Adsorption of SO2 on Activated Carbons: The Effect of Nitrogen

Jan 23, 2002 - Activated carbons of different origins were studied as sulfur dioxide adsorbents. The materials were characterized using adsorption of ...
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Adsorption of SO2 on Activated Carbons: The Effect of Nitrogen Functionality and Pore Sizes Andrey Bagreev,† Svetlana Bashkova, and Teresa J. Bandosz* Department of Chemistry and The International Center for Water Resources and Environmental Research of The City College of New York, New York, New York 10031 Received August 17, 2001 Activated carbons of different origins were studied as sulfur dioxide adsorbents. The materials were characterized using adsorption of nitrogen, titration, X-ray photoelectron spectroscopy, and thermal analysis. The investigation was focused on the role of nitrogen functionality and pore sizes in the process of SO2 adsorption/oxidation. The results showed that quaternary and pyridinic type nitrogen significantly enhance the adsorption capacity. It happens when catalytic centers are located in the small pores, which is likely to help in achieving high dispersion of these centers. Besides an oxidation effect due to the formation of active oxygen radicals, the nitrogen-containing centers attract the SO42- ions causing the gradual pore filling which is the most effective usage of the carbon pore space.

Introduction Increase in the acidity of natural waters, fast rate of abrasion of buildings and monuments, and associated health problems caused that desulfurization of fossil fuels along with removal of SO2 from stock gases are the technologies which have been developing rapidly during the last 20 years. One of the materials used to remove sulfur dioxide is activated carbon.1-19 Another popular carbonaceous sorbent used for this purpose is activated carbon fiber.7,13,17 Numerous papers published in the literature indicate good efficiency of SO2 removal on these materials at either low or high temperatures.1-17 * To whom correspondence should be addressed. E-mail: [email protected]. Tel: (212) 650-6017. Fax: (212) 650-6107. † Permanent address: Institute for Sorption and Problems of Endoecology, Ukraine. (1) Kohl, A.; Riesenfeld, F. Gas Purification, 4th ed.; Gulf Publishing Company: Houston, TX, 1985. (2) Stirling, D. The Sulfur Problem: Cleaning up Industrial Feedstocks; The Royal Society of Chemistry: Cambridge, U.K., 2000. (3) Rodriguez-Mirasol, J.; Cordero, T., Rodriguez, J. J. Abstract of 23rd Biennial Conference on Carbon, 18-23 July 1997, College Park, PA, p 376. (4) Moreno-Castilla, C.; Carrasco-Marin, F.; Utrera-Hidalgo, E.; Rivera-Utrilla, J. Langmuir 1993, 9, 1378. (5) Lisovskii, A.; Shter, G. E.; Semiat, R.; Aharoni, C. Carbon 1997, 35, 1645. (6) Davini, P. Carbon 2001, 39, 1387. (7) Mochida, I.; Miyamoto, S.; Kuroda, K.; Kawano, S.; Yatsunami, S.; Korai, Y.; Yatsutake, A.; Yashikawa, M. Energy Fuels 1999, 13, 369. (8) Davini, P. Carbon 1990, 28, 565. (9) Lisovskii, A.; Semiat, R.; Aharoni, C. Carbon 1997, 35, 1639. (10) Anurov, C. A.; Keltsev, N. V.; Smola, V. I. Usp. Khim. (Russ. Chem. Rev.) 1977, 46, 33. (11) Raymundo-Pin˜ero, E.; Cazola-Amoro´s, D.; Salinas-Martinez de Lecea, C.; Linares-Solano, A. Carbon 2000, 38, 335. (12) Molina-Sabio, M.; Mun˜ecas, Rodriguez-Reinoso, F.; McEnaney, B. Carbon 1995, 33, 1777. (13) Daley, M. A.; Mangun, C. L.; DeBarr, J. A.; Riha, S.; Lizzio, A. A.; Donnals, G. L.; Economy, J. Carbon 1997, 35, 411. (14) Lizzio, A. A.; DeBarr, J. A. Energy Fuels 1997, 11, 284. (15) Rubio, B.; Izquierdo, M. T. Carbon 1997, 35, 1005. (16) Rubio, B.; Izquierdo, M. T.; Mastral, A. M. Carbon 1998, 36, 263. (17) Mochida, I.; Korai, Y.; Shirahama, M.; Kawano, S.; Hada, T.; Seo, Y.; Yoshikawa, M.; Yasutake, A. Carbon 2000, 38, 227. (18) Davini, P.; Stoppato, G. Abstract of 23rd Biennial Conference on Carbon, 18-23 July 1997, College Park, PA, p 316. (19) Ramon, M. C.; Takarada, T.; Suzuki, Y.; Linares, A. Abstract of 23rd Biennial Conference on Carbon, 18-23 July 1997, College Park, PA, p 324.

Process of SO2 adsorption has been studied extensively and such parameters as porosity, surface chemistry, and constituents of ash were taken into consideration.5-16 The products of surface reactions were analyzed from the point of view of removal efficiency and the feasibility of regeneration. It was found that sulfur dioxide is adsorbed with two adsorption energies on activated carbons.3-12 The low energy, about 50 kJ/mol, corresponds to weak physical adsorption and the second, about 80 kJ/mol, to chemisorption.11 The weak adsorption is related to interactions of SO2 and free sites on the surface, whereas the strong adsorption is enhanced by the presence of oxygen.3,6,8,10,11,13 These oxygen-containing sites are proposed as catalytic centers for oxidation of SO2 to SO3.14 On the other hand, Raymundo-Pinero and co-workers suggested that oxidation to sulfur trioxide occurs in the 7 Å pores.11 With an increase in the size of pores, less SO2 is converted which results in smaller uptake of sulfur dioxide. Since usually the process is carried out in the presence of moisture and oxygen, sulfur dioxide is oxidized to sulfuric acid as a final product of the reaction. Adsorption/ oxidation of SO2 in oxygen atmosphere and in the presence of water occurs as follows:

SO2(gas) f SO2(ads)

(1)

O2(gas) f 2O(ads)

(2)

SO2(ads) + O(ads) f SO3(ads)

(3)

H2O(gas) f H2O(ads)

(4)

SO3(ads) + H2O(ads) f H2SO4(ads)

(5)

where “gas” and “ads” refer to the presence of reactants in the gas phase and the adsorbed state, respectively. It was also found that three forms of adsorbed sulfur oxides could be present in such a situation. They are as follows: weakly adsorbed SO2, physically adsorbed SO3 (after oxidation of SO2), and strongly adsorbed H2SO4.5-13 When basic groups containing oxygen are present on the carbon surface, the adsorption of SO2 is significantly enhanced.8,10 In such a case basic groups (pyrone-type) are responsible for strong physical adsorption of sulfur dioxide.

10.1021/la011320e CCC: $22.00 © 2002 American Chemical Society Published on Web 01/23/2002

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The removal of the adsorption products from the carbon depends on the features of the carbon surface and the chemical nature of the oxidation products and their adsorption energies.9 As indicated above, the presence of strong basic groups on the surface strengthens the bond of sulfuric acid to the surface causing the low efficiency of its water extraction. On the other hand, some researchers postulated that an increase in surface acidity may result in enhancing both adsorption of SO2 and the feasibility of water regeneration.9 The role of pore structure is not so well defined as the role of surface oxygenated groups.11,13 Although it is believed the developed porosity is important to “store” sulfuric acid as a product of oxidation, large pores decrease the conversion of SO2 to SO3, which is accompanied by a decrease in a total sorption capacity. On the other hand, when small pores are present, H2SO4 is strongly bonded to the surface20-22 and its total removal using washing is almost impossible. The process requires heating to about 400 °C when significant change in surface chemistry can occur due to the gasification of carbon and incorporation of the same sulfur to the carbonaceous matrix.23 Overall, the capacities of activated carbon and carbon fibers at ambient conditions reported in the literature are of the order of a few milligrams to a hundred milligrams per gram of carbon bed.10,18 It is interesting that high capacities were found on pitch and PAN-based activated carbon fibers.17,25 Although the role of nitrogen present in the carbon matrix was not emphasized by Lee and co-workers in their studies of SO2 adsorption on PAN-based activated carbon fibers,24 Kawabuchi and co-workers noticed a significant increase in the sorption capacity when activated carbon fibers were modified with pyridine and basic nitrogen functionalities were introduced to the surface.25 An objective of this paper is to demonstrate the role of basic nitrogen species and small pores in the process of catalytic oxidation of sulfur dioxide on activated carbons. The results are discussed in terms of adsorption capacity, oxidation products, and a specific role of the carbon surface. Experimental Section Materials. Four commercial activated carbons of various origins were used in this study. They are as follows: BAX-1500 (wood based, Westvaco), S208C (coconut shell based, Waterlink Barnabey and Sutcliffe), Centaur (catalytic carbon, Calgon), Vapure 612 (bituminous coal, Norit). Other materials used were experimental carbons obtained from polymers described in refs 26-28sSCN-500, SCN-950, SCN-2, and SCN-4. The first two samples were obtained by carbonization of nitrogen-containing macroporous vinylpyridine resin (VPR) at 500 and 950 °C, respectively.28 The last two are derived from SCN-950 using steam activation to a different extent.26,27 Expected changes in surface chemistry are presented in Figure 1. To check the effect of surface oxidation on the SO2 capacity, the test was also done on nitric acid oxidized SCN-4 sample referred to as SCN-4ox. BAX samples (20) Bagreev, A.; Rahman, H.; Bandosz, T. J. Adv. Environ. Res., in press. (21) Bagreev, A.; Rahman, H.; Bandosz, T. J. Ind. Eng. Chem. Res. 2000, 39, 3849. (22) Bagreev, A.; Rahman, H.; Bandosz, T. J. Environ. Sci. Technol. 2000, 34, 4587. (23) Davini, P. Carbon 1991, 29, 321. (24) Lee, J. K.; Shim, H. J.; Lim, J. C.; Choi, G. J.; Kim, Y. D.; Min, B. G.; Park, D. Carbon 1997, 35, 837. (25) Kawabuchi, Y.; Sotowa, C.; Kuroda, K.; Kawano, S.; Whitehurst, D.; Mochida, I. Abstracts, International Conference on Carbon, Carbon 96, Newcastle, U.K., 1996; p 431. (26) Bagreev, A.; Strelko, V.; Lahaye, J. International Conference on Carbon, Carbon 96, Newcastle, U.K., 1996; p 527. (27) Lahaye, J.; Nanse, G.; Bagreev, A.; Strelko, V. Carbon 1999, 37, 585. (28) Lahaye, J.; Nanse, G.; Fioux, P.; Bagreev, A.; Broshnik, A.; Strelko, V. Appl. Surf. Sci. 1999, 147, 153.

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Figure 1. Schematic presentation of the chemical structure of VPR and its derivatives, nitrogen-containing carbons. were modified by introducing nitrogen species to the matrix using saturated urea solution, which was followed by high-temperature treatment (HTT).29 After this process the samples are referred to as BAX-U. To further investigate the relative influence of either nitrogen or pore sizes, the carbon samples were prepared from styrenedivinylbenzene copolymer.30 They are referred to as SCS-2 and SCS-4, where 2 and 4 refer to the same conditions of activation as in the case of SCN carbons.26,27 The SCS-2 sample was oxidized with nitric acid (SCS-2ox) and then heat treated at 950 °C in nitrogen (SCS-2oxHT). The prepared materials were studied as adsorbents for sulfur dioxide in the dynamic tests described below under wet conditions, dry, and dry in the absence of oxygen. Methods. SO2 Breakthrough Capacity. Dynamic tests were carried out at room temperature to evaluate the capacity of the sorbents for SO2 removal under wet conditions.31 For the former, adsorbent samples were packed into a glass column (370 mm length, 9 mm i.d., 6 cm3 bed volume) and prehumidified with moist air (relative humidity 80% at 25 °C) for 1 h. The amount of water adsorbed was estimated from the increase in the sample weight. Moist air (relative humidity 80% at 25 °C) containing 0.3% (3,000 ppm) SO2 was then passed through the column of adsorbent at 0.5 L/min. The breakthrough of SO2 was monitored using a MicroMax monitoring system (Lumidor) with an electrochemical sensor. The test was stopped at the breakthrough concentration of 350 ppm. The adsorption capacities of each (29) Adib, F.; Bagreeev, A.; Bandosz, T. J. Langmuir 2000, 16, 1980. (30) Bagreev, A.; Strelko, V.; Dentzer. J.; Lahaye, J. Ext. Abstr, 22nd Biennal Conference on Carbon, San Diego, CA, 1995; p 610. (31) Bashkova, S.; Bagreev, A.; Locke, D. C.; Bandosz, T. J. Environ. Sci. Technol. 2001, 35, 3263.

SO2 Adsorption on Activated Carbons sorbent in terms of milligrams of SO2 per gram of carbon were calculated by integration of the area above the breakthrough curves and from the SO2 concentration in the inlet gas, flow rate, breakthrough time, and mass of sorbent. For each sample the SO2 test was repeated at least twice. The determined capacities agreed to within 4%. To check the effect of moisture, the SO2 breakthrough tests were carried out under dry conditions. Selected samples used for these measurements are designated with an additional letter “D”. To further investigate the effect of atmosphere, the capacities of chosen samples were measured in the absence of moisture and oxygen with nitrogen as a carrier gas. They are referred to as “N”. The amount of weakly adsorbed SO2 was evaluated by purging the adsorbent column with carrier gas at 0.35 L/min immediately after the breakthrough experiment. The SO2 concentration was monitored until its concentration dropped to 5 ppm. The process took about 1-2 h depending on the type of adsorbent, conditions of adsorption, and the type of carrier gas through desorption. pH of Carbon Surface. A 0.4 g portion of carbon powder was placed in 20 mL of water and equilibrated during the night. Then the pH of suspension was measured. For exhausted samples, an additional letter “E” is added (pHE). Sorption of Nitrogen. Nitrogen isotherms were measured using a ASAP 2010 (Micromeritics) at -196 °C. Before the experiment the samples were heated at 120 °C and then outgassed overnight at this temperature under a vacuum of 10-5 Torr to constant pressure. The isotherms were used to calculate the specific surface areas (S), micropore volumes (Vmic), total pore volumes (Vt), average micropore sizes (Lmic), and pore size distributions (DFT).32,33 Thermal Analysis. Thermal analysis (TA) was carried out using a TA Instruments thermal analyzer. The instrument settings were 10 deg/min heating rate and nitrogen atmosphere with 100 mL/min flow rate. Content of Nitrogen. Nitrogen content for commercial carbons was evaluated at the Kaufmann Laboratory, Golden, CO. For polymeric carbons the content of nitrogen was determined using elemental analysis and X-ray photoelectron spectroscopy (XPS).26,27 XPS. XPS characterization of surface chemistry of nitrogencontaining carbons was carried out on a LEYBOLD LHS 11 spectrometer using Mg KR radiation (energy 1253.6 eV). The techniques of sample preparation, collection of the spectra, and their processing are described in detail elsewhere.27 The values of binding energy (BE) were calibrated with respect to C1s peak at 285.0 eV. The N1s envelopes were used to characterize different forms of nitrogen. Amount of Basic and Acidic Groups. The amount of basic and acidic groups on the surface of carbon was determined by titration, with either 0.05 M HCl or 0.05 M NaOH.

Results and Discussion The SO2 breakthrough curves for the selected carbons studied and the VPR resin are presented in Figure 2 where significant differences in the performance of materials can be seen. It is interesting that for the best performing carbons tested in the presence of moisture the change in the slope is observed which may be related to the changes in the chemistry of the system and the mechanism of adsorption. The calculated capacities are collected in Table 1. The highest capacities were found for SCN-4, SCS-2, SCS-2oxHT, and Centaur, and the lowest were found for VPR and SCS-2ox. It should be mentioned here that a low level of SO2 emission has been detected ([SO2] < 0.1 ppm) during analysis (the initial concentration was 3000 ppm). This corresponds to 99.99% of SO2 removal efficiency. When the test was run without moisture, a significant decrease in the capacity was observed for SCN-4 and SCS2. For other samples selected in this test the capacities (32) Lastoskie, C. M.; Gubbins, K. E.; Quirke, N. J. Phys. Chem. 1993, 97, 7, 4786. (33) Olivier, J. P. J. Porous Mater. 1995, 2, 9.

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Figure 2. SO2 breakthrough curves for the materials studied. Table 1. The Nitrogen Content, pH of the Surface, and SO2 Breakthrough Capacity for the Materials Studied

sample

nitrogen content (%)

pH

Centaur Vapure BAX BAX-U S208 VPR SCN-500 SCN-950 SCN-2 SCN-4 SCN-4ox SCN-4(D) SCN-4ox(D) SCS-2 SCS-2(D) SCS-4 SCS-4(D) SCS-2ox SCS-2ox(D) SCS-2oxHT SCN-4(N) SCS-2(N) SCS-4(N)

1.1 0 0.2 2.4 0 5.5 10.5 4.3 3.0 2.4 2.3 2.4 2.3 0 0 0 0 0 0 0 2.4 0 0

7.03 8.58 6.85 6.34 10.1 2.23 6.51 6.42 6.64 9.05 4.26 9.05 4.26 8.18 8.18 9.86 9.86 3.79 3.79 8.12 9.05 8.18 9.86

SO2 brkthr SO2 time capacity desorbed (mg/g) (mg/g) pHE (min) 1.51 1.72 2.22 1.91 2.2 2.1 2.44 2.65 1.63 1.27 1.59 2.06 1.73 1.62 1.98 2.28 2.34 2.15 2.05 1.38 2.70 5.64 9.88

139 46 13 40 50 19 25 60 88 245 81 61 62 147 78 30 42 19 44 160 46 39 27

164 64.3 33.9 69.8 48.2 24.6 29.8 52.6 83.6 317.3 89.6 85 66 212 80.5 54 66 20 50 218 64.5 43.5 41.9

0.40 0.60 2.60 1.18 2.50 2.7 2.4 0.5 0.3 0.3 0.4 9.9 3.1 1.1 6.5 3.3 7.5 0.9 7.6 0.6 11.1 6.7 8.2

decreased only slightly. This indicates that to obtain a superior performance the presence of moisture is a necessity. The effect of oxygen in a carrier gas was checked when the breakthrough capacity experiments were run in nitrogen at dry conditions. At this situation the capacity was even lower than in dry air suggesting that oxygen chemisorbed on the carbon surface is active in the process of SO2 adsorption/oxidation.

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Table 2. Structural Parameters and Characteristic Energy of Adsorption Calculated From Adsorption of Nitrogen sample

S (m2/g)

Vmic (cm3/g)

Vt (cm3/g)

Vmic/Vt

Centaur Vapure BAX BAX-U S208 VPR SCN-500 SCN-950 SCN-2 SCN-4 SCN-4ox SCS-2 SCS-4 SCS-2ox SCS-2oxH

632 788 1400 1024 1030 25 300 861 473 973 1006 674 1023 534 598

0.24 0.34 0.56 0.37 0.39 0.01 0.13 0.23 0.29 0.43 0.39 0.23 0.35 0.16 0.21

0.29 0.43 1.21 0.71 0.41 0.15 0.29 0.49 0.42 0.83 0.86 0.58 1.43 0.47 0.84

0.82 0.79 0.46 0.52 0.95 0.07 0.45 0.42 0.83 0.64 0.62 0.51 0.33 0.30 0.77

Lmic (Å)

Eo (kJ/mol)

9.6 11.2 12.9 11.8 9.2

22.8 19.1 15.9 16.6 23 10.9 21.9 31.3 29.3 20.2 21.6 26.4 21.5 26.4 24.6

5.8 7.5 10.7 10.3 7.7 9.3 7.4 8.3

Another important feature of the system, which should be analyzed here, is the amount of SO2 desorbed from the carbon surface during air purging. As indicated elsewhere,31 it is the very weakly bonded sulfur dioxide physically adsorbed on the surface. The data collected in Table 1 suggest that higher capacity for SO2 adsorption at wet conditions is linked to the smaller amount desorbed. However, when the adsorption process was carried out under dry conditions, the amount of SO2 adsorbed decreased, especially for SCN-4, SCS-2, SCS2-oxHT, and Centaur. It was accompanied by a significant increase in the amount of SO2 desorbed (an order of magnitude). This suggests that the presence of moisture increases the apparent adsorption strength of SO2. Significantly higher capacity at wet conditions may be explained using the following interpretation. First, adsorbed water molecules can be active centers for sulfur dioxide adsorption by formation of hydrogen bonds with SO2 molecules.10 Second, enhanced capacity is likely related to the reaction of SO3 with water and formation of sulfuric acid. H2SO4 is very strongly adsorbed on the carbon surface.20,21 Since the formation of sulfuric acid is not an immediate process (SO2 has to be adsorbed, oxidized to SO3, and react with water), the above-mentioned hypothesis can explain the change in the slope observed on some of the breakthrough curves (Figure 2). Supporting for the formation of sulfuric acid is extremely low pH of samples after exhaustion (Table 1). To explain the differences in the SO2 adsorption capacity and the strength of adsorption, both porosity and surface chemistry should be taken into consideration. The structural parameters of samples studied calculated from nitrogen adsorption isotherms are collected in Table 2. We also included there the characteristic energy of adsorption calculated using the Dubinin approach.34 This parameter is related to the pore sizes of the adsorbents (Lmic) in such a way that smaller sizes result in bigger values of Eo. Analysis of the structural parameters along with the degree of microporosity, expressed as the ratio of the volume of micropores to the total pore volume (Vmic/Vt), reveals differences in porosity. Figures 3 and 4 show the pore size distributions for the materials studied. The most microporous are Centaur and S208c and the least microporus is BAX. Although very small pores are also present in SCN-500 and SCN-950, the data obtained may not be totally correct due to the problems with adsorption equilibrium. This happens when adsorption

of nitrogen occurs in “underactivated” carbons with the very small pore entrances. After activation the pore volumes of SCN carbons increased. This increase, however noticeable, cannot fully explain 4-fold difference in the adsorption capacity between SCN-2 and SCN-4. On the other hand, the capacity of BAX is very small, despite its large surface area and pore volume. For this carbon, after urea modification and heat treatment, the surface area and pore volume decreased, relative microporosity increased, and an increase in the sorption capacity was found. It has to be emphasized that structural parameters for BAX-U decreased a maximum of 50% whereas the capacity increased 100%. Results obtained for Centaur and SCN carbons suggest that not volumes but sizes of pores are important.11,17,29,35 It is intuitively understandable that to have a satisfactory capacity the carbons should have a developed porosity to “store” sulfuric acid. The capacities

(34) Dubinin, M. M. In Chemistry and Physics of Carbon; Walker, P. L., Ed.; M. Dekker: New York, 1966; Vol. 2, pp 51-120.

(35) Adib, F.; Bagreev, A.; Bandosz, T. J. Environ. Sci. Technol. 2000, 34, 686.

Figure 3. Pore size distributions for commercial carbons.

Figure 4. Pore size distributions for SCN carbons.

SO2 Adsorption on Activated Carbons

of Centaur and SCN-4 suggest that the small pore volume when used efficiently can provide a significant capacity. The efficiency here is probably related to the sizes of pores and distribution of active sites for adsorption. It is interesting that the carbon with the smallest pores (largest Eo) do not have the highest capacity. It is likely that there is an optimum size of pores where the adsorption of SO2 is enhanced. As indicated by Raymundo-Pinero and coworkers11 and Wang and Kaneko,36 this size should be around 7 Å. Even though in such pores the adsorption potential of SO2 is not at its maximum37 (σSO2 ) 4.29 Å36), the high capacity is achieved owing to the strong SO2 and SO3 adsorption followed by formation of sulfuric acid. Indeed, in such pores SO2 along with water can statistically cover the pore walls forming a monolayer. This makes oxidation and reaction with coadsorbed water molecules possible. It has to be mentioned here that H2SO4 at 25 °C has higher density (dH2SO4 ) 1.83 g/cm3) than liquid SO2 (dSO2 ) 1.27 g/cm3) but smaller than that of SO3 (dSO3 ) 1.93 g/cm3). Moreover, its boiling temperature is about 330 °C, which is much higher than that for SO2 (-10 °C) and SO3 (45 °C).38 Taking this into account, the amount of H2SO4 physically adsorbed from the vapor phase should be greater than that of SO2 and SO3 at the same pressure and temperature. It follows that the order of decreasing “ability” for physical adsorption should be H2SO4 > SO3 > SO2. If the pore size is too small, water molecules cannot be coadsorbed in the vicinity of SO2, preventing the formation of sulfuric acid. It is probable, as in the case of hydrogen sulfide adsorption,29,35 that homogeneity in microporosity enhances the dispersion of active centers where sulfur dioxide can be oxidized to SO3. This SO3 is then converted to H2SO4 and adsorption occurs until all pores are filled. The presence of various sulfur species on the surface of carbons can be studied using TA analysis. When the results are reported as DTG curves a weight loss is represented as a peak with surface area related to the amount of the species removed from the surface or decomposed. In the case of carbon/SO2 system usually two peaks are present on DTG curves.3,7,25 Since the first peak (at our experimental conditions) represents both water (predominantly) and SO2 weakly physically adsorbed on the surface we are not going to interpret it qualitatively. The second peak represents SO2, SO3 and H2SO4 chemisorbed on the surface and desorbed as SO2.3,17,25,29,31,35 The results obtained for BAX series of samples are collected in Figure 5. For the exhausted initial sample, without urea modification a new peak between 180 °C and 400 °C appears which represents sulfuric acid. For the exhausted samples after urea modification the intensity of this peak increases as a result of the catalytic effect of nitrogen functionality. Interesting results are obtained for Centaur and SCN carbons (Figure 6). We did not include in Figure 6 the curves for the initial materials since, compared to the exhausted samples, they are almost featureless. In the case of Centaur and SCN-4, the differential thermogravimetry (DTG) curves are similar and differences in the intensity of H2SO4 peak are related to the differences in the adsorption capacity. The results slightly differ when the species on the surface of SCN-4ox are analyzed. Responsible for this are likely the differences in the surface chemistry of carbons, which occurred due to its oxidation with nitric acid. (36) Wang, Z. M.; Kaneko, K. J. Phys. Chem. B 1998, 102, 2863. (37) Everett, D. H.; Powl, J. C. J. Chem. Soc., Faraday Trans. 1 1976, 72, 619. (38) Handbook of Chemistry and Physics, 67th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL 1986.

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Figure 5. DTG curves in nitrogen for BAX carbons and Centaur.

Figure 6. DTG curves in nitrogen for SCN carbons.

Figure 7 presents the DTG curves for SCN-4 sample run at various conditions. As expected, on the basis of the breakthrough capacity data at dry conditions, the amount of species thermally desorbed from the surface decreased; however, the peaks are at the same temperature ranges. Interesting are relative intensities of the peaks obtained. Comparison of the results run at dry conditions, with and without moisture, clearly shows that the contribution of SO2 to the first peak at wet conditions is very small (similar intensities for dry conditions) compared to the adsorbed water. Another important conclusion is the role of oxygen. It is apparent that oxygen helps in SO2 adsorption causing its oxidation to SO3 and more efficient adsorption on the surface even though the formation of sulfuric acid is rather impossible. Comparison of the DTG results for SCN-4 samples with those for SCS-2 shows an interesting feature. Those two samples have similar pore sizes (Table 2), and if they are the only important factor, the adsorption should be similar. The data collected in Figure 8 show that it is not the case. Although the behavior of samples run in the presence of oxygen is similar, the results significantly differ for the sample run in nitrogen. In the case of SCS-2, the peak at about 250 °C does not appear. This indicates that SO2 is

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Figure 9. Dependence of the amount of SO2 thermodesorbed (as H2SO4 or as SO3 for the dry runs) on the amount of SO2 adsorbed. Figure 7. Changes in the amount of thermally desorbed products from the surface of SCN-4 samples run at various conditions.

Table 3. Surface Acidity and Basicity (mmol/g) sample Centaur Vapure BAX BAX-U S208 SCN-500 SCN-950 SCN-2 SCN-4 SCN-4ox SCS-2 SCS-4 SCS-2ox SCS-2oxH

acidic groups basic groups total groups basic/total 0.40 0.15 1.10 0.58 0.31 0.37 0.24 0.24 0.25 1.65 0.05 0.12 1.8 0.00

0.43 0.58 0.26 0.35 0.51 0.35 0.33 0.42 0.56 0.17 0.35 0.62 0.03 0.52

0.83 0.73 1.36 0.93 0.82 0.72 0.57 0.66 0.81 1.82 0.40 0.74 1.83 0.52

0.52 0.79 0.19 0.38 0.62 0.49 0.58 0.64 0.69 0.09 0.88 0.84 0.02 1.00

Table 4. Atomic Ratio of Nitrogen to Carbon and Content of Different Nitrogen Forms Calculated from XPS Experiments sample

Figure 8. Changes in the amount of thermally desorbed products from the surface of SCS-4 samples run at various conditions.

only weakly adsorbed on the surface and oxidation to SO3 likely does not occur. Comparing this with the results for SCN-4 proves that the presence of nitrogen in the carbon matrix strengthens the adsorption process. This finding does not exclude the role of the pore sizes in the oxidation when moisture is present making the dissociation of species is possible. Additional support for the formation of sulfuric acid, besides the low pH of the exhausted carbons, is the relationship presented in Figure 9. The amount of SO2 desorbed during heat treatment is recalculated from the weight loss represented by the second DTG peak assuming that it is sulfuric acid. A remarkably good correlation with a slope close to 1 (0.97) and R2 ) 0.95 is found with the amount of SO2 adsorbed calculated from the breakthrough capacity. Although the difference in the SO2 capacities for the best performing carbons exists, the common feature of all best performing carbons is the presence of nitrogen in the

VPR SCN-500 SCN-950 SCN-2 SCN-4

forms of nitrogen, % atomic ratio [N]/[C] pyridinic quaternary pyridine-N-oxide 0.087 0.133 0.043 0.029 0.022

17 65 28 12 9

76 18 47 38 41

0 0 13 13 18

matrix. The majority of nitrogen atoms, which are incorporated into the carbon polyaromatic structure can be in the following forms: “quaternary” N (BE ) 401.3 eV) with effective positive charge +1, pyridine-N-oxide (BE ) 403.4 eV) or pyridinic N (BE ) 399.3 eV), neutral nitrogen (Figure 1). As listed in Table 1 and described elsewhere26,28 the highest content of nitrogen is found in VPR and SCN-500. The effects of this on SO2 capacity are almost negligible due to the lack of π-conjugated carbon polyaromatic system and the developed pore space to store the sulfuric acid product. After heat treatment and activation, the amount of charged nitrogen groups reaches a certain level (Table 4) (about 40% of all nitrogen species26) and they affect the adsorption process. At the same time the total amount of nitrogen built into the carbonaceous matrix decreases, which results in an increase in the surface dispersion and favors the adsorption/oxidation in small pores. Moreover, the positively charged nitrogen can enhance the ion exchange properties of activated carbon attracting the SO42- anions and thus results in strong adsorption of sulfuric acid. The process is even

SO2 Adsorption on Activated Carbons

stronger when it occurs in very small pores. Although on the surface of SCN-4ox, quaternary and pyridinic type nitrogens are also present,27 the SO2 capacity is much smaller than that for the initial counterpart. This sample was oxidized with nitric acid and the content of oxygen significantly increased. The value of pH equal to 4.26 suggests that the acidic oxygen-containing groups “neutralize” the catalytic role of nitrogen by affecting its apparent dispersion on the surface, which results in a decreased capacity. They also change the charge of carbon surface for negative by decreasing the strength of electrostatic interactions with SO42- ions. It was shown by Kelemen and co-workers that the presence of oxygencontaining groups in the vicinity of pyridinic nitrogen results in protonation of hydroxyl oxygen or carboxyl groups via formation of hydrogen bridges.39,40 Although such carbons as the SCN series or Centaur have high SO2 adsorption capacity, there are carbons which do not have nitrogen and whose capacities are high as, for instance, SCS-2. As shown in Table 3 the surface of this carbon is basic, implying the presence of pyrones whose role may be similar to that of nitrogen. Moreover, it is likely true that the sizes of pores and their certain volume are important for catalytic oxidation of sulfur dioxide and its conversion to sulfuric acid. The effect of nitrogen can be discussed in terms of its chemical action and physical location of the specific process. As indicated by Pels and co-workers,41 during high-temperature pyrolysis of nitrogen-containing precursors, nitrogen atoms are incorporated into carbon rings and located at the edges of graphene layers as pyridinic nitrogen or in the interior as quaternary nitrogen (Figure 1). When carbons after heat treatment are exposed to ambient conditions, it is possible that pyridine-N-oxides are formed. As suggested by Matzner and Boehm,42,43 and recently shown by Strelko and co-workers44 using quantum chemical calculations, the extra π-electrons of pyridinic and quaternary nitrogen occupy the high-energy states in the conduction band. It is likely that from there they can be transferred to the adsorbed oxygen and superoxide ions can be formed. Those superoxide ions can trigger the formation of OH• radicals in reaction with H2O. All of these species may oxidize SO2 to SO3. All of these likely happen gradually in the confined pore space where SO42ions are formed after dissociation of sulfuric acid, and then they are adsorbed on the positively charged surface of activated carbon. The presence of active nitrogen centers significantly enhances this process.26,44,45 When pores of small sizes are excluded from the adsorption process due to the strong adsorption of sulfuric acid, larger pores take part in the process. The reactions proceed until all small pores with positively charged nitrogen suitable to immobilize SO42- ions are filled, and then only the physical adsorption of SO2 is the remaining option. Figure 10 show the dependence of SO2 adsorption capacity on the pore volume between 6.79 and 8.58 Å (calculated from DFT). It is interesting that the two slopes can be clearly distinguished suggesting two different steps (39) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J. Prepr. ACS Div. Fuel Chem. 1994, 39, 31. (40) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J. Energy Fuel 1994, 8, 896. (41) Pels, J. R.; Kapteijn, F.; Moulijn, J. A.; Zhu, Q.; Thomas, K. M. Carbon 1995, 33, 1641. (42) Matzner, R.; Boehm, H. P. Carbon 1998, 36, 1697. (43) Schmiers, H.; Friebel, J.; Streubel, P.; Hesse, R.; Kopsel, R. Carbon 1999, 37, 1965. (44) Strelko, V. V.; Kutz, V. S.; Thrower, P. A. Carbon 2000, 38, 1499. (45) Biniak, S.; Szymanski, G.; Siedlewski, J.; Swiatkowski, A. Carbon 1997, 35, 1799.

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Figure 10. Dependence of SO2 adsorption on the volume of pore with sizes between 6.79 and 8.58 Å. Doted lines show the trend in the data.

Figure 11. Dependence of SO2 normalized capacity on the density of basic groups.

of adsorption. A similar trend is observed when specific capacity (per unit surface area) is analyzed depending on the density of basic groups (Figure 11). However, in this case, a linear relation can be found for few samples. For the majority of carbons the insensitivity of the specific capacity is noticed. Taking into account that complete separation of the roles of porosity and chemistry seems to be impossible, we plotted the dependence of the specific capacity on the incremental amount of basic groups. This value was calculated as the product of the average density of basic groups and the incremental surface area in pores between 6.79 and 8.58 Å. It is seen for Figure 12 that a relatively good linear trend exists for the majority of the samples studied. This implies that either volume of pores having sizes around 7-8 Å or basicity of the samples leads to the similar final effect, which is the enhanced SO2 adsorption capacity. This happens for the samples that may have a combination of both these factors, provided that sufficient pore volume exists for the “storage” of sulfuric acid (two slopes). The graphs indicate that both factors should be taken into account when catalytic carbon for SO2 removal is designed. Conclusions The results presented in this paper show the exceptional performance of activated carbons containing nitrogen

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dispersed on the carbon surface. This can be achieved when the relative degree of microporosity is high and pores are very small but large enough to allow the coadsorption of water and sulfur dioxide. This enables the formation of sulfuric acid. Oxidation of SO2 to SO3 and formation of sulfuric acid occurs likely gradually in pores from smaller to bigger sizes via attraction of SO42- ion to positively charged nitrogen centers. This leaves the larger pores still active and still capable of storing sulfuric acid. The process likely occurs until all centers are “poisoned” by sulfuric acid. When highly dispersed nitrogen is not present and pores are larger, the degree of conversion to sulfuric acid is smaller; a majority of SO2 is adsorbed via dispersive forces, and the capacity lasts until equilibrium is reached. When such carbons are used, weakly adsorbed SO2 is easily removed in two steps: using air purging and heating to 100 °C. It is important to emphasize here the crucial role of water for the enhancement in the amount adsorbed. Even though when SO2 is adsorbed from stock gases at temperatures higher than ambien, the carbon should be periodically moisturized to enable formation of sulfuric acid. Figure 12. Dependence of SO2 adsorption on the incremental basicity in pores between 6.79 and 8.58 Å. Doted lines show the trend in the data.

species as catalysts for SO2 adsorption/oxidation from wet air streams at room temperature. To obtain a satisfactory capacity, the positively charged nitrogen should be highly

Acknowledgment. The authors thank Dr. G. Nance from Institut de Chimie des Surfaces et Interfaces, Mulhouse, France, for the performing XPS analysis of synthetic carbon. LA011320E