Kinetic Study on Reduction of NO of Low Concentration in Air with NH

Sep 1, 1997 - Torishima, Konohana-ku, Osaka 554, Japan. Received April 13, 1995. In Final Form: July 7, 1997X. The catalytic activities of moderately ...
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Langmuir 1997, 13, 5316-5321

Kinetic Study on Reduction of NO of Low Concentration in Air with NH3 at Room Temperature over Pitch-Based Active Carbon Fibers of Moderate Surface Area Isao Mochida,*,† Shizuo Kawano,† Motohiro Hironaka,† Yuji Kawabuchi,† Yozo Korai,† Yuji Matsumura,‡ and Masaaki Yoshikawa‡ Institute of Advanced Material Study, Kyushu University, 6-1 Kasugakoen, Kasuga-shi, Fukuoka 816, Japan, and Research & Development Center, Osaka Gas Co., Ltd, 6-19-9, Torishima, Konohana-ku, Osaka 554, Japan Received April 13, 1995. In Final Form: July 7, 1997X The catalytic activities of moderately activated pitch-based carbon fibers (ACFs) were examined for the reduction of low concentration NO (10 ppm) with ammonia in dry and wet (80% humidity) air. A particular ACF calcined at 850 °C exhibited very high activity even in wet air, providing stationary NO conversions of 76 and 46% in dry and wet air, respectively, at the contact time W/F ) 5 × 10-3 g min mL-1. An optimum NH3 concentration of 15 ppm gave the largest NO conversion and the lowest leakage. The adsorptive abilities of the ACFs for NO and NH3 do not directly correlate with their catalytic activity, indicating that all adsorbed species are not active for the reduction. Catalytic activity was enhanced by calcination in an inert atmosphere, while reduced adsorptive ability suggests that the unsaturated carbon valencies induced by the elimination of oxygen functional groups act as additional active sites over the ACF surface. The hydrophobic nature, which is also enhanced by the calcination, is essential in moderating the retardation by H2O of both NO adsorption and activation. The moderately activated ACFs of 700-800 m2/g surface area after the heat treatment at 800-850 °C appear to contain the active sites of highest activity situated on the hydrophobic surface within pores not occupied by condensed H2O.

Introduction NO at concentrations of 1-10 ppm is known to accumulate in atmospheres with poor ventilation. Automobiles, especially those powered by diesel engines, release NO into the atmosphere.1 Underground highways, parking spaces, and busy traffic intersections are examples of such contaminated places where the atmosphere must be cleaned for human health.2 In these areas, NO is diluted with large amounts of humid air and is eliminated or converted at room temperature. So far, no effective method has been established to reduce NO into N2 under these conditions.3 It has been reported that active carbon and carbon fibers can reduce NO at 100-150 °C.4-11 The present authors have reported the catalytic activity of a pitch-based active carbon fiber having a low surface area (∼500 m2/g) for the reduction of NO at 10 ppm with NH3 in humid air at room temperature.11 The humidity acts as a strong inhibitor of the activated pitch-based carbon fiber (ACF) activity. †

Kyushu University. Osaka Gas Co., Ltd. X Abstract published in Advance ACS Abstracts, September 1, 1997. ‡

(1) Grove, M.; Sturn, W. Ceram. Eng. Sci. Proc. 1989, 10, 325. (2) Held, W.; Konig, A.; Riehter, T.; Puppe, L. SAE Paper 900496, 1990. (3) Sera, T. Jpn. Petrol. Inst. Symp. 1991, 32. (4) Juntgen, H.; Kuhl, H. Chemistry and Physics of Carbon; Marcel Dekker Inc.: New York, 1989; Vol. 22, 161. (5) Spirey, J. J.Annu. Rep. Prog. Chem., Sect. C: Phys. Chem. 1993, 90, 155. (6) Hamada, H.; Kintaichi, Y.; Sasaki, M.; Ito, T.; Tabata, M. Appl. Catal. 1990, 64, L1. (7) Mochida, I.; Mizojiri, T.; Fujitsu, H.; Komatsubara, Y.; Ida, S. Nippon Kagakukaishi 1985, 1676. (8) Mochida, I.; Ogaki, M.; Fujitsu, H.; Komatsubara, Y.; Ida, S. Fuel 1985, 64, 1054. (9) Mochida, I.; Fujitsu, H.; Shiraishi, I.; Ida, S. Nippon Kagaku Kaishi 1987, 5, 797. (10) Mochida, I.; Fujitsu, H.; Hisatsune, S.; Kisamori, S.; Shiraishi, I.; Ida, S. J. Jpn. Petrol. Inst. 1993, 36, 374. (11) Mochida, I.; Kawano, S.; Maeda, T. Nihon Kagaku Kaishi, 1991, 9, 1177.

S0743-7463(95)00299-X CCC: $14.00

The calcination of the ACF at 800-900 °C in an inert atmosphere, which removes a major portion of the oxygen functional groups from its surface,13-15 enhances the catalytic activity in humid air,12 probably by increasing the active sites for NO, as well as SO2,16 suggesting an important role of hydrophobic properties in the enhancement of activity. In a previous paper,17 a series of pitch-based active carbon fibers activated to varying degrees were examined after calcination up to 850 °C to identify the highest activity ACF among the fibers having moderate surface areas.17 Two fibers which had surface areas of about 800 m2/g in their as-prepared forms were known to have the highest activity. Calcination temperature significantly influenced their activity. The activation with H2SO4 followed by the calcination further increased the activity of the ACF.18 In the present study, two such pitch-based active carbon fibers, OG-7A and OG-8A, having a moderate surface area and high catalytic activity,17 were examined at 600-1000 °C and various humidities, ACF weights, flow rates, and ammonia and NO concentrations, in order to establish their performance and kinetics for the reduction of NO in ambient atmospheres. Temperature-programmed decomposition (TPDE) of the ACFs was applied to measure evolved CO and CO2, and the adsorption of ammonia, NO, and H2O were also measured to evaluate the role of the active sites and the surface properties of the ACFs subjected to the calcination. (12) Mochida, I.; Kawano, S.; Yatsunami, S.; Hironaka, M.; Matsumura, Y.; Yoshikawa, M. Energy Fuels 1995, 9, 659. (13) Mochida, I.; Kawano, S.; Fujitsu, H.; Maeda, T. Nippon Kagaku Kaishi 1991, 12, 1598. (14) Mochida, I.; Kawano, S.; Fujitsu, H.; Maeda, T. Nippon Kagaku Kaishi 1992, 3, 275. (15) Mochida, I.; Kawano, S. Ind. Eng. Chem. Res. 1991, 30, 2322. (16) Mochida, I.; Kisamori, S.; Hironaka, M., Kawano, S.; Matsumura, Y.; Yoshikawa, M. Energy Fuels 1994, 8, 1341. (17) Mochida, I.; Kawano, S.; Yatsunami, S.; Hironaka, M.; Korai, Y.; Matsumura, Y.; Yoshikawa, M. Chem. Lett. 1995, 9, 385. (18) Mochida, I.; Kawabuchi, Y.; Hironaka, M.; Kawano, S.; Matsumura, Y.; Yoshikawa, M. J. Jpn. Petrol. Inst. 1996, 39, 1551.

© 1997 American Chemical Society

Reduction of NO with NH3

Langmuir, Vol. 13, No. 20, 1997 5317

Table 1. Some Properties of Pitch-Based OG-7A and OG-8A Calcined Active Carbon Fibers surface pore area volume ash (m2/g) (mL/g)

ultimate analysis (wt %) ACF

C

H

N

O

OG-7A (as-received) OG-7A-H600a OG-7A-H700a OG-7A-H800a OG-7A-H850a OG-7A-H900a OG-7A-H1000a OG-8A (as-received OG-8A-H600a OG-8A-H700a OG-8A-H800a OG-8A-800a OG-8A-H1000a

90.8 92.1 92.4 92.8 93.2 93.5 93.9 91.2 92.6 93.0 93.4 94.0 94.4

1.0 0.9 0.9 0.9 0.8 0.7 0.7 0.9 0.8 0.8 0.6 0.6 0.5

0.6 0.5 0.5 0.5 0.5 0.5 0.4 0.6 0.5 0.5 0.4 0.3 0.3

7.4 6.3 5.9 5.5 5.2 4.9 4.5 7.0 5.8 5.4 5.2 4.7 4.3

a

0.2 0.2 0.3 0.3 0.3 0.4 0.5 0.3 0.3 0.3 0.4 0.4 0.5

690 730 690 660 640 610 570 840 920 870 750 700 670

0.35 0.37 0.35 0.34 0.32 0.30 0.28 0.41 0.45 0.42 0.37 0.34 0.32

Calcination temperature (°C). Calcined in N2 for 1 h.

Experimental Section Active Carbon Fibers and Their Characterizations. Two pitch-based active carbon fibers (OG-7A and OG-8A) were supplied by Osaka Gas Co. As-prepared active carbon fibers and calcined samples of these fibers were used in this study. The calcining treatment was carried out for 1 h at 600, 700, 800, 850, 900, and 1000 °C with a heating rate of 10 °C/min under nitrogen atmosphere. Table 1 shows the conditions of calcination and some properties of as-prepared and calcined active carbon fibers. The surface area and pore volume of OG-ACFs were measured by the BET N2 method. Temperature-programmed decomposition analysis (TPDE) of ACFs 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 heating rate of 10 °C/min, and evolved gases such as CO and CO2 were continuously analyzed, using a mass spectrometer. Reaction of NO with NH3. The reaction of NO (10 ppm) with NH3 (10-20 ppm) was carried out using either dry or wet air (10% O2 and the rest of N2 as a carrier gas, a U-shaped fixed bed flow type glass reaction(10 mm diameter) and 0.5 g of active carbon fiber closely packed into an 80 mm length. The ratio of NO to NH3, the total flow rate (F), and the contact time (W/F) were 0.5-1 ([NO] ) 10 ppm), 100 mL/min, and 5 × 10-3 g min mL-1, respectively. The influence of the total weight of active carbon fibers (W ) 0.2, 0.5, 1.0, 1.5, and 2.0 g), the contact time, and the relative humidity (0-80% rh) was also examined for some fibers. The air humidity was controlled by mixing air of 100% relative humidity(rh) with dry air. The relative humidity was measured with a humidity meter (CHINO NH-U type). Measurement of NO Conversion. The NO concentration was analyzed continuously at the inlet and the outlet of the reactor, using a NOx meter (ECL-77, Yanagimoto Co., Kyoto) which could detect 1 ppm NO. Details of this analysis have been described in previous papers.12,13 Stationary NO conversion was determined when the NO concentration at the outlet of the reactor became stable. Adsorption of reductants and products appeared to continue for several hours after initial the startup of the reaction. The NO2 concentration was analyzed hourly. The measurement of N2 and N2O concentrations was not carried out due to their low concentrations, although the reduction of NO to N2 was quantitatively confirmed at its high concentration of 400 ppm. Measurement of the Adsorption Capacities of NO, NH3, and H2O. The NO adsorption capacity of the ACF was estimated by measuring the NO concentration at the inlet and the outlet of the reactor to obtain the time for 100% breakthrough (T100) and quantify the amount by T100, using the same apparatus for the reaction. The NO concentration, the total flow rate, W/F, the temperature, and the relative humidity for NO adsorption were 10 ppm, 100mL/min, 5 × 10-3 g min ml-1, 20 °C, and 80%, respectively. The NH3 adsorption capacity was measured in a similar manner. The NH3 concentration, the total flow, W/F, the temperature, and the relative humidity for NH3 adsorption were

Figure 1. Conversion of NO in dry and wet air at room temperature over calcined OG-7A and OG-8A: NO:NH3 ) 1:2; [NO] ) 10 ppm; O2 ) 10% in N2; temp ) 20 °C; W/F ) 5 × 10-3 g‚min‚mL-1; (1-8) dry air (rh ) 0%); (9-16) wet air (rh ) 80%); (1) OG-7A; (2) OG-H600; (3) OG-H700; (4) OG-H800; (5) OGH850; (5) OG-H900; (7) OG-H1000; (8) OG-8A-H850; (9) OG7A; (10) OG-H600; (11) OG-H700; (12) OG-H800; (13) OG-H850; (14) OG-H900; (15) OG-H1000; (16) OG-8A-H850. 20 ppm, 100 mL/min, 5 × 10 -3 g min mL-1, 20 °C, and 80%, respectively. The adsorbed weight of H2O after flowing nitrogen gas of 100% relative humidity at 20 °C for 3 h was measured using a Cahn balance to estimate the H2O adsorption capacity.

Results Catalytic activities of Calcined OG-7As and OG8As. Figure 1 illustrates conversion of NO (10 ppm) in the reaction with NH3 (20 ppm) in dry and wet (80% rh) air over as-prepared, calcined OG-7As and OG-8As. NO in dry air was completely removed (100% conversion) 10 h after the reaction started over the as-prepared OG7A ACF, and then NO started to break through. Its conversion decreased gradually for the successive 10 h to be 60% at the stationary state. The conversion was stable for a further 10 h at the 60% stationary value, which was stable for at least another 10 h. The calcination shortened the period of the initial complete removal, depending on the calcination temperature. The stationary conversion increased to a value as high as 74% by the calcination at 850 °C. Higher calcination temperatures gradually decreased the stationary conversion to 53% for calcination at 1000 °C. Calcination at a specific temperature invariably increased the activity of the ACF for NO reduction in dry air. NO2 was not detected in the product, while the excess of unreacted NH3 was emitted from the ACF bed under stationary conditions. Hence the formation of a little NH4NO3 on the ACF surface could not be excluded. The humidity of 80% very drastically reduced the period of the initial complete removal of NO, regardless of the calcination temperatures. The stationary conversion over the as-prepared OG-7A, which was obtained within 30 min after the reaction started, was as low as 8%. A severe retardation by humidity of the conversion at this temperature was observed. Calcination at 850 °C increased markedly the stationary activity in the humid air to 38% of the highest performance. Further increases of calcination temperature decreased the activity gradually. OG-8A exhibited a very similar behavior in the NO + NH3 reaction to that of OG-7A. The highest activity was also obtained by the calcination at 850 °C in both dry and humid air, the conversions being 76 and 46%, respectively, as illustrated in Figure 1. These are the highest activities so far obtained with commercially available ACFs. The period of initial NO removal appeared to be almost the same in dry air for OG-8A and OG-7A fibers. The increase

5318 Langmuir, Vol. 13, No. 20, 1997

Figure 2. NO conversion in humid air over the calcined pitchbased ACFs OG-7A and OG-8A: NO:NH3 ) 1:2; [NO] ) 10 ppm; O2 ) 10% in N2; temp ) 20 °C; time ) 15-30 h; W/F ) 5 × 10-3 g‚min‚mL-1; (O) OG-7A-H800; (]) OG-7A-H800; (b) OG-7A-H800; (4) OG-7A-H800.

Figure 3. Conversion of NO in dry and wet air at room temperature over calcined ACF-OG-7A-H850 for variable weights: NO:NH3 ) 1:2; [NO] ) 10 ppm; O2 ) 10% in N2; temp ) 20 °C; W/F ) 5 × 10-3 g‚min‚mL-1; (1-4) dry air (rh ) 0%); (5-8) wet air (rh ) 80%); (1) W ) 0.2 g; (2) W ) 0.5 g; (3) W ) 1.0 g; (4) W ) 2.0 g; (5) W ) 0.2 g; (6) W ) 0.5 g; (7) W ) 1.0 g; (8) W ) 2.0 g.

in activity of OG-8A by calcination at 850 °C was marked in wet air. OG-8A’s were certainly superior to OG-7A’s in wet air. Influence of Humidity. The stationary conversion of NO in air of variable humidity over OG-7A H800, H850 and OG-8A H850, H900 is illustrated in Figure 2. Humidity of less than 50% hardly influenced the activity, regardless of the type of the ACFs, but the activity decreased linealy with humidity levels above 50%. Higher calcination temperature reduced the slope of the activity decrease, the smallest decrease being observed with the ACF calcined at 900 °C. The orders of catalytic activity in dry, 60% rh, and 80% rh air were as follows:

dry

OG-8A-H850 > OG-7A-H850 > OG-7A-H800 . OG-8A-H900

60% rh

OG-8A-H850 > OG-7A-H850 > OG-7A-H800 g OG-8A-H900

80% rh

OG-8A-H850 > OG-7A-H850 g OG-8A-H900 > OG-7A-H800

Thus, OG-8A-H850 was found to be the most active in the whole humidity range due to its largest intrinsic activity and modest conversion retardation by humidity. Kinetic Study. Figure 3 illustrates the conversion of NO (10 ppm) with NH3 (20 ppm) over variable amounts of OG-7A-H850 with a fixed flow rate of 100mL/min in dry and humid air at 20 °C. Increasing the amount of ACF to 2.0 g prolonged the period of the initial complete

Mochida et al.

Figure 4. NO conversion vs weight (W) of OG-7A-H850: NO: NH3 ) 1:2; [NO] ) 10 ppm; O2 ) 10% in N2; temp ) 20 °C; F ) 100 mL/min; reaction time ) 40 h; (O) dry air (rh ) 0%); (4) wet air (rh ) 80%).

Figure 5. Relation between NH3/NO and NO conversion and outlet amount of unreaction NH3: ACF, OG-8A-H850; W ) 0.5 g; F ) 100 mL/min; W/F ) 5 × 10-3 g‚min‚mL-1; room temp (20 °C); flow gas, NO/NH3 mix ([NO] ) 10 ppm); O2 ) 10% in N2; (1) dry (rh ) 0%); (2) wet (rh ) 80%).

conversion to values as long as 10 and 3.5 h, respectively, in dry and wet air. The stationary conversion increased up to 98% with 2.0 g. High humidity decreased the stationary conversion and shortened the periods of initial complete conversion. Figure 4 correlates the stationary conversion in dry and wet air with the amount of OG-7A-H850. The conversion increased with the amount of ACF sharply to 55% in dry air and then gradually to 98% for 2.0 g of ACF. In comparison, the conversion increased to 55% in wet air and very gradually to 72% with 2.0 g of ACF. The reaction orders in NO are calculated to be 1.2 and 2.0, respectively, in dry and wet air according to the integrated kinetic equations. It must be noted that the wet air increased the reaction order significantly. The reaction order may reflect the inhibition of NO activation by the humidity. Figure 5 illustrates the stationary conversion over OG8A-H850 with changing NH3 concentration at a fixed NO concentration of 10 ppm. The coversion in dry air increased linearly with increasing concentration of NH3 up to 15 ppm (NH3/NO mole ratio ) 1.5) until it achieved the highest value of 76% and was saturated at higher concentration. The conversion in wet air increased linearly with increasing NH3 concentration also up to 15 ppm of NH3 to the highest value of 43%. Higher concentrations of NH3 increased the conversion very gradually. Figure 5 also illustrates the emission of unreacted NH3 from the ACF bed at stationary conversion of NO. The emission stayed at 43% when NH3 up to 12 ppm was

Reduction of NO with NH3

Langmuir, Vol. 13, No. 20, 1997 5319

Table 2. Adsorption of NO, NH3, and H2O in Air over the As-Received and Calcined Pitch-Based ACFs OG-7A and OG-8A adsorption amount (mmol/g) NOa

NH3a

ACF

dryd

wete

dryd

wete

H2Ob (mmol/g)

OG-7A (as-received) OG-7A-H600c OG-7A-H700c OG-7A-H800c OG-7A-H850c OG-7A-H900c OG-7A-H1000c OG-8A (as-received) OG-8A-H600c OG-8A-H700c OG-8A-H800c OG-8A-H850c OG-8A-H900c OG-8A-H1000c

1.4 0.9 0.8 0.5 0.4 0.2 0.1 1.6 1.2 1.0 0.8 0.6 0.4 0.3

0.4 0.6 0.5 0.3 0.2 0.1

1.6 2.4 2.0 1.8 1.5 1.1 0.6 1.8 2.7 2.2 2.0 1.7 1.3 0.9

1.8 2.8 2.5 2.3 2.0 1.4 0.9 2.3 3.1 2.8 2.6 2.2 1.8 1.4

9.1 8.3 7.3 7.0 5.1 4.9 3.1 8.9 8.1 7.1 6.8 5.0 4.8 3.0

0.5 0.6 0.5 0.4 0.3 0.2 0.1

a Adsorbed amount before 100% breakthrough (T b 100). Adsorbed amount in equilibrium when the relative humidity of air was 100%. Three hours was required for equilibrium adsorption. Adsorbed temperature: 20 °C. Concentration of NO and NH3: 10 and 20 ppm. Flow rate: 100 mL/min. c Calcination temperature (°C). d Dry air (rh ) 0%). e Wet air (rh ) 80%).

charged in dry air and increased almost proportionally with more amounts of NH3. Greater emission of NH3 and a small increase of NO conversion with concentrations of NH3 higher than 12 ppm were seen under the present conditions. Ammonia leakage increased to 70% in wet air when 10-15 ppm NH3 was charged. Higher amounts of NH3 caused more emission. At 21 ppm NH3, the amount of emission was 80% of the charged concentration. Thus, the amount of NH3 charged should be optimized by taking into account NO conversion, the accepted concentration of unreacted NH3, and the trapping efficiency of NH3 after the reaction. Adsorption of NO, NH3, and H2O. Table 2 summarizes the adsorption of NO, NH3, and H2O at 20 °C over OG-7A, OG-8A, and their calcined forms. As-prepared OG-7A adsorbed 1.4 mmol/g of NO in dry air before its 100% breakthrough. A relative humidity of 80% markedly reduced the adsorption to 0.4 mmol/g. Calcination reduced the adsorption in dry air according to the calcination temperature, while the adsorption in wet air (80% rh) exhibited a small increase to 0.6 mmol/g by the calcination at 600 °C and then decreased gradually by calcinations at higher temperatures. As-prepared OG-8A adsorbed a little more NO (1.6 mmol/g in dry air and 0.5 mmol/g in wet air). The adsorption amount of NH3 in dry air on OG-7A was 1.6 mmol/g, being slightly larger than that of NO. Calcination increased the adsorption when the calcination temperature reached 600-700 °C. Higher temperatures of calcination, up to 900 °C, gave 1.1 mmol/g of adsorption, while calcination at 1000 °C markedly reduced the adsorption to 0.6 mmol/g. A relative humidity of 80% was found to slightly increase the adsorption of NH3 compared to that in dry air regardless of the calcination temperatures. Larger adsorption by calcinations at 600800 °C was again observed. Calcination at higher temperature decreased the adsorption to the same level as that of the as-prepared ACF. As-prepared OG-7A adsorbed 9.1 mmol/g of H2O when humidified air of 100% rh was passed over it for 3 h. Calcination reduced the adsorption gradually:7.3 mmol/g at 700 °C, 7.0 mmol/g at 800 °C, and 4.9 mmol/g at 900 °C, respectively. A marked reduction to 3.1 mmol/g was observed when the calcination temperature reached 1000

Figure 6. TPDE spectra of CO2 and CO evolution from asreceived OG-7A and OG-8A.

°C. As-prepared OG-8A adsorbed a slightly smaller amount of H2O (8.9 mmol/g) than as-prepared OG-7A in spite of its larger surface area. Calcination generally reduced the adsorption of H2O over OG-8A, giving the values of 6.8 and 4.8 mmol/g for calcinations at 800 and 900 °C, respectively. Characterization of ACF. Table 1 summarizes the microanalysis, surface area, and pore volume of OG-7As and OG-8As. The surface area of as-prepared OG-7A was 690 m2/g. The surface area of OG-7A calcined at 600 °C was 730 m2/g, showing a 5% increase compared to that of the as-received one. The area of calcined ACFs at 700800 °C decreased slightly compared to that of the ACF calcined at 600 °C. The surface area of the calcined ACFs at 800, 900, and 1000 °C decreased gradually to 660, 610, and 570 m2/g, respectively. The pore volume of the as-prepared OG-7A was 0.35 mL/g. It was reduced by calcination: 0.34 mL/g at 800 °C, 0.30 mL/g at 900 °C, and 0.28 mL/g at 1000 °C, respectively. As-prepared OG-8A had a larger surface area of 840 m2/g, a larger pore volume of 0.41 mL/g, and a smaller oxygen content (difference) of 7.0% than the as-prepared OG-7A. Calcination influenced the properties in a manner similar to that for OG-7A. Calcination decreased the contents of oxygen (difference), nitrogen, and hydrogen, although the latter two values were always very small compared to those of carbon and oxygen. Figure 6 illustrates the TPDE profiles of OG-7A and OG-8A up to 1100 °C. Both fibers liberated CO2 and CO in the temperature ranges 250-800 °C and 400-1100 °C with the peaks at 240 and 800 °C, respectively. OG-8A carried slightly more CO2- and COproducing groups. All the CO2-producing groups and 80% of the CO-producing oxygen groups evolving up to 1100 °C appear to be decomposed over both ACFs by 850 °C. Thus, calcination to achieve the highest activity removed the major portion of oxygen functional groups from the ACF surface as well. Discussion The present study clarified several features of the NO + NH3 reaction over pitch based active carbon fibers of moderate surface area, reporting the largest activity so far obtained among the commercially available ACFs for NO reduction in wet and dry air at room temperature.17 The reaction was severely retarded by humidity, as reported previously.12 Even H2O produced during the reduction appeared to retard the reaction by being adsorbed on the ACF, as observed for the initial several hours of the reaction in dry air before stationary conversion was achieved. Such retardation may be more noticeable at higher stationary conversions, over a larger amount of

5320 Langmuir, Vol. 13, No. 20, 1997

Figure 7. Relation between NO conversion and amount of adsorbed NO conversion and amount of adsorbed NO over calcined OG-7A-H (600-1000) and OG-8A-H (600-1000): flow gas, NO:NH3 ) 1:2 ([NO] ) 10 ppm); O2 ) 10%, in N2; temp ) 20 °C; W/F ) 5 × 10-3 g‚min‚mL-1; NO conversion, stationary activity was measured about 15-30 h after the reaction started; (1 ans 2) dry (rh ) 0%); (3 and 4) wet (rh ) 80%); (large open circle) OG-7A-H (600-1000); (small open triangle) OG-8A-H (600-1000); (small open circle) OG-7A-H (600-1000); (small open triangle) OG-8A-H (600-1000).

Figure 8. Relation between NO conversion and amount of adsorbed NH3 over Calcined OG-7A-H (600-1000) and OG8A-H (600-1000): flow gas, NO:NH3 ) 1:2 ([NO] ) 10 ppm); O2 ) 10% in temp ) 20 °C; W/F ) 5 × 10-3 g‚min‚mL-1; NO conversion, stationary activity was measured about 15-30 h after the reaction started; (1 and 2) dry (rh ) 0%); (3 and 4) wet (rh ) 80%); (large open circle) OG-7A-H (600-1000); (large open triangle) OG-8A-H (600-1000); (small open circle) OG7A-H (600-1000); (small open triangle) OG-8A-H (600-1000).

ACF, increasing the reaction order to a value greater than 1.0. Lower concentrations of NO achieved by higher conversions suffer even more retardation by H2O. The retardation of the as-prepared ACF by humidity was found to be significantly moderated by calcination in an inert atmosphere at 600-900 °C. Such a simple procedure improves the practical applicability of ACFs in wet atmospheres. Calcination decreased the oxygen functional groups, as shown by TPDE, reducing the adsorption of H2O, which strongly retards the adsorption of NO, although calcination also decreased the adsorption of NO. Thus, for high conversion efficiency, the required factor is an increase in the number of surface active sites of the pitch-based ACF, obtainable by the selection of precursors as well as the choice of appropriate activation and postcalcination conditions. It should be noted that the period of initial complete NO removal was observed even in humid air when a large amount of ACF was used. Rapid regeneration may allow another efficient process for NO removal. Figures 7 and 8 correlate the stationary conversion and adsorption amounts of NO and NH3 over OG-7As and OG8As, respectively. It is indicated that there is an optimum range of adsorption for both NO and NH3. In other words,

Mochida et al.

larger adsorption ability is not always related to higher activity. The OG-8A series of ACFs exhibited higher conversion and larger adsorptive ability for both NO and NH3 in both dry and wet air than the OG-7A series. The adsorption of NO is significantly reduced by calcination, indicating that the majority of its adsorption may take place at the surface oxygen groups on the pore wall. The basic sites among the oxygen functional groups have been reported to be increased by calcination above 600 °C by the removal of acidic oxygen functional groups, increasing the adsorption of acidic substrates such as SO2 and HCl.19,20 A form of condensation of NO is also postulated in very small pores.21 Such a process may not contribute to the stationary reduction of NO. The catalytic activity of the ACF can be discussed from two points of view, moderation of H2O inhibition and the number of active sites. The lower adsorption of H2O, leading to smaller retardation of conversion over the calcined ACFs, may explain partly the favorable influence of the calcination up to 850 °C, where the excess of oxygen functional groups is removed from the ACF surface. The oxygen functional groups of PAN-ACF have been postulated to be the acidic active sites, activating NH3 for reaction with NO around 100 °C.22-24 However, the calcination decreased or completely eliminated such groups in pitch ACFs, as shown in Figure 6, whereas the activity for NO reduction with NH3 was markedly enhanced, as shown in Figure 1. Thus, such oxygencontaining groups are not all catalytically active, especially for the reaction in humid air at room temperature. The correlation between the activity and NO adsorption in Figure 7 indicates that a very small proportion of adsorption sites is necessary for effective reduction of NO with carbon or char at 300-700 °C. Tomita et al.25 suggested that the reactive surface C-O complex is a possible active site, although the initial rapid reaction of NO and carbon is postulated to produce such a reactive intermediate. Radovic et al.26-28 assumed that a surface C(O) complex produced through the NO + carbon reaction is the intermediate, releasing CO or CO2 for the regeneration of the active carbon sites. The catalyst is believed to accelerate dissociative adosorption of NO to form the C(O) complex. Thus, in spite of the extensive study, there is no unified theory on the nature of the active carbon site which is oxidized by NO to form a C(O) complex. In addition, it should be noted that the adsorbed NO is reduced with NH3 without producing C(O) groups on the ACF surface under the present conditions. The active sites for NO reduction appear to be produced in increasing amount over the ACF surface by calcination at high temperatures up to 850 °C, since it leads to increasing activity in dry air as well as in wet air. The unsaturated valency of the surface introduced by the elimination of oxygen groups can constitute the active (19) Voll, M.; Boehm, H. P. Carbon 1970, 8, 741. (20) Davini, P. Carbon 1990, 28, 565. (21) Kaneko, K.; Fukuzaki, N.; Kakei, K.; Suzuki, T.; Ozeki, S. Langmuir 1989, 5, 960. (22) Mochida, I.; Ogaki, M.; Fujitsu, H.; Komatsubara, Y.; Ida, S. Nippon Kagaku Kaishi 1985, 4, 680. (23) Komatsubara, Y.; Ida, S.; Fujitsu, H.; Mochida, I.; Fuel, 1984, 63, 1738. (24) Kawano, S.; Kisamori, S.; Mochida, I.; Fujitsu, H.; Maeda, T. Nippon Kagaku Kaishi 1993, 6, 694. (25) Yamashita, H.; Tomita., A.; Yamada, H.; Kyotani, T.; Radovic, L. R. Energy Fuels 1993, 7, 85. (26) Illan-Gomez, M. J.; Linares-Solano, A.; Radovic, L. R.; de Lecea, C. S. Energy Fuels 1995, 9, 97. (27) Illan-Gomez, M. J.; Linares-Solano, A.; Radovic, L. R.; de Lecea, C. S. Energy Fuels 1995, 9, 97. (28) Illan-Gomez, M. J.; Linares-Solano, A.; Radovic, L. R.; de Lecea, C. S. Energy Fuels 1995, 9, 112.

Reduction of NO with NH3

sites which are also active for the oxidation of SO2.29,30 The activity of the ACF was further increased by H2SO4 oxidation and successive calcination where more evolution of CO and CO2 may increase the number of such active sites18 on the carbon. Leon et al.31 proposed the existence of oxygen-free basic sites on the basal plane. Miller and co-workers32,33 proposed a nascent active site for the uncatalyzed hydrogasification of HNO3-preoxidized carbon. The present authors proposed that the active site is a benzyne type group on the edge of a hexagonal plane which may be produced by the decomposition of C(O) groups on the calcined ACF surface.34 On the other hand, the free spin concentration of the ACF was reduced markedly by calcination above 800 °C, ruling out the dangling bond as the active site. NO can be oxidized on such an active carbon site to NO2 by reaction with NH3, explaining the favorable role of oxygen in air on NO reduction. (29) Kisamori, S.; Mochida, I.; Fujitsu, H. Langmuir 1994, 10, 1241. (30) Mochida, I.; Kisamori, S.; Kuroda, K.; Kawano, S.; Matsumura, Y.; Yoshikawa, M. Energy Fuels 1994, 8, 1337. (31) Leon, C. A.; Leon, Y.; Solar, J. M.; Calemma, V.; Radovic, L. R.; Carbon 1992, 30, 797. (32) Heiji, H. Zo.; Miller, D. J. Carbon 1989, 25, 809. (33) Treptau, M. H.; Miller, D. J. Carbon 1991, 29, 531. (34) Mochida, I.; Kuroda, K.; Miyamoto, S.; Sotowa, C.; Korai, Y.; Kawano, S.; Sakanishi, K.; Yasutake, A.; Yoshikawa, M. Energy Fuels, 1997, 11, 307.

Langmuir, Vol. 13, No. 20, 1997 5321

Basic NH3 can be adsorbed on polar oxygen functional groups. Its adsorption at its present concentration is influenced much less by calcination below 900 °C and humidity than that of NO. Hence NH3 can stay on the ACF surface or be dissolved in the adsorbed H2O at room temperature. Such NH3 species present in sufficient quantities on the ACF surface can react with NO2 derived from NO on the ACF surface, reducing it N2. Adsorption of NH3 does not appear to be directly correlated to the activity. So far, sufficient adsorption of NH3 appears to take place to accomplish the reduction of NO over the ACF surface with a minimum amount of oxygen functional groups at room temperature. In conclusion, a particular pitch-based active carbon fiber OG-8A of moderate surface area, exhibited an excellent NO reduction activity with NH3 even in wet air after calcination at 850 °C. The optimization of reaction conditions, including the control of NH3 concentration and trapping the unreacted NH3, is expected to solve the problem of the presence of a low concentration of NO in the urban atmosphere. The adsorbed NO species on the ACF surface is to be characterized in more detail to clarify the nature of the active sites. LA950299C