Reduction of Nitrogen Oxide Emissions - American Chemical Society

Chapter 9

IR Study of Catalytic Reduction of NO by Propene in the Presence of O over CeZSM-5 Zeolite

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Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on November 21, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0587.ch009

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H. Yasuda , T. Miyamoto , and M . Misono

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Department of Synthetic Chemistry, Faculty of Engineering, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan

The adsorbed species formed on Ce-ZSM-5 during the catalytic reduction of nitrogen monoxide (NO) by propene (C3H6) in the presence of oxygen using a closed circulation system and their reactivities have been investigated by means of infrared spectroscopy coupled with the quantitative analysis of the gas phase. Organic nitro(1558 cm ), nitrito- (1658 cm ) compounds, and isocyanate (2266 and 2241 cm ) species were formed in addition to carbonyl, nitrate, etc. on the surface during the NO -C H -O reaction at 373 K, and the band due to the nitro-compounds rapidly decreased correspondingly to the formation of N in the gas phase when the temperature was raised to 423 and 473 Κ in the presence of NO . The weak band due to isocyanate species (2241 cm ) also decreased gradually, but the change was in general much smaller. Similar behavior was observed for the reaction between adsorbed CH NO and NO . Based on these results, it was presumed that organic nitro compounds formed rapidly from NO and C H were possible intermediates and that Ν was mainly produced by the reactions between the nitro-compounds and NO (or NO and Ο ) in the NO C H -O reaction over Ce-ZSM-5. -1

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The catalytic removal of nitrogen oxides ( N O ) is a technology urgently required for the protection of our atmospheric environment. The catalytic reduction of nitrogen x

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Current address: Chemical Technology Division, Institute of Research and Innovation, Takada 1201, Kashiwa-shi, Chiba 227, Japan

0097-6156/95/0587-0110$12.00/0 © 1995 American Chemical Society

In Reduction of Nitrogen Oxide Emissions; Ozkan, U., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


9. Y A S U D A E T A L

IR Study of Catalytic Reduction ofNO

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by Propene

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monoxide (NO) by hydrocarbons in the presence of excess oxygen has recently attracted much attention for the depollution of exhaust gases from diesel or lean-burn gasoline engines. Various catalysts such as zeolites (1-12), metal oxides (13,14), metallosilicates (15,16), and noble metals (2,17,18) have been reported to be active for this reaction. We previously found that rare earth ion-exchanged zeolites, especially Ce-ZSM-5, showed a high catalytic activity (19), and that the increase of cerium content or the addition of alkali earth metals further enhanced the activity (20). However, the mechanism of this reaction, particularly the nature of the reaction intermediates formed on the surface, has not yet been elucidated. From the results of infrared spectroscopic measurements, it has been suggested that isocyanate species (NCO) was an intermediate of the reaction on Q1-CS/AI2O3 (21,22) and Cu-ZSM-5 (23). We recently reported the formation of organic nitro- and nitrito-compounds from N02and C3H6over Pt/Si02 (24) and Ce-ZSM-5 (25,26). In the present study, we investigated the adsorbed species formed from NO2-C3H6-O2 and their reactivities on Ce-ZSM-5 catalyst by means of infrared spectroscopy coupled with the quantitative analysis of the gas phase. Experimental Ce ion-exchanged ZSM-5 used in this study was prepared in the following way by the repeated ion-exchange at 298 Κ of parent ZSM-5 zeolite (S1O2/AI2O3 = 23.3) supplied by Tosoh Corporation. About 20 g of parent ZSM-5 was first ion-exchanged in an aqueous sodium nitrate solution, washed with water, and dried to obtain NaZSM-5, which was then ion-exchanged in 1.2 dm of aqueous cerium acetate solution (0.1 mol-dm" ) for two days. The zeolite was then filtered, washed with water, and dried at 393 Κ overnight. Ce-ZSM-5 thus obtained was ion-exchanged again in a new cerium acetate solution in order to increase the cerium content in the zeolite. The ionexchange level was estimated by the amount of the eluted sodium ion, which was measured with atomic absorption spectroscopy by assuming that cerium was present as Ce +. The exchange level of Ce-ZSM-5 used in this study was 18.5 % (1.3 wt% as Ce02). The powder X-ray diffraction pattern of the ion-exchanged Ce-ZSM-5 was the same as that of Na-ZSM-5, no diffraction peaks due to Ce02 being observed. Infrared spectra were collected by using an FTIR-8500 spectrometer (Shimadzu Co., Ltd.) and an IR cell (with CaF2 windows) connected to a closed circulation system (ca. 620 cm ). The catalyst was pressed into a wafer (ca. 40 mg), and supported by a glass-made holder which could be moved up and down in the cell to bring the wafer in and out of the optical path. A small basket which contained about 60 mg of catalyst was placed near the catalyst wafer but out of optical path in order to increase the rate of reaction. Thus, it was possible to measure the IR spectra and the composition of the gas phase simultaneously. The catalyst was evacuated at 773 Κ for 30 min, exposed to oxygen (P02 = 100 Torr, 1 Torr = 0.133 kPa) for 30 min, evacuated again for 30 min, cooled to a desired temperature, and then exposed to reactant gases. IR spectra from the adsorbed species were obtained by subtracting the spectrum of the pretreated wafer and the gas phase at each temperature from the spectrum obtained after adsorption or reaction. Products in the gas phase were analyzed by a gas chromatograph which was directly connected to the circulation system. 3

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Results and Discussion Adsorbed Species Formed from NO2-C3H6-O2 at 373 K . Figure 1 (A) and (B) shows the IR spectra of the adsorbed species formed after the introduction of the gas


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N O REDUCTION x

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Figure 1. IR spectra in the range 2350 - 2050 cm" (A) and 1800 - 1300 cm" (B) of the adsorbed species formed on Ce-ZSM-5 at 373 Κ after exposure to the N O (10 Torr)-C3H6 (10 Torr)-02 (50 Torr) mixed gas for 1 min (a), 60 min (b), and ^NO-C3H6-02for 59 min (c) (note that the actual composition of reactant was close to NO2 (10 Torr)-C3H6 (10 Torr)-C>2 (45 Torr), see text). 1 4

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Figure 2. IR spectra of the adsorbed species formed on Ce-ZSM-5 during the NO2-C3H6-02 reaction at 373 Κ (a), the adsorbed CH3N02on Ce-ZSM-5 at 423 Κ (b), and the adsorbed C4H9ONO on S1O2 at 298 Κ (c).


9. YASUDA ET AL.


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mixture of NO (10 Torr), C3H6 (10 Torr), and 0 2 (50 Torr) on Ce-ZSM-5. Spectra a and b were obtained at 373 Κ after 1 min and 1 h, respectively. Several bands appeared in the range 2400 - 1300 cm"l, while no significant bands were observed in the range 2100 - 1800 cm" . As shown in spectrum c, the bands at 2266, 2241, 2112, 1658, 1558, 1418, and 1385 cm~l shifted to lower wavenumbers when ^ N O was used in place of l^NO. Tjpori the introduction of the NO-C3H6-O2 gas mixture, N2, N2O, CO2 and CO were formed at a significant rate in the gas phase in the initial stage, but the rates of formation slowed down with time. In addition to these products, a small amount of HCN, which was observed in flow reactor experiments (25,27), was also produced in the gas phase. The band at 1558 c m ' l , which is the possible reaction intermediate of the N2 formation as discussed below, developed with time. In this experiment, NO in the gas phase was mostly oxidized to NO 2 by the coexisting oxygen, before the adsorption was initiated, as the NO-C3H6-O2 gas mixture showed red-brown color. It was also confirmed by the IR spectra (N02(g): 1618 cm "1) of the gas phase. It was previously concluded (25), on the basis of the comparison of the rates of NO-C3H6-O2, N02-C3H6(-02), and NO-O2 reactions in flow reactor experiments, that the NO-C3H6-O2 reaction on Ce-ZSM-5 proceeded via the reaction between NO2 and C3H6. Hence, the NO2-C3H6-O2 reaction studied in a closed system in this study would reasonably reflect the NO-C3H6-O2 reaction in flow experiments. The adsorbed species formed from C2H4 [NO 2 (10 Torr)-C2H4 (9 Torr)-02 (45 Torr)] and C3H8 [NO2 (10 Torr)-C3H.8 (10 Torr)-02 (45 Torr)] were also investigated. The IR spectra of the adsorbed species for C2H4 were similar to those for C3H6, except that the bands at 2266 and 2241 cm"* were much smaller for C2H4. On the other hand, the IR spectra were quite different in the case of C3H8; bands observed for the adsorption of NO 2 alone were dominant. The amounts of N2 and N2O formed were in the order of C3H6 > C2H4 » C3H8.


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Assignments of the IR Bands. The bands of spectra a and b in Figure 1 are assigned as follows. As shown in spectrum b in Figure 2, CH3NO2 adsorbed on Ce-ZSM-5 exhibited a band at 1560 c m ' l , which was assigned to (NO2) (24,28). A band assignable to ν (NO) appeared at 1654 c m for adsorbed C4H9ONO on S1O2 (spectrum c in Figure 2). Therefore the bands at 1558 and 1658 cm"! of spectra a and b in Figure 1 are attributed respectively to v (NO2) and ν (NO) of organic nitroand nitrito-compound(s) formed by the reaction between NO2 and C3H6. Although the detailed structure of these nitro- and nitrito-compounds are not clear yet, the formation of those compounds is known in the reactions between alkenes and NO2 (29). These bands did not appear when only NO2 was adsorbed. Iwamoto, et al., (23) have reported two isocyanate (NCO) bands at 2189 and 2251 cm" for Cu-ZSM5 and assigned them to NCO on Cu and on the zeolite framework, respectively. So, the small bands at 2266 and 2241 cm"l of spectra a and b in Figure 1 are probably due to two NCO species (note that the scale of Figure 1 (A) is one tenth that in Figure 1 (B)). The band at 2112 c m may be due to adsorbed cyanide (30) or NO2 (31). The broad doublet at 1418 and 1385 cm"l is assigned to nitrate (30). The bands at 1724 and 1706 cm""* are tentatively assigned to carbonyl species formed under the reaction conditions, since the position of these bands did not shift when l ^ N O used, and carbonyl species have a characteristic band at around 1700 c m ' l . The band at 1637 cm"l is due to adsorbed water. - 1

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N O REDUCTION x


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Time / min Figure 3. Changes of the amounts of N2 ( · ) , N2O (•) and CO2 (O) formed (A) and the absorbances of the bands at 2241 (Δ), 1724 (•), 1658 (O), 1558 ( · ) , and 1418 ( • ) cm"l (B) during the elevation of the temperature in a closed system after the NO2 (10 Torr)-C3H6 (10 Ton)-02 (45 Torr) reaction over CeZSM-5 at 373 Κ and subsequent evacuation.


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Thermal Desorption of the Adsorbed Species. The spectrum b in Figure 1 little changed upon evacuation at 373 K, except that the band at 2112 c m disappeared. When the temperature was raised after evacuation to 473 Κ in a closed system, N2, N2O, and CO2 evolved in the gas phase, as shown in Figure 3 (A). Concomitantly, the band at 1558 c m and the doublet at around 1400 cm" decreased, while the bands at 2266, 2241, 1724, 1706, and 1658 c m ' little changed (Figure 3 (B)). In this experiment, it took about 3 min to increase the temperature by 50 degrees, and the vertical dotted lines in Figure 3 (A) and (B) show the time when the desired temperature was reached. As shown in Figure 3 (A) and (B), good correspondences exist between the decrease of the band at 1558 cm" and the increase of N2, N2O, and CO2. By contrast, the correlations were not found for the bands at 1658 cm" (nitritocompound), 2266 and 2241 cm" (NCO), or 1724 cm" (carbonyl). The doublet at around 1400 cm" (nitrate) also decreased. However, the relative change of the band intensity was smaller than that of the band at 1558 cm" and moreover it was confirmed by a separate experiment that Ν2 was not produced by thermal desorption of nitrate species which was formed only from Νθ2· Therefore, it is unlikely that N2 observed in the above experiment was produced from nitrate. Thus, N2, N20> and CO2 were most probably produced by the decomposition of the nitro-compounds formed during the NO2-C3H6-O2 reaction. - 1

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Reactivities of the Adsorbed Species with NO2, O2, or NO. After the NO2-C3H602 reaction at 373 Κ for 1 h and subsequent evacuation as in the above experiment, the temperature was raised in the presence of NO 2 (10 Torr), O2 (50 Torr), or NO (10 Torr) to 473 K, and then kept for 1 h at 473 K. The amounts of N2, N2O, CO2, and CO produced in the gas phase during this experiment are summarized in Table I, together with the atomic ratios of nitrogen to carbon (N/C) in the gaseous products. The amounts of products formed by the experiment described in the preceding section are also included in the table for comparison. As shown in Table I, the reactivity of Table I. Products Formed in the Gas Phase during the Reaction of the Adsorbed Species Formed from N02-C3H6-Q2 with NO2, θ 2 or NO Products 1 Torr* N/C*> CO Reactions N2O CO2 N2 1.0 1.1 3.2 1.6 0.5 NO2 (10 Torr) 1.3 0.3 1.0 0.1 0.7 02 (50 Torr) NO (10 Torr) 0.4 0.4 1.0 0.5 1.1 _d 0.6 n.d. 1.0 0.1 0.2 Amounts of N2, N2O, CO2, and CO formed when the temperature was increased in the presence of NO2, O2, NO or in vacuum and then kept at 473 Κ for 1 h in the same atmosphere after the NO2-C3H6-O2 reaction at 373 Κ over Ce-ZSM-5. b Atomic ratios of nitrogen to carbon (N/C) in the products. figures in parentheses are the partial pressure. Without the addition of NO2, O2, or NO. ^ . d . means that CO was not detected by gas chromatography. c

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Time / min Figure 4. Changes of the amounts of N2 ( · ) , N2O (•), CO2 (O), and CO (•) formed (A) and the absorbances of the bands at 2241 (Δ) and 1558 ( · ) cm" (B) during the elevation of the temperature in NO2 (10 Torr) after the NO2 (10 Torr)-C3H6 (10 Torr)-02 (45 Torr) reaction over Ce-ZSM-5 at 373 Κ and subsequent evacuation. 1


9. Y A S U D A E T A L

IR Study of Catalytic Reduction of NO by Propene x

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the adsorbed species was greatest in the presence of Νθ2· It is noted that N/C ratios were approximately unity except for the experiment without the addition of NO2, etc. The changes of the products in the gas phase and the bands at 1558 and 2241 cm" are shown for the case of NO2 in Figure 4 (A) and (B), respectively. The vertical dotted lines in Figure 4 mean the same as in Figure 3. Upon the introduction of NO2 at 373 K, there were little changes in the IR spectra which were very similar to the spectrum b in Figure 1. When the temperature was raised in NO2, the band at 1558 cm" decreased remarkably at 423 Κ and disappeared completely at 473 K. The band at 2241cm also decreased gradually, but the change was small. On the other hand, the bands at 2266 and 1658 c m little varied. As shown in Figure 4 (A), N2, N2O, CO2, and CO were produced in the gas phase during this period. Figure 5 shows that linear correlations approximately exist between the rates of N2 and CO2 formation and the absorbance of the IR band due to nitro-compounds (1558 cm" ) at 423 K. The band at 1558 cm" also decreased in O2 or NO at elevated temperatures, but the decrease of the band intensity and the formation of gaseous products were much slower in O2 or NO than in NO2. The band at 2241 cm" remained almost unchanged in O2 or NO. 1


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These results indicate that the reaction between nitro-compounds and NO2 is the main route of the N2 formation for the NO2-C3H6-O2 reaction on Ce-ZSM-5 at 373 473 K. As described above, N2and N2O were formed upon the introduction of NO2C3H6-O2 onto Ce-ZSM-5 at 373 - 423 Κ and the rate of their formation slowed down with time. It is noteworthy that the rate of N2 and N2O formation for the NO 2C3H6-O2 reaction was of the comparable order at 423 Κ with that observed here at 423 Κ (Figure 4 (A)) and the N2/N2O and N/C ratios were also similar, while the direct comparison must be made cautiously, since the reaction conditions are different between the two cases. The relative slow development of the band at 1558 cm" at 373 Κ (spectrum a to b in Figure 1), accompanied by the decrease of the rates of N2 and N2O formations, may be explained as follows. The decomposition of the nitro-compounds (1558 cm" !) is rapid on fresh catalyst and hence they are hardly observable, but as the decomposition activity decreases possibly due to the accumulation of polymerization or oxidation products on the surface, the band gradually develops. The accumulation may be more pronounced at low temperatures. Rapid reaction between NO2 and C3H6 has already been demonstrated for Ce-ZSM-5 (25,26) and S1O2 (24). It may also be possible that a part of the nitro-compounds are transformed to N2 via isocyanate species (and/or cyanide), since the band at 2241 cm" also slowly decreased in NO 2- On the contrary, it is unlikely that nitrito-compounds participate in the formation of N2- Slight increase of the bands at around 1700 cm" (Figure 3 (B)) suggests the possibility of the formation of carbonyl species from the nitro compounds as intermediates. However, the detailed mechanism subsequent to the formation of the nitro-compounds is not very clear yet. 1

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Reactivity of Adsorbed CH3NO2. To confirm the above mechanism, the reactivity of adsorbed CH3NO2, which is an example of organic nitro-compounds, with NO 2, O2 or NO was examined. First, CH3NO2 (2 Torr) was adsorbed on Ce-ZSM-5 at 423 Κ and subsequently evacuated. During this procedure the band at 1560 cm" developed rapidly (see spectrum b in Figure 2), but the amounts of products detectedin the gas phase were small. Then, NO2 (10 Torr) was introduced at 423 K . N2, N2O, CO2, and CO were formed in the gas phase, accompanied by rapid 1



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Figure 5. Correlations between the absorbance of the band at 1558 cm" and the rates of N2 ( · ) and CO 2 ( O ) formation at 423 Κ in the experiment for the elevation of the temperature in NO2 (10 Torr) after the NO2 (10 Torr)-C3H6 (10 Torr)-02 (45 Torr) reaction over Ce-ZSM-5 at 373 K.


9. Y A S U D A E T A L .

IR Study of Catalytic Reduction of N0

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Table II. Products Formed in the Gas Phase during the Reaction of Adsorbed CH3NO2 with NO2, θ 2 or NO Products 1 Torr* Reactions CO N2O CO2 N2 0.7 NO2 (10 Torr)b 0.9 0.4 0.8 0.1 0.1 0.1 n.d. 02 (62 Torr) NO (10 Torr) 0.1

Reduction of Nitrogen Oxide Emissions - American Chemical Society

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