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Energy & Fuels 2009, 23, 1674–1682
Selective Catalytic Oxidation of Hydrogen Sulfide on Activated Carbons Impregnated with Sodium Hydroxide Svetlana Bashkova,† Timothy R. Armstrong,‡ and Viviane Schwartz*,‡ Department of Chemistry, The City College of the City UniVersity of New York, 160 ConVent AVenue, New York City, New York 10031, and Oak Ridge National Laboratory, Post Office Box 2008, Oak Ridge, Tennessee 37831 ReceiVed August 27, 2008. ReVised Manuscript ReceiVed NoVember 4, 2008
Two activated carbons of different origin were impregnated with the solution of sodium hydroxide (NaOH) of various concentrations up to 10 wt %, and the effect of impregnation on the catalytic performance of the carbons was evaluated. The catalytic activity was analyzed in terms of the capacity of carbons for hydrogen sulfide (H2S) conversion and removal from hydrogen-rich fuel streams and the emission times of H2S and the products of its oxidation [e.g., sulfur dioxide (SO2) and carbonyl sulfide (COS)]. The results of impregnation showed a significant improvement in the catalytic activity of both carbons proportional to the amount of NaOH introduced. NaOH introduces hydroxyl groups (OH-) on the surface of the activated carbon that increase its surface reactivity and its interaction with sulfur-containing compounds.
1. Introduction Hydrogen-rich fuel streams are the primary fuel for both fuel cells and integrated gasification combined cycle (IGCC) processes to produce electric power. In both cases, complete desulfurization of the fuel is required. For fuel cells, sulfur removal prevents poisoning of the anode catalysts by sulfur, and for the case of IFCCs, sulfur removal is necessary to minimize corrosion and maintain environmental emission limits. Because H2S is a major component of fuel gases and causes a great concern for the successful operation of the abovementioned processes, it must be reduced to levels less than 1 part per million, preferably parts per billion. The use of activated carbons as catalysts for H2S oxidation to sulfur has been studied by many research groups1-17 in recent years. Activated carbon * To whom correspondence should be addressed. Telephone: +1 (865) 576-6749. Fax: +1 (865) 576-6749. E-mail:
[email protected]. † The City College of the City University of New York. ‡ Oak Ridge National Laboratory. (1) Cariaso, O. C.; Walker, P. L., Jr. Carbon 1975, 13, 233–239. (2) Gosh, T. K.; Tollefson, E. L. Can. J. Chem. Eng. 1986, 64, 960– 968. (3) Steijns, M.; Mars, P. Ind. Eng. Chem. Prod. Res. DeV. 1977, 16, 35–41. (4) Kaliva, A. N.; Smith, J. W. Can. J. Chem. Eng. 1983, 61, 208–212. (5) Sreeramamurthy, R.; Menon, P. G. J. Catal. 1975, 37, 287–296. (6) Mikhalovsky, S. V.; Zaitsev, Y. P. Carbon 1997, 35, 1367–1374. (7) Steijns, M.; Mars, P. J. Catal. 1974, 35, 11–17. (8) Steijns, M.; Derks, F.; Verloop, A.; Mars, P. J. Catal. 1976, 42, 87–95. (9) Klein, J.; Henning, K. D. Fuel 1984, 63, 1064–1067. (10) Puri, B. R.; Kumar, B.; Kalra, K. C. Indian J. Chem. 1971, 9, 970. (11) Bagreev, A.; Katikaneni, S.; Parab, S.; Bandosz, T. J. Catal. Today 2005, 99, 329–337. (12) Lampert, J. J. Power Sources 2004, 131, 27–34. (13) Gardner, T. H.; Berry, D. A.; Lyons, K. D.; Beer, S. K.; Freed, A. D. Fuel 2002, 81, 2157–2166. (14) Wu, X.; Kercher, A. K.; Schwartz, V.; Overbury, S. H.; Armstrong, T. R. Extended Abstracts, 228th American Chemical Society (ACS) National Meeting, Philadelphia, PA, Aug 22-26, 2004; Vol. 49, p 893. (15) Wu, X.; Schwartz, V.; Overbury, S. H.; Armstrong, T. R. Energy Fuel 2005, 19, 1774–1782. (16) Wu, X.; Kercher, A. K.; Schwartz, V.; Overbury, S. H.; Armstrong, T. R. Carbon 2005, 43, 1087–1090.
plays a dual role, being both a catalyst for the direct oxidation of H2S by air and as an adsorbent for removing sulfur via chemisorption and maintaining it within its pore structure allowing for easy disposal of the sorbent. Unfortunately, carbon catalysts can also promote the oxidation of sulfur to SO2 and cause the formation of COS.14-17 Both of these compounds contain sulfur and may cause damaging effects to fuel cells or the environment. Therefore, an ideal activated carbon catalyst should be capable of complete H2S conversion with high selectivity to elemental sulfur, opposed to other sulfur-containing compounds (i.e., SO2 and COS).15 Such a process has been termed “selective catalytic oxidation” and is a very promising approach for H2S removal from hydrogen-containing gas streams.1,3,12-17 Previous research by Bashkova et al. and Wu et al. on H2S selective catalysts has been concentrated on understanding the properties of activated carbon of different origins and the effects of processing conditions14-16 and microstructural characteristics17 of these materials on their catalytic activity and selectivity. One of the carbons investigated and reported previously by our group,15,17 WSC, exhibited particularly good activity and selectivity with respect to the catalytic oxidation of H2S to sulfur. To further improve the catalytic properties of this material, it was subjected to the impregnation with alkaline hydroxides. Impregnation with such active reagents as metal oxides,6,18 potassium iodide,19 potassium carbonate,20 ammonia,21,22 urea,23,24 (17) Bashkova, S.; Baker, F. S.; Wu, X.; Armstrong, T. R.; Schwartz, V. Carbon 2007, 45, 1354–1363. (18) Katoh, H.; Kuniyoshi, I.; Hirai, M.; Shoda, M. Appl. Catal., B 1995, 6, 255–262. (19) Shin, C. S.; Kim, K. H.; Yu, S. H.; Ryu, S. K. Presented at 7th International Conference on Fundamentals of Adsorption, Nagasaki, Japan, May 20-25, 2001. (20) Przepiorski, J.; Yoshida, S.; Oya, A. Carbon 1999, 37, 1881–1890. (21) Turk, A.; Sakalis, E.; Lessuck, J.; Karamitsos, H.; Rago, O. EnViron. Sci. Technol. 1989, 23, 1242–1245. (22) Boudou, J. P.; Chehimi, M.; Broniek, E.; Siemieniewska, T.; Bimer, J. Carbon 2003, 41, 1999–2007. (23) Adib, F.; Bagreev, A.; Bandosz, T. J. Langmuir 2000, 16, 1980– 1986.
10.1021/ef800711c CCC: $40.75 2009 American Chemical Society Published on Web 02/27/2009
SelectiVe Catalytic Oxidation of Hydrogen Sulfide
and alkaline materials21,25-28 has been applied to improve catalyst performance. Besides the known beneficial effects of NaOH impregnation on H2S adsorption,25-27 it has also been shown to minimize the emissions of SO2 and COS. According to results reported by George,29 deposition of 3.9 wt % NaOH on Chromosorb-A increased the adsorption of COS from 0.011 to 0.018 mmol/g and also increased the initial rate of COS hydrolysis 25 times at 230 °C. George29 suggested that COS could be adsorbed on the surface of a catalyst via ion-dipole interactions between COS and OH-. Similarly, Fiedorow et al.30 observed that COS was adsorbed on the alumina surface at basic sites. In relation to the activated carbon, COS may also react with NaOH on the carbon surface to yield some sodium sulfide (Na2S) and sodium carbonate (Na2CO3).31 It is also well-known that the increase in basicity of the carbon surface leads to the increase of SO2 adsorption.32-35 Further, Guo et al.33 reported that impregnation of the activated carbon with alkaline materials can enhance the amount of SO2 adsorbed via the formation of sulfites. The objective of this work was to investigate the effects of sodium hydroxide additions on the catalytic properties, specifically toward conversion of H2S to Sn, of activated carbon. 2. Experimental Section Two activated carbon materials were chosen for this study: “WSC”, a laboratory-synthesized, physically activated, cellulosicbased carbon, and “VA”, a physically activated, coconut shell-based carbon (PICA, Saint Maurice Cedex, France). Impregnation with NaOH was performed by mixing 20 mL of carbon with 20 mL of a solution of NaOH of variable concentration for 2 h followed by drying at 110 °C for 24 h. The weight percent of NaOH in the impregnated activated carbon was calculated as the weight of NaOH divided by the weight of carbon. The impregnated activated carbons were labeled as “WSC5” or “VA5”, where the number added to the sample name represents the weight percent of NaOH in the solution used during impregnation. The spent catalysts after desulfurization (the term desulfurization will be referred throughout the text as the process of the removal of H2S from the feed gas) were identified by the addition of the letter “d”. For example, WSC7d is the 7 wt % NaOH/WSC catalyst after the desulfurization activity test. Surface Area and Pore Size Distribution Measurements. Nitrogen adsorption isotherms on the carbon samples were determined at 77 K using an AUTOSORB-1C instrument (Quantachrome Corporation, Boynton Beach, FL). The samples were outgassed overnight at 280 °C under medium vacuum, immediately prior to the isotherm measurements. For the spent samples, the temperature of 100 °C was used. The specific surface area (SBET) was calculated from the isotherm data using the Brunauer, Emmett, and Teller (24) Bagreev, A.; Menendez, J. A.; Dukhno, I.; Tarasenko, Y.; Bandosz, T. J. Carbon 2004, 42, 469–476. (25) Bagreev, A.; Bandosz, T. J. Ind. Eng. Chem. Res. 2002, 41, 672– 679. (26) Chang, H. L.; Tsai, J. H.; Tsai, C. L.; Hsu, Y. C. Sep. Sci. Technol. 2000, 35, 903. (27) Bandosz, T. J.; Bagreev, A.; Adib, F.; Turk, A. EnViron. Sci. Technol. 2000, 34, 1069–1074. (28) Yan, R.; Liang, D. T.; Tsen, L.; Tay, J. H. EnViron. Sci. Technol. 2002, 36, 4460–4466. (29) George, Z. M. J. Catal. 1974, 35, 218–224. (30) Fiedorow, R.; Le´aute´, R.; Dalla Lana, I. G. J. Catal. 1984, 85, 339–348. (31) Ferm, R. J. Chem. ReV. 1957, 57, 621–640. (32) Guo, J.; Lua, A. C. Sep. Purif. Technol. 2003, 30, 265–273. (33) Guo, J.; Lua, A. C. J. Colloid Interface Sci. 2002, 254, 227–233. (34) Davini, P. Carbon 1990, 28, 565–571. (35) Lisovskii, A.; Semiat, R.; Aharoni, C. Carbon 1997, 35, 1639– 1643.
Energy & Fuels, Vol. 23, 2009 1675 (BET) model.36 Total pore volume (Vtot), volume of pores less than 1 nm (V
