Energy & Fuels 1994,8, 1341-1344
1341
Oxidation of NO into NO2 over Active Carbon Fibers Isao Mochida,* Seiki Kisamori, Motohiro Hironaka, Shizuo Kawano, Yuji Matsumura,? and Masaaki Yoshikawat Institute of Advanced Material Study, Kyushu University, 6-1 Kasugakoen, Kasuga-shi, Fukuoka 816, Japan Received May 18, 1994@
Catalytic oxidation of NO (400 ppm) into NO2 was studied over active carbon fibers at room temperature to trap the unreacted NO as well as the oxidized product in forms of nitric acid or its salts to develop an oxidative removal of NO in the flue gas. The heat treatment of a particular pitch-based active carbon fiber of rather small surface area was found to enhance markedly the activity to allow the conversion of 82% in dry, 65% in 80%relative humidity (rh) and 21% in wet (100%rh) air, respectively, at 25 "C by W/F of 1.0 x low2g min mL-l. The rate of oxidation was examined by varying humidity to reveal marked decrease of the activity above 80% rh. The strong retardation by humidity of the activity of as-received fiber was moderated by the heat treatment of the fiber at 800 "C. The catalytic activities of other pitch- and poly(acrylonitri1e)based fibers were also examined in dry and wet air to find their inferior activity in wet air above 80% rh.
Introduction NO in flue gas or even in the atmosphere has been continuously expected to be removed or converted into unharmful species more extensively and more efficiently. Selective catalytic reduction (SCR)of NO in flue gases from various sources has been performed in a commercial scale, using vanadium on Ti02 catalyst in a honeycomb form at a temperature range of 300-400 "C, where the complete decomposition of (NH4)2S04is achieved even if it is produced.' Although such a SCR process appears proven and established, there remain several problems such as a high reaction temperature which requires sometimes reheating of the flue gas, large volume of reactor, and necessity of reductant ammonia. The process is not applicable to NO from mobile sources or in very low concentration at ambient temperature as observed in urban areas. Low-temperature SCR processes have been explored around 100-150 "C, using zeolite and active carbon as the catalysts and ammonia as the r e d ~ c t a n t . ~ Com,~ plete removal of SO2 before SCR is strictly necessary because (NH4)2S04tends t o plug the reactor as well as the catalyst bed. The present authors have proposed such a process catalyzed by active carbon fiber (ACF) t o reduce NO of 10 ppm at room temperature in the a t m ~ s p h e r e . However, ~,~ severe retardation by humidity, necessity of ammonia, and further activation of ACF with H2S04 may increase the cost or restrict the application. In the present paper, we describe the catalytic activity of active carbon fibers and their heat-treated ones for ~~~~~
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Research & Development Center, Osaka Gas Co., Ltd., 6-19-9, Torishima, Konohana-ku, Osaka 554,Japan. @Abstractpublished in Advance ACS Abstracts, September 1,1994. (1)Grove, M.; Sturn, W. Ceram. Eng. Sei. Pore 1989,10,325. Konig, A.; Richte, T.; Puppe, L. S E A Pap. 1990,13. (2)Held, W.; (3)Iwamoto, M.; Mizuno, N. Catalysis 1990,32, 462. (4)Mochida, I.; Kawano, S.; Fujitsu, H.; Maeda, T. Nippon Kagaku +
Kaishi 1992,275. (5) Kawano, S.; Kisamori, S.; Mochida, I.; Fujitsu, H.; Maeda, T. Nippon Kagaku Kaishi 1993,694.
the oxidation of NO in flue gas to be captured as nitric acid or its salts. NO of high concentration is very reactive to be oxidized into NO2 with 0 2 forming HN03 in water.6 However, NO of low concentration below 0.1% is very stable and not is oxidized kinetically in air, probably because the oxidation is second order in NO7 and the rate constant decreases with the reaction temperature. Thus, 400-1000 ppm NO was found in the flue gas from the power plant, causing an environmental problem of acid rain even if the flue gas contains 5-10% oxygen. The slow oxidation of NO inhibits the development of its oxidative removal technology, requiring expensive oxidants such as 0 3 . An oxidation catalyzed by active carbon has been reported for a long time.8p9 The unsatisfactory activity and strong retardation by humidity so far restricts further development in the practical application for the capture of low concentration NO. The present authors have revealed that the surface properties of active carbon fiber can be controlled by the selection of precursor, activation conditions, and postmodulation such as heat treatment as well as chemical treatment.1°-13 Hence we examined in the present study the catalytic activity of pitch and PAN based active fibers and their heat-treated ones to explore the catalytic activity of humid resistivity. Very preliminary trials to capture the oxidized nitric species in water and aqueous bases were also included.14J5 ( 6 )Cotton, F. A,; Wilkinson, G. Advanced Inorganic Chemistry; Intersciences: New York, 1972;p 355. (7)Bodenstein, M. Helv. Chem. Acta 1935,18,743. ( 8 ) Kircher, 0.; Hougen, 0. A. MChE J . 1967,3,331. (9)Chu, C.; Hougen, 0. A. Chem. Eng. Prog. 1961,57,51. (10)Mochida, I.; Kisamori, S.; Kawano, S.; Fujitsu, H. Nippon Kagaku Kaishi 1992,1429. (11)Mochida, I.; Hirayama, T.; Kisamori, S.; Kawano, S.; Fujitsu, H. Langmuir 1992,8,2290. (12)Kisamori, S.;Kawano, S.; Mochida, I. Chem. Lett. 1993,1899. (13)Mochida, I.; Kisamori, S.; Kawano, S. Langmuir 1994,10,1241. (14)Mirev, D.;Balarev, K.; Boyadzhiev, L.; Lambiev, D. Compt. Rend. Acad. Bulgare Sei. 1961,14, 250. (15)Koval, E.J.; Peters, M. S. Znd. Eng. Chem. 1960,52, 1011.
0887-0624/94/2508-1341~04.50~0 0 1994 American Chemical Society
Mochida et al.
1342 Energy & Fuels, Vol. 8, No. 6, 1994
Table 1. Some Properties of ACFs ACFs OG-5A (as-received) OG-1OA (as-received) OG-20A (as-received) PAN-EF300 (as-received) OG-5A-H800a OG-10A-H800a OG-20A-HSOW PAN-FE300-H80Oa a
C 89.6 91.6 93.9 83.4 92.1 94.0 95.8 86.9
ultimate analysis (wt %) N 0 1.1 0.7 8.3 0.9 0.5 6.7 0.9 0.3 4.6 4.7 9.2 0.9 1.0 0.5 5.8 0.8 0.4 4.4 0.3 2.8 0.6 0.7 3.6 5.8 H
S tr tr tr 0 0 0 0 0
ash 0.3 0.3 0.5 1.8 0.6 0.4 0.5 3.0
surface area (m2/g) 480 710 1550 875 450 620 1320 810
pore vol. (mug) 0.25 0.37 0.81 0.46 0.23 0.32 0.68 0.42
Calcined at 800 "C in Nz.
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Time (hr) Figure 1. Oxidation of NO into NO2 over as-received pitchbased ACF OG-5A. ACF, OG-5A (as-received),W = 1.0 g. NO, g min mL, 380 ppm (Nz balance); 0 2 , 4.0%. W/F = 1 x reaction temperature 25 "C. Relative humidity: (1) 0%, (2) 60%, (3) 80%, (4) 100%. Outlet gas passed through washing a bottle. HzO, 100 mL; depth 70 mm, T = 25 "C. Aqueous KOH (1mol), 100 mL; depth 70 mm, T = 25 "C.
Experimental Section Pitch-based (OG-5A,-lOA,-20A)and PAN-based (PAN-FE-
300) active carbon fibers (ACFs) were supplied by Osaka Gas and Toho Rayon, respectively, in yarn forms. Their properties are summarized in Table 1. The fibers were heat-treated in Nz at 200-1000 "C for 4 h. Properties of the fibers heattreated at 800 "C (H8OO) are listed also in Table 1. Oxidation of NO was preformed in a fixed bed U-shaped flow type reactor. ACF mat was chopped into cubes (5 mm (width) x 5 mm (length) and 3 mm (thickness)) to be packed densely in the glass tube of 10 mm diameter. The weight and length of fiber bed, flow rate, the concentrations of NO in Nz containing 4-15% 0 2 , and reaction temperatures were 0.5 g, 70 mm, 50 mL min-', 380 ppm, and 25-125 "C, respectively. Air was humidified at 25 "C. Reactant and product gases were analyzed by NO, meter (ECL-77A, Yanagimoto Co., Ltd.). The oxidized outlet gas was washed in a washing bottle after the reaction before the analyzer to examine the capture of NO2 and remaining NO. Water and aqueous KOH (1mom) were placed in the bottle by the depth of 70 mm (100 mL). Temperature-programmed decomposition profiles of ACFs were recorded by mass spectrometer (TE-600, Anoelba Co., Ltd.).'lJ3 The heating rate was 10 W m i n up to 1000 "C.
Results Catalytic Activity of a Pitch-BasedACF, OG-5A. Figure 1illustrates the compositional change of a model flue gas at the outlet of the reactor at 25 "C over asreceived OG-5A by W/F of 1 x g min mL-l. NO in dry air decreased its concentration from 380 to 75 ppm by passing through the OG-5A for the first 2 h and then increased gradually to 170 ppm by 20 h. Up to this time, no nitric species except for NO was found in the
Figure 2. Oxidation of NO into NO2 over OG-5A heat treated at 800 "C (OG-5A-H800). ACF, OG-5A-H800; W = 1.0 g. NO, 380 ppm (Nz balance); 0 2 , 4.0%. W/F = 1 x g min mL, reaction temperature 25 "C. Relative humidity: (1)0%, (2) 60%, (3) 80%, (4) 100%.
outlet gas, adsorption of the species being suggested although so far their form is not identified. At 20 h after the gas flow started, NO2 started to be found in the outlet gas. The concentration of NO2 steadily increased, while NO both found and adsorbed, being calculated from N balance, decreased. The saturation of NO adsorption was suggested at 40 h, when the concentrations of NO and NO2 became stationary t o be 100 and 280 ppm, respectively, N balance being now obtained. Such stationary oxidation of NO continued over OG-5A at least for 40 h. Increasing humidity in air decreased the adsorption amount of NO and its oxidation into NO2 in the stationary state, shortening the time to reach the stationary state. Although the retardation of humidity was slight when the humidity was below 60%, humidity above 80% retarded very markedly and humidity of 100% decreased severely the NO oxidation, the conversion into NO2 becoming essentially null. The last result indicates the low reactivity of NO in low concentration for the oxidation. Influence of Heat Treatment. Figure 2 illustrates the activity of OG-5A heat-treated at 800 "C (OG-5AH800) for the oxidation of NO at 25 "C. Although the profile for NO adsorption and oxidation over OG-5AH8OO was similar to that over as-received OG-5A, the activity was certainly much higher by the heat treatment. The stationary oxidation of NO into NO2 in dry air increased to 80% by the same W/F. The humidity in air reduced the conversion to 87% at 60% rh, 55% at 80% rh, and 21% at 100% rh. Such reduction was certainly much less with the heat-treated OG-5A than that with the as-received one. The significant activity a t 80% rh should be noted.
Energy & Fuels, Vol. 8, No.6, 1994 1343
Oxidation of NO into NO2
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As-received Calcination temp. ("C ) Figure 3. Catalytic activities of heattreated OG5A for NO oxidation stationary activity was measured 40 h after the reaction started. ACF, OG-5A (pitch base); W = 1.0 g; NO, 380 ppm (N2 balance); 02,4.0%. W/F = 1 x g min mL-l; reaction temperature 25 "C. Conversion was observed 40 h after the reaction started. Relative humidity: (1)0%,(2) 80%, (3) 100%. 100I
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Reaction temp. ("c ) Figure 6. Activity of OG5A-H800 for humidified NO oxidation in a temperature range of 30-125 "C. ACF,OG5A-HSOO; W = 1.0 g. (1) 0 2 = 4%,H2O = 0. NO, 380 ppm (N2 balance). (2) 0 2 = 4%,H20 = 30.4 g/m3. W/F = 1 x g min mL-l. (3) 0 2 = 15%, H20 = 30.4 g/m3. H2O = constants (H2O amount in gas = 30.4 g/m3). Conversion was observed 40 h after the reaction started. Table 2. Catalytic Activities of ACFs for NO Oxidation in Dry and Humidified Carriev NO conversion (%) ACFs rh=O% rh=60% rh=80% 18 OG5A (as-received) 73 58 14 56 71 OGlOA (as-received) 16 49 OG20A (as-received) 64 4 43 PAN-FE300 (as-received) 60 OG-5A-H8006 OG10A-H8006 OG20A-H8006 PAN-FE300-H8006
Relative humidity (%) Figure 4. Influence of humidity on NO oxidation over OG 5A as-received and H800. NO, 380 ppm (N2 balance); 02,4.0%. W/F = 1 x g min mL-l; W = 1.0 g. Conversion was observed 40 h after the reaction started. (1) OG5A (asreceived), (2) OG-5A-H800. Reaction temperature 25 "C.
Figure 3 summarizes the stationary conversions of NO into NO2 over OG5A heat-treated a t 400-1000 "C. The heat treatment increased slightly the activity in dry air, providing the largest activity at 800 "C. Either lower or higher temperature decreased the conversion. The increase of activity by the heat treatment at 800 "C was most marked when the relative humidity was 80%. Activity increase of 4 times in comparison with that of as-received OG-5As was very significant. Further higher humidity reduced the conversion over all OG5A regardless of the heat treatment. Nevertheless, OG5A-H800 exhibited the highest conversion of only 21%, although the activity increase may be remarkable even under 100% rh because the as-received OG-5A exhibited no activity at all under the same conditions. Influences of Reaction Conditions. Figure 4 summarizes the influences of relative humidity on the oxidation of NO over OG5A as-received and heattreated (H800). Although the humidity reduced the conversion slightly below 60% but severely above 60% rh especially over 80% rh, the marked influence of the heat treatment on the higher activity was observed at 80% rh. Figure 5 illustrates the conversion of NO in a temperature range of 30-125 "C over OG5A-H800 a t 4%
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a NO, 380 ppm (N2balance); 0 2 ; 4.0%. W/F = 1 x g min mL-', W = 1.0 g. Reaction temperature 25 "C. Calcined at 800
"Cin N2. 0 2 in dry and humid air. Air was humidified at 30 "C to carry 30.4 g/m3 H20 regardless of the reaction
temperature. Hence, the relative humidity decreased a t the higher reaction temperature. The conversion in dry air decreased monotonously with rising the reaction temperature as reported b e f ~ r e . ~The . ~ decrease was marked above 50 "C. In contrast, the conversion increased with rising temperature up to 75 "C in the humid air. The conversion decreased above 75 "C as observed in dry air. Higher oxygen concentration of 15% increased markedly the conversion of NO in humid air. The influence of ortygen concentration was more emphasized at higher temperatures up to 75 "C where the conversion was doubled. Further higher temperature decreased the conversion as well. Activities of Other Pitch and PAN ACFs. Table 2 summarizes the conversion of NO into NO2 over PAN, pitch ACFs, and their heat-treated forms. OG-5A exhibited the largest activity in dry air among the asreceived ones. The larger surface areas of pitch ACFs tended to show lower activity. Pitch ACF was more active than PAN-ACF. The heat treatment at 800 "C enhanced the activity of all ACFs, OG5A-HSOO exhibiting the largest activity in dry air. The superiority of pitch ACFs, especially heat-treated ones, was more marked in wet air of 80%rh, where O G 5A-HSOO and PAN-FE-300-H800 exhibited the conversions of 65 and 38%, respectively.
Mochida et al.
1344 Energy &Fuels, Vol. 8, No. 6,1994
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Temperature ("C ) Figure 6. TPDE spectra of CO and COZ evolution from asreceived pitch-ACF-OG-5A and PAN-ACF-FEBOO. Sample weight, 100 mg; carrier gas, helium; flow rate, 100 mL min-l. COz (-1, CO (- -1. (1)PAN-FEBOO, (2) OG-5A, (3) PAN-FE300, (4) OG-5A.
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Capture of Oxidized NO. Figure 1 illustrates the extent of captured NO2 and NO at the outlet of the reactor after the catalytic oxidation of NO over asreceived OG-5A. Under the conditions described above, NO of 380 ppm was stationarily oxidized into NO2 by the conversion of 73% as described above. Water in the washing bottle (100 mL, depth the of water 70 mm) captured 70% of produced NO2, passing remaining NO2 and unreacted NO. Aqueous KOH (1moW) of 100 mL captured all NO2 produced and 58% of unreacted NO, removing 91% of NO of the inlet gas. More efficient contact of gas with water or aqueous KOH is expected to remove more NO as well as NO2 according to literature12J3which report that a mixture of NO and NO2 at their mixing ratio of unity was completely captured by the base. TPDE of OG-5Aand FE-300. Figure 6 illustrates the TPDE profiles of OG-5A and FE-300 up to 1000 "C. OG-5A liberated C 0 2 and CO in the respective temperature ranges of 300-850 "C and 500-1000 "C with the peaks at 650 and 750 "C, respectively. FE300 did the same in the ranges of 150-900 "C and 250 to above 1000 "C with peaks at 250,800, and above 1000 "C, respectively. FE-300 liberated much more C02 and CO in the temperature range up to 800 "C where the largest catalytic activity was observed with both ACFs. 80% of CO-producing oxygen functional groups appears to be decomposed over OG-5A, while more than half of the groups remains on the FE-300 surface. It is of value t o note that OG-1OA and 20A of large surface area liberated much smaller amounts of both CO and C 0 2 than OG-5A. Discussion The present paper describes the significant catalytic activity of a particular pitch-based active carbon fiber, especially after the heat treatment at 800 "C, for the oxidation of NO in a humid air into NO2 at 25 "C, which is captured rather easily with water or aqueous basic solution. Since NO of 380 ppm is kinetically stable in the flue g a ~ ,causing ~ , ~ pollution in the form of acid rain, the catalytic activity of the present study may provide a base for the oxidative removal of NO. Low concentration of NO can be adsorbed on ACF to be catalytically oxidized. Three points may be worthwhile for d i s c u ~ s i o n ~ ~ ~ The first point is related to the retardation by humidity as reported in the p a ~ t : The ~ , ~important features
are that a particular pitch-based fiber showed higher activity in a humid air and that heat treatment improved the resistivity of the ACF against the humidity, allowing significant activity at room temperature in air of humidity up to 80%, whereas the as-received ACFs lost the activity very severely as observed with conventional active carbons. The hydrophobic surface may be induced on the particular pitch ACF surface of potentially high graphitizability. The removal of oxygen functional groups in forms of CO or C02 is well performed by the heat treatment from the ACF surface.10-12As clearly indicated by TPDE, OG-5A lost its oxygen functional groups completely with CO2-forming groups and substantially with CO-forming groups by the heat treatment up to 800 "C. No significant reduction of surface areas was observed up to this temperature. PAN-ACF FE-300 carries much more amount of CO and C02 producing groups and retains more than a half of the latter groups even after the heat treatment, up to 800 "C. Certain graphitization may be also achieved by such a treatment, although the ACF experienced such a temperature under the oxidative conditions in the activation stage. In contrast, PAN-ACF is basically nongraphitizable. The preference of the pitch-based fiber to PAN based fiber in humid air may be partly ascribed to less oxygen functional groups remaining and the graphitization potential of the former fiber both of which bring about hydrophobic nature to the carbon surface. The second point is related to the active site for the oxidation on the ACF surface. The heat treatment appears to induce more number of more active sites for the oxidation of NO as well as SO2 as described in preceding p a p e r ~ . ~ ~ The J ~site J ~appears to be induced by the liberation of CO and C02 in contrast to the expectation that the oxidation active site is connected to the oxygen functional group. Unidentified sites for oxidation appear to be introduced by the heat treatment. Surface carbon of unsaturation in its valence may be induced. The third point is that the pitch ACF of lower surface area exhibited the largest activity. Pore size, the oxygen functional groups, and graphitization potential may be delicately varied by the extent of activation. The interesting point is that PAN- and pitch-based ACFs exhibited the reverse orders of catalytic activity for the oxidation of SO2 and N0.13J6J7 Different ways of H2O intervention in the two oxidation reactions may cause the different activity in the humid air. The activity in dry air may reflect the different interactions of such substrates with the ACF surface. Detailed characterization of the ACF surface is necessary t o clarify its interaction with NO, Son, 0 2 , and H20, independently, competitively, or cooperatively. In conclusion, the combination of PAN-ACF and pitch ACF after the heat treatment at 800 "C may allow the oxidative removal of both SO2 and NO at room temperature in their acid forms, providing a base for novel technology for treatment SO2 and NO in flue and atmosphere. The successive application of both ACFs can recover H2S04 and HN03, separately. (16)Mochida, I.; Kawano, S.; Kisamori, S. Prepr. Pup.-Am. Chem. SOC.,Diu. Fuel Chem. 1993,38, 421. (17) Mochida, I.; Kawano, S.; Kisamori, S. Prepr. Pap.-Am. Chem. SOC.,Diu.Fuel Chem. 1994, 39, 219.
