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Subcritical Water Regeneration of Supported Ruthenium Catalyst Poisoned by Sulfur Mitsumasa Osada,†,‡ Norihito Hiyoshi,‡ Osamu Sato,‡ Kunio Arai,‡ and Masayuki Shirai*,‡ Department of Chemical Engineering, Ichinoseki National College of Technology, Takanashi, Hagisho, Ichinoseki, Iwate 021-8511, Japan, and Research Center for Compact Chemical Process, National Institute of AdVanced Industrial Science and Technology (AIST), 4-2-1, Nigatake, Miyagino, Sendai 983-8551, Japan ReceiVed August 29, 2007. ReVised Manuscript ReceiVed NoVember 26, 2007
A titania-supported ruthenium (Ru/TiO2) catalyst was effective for the gasification of lignin to methane, carbon dioxide, and hydrogen in supercritical water. For the Ru/TiO2 catalyst poisoned by sulfur (S-Ru/TiO2), the overall gas yield decreased and the tetrahydrofuran-insoluble products (namely, char) were slightly formed in supercritical water gasification. After subcritical water treatments of the S-Ru/TiO2 catalyst at 523 and 573 K, its gasification activity was found to be higher than that without the treatment. On the other hand, the treatment of the S-Ru/TiO2 catalyst in supercritical water at 673 K showed lower activity than that in subcritical water. X-ray photoelectron spectroscopy analysis showed that three-fourths of the amount of sulfur was removed from the catalyst surface after the subcritical water treatment. It was concluded that deactivated catalysts poisoned by sulfur species could be regenerated by the removal of the sulfur by the treatment with subcritical water.
Introduction The gasification processes of woody biomass have received great attention as an efficient technology for the conversion of biomass into energy. The gas obtained by biomass gasification has wide ranging applications, for example, in gas turbines, in fuel cells, and for the production of synthesis gas. Generally, very high temperatures of over 1073 K are needed for steam reformation of lignin in the gasification process.1,2 Hence, lowtemperature methods for lignin gasification are the most desirable so that the waste heat from high-temperature processes in industry can be utilized for energy generation. From this point of view, catalytic gasification in supercritical water (Tc ) 647.3 K, Pc ) 22.1 MPa) could be a potential technology that can operate at low temperatures3–8 because of the high solubility and hydrolysis rates of biomass in supercritical water. This would also lead to the minimization of the mass-transfer limitations of the reactant9,10 and the rinsing effect of supercritical water for washing out coke precursors from the active * To whom correspondence should be addressed. Telephone: +81-22237-5219. Fax: +81-22-237-5224. E-mail:
[email protected]. † Ichinoseki National College of Technology. ‡ National Institute of Advanced Industrial Science and Technology. (1) Antal, M. J., Jr. Solar Energy; Plenum Press: New York, 1983; pp 175–255.. (2) Antal, M. J., Jr Ind. Eng. Chem. Prod. Res. DeV. 1983, 22, 366– 375. (3) Osada, M.; Sato, T.; Watanabe, M.; Shirai, M.; Arai, K. Combust. Sci. Technol. 2006, 178, 537–552. (4) Matsumura, Y.; Minowa, T.; Potic, B.; Kersten, S. R. A.; Prins, W.; van Swaaij, W. P. M.; van de Beld, B.; Elliott, D. C.; Neuenschwander, G. G.; Kruse, A.; Antal, M. J. Biomass Bioenergy 2005, 29, 269–292. (5) Watanabe, M.; Inomata, H.; Osada, M.; Sato, T.; Adschiri, T.; Arai, K. Fuel 2003, 82, 545–552. (6) Park, K. C.; Tomiyasu, H. Chem. Commun. 2003, 6, 694–695. (7) Antal, M. J., Jr.; Allen, S. G.; Schulman, D.; Xu, X.; Divilio, R. J. Ind. Eng. Chem. Res. 2000, 39, 4040–4053. (8) Kruse, A.; Gawlik, A. Ind. Eng. Chem. Res. 2003, 42, 267–279. (9) Savage, P. E. Chem. ReV. 1999, 99, 603–621. (10) Watanabe, M.; Sato, T.; Inomata, H.; Smith, R. L., Jr.; Arai, K.; Kruse, A.; Dinjus, E. Chem. ReV. 2004, 104, 5803–5821.
catalyst sites.11 These characteristics of supercritical water gasification are different from the steam reformation in which the gasification proceeds mainly through high temperature pyrolysis.1–3 The decomposition behavior of lignin in supercritical water around 673 K was reported in which lignin was converted to alkylphenols and formaldehyde via hydrolysis12–16 and then via the decomposition of alkylphenols to gases. Cross-linking reactions between the alkylphenols and formaldehyde16 also take place to produce solid materials, insoluble in water and tetrahydrofuran (THF).14–16 The gasification of lignin in supercritical water around 673 K is enhanced by several metal catalysts, among which titania or carbon supported ruthenium catalysts were found to be highly active.17–20 However, not only the initial activity and selectivity but also the duration of the catalytic activity with respect to support stability and poisoning by impurities present in biomass (such as sulfur compounds) for lignin gasification in supercritical water is very important.20–27 (11) Savage, P. E. Catal. Today 2000, 62, 167–173. (12) Bobleter, O.; Consin, R. Cellul. Chem. Technol. 1979, 13, 583– 593. (13) Funazukuri, T.; Wakao, N.; Smith, J. M. Fuel 1990, 69, 349–353. (14) Yokoyama, C.; Nishi, K.; Nakajima, A.; Seino, K. Sekiyu Gakkaishi 1998, 41, 243–250. (15) Ehara, K.; Saka, S.; Kawamoto, H. J. Wood Sci. 2002, 48, 320– 325. (16) Saisu, M.; Sato, T.; Watanabe, M.; Adschiri, T.; Arai, K. Energy Fuels 2003, 17, 922–928. (17) Sato, T.; Osada, M.; Watanabe, M.; Shirai, M.; Arai, K. Ind. Eng. Chem. Res. 2003, 42, 4277–4282. (18) Osada, M.; Sato, T.; Watanabe, M.; Adschiri, T.; Arai, K. Energy Fuels 2004, 18, 327–333. (19) Osada, M.; Sato, O.; Watanabe, M.; Arai, K.; Shirai, M. Energy Fuels 2006, 20, 930–935. (20) Osada, M.; Sato, O.; Arai, K.; Shirai, M. Energy Fuels 2006, 20, 2337–2343. (21) Elliott, D. C.; Sealock, L. J.; Baker, E. G. Ind. Eng. Chem. Res. 1993, 32, 1542–1548.
10.1021/ef7005194 CCC: $40.75 2008 American Chemical Society Published on Web 01/11/2008
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Figure 1. Apparatus for catalyst with subcritical and supercritical water treatment.
In general, the amount of sulfur contained in biomass is small; however, some biomass contained as much as 0.5 wt % sulfur, for example, bluegrass, giant brown kelp, and so forth.28 For the commercial operation of the supercritical water gasification of real biomass, the consideration of the duration of the catalytic activity against sulfur should be significant. We have reported that the presence of sulfur in the biomass lowered the gasification activities of the titania or carbon supported ruthenium catalysts in supercritical water because the sulfur atoms were adsorbed on the ruthenium metal surface during the gasification process. We also observed that the amount of sulfur that remained on the catalyst surface after the supercritical water gasification at 0.65 g · cm-3 of water density was lower than that at 0.50 g · cm-3 of water density, indicating that the water density affects the state and amount of sulfur adsorbed on the ruthenium atoms.27 Therefore, a simple and convenient technique, which can regenerate the catalysts poisoned by sulfur species, is desirable. We report here the regeneration technique for sulfur poisoned supported ruthenium catalysts involving the subcritical and supercritical treatments in a temperature and pressure range of 523-673 K and 25-40 MPa, respectively. Experimental Section Preparation of the Catalyst. The preparation procedure of the sulfur poisoned catalysts is as follows. A titania supported ruthenium catalyst (Ru/TiO2, 2 wt % ruthenium on titania) was soaked in an aqueous sulfuric acid solution followed by drying by evaporation. The molar ratio of sulfur atoms to surface ruthenium atoms was kept as 3.7, giving the molar ratio of sulfur to total ruthenium atoms as 1.0 (this catalyst is designated as S-Ru/TiO2 in this paper). Subcritical or supercritical water treatment of the S-Ru/TiO2 catalyst was given using equipment that consisted of a water loading unit, a water preheating unit, a packed bed reactor for the catalyst, a heat exchanger, a pressure control unit, and a solution recovery unit (Figure 1). After 1.0 g of the S-Ru/TiO2 catalyst was placed in a packed bed reactor, stainless steel filters were fitted at both ends of the reactor. Distilled water was introduced into the reactor at a flow (22) Elliott, D. C.; Sealock, L. J.; Baker, E. G. Ind. Eng. Chem. Res. 1994, 33, 558–565. (23) Elliott, D. C.; Phelps, M. R.; Sealock, L. J.; Baker, E. G. Ind. Eng. Chem. Res. 1994, 33, 566–574. (24) Elliott, D. C.; Neuenschwander, G. G.; Hart, T. R.; Scott Burner, R.; Zacher, A. H.; Engelhard, M. H.; Young, J. S.; McCready, D. E. Ind. Eng. Chem. Res. 2004, 43, 1999–2004. (25) Elliott, D. C.; Hart, T. R.; Neuenschwander, G. G. Ind. Eng. Chem. Res. 2006, 45, 3776–3781. (26) Davda, R. R.; Shabaker, J. W.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Appl. Catal., B 2003, 43, 13–26. (27) Osada, M.; Hiyoshi, N.; Sato, O.; Arai, K.; Shirai, M. Energy Fuels 2007, 21, 1400–1405. (28) Klass, D. L. Biomass for Renewable Energy, Fuels, and Chemicals; Academic Press: San Diego, 1998; pp 72–90. (29) Wagner, W.; Purb, A. J. Phys. Chem. Ref. Data 2002, 31, 387– 535.
Osada et al. rate of 3 mL · min-1 by high performance liquid chromatography pumps to pressurize the system up to 40 MPa and was maintained at the desired pressure by a back-pressure regulator, and the reactor was heated by an electric furnace. The reactor was kept in an air bath at 373 K, and the temperature of both the air bath and the extraction solvent unit were then increased up to 673 K. An aqueous solution at the outlet of the reactor was rapidly quenched by a cooling jacket and subsequently collected continuously in the sampling bottles. The amount of sulfur in the aqueous solutions was analyzed by an inductively coupled plasma (ICP) mass spectrometer (Yokogawa, model HP4500). After cooling, catalysts were recovered by opening the reactor and dried by evaporation. These catalysts were then used for lignin gasification in a different reactor described below. The catalysts before and after treatment by subcritical and supercritical water were also analyzed by X-ray photoelectron spectroscopy (XPS, UVLAC-PHI, Inc., PHI 5700ESCA). Evaluation of Catalytic Activity for Lignin Gasification. Organosolv-lignin powder (lignin) was purchased from Aldrich, and its molecular formula was C42.39H45.46O12.15, as determined by an ultimate CHNS analyzer (Perkin-Elmer, model 2400). Its molecular weight was between 1000 and 1500, as determined by a gel permeation chromatography (GPC) system (Waters, GPC150Cplus). The lignin powder was completely soluble in THF. THF (+97%) and 5 wt % sulfuric acid solution were purchased from Wako Pure Chemicals Industries, Ltd. All chemicals were used without further purification. Distilled water was obtained from a water distillation apparatus (Yamato Co., model WG-220). Catalytic gasification was conducted in a stainless-steel 316 reactor having an internal volume of 6 cm3. A known amount of catalyst (typically 0.375 g), lignin (0.1 g), and water (3.0 g) were loaded into the reactor. This amount of water corresponded to a density of 0.5 g · cm-3 for a 37.1 MPa partial pressure of water at 673 K.29 Air inside the reactor was purged with argon gas. The reactor was submerged into a preheated sand bath (Takahashi Rica Co., model TK-3), and it required about 4 min to heat the reactor to 673 K. The reaction time reported here includes this heat-up time. After a given reaction time, the reactor was taken out of the sand bath and submerged into a water bath for its rapid cooling to room temperature. Gaseous products were collected by a syringe through sampling loops attached to a gas chromatograph for analysis by a thermal conductivity detector (Shimadzu, model GC-8A). After sampling the gaseous products, the other products from the reactor were recovered with pure water and separated as water-soluble and water-insoluble fractions. The water-insoluble fraction was again separated as tetrahydrofuran-soluble (THF-soluble) and solid fractions (THF-insoluble). Since the solid fraction also contained the catalysts used, we evaluated the amount of THF-insoluble products by subtracting the weight of the catalyst loaded from the solid fraction. Water-soluble and THF-soluble products were analyzed by gas chromatography using a flame ionization detector (Hewlett-Packard, model HP-6980) and gas chromatography–mass spectroscopy (JEOL, Automass 20). The amount of organic carbon in the watersoluble fractions was evaluated using the total organic carbon analyzer (Shimadzu, model TOC-5000A). Ultimate analysis of the solid product was conducted by a CHNS analyzer. The product yield based on carbon and gas composition are defined as given below, product yield based on carbon (C%) ) moles of carbon in the product × 100 (1) moles of carbon in lignin loaded moles of gas product gas composition (%) ) × 100 (2) sum of moles of gas product
Results and Discussion Effect of Subcritical and Supercritical Water Treatment on Catalytic Activity for Gasification. The gasification activities of the catalysts as prepared above were investigated in detail. Table 1 shows the product yields and the gas composition
Regeneration of Supported Ruthenium Catalyst
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Table 1. Treatment Conditions of Catalyst and Product Yields and Composition of Gas Products from Catalytic Lignin Gasification in Supercritical Watera treatment conditions run 1 2 3 4 5 6 7 8 9 10 11
3% H2O2 without catalyst Ru/TiO2 S-Ru/TiO2 S-Ru/TiO2 S-Ru/TiO2 S-Ru/TiO2 S-Ru/TiO2 S-Ru/TiO2 S-Ru/TiO2 S-Ru/TiO2 S-Ru/TiO2
O O
yield (C%)
temperature pressure water density time (h) (K) (MPa) (g · cm-3)
298 523 573 573 673 673 523 573
0.1 25 25 25 25 40 25 25
0.99 0.83 0.75 0.75 0.17 0.52 0.83 0.75
3 3 3 10 3 3 3 3
gas 7.9 97.7 21.0 20.8 47.3 47.2 57.1 26.1 41.5 49.5 46.3
composition of gas products (%)
waterTHFTHFsoluble insoluble solubleb 20.6 0.7 18.3 10.8 2.6 1.7 1.5 8.1 4.5 2.7 1.7
20.9 0.0 3.1 3.2 0.0 0.0 0.0 2.1 0.2 0.0 0.0
50.6 1.6 57.6 65.2 50.1 51.1 41.4 63.7 53.8 47.8 52.0
H2
CH4
CO2
C2-C4 gases
21.5 4.5 14.5 19.6 7.4 6.4 5.4 17.2 7.8 6.4 7.5
30.6 49.0 24.5 23.4 46.7 45.4 47.6 21.6 36.4 41.7 45.4
43.8 46.5 57.7 52.7 42.5 44.4 43.6 52.4 50.6 48.3 42.5
4.1 0.0 3.3 4.3 3.4 3.8 3.4 8.8 5.2 3.6 4.6
a Gasification conditions: 0.1 g of lignin, 7.4 × 10-5 mol of ruthenium in catalyst at 673 K, 0.50 g · cm-3 of water density, and 180 min reaction time. b (THF-soluble (C%)) ) 100 - ((gas yield (C%)) + (water-soluble yield (C%)) + (THF-insoluble (C%))).
obtained by the gasification of lignin over the Ru/TiO2 and S-Ru/TiO2 catalysts at 673 K and 0.5 g · cm-3 of water density after 180 min. The gas yield was only 8 C% and THF-insoluble (namely char) yield was 20 C% in the absence of the catalyst (run 1). The major gas composition was carbon dioxide, methane, hydrogen, and C2-C4 gases (ethane, propane, and butane). Lignin was completely gasified over the Ru/TiO2 catalyst (run 2). However, the gas yield decreased to 21 C%, and the THF-insoluble products obtained were 3.1 C% over the S-Ru/TiO2catalyst (run 3). In comparison with run 2, the methane composition decreased, and those of carbon dioxide and hydrogen increased in run 3. We have already reported that the lowering of the gas yield and the methane composition and also the formation of the THF-insoluble products were observed by the addition of sulfur to Ru/TiO2 in catalytic gasification, because of the decrease of the surface ruthenium ensembles which are indispensable for the carbon-carbon bond breaking and the methanation reactions.30 The yield and the composition of gas products and the yield of THF-insoluble products of run 4 were almost the same as those of run 3, indicating that the water treatment at 298 K and 0.1 MPa was not effective for the regeneration of the S-Ru/ TiO2 catalyst. On the other hand, the gas yields and the methane composition for the S-Ru/TiO2 catalyst treated with subcritical water treatments (runs 5 and 6) were higher than those of run 3. The THF-insoluble products were also not formed in runs 5 and 6. These results suggest that the activity of the S-Ru/TiO2 catalyst for gasification and methanation was regenerated by the subcritical water treatment. Also, the gas yield and the methane composition for the S-Ru/TiO2 catalyst treated with subcritical water for 10 h (run 7) was higher than those for the S-Ru/TiO2 catalyst treated with subcritical water for 3 h (run 6), indicating that a longer treatment time was effective for the regeneration of the S-Ru/TiO2 catalyst. The gas yield for the S-Ru/TiO2 catalyst after the supercritical water treatment (run 8) was higher than that for the untreated S-Ru/TiO2 catalyst (run 3); however, it was lower than that for the catalysts treated with subcritical water (run 5 and 6), indicating that the subcritical water was more effective than the supercritical water for the regeneration of the Ru/TiO2 catalyst poisoned by sulfur. Furthermore, 2.1 C% of the THFinsoluble product was formed in run 8, and the methane composition for run 8 was lower than those for runs 5 and 6. The gas yield and the methane composition of the S-Ru/TiO2 catalyst treated with supercritical water at 673 K and 40 MPa (30) Osada, M.; Hiyoshi, N.; Sato, O.; Arai, K.; Shirai, M. Energy Fuels 2007, 21, 1854–1858.
Figure 2. (a) Pressure and temperature dependence of water density. (b) Relation between water density and gas yield. Treatment conditions of subcritical and supercritical water in Table 1: run 4 (O), runs 5 and 10 (4), runs 6, 7, and 11 (0), run 8 (]), run 9 (3), and run 3 (×).
(run 9) were higher than those of run 8, indicating that the supercritical water treatment at a higher water density at 673 K was more effective for the regeneration of the S-Ru/TiO2 catalyst. Figure 2a shows the temperature and pressure dependence of the water density, indicating a comparison of the subcritical water and supercritical water treatment conditions. The water density of subcritical water is almost constant over the saturated vapor pressure; on the other hand, the water density under the supercritical condition at 673 K increases continuously and significantly with increase in pressure. Figure 2b shows the relation between the water density and the gas yield. Compared with the untreated S-Ru/TiO2 catalyst (run 3), the gas yield of the S-Ru/TiO2 catalyst increased with increase in the water density, indicating that the water density significantly affected the degree of regeneration of the catalytic activity. We also conducted the regeneration of the S-Ru/TiO2 catalyst in the presence of hydrogen peroxide (runs 10 and 11), which decomposes to give oxygen and water, in which the gas
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Table 2. Summary of Results from XPS of the Ru/TiO2 Surfacea treatment conditions run 2 3 5 7 8
catalyst Ru/TiO2 S-Ru/TiO2 S-Ru/TiO2 S-Ru/TiO2 S-Ru/TiO2
a
temperature (K)
water density (g · cm-3)
523 573 673
0.83 0.75 0.17
S(2p) time (h)
3 10 3
SO320.5 3.0 0.2 0.2 0.7
SO422.8 4.4 2.3 1.5 3.4
Ru(3d) total
Ru0
Ru4+
Ru6+ or Ru8+
total
S(2p)/Ru(3d)
3.3 7.4 2.5 1.7 4.1
NDb
1.1 1.1 0.6 0.6 0.5
0.9 0.8 0.5 0.6 0.4
2.0 1.9 2.3 2.5 1.7
1.7 3.9 1.1 0.7 2.4
ND 1.2 1.3 0.8
The unit is atom %. b ND ) not detected.
yields and the composition were almost the same as those of runs 5 and 6, indicating that the presence of oxygen did not affect the regeneration of the S-Ru/TiO2 catalyst. Characterization of the Sulfur Species on the Ruthenium Surface. An ICP analysis showed the presence of sulfur species and the absence of ruthenium species in the solution recovered from the S-Ru/TiO2 catalyst treated with subcritical and supercritical water, indicating that only sulfur leached out into the water and that the ruthenium metal existed on the titania support during the treatment process. The amount and state of the sulfur present on the catalyst surface before and after the subcritical and supercritical water treatments were characterized by an XPS analysis, and the results are shown in Table 2. The XPS analysis showed the presence of titanium, oxygen, ruthenium, and sulfur. In the fresh Ru/TiO2 catalyst (run 2), small amounts of various sulfur species (SO32- and SO42-) and ruthenium species (Ru4+, Ru6+, and Ru8+) were observed because the titania support used in this study contained sulfur species to obtain a large surface area in the manufacturing process. The S/Ru value (1.1) of the S-Ru/TiO2 catalyst treated by subcritical water at 523 K for 3 h (run 5) was lower than that (3.9) of the untreated S-Ru/TiO2 sample (run 3). Also, Ru0 species were observed in the S-Ru/TiO2 catalysts after treatment by subcritical water at 523 K for 3 h (run 5), indicating that the reduction of ruthenium also proceeded during the subcritical water treatment. For the S-Ru/TiO2 catalyst after the subcritical water treatment at 573 K for 10 h (run 7), which showed high activity for gasification, the S/Ru ratio observed was 0.7 which was the lowest among all the pretreatment runs carried out in this study (Table 2). The S/Ru ratio of the S-Ru/ TiO2 catalyst after the supercritical water treatment (run 8) was lower than that of the untreated S-Ru/TiO2 catalyst (run 3); however, it was higher than for those catalysts treated under subcritical water (runs 5 and 7) conditions. We also evaluated the S/Ru ratio of the S-Ru/TiO2 catalysts from the amount of sulfur in the solution recovered during the subcritical and supercritical water treatments by ICP analysis. The results are shown in Figure 3. As mentioned in the Experimental Section, the S/Ru ratio of the S-Ru/TiO2 catalysts was prepared as 3.7. Because we detected only sulfur and did not detect ruthenium by the ICP analysis, the S/Ru ratio was evaluated on the assumption that the number of surface rutheniums was constant. The S/Ru ratio obtained after the subcritical water treatment at 573 K and 25 MPa (run 6) was lower than that after the supercritical water treatment at 673 K and 25 MPa (run 8), indicating that the amount of sulfur removal from the catalyst surface by the subcritical water treatment is higher than that by the supercritical water treatment. Most of the sulfur removal occurred within 1 h, and the remaining sulfur was removed gradually after 1 h during the subcritical and supercritical water treatments. The S/Ru ratio values for the S-Ru/TiO2 catalysts after the subcritical and supercritical water treatments for 3 h were 0.6 and 1.6, respectively, thus confirming the trend as shown by the XPS results in Table 2.
Figure 3. Sulfur to ruthenium ratio during the catalyst treatment under subcritical and supercritical water conditions.
Effect of the Regeneration of Catalyst Poisoned by Sulfur by Subcritical and Supercritical Water Treatment. As shown in Table 1, we found that the gasification activities of the catalyst after the subcritical water treatment were greater than those after the supercritical water treatment. From XPS analysis of the catalyst surface (Table 2), the amount of sulfur after the subcritical water treatment was lower than that by the supercritical water treatment, which would be due to a high solubility of the sulfur species in subcritical water. We found that treatment under higher water density and longer treatment time were effective for the regeneration of the catalytic activity for gasification. We reported that the adsorption of sulfur atoms on ruthenium metal particles inhibited the carbon-carbon bond breaking and the methanation reaction because the ruthenium ensembles, responsible for both these reactions, decreased.30 A part of the ruthenium ensembles were regenerated by the removal of sulfur during the subcritical water treatment, which was supported by the result that the gas yield and the methane composition for the S-Ru/TiO2 catalyst after the subcritical water treatment was higher than that without the treatment. Hence, we can propose a promising gasification process for the waste biomass containing sulfur over the Ru/TiO2 catalyst in supercritical water at 673 K. The activity of the supported ruthenium catalysts could be higher initially; however, it would decrease because of the adsorption of sulfur during gasification under the supercritical water condition. After the subcritical water treatment of the used catalyst, its gasification activity can be recovered. Alternation of the supercritical water gasification and the subcritical water treatment of the catalyst would provide a consistent high performance of the Ru/TiO2 catalyst. Conclusion The regeneration of a titania-supported ruthenium catalyst poisoned by sulfur (S-Ru/TiO2) was examined using subcritical and supercritical water treatments in a temperature and pressure range of 523-673 K and 25-40 MPa, respectively. The
Regeneration of Supported Ruthenium Catalyst
catalytic activities of the S-Ru/TiO2 catalyst for the gasification and the methanation reactions increased after the subcritical and supercritical water treatment of the catalyst. The catalytic activities and the amount of sulfur species of the S-Ru/TiO2 removed by the subcritical water treatment were higher than those after the supercritical water treatment. The subcritical water treatment is a promising method for the regeneration of
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the ruthenium catalyst poisoned by sulfur species during supercritical water gasification of waste biomass. Acknowledgment. This research was partially supported by the Japan Science and Technology Agency (JST). EF7005194
