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Involvement of NCO species in promotion effect of water vapor on propane-SCR over Co-MFI zeolite
Applied Catalysis B: Environmental 77 (2007) 92–99 www.elsevier.com/locate/apcatb
Involvement of NCO species in promotion effect of water vapor on propane-SCR over Co-MFI zeolite Akira Shichi, Tadashi Hattori, Atsushi Satsuma * Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan Received 4 December 2006; received in revised form 9 July 2007; accepted 12 July 2007 Available online 18 July 2007
Abstract The effect of water vapor on the SCR of NO by propane over Co-MFI zeolite was investigated with respect to behavior of surface NCO species. In the absence of water vapor, the NO conversion at lower temperatures below 648 K decreased with time-on-stream. In the presence of water vapor, on the other hand, the NO conversion was higher than in the absence of water vapor and the catalyst deactivation was not observed. From the measurement of in situ IR spectra, it was found that NCO species accumulates on the catalyst in the absence of water vapor and then leads to deactivation of catalyst. In the presence of water vapor, however, it was found that the NCO species were easily hydrolyzed to ammonia and carbon dioxide, and therefore, the higher SCR activity was obtained. Thus, NCO species, which are known to be an important intermediate to form nitrogen in HC-SCR, could cause self-poisoning in the absence of water vapor. # 2007 Elsevier B.V. All rights reserved. Keywords: Selective catalytic reduction; Water vapor; Co-MFI; IR spectra; Deactivation
1. Introduction Selective catalytic reduction of NO by hydrocarbon (HCSCR) under excess oxygen has received much attention because of its potential applicability to removal of NOx from exhaust such as lean-burn gasoline and diesel engines. Since the first report on Cu-MFI [1,2], a large number of studies have been made on the development of de-NOx catalysts and many types of catalyst have been reported [3–5]. Co2+-exchanged zeolites, as firstly reported by Li and Armor [6], are known to be very active for the SCR of NO by lower alkanes, such as CH4 [6–9] and C3H8 [10,11], while their catalytic activity is strongly inhibited by the coexistence of water vapor which is a serious concern for the development of these materials for practical purposes [9,10]. On the contrary, we found that the effect of water vapor on C3H8-SCR activity greatly depends on Co2+ exchange level and the coexisting cation: The presence of water vapor remarkably enhances the catalytic activity for C3H8-SCR over Co-MFI zeolite prepared from H-MFI zeolite, but inhibits the C3H8-SCR reaction over excess co-exchanged MFI and Co–
* Corresponding author. Tel.: +81 52 789 4608; fax: +81 52 789 3193. E-mail address: [email protected] (A. Satsuma). 0926-3373/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2007.07.010
Na-MFI prepared from Na-MFI zeolite [12]. In the case of lower alkanes as reductants, however, the presence of water vapor inhibits the SCR reaction and therefore the promoting effect of water is not well known. The inhibiting effect of water can be explained by the competitive adsorption between H2O and reactant [8,10]. A few examples of the promoting effect of water vapor on the HC-SCR activity have been reported on some of catalysts by using C3H6 [13–15], iso-C4H10 [16] and noctane [17] as reductants. These promoting effects can be considered that water suppresses the formation of carbonaceous materials and/or promotes the removal of carbonaceous materials deposited on the catalyst. Although such explanations can be applied to the promoting effect observed in the present case, the color change of catalyst into brown or black, as usual observed in the formation of carbonaceous materials, was not found at all after the reaction test on each catalysts. Therefore, the inhibiting materials may not be carbonaceous materials, but possibly the strongly adsorbed reaction intermediates on catalyst surface. However, the promoting effect of water and the poisoning material are unclear. In the present study, the adsorbed species on Co-MFI zeolite during the C3H8-SCR were investigated by in situ IR and the promoting effect of water on the C3H8-SCR activity is clarified.
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2. Experimental Parent H-MFI (Si/Al = 22) zeolite was supplied by Tosoh Corp. Co-MFI zeolite was prepared by ion exchanging in an aqueous solution of cobalt(II) acetate at 353 K. After filtration, the sample was washed with distilled water and dried at 393 K, followed by calcination at 773 K for 6 h in a flow of dried air. The amount of cobalt ions exchanged in the zeolite was determined by inductively coupled plasma emission spectroscopy (ICP, Jarrel-Ash MODEL 975). The catalyst has the following composition; Si/Al = 22, Co/Al = 0.35 and Na/Al = 0.0. Prior to the catalytic test, the catalyst was pretreated in a flow of 20% O2/He at 773 K for 1 h. The catalytic test was performed with a fixed-bed flow reactor at atmospheric pressure by passing a mixture of 1000 ppm NO, 2000 ppm C3H8, 6.7% O2 and 0 or 2% H2O diluted in He at a rate of 100 cm3 min1 over 0.10 g of catalyst (GHSV = 38,000 h1). The effluent gas was analyzed by on-line GC-TCD with molecular sieve 13 and Porapak Q columns and by a chemiluminescence NOx analyzer (Best BCL-100uH). It should be noted that only negligible amount of N2O was detected. The temperature dependence of the catalytic performance was measured by lowering the temperature stepwise from 773 to 598 K. Sampling was done 60 min after a preset temperature had been reached, thus the obtained activity may not represent a true steady state if the catalyst performance changed with time. Time dependence of the catalytic activity was measured in a separate run. For the measurement of the NO reduction rate, higher space velocities (GHSV = 76,000–150,000 h1) were used to obtain low conversion below 30%. In situ IR spectra were collected on a JASCO FT/IR-620 equipped with a quartz IR cell connected to a conventional flow reaction system, which was used in our previous studies [18,19]. The sample was pressed into a 0.05 g self-supporting wafer and mounted into the IR cell with CaF2 windows. All the spectra were measured at the reaction temperatures accumulating 23 scans at a resolution of 2 cm1. A reference spectrum of Co-MFI was subtracted from each spectrum. Prior to each experiment, the catalyst was heated in 20% O2/He at 773 K for 1 h, cooled to the desired temperature in
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He, and then various gas mixtures were fed at a rate of 42 cm3 min1. The concentration of feed gas was the same as catalytic tests. 3. Results 3.1. Catalytic tests Fig. 1 shows the catalytic activity of Co-MFI zeolite for C3H8-SCR in the absence and presence of water vapor as a function of reaction temperature. Under both dry ([H2O] = 0%) and wet ([H2O] = 2%) conditions, NO conversion displayed a volcano-type correlation as a function of reaction temperature, and the maximum NO conversion was obtained at 648 K in the dry conditions and at 673 K in the wet conditions, respectively. Above 673 K, the conversion of C3H8 to COx (=CO + CO2) exceeded 90%. The bending over of NO conversion may be due to the depletion of C3H8. Interestingly, it was found that the addition of 2% H2O in the feed gas significantly promoted the conversion of NO to N2 at lower temperatures below 673 K, while at higher temperatures above 673 K the presence of water vapor does not much affect the catalytic activity. Addition of water vapor also promoted the C3H8 conversion in the C3H8SCR. The promotion effect was the most significant at 648 K. It should be noted that, however, the C3H8 conversion during the C3H8 combustion (square symbols in Fig. 1) was greatly inhibited by the presence of water at whole temperatures. Fig. 2 shows the effect of H2O concentration on the rate of NO reduction at 648 K. Below 1% of water vapor, the addition of water vapor in the feed gas increased sharply the rates of NO reduction. The NO reduction rate was not much affected by further increase in H2O concentration above 1%. Under wet conditions in the whole temperature region, the catalytic performance immediately reached a steady state within 10 min after a preset temperature had been reached and no decline of NO conversion was observed with time-on-stream. Under dry conditions, it was found that the catalytic performance was also stable at higher temperatures. On the other hand, the deactivation immediately sets in below 648 K. Fig. 3 shows the NO conversion at 648 K as a function of time. The reaction test was conducted in a flow of
Fig. 1. Temperature dependence of catalytic activity for the SCR of NO by C3H8 over Co-MFI in the absence and presence of water vapor. [NO] = 0 or 1000 ppm, [C3H8] = 2000 ppm, [O2] = 6.7%, [H2O] = (*, &) 0% or (*, &) 2%, W = 0.10 g and F = 100 cm3 min1 (W/F = 0.06 g s cm3).
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Fig. 3. Time course of (*) NO conversion to N2 and (&) intensity of IR band at 2258 cm1. [NO] = 1000 ppm, [C3H8] = 2000 ppm, [O2] = 6.7%, [H2O] = 0 or 2%, reaction temperature = 648 K and W/F = 0.06 g s cm3.
Fig. 2. Reaction rate of NO to N2 at 648 K over Co-MFI as a function of H2O concentration. [NO] = 1000 ppm, [C3H8] = 2000 ppm, [O2] = 6.7%, [H2O] = 0– 10% and W/F = 0.015–0.06 g s cm3.
NO + C3H8 + O2 without water for 150 min, and subsequently water was added in the feed gas for 90 min, followed by eliminating water from the feed. Under the dry conditions, the NO conversion was initially 72%, then readily decreased with time-on-stream, and finally to 18% after 150 min. Subsequent addition of 2% water in the feed gas resulted in rapid increase of the NO conversion and then stabilized the NO conversion at 77%. After that, by eliminating water from the feed gas, the NO conversion again decreased with time-on-stream. The open square in the figure will be explained in the next section. 3.2. Adsorbed species during C3H8-SCR reaction In situ IR was conducted to identify the cause of the catalyst deactivation under dry conditions. Fig. 4 shows in situ IR
spectra of adsorbed species on Co-MFI during the C3H8-SCR reaction at various temperatures. Under the dry conditions at lower temperatures, 623 and 648 K (spectra 4a and 4b), strong bands were observed at 2933, 2314, 2285, 2258, 2179, 1624, 1570–1540, 1446, 1414, and 1375 cm1. Bands in the region of 2800–3000 cm1 are due to the C–H stretching vibrations of gaseous and/or adsorbed C3H8. The band at 2179 cm1 can be assigned to CN species bound to Co2+ ion (Co2+-CN) [20–22], and the band at 2258 cm1 can be assigned to NCO species bound to Al3+ (Al3+-NCO) [20–22]. The bands at 2285 and 2314 cm1 can be assigned to CN species bound to Bronsted and Lewis Al3+ sites, respectively (abbreviated as AlB-CN and AlL-CN) [23–25]. The broad bands in the region of 1350–1750 cm1 show a few maxima for which unambiguous assignment is difficult. The same bands around 1570– 1540 cm1 were observed in a flow of C3H8 + O2, and were in agreement with the bands of acetic acid adsorbed on the catalyst (results not shown). Thus, the bands at 1570–1540 with a sharp band at 1446 cm1 may be assigned to nCOO of adsorbed acetate anion. A shoulder around 1624 cm1 is most
Fig. 4. In situ IR spectra of Co-MFI upon exposing a flow of NO + C3H8 + O2 without or with H2O for 120 min, subsequently purged with He for 30 min. a–c) without H2O at 623 K, 648 K, 673 K, d–f) with 2% H2O at 623 K, 648 K, 673 K. [NO] = 1000 ppm, [C3H8] = 2000 ppm, [O2] = 6.7%, [H2O] = 0 or 2% and F = 42 cm3 min1.
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Fig. 5. In situ IR spectra of Co-MFI taken at 648 K upon exposing a flow of NO + C3H8 + O2 for a) 5 min, b) 10 min, c) 30 min, d) 60 min and e) 120 min. [NO] = 1000 ppm, [C3H8] = 2000 ppm, [O2] = 6.7%, [H2O] = 0% and F = 42 cm3 min1.
probably due to the O–H deformation vibration of adsorbed water [26]. A weak band at 1375 cm1 can be assigned to propane enolate or carbonate [27]. Negative bands at 3590– 3700 cm1 indicate the adsorption of some of these species on surface OH species. At 673 K in dry conditions (spectrum 4c), these bands are rarely observed, suggesting the promotion of surface reaction and/or thermal decomposition of these species at higher temperatures. In the presence of 2% of water vapor, on the other hand, the bands became very weak, especially the bands at 2314–2179 cm1 assignable to N-containing surface species. These N-containing species disappeared at 648 K (spectrum 4e), thought trace of the bands at 1570–1440 cm1 assignable to oxygenated species was observed even at 673 K (spectrum 4f). The trends in the adsorbed species under the periodic operation shown in Fig. 3 were also examined by in situ IR. Fig. 5 shows the change of the IR spectrum with reaction time in flowing dry feed (NO + C3H8 + O2 without water) at 648 K.
Strong bands in the region of 2800–3000 cm1 (nCH ) and of 1400–1600 cm1 (nCOO ) were observed from the beginning of the HC-SCR reaction, and the band intensities only slightly increased with time. In the region of 2100–2400 cm1, on the other hand, the changes in the bands were significant with timeon-stream. At 5 min, broad bands were observed in the range of 2250–2350 cm1. The band may be mainly due to CO2, arisen from the conversion of propane, although the bands attributable to NCO and CN species are overlapped. With increasing the exposure time, the bands at 2179 cm1 (Co2+-CN), 2258 cm1 (Al3+-NCO), 2285 cm1 (AlB-CN) and 2314 cm1 (AlL-CN) increased, while the contribution of CO2 decreased. Simultaneously, the bands of nCH (3074, 3124 cm1) from unsaturated group and the negative bands of OH group emerged. Fig. 6 shows in situ IR spectra after the HC-SCR reaction under the dry conditions for 150 min (spectrum 6a), and under subsequent addition of 2% water vapor (spectra 6b–e). When
Fig. 6. In situ IR spectra of Co-MFI taken at 648 K upon exposing a flow of NO + C3H8 + O2 in the absence of water for (a) 150 min, followed by introducing 2% H2O in the feed gas for b) 5 min, c) 10 min, d) 20 min and e) 60 min. [NO] = 1000 ppm, [C3H8] = 2000 ppm, [O2] = 6.7%, [H2O] = 0–2% and F = 42 cm3 min1.
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The time course of the intensity of the band at 2258 cm1 assignable to Al3+-NCO species shown in Figs. 4 and 5 are plotted in Fig. 3 as open squares. The band intensity showed entirely opposite trend to the NO conversion, i.e., the band gradually increased with time in the dry conditions while sharply decreased and almost entirely disappeared in the wet conditions. The accumulation of NCO species was entirely reversible depending on the presence of water vapor. 3.3. Reactivity of adsorbed species
Fig. 7. Height intensity of IR band upon exposing NO + C3H8 + O2 at 648 K as a function of time.
2% water was added to the feed gas, the bands of NCO (2258 cm1), CN (2179, 2285 and 2314 cm1) and C–H (3074, 3124 cm1) from unsaturated group immediately decreased and almost disappeared within 20 min, while the bands of CO2 (2360 cm1) and OH (3595 cm1) increased. The broad bands of acetate, nðCOO Þ at 1400–1600 cm1, once increased and then decreased in the presence of water vapor. After 20 min, the IR spectra were in agreement with the spectrum observed under wet condition in Fig. 4, and there was not much change in the bands with time-on-stream. Fig. 7 shows the time course of the band intensities of NCO and CN species during the C3H8-SCR reaction under the dry conditions. The intensities of CN band initially increased with time until 60 min, and then reached a steady state. On the other hand, the intensity of NCO band gradually increased with time at first 60 min, and then NCO band continued to increase gradually with time.
Fig. 8 shows responses of the adsorbed species in a flow of 1000 ppm NO + 6.7% O2 at 648 K. After the C3H8-SCR reaction under dry conditions for 120 min, the catalyst was flushed with He for 30 min to eliminate the gaseous and weakly adsorbed species, and then the flowing gas was switched. It should be noted that these adsorbed species are relatively stable to pure O2 or pure NO because IR spectrum was not changed when the gas flow was switched to 6.7% O2 or 1000 ppm NO (results not shown). In flowing NO + O2, however, the most of the bands assigned to NCO, CN, C–H, and COO disappeared and broad bands of NO2 species at around 1400–1650 cm1 appeared. These results indicate that adsorbed species are reactive to NO2 formed from oxidation of NO by O2. As shown in Fig. 9, in flowing 2% of water vapor, the bands of NCO (2258 cm1) and Co-CN (2179 cm1) almost disappeared within 30 min and the bands of CN at 2285 and 2314 cm1 also decreased. Simultaneously, new bands at 3360, 3277 and 3190 cm1 due to N–H stretching vibrations of NH4+ or amine appeared and also the bands of acetate (1570–1540, 1446 cm1) greatly increased. These results indicate that NCO and CN species were hydrolyzed to NH3 and CO2. In fact, many papers have been reported that NCO and CN species can be easily hydrolyzed to NH3 and CO2 [20,28–31]. In the present case, the simultaneous increase in acetate and N–H bands indicate that amido species are hydrolyzed to carboxylates and amines by addition of water vapor [28,32,33].
Fig. 8. In situ IR spectra of Co-MFI taken at 648 K upon exposing a flow of NO + C3H8 + O2 for 120 min, subsequently purged with He for (a) 30 min, followed by purging with NO + O2 for b) 5 min, c) 10 min, d) 15 min, e) 20 min and f) 25 min. [NO] = 1000 ppm, [C3H8] = 2000 ppm, [O2] = 6.7%, [H2O] = 0–2% and F = 42 cm3 min1.
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Fig. 9. In situ IR spectra of Co-MFI taken at 648 K upon exposing a flow of NO + C3H8 + O2 for 120 min, subsequently purged with He for a) 30 min, followed by purging with 2% H2O/He for (b) 3 min, c) 10 min, d) 20 min and e) 30 min. [NO] = 1000 ppm, [C3H8] = 2000 ppm, [O2] = 6.7%, [H2O] = 0–2% and F = 42 cm3 min1.
4. Discussion 4.1. Reversible deactivation at low temperatures As shown in Fig. 3, the Co-MFI catalyst is deactivated under the dry conditions and the catalytic activity is recovered by the addition of water vapor. The reversible deactivation of catalyst suggests that the deactivation was not caused by some modifications of catalyst structure but by deposition of poisoning materials. So far, the promotion effect of water vapor on the HC-SCR activity has been reported when C3H6 [13–15], iso-C4H10 [16] and n-octane [17] are used as reductants. In such cases, the deactivation is caused by the formation of carbonaceous materials, and the promoting effect of water vapor is attributed to the suppression of the formation and/or the removal of deposited carbonaceous materials [13–16]. Although such explanations might be applied to the present case, the color change of catalyst due to the formation of carbonaceous materials was not found at all after the reaction test. Furthermore, even in the presence of water vapor, the broad IR bands were observed in the region of 1350– 1700 cm1, in which the absorption bands due to various hydrocarbon derived species (C O, C C, C N, and so on) could be observed (Fig. 4). Therefore, the contribution of carbonaceous materials as the inhibiting materials can be ruled out. The poisoning by other adsorbed species should be the main reason for the deactivation. 4.2. Involvement of NCO species in deactivation The results obtained from in situ IR spectra indicated that the presence of two types of strongly adsorbed hydrocarbon derived species on catalyst surface under the dry reaction conditions, i.e., oxygenated species (mainly acetate) and Ncontaining species (CN and NCO). As shown in Fig. 5, these species accumulated during the reaction under the dry conditions. The bands due to oxygenated species are intense
from the initial stage of the reaction, while the bands assignable to N-containing species gradually increased with time-onstream. Since the NO conversion gradually decreased with time-on-stream as shown in Fig. 3, the deactivation of catalyst is in agreement with the accumulation of N-containing species. After the introduction of water vapor, the band of oxygenated species once increased and then slightly decreased. Even after 20 min, when the catalytic activity entirely recovered, the intensities of the bands of oxygenated species are not much different from those under the dry conditions. On the other hand, the bands of N-containing species drastically diminished. These quick responses of the bands against water vapor clearly indicate that the deactivation under the dry conditions is caused by the strong adsorption of N-containing species observed as bands around 2170–2360 cm1. After the introduction of the water vapor, the band of Ncontaining species showed some differences in the response. Under the dry conditions shown in Figs. 5 and 7, surface CN species (AlB-CN, AlL-CN and Co-CN) were saturated within 60 min, while surface Al-NCO species gradually increased up to 150 min. The trend of the band of Al-NCO is in good agreement with the decrease in the NO conversion shown in Fig. 3. Actually, the time course of NO conversion to N2 is clearly contrast to that of the band intensity of NCO in the periodic operation between the dry and wet conditions. By addition of water vapor to the feed gas, NO conversion rapidly recovered to the initial level and kept stable under wet condition, while the intensity of the NCO band immediately decreased and disappeared. Therefore, the deactivation of Co-MFI under the dry conditions should be due to the strong adsorption of NCO species on Al-site, i.e., acid site of zeolite. The deposition of NCO species on zeolitic acid sites was also supported by the negative bands at 3668 and 3595 cm1. These negative bands gradually became intense with time-on-stream, indicating adsorption of surface species on OH groups under the dry reaction conditions. A good agreement of the responses of the OH
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Fig. 10. Proposed reaction pathway of C3H8-SCR over Co-MFI.
bands with the Al-NCO band indicates the formation of NCO species on Al–OH site. In the case of nitromethane decomposition over Co-MFI, we have already reported that the oligomerization of NCO species followed by polymerization causes strong deactivation of the HC-SCR over Co-MFI [20]. In the present case, however, since there was no band at 1723 cm1, assignable to cyanuric acid, oligomerization or polymerization of NCO species can be neglected. Since the theoretical water production at 100% conversion is five times higher in this case than the nitromethane decomposition, the difference may be due to the difference in the partial pressure of water vapor. 4.3. Reaction mechanism of C3H8-SCR over Co-MFI Based on the results in this study and the previous investigation in the literature, the deactivation in the dry conditions and the positive role of water vapor can be proposed as shown in Fig. 10. At the initial stage of the HC-SCR reaction, it has been generally accepted that the HC-SCR reaction begins with NO oxidation to NO2 [8–10,34,35]. NO2 react with hydrocarbons to form organic nitroso or nitro compounds (R– NO, R–NO2) [28,32] via gas phase radicals [11] or surface species. Adsorbed NCO and CN species observed in the IR spectra may be formed from these organic nitroso or nitro compounds probably via very reactive amido species (R0 C(O)NHR00 ) [20–22,28,31–33]. In the case of CH4-SCR, Cowan et al. proposed that formohydroxamic acid and formamido are the possible intermediates to form NCO and CN species, respectively [28]. In the case of higher hydrocarbons such as propane, Beutel et al. [33] proposed that 2-nitrosopropane with subsequent formation to Nmethylacetamide followed by hydrolysis to acetic acid and methylamine is the possible pathway. The presence of the bands at 1446 and 1570 cm1 assignable to carboxylate species represents the presence of this reaction pathway (path B in Fig. 10). The reaction rate of this path might be influenced by the presence of water vapor, however, the deposition of amido or amine species on the deactivated catalyst can be excluded. This is because (1) the intensity of the bands at 1446 and 1570 cm1 was not much affected by the presence of water and (2) the band at 3300–3500 cm1, at which amido or amine species give N–H stretching band, did not increase under the dry conditions at low temperatures (Fig. 4a–c) or with time-onstream (Fig. 5a–e).
As discussed in the previous section, from the in situ IR spectra, it is clear that deposition of surface NCO species on Co-MFI is the cause of deactivation (path A in Fig. 10). Although the NCO species can be oxidized by NO2 even in the dry conditions as shown in Fig. 8, the reaction rate of NCO species by NO2 is slower than that in the presence of water vapor. This is because the complete consumption of NCO species was achieved after 20 min of the introduction of water vapor (Fig. 6) while it took more than 20 min by the reaction with NO2 (Fig. 8). Therefore, the NCO species are gradually accumulated with time-on-stream at the dry conditions and lower temperatures. Higher temperature, and/or inclusion of water in the feed, can prevent or delay the deactivation. This interpretation seems reasonable given that cyanuric acid melts and decomposes at the temperature range of interest here and HCNO is readily hydrolyzed on ion-exchanged zeolites and metal oxides [28,29,32,36]. Piazzesi et al. reported investigated the decomposition of HNCO on titania by DRIFT spectroscopy [36]. HNCO adsorbs dissociatively on titania to yield NCO species, bound to Ti4+ sites, and hydroxyl (OH) groups. The stability of the NCO species decreases with increasing temperatures, and above 473 K they are very easily removed from the surface. In the presence of water vapor, the hydrolysis to ammonia easily proceeds on titania. It can be rationalized that the recovery of the NO conversion in the presence of water vapor is caused by the hydrolysis of the deposited NCO species and the increase in the contribution of the reaction pathway C in Fig. 10, i.e., the formation of NH3 from NCO species followed by effective NO reduction by NH3 reaction [11–14]. Actually, Cowan et al. have reported the high activity of NO reduction with NH3 over Co-MFI, i.e., the conversion of NO achieves to 100% in the temperature range of 525–723 K [28]. 5. Conclusion The deactivation and reactivation of Co-MFI in the C3H8SCR was investigated in the light of the deposited species in the dry reaction conditions. In the absence of water vapor below 648 K, the NO conversion decreased with time-on-stream by the accumulation of NCO species Al sites of zeolites. In the presence of water vapor, however, the deposited NCO species were easily hydrolyzed to NH3 and CO2, and therefore, the higher HC-SCR activity was obtained. NCO species, which are known to be an important intermediate to form N2 for the HCSCR, could be self-poisoning substance depending on the
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reaction conditions. The water vapor act as an important role in the decomposition of the poisoning species and simultaneously the formation of another reaction intermediate, NH3, as an effective reductant for the selective reduction of NOx to N2. Acknowledgements This work was partly supported by a Grant-in-Aid for Scientific Research (B) and Priority Area ‘‘Molecular Nano Dynamics’’ from the Ministry of Education, Science and Culture, Japan. References [1] M. Iwamoto, H. Yahiro, Y. Yu-u, S. Shundo, N. Mizuno, Shokubai (Catalyst) 32 (1990) 430. [2] W. Held, A. Ko¨nig, T. Richter, L. Pupper, SAE Paper 900496 (1990). [3] M. Iwamoto, H. Yahiro, Catal. Today 22 (1994) 5. [4] M. Shelef, Chem. Rev. 95 (1995) 209. [5] Y. Traa, B. Burger, J. Weitkamp, Micropor. Mesopor. Mater. 30 (1999) 3. [6] Y. Li, J.N. Armor, Appl. Catal. B 1 (1992) L31. [7] Y. Li, J.N. Armor, J. Catal. 150 (1994) 376. [8] Y. Li, T.L. Slager, J.N. Armor, J. Catal. 150 (1994) 388. [9] Y. Li, P.J. Battavio, J.N. Armor, J. Catal. 142 (1993) 561. [10] T. Tabata, M. Kokitsu, H. Ohtsuka, O. Okada, L.M.F. Sabatino, G. Bellussi, Catal. Today 27 (1996) 91. [11] F. Witzel, G.A. Sill, K.W. Hall, J. Catal. 149 (1994) 229. [12] A. Shichi, A. Satsuma, T. Hattori, Chem. Lett. 1 (2001) 44. [13] Y. Hirao, C. Yokoyama, M. Misono, Chem. Commun. 597 (1996). [14] M. Haneda, Y. Kintaichi, H. Hamada, Catal. Lett. 55 (1998) 47.
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Involvement of NCO species in promotion effect of water vapor on propane-SCR over Co-MFI zeolite Akira Shichi, Tadashi Hattori, Atsushi Satsuma * Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan Received 4 December 2006; received in revised form 9 July 2007; accepted 12 July 2007 Available online 18 July 2007
Abstract The effect of water vapor on the SCR of NO by propane over Co-MFI zeolite was investigated with respect to behavior of surface NCO species. In the absence of water vapor, the NO conversion at lower temperatures below 648 K decreased with time-on-stream. In the presence of water vapor, on the other hand, the NO conversion was higher than in the absence of water vapor and the catalyst deactivation was not observed. From the measurement of in situ IR spectra, it was found that NCO species accumulates on the catalyst in the absence of water vapor and then leads to deactivation of catalyst. In the presence of water vapor, however, it was found that the NCO species were easily hydrolyzed to ammonia and carbon dioxide, and therefore, the higher SCR activity was obtained. Thus, NCO species, which are known to be an important intermediate to form nitrogen in HC-SCR, could cause self-poisoning in the absence of water vapor. # 2007 Elsevier B.V. All rights reserved. Keywords: Selective catalytic reduction; Water vapor; Co-MFI; IR spectra; Deactivation
1. Introduction Selective catalytic reduction of NO by hydrocarbon (HCSCR) under excess oxygen has received much attention because of its potential applicability to removal of NOx from exhaust such as lean-burn gasoline and diesel engines. Since the first report on Cu-MFI [1,2], a large number of studies have been made on the development of de-NOx catalysts and many types of catalyst have been reported [3–5]. Co2+-exchanged zeolites, as firstly reported by Li and Armor [6], are known to be very active for the SCR of NO by lower alkanes, such as CH4 [6–9] and C3H8 [10,11], while their catalytic activity is strongly inhibited by the coexistence of water vapor which is a serious concern for the development of these materials for practical purposes [9,10]. On the contrary, we found that the effect of water vapor on C3H8-SCR activity greatly depends on Co2+ exchange level and the coexisting cation: The presence of water vapor remarkably enhances the catalytic activity for C3H8-SCR over Co-MFI zeolite prepared from H-MFI zeolite, but inhibits the C3H8-SCR reaction over excess co-exchanged MFI and Co–
* Corresponding author. Tel.: +81 52 789 4608; fax: +81 52 789 3193. E-mail address: [email protected] (A. Satsuma). 0926-3373/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2007.07.010
Na-MFI prepared from Na-MFI zeolite [12]. In the case of lower alkanes as reductants, however, the presence of water vapor inhibits the SCR reaction and therefore the promoting effect of water is not well known. The inhibiting effect of water can be explained by the competitive adsorption between H2O and reactant [8,10]. A few examples of the promoting effect of water vapor on the HC-SCR activity have been reported on some of catalysts by using C3H6 [13–15], iso-C4H10 [16] and noctane [17] as reductants. These promoting effects can be considered that water suppresses the formation of carbonaceous materials and/or promotes the removal of carbonaceous materials deposited on the catalyst. Although such explanations can be applied to the promoting effect observed in the present case, the color change of catalyst into brown or black, as usual observed in the formation of carbonaceous materials, was not found at all after the reaction test on each catalysts. Therefore, the inhibiting materials may not be carbonaceous materials, but possibly the strongly adsorbed reaction intermediates on catalyst surface. However, the promoting effect of water and the poisoning material are unclear. In the present study, the adsorbed species on Co-MFI zeolite during the C3H8-SCR were investigated by in situ IR and the promoting effect of water on the C3H8-SCR activity is clarified.
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2. Experimental Parent H-MFI (Si/Al = 22) zeolite was supplied by Tosoh Corp. Co-MFI zeolite was prepared by ion exchanging in an aqueous solution of cobalt(II) acetate at 353 K. After filtration, the sample was washed with distilled water and dried at 393 K, followed by calcination at 773 K for 6 h in a flow of dried air. The amount of cobalt ions exchanged in the zeolite was determined by inductively coupled plasma emission spectroscopy (ICP, Jarrel-Ash MODEL 975). The catalyst has the following composition; Si/Al = 22, Co/Al = 0.35 and Na/Al = 0.0. Prior to the catalytic test, the catalyst was pretreated in a flow of 20% O2/He at 773 K for 1 h. The catalytic test was performed with a fixed-bed flow reactor at atmospheric pressure by passing a mixture of 1000 ppm NO, 2000 ppm C3H8, 6.7% O2 and 0 or 2% H2O diluted in He at a rate of 100 cm3 min1 over 0.10 g of catalyst (GHSV = 38,000 h1). The effluent gas was analyzed by on-line GC-TCD with molecular sieve 13 and Porapak Q columns and by a chemiluminescence NOx analyzer (Best BCL-100uH). It should be noted that only negligible amount of N2O was detected. The temperature dependence of the catalytic performance was measured by lowering the temperature stepwise from 773 to 598 K. Sampling was done 60 min after a preset temperature had been reached, thus the obtained activity may not represent a true steady state if the catalyst performance changed with time. Time dependence of the catalytic activity was measured in a separate run. For the measurement of the NO reduction rate, higher space velocities (GHSV = 76,000–150,000 h1) were used to obtain low conversion below 30%. In situ IR spectra were collected on a JASCO FT/IR-620 equipped with a quartz IR cell connected to a conventional flow reaction system, which was used in our previous studies [18,19]. The sample was pressed into a 0.05 g self-supporting wafer and mounted into the IR cell with CaF2 windows. All the spectra were measured at the reaction temperatures accumulating 23 scans at a resolution of 2 cm1. A reference spectrum of Co-MFI was subtracted from each spectrum. Prior to each experiment, the catalyst was heated in 20% O2/He at 773 K for 1 h, cooled to the desired temperature in
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He, and then various gas mixtures were fed at a rate of 42 cm3 min1. The concentration of feed gas was the same as catalytic tests. 3. Results 3.1. Catalytic tests Fig. 1 shows the catalytic activity of Co-MFI zeolite for C3H8-SCR in the absence and presence of water vapor as a function of reaction temperature. Under both dry ([H2O] = 0%) and wet ([H2O] = 2%) conditions, NO conversion displayed a volcano-type correlation as a function of reaction temperature, and the maximum NO conversion was obtained at 648 K in the dry conditions and at 673 K in the wet conditions, respectively. Above 673 K, the conversion of C3H8 to COx (=CO + CO2) exceeded 90%. The bending over of NO conversion may be due to the depletion of C3H8. Interestingly, it was found that the addition of 2% H2O in the feed gas significantly promoted the conversion of NO to N2 at lower temperatures below 673 K, while at higher temperatures above 673 K the presence of water vapor does not much affect the catalytic activity. Addition of water vapor also promoted the C3H8 conversion in the C3H8SCR. The promotion effect was the most significant at 648 K. It should be noted that, however, the C3H8 conversion during the C3H8 combustion (square symbols in Fig. 1) was greatly inhibited by the presence of water at whole temperatures. Fig. 2 shows the effect of H2O concentration on the rate of NO reduction at 648 K. Below 1% of water vapor, the addition of water vapor in the feed gas increased sharply the rates of NO reduction. The NO reduction rate was not much affected by further increase in H2O concentration above 1%. Under wet conditions in the whole temperature region, the catalytic performance immediately reached a steady state within 10 min after a preset temperature had been reached and no decline of NO conversion was observed with time-on-stream. Under dry conditions, it was found that the catalytic performance was also stable at higher temperatures. On the other hand, the deactivation immediately sets in below 648 K. Fig. 3 shows the NO conversion at 648 K as a function of time. The reaction test was conducted in a flow of
Fig. 1. Temperature dependence of catalytic activity for the SCR of NO by C3H8 over Co-MFI in the absence and presence of water vapor. [NO] = 0 or 1000 ppm, [C3H8] = 2000 ppm, [O2] = 6.7%, [H2O] = (*, &) 0% or (*, &) 2%, W = 0.10 g and F = 100 cm3 min1 (W/F = 0.06 g s cm3).
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Fig. 3. Time course of (*) NO conversion to N2 and (&) intensity of IR band at 2258 cm1. [NO] = 1000 ppm, [C3H8] = 2000 ppm, [O2] = 6.7%, [H2O] = 0 or 2%, reaction temperature = 648 K and W/F = 0.06 g s cm3.
Fig. 2. Reaction rate of NO to N2 at 648 K over Co-MFI as a function of H2O concentration. [NO] = 1000 ppm, [C3H8] = 2000 ppm, [O2] = 6.7%, [H2O] = 0– 10% and W/F = 0.015–0.06 g s cm3.
NO + C3H8 + O2 without water for 150 min, and subsequently water was added in the feed gas for 90 min, followed by eliminating water from the feed. Under the dry conditions, the NO conversion was initially 72%, then readily decreased with time-on-stream, and finally to 18% after 150 min. Subsequent addition of 2% water in the feed gas resulted in rapid increase of the NO conversion and then stabilized the NO conversion at 77%. After that, by eliminating water from the feed gas, the NO conversion again decreased with time-on-stream. The open square in the figure will be explained in the next section. 3.2. Adsorbed species during C3H8-SCR reaction In situ IR was conducted to identify the cause of the catalyst deactivation under dry conditions. Fig. 4 shows in situ IR
spectra of adsorbed species on Co-MFI during the C3H8-SCR reaction at various temperatures. Under the dry conditions at lower temperatures, 623 and 648 K (spectra 4a and 4b), strong bands were observed at 2933, 2314, 2285, 2258, 2179, 1624, 1570–1540, 1446, 1414, and 1375 cm1. Bands in the region of 2800–3000 cm1 are due to the C–H stretching vibrations of gaseous and/or adsorbed C3H8. The band at 2179 cm1 can be assigned to CN species bound to Co2+ ion (Co2+-CN) [20–22], and the band at 2258 cm1 can be assigned to NCO species bound to Al3+ (Al3+-NCO) [20–22]. The bands at 2285 and 2314 cm1 can be assigned to CN species bound to Bronsted and Lewis Al3+ sites, respectively (abbreviated as AlB-CN and AlL-CN) [23–25]. The broad bands in the region of 1350–1750 cm1 show a few maxima for which unambiguous assignment is difficult. The same bands around 1570– 1540 cm1 were observed in a flow of C3H8 + O2, and were in agreement with the bands of acetic acid adsorbed on the catalyst (results not shown). Thus, the bands at 1570–1540 with a sharp band at 1446 cm1 may be assigned to nCOO of adsorbed acetate anion. A shoulder around 1624 cm1 is most
Fig. 4. In situ IR spectra of Co-MFI upon exposing a flow of NO + C3H8 + O2 without or with H2O for 120 min, subsequently purged with He for 30 min. a–c) without H2O at 623 K, 648 K, 673 K, d–f) with 2% H2O at 623 K, 648 K, 673 K. [NO] = 1000 ppm, [C3H8] = 2000 ppm, [O2] = 6.7%, [H2O] = 0 or 2% and F = 42 cm3 min1.
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Fig. 5. In situ IR spectra of Co-MFI taken at 648 K upon exposing a flow of NO + C3H8 + O2 for a) 5 min, b) 10 min, c) 30 min, d) 60 min and e) 120 min. [NO] = 1000 ppm, [C3H8] = 2000 ppm, [O2] = 6.7%, [H2O] = 0% and F = 42 cm3 min1.
probably due to the O–H deformation vibration of adsorbed water [26]. A weak band at 1375 cm1 can be assigned to propane enolate or carbonate [27]. Negative bands at 3590– 3700 cm1 indicate the adsorption of some of these species on surface OH species. At 673 K in dry conditions (spectrum 4c), these bands are rarely observed, suggesting the promotion of surface reaction and/or thermal decomposition of these species at higher temperatures. In the presence of 2% of water vapor, on the other hand, the bands became very weak, especially the bands at 2314–2179 cm1 assignable to N-containing surface species. These N-containing species disappeared at 648 K (spectrum 4e), thought trace of the bands at 1570–1440 cm1 assignable to oxygenated species was observed even at 673 K (spectrum 4f). The trends in the adsorbed species under the periodic operation shown in Fig. 3 were also examined by in situ IR. Fig. 5 shows the change of the IR spectrum with reaction time in flowing dry feed (NO + C3H8 + O2 without water) at 648 K.
Strong bands in the region of 2800–3000 cm1 (nCH ) and of 1400–1600 cm1 (nCOO ) were observed from the beginning of the HC-SCR reaction, and the band intensities only slightly increased with time. In the region of 2100–2400 cm1, on the other hand, the changes in the bands were significant with timeon-stream. At 5 min, broad bands were observed in the range of 2250–2350 cm1. The band may be mainly due to CO2, arisen from the conversion of propane, although the bands attributable to NCO and CN species are overlapped. With increasing the exposure time, the bands at 2179 cm1 (Co2+-CN), 2258 cm1 (Al3+-NCO), 2285 cm1 (AlB-CN) and 2314 cm1 (AlL-CN) increased, while the contribution of CO2 decreased. Simultaneously, the bands of nCH (3074, 3124 cm1) from unsaturated group and the negative bands of OH group emerged. Fig. 6 shows in situ IR spectra after the HC-SCR reaction under the dry conditions for 150 min (spectrum 6a), and under subsequent addition of 2% water vapor (spectra 6b–e). When
Fig. 6. In situ IR spectra of Co-MFI taken at 648 K upon exposing a flow of NO + C3H8 + O2 in the absence of water for (a) 150 min, followed by introducing 2% H2O in the feed gas for b) 5 min, c) 10 min, d) 20 min and e) 60 min. [NO] = 1000 ppm, [C3H8] = 2000 ppm, [O2] = 6.7%, [H2O] = 0–2% and F = 42 cm3 min1.
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The time course of the intensity of the band at 2258 cm1 assignable to Al3+-NCO species shown in Figs. 4 and 5 are plotted in Fig. 3 as open squares. The band intensity showed entirely opposite trend to the NO conversion, i.e., the band gradually increased with time in the dry conditions while sharply decreased and almost entirely disappeared in the wet conditions. The accumulation of NCO species was entirely reversible depending on the presence of water vapor. 3.3. Reactivity of adsorbed species
Fig. 7. Height intensity of IR band upon exposing NO + C3H8 + O2 at 648 K as a function of time.
2% water was added to the feed gas, the bands of NCO (2258 cm1), CN (2179, 2285 and 2314 cm1) and C–H (3074, 3124 cm1) from unsaturated group immediately decreased and almost disappeared within 20 min, while the bands of CO2 (2360 cm1) and OH (3595 cm1) increased. The broad bands of acetate, nðCOO Þ at 1400–1600 cm1, once increased and then decreased in the presence of water vapor. After 20 min, the IR spectra were in agreement with the spectrum observed under wet condition in Fig. 4, and there was not much change in the bands with time-on-stream. Fig. 7 shows the time course of the band intensities of NCO and CN species during the C3H8-SCR reaction under the dry conditions. The intensities of CN band initially increased with time until 60 min, and then reached a steady state. On the other hand, the intensity of NCO band gradually increased with time at first 60 min, and then NCO band continued to increase gradually with time.
Fig. 8 shows responses of the adsorbed species in a flow of 1000 ppm NO + 6.7% O2 at 648 K. After the C3H8-SCR reaction under dry conditions for 120 min, the catalyst was flushed with He for 30 min to eliminate the gaseous and weakly adsorbed species, and then the flowing gas was switched. It should be noted that these adsorbed species are relatively stable to pure O2 or pure NO because IR spectrum was not changed when the gas flow was switched to 6.7% O2 or 1000 ppm NO (results not shown). In flowing NO + O2, however, the most of the bands assigned to NCO, CN, C–H, and COO disappeared and broad bands of NO2 species at around 1400–1650 cm1 appeared. These results indicate that adsorbed species are reactive to NO2 formed from oxidation of NO by O2. As shown in Fig. 9, in flowing 2% of water vapor, the bands of NCO (2258 cm1) and Co-CN (2179 cm1) almost disappeared within 30 min and the bands of CN at 2285 and 2314 cm1 also decreased. Simultaneously, new bands at 3360, 3277 and 3190 cm1 due to N–H stretching vibrations of NH4+ or amine appeared and also the bands of acetate (1570–1540, 1446 cm1) greatly increased. These results indicate that NCO and CN species were hydrolyzed to NH3 and CO2. In fact, many papers have been reported that NCO and CN species can be easily hydrolyzed to NH3 and CO2 [20,28–31]. In the present case, the simultaneous increase in acetate and N–H bands indicate that amido species are hydrolyzed to carboxylates and amines by addition of water vapor [28,32,33].
Fig. 8. In situ IR spectra of Co-MFI taken at 648 K upon exposing a flow of NO + C3H8 + O2 for 120 min, subsequently purged with He for (a) 30 min, followed by purging with NO + O2 for b) 5 min, c) 10 min, d) 15 min, e) 20 min and f) 25 min. [NO] = 1000 ppm, [C3H8] = 2000 ppm, [O2] = 6.7%, [H2O] = 0–2% and F = 42 cm3 min1.
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Fig. 9. In situ IR spectra of Co-MFI taken at 648 K upon exposing a flow of NO + C3H8 + O2 for 120 min, subsequently purged with He for a) 30 min, followed by purging with 2% H2O/He for (b) 3 min, c) 10 min, d) 20 min and e) 30 min. [NO] = 1000 ppm, [C3H8] = 2000 ppm, [O2] = 6.7%, [H2O] = 0–2% and F = 42 cm3 min1.
4. Discussion 4.1. Reversible deactivation at low temperatures As shown in Fig. 3, the Co-MFI catalyst is deactivated under the dry conditions and the catalytic activity is recovered by the addition of water vapor. The reversible deactivation of catalyst suggests that the deactivation was not caused by some modifications of catalyst structure but by deposition of poisoning materials. So far, the promotion effect of water vapor on the HC-SCR activity has been reported when C3H6 [13–15], iso-C4H10 [16] and n-octane [17] are used as reductants. In such cases, the deactivation is caused by the formation of carbonaceous materials, and the promoting effect of water vapor is attributed to the suppression of the formation and/or the removal of deposited carbonaceous materials [13–16]. Although such explanations might be applied to the present case, the color change of catalyst due to the formation of carbonaceous materials was not found at all after the reaction test. Furthermore, even in the presence of water vapor, the broad IR bands were observed in the region of 1350– 1700 cm1, in which the absorption bands due to various hydrocarbon derived species (C O, C C, C N, and so on) could be observed (Fig. 4). Therefore, the contribution of carbonaceous materials as the inhibiting materials can be ruled out. The poisoning by other adsorbed species should be the main reason for the deactivation. 4.2. Involvement of NCO species in deactivation The results obtained from in situ IR spectra indicated that the presence of two types of strongly adsorbed hydrocarbon derived species on catalyst surface under the dry reaction conditions, i.e., oxygenated species (mainly acetate) and Ncontaining species (CN and NCO). As shown in Fig. 5, these species accumulated during the reaction under the dry conditions. The bands due to oxygenated species are intense
from the initial stage of the reaction, while the bands assignable to N-containing species gradually increased with time-onstream. Since the NO conversion gradually decreased with time-on-stream as shown in Fig. 3, the deactivation of catalyst is in agreement with the accumulation of N-containing species. After the introduction of water vapor, the band of oxygenated species once increased and then slightly decreased. Even after 20 min, when the catalytic activity entirely recovered, the intensities of the bands of oxygenated species are not much different from those under the dry conditions. On the other hand, the bands of N-containing species drastically diminished. These quick responses of the bands against water vapor clearly indicate that the deactivation under the dry conditions is caused by the strong adsorption of N-containing species observed as bands around 2170–2360 cm1. After the introduction of the water vapor, the band of Ncontaining species showed some differences in the response. Under the dry conditions shown in Figs. 5 and 7, surface CN species (AlB-CN, AlL-CN and Co-CN) were saturated within 60 min, while surface Al-NCO species gradually increased up to 150 min. The trend of the band of Al-NCO is in good agreement with the decrease in the NO conversion shown in Fig. 3. Actually, the time course of NO conversion to N2 is clearly contrast to that of the band intensity of NCO in the periodic operation between the dry and wet conditions. By addition of water vapor to the feed gas, NO conversion rapidly recovered to the initial level and kept stable under wet condition, while the intensity of the NCO band immediately decreased and disappeared. Therefore, the deactivation of Co-MFI under the dry conditions should be due to the strong adsorption of NCO species on Al-site, i.e., acid site of zeolite. The deposition of NCO species on zeolitic acid sites was also supported by the negative bands at 3668 and 3595 cm1. These negative bands gradually became intense with time-on-stream, indicating adsorption of surface species on OH groups under the dry reaction conditions. A good agreement of the responses of the OH
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Fig. 10. Proposed reaction pathway of C3H8-SCR over Co-MFI.
bands with the Al-NCO band indicates the formation of NCO species on Al–OH site. In the case of nitromethane decomposition over Co-MFI, we have already reported that the oligomerization of NCO species followed by polymerization causes strong deactivation of the HC-SCR over Co-MFI [20]. In the present case, however, since there was no band at 1723 cm1, assignable to cyanuric acid, oligomerization or polymerization of NCO species can be neglected. Since the theoretical water production at 100% conversion is five times higher in this case than the nitromethane decomposition, the difference may be due to the difference in the partial pressure of water vapor. 4.3. Reaction mechanism of C3H8-SCR over Co-MFI Based on the results in this study and the previous investigation in the literature, the deactivation in the dry conditions and the positive role of water vapor can be proposed as shown in Fig. 10. At the initial stage of the HC-SCR reaction, it has been generally accepted that the HC-SCR reaction begins with NO oxidation to NO2 [8–10,34,35]. NO2 react with hydrocarbons to form organic nitroso or nitro compounds (R– NO, R–NO2) [28,32] via gas phase radicals [11] or surface species. Adsorbed NCO and CN species observed in the IR spectra may be formed from these organic nitroso or nitro compounds probably via very reactive amido species (R0 C(O)NHR00 ) [20–22,28,31–33]. In the case of CH4-SCR, Cowan et al. proposed that formohydroxamic acid and formamido are the possible intermediates to form NCO and CN species, respectively [28]. In the case of higher hydrocarbons such as propane, Beutel et al. [33] proposed that 2-nitrosopropane with subsequent formation to Nmethylacetamide followed by hydrolysis to acetic acid and methylamine is the possible pathway. The presence of the bands at 1446 and 1570 cm1 assignable to carboxylate species represents the presence of this reaction pathway (path B in Fig. 10). The reaction rate of this path might be influenced by the presence of water vapor, however, the deposition of amido or amine species on the deactivated catalyst can be excluded. This is because (1) the intensity of the bands at 1446 and 1570 cm1 was not much affected by the presence of water and (2) the band at 3300–3500 cm1, at which amido or amine species give N–H stretching band, did not increase under the dry conditions at low temperatures (Fig. 4a–c) or with time-onstream (Fig. 5a–e).
As discussed in the previous section, from the in situ IR spectra, it is clear that deposition of surface NCO species on Co-MFI is the cause of deactivation (path A in Fig. 10). Although the NCO species can be oxidized by NO2 even in the dry conditions as shown in Fig. 8, the reaction rate of NCO species by NO2 is slower than that in the presence of water vapor. This is because the complete consumption of NCO species was achieved after 20 min of the introduction of water vapor (Fig. 6) while it took more than 20 min by the reaction with NO2 (Fig. 8). Therefore, the NCO species are gradually accumulated with time-on-stream at the dry conditions and lower temperatures. Higher temperature, and/or inclusion of water in the feed, can prevent or delay the deactivation. This interpretation seems reasonable given that cyanuric acid melts and decomposes at the temperature range of interest here and HCNO is readily hydrolyzed on ion-exchanged zeolites and metal oxides [28,29,32,36]. Piazzesi et al. reported investigated the decomposition of HNCO on titania by DRIFT spectroscopy [36]. HNCO adsorbs dissociatively on titania to yield NCO species, bound to Ti4+ sites, and hydroxyl (OH) groups. The stability of the NCO species decreases with increasing temperatures, and above 473 K they are very easily removed from the surface. In the presence of water vapor, the hydrolysis to ammonia easily proceeds on titania. It can be rationalized that the recovery of the NO conversion in the presence of water vapor is caused by the hydrolysis of the deposited NCO species and the increase in the contribution of the reaction pathway C in Fig. 10, i.e., the formation of NH3 from NCO species followed by effective NO reduction by NH3 reaction [11–14]. Actually, Cowan et al. have reported the high activity of NO reduction with NH3 over Co-MFI, i.e., the conversion of NO achieves to 100% in the temperature range of 525–723 K [28]. 5. Conclusion The deactivation and reactivation of Co-MFI in the C3H8SCR was investigated in the light of the deposited species in the dry reaction conditions. In the absence of water vapor below 648 K, the NO conversion decreased with time-on-stream by the accumulation of NCO species Al sites of zeolites. In the presence of water vapor, however, the deposited NCO species were easily hydrolyzed to NH3 and CO2, and therefore, the higher HC-SCR activity was obtained. NCO species, which are known to be an important intermediate to form N2 for the HCSCR, could be self-poisoning substance depending on the
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