K and increasing at 4 K min−1 (Fig. 3)a Adsorbent
Ba-Y Cs-Y Na-Y Li-Y
mol NOx /g sorbent
NOx /Mn+
NOx /cage
a
a
a
970 750 410 –
b 630 500 360 290
0.42 0.24 0.10 –
1.7 1.6 0.64 –
a Gas
composition: 5% CO2 , 5% H2 O, 10% O2 and 1000 ppm Nox (50% NO2 and 50% NO).
integration of the NOx concentration curves (Fig. 2) are in reasonable agreement, suggesting that no other nitrogen containing species are formed, and that there is no irreversible NOx adsorption. The zeolites remained crystalline as verified with XRD. The temperature up to which NOx adsorption occurs, and, consequently, the NOx adsorption capacity, is dependent on the nature of the cations exchanged in the Y zeolite (Fig. 2). The NOx adsorption capacity of Ba-Y and Cs-Y in these temperature-swing conditions calculated from the ‘missing’ NOx and estimated by integration of the NOx concentration curves is 630 and 500 mol/g, respectively. On Na-Y and Li-Y the capacities are lower, viz. 360 and 290 mol/g. The adsorption capacities increase with decreasing electrostatic potential of the cation [15]. NOx saturation capacities were also determined under isothermal conditions at 408 K (Table 1and Fig. 3). The saturation capacities of Ba-Y, Cs-Y and NaY at 408 K are 970, 750 and 410 mol/g, respectively (Table 1). It corresponds to 1.7, 1.6 and 0.64 NOx
Fig. 3. NOx concentration in the outlet of Ba-Y (a), Cs-Y (b) and Na-Y (c) adsorbent bed contacted with synthetic exhaust gas at 408 K. Inlet gas composition: 500 ppm NO, 500 ppm NO2 , 5% CO2 , 5% H2 O, 10% O2 , balanced with helium. VHSV: 30 000 h−1 .
Fig. 4. NOx concentration in the outlet of the Ba-Y adsorbent bed contacted at 373 K with synthetic exhaust gas and heated to 823 K at 4 K/min. Inlet gas composition: 100 ppm NO, 900 ppm NO2 , 5% CO2 , 5% H2 O, 10% O2 , balanced with helium. VHSV: 30 000 h−1 .
molecules per supercage. Irrespective of the nature of the cations in zeolite Y, at 373 K (Fig. 2) and 408 K (Fig. 3), the uptake of NOx from the gas mixtures with 500 ppm NO2 and 500 ppm NO is limited to ca. 330 ppm, leaving ca. 670 ppm not adsorbed. In experiments with pure NO, no NOx adsorption occurred in any of the Y zeolites used. When Ba-Y is exposed to a gas mixture containing 900 ppm NO2 and 100 ppm NO, the NOx uptake in the temperature window 373–450 K corresponds to ca. 600 ppm (Fig. 4). In an isothermal adsorption experiment at 408 K on Ba-Y, using a gas composition with ca. 600 ppm NO2 and ca. 400 ppm NO, the NOx compounds and N2 O were analysed individually (Fig. 5). During the first 5 min the adsorbent bed was by-passed and the inlet
Fig. 5. NOx (a), NO2 (b) and NO (c) concentrations in the outlet of Ba-Y adsorbent bed contacted with synthetic exhaust gas at 408 K. Inlet gas composition (verified in the first 5 min): 400 ppm NO, 600 ppm NO2 , 5% CO2 , 5% H2 O, 10% O2 , balanced with helium. VHSV: 30 000 h−1 .
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Fig. 6. FTIR spectra of Ba-Y after contact with the gas mixture and heating from 373 to 833 K (a); FTIR spectra of Ba-Y (b), Na-Y (c), Li-Y (d) and Cs-Y (e) after 150 min contact with gas mixture at 408 K. Gas mixture: 500 ppm NO, 500 ppm NO2 , 5% CO2 , 5% H2 O, 10% O2 , balanced with helium. VHSV: 30 000 h−1 .
NO, NO2 and NOx concentrations verified. The gas mixture was subsequently run through the adsorbent bed. For about 40 min, the NO2 was almost completely adsorbed, while the NO concentration in the outlet of the adsorbent bed was increased to ca. 600 ppm. The N2 O concentration was always below the detection limit (2 ppm) and is not reported in the figure. The experiment of Fig. 5 together with those of Figs. 2–4 teach that at temperatures of 373–450 K and in presence of large amounts of water (5%), only twothirds of the NO2 can be adsorbed, while one-third is converted into NO. This stoichiometry can be rationalised by invoking a reaction with water (reaction (2)): 3NO2 + H2 O → 2HNO3 + NO
(2)
The maximum NOx uptake observed is ca. 330 ppm in the experiments with 500 ppm NO2 in the feed (Figs. 2 and 3), ca. 400 ppm with 600 ppm NO2 (Fig. 5) and ca. 600 ppm when the gas contains 900 ppm NO2 (Fig. 4). This can be explained by assuming that reaction (2) goes to completion and that the nitric acid reacts with the basic zeolite and is retained. FTIR spectra of the Y zeolites saturated with NOx from synthetic exhaust gas at 408 K are shown in Fig. 6. In all samples, vibrations present at ca. 1384 cm−1 can be assigned to the ν 3 stretching band of the nitrate anion which appears as the most intense band in many nitrate salts [16,17]. The presence of this band suggests that in all these basic zeolites, nitric acid is converted into nitrate species: Mn+ -nZ− + nHNO3 → M(NO3 )n + nH-Z
219
in which Mn + represents the alkali metal or alkaline earth metal cation and Z− the negatively charged zeolite framework. The M(NO3 )n must not be considered as bulk nitrate, but rather as ion pairs occluded in the zeolite pores, as suggested by the small concentration of nitrate anions formed (1.7, 1.6 and 0.64 per supercage in Ba-Y, Cs-Y and Na-Y, respectively, Table 1). In the FTIR spectrum of Ba-Y recorded after NOx desorption at 833 K (Fig. 6), the absorption at ca. 1384 cm−1 has disappeared, indicating that all NOx species were removed. It is consistent with the observed reversibility of the NOx adsorptions and desorptions in consecutive adsorption–desorption cycles (Fig. 2). This behaviour of Y zeolites is totally different from that observed in literature in absence of water and at lower temperatures where NO disproportionation and co-adsorption of NO and NO2 do occur (see Section 1). It is concluded that disproportionation reactions an co-adsorptions of NOx are suppressed in presence of large amounts of water. The proposed chemistry (reactions (2) and (3)) suggests that carbon dioxide and oxygen do not interfere with the NOx adsorption chemistry in Y type zeolites at temperatures of 373–450 K and in presence of 5% water. 4. Conclusions At temperatures exceeding 373 K and at high volumetric space velocities, Y zeolites that are ion exchanged with sodium, lithium, caesium and barium can selectively adsorb NO2 from synthetic exhaust gas mixtures containing 5% water, 10% oxygen, 5% carbon dioxide and 1000 ppm NOx . These zeolites therefore, can be used as NOx traps in oxidised exhaust gas from lean burn internal combustion engines when temperature-swing regeneration of the trap is applied. Mechanistically, in all zeolites investigated, the NO2 molecules are quantitatively converted with water into nitric oxide and nitrate species. Owing to the transformation of one-third of the nitrogen dioxide into nitric oxide, the NOx removal from a gas stream is limited to two-thirds of its NO2 concentration. NO disproportionation reactions and co-adsorptions of NO and NO2 , often reported in literature at low temperatures and in absence of water do not occur under the present more realistic adsorption conditions.
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Acknowledgements This work was performed in the Brite Euram III project No.BRPR-CT95-0054 ‘SNR-Technique’, sponsored by the EC. O.M. acknowledges the EC for a TMR fellowship.
References [1] M. Iwamoto, in: M. Misono, Y. Moro-oka, S. Kimura (Eds.), Future Opportunities in Catalytic and Separation Techonology, Elsevier, Amsterdam, 1990, p. 121. [2] S. Sato, Y. Yu-u, H. Yahiro, M. Iwamoto, Appl. Catal. 70 (1991) L1. [3] Y. Li, T.L. Slager, J.N. Armor, J. Catal. 150 (1994) 388. [4] M. Iwamoto, Catal. Today 28 (1996) 29. [5] M. Amidiris, T. Zhang, R. Farrauto, Appl. Catal. B 1 (1996) 203.
[6] M. Miyoshi, S. Matsumoto, K. Katoh, T. Tanaka, J. Harada, N. Takahashi, K. Yokota, M. Sigiura, K. Kasahara, SAE 950809, 1995. [7] J.A. Ritter, R.T. Yang, Ind. Eng. Chem. Res. 29 (1990) 1023. [8] W.E. Addison, R.M. Barrer, J. Chem. Soc. 757 (1955). [9] P.H. Kasai, R.J. Bishop, J. Am. Chem. Soc. 94 (1972) 5560. [10] E. Ito, Y.J. Mergler, B.E. Nieuwenhuys, H. van Bekkum, C.M. Van den Bleek, Microporous Materials 4 (1995) 455. [11] B.J. Adelman, G.D. Lei, W.M.H. Sachtler, Catal. Lett. 28 (1994) 119. [12] K. Hadjiivanov, J. Saussey, J.L. Freysz, J.C. Lavalley, Catal. Lett. 52 (1998) 103. [13] C.C. Chao, J.H. Lunsford, J. Am. Chem. Soc. 93 (1971) 71. [14] W.X. Zhang, H. Yahiro, N. Mizuno, J. Izumi, M. Iwamoto, Langmuir 9 (1993) 2337. [15] D. Barthomeuf, in: J. Fraissard, L. Petrakis (Eds.), Acidy and Basicity of Solids, Kluwer Academic Publishers, Dordrecht, 1994, p. 181. [16] C.C. Addison, B.M. Gatehouse, J. Chem. Soc. (1960) 613. [17] K. Eguchi, M. Wase, M. Machida, H. Arai, Catal. Today 27 (1996) 297.
NOx removal from exhaust gas from lean burn internal combustion engines through adsorption on FAU type zeolites cation exchanged with alkali metals and alkaline earth metals Orietta Monticelli, Raf Loenders, Pierre A. Jacobs, Johan A. Martens ∗ Centrum voor Oppervlaktechemie en Katalyse, K.U. Leuven, Kard. Mercierlaan 92, B-3001 Heverlee, Belgium Received 13 December 1998; received in revised form 3 March 1999; accepted 5 March 1999
Abstract The adsorption of NOx from gas mixtures containing 10% of oxygen, 5% of carbon dioxide, 5% of water and 1000 ppm of NOx of different compositions (500 ppm NO and 500 ppm NO2 ; 100 ppm NO and 900 ppm NO2 ; 400 ppm NO and 600 ppm NO2 ) is investigated in the temperature range 373–833 K on Y zeolites, cation exchanged with barium, caesium, sodium or lithium. In a typical experiment the gas mixture is conducted at a VHSV of 30 000 h−1 over the adsorbent bed at 373 K, the temperature of which is increased at 4 K/min. A period of NOx adsorption at the lower temperatures is followed by NOx desorption at higher temperatures. The temperature window for NOx adsorption and desorption and the NOx adsorption capacity are dependent on the nature of the cation exchanged into Y zeolite. The NOx adsorption capacity decreases in the order: Ba-Y > Cs-Y > Na-Y > Li-Y. With the present gas mixtures and at temperatures of 373–450 K, nitrogen dioxide is selectively adsorbed, while nitric oxide is inert to these adsorbents. Nitrogen dioxide contained in the gas stream reacts quantitatively with water to nitric acid and nitric oxide, the former being transformed into ‘zeolitic’ nitrate species, giving rise to an infrared band at 1384 cm−1 . The NO is released from the zeolite. This chemistry is observed independently of the NO/NO2 composition and of the nature of the cations in the Y zeolite. Accordingly, the maximum NOx uptake level is limited to two-third of the NO2 present in the gas mixture. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Adsorption; Nitrogen monoxide; Nitrogen dioxide; Y zeolites; Nitrate formation
1. Introduction There is a large interest in developing systems to abate NOx emissions from stationary and mobile sources [1]. Whereas the use of three-way catalyst is an established technology for the catalytic reduction of NOx produced by gasoline engines operating at stoichiometry, there is still no appropriate catalytic ∗
Corresponding author. Tel.: +32-16-321637; fax: +32-16321998; e-mail: [email protected]
technology for NOx emission abatement in vehicles with diesel and lean burn gasoline engines [2,3]. The main difficulties with potential catalysts is their narrow temperature window of operation and their poisoning by sulphur oxides [4,5]. An alternative to the catalytic approach is based on NOx adsorption. In the NOx Storage and Reduction Systems (NSR), the adsorbent is a basic oxide combined with an oxidation catalyst such as platinum, favouring the oxidation of NO into NO2 , which is adsorbed. The NOx trap is regenerated by periodically
0926-3373/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 9 9 ) 0 0 0 2 5 - 9
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running the engine rich, whereby the adsorbed nitrate is chemically reduced into nitrogen [6]. In the Selective NOx Recirculation technique (SNR), the NOx is temporarily stored on an adsorbent and periodically circulated to the combustion chamber where it is decomposed in the combustion process. The NOx storage concept is also applicable in other de-NOx processes in which NOx enrichment is required [7]. The success of adsorbent systems is connected with the development of a NOx adsorbent able to temporarily store high quantities of NOx at suitable temperatures and in the presence of other components, including water, oxygen, carbon oxides, traces of sulphur oxides and hydrocarbons. The peculiar chemical reactivity of NOx molecules in zeolites has been known for a long time [8]. In a variety of zeolites including Y zeolite, exchanged with sodium and calcium (Na-Y and Ca-Y, [8]) barium and zinc (Ba-Y, Zn-Y [9]), chabasite exchanged with calcium (Ca-CHA, [8]), and sodium mordenite partially exchanged with cerium and lanthanum (CeNa-MOR and LaNa-MOR [10]), nitric oxide undergoes a disproportionation reaction (1): 4NO → N2 O + N2 O3
2. Experimental 2.1. Adsorbents NaY zeolite powder with Si/Al atomic ratio of 2.7 from Zeocat was cation exchanged by slurring 5 g of zeolite in 1 dm3 of a 0.05 M aqueous solution of the respective chloride salts for 16 h at room temperature. The zeolites, recovered by filtration, were washed with deionised water and dried at 333 K. In this way 70% of the cation exchange capacity (CEC) of Na-Y was exchanged with Cs+ or Ba2+ . To prepare Li-Y, the ion exchange operation was repeated three times to achieve almost complete exchange of lithium for sodium. 2.2. NOx adsorption–desorption procedure A flow chart of the experimental set-up is shown in Fig. 1. NO is monitored with a non-dispersive infrared detector (NDIR, Maihak, Multor 610). The same in-
(1)
While the nitrous oxide is easily desorbed, the N2 O3 species appearing as an NO+ –NO2 − ion pair is much stronger retained in the zeolite. The N2 O3 species have also been generated in Na-Y, H-ZSM-5, Na-ZSM-5, CeNa-MOR and LaNa-MOR by contacting the zeolite with mixtures of NO and NO2 , or NO and O2 [10–12]. On CeNa-MOR and LaNa-MOR zeolites, NO2 adsorption leads to the formation of N2 O4 and ionization into NO+ + NO3 − [10]. All these studies involve NOx adsorptions at room temperature or below, using low pressures of pure NO or mixtures of NO and O2 diluted with helium. The presence of residual intracrystalline water strongly interferes with the NOx chemistry, giving rise to the formation of nitrous and nitric acid among other compounds [8,13], leading to a significant reduction of the NOx adsorption capacity [14]. In this study, the NOx adsorption–desorption behavior of Y zeolites has been studied using realistic temperatures of technological significance and synthetic exhaust gas mixtures mimicking oxidised exhaust from a lean burn internal combustion engine.
Fig. 1. Experimental setup for performing NOx adsorption and desorption experiments (MFC: mass flow controller).
O. Monticelli et al. / Applied Catalysis B: Environmental 21 (1999) 215–220
strument in combination with a molybdenum based catalytic converter for NO2 is used to determine continuously the total NOx concentration. Simultaneous monitoring of NO and NO2 concentrations is done with a chemiluminescence detector (Eco Physics, CLD 700 EL ht). N2 O was analysed by a NDIR (Maihak, Unor 610). An amount of 0.6 ml of zeolite particles (0.25– 0.50 mm) consisting of compressed, crushed and sieved powder, is mounted in a quartz tube fitted in a tubular furnace. The gases are heated to the adsorbent temperature in the first section of the quartz tube filled with quartz particles of the same size as the zeolite pellets. He, O2 , CO2 and NO are fed from gas cylinders using mass flow controllers. A stream of water saturated helium is added after mixing of all other gases. All gas tubes beyond the water saturator are heated at 373 K to avoid water condensation. The oxidised synthetic exhaust gas is composed of 5% of carbon dioxide, 5% of water, 10% of oxygen and 1000 ppm of NOx and helium as balance. To achieve a NOx composition of 900 ppm NO2 –100 ppm NO, the NO stream is premixed with oxygen and combined with the other components down-stream. In experiments with pure NO, the oxidation of NO in the tubes is avoided by adding it after dilution of the other components with oxygen. Other NOx compositions are achieved by combining the two ways of NO introduction. The different NOx compositions used were verified with the chemiluminescence detector. The VHSV (volume hourly space velocity) is 30 000 h−1 . In an adsorption–desorption experiment, the adsorber is first by-passed for 5 min. to verify the NOx concentration and composition. After admitting the gas mixture to the adsorber at 373 K, the temperature is raised to 833 K at a rate of 4 K/min. Cooling to the initial temperature is performed under helium containing 5.8% of water vapour. The temperature cycle is repeated until the NOx concentration profile in the outlet of the adsorber does no longer change (typically after two cycles). The NOx adsorption–desorption profiles are reproducible within 5%.
2.3. Physico-chemical characterisation of adsorbents FTIR spectra of zeolite samples saturated with NOx at 408 K and after NOx desorption were recorded with
217
Fig. 2. NOx concentration in the outlet of BaY (a), CsY (b), NaY (c) and LiY (d) adsorbent bed contacted with synthetic exhaust gas at 373 K and heated at 4 K/min. Inlet gas composition: 500 ppm NO, 500 ppm NO2 , 5% CO2 , 5% H2 O, 10% O2 , balanced with helium. VHSV: 30 000 h−1 .
a Nicolet 730 FTIR spectrometer using the KBr technique. The spectra were recorded in the transmission mode with a resolution of 2 cm−1 and using an accumulation of 128 scans. The crystallinity of the powders before and after the adsorption experiments were verified using powder XRD (Siemens D5000 matic).
3. Results and discussion NOx concentration profiles at the outlet of the adsorber filled with Li-Y, Na-Y, Ba-Y or Cs-Y samples and recorded during a temperature programmed experiment using a gas mixture with 500 ppm NO and 500 ppm NO2 are shown in Fig. 2. On the four samples, the initial NOx concentration is around 670 ppm, indicating that ca. 330 ppm NOx is being adsorbed. At a certain moment upon heating, the NOx concentration raises above 1000 ppm, indicating that NOx is desorbing. NOx desorption occurs over a wide temperature range. On Li-Y and Na-Y, two desorption steps can be distinguished, a first one following rapidly the adsorption phase, a second one being spread over a broad range of higher temperatures. The origin of these two desorption steps is presently unknown. As the NOx concentration profiles in Fig. 2 are reproducible in consecutive cycles, it is concluded that zeolite Y, cation exchanged with Na+ , Li+ , Ba2+ and Cs+ is capable of performing reversible NOx adsorption/desorption in temperature swing cycles. The adsorbed and desorbed amounts of NOx estimated by
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Table 1 Adsorption capacities of zeolite Y adsorbent traps (a) at 408 K (Fig. 2); (b) starting at 373 K and increasing at 4 K min−1 (Fig. 3)a Adsorbent
Ba-Y Cs-Y Na-Y Li-Y
mol NOx /g sorbent
NOx /Mn+
NOx /cage
a
a
a
970 750 410 –
b 630 500 360 290
0.42 0.24 0.10 –
1.7 1.6 0.64 –
a Gas
composition: 5% CO2 , 5% H2 O, 10% O2 and 1000 ppm Nox (50% NO2 and 50% NO).
integration of the NOx concentration curves (Fig. 2) are in reasonable agreement, suggesting that no other nitrogen containing species are formed, and that there is no irreversible NOx adsorption. The zeolites remained crystalline as verified with XRD. The temperature up to which NOx adsorption occurs, and, consequently, the NOx adsorption capacity, is dependent on the nature of the cations exchanged in the Y zeolite (Fig. 2). The NOx adsorption capacity of Ba-Y and Cs-Y in these temperature-swing conditions calculated from the ‘missing’ NOx and estimated by integration of the NOx concentration curves is 630 and 500 mol/g, respectively. On Na-Y and Li-Y the capacities are lower, viz. 360 and 290 mol/g. The adsorption capacities increase with decreasing electrostatic potential of the cation [15]. NOx saturation capacities were also determined under isothermal conditions at 408 K (Table 1and Fig. 3). The saturation capacities of Ba-Y, Cs-Y and NaY at 408 K are 970, 750 and 410 mol/g, respectively (Table 1). It corresponds to 1.7, 1.6 and 0.64 NOx
Fig. 3. NOx concentration in the outlet of Ba-Y (a), Cs-Y (b) and Na-Y (c) adsorbent bed contacted with synthetic exhaust gas at 408 K. Inlet gas composition: 500 ppm NO, 500 ppm NO2 , 5% CO2 , 5% H2 O, 10% O2 , balanced with helium. VHSV: 30 000 h−1 .
Fig. 4. NOx concentration in the outlet of the Ba-Y adsorbent bed contacted at 373 K with synthetic exhaust gas and heated to 823 K at 4 K/min. Inlet gas composition: 100 ppm NO, 900 ppm NO2 , 5% CO2 , 5% H2 O, 10% O2 , balanced with helium. VHSV: 30 000 h−1 .
molecules per supercage. Irrespective of the nature of the cations in zeolite Y, at 373 K (Fig. 2) and 408 K (Fig. 3), the uptake of NOx from the gas mixtures with 500 ppm NO2 and 500 ppm NO is limited to ca. 330 ppm, leaving ca. 670 ppm not adsorbed. In experiments with pure NO, no NOx adsorption occurred in any of the Y zeolites used. When Ba-Y is exposed to a gas mixture containing 900 ppm NO2 and 100 ppm NO, the NOx uptake in the temperature window 373–450 K corresponds to ca. 600 ppm (Fig. 4). In an isothermal adsorption experiment at 408 K on Ba-Y, using a gas composition with ca. 600 ppm NO2 and ca. 400 ppm NO, the NOx compounds and N2 O were analysed individually (Fig. 5). During the first 5 min the adsorbent bed was by-passed and the inlet
Fig. 5. NOx (a), NO2 (b) and NO (c) concentrations in the outlet of Ba-Y adsorbent bed contacted with synthetic exhaust gas at 408 K. Inlet gas composition (verified in the first 5 min): 400 ppm NO, 600 ppm NO2 , 5% CO2 , 5% H2 O, 10% O2 , balanced with helium. VHSV: 30 000 h−1 .
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Fig. 6. FTIR spectra of Ba-Y after contact with the gas mixture and heating from 373 to 833 K (a); FTIR spectra of Ba-Y (b), Na-Y (c), Li-Y (d) and Cs-Y (e) after 150 min contact with gas mixture at 408 K. Gas mixture: 500 ppm NO, 500 ppm NO2 , 5% CO2 , 5% H2 O, 10% O2 , balanced with helium. VHSV: 30 000 h−1 .
NO, NO2 and NOx concentrations verified. The gas mixture was subsequently run through the adsorbent bed. For about 40 min, the NO2 was almost completely adsorbed, while the NO concentration in the outlet of the adsorbent bed was increased to ca. 600 ppm. The N2 O concentration was always below the detection limit (2 ppm) and is not reported in the figure. The experiment of Fig. 5 together with those of Figs. 2–4 teach that at temperatures of 373–450 K and in presence of large amounts of water (5%), only twothirds of the NO2 can be adsorbed, while one-third is converted into NO. This stoichiometry can be rationalised by invoking a reaction with water (reaction (2)): 3NO2 + H2 O → 2HNO3 + NO
(2)
The maximum NOx uptake observed is ca. 330 ppm in the experiments with 500 ppm NO2 in the feed (Figs. 2 and 3), ca. 400 ppm with 600 ppm NO2 (Fig. 5) and ca. 600 ppm when the gas contains 900 ppm NO2 (Fig. 4). This can be explained by assuming that reaction (2) goes to completion and that the nitric acid reacts with the basic zeolite and is retained. FTIR spectra of the Y zeolites saturated with NOx from synthetic exhaust gas at 408 K are shown in Fig. 6. In all samples, vibrations present at ca. 1384 cm−1 can be assigned to the ν 3 stretching band of the nitrate anion which appears as the most intense band in many nitrate salts [16,17]. The presence of this band suggests that in all these basic zeolites, nitric acid is converted into nitrate species: Mn+ -nZ− + nHNO3 → M(NO3 )n + nH-Z
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in which Mn + represents the alkali metal or alkaline earth metal cation and Z− the negatively charged zeolite framework. The M(NO3 )n must not be considered as bulk nitrate, but rather as ion pairs occluded in the zeolite pores, as suggested by the small concentration of nitrate anions formed (1.7, 1.6 and 0.64 per supercage in Ba-Y, Cs-Y and Na-Y, respectively, Table 1). In the FTIR spectrum of Ba-Y recorded after NOx desorption at 833 K (Fig. 6), the absorption at ca. 1384 cm−1 has disappeared, indicating that all NOx species were removed. It is consistent with the observed reversibility of the NOx adsorptions and desorptions in consecutive adsorption–desorption cycles (Fig. 2). This behaviour of Y zeolites is totally different from that observed in literature in absence of water and at lower temperatures where NO disproportionation and co-adsorption of NO and NO2 do occur (see Section 1). It is concluded that disproportionation reactions an co-adsorptions of NOx are suppressed in presence of large amounts of water. The proposed chemistry (reactions (2) and (3)) suggests that carbon dioxide and oxygen do not interfere with the NOx adsorption chemistry in Y type zeolites at temperatures of 373–450 K and in presence of 5% water. 4. Conclusions At temperatures exceeding 373 K and at high volumetric space velocities, Y zeolites that are ion exchanged with sodium, lithium, caesium and barium can selectively adsorb NO2 from synthetic exhaust gas mixtures containing 5% water, 10% oxygen, 5% carbon dioxide and 1000 ppm NOx . These zeolites therefore, can be used as NOx traps in oxidised exhaust gas from lean burn internal combustion engines when temperature-swing regeneration of the trap is applied. Mechanistically, in all zeolites investigated, the NO2 molecules are quantitatively converted with water into nitric oxide and nitrate species. Owing to the transformation of one-third of the nitrogen dioxide into nitric oxide, the NOx removal from a gas stream is limited to two-thirds of its NO2 concentration. NO disproportionation reactions and co-adsorptions of NO and NO2 , often reported in literature at low temperatures and in absence of water do not occur under the present more realistic adsorption conditions.
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Acknowledgements This work was performed in the Brite Euram III project No.BRPR-CT95-0054 ‘SNR-Technique’, sponsored by the EC. O.M. acknowledges the EC for a TMR fellowship.
References [1] M. Iwamoto, in: M. Misono, Y. Moro-oka, S. Kimura (Eds.), Future Opportunities in Catalytic and Separation Techonology, Elsevier, Amsterdam, 1990, p. 121. [2] S. Sato, Y. Yu-u, H. Yahiro, M. Iwamoto, Appl. Catal. 70 (1991) L1. [3] Y. Li, T.L. Slager, J.N. Armor, J. Catal. 150 (1994) 388. [4] M. Iwamoto, Catal. Today 28 (1996) 29. [5] M. Amidiris, T. Zhang, R. Farrauto, Appl. Catal. B 1 (1996) 203.
[6] M. Miyoshi, S. Matsumoto, K. Katoh, T. Tanaka, J. Harada, N. Takahashi, K. Yokota, M. Sigiura, K. Kasahara, SAE 950809, 1995. [7] J.A. Ritter, R.T. Yang, Ind. Eng. Chem. Res. 29 (1990) 1023. [8] W.E. Addison, R.M. Barrer, J. Chem. Soc. 757 (1955). [9] P.H. Kasai, R.J. Bishop, J. Am. Chem. Soc. 94 (1972) 5560. [10] E. Ito, Y.J. Mergler, B.E. Nieuwenhuys, H. van Bekkum, C.M. Van den Bleek, Microporous Materials 4 (1995) 455. [11] B.J. Adelman, G.D. Lei, W.M.H. Sachtler, Catal. Lett. 28 (1994) 119. [12] K. Hadjiivanov, J. Saussey, J.L. Freysz, J.C. Lavalley, Catal. Lett. 52 (1998) 103. [13] C.C. Chao, J.H. Lunsford, J. Am. Chem. Soc. 93 (1971) 71. [14] W.X. Zhang, H. Yahiro, N. Mizuno, J. Izumi, M. Iwamoto, Langmuir 9 (1993) 2337. [15] D. Barthomeuf, in: J. Fraissard, L. Petrakis (Eds.), Acidy and Basicity of Solids, Kluwer Academic Publishers, Dordrecht, 1994, p. 181. [16] C.C. Addison, B.M. Gatehouse, J. Chem. Soc. (1960) 613. [17] K. Eguchi, M. Wase, M. Machida, H. Arai, Catal. Today 27 (1996) 297.