Bituminous coal-based activated carbons modified with nitrogen as adsorbents of hydrogen sulfide

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1 Carbon 42 (2004) Bituminous coal-based activated carbons modified with nitrogen as adsorbents of hydrogen sulfide Andrey Bagreev a, J. Angel Menendez b, Irina Dukhno c, Yuriy Tarasenko c, Teresa J. Bandosz a, * a Department of Chemistry, The City College of New York, The Graduate School of the City University of New York, New York, NY 10031, USA b Instituto Nacional del Carbon (INCAR) C.S.I.C., Apartado Oviedo, Spain c Institute for Sorption and Problems of Endoecology, Kiev, Ukraine Received 10 April 2003; accepted 13 October 2003 Abstract Bituminous coal-based activated carbon was modified by impregnation with melamine and heat treatment at 850 C. Another sample was impregnated with melamine and urea and heat treated at 650 and 850 C. Chemical and physical properties of the materials were determined using Boehm titration, thermal analysis, sorption of nitrogen and SEM with EDX. Then the H 2 S breakthrough capacity tests were carried out and the sorption capacity was calculated. The results revealed that carbons modified with nitrogen-containing species and heat-treated at 850 C have a hydrogen sulfide removal capacity exceeding more then 10 times the capacity of unmodified sample. H 2 S on the surface of these materials is oxidized to sulfuric acid and elemental sulfur and stored in the pore system. New carbons are hypothesized to act as catalytic reactors promoting two different pathways of hydrogen sulfide oxidation in two different locations. In small micropores, where water is present, hydrogen sulfide dissociate to HS ions and those ions are oxidized to sulfur radicals and sulfur dioxide leading to the formation of sulfuric acid. In larger pores with incorporated nitrogen, basic sites promote dissociation and formation of sulfur polymers, which are resistant to further oxidation. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: A. Activated carbon; C. Adsorption; Thermal analysis; D. Catalytic properties 1. Introduction * Corresponding author. Tel.: ; fax: address: tbandosz@scisun.sci.ccny.cuny.edu (T.J. Bandosz). The specific application of activated carbons as adsorbents of gases and vapors depends on the properties of molecules to be removed/adsorbed. For only physical adsorption the sizes and volumes of pores are important [1 3] whereas for specific adsorption, surface reactions and chemisorption surface chemistry plays a significant role [3 6]. The surface chemistry of activated carbon is preliminary the result of the presence of heteroatoms such as oxygen, hydrogen, nitrogen and phosphorus coming from the precursor or activation agent used [1]. They form functional groups, acidic, basic or neutral, analogous to those classified in organic chemistry [4 7]. Delocalized p electrons of aromatic rings and unsaturated valences also contribute to the basicity of carbonaceous adsorbents [3,8]. Very often adsorbate adsorbent interactions or catalytic properties of activated carbon surfaces are enhanced by additional modifications as, for instance, impregnation [9]. As impregnants either oxidants such as potassium permanganate or basic compounds, such as sodium or potassium hydroxide, are used. Depending on the chemical properties of the molecule to be removed, such impregnants convert the pollutants to harmless species and/or enhance their adsorption. A well known example are whetlerites [10], carbons impregnated with caustics used for adsorption of acidic gases such as H 2 S [11,12] or carbons impregnated with KMnO 4 and KIO 3 which contribute to the oxidation process [13]. Another modification is based on the alteration of surface chemistry of the carbon matrix and incorporation of heteroatoms such as oxygen [14], nitrogen [15], phosphorus [16] or chlorine [17]. These /$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi: /j.carbon

2 470 A. Bagreev et al. / Carbon 42 (2004) heteroatoms not only change the acidic/basic character of carbon but also contribute to changes in the electrochemical and catalytical properties of the matrix [3,15,18]. The results published in the literature demonstrate the effect of nitrogen incorporation on the adsorptive properties of activated carbons [15,19 25]. Nitrogen within the carbon matrix can cause an increase in the number of basic groups and changes the charge distribution within graphene layers [18,21]. It was shown that its presence enhances the process of hydrogen sulfide [19] and sulfur dioxide removal [20]. In fact, one of the best commercial catalytic carbons on the market, Centaur â, especially designed to adsorb H 2 S, is obtained by urea modification of low temperature char followed by heat treatment [24,25]. As proposed elsewhere [19] this process leads to the high dispersion of basic nitrogen centers (quaternary nitrogen) in small pores [18], which is important for formation of sulfur radicals and their oxidation to sulfuric acid. It is generally assumed that to have efficient nitrogen incorporation at low temperature, a char susceptible to chemical changes in aromatic rings has to be used [19,25]. The objective of this paper is to investigate the effect of modification of bituminous coal-based carbons with nitrogen-containing species such as melamine and urea and its effect on the removal of hydrogen sulfide. In a previous study the catalytic effect of urea modification on adsorption and oxidation of hydrogen sulfide by low temperature chars [25] or wood-based carbons was shown [19]. On those materials H 2 S was oxidized mainly to water-soluble sulfur species (sulfur dioxide and sulfuric acid) as a result of reaction of HS ions with chemisorbed O 2 on basic nitrogen centers located likely in small pores. Activated carbon used in this study is a bituminous coal-based carbon obtained at 900 C. Incorporation of heteroatoms can be inhibited on such materials due to their high degree of aromatization (atomic ratio H/C equals to 0.11). Melamine precursor contains more weight percent of nitrogen in its structure than urea. Moreover the chemical states of N atoms are different; in urea this is only amine-type nitrogen, whereas in melamine the heterocyclic nitrogen is also present. Choosing these materials we expect efficient incorporation of nitrogen at the edges of carbon crystallites where it is thermodynamically favorable, which may result in a high hydrogen sulfide capacity combined with high oxidizing power. This is an important goal since the H 2 S breakthrough capacity of the above mentioned Centaur â is not high compared to caustic impregnated carbons [9,11,19]. The advantage of Centaur â lies in the possibility of its water regeneration. This results in the removal of sulfuric acid which is the product of surface reaction [25]. Easy regeneration of spent materials makes the process of hydrogen sulfide removal more feasible. 2. Experimental 2.1. Materials The initial activated carbon (SBC) was obtained by activation of a bituminous coal. The coal was first oxidized in air at 200 C for 24 h. Then it was pyrolyzed in N 2 (150 ml/min) at 900 C for 1 h. Then the temperature was reduced to 850 C and the N 2 flow switched to CO 2 (150 ml/min). This activation process lasted for 16.5 h with a 51.6 wt.% burnoff. Pyrolysis and activation were carried out in a laboratory furnace, which holds a stainless steel reactor, of ca. 100 g capacity, designed to ensure homogeneous gas flow through the sample. Before modification with nitrogen-containing species, SBC was oxidized with 50% HNO 3 for 4 h and then washed out with water to remove excess acid and watersoluble products of oxidation. A 30 g sample of oxidized carbon was mixed with a melamine suspension (20 g of melamine in 100 ml of 80% ethanol) and stirred at room temperature for 5 h. Then the mixture was boiled to evaporation of alcohol and water and the carbon sample was dried at 100 C. The sample impregnated with melamine was carbonized in argon at 10 C/min to 850 C and held at this temperature for 30 min. Then the sample was washed with boiling water and dried to constant weight. After this treatment the sample is referred to as SBC-M1. Another sample of the initial carbon was impregnated with melamine and urea. The preparation procedure was practically the same as for the melamine-modified counterpart. The carbon sample was impregnated with a suspension of 30 g of melamine and 10 g of urea in 80% alcohol. We refer to this sample as SBC-M2. To check the effect of temperature, another sample was prepared using heating at 650 C. This sample is designated as SMC-M3. After modification the samples were water-washed to remove excess decomposition products. The exhausted carbons after the hydrogen sulfide breakthrough tests are designated by an additional letter, E, in their name Methods H 2 S breakthrough capacity Dynamic tests were carried out to evaluate the capacity of carbons for H 2 S removal. Moist air (relative humidity 80% at 25 C) containing 0.3% (3000 ppm) H 2 S was passed through a column of carbon (length 370 mm, diameter 9 mm) at 0.5 l/min. The experiments were carried out at room temperature. The H 2 S evolution was monitored by an InterScan LD-17 H 2 S continuous monitor system interfaced with a computer data acquisition program. The test was stopped at the breakthrough concentration of 500 ppm. The adsorption capacities of each carbon were calculated by integration of the area above the breakthrough curves, and from the

3 A. Bagreev et al. / Carbon 42 (2004) H 2 S concentration in the inlet gas, flow rate, breakthrough time, and mass of carbon Boehm titration The acidic and basic surface groups were determined according to the method of Boehm [6]. One gram of carbon sample was placed in 50 ml of 0.05 N solutions of either sodium hydroxide or hydrochloric acid. The vials were sealed and shaken for 24 h and then 5 ml of each filtrate was pipetted and the excess of base or acid was titrated with HCl or NaOH, as required. The numbers of acidic sites of various types were calculated under the assumption that NaOH neutralizes all acidic groups (carboxylic, phenolic and lactonic groups) and HCl reacts with all basic groups ph of carbon surface A sample of 0.4 g of dry carbon powder was added to 20 ml of water and the suspension was stirred overnight to reach equilibrium. Then the ph of suspension was measured Elemental analyses A LECO CNHS-932 instrument was used. A 1 mg sample was burned at 1005 C and the released gases were analyzed. Taking into account the much higher content of N and S in some of the modified carbons compared to other non-treated activated carbons, sulfamethazine was used as a calibration standard. The content of oxygen was measured directly using a LECO VTF900 apparatus operated at 1350 C. This temperature was chosen to completely desorb CO 2 and CO from the carbon surface. Benzoic acid was used as a calibration standard Thermal analysis In order to evaluate either the species present on the initial carbon surface or surface reaction products, thermal analysis was carried out using a TA Instruments Thermal Analyzer. The instrument settings were: heating rate, 10 C/min; oxygen or nitrogen atmosphere at 100 ml/min. To evaluate the ash content, the experiments were done in air SEM and EDX A scanning electron microscope DSM 942 from Zeiss with an EDX detector (OXFORD LINK-ISIS) was used to study the texture and chemical elements present on the surface of activated carbons Sorption of nitrogen Nitrogen isotherms were measured using a ASAP 2010 apparatus (Micromeritics) at )196 C. Before the experiment the samples were heated at 120 C and then outgassed at this temperature to 10 5 Torr. The isotherms were used to calculate the specific surface area, S N2, micropore volume, V mic, and total pore volume, V t. All of the above parameters were calculated using the density functional theory (DFT) [26,27]. The Dubinin Radushkevich equation was used to calculate the characteristic energy of adsorption, E 0 [28]. 3. Results and discussion As expected, modifications with melamine (SBC-M1) and melamine plus urea (SBC-M2,SBC-M3) resulted in a significant increase in the nitrogen content (Table 1). High content in SBC-M3 is likely due to the presence of melamine and urea decomposition products. The relatively low temperature of 650 C is likely not sufficient for complete decomposition of nitrogen-containing organic compounds in the presence of carbon [19,29] which was confirmed by thermal analysis. This temperature was first selected by assuming that melamine decomposition should occur at 345 C as indicated elsewhere [29] for a pure compound. It is also worth to mention that the initial SBC carbon, owing to its origin and relatively high carbonization temperature, is expected to have high degree of aromatization and carbon content above 95%. As seen from Table 1, the SBC carbon has a very low H/C ratio ( by weight and 0.11 and atomic ratio). An increased oxygen content after modification has its origin in the nitric acid oxidation step during carbon modification procedure. Introduction of nitrogen did resulted in changes in the surface chemistry of carbons, as shown in Table 2. In spite of nitrogen incorporation, the ph of the carbon surface decreased almost one ph unit. As mentioned above, oxidation of carbon before modification with nitrogen compounds also contributes to this change. Table 1 Elemental analysis of carbon samples and ash content [%] Sample C H O a N S Ash SBC SBC-M SBC-M SMC-M SBCE SBC-M1E SBC-M2E SBC-M3E a Oxygen is determined by a direct analysis. Table 2 ph and surface chemistry from Boehm titration (number of groups in meq/g) Sample ph Acidic Basic All Basic/all SBC SBC-M SBC-M

4 472 A. Bagreev et al. / Carbon 42 (2004) The more detailed changes in surface chemistry are seen from Boehm titration results (Table 2) [6]. We report here only acidic and basic groups, without presenting the distinctions for carboxylic, lactonic or phenolic groups. This is done taking into account the fact that nitrogen-containing groups can have similar pk a to oxygen-based groups [19,23,30]. After modifications with nitrogen, the number of basic groups significantly increased, as expected. This increase is greatest for SBC- M2 and is twice the value obtained for SBC-M1. (We do not discuss here the chemistry of SBC-M3 due it is complex structure related to the low temperature during heat treatment.) This increase can be linked to the presence of basic oxygen functional groups such as pyrones or chromenes [6] since the amount of nitrogen in both samples is similar and the content of oxygen is higher for SBC-M2 than for SBC-M1. The average effect of the amount and strength of basic groups reflects on the surface ph values. While for the modified samples the ph is in agreement with the average basicity (ratio of the number of basic groups to total number of groups), the initial sample is an exception. Its relatively high ph can be linked to the presence of weak acids and /or to the contribution of ash to the ph of water suspension. In the case of modified samples, their oxidation with nitric acid resulted in the removal of water-soluble nitrates of metals present in the mineral matter of the original sample. As seen from Table 1, after the treatments applied the ash content decreased twice. One has to be aware that the surface ph represents the average apparent surface acidity and a direct link with the number of acidic or basic groups determined from Boehm titration is difficult to establish. This is because those groups are thought to contribute to acidity with both their number and their strength (represented by acidity constants). Modifications with nitrogen-containing species may also result in changes in the porous structure. The nitrogen adsorption isotherms are presented in Fig. 1. Their shapes confirm the microporous nature of the adsorbents. As expected, for SBC-M3 the sorption capacity significantly decreased after modification, indicating pore blocking by product of melamine and/or urea partial decomposition. From the nitrogen isotherms the textural parameters were calculated. They are summarized in Table 3. Carbon SBC shows the typical characteristics of a bituminous coal-based carbon [10] with surface area of about 850 m 2 /g and pore volume of about 0.4 cm 3 /g. After modification and high-temperature treatment about 25% decrease in textural parameters is noticed. Taking into account the fact that the temperature of heat treatment was lower than the carbonization temperature, it is possible that the thermodynamically favorable reaction is incorporation of nitrogen species to the edges of graphene layers. Such species can create steric hindrances and partially block Fig. 1. Nitrogen adsorption isotherms for the carbons studied. Closed symbols represent the desorption branch of the isotherm. Table 3 Textural parameters calculated from sorption of nitrogen Sample S (DFT) [m 2 /g] V mic (DFT) [cm 3 /g] V mic =V t L mic [ A] E 0 [kj/mol] SBC SBC-M SBC-M SMC-M SBCE SBC-M1E SBC-M2E SBC-M3E the access of nitrogen molecule to some small pores [31,32]. Incorporation of nitrogen in SBC-M1 and SBC- M2 is seen as a slight increase in the value of specific adsorption energy, E 0, and a slight decrease in the average size of micropores, L mic, especially for the sample modified with melamine only. Fig. 2 shows that for the SBC sample all pores are smaller than 30 A and a significant volume is in pores smaller than 10 A. After modification with melamine Fig. 2. Pore size distributions for the initial and modified samples.

5 A. Bagreev et al. / Carbon 42 (2004) (M1) and melamine and urea (M2) and temperature treatment at 850 C the volume of pores larger than 10 A decreases while the volume of small micropores (<10 A) is practically unchanged. If even incorporation of nitrogen is assumed, it is likely that some previously existing small pores are blocked and the larger pores present in SBC decreased in size and became smaller than 10 A thus resulting in the observed changes. A lack of change in the volume of small pores may also suggest that nitrogen-containing species probably are not present there and that nitrogen is present in larger micropores where either melemine or melamine plus urea were adsorbed in a sufficient amount during the pretreatment (critical size of melamine molecule is about 7 A and urea molecule about 4 A). The H 2 S breakthrough results are collected in Fig. 3 and Table 4. For SBC-M1 and SBC-M2 a significant enhancement in the performance was found. The capacity of high-temperature treated samples is 250% higher than that of Centaur â carbon [19]. For all exhausted samples the ph becomes acidic after H 2 S adsorption indicating oxidation of hydrogen sulfide to sulfuric acid. The conversion to sulfuric acid has to be important for SBC and SBC-M3 since the ph decreased 5 units in spite of the small capacity. After nitrogen modification the carbons become more hydrophilic as reflected in the amount of adsorbed water during prehumidification [18]. The effect of hydrogen sulfide adsorption on porosity is presented in Fig. 4 and Table 3. There is a significant decrease in the volume of all pores. In the case of SBC- M1E and SBC-M2E the volume of micropores decreased four-fold as a result of the accumulation of a large amount of sulfur-containing species (high H 2 S breakthrough capacity). To identify the products of H 2 S adsorption/oxidation, thermal analysis in nitrogen was performed. The comparison of the DTG curves for initial and exhausted samples is presented in Fig. 5. While the curves for the initial samples are almost featureless (except for a peak representing decomposition in SBC-M3) the exhausted samples have peaks at about 250 C (SBCE, SBC-M1E, and SBC-M2E) and between 350 and 500 C (SBC- M1E, SBC-M2E, and SBC-M3E). The peak at 100 C represents desorption of water [33 36]. Following the temperatures of desorption of various sulfur species [34 38] we assign the first peak at 250 C to desorption of SO 2 and SO 3 and decomposition of sulfuric acid and the second one to the removal of elemental sulfur. As suggested elsewhere [34 36], the presence of very small pores enhances oxidation of hydrogen sulfide to SO 2 and sulfuric acid. In those pores bulky sulfur crystals cannot be formed due to the steric effect and small sulfur clusters are easily oxidized by adsorbed oxygen. The highest activity for sulfuric acid formation on SBC-M1 is also reflected on its highest content of oxygen after H 2 S adsorption (see Table 1). The location of the first peak in Fig. 5 is identical for all three carbons having high volume of pores smaller than 10 A (SBC, SBC-M1 and SBC-M2). Differences in the intensity must be related to differences in the catalytic activities of these carbons toward sulfuric acid formation. For unmodified carbon the activity is the lowest while for carbons treated with melamine it is the highest. As argued elsewhere [19], both the amount of basic centers and their high dispersion in small pores are responsible for high activity and H 2 SO 4 formation. In the case of SBC only the size of pores plays a role since Fig. 3. H 2 S breakthrough curves. Table 4 ph of exhausted carbons, their H 2 S breakthrough capacity and the amount of water preadsorbed on the carbon surface Sample H 2 S breakthrough capacity [mg/g] Water preadsorbed a [mg/g] ph E SBCE SBC-M1E SBC-M2E SBC-M3E a During prehumidification. Fig. 4. Pore size distributions for exhausted samples.

6 474 A. Bagreev et al. / Carbon 42 (2004) Normalized Intensity (%) Normalized Intensity (%) Normalized Intensity (%) SBC SBC-E Energy [kev] SBC-M1 SBC-M1E Energy [kev] C C O C O N O AL AL Si Si S S S SBC-M3 SBC-M3E Energy [kev] Fig. 6. Results of EDX study of initial and exhausted carbon samples. Fig. 5. DTG curves in nitrogen for the samples before (A) and after (B) the H 2 S breakthrough tests. nitrogen-containing groups were not introduced. For nitrogen-modified carbons, an additional factor, nitrogen-containing active centers, should be considered. The highest activity in the case of SBC-M1 is likely related to smallest sizes of pores (Table 3) where sulfuric acid can be formed from small sulfur clusters. Their high surface area makes the oxidation reaction more feasible than in the case of bulky crystals. Indeed, for this samples the biggest ph decrease was found after H 2 S adsorption (Table 4). For both samples SBC-M1 and SBC-M2 elemental sulfur is also a major product. The high-temperature Table 6 Relative proportions of the EDX signal of the elements (referred to the C) Sample O/C S/C N/C SBC SBCE SBC-M SBC-M1 E SBC-M SBC-M3E peaks almost overlap. Elemental sulfur is more likely to be present in larger pores where bulk polymers can exist as a result of polysulfide formation [39,40]. The distributions of those pores are identical for both samples. Table 5 Balance of sulfur species for exhausted samples (Dw weight loss [%], S TA amount of sulfur calculated from TA [%]; S Bth: cap amount of sulfur calculated from H 2 S breakthrough capacity [%]) Sample Dw C Dw C Dw C Dw C Dw Total S TA S Bth: cap SBCE SBC-M1E SBC-M2E SBC-M3E S TA is calculated as S TA ¼ S e1 þ SO 2 M S =M SO2, where S e1 is the content of elemental sulfur; SO 2 is the content of sulfur dioxide; M S and M SO2 are molecular weights of sulfur and sulfur dioxide. SO 2 and S e1 are calculated from the weight losses, Dw, corresponding to the temperature ranges and C, respectively, and normalized to the initial weight of sample.

7 A. Bagreev et al. / Carbon 42 (2004) The balance of sulfur species present on the surface was calculated taking into account the above mentioned assignment of surface reaction products and it is presented in Table 5. The sulfur contents are in agreement with the breakthrough capacity results and with elemental analysis data. SEM images did not reveal differences in surface morphology before and after H 2 S adsorption. Since no sulfur crystals are seen, the about 25% of sulfur in samples SBC-M1E and SBC-M2E is likely deposited inside the micropores, as indicated from the analysis of the porosity. The results of EDX analysis are collected in Fig. 6. Since SBC-M1 and SBC-M2 gave similar results, only SBC-M1 was tested. The intensity of the signals of all elements was normalized to the intensity of carbon signal. The ratios of the intensities of oxygen, sulfur and nitrogen to carbon are listed in Table 6. They represent relative concentrations of those elements. The signal for nitrogen is seen only for the SBC-M3 sample and is related to an exceptionally high nitrogen concentration. (When working with carbons the N signal is usually hidden by the C signal.) For exhausted samples after H 2 S adsorption a significant increase in sulfur signal intensity is observed, in agreement with the increased sulfur content. 4. Conclusions Modification of bituminous coal-based activated carbon with melamine and melamine with urea followed by heat treatment at 850 C results in very efficient adsorbents for H 2 S removal. Their performance is 250% better than that of Centaur â carbon, which is obtained using urea modification of a low temperature char (obtained at 600 C). The unique feature of materials studied here is the relatively high content of nitrogen presumably highly dispersed in the microporous system (in pores between 10 and 30 A). In such pores oxidation of a notable amount of hydrogen sulfide to elemental sulfur is possible owing to the formation of bulky crystals which are resistant to further oxidation. A significant enhancement in the performance of bituminous-coal based carbon after modification with nitrogen-containing species proves that incorporation of active nitrogen groups can be successful in the case of high-temperature carbons (obtained at T > 850 C). Acknowledgements This research is supported by NATO Collaborative Linkage grant NATO EST CLG References [1] Bansal RC, Donnet JB, Stoeckli F. Active carbon. New York: Marcel Dekker; [2] Gregg SJ, Sing KSW. Adsorption, surface area, and porosity. New York: Academic Press; [3] Leon y Leon CA, Radovic LR. Interfacial chemistry and electrochemistry of carbon surfaces. In: Thrower PA, editor. Chemistry and physics of carbon, vol. 24. New York: M. Dekker; p [4] Puri BR. In: Walker Jr PJ, editor. Chemistry and physics of carbon, vol. 6. New York: M. Dekker; p [5] Boehm HP. Some aspects of the surface chemistry of carbon blacks and other carbons. Carbon 1994;32: [6] Boehm HP. Chemical identification of surface groups. In: Advances in catalysis, vol. 16. New York: Academic Press; p [7] Fanning PE, Vannice MA. A DRIFT study of the formation of surface groups on carbon by oxidation. Carbon 1993;31: [8] Montes-Moran MA, Menendez JA, Fuente E, Suarez D. On the contribution of the basal planes to carbon basicity: an ab-initio study of the H 3 O þ p interaction in cluster models. J Phys Chem B 1998;102: [9] Bagreev A, Bandosz TJ. A role of sodium hydroxide in the process of hydrogen sulfide adsorption/oxidation on caustic-impregnated activated carbons. Ind Eng Chem Res 2002;41: [10] Jankowska H, Swiatkowski A, Choma J. Active carbon. New York: Ellis Horwood; [11] Turk A, Sakalis E, Rago O, Karamitsos H. Activated carbon systems for removal of light gases. Ann NY Acad Sci 1992;66: [12] Turk A, Mahmood K, Mozaffari J. Activated carbon for air purification in New York City s sewage treatment plants. Water Sci Technol 1993;27: [13] Shin CS, Kim KH, Ryu SK. Adsorption of methyl mercaptan and hydrogen sulfide on the impregnated activated carbon fibre and activated carbon. Presented at 7th International Conference on Fundamentals of Adsorption, Nagasaki, Japan May [14] Bandosz TJ, Jagiello J, Schwarz JA, Krzyzanowski A. Effect of surface chemistry on sorption of water and methanol on activated carbons. Langmuir 1996;12: [15] Stohr B, Boehm HP, Schlogl R. Enhancement of the catalytic activity of activated carbons in oxidation reactions by the thermal treatment with ammonia or hydrogen cyanide and observation of a superoxide species as a possible intermediate. Carbon 1991;29:707 20; Matzner R, Boehm HP. Influence of nitrogen doping on the adsorption and reduction of nitric oxide by activated carbons. Carbon 1998;36: [16] Bourbigot S, Le Bras M, Delobel R. Carbonization mechanisms resulting from intumescence association with the ammonium polyphosphate-pentaerythritol fire retardant system. Carbon 1993;31: [17] Cadenas-Perez AF, Maldonado-Hodar FJ, Moreno-Castilla C. On the nature of surface acid sites of chlorinated activated carbons. Carbon 2003;41: [18] Lahaye J, Nanse G, Bagreev A, Strelko V. Porous structure and surface chemistry of nitrogen containing carbon from polymers. Carbon 1999;37: [19] Adib F, Bagreev A, Bandosz TJ. Adsorption/oxidation of hydrogen sulfide on nitrogen containing activated carbons. Langmuir 2000;16: [20] Bagreev A, Bashkova S, Bandosz TJ. Adsorption of SO 2 on activated carbons: the effect of nitrogen functionality and pore sizes. Langmuir 2002;18: [21] Pels JR, Kapteijn F, Moulijn JA, Zhu Q, Thomas KM. Evolution of nitrogen functionalities in the carbonaceous materials during pyrolysis. Carbon 1995;33:

8 476 A. Bagreev et al. / Carbon 42 (2004) [22] Mochida I, Korai Y, Shirahama M, Kawano S, Hada T, Seo Y, Yoshikawa M, Yasutake A. Removal of SO x and NO x over activated carbon fibers. Carbon 2000;38: [23] Elsayed Y, Bandosz TJ. Acetaldehyde adsorption on nitrogencontaining activated carbons. Langmuir 2002;18: [24] Hayden RA. Method for reactivating nitrogen-treated carbon catalysts. US patent 5,466,645 (1995). [25] Matviya TM, Hayden RA. Catalytic carbon; US patent 5,356,849 (1994). [26] Olivier JP. Modeling physical adsorption on porous and nonporous solids using density functional theory. J Porous Mater 1995;2:9 17. [27] Lastoskie CM, Gubbins KE, Quirke N. Pore size distribution analysis of microporous carbons: a density functional theory approach. J Phys Chem 1993;97: [28] Dubinin MM. Porous structure and adsorption properties of active carbons. In: Walker PL, editor. Chemistry and physics of carbon, vol. 2. New York: M. Dekker; p [29] Weast RC, editor. Handbook of chemistry and physics. 67th ed. Boca Roton, FL: CRC Press; [30] Kortum G, Vogel W, Andrussov K. Dissociation constants of organic acids in aqueous solution. London: Butterworth; [31] Kyotani T. Control of pore structure in carbon. Carbon 2000;38: [32] Raymundo-Pinero E, Cazorla-Amoros D, Linares-Solano A. The role of different nitrogen functional groups in the removal of SO 2 from the gases by N-doped activated carbon powders and fibres. Carbon 2003;41: [33] Bandosz TJ. Effect of pore structure and surface chemistry of virgin activated carbons on removal of hydrogen sulfide. Carbon 1999;37: [34] Adib F, Bagreev A, Bandosz TJ. Effect of surface characteristics of wood-based activated carbons on adsorption of hydrogen sulfide. J Coll Interf Sci 1999;214: [35] Adib F, Bagreev A, Bandosz TJ. Effect of ph and surface chemistry on the mechanism of H 2 S removal by activated carbons. J Coll Interf Sci 1999;216: [36] Adib F, Bagreev A, Bandosz TJ. Analysis of the relationship between H 2 S removal capacity and surface properties of unimpregnated activated carbons. Environ Sci Technol 2000;34: [37] Rodriguez-Mirasol J, Cordero T, Rodriguez JJ. Effect of oxygen on the adsorption of SO 2 on activated carbon. Extended Abstracts of 23rd Biennal Conference on Carbon, Penn State University, July 1997, pp [38] Chang CH. Preparation and characterization of carbon sulfur surface compounds. Carbon 1981;19: [39] Stejns M, Derks F, Verloop A, Mars P. The mechanism of the catalytic oxidation of hydrogen sulphide. II. Kinetics and mechanism of hydrogen sulphide oxidation catalyzed by sulfur. J Catal 1976;42: [40] Stejns M, Mars P. Catalytic oxidation of hydrogen sulphide. Influence of pore structure and chemical composition of various porous substances. Ind Eng Chem Prod Res Dev 1977;16: