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Langmuir 2000, 16, 1980-1986
Adsorption/Oxidation of Hydrogen Sulfide on Nitrogen-Containing Activated Carbons Foad Adib, Andrey Bagreev,† and Teresa J. Bandosz* Department of Chemistry, The City College of New York, The Graduate School of the City University of New York, and Center for Water Resources and Environmental Research of the City College of New York New York, New York 10031 Received July 13, 1999. In Final Form: October 21, 1999 Wood-based activated carbon was modified by impregnation with urea and heat treatment at 450 and 950 °C. The chemical and physical properties of materials were determined using acid/base titration, FTIR, thermal analysis, IGC, and sorption of nitrogen. The surface features were compared to those of a commercial urea-modified carbon. Then, the H2S breakthrough capacity tests were carried out, and the sorption capacity was evaluated. The results showed that urea-modified sorbents have a capacity similar to that of the received material; however, the conversion of hydrogen sulfide to a water-soluble species is significantly higher. It happens due to a high dispersion of basic nitrogen compounds in the small pores of carbons, where oxidation of hydrogen sulfide ions to sulfur radicals followed by the creation of sulfur oxides and sulfuric acid occurs. It is proposed that the process proceeds gradually, from small pores to larger, and that the degree of microporosity is an important factor.
Introduction Activated carbons are well known as adsorbents of gases and vapors.1 Their specific application depends on the properties of molecules to be removed/adsorbed. Microporous carbons are used for the sorption/separation of light gases, whereas carbons with broad pore size distributions are applied for removal of toxins or other large organic molecules.1-3 When the specific interactions of adsorbate/adsorbents play a role in the adsorption process, other features of activated carbons such as surface chemistry should also be taken into consideration.3-6 The surface chemistry of activated carbon is governed by the presence of heteroatoms such as oxygen, hydrogen, nitrogen, and phosphorus. Their origin is in the chemical nature of the organic precursor and the method of carbon preparation/activation.1 They exist in the form of acidic, basic, or neutral organic functional groups.4-7 Moreover, delocalized π electrons of aromatic rings and unsaturated valences contribute to the basicity of carbonaceous sorbents.3 Sometimes the “natural” chemistry of the activated carbon surface is not potent enough to enhance the specific adsorbate-adsorbent interactions or catalytic processes. In such cases, the special chemical modifications based mainly on impregnation processes are used. Examples
are carbons impregnated with caustics used for adsorption of acidic gases such as H2S8,9 or carbons impregnated with KMnO4 and KI, which contribute to the oxidation process. When those modifications are used, the carbon surface acts as a catalyst support. The objective of this paper is to demonstrate the effect of modification with urea on the removal of hydrogen sulfide by activated carbons. In a previous study, we showed the catalytic effect of unmodified carbons on adsorption and oxidation of hydrogen sulfide.10-12 A mechanism of the process was proposed, and the influence of local pH and affinity of carbon to retain water was pointed out.11,12 It was demonstrated that even carbons with average pH in the acidic range can work as effective H2S adsorbents; when pH was very low (lower than a certain threshold value), despite a significant decrease in the adsorption capacity, a high yield of sulfuric acid was observed.12 Activated carbons used in this study are modified with urea. The conditions of modification were chosen to see the differences in the form of nitrogen incorporated in the carbon matrix.13 The results of our study are presented in comparison with Centaur carbon which, according to Calgon specifications,14,15 is able to convert all H2S into sulfuric acid, which can then be removed from the carbon by washing with water. Experimental Section
* To whom correspondence should be addressed Tel: (212) 650-6017. Fax: (212) 650-6107. E-mail: tbandosz@ scisun.sci.ccny.cuny.edu. † Permanent address: Institute for Sorption and Problems of Endoecology, General Naumova Str. 13, Kiev-164, 252180, Ukraine. (1) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. Active Carbon; Marcel Dekker: New York, 1988. (2) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: New York, 1982. (3) Leon y Leon, C. A.; Radovic, L. R. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; M. Dekker: New York, 1992; Vol. 24, p 213. (4) Puri, B. R. In Chemistry and Physics of Carbon; Walker, P. J., Jr., Ed.; M. Dekker: New York, 1970; Vol. 6, p 191. (5) Boehm, H. P. Carbon 1994, 32, 759. (6) Boehm, H. P. In Advances in Catalysis; Academic Press: New York, 1966; Vol. 1, pp 179-274. (7) Carrasco-Marin, F.; Mueden, A.; Centeno, T. A.; Stoeckli, F.; Moreno-Castilla, C. J. Chem. Soc., Faraday Trans. 1997, 93, 2211.
Materials. Wood-based activated carbon was used in this study. The initial sample is designated as W. Two samples of W were impregnated with urea (saturated solution) and heated in nitrogen at the rate of 10 °C/min, one sample to 450 °C and the other to 950 °C, and maintained at these temperatures for 0.5 (8) Turk, A.; Sakalis, E.; Rago, O.; Karamitsos, H. Ann. N.Y. Acad. Sci. 1992, 661, 221. (9) Turk, A.; Mahmood, K.; Mozaffari, J. Water Sci. Technol. 1993, 27, 121. (10) Bandosz, T. J. Carbon 1999, 37, 483. (11) Adib, F.; Bagreev, A.; Bandosz, T. J. J. Colloid Interface Sci. 1999, 214, 407. (12) Adib, F.; Bagreev, A.; Bandosz, T. J. J. Colloid Interface Sci. 1999, 216, 360. (13) Strohr, B.; Boehm, H. P.; Schlogl, R. Carbon 1991, 26, 707. (14) Hayden, R. A. U.S. Patent 5,444,031, 1995. (15) Matviya, T. M.; Hayden, R. A. U.S. Patent 5,356,849, 1994.
10.1021/la990926o CCC: $19.00 © 2000 American Chemical Society Published on Web 12/15/1999
Adsorption/Oxidation of Hydrogen Sulfide h in order to introduce nitrogen groups. After modification, the samples were water-washed to remove any excess of urea decomposition products. The modified carbons are referred to as Wu-450 and Wu-950. Another sample used for experiments is a commercial carbon modified with urea manufactured by Calgon Carbon, Centaur.14,15 The exhausted carbons after the hydrogen sulfide breakthrough tests are designated by the additional letter, E, in their name. To distinguish between the effects of thermal treatment and incorporation of nitrogen, two untreated wood-based carbon samples were heated under the same conditions as those used for samples impregnated with urea. Those samples are referred to as WT-450 and WT-950. Methods. H2S Breakthrough Capacity. The dynamic test was carried out to evaluate the capacity of carbons for H2S removal. Moist air (relative humidity 80% at 25 °C) containing 0.3% (3000 ppm) H2S 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 H2S emission was monitored by an Interscan LD-17 H2S continuous monitor system interfaced with a computer data acquisition program. The test was stopped at the breakthrough concentration of 500 ppm. In the case of W carbon, the prehumidification was necessary to reach high H2S breakthrough capacity. Other samples did not require prehumidification. The detailed conditions of the test are described elsewhere.11 The adsorption capacities of each carbon in terms of grams of H2S per gram of carbon were calculated by integration of the area above the breakthrough curves and from the H2S concentration in the inlet gas, flow rate, breakthrough time, and mass of carbon. Boehm Titration. The oxygenated surface groups were determined according to the method of Boehm.6 One gram of carbon sample was placed in 50 mL of each of the following 0.05 N solutions: sodium hydroxide, sodium carbonate, sodium bicarbonate, and hydrochloric acid. The vials were sealed and shaken for 24 h; 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 carboxylic, phenolic and lactonic groups; Na2CO3 neutralizes carboxylic and lactonic groups; and NaHCO3 neutralizes only carboxylic groups. The number of surface basic sites was calculated from the amount of hydrochloric acid that reacted with the carbon. pH of Carbon Surface. The 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 sample was filtered, and the pH of the solution was measured. Potentiometric Titration. Potentiometric titration measurements were performed with a DMS Titrino 716 automatic titrator (Metrohm). The instrument was set at the mode when the equilibrium pH was collected. Subsamples of the carbons of about 0.100 g in 50 mL of 0.01 M NaNO3 were placed in a container thermostated at 298 K and equilibrated overnight with the electrolyte solution. To eliminate the influence of atmospheric CO2, the suspension was continuously saturated with N2. The carbon suspension was stirred throughout the measurements. The volumetric standard NaOH (0.1 M) was used as the titrant. The experiments were done in the pH range of 3-10. Each sample was titrated with base after acidifying the carbon suspension. Thermal Analysis. Thermal analysis was carried out using a TA Instruments Thermal Analyzer. The instrument settings were a heating rate of 10 deg/min and a nitrogen atmosphere with a 50 mL/min flow rate. To evaluate ash content, the experiments were done in air. Elemental Analysis. The content of carbon, hydrogen, and nitrogen was determined by Huffman Laboratories, Golden CO. Sorption of Nitrogen. Nitrogen isotherms were measured using an ASAP 2010 (Micromeritics) at -196 °C. Before the experiment, the samples were heated at 120 °C and then outgassed at this temperature to a vacuum of 10-5 Torr. The isotherms were used to calculate the specific surface area, SN2, the micropores volume, Vmic, and the total pore volume, Vt. All of the above parameters were calculated using density functional theory (DFT).16,17 The Dubinin-Raduskevich equation was used to calculate the characteristic energy of adsorption, E.18
Langmuir, Vol. 16, No. 4, 2000 1981 FTIR. IR spectra were collected using a Nicolet Impact 410 FTIR spectrometer equipped with a diffuse reflectance unit. The instrument resolution was set at 4 cm-1. Carbon powder was placed in a micro-sample holder. Before each measurement, the instrument was run to establish the background, which was then automatically subtracted from the sample spectrum. Inverse Gas Chromatography. The chromatographic experiments were performed with an SRI gas chromatograph equipped with a flame ionization detector. The stainless steel columns (20 cm long, 2.17 mm in diameter) were filled with carbon particles of sizes ranging from 0.2 to 0.4 mm. Helium was used as a carrier gas with a flow rate of 30-120 cm3/min, and methane was used as a nonretained species. The samples were conditioned at 473 K in the chromatographic column under helium gas flow for 12 h prior to the measurement. Injection volume was in the range of 0.1-1.0 mL of a 0.5% mixture of H2S in nitrogen. The range of experimental temperatures was 40-200 °C (20 °C steps). Retention volumes were corrected for the gas compressibility. The precision of the measurement of retention times was 5%. The temperature of the column was stabilized with accuracy an of (0.1 °C. Gas-solid chromatography, when applied to the investigation of solid surface properties, is usually called inverse gas chromatography. It is assumed, in the case of infinite dilution chromatography, that when very small amounts of solutes are injected, the adsorption is described by Henry’s law. This assumption is fulfilled when measured retention volume, VN, is independent of the amount injected, and this can be easily verified experimentally.19-21 Another quantity used to derive thermodynamic parameters of the adsorption process is the specific retention volume, Vs, which is calculated from the equation:
VS )
VN Sm
(1)
where S is specific surface area and m mass of carbon. Using VS, the isosteric heat of adsorption can be easily calculated from the measurements at various temperatures:
Qst ) R
ln(VS/T) (1/T)
(2)
Analysis of the thermodynamic functions obtained under the conditions of infinite dilution provides information about the interactions of the probe molecules with the surface only, since the interaction between adsorbed molecules is neglected. In the case of the energetically uniform surface, these quantities are directly related to the adsorption energy, whereas for energetically heterogeneous surfaces such as activated carbons, they should be considered to be related to some average value of the adsorption energy.21
Results and Discussion The results of elemental analysis presented in Table 1 show a significant increase in nitrogen content for woodbased carbon samples after modification with urea. As expected, for Wu-950 nitrogen content is lower than for Wu-450. In the case of the latter, due to the low temperature of modification, nitrogen is likely to be in the form of amides, free NH and NH2, bonded NH and NH2, or NH4+ species (Figure 1). They decompose at high temperature.13 After heat treatment at 950 °C, the (16) Olivier, J. P.; Conklin, W. B. Presented at the 7th International Conference on Surface and Colloid Science, Compiegne, France, 1991 (17) Lastoskie, Ch. M.; Gubbins, K. E.; Quirke, N. J. Phys. Chem. 1993, 97, 4786. (18) Dubinin, M. M. In Chemistry and Physics of Carbon; Walker, P. L., Ed.; M. Dekker: New York, 1966; Vol. 2, p 51-120. (19) Conder J. R.; Young C. L. Physicochemical Measurement by Gas Chromatography; John Wiley & Sons: New York, 1979. (20) Tijburg, I.; Jagiełło, J.; Vidal A.; Papirer E. Langmuir 1991, 7, 2243. (21) Jagiełło, J.; Bandosz, T. J.; Schwarz, J. A. J. Colloid Interface Sci. 1992, 151, 433.
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Langmuir, Vol. 16, No. 4, 2000
Adib et al. Table 2. pH and Surface Chemistry from Boehm Titration (Number of Groups in mequiv/g)
Figure 1. Schematic representation of changes in surface chemistry of carbons due to saturation with urea and heat treatment. Table 1. Elemental Analysis of Carbon Samples and Ash Content (%) sample
C
H
N
ash
W Wu-450 Wu-950 Centaur
85.1 82.3 91.3 90.6
2.3 2.0 0.7 0.7
0.2 7.5 2.4 1.1
3.15 3.23 1.47 4.34
majority of nitrogen is incorporated into the carbon matrix as a component of an aromatic ring in a pyrdine-like configuration22 (Figure 1). It is noteworthy that the nitrogen content for Wu-950 is higher than for Centaur. This is likely a result of differences in the surface area of the two initial carbons. When surface area is higher, the accessibility of crystallite edges to incorporate nitrogen is greater, as happened in the case of Wu-950. An increase in carbon content accompanied by a decrease in hydrogen content indicates changes in the degree of carbonization (W carbon was manufactured at about 600 °C). (22) Biniak, S.; Szymanski, G.; Siedlewski, J.; Swiatkowski, A. Carbon 1997, 35, 1799.
sample
pH
acidic
basic
all
basic/all
W Wu-450 Wu-950 Centaur
4.41 6.45 6.71 8.30
1.100 0.595 0.575 0.400
0.260 0.570 0.350 0.425
1.360 1.165 0.925 0.825
0.20 0.49 0.38 0.51
The changes in surface chemistry as a result of nitrogen incorporation can be evaluated using “wet” and “dry” methods. “Wet” methods used in this study are based on acid/base titration in aqueous solutions. It is noteworthy that although the pH significantly increased for woodbased carbon samples after modification with urea, it is still lower than that of Centaur (Table 2). The surface functional groups should be changed not only owing to the presence of nitrogen but also, as mentioned above, as a result of treatment at 950 °C. Such high temperature is expected to significantly alter the degree of carbonization and to reduce the number of acidic groups, which decarboxylate above about 200 °C.23 On the other hand, exposure of carbons to air after thermal treatment promotes oxidation as a consequence of the thermodynamically unstable edges of crystallite layers.3 Boehm titration was used here to determine the number of acidic and basic groups (Table 2). We did not distinguish between Boehm’s acid classes because nitrogen-containing groups can behave as acids in water solution and do not fit under Boehm’s categories.6 Since the determination is based on the strength of the acids (pKa values), our species containing amines can behave as either bases or strong (carboxylic) or weak (phenols) acids. Nevertheless, the results summarized in Table 2 show significant changes in acidity after thermal treatment with urea. In the case of the Wu-450 sample, the number of acids significantly decreased. This change is accompanied by a more than 2-fold increase in the number of bases. For Wu-950, the content of surface acids is slightly smaller than for Wu450, but the decrease in the content of bases is more pronounced. In the case of Centaur, the number of bases is higher than that of acids, yielding an average pH > 7. To evaluate the overall changes in the chemical character of the surface, we defined the degree of basicity as the ratio of the number of basic groups to all species detected on the surface. Analysis of these values indicates a significant increase in the basicity for the urea treated wood-based carbon samples. Potentiometric titration combined with the SAIEUS procedure (Solution of Adsorption Integral Equation Using Splines)24 provides information about the acidity constant distributions of species with 3 < pKa
