Energy & Fuels 1997, 11, 307-310
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Initial Period of NO-NH3 Reaction over a Heat-Treated Pitch-Based Active Carbon Fiber Isao Mochida,*,† Masahiro Kishino,† Sizuo Kawano,† Hideki Iwaizono,† Akinori Yasutake,‡ and Masa-aki Yoshikawa§ Institute of Advanced Material Study, Kyushu University, 6-1 Kasugakoen, Kasuga-shi, Fukuoka 816, Japan; Technical Headquarters, Nagasaki Research and Development Center, Chemical Reserch Laboratory, Mitsubishi Heavy Industries, Ltd., 5-717-1 Fukahori-machi, Nagasaki 851-03, Japan; and Research and Development Center, Osaka Gas Company, Ltd., 6-19-9 Torishima, Konohana-ku, Osaka 554, Japan Received September 18, 1996. Revised Manuscript Received December 19, 1996X
The initial period of NO-NH3 reaction within 2-6 h over a heat-treated activated carbon fiber (ACF) was studied at several reaction temperatures to clarify the material balances during the complete removal of NO in this period. NO was found to be reduced and adsorbed of the ACF during the period to achieve the complete removal. NO adsorption to reach saturation decreased the initial conversion to the stationary one. Adsorbed NO provided desorption at the two temperature ranges, suggesting two species over the ACF. The species of higher temperature desorption appears to originate from NO2 species. Both species appear to be reduced with NH3. The influences of the reaction temperature and humidity on both reduction and adsorption of NO in the initial period were also observed. Competitive adsorption of NO against H2O over the ACF appears to reduce the amounts of both reduction and adsorption.
Introduction We have reported that a particular pitch-based active carbon fiber (ACF) after calcination at 850 °C exhibited the highest catalytic activity for the NO-NH3 reaction at room temperature among ACFs so far examined, although humidity retarded severely the reaction.1,2 Such an activity was found stationart; however, an interesting point is that the same ACF showed in the initial period much higher activity, which is reduced gradually to the stationary one within several hours. Such a stationary activity was kinetically analyzed in a previous paper.3 In the present study, the complete removal of NO in the initial period was studied by counting the reacted, unreacted, and adsorbed NO over the ACF. The adsorbed NO species was quantified by temperatureprogrammed desorption (TPD). The analyses of the adsorbed species may tell the active species of NO reducible with NH3. Experimental Section ACF Characterization. As-prepared ACF (OG-8A) was supplied by Osaka Gas Co. The ACF was used after calcination at 850 °C in N2 atmosphere. Its surface area and pore volume were measured by BET method to 750 m2/g and 0.37 mL/g, respectively. Calcination at this temperature eliminated almost completely the surface oxygen function groups from the ACF. †
Kyushu University. Mitsubishi Heavy Industries, Ltd. Osaka Gas Co. Ltd. X Abstract published in Advance ACS Abstracts, February 1, 1997. (1) Mochida, I.; Kawano, S.; Hironaka, M.; Yatsunami, S.; Korai, Y.; Yoshikawa, M. Chem. Lett. 1995, 385. (2) Mochida, I.; Kawabuchi, Y.; Kawano, S.; Matsumura, Y.; Yoshikawa, M. Fuel 1997, in press. (3) Mochida, I.; Kawano, S.; Hironaka, M.; Kawabuchi, Y.; Matsumura, Y.; Korai, Y. Langmuir 1997, in press. ‡ §
S0887-0624(96)00159-4 CCC: $14.00
Reaction of NO-NH3. The reaction of NO (200 ppm) with NH3 (200-260 ppm) was carried out using dry [relative humidity (RH) 0%] and wet air (RH 80%) as a carrier gas, a U-shaped fixed bed flow type glass reactor (10 mm diameter), and 0.5-3 g of ACF closely packed into the reactor. The ratio of NO to NH3, total flow rate (F), and the contact time (W/F) were 1-1.35, 100 mL/min, and 5 × 10-3- 3 × 10-2 g‚min‚mL-1, respectively. The humidity was controlled by mixing air of 100% RH with dry air. The relative humidity was measured by a humidity meter (CHINO NH-U type). Measurement of NO Conversion. NO concentration was analyzed continuously at the inlet and outlet of the reactor, using a NOX meter (ECL-77, Yanagimoto Co., Kyoto). Details have been described in previous papers.1,2 Stationary NO conversion was determined when NO concentration at the outlet of the reactor became stable. Adsorption of reductants and products appeared to continue for several hours after the start of the reaction. NO2 concentration was analyzed at each hour. The measurement of N2 and N2O concentrations was not intended due to their low concentration, although the reaction in He carrier proved the quantitative formation of N2 in the stationary state. Measurement of the Adsorption Capacities of NO, NH3, and H2O. NO adsorption capacity of the ACF was estimated by measuring NO concentration at the inlet and outlet of the reactor to define the breakthrough of 100% (T100) and quantify the amount by T100, using the same apparatus for the reaction. The NO concentration, total flow rate, W/F, temperature, and RH for NO adsorption were 200 ppm, 100 mL/min, 5 × 10-3 g‚min‚mL-1, 20 °C, and 80%, respectively. The NH3 adsorption capacity was also measured in a similar manner. NH3 concentration, total flow, W/F, temperature, and RH for NH3 adsorption were 200 ppm, 100 mL/min, 5 × 10-3 g‚min‚mL-1, 20 °C, and 80%, respectively. TPD. TPD of NO and NH3 adsorbed over the ACF was carried out using a quartz-glass apparatus equipped with a mass spectrometer (TE-600, Nichiden Anelva Inc.). The sample (0.1 g) was heated in a helium flow to 1000 °C at a © 1997 American Chemical Society
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Figure 1. NO-NH3 reaction and NO adsorption profiles at some temperatures over ACF OG-8A-H850 in dry and wet air: NO:NH3 ) 1:1 (NO ) 200 ppm); O2 ) 4.0%, N2 balance; W/F ) 5 × 10-3 g‚min‚mL-1; W ) 0.5 g; time ) 180 min; (dry) (1) 25 °C, (2) 40 °C, (3) 55 °C, (4) 70 °C; (wet) (5) 25 °C, (6) 40 °C, (7) 55 °C, (8) 70 °C. Adsorption of NO alone: NO ) 200 ppm, N2 balance; W/F ) 5 × 10-3 g‚min‚mL-1; W ) 0.5 g; time ) 180 min; temp ) 25 °C; (9) dry, (10) wet (RH ) 80%).
Figure 3. NO-NH3 reaction profile over pitch-based OG-8AH850 of larger weight heat-treated at 850 °C in wet air. NO: NH3 ) 1:1 (NO ) 200 ppm); O2 ) 4.0%, N2 balance; W/F ) 0.5-3.0 × 10-2 g‚min‚mL-1; RH ) 80%; temp ) 22 °C; (1) W ) 0.5 g, (2) W ) 1.0 g, (3) W ) 2.0 g, (4) W ) 3.0 g.
Figure 2. NO conversion over pitch-based activated carbon fiber OG-8A-H850 in dry and wet air. NO:NH3 ) 1:1.35 (NO ) 200 ppm); O2 ) 4.0%, N2 balance; W/F ) 5 × 10-3 g‚min‚mL-1; W ) 0.5 g; time ) 180 min; (1) dry (RH ) 0%), (2) wet (RH ) 80%).
Figure 4. Relation between NO conversion and weight of ACF over pitch-based OG-8A heat-treated at 850 °C in wet air. ACF ) OG-8A-H850; NO:NH3 ) 1:1 (NO ) 200 ppm); O2 ) 4.0%, N2 balance; W/F ) 0.5-3.0 × 10-2 g‚min‚mL-1; RH ) 80%; temp ) 22 °C; (1) NO conversion at 6 h, (2) stationary NO conversion.
heating rate of 10 °C/min, and evolved gases such as NO and NH3 were continuously analyzed using a mass spectrometer.
Results Activity of the ACF Calcined at 850 °C. Figure 1 illustrates the conversions of NO over the ACF of 0.5 g at 25-70 °C in dry and wet air, respectively. The reaction in dry air showed a complete removal of NO for the initial 30 min at 25 °C, and then the conversion reduced gradually to the stationary one of 95% by 120 min. Higher reaction temperatures eliminated the period of the complete removal and accelerated the decrease of the conversion to reach the stationary ones of 85, 78, and 55% at 40, 55, and 70 °C, respectively. Reduced activity at higher temperature is definite. In a wet air of 80% RH, the period of complete removal was not observed to reach the stationary conversion within 30 min over the ACF of this amount regardless of the reaction temperature. The dependence of the stationary conversion on the reaction temperature was definitely different from that in the dry air, the largest conversion of 60% being observed at 40 °C. Higher temperature reduced the conversion, although the stationary conversion at 70 °C was still higher than that at 25 °C. The severe retardation of H2O and its temperature dependence are suggested. Figure 2 illustrates the conversions vs temperature. The conversion in dry air decreased with the reaction temperature, while that in wet air increased up to 40
°C and then decreased at further higher temperatures. The conversions in dry and wet air became similar at temperatures >70 °C. Figure 3 illustrates the conversion of NO in wet air at 25 °C over ACF of much larger amount (3 g). The increase of the ACF amount prolonged the initial period of complete removal and the stationary conversion. The ACF of 3 g allowed 6 h of complete removal and 65% of stationary conversion when 200 ppm of NO was flowed by 100 mL/min; 0.64 mmol of NO was removed over 1 g of ACF for the initial 6 h. Figure 4 correlates the initial and stationary conversions in the wet air against the weight of the ACF by the fixed flow rate of 100 mL/g at 22 °C. Both conversions increased with the increase of the weight as expected, the increase of the stationary conversion being much slower above 40% conversion. The stronger retardation by humidity is attributed to the lower concentration of NO in the stationary stage, where H2O adsorption is saturated. Desorption after Reaction. Figure 5 illustrates the desorption of NO and NH3 from the ACF after the reaction of 3 h in dry and wet air, respectively. NO was found to desorb from the ACF surface after the reaction in dry air at 40-120 and 140-260 °C, where the peaks were observed at 95 and 200 °C, respectively. The amounts of desorbed NO at two ranges were 4.2 × 10-2 and 5.6 × 10-2 mmol/g, respectively, as summarized in Table 1. The reaction in wet air provided also two peaks
NO-NH3 Reaction
Energy & Fuels, Vol. 11, No. 2, 1997 309
Table 1. NO Reaction and Adsorption at Some Reaction Temperatures afer 3 h of Reaction over OG-8A-H850 25 °C condition of regeneration
mmol/g (× 10-2)
initial amt desorbed (adsorbed)a desorb temp 22-130 °C desorb temp 130-300 °C reactedb breakthroughc
32.14 9.76 4.16 5.60 21.53 0.85
initial amt desorbed (adsorbed)a desorb temp 22-130 °C desorb temp 130-300 °C reactedb breakthroughc
32.14 6.71 2.68 4.03 11.01 14.42
40 °C
55 °C mmol/g (× 10-2)
30.37 12.94 17.42 66.99 2.64
NO (Dry, RH ) 0%) 32.14 8.14 25.33 3.66 11.39 4.48 13.94 19.49 60.64 4.51 14.03
22.88 8.35 12.53 34.26 44.87
NO (Wet, RH ) 80%) 32.14 8.80 27.38 3.52 10.95 5.28 16.43 8.18 25.45 15.16 47.17
ratio (%)
mmol/g (× 10-2)
ratio (%)
70 °C
ratio (%)
mmol/g (× 10-2)
ratio (%)
32.14 5.43 2.17 3.26 19.39 7.32
16.89 6.75 10.14 60.33 22.78
32.14 2.50 1.12 1.38 17.26 12.38
7.78 3.48 4.30 53.70 38.52
32.14 4.00 1.60 2.40 9.30 18.84
12.45 4.98 7.47 28.94 58.62
32.14 1.50 0.65 0.85 8.65 21.99
4.57 2.02 2.65 26.91 68.42
a Amount of adsorbed NO in the reaction (amount of desorbed NO after reaction). b Amount of reacted NO in the reaction. c Amount of breakthrough NO in the reaction. NO:NH3 ) 1:1.35 (NO ) 200 ppm); N2 balance, O2 ) 4.0%; W ) 0.5 g; W/F ) 5 × 10-3 g‚min‚mL-1; time ) 180 min.
Table 2. Amount of Desorbed NH3 after 25 °C-3 h Reactiona,b RH dry (0%) wet (80%)
desorbed temp (°C)
NH3 (× 10-2 mmol/g)
22-130 130-300 22-130 130-300
4.28 1.61 5.82 1.07
a Reaction: NO:NH ) 1:1.35 (NO ) 200 ppm); O ) 4.0%; N 3 2 2 balance; W ) 0.5 g; W/F ) 5 × 10-3 g‚min‚mL-1; temp ) 25 °C; b time ) 180 min. Desorption: flow gas (He) ) 100 mL/min; temp ) 25-300 °C; HR ) 3.3 °C/min.
Figure 5. Desorption profiles of NO and NH3 after NO-NH3 reaction over OG-8A-H850. Reaction: NO:NH3 ) 1:1.35 (NO ) 200 ppm); O2 ) 4.0%, N2 balance; W ) 0.5 g; W/F ) 5 × 10-3 g‚min‚mL-1; temp ) 25 °C; time ) 180 min. Desorption: flow gas (He) ) 100 mL/min; temp ) 25-300 °C; HR ) 3.3 °C/min; (1) NO, dry; (2) NO, wet; (3) NH3, dry; (4) NH3, wet.
at 75 and 180 °C, respectively. The amounts were 2.7 × 10-2 and 4.0 × 10-2 mmol/g, respectively. Lower temperatures and lesser amounts of desorption are noticed after the reaction in wet air. NH3 was also found to desorb from the surface at the peak temperatures of 40 and 280 °C in dry air and at 35 and 220 °C in wet air, respectively. The amounts of desorbed NH3 are summarized in Table 2, where greater amounts of desorption were observed at a lower temperature range after the reaction in wet air. The reactions at higher temperatures allowed similar desorption of NO at two temperature ranges regardless of the humidity as summarized in Table 1. The desorption in the higher temperature range was always higher in all cases; however, the desorbed amounts of NO decreased with the rising reaction temperature in the
Figure 6. Adsorption amount of NO alone over OG-8A-H850. NO ) 200 ppm; N2 balance; W/F ) 5 × 10-3 g‚min‚mL-1; W ) 0.5 g; time ) 180 min; (1) dry, (2) wet.
dry air, while the largest amount was observed at 40 °C in wet air. Desorption of CO and CO2 from ACF after Reaction. CO and CO2 were found in the desorbed gas from the ACF after the reaction, both CO and CO2 evolving at 120 °C while CO was found also around 900 °C. Such evolution of CO and CO2 indicates the oxidation of the ACF surface during desorption of NO. The amount of CO/CO2 evolution appears smaller than that of NO. Adsorption of NO Alone over ACF. The adsorption amounts of NO alone in dry and wet N2 over the ACF are plotted against temperature in Figure 6, where the adsorption amount was counted until 100% breakthrough as shown in Figure 1. The adsorption of NO in dry N2 decreased along with the temperature. In
310 Energy & Fuels, Vol. 11, No. 2, 1997
Mochida et al.
NO ad + O ad f NO2 ad
(5)
6NO2 ad + 8NH3 ad f 7N2 + 12H2O
(6)
contrast, the wet N2 provided the largest adsorption of NO at 40 °C. Discussion The present study describes the initial stage of complete NO removal in the NO-NH3 reaction over the ACF and the dependence of the reaction on the reaction temperature. Table 1 summarizes of material balances of NO over the ACF for the initial 3 h at several temperatures. During the period, NO of 6.1 × 10-1 to 2.2 × 10-1 mmol/g of ACF is removed during the initial period by reduction with NH3 and adsorption while the rest of the NO breaks through the ACF bed, varying with the temperature and humidity. By quantifying the amount of NO desorbed by heating after the reaction, the reacted NO is estimated as shown in Table 1. The major portion of supplied NO (67%) is reduced over the ACF in dry air at 25 °C and 30% is adsorbed on the ACF, leaving very little NO breakthrough for 3 h, although the adsorption is very rapid to be dominant for the initial period. The higher temperature decreases both reduction and adsorption, increasing the amount of breakthrough NO. Hence, it is concluded that the initial removal of NO is ascribed to reduction and adsorption. The contribution of the adsorption becomes zero when the adsorption is saturated, leading to the stationary conversion. An increasing amount of water produced through the reduction over the ACF may retard the reaction and adsorption, leading also to the stationary conversion. NO was found to desorb at two temperature ranges by heating, suggesting two adsorbed species present during the reaction over the ACF. NO desorption accompanies the CO/CO2 evolution at 100-800 °C from the ACF; although the ACF has been calcined at 850 °C, some oxidation of the ACF surface is suggested.3 Although no NO2 was found at the outlet of the reaction in the presence of NH3, NO2 is suspected to be present on the ACF. NO2 is reduced by carbon as well as NH3 on the surface of the ACF,4,5 desorbing NO and leading some of the oxygen as surface functional groups. Amounts of CO/CO2 evolve that are smaller than the amount of NO desorbing at the higher temperature range, suggesting the significant contribution of its reduction by adsorbed NH3. Oxygen of 4% alone at room temperature did not introduce any oxygen functional groups over the ACF surface as the TPD after the flow of O2 in N2 did not give any CO or CO2. Adsorption of O2 may not lead to formation of surface oxygen functional groups which decompose to produce CO and CO2. Such a discussion leads to the following scheme, which has been reported for some catalysts.4,5
O2 f 2O ad
(1)
NO f NO ad
(2)
NH3 f NH3 ad
(3)
6NO ad + 4NH3 ad f 5N2 + 6H2O
(4)
The positive roles of O2 in the acceleration of the reduction are reasoned by eq 6. When the second step is rate-determining, the reaction order in NO is first. No NO2 in the gaseous product at room temperature indicates that eq 6 does not allow eq 5 to be saturated. The fact that a slightly larger amount of NH3 is required to give the largest conversion of NO supports the contribution of eq 6. NO-O2 reaction gave NO2 in the stationary state over ACF at room temperature, although the reaction was strongly retarded by humidity.6,7 Intermediate ad.NO2 has been postulated in the reduction of NO on carbon at elevated temperature.8,9 The humidity and temperature cooperatively influence both the adsorption and the reaction as shown in Figures 6, 1, and 2, respectively. Humidity decreases more severely both adsorption and reduction at lower reaction temperatures. Humidity prohibits the NO adsorption over the ACF through the adsorption of H2O above 70% RH even if the calcination causes the ACF surface to be much more hydrophobic than that of the as-received form. Prohibit of NO adsorption leads to less formation of NO2 and hence its reaction with NH3. The reaction temperature appears to influence the reaction through six factors, NO, NH3, and H2O adsorption and activation of NO, NH3, and NO2. In dry air, the adsorption of NO is strongly influenced by the reaction temperature. Decrease of adsorption slows the rates of its oxidation into NO26,7 and reduction of NO2. Thus, the dependence of the rate on the temperature is very similar to that of adsorption. NO oxidation in the gas phase has been postulated to proceed through its dimer, thus, the dimerization equilibrium does not favor the higher reaction temperature and leads to the second order in NO.10,11 The reaction orders in the gas phase and catalytic reactions are different as described above. The situation in wet air is more complex because the adsorption and hence inhibition of water are dependent also upon the reaction temperature. As the sum of such influences, a particular temperature of 40 °C allowed the largest activity, adsorption, and reduction of NO. The profiles of NO adsorption during the reaction are very similar to those of its single adsorption, although amounts are different because of different extents of saturation, probably due to the oxidation by O2, suggesting the common aspects. Finally, when the mechanism above discussed is true, the surface reaction of adsorbed NO and NH3 remaining unreacted over the ACF during the period can proceed without the supply of NO and NH3 at the reaction temperature, regenerating the adsorption ability of the ACF for the initial complete removal of NO. Switching of removal and regeneration steps over the ACF can be designed for the high efficiency of NO removal. EF960159Q
(4) Juntgen, H.; Kuhl, H. Chemistry and Physics of Carbon; Dekker: New York, 1989; Vol. 22, p 161. (5) Chemistry and Physics of Carbon; Dekker: New York, 1996; Vol. 25.
(6) Mochida, I.; Kisamori, S.; Hironaka, M.; Kawano, S.; Matsumura, Y.; Yoshikawa, M. Energy Fuels 1994, 8, 1341. (7) Mochida, I.; Kawano, S.; Hironaka, M.; Yatsunami, S.; Korai, Y.; Matsumura, Y. Energy Fuels 1995, 9, 659. (8) Teng, H.; Snuberg, E. M.; Calo, J. M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1990, 35, 592. (9) Teng, H.; Snuberg, E. M.; Calo, J. M. Energy Fuel 1992, 6, 398. (10) Mochida, I.; Kawano, S.; Fujitsu, H.; Maeda, T. Nippon Kagaku Kaishi 1992, 275. (11) Mochida, I.; Kawano, S.; Kisamori, S.; Fujitsu, H.; Maeda, T. Carbon 1994, 32, 175.
