Active Carbons Obtained from Bituminous Coal for NO2 Removal

Energy & Fuels 2009, 23, 3617–3624

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Active Carbons Obtained from Bituminous Coal for NO2 Removal under Dry and Wet Conditions at Room Temperature Robert Pietrzak* Laboratory of Coal Chemistry and Technology, Faculty of Chemistry, Adam Mickiewicz UniVersity, Grunwaldzka 6, 60-780 Poznan´, Poland ReceiVed March 31, 2009. ReVised Manuscript ReceiVed May 18, 2009

The adsorption of NO2 on active carbons obtained from Polish bituminous coal was studied. Carbonaceous adsorbents were obtained by pyrolysis of coal at 500, 600, and 700 °C followed by activation with KOH at 700 °C. Adsorption of NO2 was carried out from dry and moist (70% humidity) air. The surface structure and chemistry of the initial and exhausted adsorbents were analyzed using adsorption of nitrogen, Boehm method, and elemental and thermal analyses. All adsorbents obtained were found to have acidic surfaces. The best NO2 sorption capacity in both dry and wet conditions was obtained for the active carbon pyrolyzed at 700 °C, with respective values of 25.0 and 43.5 mg/g, respectively. For all materials a considerable amount of NO2 was reduced to NO. Differential thermogravimetry curves of samples after exposure to NO2 showed two new peaks, the first at 90-180 °C, assigned to the removal of physically adsorbed NO2 and NO, and the second at 200-400 °C, assigned to the decomposition of potassium nitrates and nitrites.

1. Introduction Air pollution caused by emission of toxic gases to the atmosphere is a serious global problem. Air pollutants are divided into two main categories: primary and secondary ones. Primary pollutants are those emitted directly into the air, in contrast to secondary pollutants, which are created in the atmospheric air by the reactions among the primary pollutants.1 One of the most threatening air pollutants is the group of nitrogen oxides (NOx). The main sources of NOx are combustion of nitrogen-containing fuels in industry and household, car engine, and electric utilities. They contribute to such phenomena as global warming (greenhouse effect), acidification (acid rain), photochemical smog, and ozone layer depletion.1,2 Although the main component of NOx is NO, much more harmful is NO2. It is a toxic and fast-acting compound whose harmful effect is a few times greater than that of CO and SO2. It is formed in the atmosphere as a result of photochemical oxidation of NO3 and directly in car engines. Emission of pollutants can be limited at the source of their formation or by purification of exhaust gases by reduction or oxidation in the liquid or gas phase with simultaneous sorption in or on solid-state sorbents. The most popular method of gas purification from nitrogen oxides is selective catalytic reduction (SCR), although this method is marred with high corrosivity of the equipment needed and toxicity of ammonia that is used as a reducing agent.4 Another method is based on the use of active carbons enriched in nitrogen as the fluidal bed. In this method the reduction of * Phone: +48-618291476, Fax: +48-618291505, E-mail: pietrob@ amu.edu.pl. (1) Inglezaks, V. J.; Poulopoulos, S. G. Adsorption, Ion Exchange and catalysis Design of Operations and EnVironmental Applications, First ed.; Elsevier: Amsterdam, The Netherlands; 2006; Ch 1, pp 1-30. (2) Manahan, S. E. EnVironmental Chemistry, 7th ed.; CRC Press LLC: 2000; pp 405-430. (3) Shirahama, N.; Moon, S. H.; Choi, K.-H.; Enjoji, T.; Kawano, S.; Korai, Y.; Tanoura, M.; Mochida, I. Carbon 2002, 40, 2605–2611. (4) Madia, G.; Koebel, M.; Elsener, M.; Wokaun, A. Ind. Eng. Chem. Res. 2002, 41, 4008–4015.

NOx on the active carbon bed leads to formation of free and nontoxic N2.5,6 An alternative approach is based on removal of nitrogen oxides with the help of adsorption and reducing properties of carbon adsorbents.3,7-15 Investigation of NO adsorption on active carbon in the presence of oxygen and steam performed at 100-150 °C has proved the simultaneous occurrence of the following processes: adsorption, reduction, and catalytic oxidation of NO and adsorption of the NO2 formed.16 Suzuki at el.17 have reported that the presence of oxygen increases the rate of the surface reaction between carbon and NO and proved that the formation of surface C-NO groups requires the preliminary formation of the oxygen complexes C-O2. The FTIR18 study has shown that the reaction of NO with the carbon surface leads to formation of nitrates and small amounts of NO2 and (NO)2 dimers. The heterogenic reaction of NO2 with carbon has been mostly studied on soot.19,20 The results have shown that the reaction (5) Matzner, S.; Boehm, H. P. Carbon 1998, 36, 1697–1703. (6) Sanchez-Cortezon, E.; Pfa¨nder, N.; Wild, U.; Mestl, G.; Find, J.; Schlo¨gl, R. Carbon 2000, 38, 2029–2039. (7) Rubel, A. M.; Stencel, J. M. Energy Fuels 1996, 10, 704–708. (8) Izquierdo, M. T.; Rubio, B. EnViron. Sci. Technol. 1998, 32, 4017– 4022. (9) Xia, B.; Philipps, J.; Chen, C.-K. Energy Fuels 1999, 13, 903–906. (10) Muckenhuber, H.; Hinrich, G. Carbon 2007, 45, 321–329. (11) Jeguirim, M.; Tschamber, V.; Brilhac, J. F.; Ehrburger, P. J. Anal. Appl. Pyrol. 2004, 72, 171–181. (12) Kong, Y.; Cha, C. Y. Carbon 1996, 34, 1027–1033. (13) Ellison, M. D.; Crotty, M. J.; Koh, D.; Spray, R. L.; Tate, K. E. J. Phys. Chem. B 2004, 108, 7938–7943. (14) Lee, Y.-W.; Kima, H.-J.; Parka, J.-W.; Choia, B.-U.; Choi, D.-K.; Park, J.-W. Carbon 2003, 41, 1881–1888. (15) Illań-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartıńez de Lecea, C. Energy Fuels 1996, 10, 158–168. (16) Klose, W.; Rincoń, S. Fuel 2007, 86, 203–209. (17) Suzuki, T.; Kyotani, T.; Tomita, A. Ind. Eng. Chem. Res. 1994, 33, 2840–2845. (18) Long, R. Q.; Yang, R. T. Ind. Eng. Chem. Res. 2001, 40, 4288– 4291. (19) Al-Abadleh, H. A.; Grassian, V. H. J. Phys. Chem. A 2000, 104, 11926–11933.

10.1021/ef9002796 CCC: $40.75  2009 American Chemical Society Published on Web 06/09/2009

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of carbon with NO2, NO/O2, or NO2/O2 in low temperatures leads to formation of the surface complexes such as C-NO2, C-ONO, C-ONO2, and C-O. On the basis of the hitherto studies of the removal of gas pollutants, the most promising materials used for adsorption and reduction of NO2 seem to be activated carbons. They have exceptional physicochemical properties related to the very well developed surface area and the presence of heteroatoms in their structure.21-23 Some properties of active carbons, in particular their adsorption abilities, are determined by the type of precursor, the method, and conditions applied to get the final product (carbonization, activation).24 Active carbons can be obtained from different materials of mainly organic origin,25-28 but the most suitable are fossil coals representing the whole scale of coalification.26,29-31 The main argument for the use of fossil coals as raw materials for production of adsorbents is the facility of generation in them a highly developed porous structure as they originally have some inherent. Despite having this inherent porous structure, fossil coals cannot be used directly as industrial adsorbers as their innate pores are too fine and hence inaccessible for the majority of molecules needed to be adsorbed. The desired porous structure can be achieved after additional treatment in the processes of carbonization and activation. The main aim of the study was to obtain active carbon adsorbents from bituminous coal by their treatment with KOH and test the adsorbents obtained in the process of nitrogen dioxide removal. Also, the effects of the pyrolysis temperature and the activating agent on the sorption properties of the active carbon adsorbents obtained were analyzed. Pyrolysis was performed at three temperatures of 500, 600, and 700 °C, and the active carbon samples obtained by the treatment with KOH were tested for NO2 adsorption in dry and wet conditions. 2. Experimental Section Samples. The starting raw sample (JM) was prepared from a Polish bituminous coal (Jas-Mos mine; moisture ) 0.8 wt %, ashd ) 2.5 wt %, VMdaf ) 19.4 wt %). It was milled and sieved to the grain size of 0.2 mm. All measurements were carried out on samples demineralised (D) by concentrated hydrochloric and hydrofluoric acids according to the Radmacher and Mohrhauer method.32 Carbonization. Carbonization was performed in a horizontal furnace under argon flow (flow rate of 170 mL/min). The samples (20) Arenas, F.; Gutzwiller, L.; Baltensperger, U.; Ga¨ggeler, H. W.; Ammann, M. EnViron. Sci. Technol. 2001, 35, 2191–2199. (21) Pietrzak, R.; Jurewicz, K.; Nowicki, P.; Babeł, K.; Wachowska, H. Fuel 2007, 86, 1086–1092. (22) Bandosz, T. J.; Ania, C. O. Activated carbon surfaces in environmental remediation. Surface chemistry of activated carbons and its characterization. In Interface Science and Technology; Bandosz, T.J. Ed.; Elsevier: 2006; Ch 7, pp 159-229. (23) Radovic, L. R.; Sudhakar, C. Carbon as Catalyst Support: Introduction, properties and applications. In Introduction to Carbon Technologies; Marsh, H.; Heintz, E. A.; Rodriquez-Reinoso, F. Eds.; University of Alicante: Alicante, Spain, 1997. (24) Hayashi, J.; Horikawa, T.; Muroyama, K.; Gomes, V. G. Microporous Mesoporous Mater. 2002, 55, 63–68. (25) Diaz-Teran, J.; Nevskaia, D. M.; Fierro, J. L. G.; Lopez-Peinado, A. J.; Jerez, A. Microporous Mesoporous Mater. 2003, 60, 173–181. (26) Hayashi, J.; Kazehaya, A.; Muroyama, K.; Watkinson, A. P. Carbon 2000, 38, 1873–1878. (27) Hu, Z.; Srinivasan, M. P. Microporous Mesoporous Mater. 1999, 27, 11–18. (28) Wu, M.; Zha, Q.; Qiu, J.; Guo, Y.; Shang, H.; Yuan, A. Carbon 2004, 42, 205–210. (29) Nowicki, P.; Pietrzak, R.; Wachowska, H. Energy Fuels 2008, 22, 4133–4138. (30) Hsu, L.-Y.; Teng, H. Fuel Process. Technol. 2000, 64, 155–166. (31) Nowicki, P.; Pietrzak, R.; Wachowska, H. Fuel 2008, 87, 2037– 2040. (32) Radmacher, W.; Mohrhauer, O. Brennstoff-Chemie 1956, 37, 353– 358.

Pietrzak were heated (5 °C/min) from room temperature to final carbonization temperatures of 500, 600, and 700 °C, designated as JMD5, JMD6, and JMD7, respectively. At the final temperature, samples were maintained for 1 h and then cooled in an inert gas atmosphere. Activation. Activation (A) was applied to carbonization products. KOH was directly mixed at room temperature with samples at the weight ratio of 4:1. After the physical mixing, the samples were activated at 700 °C for 45 min. This process was carried out in a laboratory furnace in argon flow (330 mL/min). The obtained activated carbons were washed first with 5% HCl solution and then with distilled water until free of chloride ions. The washed activated carbons were dried at 110 °C for 24 h. Analytical Procedures. Proximate analysis (moisture, ash, volatile matter) was performed according to the Polish Standards.33-35 Elemental analysis (C,H,N,S) of the products obtained at each stage of the processing was made on an elemental analyzer CHNS Perkin Elmer 2400 Series II. The surface oxide functional groups were determined by the Boehm method.36,37 The home-designed dynamic test was used to evaluate NO2 adsorption from gas streams.38 Samples were packed into a glass column (length 350 mm, internal diameter 9 mm, bed volume between 0.5 cm3 for carbonizates and 1 cm3 for active carbons). Dry (designation “Ed” as exhausted in dry condition) or moist (70% humidity) air (designation “Em” as exhausted in moist/wet condition) with 0.1% of NO2 (accelerated test) was passed through the column of adsorbent at 0.450 L/min for NO2. The flow rate was controlled using Cole Parmer flow meters. The breakthrough of NO2 and the concentration of NO were monitored using Q-RAE PLUS PGM-2000/2020 with electrochemical sensors. The tests were stopped at the breakthrough concentration of 20 ppm. The interaction capacities of each sorbent in terms of milligram of toxic gases per gram of adsorbent were calculated by integration of the area above the breakthrough curves and from the NO2 concentration in the inlet gas, flow rate, breakthrough time, and mass of sorbent. To check the NO2 reduction, the concentration of NO was also monitored until 200 ppm (electrochemical sensor limit). Characterization of the pore structure of activated carbons was performed on the basis of low-temperature (77K) nitrogen adsorption-desorption isotherms measured on a sorptometer ASAP 2010 manufactured by Micromeritics Instrument Corp. (USA). Before the isotherm measurements, samples were outgassed at 120 °C for 10 h. Surface area and pore size distribution were calculated by BET and BJH methods, respectively. Total pore volume and average pore diameter were determined as well. Micropore volume and micropore area were calculated using t-plot method. A 0.4 g sample of dry carbon powder was added to 20 mL of distilled water, and the suspension was stirred overnight to reach equilibrium. Then the pH of suspension was measured. Thermogravimetric analysis of the initial carbon samples and after adsorption were performed on an SETSYS 12 made by Setaram. The samples (10 mg, particle size below 0.06 mm) were heated at the rate 10 °C/min, in the helium atmosphere. Analysis (33) Polish Standards PN-80/G-04511. (34) Norma Polish Standards PN-ISO 1171: 2002. (35) Norma Polish Standards PN-ISO 562: 2000. (36) Boehm, H. P.; Diehl, E.; Heck, W.; Sappok, R. Angew. Chem. Inte. Edit in English 3 1964, 669–677. (37) Boehm, H. P. Carbon 1994, 32, 759–769. (38) Pietrzak, R.; Bandosz, T. J. Carbon 2007, 45, 2537–2546. (39) Otowa, T.; Nojima, Y.; Miyazaki, T. Carbon 1997, 35, 1315–1319. (40) Stanmore, B. R.; Tschamber, V.; Brilhac, J. F. Fuel 2008, 87, 131– 146. (41) Zhang, W. J.; Bagreev, A.; Rasouli, F. Ind. Eng. Chem. Res. 2008, 47, 4358–4362. (42) Chugtai, A. R.; Gordon, S. A.; Smith, D. M. Carbon 1994, 32, 405–416.

ActiVe Carbons Obtained from Bituminous Coal

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hydrogen and nitrogen, as well as total removal of sulfur. The decrease in the content of nitrogen is explained as a consequence of low resistance of the nitrogen species to the activating agentsKOH. An increase in the content of oxygen in the active carbon samples relative to that in the initial coal is in agreement with literature data.39 This increase is a consequence of decomposition of KOH and the reaction of K2O with carbon, which is consistent with the mechanism of the reaction of coal gasification upon reaction with alkaline compounds. On the other hand, the content of oxygen decreases with increasing temperature of pyrolysis, which is a consequence of the fact that the higher the pyrolysis temperature the greater the structure ordering and the lower the susceptibility of the chars obtained to the effect of K2O. In the chars obtained and in the active carbons obtained from them, the content of the surface oxygen functional groups was determined by the Boehm method.37,38 The results collected in Table 2 imply that although the total content of surface oxides in the active carbon samples is many times higher than in the corresponding chars, the surfaces of all samples have acidic character. The greatest content of the surface acidic oxygen groups was determined for the char obtained at 600 °C (JMD6) and the active carbon obtained from this char (JMD6A). The structural parameters of the chars obtained and the active carbons obtained from them are presented in Table 3. They prove that the activation with KOH leads to the active carbon samples of well-developed porous structure with the domination of micropores. The higher the temperature of carbonization, the smaller the surface area of the active carbon obtained. This observation can be explained by the increasing ordering of the carbon structure with increasing temperature of carbonization, which makes the char obtained increasingly more resistant to the activating agent and, first of all, to the K2O formed. All the chars and active carbon samples obtained from them were tested for the NO2 adsorption in dry and wet conditions. The NO2 breakthrough curves obtained for all samples are presented in Figures 1-4, showing also the NO concentration curves, as nitrogen monoxide is known to be the product of surface reduction of NO2, on carbonaceous adsorbents;10 moreover, the activated carbons also catalyze the decomposition of NO2 to NO.40 The NO emission is very high both for the chars and the active carbons obtained in this work, which indicates a high reduction potential of the surface or low adsorption capacity for NO. A very fast increase in the NO concentration in the outlet gas at the beginning of the process is consistent with the results obtained by Jequirim et al.11 They studied the adsorption/ desorption products formed upon NO2 adsorption on commercial carbon black, Vulcan 6 from Cabotat at 25-50 °C and found that NO is formed during the initial adsorption of NO2. This observation was confirmed by Zhang

Table 1. Elemental Analysis of Initial Coals, Chars, and Active Carbons (wt %) coal

ashd

Cdaf

Hdaf

Ndaf

Sdaf

Odaf a

JM JMD JMD5 JMD6 JMD7 JMD5A JMD6A JMD7A

2.5 0.6 0.4 0.3 0.6 0.5 0.6 0.3

89.7 89.8 90.0 91.5 95.7 93.4 94.2 96.4

5.0 4.8 3.8 2.7 1.8 0.8 0.6 0.6

1.6 1.1 1.2 1.4 1.3 0.1 0.3 0.4

0.4 0.3 0.3 0.3 0.2 0.0 0.0 0.0

3.2 4.0 4.7 4.1 1.0 5.6 4.9 2.6

a

By difference.

Table 2. Acid-Base Properties of the Chars and Active Carbons Obtained sample

total content of surface oxides [mmol/g]

acidic groups [mmol/g]

basic groups [mmol/g]

JMD5 JMD6 JMD7 JMD5A JMD6A JMD7A

0.20 0.35 0.30 1.62 1.80 1.32

0.20 0.25 0.20 0.85 1.05 0.79

0.00 0.10 0.10 0.77 0.75 0.53

lasted for 6 h, and the temperature during the decomposition varied from 20 to 1100 °C.

3. Results and Discussion As follows from the results presented in Table 1, demineralization of raw coal with hydrochloric and hydrofluoric acids leads to a considerable decrease in the ash content. On the basis of the ash content in the raw and demineralized coal the effectiveness of demineralization was estimated as 86%. Despite so significant demineralization the results of elemental analysis prove that this process has no significant effect on the content of particular elements in the initial coal. The demineralized coal was subjected to pyrolysis at 500, 600, or 700 °C. According to Table 1, the temperature of pyrolysis had no significant influence on the ash content in the samples obtained, but with increasing temperature of pyrolysis the content of Cdaf increases, while the contents of Hdaf and Odaf (calculated from difference) decrease. The contents of nitrogen and sulfur in the chars obtained were not affected significantly by the pyrolysis temperature. As to the content of nitrogen, the results confirmed the thermal stability of the nitrogen groups originally present in the structure of the initial coal. The chars were subjected to activation by KOH. As indicated by Table 1 data, the activation of the chars (JMD5A, JMD6A, and JMD7A) with KOH aimed at removal of amorphous carbon formed in the process of pyrolysis, development of the surface area, and generation of new pores, leads to an increase in the content of carbon, a significant decrease in the content of

Table 3. Surface Area, Pore Volume, and Pore Diameter of Obtained Initial and Exhausted Samples surface area [m2/g]

pore volume [cm3/g]

sample

total surface area (BET)

micropore area

total pore volume

micropore volume

Vmic/Vt

average pore diameter [nm]

JMD5 JMD6 JMD7 JMD5A JMD6A JMD7A JMD5A-Ed JMD6A-Ed JMD7A-Ed JMD5A-Em JMD6A-Em JMD7A-Em

2 2 2 2634 2249 1856 2196 1945 1552 2099 1904 1324

2 2 1 2560 2212 1832 2153 1913 1532 2051 1878 1308

0.003 0.005 0.002 1.399 1.192 0.875 1.151 1.016 0.765 1.066 0.994 0.649

0.001 0.001 0.001 1.292 1.130 0.863 1.079 0.963 0.727 0.991 0.950 0.618

0.33 0.20 0.50 0.92 0.95 0.99 0.94 0.94 0.95 0.93 0.96 0.95

5.39 10.16 2.63 2.13 2.12 1.98 2.10 2.09 1.97 2.03 2.09 1.96

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Figure 1. NO2 breakthrough curves and NO concentration curves for chars studied in dry condition.

Figure 2. NO2 breakthrough curves and NO concentration curves for active carbons studied in dry condition.

et al.,41 who conducted the reaction of NO2 with commercial activated carbon at ambient temperature. On the basis of their studies they proved that NO2 reduction on the carbon surface leading to formation of NO, results in the surface oxidation and/ or the formation of molecular O2 coadsorbed with NO, which they proposed to describe by the following reaction: 4NO2 + 2C f 2NOgas + (2NO-O2)ads + 2C(O)

(1)

According to the authors, in the above reaction 50% of the consumed NO2 released as NOgas and the other 50% converts to NO adsorbed as (2NO-O2)ads on carbon surface. Figures 1 and 2 present the NO2 breakthrough curves and NO concentration curves recorded in dry conditions. The shapes of NO2 breakthrough curves presented in Figure 1 and very short experimental time indicate that the chars cannot be used as NO2 adsorbers. The shapes of the NO2 breakthrough curves for active carbons (Figure 2) differ from those of the NO2 breakthrough curves for chars, which means that the former show much better performance as NO2 adsorbents. The higher the temperature at which a given char was obtained the better the performance of the active carbon obtained from this char. The shapes of the curves obtained are different, which suggests different courses of the processes of adsorption on different adsorbers. On the basis of the shapes of the NO2 desorption curves it can be concluded that some NO2 is weakly adsorbed and is detected in the outlet gas when the source of NO2 is disconnected. It is most probably the reversibly physisorbed NO2, which at this

Table 4. NO2 Breakthrough Capacities in Dry Condition and Surface pH Values for the Initial and Exhausted Samples NO2 breakthrough capacity

pH

sample

mg/g of ads

mg/cm3 of ads

initial

exhausted

JMD5-Ed JMD6-Ed JMD7-Ed JMD5A-Ed JMD6A-Ed JMD7A-Ed

0.3 0.4 0.5 16.2 20.4 25.0

0.2 0.2 0.4 3.5 5.4 8.8

6.54 6.33 6.30 7.47 6.92 6.66

5.44 4.78 4.96 3.42 3.10 2.94

stage of the reaction is released from the carbon surface. This supposition is supported by Chugtai et al.,42 who studied the NO2 adsorption at 22 °C and proved that at ambient temperature only about 10% of NO2 is reversibly physisorbed, while the rest is chemisorbed. The calculated breakthrough capacities in dry condition and pH values for the initial and exhausted samples are collected in Table 4. As follows from these data, the NO2 breakthrough capacity of the chars is very low, which confirms that they cannot be used as NO2 adsorbents. However, even small amounts of the NO2 adsorbed lead to a significant decrease in the pH value of the exhausted samples relative to the values for the initial samples. The activation of the chars with KOH substantially increases their NO2 breakthrough capacity. The sorption abilities of the active carbon samples clearly improve with increasing temperature of pyrolysis of the initial material and with increasing initial acidity (pH) of the adsorbent. As a result of NO2 adsorption in dry condition, the pH of all samples



Figure 3. NO2 breakthrough curves and NO concentration curves for chars studied in wet condition.

Figure 4. NO2 breakthrough curves and NO concentration curves for active carbons studied in wet condition.

significantly decreases, which is most probably a consequence of NO2 adsorption and formation of nitric acid, despite the dry conditions of the process. Formation of nitric acid requires the presence of water, however, according to Lee et al.,43 in dry conditions the source of water molecules needed for HNO3 are the surface OH groups. Figures 3 and 4 present the NO2 breakthrough curves and NO concentration curves recorded in wet conditions. Similarly as for the process in dry conditions, in wet conditions the reaction of NO2 with char (Figure 3) leads to a quick increase in the concentration of NO2 in the outlet gas. This fact means that chars are unsuitable as adsorbents for NO2 removal both in dry and wet conditions. The shapes of the NO2 breakthrough curves for wet conditions (Figure 4) indicate that the best performance as NO2 adsorbent is shown by the active carbon obtained from the char treated at 700 °C (JMD7A). The shape of the NO2 breakthrough curve for this active carbon is much different from those of the other (43) Lee, M. R.; Allen, E. R.; Wolan, J. T.; Hoflund, G. B. Ind. Eng. Chem. Res. 1998, 37, 3375–3381. (44) Lozano-Castello, D.; Lillo-Rodenas, M. A.; Cazorla-Amoros, D.; Linares-Solano, A. Carbon 2001, 39, 741–749. (45) Lillo-Rodenas, M. A.; Lozano-Castello, D.; Cazorla-Amoros, D.; Linares-Solano, A. Carbon 2001, 39, 751–759. (46) Lillo-Rodenas, M. A.; Cazorla-Amoros, D.; Linares-Solano, A. Carbon 2003, 41, 267–275. (47) Macia´-Agullo´, J. A.; Moore, B. C.; Cazorla-Amoro´s, D.; LinaresSolano, A. Carbon 2004, 42, 1365–1369. (48) Pietrzak, R.; Bandosz, T. J. J. Hazard. Mater. 2008, 154, 946– 953.

Table 5. NO2 Breakthrough Capacities in Wet Condition and Surface pH Values for the Initial and Exhausted Samples NO2 breakthrough capacity

pH

sample

mg/g of ads

mg/cm3 of ads

initial

exhausted

JMD5-Em JMD6-Em JMD7-Em JMD5A-Em JMD6A-Em JMD7A-Em

0.4 0.6 0.7 23.7 23.5 43.5

0.2 0.5 0.7 5.0 7.8 15.7

6.54 6.33 6.30 7.47 6.92 6.66

5.26 4.89 4.77 2.87 2.88 2.34

curves, which points to a change in the mechanism of reactive adsorption in this sample. For each active carbon sample, at the beginning of the reaction the concentration of NO2 in the outlet gas rapidly increases, and then for samples JMD5A and JMD6A still systematically increases throughout the experiment, whereas for JMD7A at a certain moment the increase in the concentration of NO2 considerably slows down. This shape is most probably related to the surface chemistry being a consequence of the properties of this sample and the conditions of the adsorption process. The calculated breakthrough capacities in wet conditions presented in Table 5 confirm that chars are not suitable NO2 adsorbents. As follows from analysis of our results, samples JMD5A and JMD6A show almost the same adsorption capacities expressed in mg/g, whereas when the capacities are expressed in mg/cm3 sample JMD6A has a higher capacity. This observation is related to the physical properties of this sample, primarily with the bulk

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Table 6. Elemental Analysis of Active Carbons Exhausted Samples [wt %] coal

ashd

Cdaf

Hdaf

Ndaf

Sdaf

Odaf *

JMD5A-Ed JMD6A-Ed JMD7A-Ed JMD5A-Em JMD6A-Em JMD7A-Em

5.3 1.9 2.6 3.2 1.3 1.5

91.7 89.3 88.4 88.2 87.9 82.5

0.6 0.5 0.7 0.7 0.6 1.5

0.9 1.2 1.3 1.2 1.2 1.8

0.0 0.0 0.0 0.0 0.0 0.0

6.8 9.0 9.6 9.9 10.3 14.2

* By difference.

density of the active carbons. The bulk density of JMD5A is 0.21 g/cm3, whereas that of JMD6A is 0.35 g/cm3. Analysis of the data obtained in wet conditions reveals that JMD7A has almost twice greater NO2 breakthrough capacity expressed in mg/g than the other two samples. When the NO2 breakthrough capacity is expressed in mg/cm3 the difference is even greater: JMD7A capacity is over twice greater than that of JMD6A and over 3 times greater than that of JMD5A. A comparison of the NO2 breakthrough capacity in dry (Table 4) and wet conditions (Table 5) shows that the presence of water causes its increase. As a result of NO2 adsorption in wet conditions, the acidity of the materials increases relative to that of the materials adsorbing in dry condition (Table 4), which is related to the presence of water in the former systems favoring the formation of nitric acid.43 This acid is also formed upon adsorption in dry conditions, but then the reaction and amount of HNO3 is limited by the amount of the surface hydroxyl groups. Results of the elemental analysis of the exhausted active carbons are presented in Table 6. Each exhausted sample shows a decrease in the content of carbon and an increase in the content of nitrogen and oxygen relative to those in the initial samples (Table 1). As reported by Klose and Rincoń,16 who studied NO adsorption at 100-150 °C, this process takes place simultaneously with reduction and catalytic oxidation of NO and the adsorption of NO2 formed. In view of their results and those presented in Table 6, it is reasonable to expect that the adsorption of NO2 on active carbon occurs simultaneously with reduction and oxidation. The smallest differences in the elemental composition between the exhausted and initial samples are noted in the content of hydrogen. Only in JMD7A-Em is the content of hydrogen much greater than in JMD7A, whereas for all other samples the content of hydrogen before and after the adsorption is almost the same. This considerable increase in the content of hydrogen in JMD7A-Em is most probably a consequence of formation of much greater amounts of nitric acid than in the other samples. This supposition is confirmed by a substantial increase in the contents of nitrogen and oxygen in this sample relative to the initial sample and the highest acidity of JMD7A-Em (Table 5) of all exhausted samples. Formation of much greater amounts of nitric acid can also explain the shape of the NO2 breakthrough curve obtained for JMD7A-Em much different than those of the curves obtained for the other samples (Figure 4). Recorded for this sample, a small increase in the concentration of NO2 in the outlet gas should not be explained as a sudden increase in the NO2 sorption capacities of this adsorbent but as a consequence of HNO3 formation. As mentioned earlier, it is most probably related to the surface chemistry of JMD7A. As follows from the pH values of all exhausted samples, nitric acid is formed during each adsorption process in all samples, but only in JMD7A is the nonadsorbed NO2 not removed from the system but in almost total amount reacts with the water in the sample, which is reflected by a plateau at a certain section of the NO2 breakthrough curve.

On the basis of our results and those of the earlier studies11,16 the processes taking place on the surfaces of the active carbon samples upon NO2 adsorption taking into regard reaction 1 can be presented as: C + NO2 f C(NO2)

(2)

C(NO2) f C(O) + NO

(3)

C + NO f C(NO)

(4)

C(NO) f C(O) + N

(5)

2C + O2 f 2C(O)

(6)

C(O) + NO2 f C(ONO2)

(7)

C(O) + NO f C(NO2) f C + NO2

(8)

C(ONO2) f NO + CO2 + C

(9)

C(ONO2) f NO2 + CO + C

(10)

Moreover, in wet conditions the reaction of NO2 with water leads to formation of nitric acid:43 3NO2 + H2O f 2HNO3 + NO

(11)

or a mixture of nitric and nitrous acids: 2NO2 + H2O f HNO3 + HNO2

(12)

The structural parameters of all exhausted active carbon samples are presented in Table 3. After adsorption in dry conditions, the surface area and pore diameter decrease by 14-17%, whereas the pore volume decreases by 13-18%. For the samples exhausted in wet condition the decrease in the structural parameters is more pronounced; surface area decreases by 15-20% and pore diameter by 16-28%. The changes are caused by NO2 adsorption on the surface and by oxidation of the surface. Different surface chemistry of the initial and exhausted samples is seen on differential thermogravimetry (DTG) curves obtained in nitrogen and collected in Figure 5. The first peak at 60-90 °C, present in all DTG curves, corresponds to elimination of the adsorbed water. The DTG curves of the active carbons before adsorption do not show any other important peaks corresponding to the mass loss upon heating, which is a consequence of the applied conditions of pyrolysis and activation of the carbon samples. The small and very broad peak covering the range 500-900 °C on the curve of JMD5A and a similar one covering the range 600-900 °C on the curve of JMD6A can be assigned to the secondary degassing that takes place at about 700 °C and involves liberation of hydrogen and methane from the sample undergoing decomposition. As such a peak was not observed for JMD7A, and for the two other samples it started growing at the final temperatures of their pyrolysis, it is reasonable to suppose that the changes taking place in the original structure of the coals depend mainly on the process of pyrolysis, whereas the process of activation only induces development of the already shaped structure. The DTG curves of the samples after exposure to NO2 show a distinct peak at 90-180 °C, interpreted as corresponding to the removal of physically adsorbed NO2 and NO forming according to the reactions 2, 4, and 7. Another peak observed on the DTG curves of all exhausted samples at 200-400 °C



6KOH + C f 2K + 3H2 + 2K2CO3

(13)

4KOH + C f 4K + CO2 + 2H2O

(14)

4KOH + 2CO2 f 2K2CO3 + 2H2O

(15)

Moreover, as proved by Ottowa et al.,39 the carbon activation with KOH leads to formation of K2O but also K2CO3. In view of the above, the reactions taking place during the activation process can be described as: 4KOH + C f K2CO3 + K2O + 2H2

(16)

8KOH + 2C f 2K2CO3 + 2K2O + 4H2

(17)

Analysis of the above reactions implies with a high probability that the peak covering the range 200-400 °C should be assigned to the decomposition of potassium nitrates and nitrites forming during adsorption as a result of the reaction of NO2 with the oxides and carbonates formed. The formation of these compounds can occur according to the reactions: K2O + 3NO2 f 2KNO3 + NO

(18)

K2O + 2HNO3 f 2KNO3 + H2O

(19)

K2O + 2HNO2 f 2KNO2 + H2O

(20)

K2CO3 + 2HNO3 f 2KNO3 + CO2 + H2O

(21)

K2CO3 + 2HNO2 f 2KNO2 + CO2 + H2O

(22)

The formation of the above potassium nitrates and nitrites is confirmed by the content of ashes in all exhausted samples (Table 6) relative to their content in the initial activated carbons (Table 1). Analysis of the results obtained should be performed taking into account that, although the content of potassium in the activated samples is probably not large because the materials have been washed with HCl, the amount of potassium left in the samples (mainly in the form of K2O and K2CO3) proved enough for formation of nitrates in the above-mentioned reactions during the process of adsorption. Taking into regard the above, this fact confirms that the peak at 200-400 °C can be assigned to decomposition of potassium nitrates and nitrites forming in the process of adsorption. Moreover, according to Pietrzak and Bandosz38,48 an increase in the efficiency of NO2 removal can be related to both formation of nitric and nitrous acid as a result of NO2 oxidation and reactions of NOx with water and formation of hydroxides from metal oxides as a result of exposure to water and formation of water film. 4. Conclusions

Figure 5. DTG curve in helium obtained for the initial and exhausted samples.

should be assigned to the liberation of low-thermally stable nitrogen species. According to Linares-Solano at el.44-47 the activation of carbon with KOH involves the following reactions:

Analysis of the above presented and discussed results has shown that the chars obtained from bituminous coal show unsatisfactory performance as adsorbents of NO2 both in dry and wet conditions. The activation of the chars with KOH significantly increases their NO2 breakthrough capacity, which means that the active carbons obtained can be used as efficient adsorbents for NO2 removal. The sorptive abilities of the active carbon samples increase with increasing temperature of the initial material pyrolysis and with increasing acidity of the initial pH adsorbent. Analysis of the NO2 breakthrough capacity obtained in dry and wet conditions has proved that the presence of water increases this parameter. The best NO2 sorption

3624


capacity in both dry and wet conditions was obtained for the active carbon pyrolyzed at 700 °C, with respective values of 25.0 and 43.5 mg/g, respectively. The DTG curves of the samples after exposure to NO2 show two new peaks, the first at 90-180 °C, corresponding to the removal of physically adsorbed NO2 and NO, and the second at 200-400 °C, corresponding to the decomposition of potassium nitrates and nitrites forming

Pietrzak

upon adsorption as a result of the reaction of NO2 with oxides and carbonates formed during the activation process. Acknowledgment. This work was supported by The Polish Ministry of Science and Higher Education project No. N N204 056235. EF9002796

Active Carbons Obtained from Bituminous Coal for NO2 Removal

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