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SiO2 catalysts
Applied Catalysis B: Environmental 16 (1998) 235±243
Selective oxidation of H2S to elemental sulfur over TiO2/SiO2 catalysts Sung Woo Chuna, Jae Yeol Janga, Dae Won Parka,*, Hee Chul Woob, Jong Shik Chungc a Department of Chemical Engineering, Pusan National University, Pusan 609-735, South Korea Department of Chemical Engineering, Pukyung National University, Pusan 608-739, South Korea c Department of Chemical Engineering, School of Environmental Engineering, Pohang University of Science and Technology, Pohang 790-784, South Korea b
Received 21 March 1997; received in revised form 30 August 1997; accepted 1 September 1997
Abstract Selective catalytic oxidation of H2S to elemental sulfur using TiO2/SiO2 catalysts was investigated in this study. The reaction test with pure TiS2 and Ti(SO4)2 and cyclic temperature operation showed that TiO2 had good resistance to sulfation and sul®dation, which are known as the main causes of catalytic deactivation in sulfur recovery process. Catalyst deactivation caused by deposition of sulfur on the catalyst surface resulted in a decrease in the conversion of H2S, not in the selectivity to sulfur, at low reaction temperatures. With an increase in the O2/H2S ratio from 0.5 to 4, the conversion was slightly increased, but the selectivity to elemental sulfur was remarkably decreased. The presence of water vapor decreased both the activity and selectivity. Temperature programmed desorption with NH3 and CH3COOH and reaction tests after doping K2O and B2O3 to TiO2/SiO2 revealed that the selective oxidation of H2S occurred on acidic sites and the reverse Claus reaction proceeded on basic sites. # 1998 Elsevier Science B.V. Keywords: Hydrogen sul®de; Sulfur; Catalytic oxidation; TiO2/SiO2
1. Introduction In these days, air pollution is a serious global problem and legal regulation has been tightened. Especially, the emission limits for SOx become stringent since sulfur oxides are considered responsible for acid rain. Recently, a large amount of hydrogen sul®de is released from crude oil, natural gas re®neries and metal smelting process in steel production. The coal *Corresponding author. Tel: +82 51 510 2399; fax: +82 51 512 8563; e-mail: [email protected] 0926-3373/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-3373(97)00078-7
liquefaction process is considered to be the major source of H2S emissions in the near future. Hydrogen sul®de is usually removed by the wellknown Claus process [1,2]. The Claus process consists of two steps: thermal oxidation and catalytic reaction. Step I 2H2 S 3O2 ! 2SO2 2H2 O thermal oxidation 3 Step II 2H2 S SO2 $ Sn 2H2 O n catalytic reaction
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In the thermal oxidation step, one-third of the hydrogen sul®de is ®rst burned with air to sulfur dioxide in a waste heat furnace. In the subsequent catalytic reaction step, SO2 is reacted with unconverted H2S to elemental sulfur over an Al2O3 catalyst. However, due to thermodynamic limitations, typically 3±5% of H2S is not converted to sulfur. As the environmental protection law becomes more strict, it is necessary to further treat the residual gas of the Claus installation, the so-called tail gas. Various commercial tail gas treating (TGT) processes have been developed. Conventional Claus TGT processes involve a hydrogen sul®de absorption step, in which a tail gas containing unreacted hydrogen sul®de is introduced in an alkaline bath. The most attractive process that has been recently developed is MODOP (mobil direct oxidation) process [3±5] or Super-Claus process [6±9], both of which are based on the direct catalytic oxidation of H2S to elemental sulfur. 2 2H2 S O2 ! Sn 2H2 O n
direct oxidation
High conversion can be obtained in direct oxidation of H2S to elemental sulfur because it is not a thermodynamic reversible reaction. In the MODOP process, H2S is oxidized to elemental sulfur with a stoichiometric amount of oxygen on a TiO2 catalyst. Drawback of the MODOP process is removal of water from the tail gas before the reaction owing to the deactivation of the catalyst by the presence of water. In the Super-Claus process, tail gas can be treated without dehydration step since Fe2O3/SiO2 catalyst is not deactivated even in the presence of 30 vol% of water vapor. However, the Super-Claus process cannot treat high concentrations of H2S above 2 vol% because it is necessary to supply excess oxygen (usually 10 times the stoichiometric amount) to overcome catalytic deactivation caused by the water. To treat a high concentration of H2S using a stoichiometric amount of oxygen, TiO2/SiO2 catalyst was selected in this study. Although the MODOP process is a well-known commercial process [3±5], there is no information available about the role of TiO2/SiO2 catalyst. Therefore the primary object of this paper is to test the stability of TiO2/SiO2 catalyst and to understand the effects of O2/H2S ratio and water vapor
on the direct oxidation of hydrogen sul®de to elemental sulfur. Kinetic studies on the effects of surface acidity are also carried out to elucidate the reaction path. 2. Experimental 2.1. Catalyst preparation Catalysts were prepared by an incipient wetness method in a glove chamber under ¯owing inert gas (N2), to avoid hydrolysis of precursor. TiO2/SiO2 catalysts were prepared by impregnating a solution of Ti(OCH(CH3)2)4 (titanium isopropoxide; Aldrich) dissolved in anhydrous toluene (Junsei) onto an SiO2 support (JRC-SIO-5), followed by drying for 12 h in an oven at 1208C. After impregnation, toluene was evaporated with a rotary vacuum evaporator at 608C. Then the catalysts were dried in a vacuum oven at 1308C for 12 h and calcined in ¯owing air at 5008C for 5 h. The K2O/[TiO2/SiO2] and B2O3/ [TiO2/SiO2] catalysts were prepared by impregnating aqueous solutions of KNO3 (potassium nitrate; Katayama) and HBO3 (boric acid; Junsei), respectively, on the previously calcined 10 wt% TiO2/SiO2 catalyst. 2.2. Characterization of catalysts The speci®c surface areas of the catalysts were determined by the continuous ¯ow technique using a BET apparatus (Quantachrom, Autosorb-1). The phase analysis was performed by X-ray diffraction crystallography with Cu K radiation (Rigaku, DMAX 2400). The morphology of the catalysts was monitored from scanning electron microscopy (JSM, JEOL 5400). In order to check acid/base site distribution, temperature programmed desorption (TPD) experiments using NH3 and CH3COOH were carried out with a thermal conductivity detector. 2.3. Reaction test The reaction test apparatus is illustrated in Fig. 1. A vertical continuous ¯ow ®xed bed reactor made of
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Conversion of H2 S %
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H2 Sinlet ÿH2 Soutlet 100 H2 Sinlet
Selectivity to sulfur %
H2 Sinlet ÿ H2 Soutlet ÿ SO2 outlet 100 H2 Sinlet ÿ H2 Soutlet
3. Results and discussion 3.1. The activity test of TiO2 =SiO2 catalysts
Fig. 1. Schematic diagram of fixed bed reactor system: 1, moisture trap; 2, oxygen trap; 3, MFC; 4, cut-off valve; 5, three-way valve; 6, bubble flowmeter; 7, sulfur condenser; 8, reactor heater; 9, syringe pump; 10, sulfur removal filter; 11, six-port valve; 12, exhaust gas trap.
Pyrex tube (i.d.1 in.) was used for the reaction test at atmospheric pressure. The gas ¯ow rate was controlled by mass ¯ow controllers (Brooks MFC, 5850E). A mixture of H2S and O2 diluted with helium was used as reactant. A sulfur condenser was attached at the ef¯uent side of the reactor, and its temperature was constantly maintained at 1108C to condense only sulfur vapor. A line ®lter was installed after the condenser to trap any sulfur mist which had not been captured by the condenser. From the condenser up to gas chromatograph, all the lines and ®ttings were heated above 1208C to prevent condensation of water vapor. In a typical experiment, the reactant composition consisted of 5 vol% H2S, 2.5 vol% O2 and balance He. The ratio of O2/H2S was varied between 0.5 and 4, and the gas hourly space velocity (GHSV) was ®xed at 3000 hÿ1. Water vapor was introduced to the reactant stream using a steam evaporator ®lled with small glass beads, and its amount was controlled by a syringe pump (Sage Instruments, 341A). The O2, H2S and SO2 content of ef¯uent gas were analyzed by a gas chromatography (HP 5890) equipped with a thermal conductivity detector and a 6 ft Porapak T column (80±100 mesh) at 1008C. The exit gas from the analyzer was passed through a trap containing a concentrated NaOH solution and vented out to a hood. The conversion of H2S and the selectivity to sulfur are de®ned as follows:
TiO2/SiO2 catalysts prepared with different loadings of TiO2 (2.5, 5, 10, 30 wt%) showed about the same value in the BET area; 194 m2/g (2.5 wt%), 192 m2/g (5 wt%) 195 m2/g (10 wt%), and 195 m2/g (30 wt%), respectively. Therefore, one can see that TiO2 is dispersed as very small particles, and it has been con®rmed from mapping images of TiO2/SiO2 catalyst by a SEM/EDX. The results of the reaction test at 2758C under typical reaction condition (5 vol% H2S, 2.5 vol% O2, 92.5 vol% He) is shown in Fig. 2. The SiO2 support showed very low conversion of H2S. The conversion of H2S increased from 82% to 94% with an increase in TiO2 loading amount from 2.5 to 30 wt%, and bulk TiO2 showed 95% of H2S conversion. However, the selectivity to elemental sulfur remained almost constant around 98±99% for all the catalysts tested.
Fig. 2. Conversion and selectivity of various catalysts at 2758C: (a) SiO2; (b) 2.5 wt% TiO2/SiO2; (c) 5 wt% TiO2/SiO2; (d) 10 wt% TiO2/SiO2; (e) 30 wt% TiO2/SiO2; (f) bulk TiO2.
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Fig. 3. Conversion and selectivity with time on stream for TiS2 at 2758C (5 vol% H2S, 2.5 vol% O2, 92.5 vol% He, GHSV 3000 hÿ1).
Catalysts for sulfur treating process (Claus reaction, COS hydrolysis, H2S oxidation) are generally known to be deactivated mainly by sulfation or sul®dation [10±12]. In order to test the stability of TiO2/SiO2 catalyst, we examined TiS2 and Ti(SO4)2 in the reaction system. These two sulfur compounds are more easily available than other compounds like TiS, TiS3, Ti2S, Ti2S3, Ti(SO4)2, TiOSO4 or TiOSO42H2O. The reaction test results on TiS2 (titanium disul®de) for 15 h are shown in Fig. 3. The conversion of H2S increased continuously from 22% to 87% up to 7 h and remained almost constant. Concerning the selectivity to elemental sulfur, it is interesting to observe that the selectivity was maintained at 100% even though the conversion is varied over a wide range. XRD patterns of fresh and used TiS2 catalyst shown in Fig. 4 indicate that the TiS2 structure was changed to anatase type of TiO2 after reaction. However, TiO2 catalysts before reaction and after 11 h reaction showed the same XRD patterns. It was previously reported that TiS2 could be decomposed into H2S and TiO2 upon heating in moist air [13]. Hence, the structural change of TiS2 to TiO2 during the oxidation of H2S may be proceeded by the reaction of TiS2 with unreacted oxygen or H2O formed at the reaction condition. The gradual phase change of TiS2 to active TiO2 results in the continuous increase of conversion during 7 h from the beginning of the reaction. On the other hand, Ti(SO4)2 (titanium sulfate) was completely decomposed when preconditioning of the
Fig. 4. X-ray diffraction patterns of catalysts: (a) fresh TiS2; (b) TiS2 after 15 h reaction; (c) fresh and used TiO2.
catalyst was accomplished at 2758C under a helium ¯ow for 2 h. The results are in agreement with the report that Ti(SO4)2 is readily decomposed to titanyl sulfate TiOSO4 and then TiO2 by heating [13]. Saur et al. [14] studied the structural stability of sulfated alumina and titania, and reported that Ti(SO4)2 was less thermally stable, and more easily reduced by hydrogen than Al2(SO4)3. However, SiO2 unlike Al2O3, MgO or Fe2O3 could not be easily sulfated by heating in the presence of H2S or SO2 with excess oxygen [15]. The TiO2, employed as an active component in this research, was known to have a good resistance to sulfation and sul®dation during hydrolysis of carbonyl sul®de [16±18]. Consequently one can suppose that catalytic deactivation by sulfation or sul®dation is not serious in the TiO2/SiO2 catalyst. A series of cyclic sweeping of temperature was also carried out to con®rm the structural durability of TiO2. The temperature of the catalyst bed was increased stepwise (208C) from 1608C to 3608C, and then decreased back down to 1608C. When the temperature arrived at a preset value, catalytic performance was measured at each step after waiting for 12 min. This cycle was repeated three times for total 11 h. Fig. 5 shows the catalytic performance of the fresh bulk TiO2 with repeated cycles. At temperatures above
S.W. Chun et al. / Applied Catalysis B: Environmental 16 (1998) 235±243
Fig. 5. Catalytic performance of bulk TiO2 in cyclic temperature operations (5 vol% H2S, 2.5 vol% O2, 92.5 vol% He, and GHSV3000 hÿ1).
2608C, the conversion did not change even though the cycle was repeated. Below 2608C, there was a hysteresis in the H2S conversion between the temperaturedown cycle (Run 2 and Run 4) and the subsequent temperature-up cycle (Run 3 and Run 5); the former always exhibited higher conversion than the latter. However, the conversion of H2S at Run 2 and Run 4 (temperature-down cycle) was almost equal over all the ranges of temperature. The same phenomenon was also observed at Run 3 and Run 5 (temperature-up cycle). Accordingly, one can see that the difference in conversion between temperature-up and temperaturedown cycle does not result from irreversible catalytic deactivation. We suggest that the hysteresis is caused by deposition of elemental sulfur on the catalyst during the reaction. Assuming that the sulfur deposition rate is very slow at low temperatures and the deposition becomes reversible only at temperatures higher than 2608C, the catalyst at the temperature-down cycle must have shorter period of experiencing the irreversible deposition of sulfur than the catalyst at the next temperature-up cycle which experiences longer period
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for the irreversible sulfur deposition. Compared to used catalysts after Run 1, the ®rst run with fresh catalyst as starting material has shorter period of experiencing the irreversible sulfur deposition, yielding much higher conversion especially at the beginning stage of the ®rst cycle. The same conversion between Run 2 and Run 4 (or Run 3 and Run 5) means that irreversible deactivation of the catalyst by structural deformation did not occur in spite of long-term operation at high temperature. To verify the physical deposition of sulfur on the catalyst surface, continuous reaction was carried out as a function of reaction time at 1808C. The conversion decreased continuously with time. During the reaction, the color of the catalyst was changed from white to yellow, indicating the deposition of elemental sulfur. The catalytic activity was readily recovered by simply heating the used catalyst at 2758C under inert gas (He) condition. However, XRD patterns of TiO2 before and after use showed that the phase and particle size of TiO2 had not changed after the reaction. From the above results, it is concluded that TiO2 was not deactivated irreversibly by sulfation or sul®dation but by reversible sulfur deposition especially at low reaction temperatures. 3.2. Effect of O2/H2S ratio We have studied the in¯uence of the partial pressure of oxygen by varying the oxygen concentration in the feed from 2.5 to 20 vol% while maintaining the concentration of H2S to 5 vol%. Fig. 6 shows the conversion of H2S and selectivity of sulfur as a function of the O2/H2S ratio. For 10 wt% TiO2/SiO2 catalyst, the conversion increased from 93.3% to 99.6% by increasing the ratio of O2/H2S from 0.5 to 2. Bulk TiO2 showed 94.7% of the conversion at O2/H2S 0.5 and 98.9% at O2/H2S1. The selectivity was drastically decreased with the increase of O2/H2S ratio and reached to about 0 when the ratio of O2/H2S was 4. One can propose that the overall reaction is composed of four individual reactions: selective oxidation (1), oxidation of sulfur (2), deep oxidation (3), and the Claus reaction (4). Reactions (1)±(3) are known as irreversible, while reaction (4) is reversible [19]. H2S is mainly converted to elemental sulfur by reaction (1)
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Fig. 7. NH3 TPD profiles of modified catalysts with B2O3 or K2O: (a) 2 wt% B2O3/[TiO2/SiO2]; (b) 1 wt% B2O3/[TiO2/SiO2]; (c) 10 wt% TiO2/SiO2; (d) 1 wt% K2O/[TiO2/SiO2]; (e) 2 wt% K2O/ [TiO2/SiO2].
Fig. 6. Effect of O2/H2S ratio on conversion and selectivity with 10 wt% TiO2/SiO2 catalyst at 2758C.
where the conversion remains constant. This indicates that the elemental sulfur produced by reaction (1) is converted to SO2 by the consecutive reaction (2), and that reactions (1) and (3) are proceeded competitively.
and partly to SO2 by reaction (3).
3.3. Addition of B2O3/K2O and effect of water vapor
2 2H2 S O2 ! Sn 2H2 O n 1 Sn O2 ! SO2 n 2H2 S 3O2 ! 2SO2 2H2 O 3 2H2 S SO2 $ Sn 2H2 O n
(1) (2) (3) (4)
SO2, which is produced by reactions (2) and (3), can be converted into elemental sulfur by Claus reaction (4). The high selectivity can be achieved by suppressing reactions leading to SO2. An excess of O2 can lead to the formation of SO2 according to either the consecutive reaction (2) or parallel reaction (3). In addition, the hydrolysis of the sulfur (reverse Claus reaction) can also produce SO2. It might be expected that in the presence of excess oxygen, reactions (2) and (3) could be promoted. It is interesting to observe that, for the bulk TiO2, there exists a drastic decrease in the selectivity even in the region of O2/H2S ratio
To investigate the in¯uence of acidity on the reaction path, B2O3 and K2O was added to 10 wt% TiO2/ SiO2 catalyst. The acid and base amount of catalysts were measured by temperature programmed desorption of NH3 and CH3COOH, in which NH3 was adsorbed at 508C and CH3COOH at 1508C. As shown in the NH3 TPD pro®le (Fig. 7), the amount of acid sites decreased with increasing the addition of K2O, compared to the standard catalyst (10 wt% TiO2/ SiO2). TPD pro®les of CH3COOH (Fig. 8) indicated that the amount of basic sites decreased by addition of B2O3. It was also observed for the catalysts modi®ed with K2O that Tmax (temperature at peak maximum) in the TPD pro®les of CH3COOH was shifted toward lower temperatures, about 308C. The surface area determined by the BET method and amount of acid and base measured by NH3 and CH3COOH TPD are listed in Table 1. It shows that the surface area of the catalysts had only a little change. One can also see that the acid amount of K2O modi®ed catalysts are
S.W. Chun et al. / Applied Catalysis B: Environmental 16 (1998) 235±243
Fig. 8. CH3COOH TPD profiles of modified catalysts with B2O3 or K2O: (a) 2 wt% B2O3/[TiO2/SiO2]; (b) 1 wt% B2O3/[TiO2/ SiO2]; (c) 10 wt% TiO2/SiO2; (d) 1 wt% K2O/[TiO2/SiO2]; (e) 2 wt% K2O/[TiO2/SiO2].
decreased with increasing K2O, and the base amount of B2O3 modi®ed catalysts are also decreased with increasing B2O3. The meaning of `relative acid/base ratio' in Table 1 is the relative value after setting the acid/base ratio of 10 wt% TiO2/SiO2 as unity. Fig. 9 shows the effect of modifying acid/base properties on the catalytic activity. The conversion was nearly independent upon addition of B2O3 but the selectivity decreased from 98% (TiO2/SiO2 catalyst) to 92.7% (2 wt% B2O3/[TiO2/SiO2] catalyst). In the case of 2 wt% K2O/[TiO2/SiO2] catalyst, the conversion was drastically decreased while maintaining the selectivity very high at about 100%. These results con®rm that the selective oxidation to elemental sulfur is proceeded on acidic sites and that reaction (1) is dominant over reaction (2).
241
Fig. 9. Conversion and selectivity of modified catalysts with K2O or B2O3 as a function of the relative acid/base ratio indicated in Table 1 (5 vol% H2S, 2.5 vol% O2, 92.5 vol% He, GHSV 3000 hÿ1).
Water vapor was added into the reactant stream to study the effects of water on the reverse Claus reaction. For the unmodi®ed catalyst (10 wt% TiO2/SiO2) in Fig. 10, increased amount of water vapor from 5 to 10 vol% decreased the conversion and selectivity. When 10 vol% of water was added, both the conversion and selectivity were decreased simultaneously for all the catalysts of different acid amount. This is different from the general trend, that the lower the conversion the higher the selectivity. The simultaneous decrease of both the conversion and selectivity may result from the reverse Claus reaction that is promoted by adding water vapor. It means that the reaction path is by parallel and not consecutive. In the presence of water vapor, 1 wt% B2O3/[TiO2/ SiO2] and TiO2/SiO2 catalyst showed about the same degree of decrease in both the conversion and selectivity, therefore one can suggest that an increase of acidic site does not affect the reverse Claus reaction.
Table 1 The surface area and amount of acid and base site of TiO2/SiO2 catalysts modified with B2O3 or K2O Catayst
Surface area (m2/g)
Acid amounta (mmol/g)
Base amountb (mmol/g)
Relative acid/base ratio
TiO2/SiO2 (10% TiO2) 1 wt% B2O3/[TiO2/SiO2] 2 wt% B2O3/[TiO2/SiO2] 1 wt% K2O/[TiO2/SiO2] 2 wt% K2O/[TiO2/SiO2]
195.3 198.3 191.7 196.0 182.2
0.217 0.222 0.251 0.175 0.163
0.119 0.101 0.091 0.122 0.141
1.00 1.21 1.53 0.75 0.64
a
Measured by NH3 TPD experiment. Measured by CH3COOH TPD experiment.
b
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Fig. 10. Effects of water vapor on conversion and selectivity of modified catalysts with K2O or B2O3 as a function of the relative acid/base ratio indicated in Table 1.
cyclic sweeping experiments of temperature revealed that they had good resistance to sul®dation and sulfation, both of which are known as main causes of catalytic deactivation in sulfur recovery process. Kinetic experiments with TiS2 and Ti(SO4)2 also con®rmed that the catalytic deactivation was not serious in TiO2/SiO2 catalysts. The conversion of H2S increased with increasing the molar ratio of O2/H2S, but the selectivity to sulfur was remarkably decreased. Therefore, the O2/H2S ratio has to be maintained in stoichiometry in order to decrease SO2 emission effectively. By decreasing the acidic sites of TiO2/SiO2 catalyst with K2O, the conversion of H2S was drastically decreased, whereas adding B2O3 did not affect the conversion signi®cantly. In the presence of 10 vol% water to the reactant stream, the catalyst modi®ed by K2O showed remarkable decreases in both the conversion and selectivity, compared with the unmodi®ed TiO2/SiO2 and the catalyst modi®ed with B2O3. From these results, it can be concluded that selective oxidation of H2S occurs on acidic sites and the reverse Claus reaction proceeds on basic sites. Acknowledgements
Meanwhile, 1 wt% K2O/[TiO2/SiO2] catalyst showed a more drastic decrease in both the conversion and selectivity, compared to 1 wt% B2O3/[TiO2/SiO2] and TiO2/SiO2. The remarkable decrease of both conversion and selectivity with alkali promoted catalyst (1 wt% K2O/[TiO2/SiO2]) allows us to suppose that the reverse Claus reaction may be activated on basic sites. It is known in the MODOP process that, after hydration, the concentration of water vapor is less than 5% in the feed stream [4]. Therefore, in view of the development of commercial catalyst one can conclude that the 10 wt% TiO2/SiO2 catalyst can be a good candidate for the selective oxidation of H2S to elemental sulfur at the MODOP process condition because it shows around 85% of H2S conversion and more than 88% of sulfur selectivity even in the presence of 10 vol% of water vapor. 4. Conclusions TiO2/SiO2 catalysts showed good activity in the selective oxidation of H2S to elemental sulfur, and
The authors express their grateful appreciation to the Research Center for Catalytic Technology of POSTECH for providing ®nancial support. References [1] Kirk-Othmer, Encyclopedia of Chemical Technology, vol. 22, 3rd ed., Wiley/Interscience, New York, 1983, p. 276. [2] J.A. Lagas, J. Borsboom, P.H. Berben, Oil and Gas J. 10 (1988) 68. [3] R. Kettner, N. Liermann, Oil and Gas J. 11 (1983) 63. [4] R. Kettner, T. Lubcke, N. Liermann, EP 78 (1982) 690. [5] T. Chopin, J.L. Hebrard, E. Quemere, EP 422 (1990) 999. [6] P.J. Brink, J.W. Geus, EP 409 (1990) 353. [7] P.H. Berben, J.W. Geus, US Patent 4 818 740 (1989). [8] P.J. van den Brink, R.J.A.M. Terode, J.H. Moors, A.J. van Dillen, J.W. Geus, in: P. Ruiz, B. Delmon (Eds.), New Developments in Selective Oxidation by Heterogeneous Catalysis, Elsevier, Amsterdam, 1991, p. 123. [9] R.J.A.M. Terorde, P.J. van den Brimk, L.M. Visser, A.J. van Dillen, Catal. Today 17 (1993) 217. [10] P.H. Berben, A. Scholten, M.K. Titulaer, W.J.J. van der Wal, J.W. Geus, in: B. Delmon, G.F. Froment (Eds.), Catalyst Deactivation 1987, Elsevier, Amsterdam, 1987, p. 303.
S.W. Chun et al. / Applied Catalysis B: Environmental 16 (1998) 235±243 [11] R. Fiedorow, R. Leaute, I.G. Dalla Lana, J. Catal. 85 (1984) 339. [12] P.J. van den Brink, A. Scholten, A.J. van Dillen, J.W. Geus, in: C.H. Bartholomew, J.B. Butt (Eds.), Catalyst Deactivation 1991, Elsevier, Amsterdam, 1991, p. 515. [13] Kirk-Othmer, Encyclopedia of Chemical Technology, vol. 23, 3rd ed., Wiley/Interscience, New York, 1983, p. 168. [14] O. Saur, M. Bensitel, A. Mohammed Saad, J.C. Lavalley, C.P. Tripp, B. Morrow, J. Catal. 99 (1986) 104.
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[15] B.A. Morrow, R.A. McFrane, M. Lion, J.C. Lavalley, J. Catal. 107 (1987) 232. [16] H.M. Huisman, P. van der Berg, R. Mos, A.J. van Dillen, J.W. Geus, in: J.N. Armor (Ed.), Environmental Catalysis, Marple, New York, 1994, p. 393. [17] R.S. Coward, W.M. Skarat, Oil and Gas J. 8 (1985) 86. [18] T. Dupin, R. Voirin, Hydrocarbon Proc. 61 (1982) 189. [19] B.W. Gamson, R.H. Elkins, Chem. Eng. Progr. 49 (1953) 203.
Selective oxidation of H2S to elemental sulfur over TiO2/SiO2 catalysts Sung Woo Chuna, Jae Yeol Janga, Dae Won Parka,*, Hee Chul Woob, Jong Shik Chungc a Department of Chemical Engineering, Pusan National University, Pusan 609-735, South Korea Department of Chemical Engineering, Pukyung National University, Pusan 608-739, South Korea c Department of Chemical Engineering, School of Environmental Engineering, Pohang University of Science and Technology, Pohang 790-784, South Korea b
Received 21 March 1997; received in revised form 30 August 1997; accepted 1 September 1997
Abstract Selective catalytic oxidation of H2S to elemental sulfur using TiO2/SiO2 catalysts was investigated in this study. The reaction test with pure TiS2 and Ti(SO4)2 and cyclic temperature operation showed that TiO2 had good resistance to sulfation and sul®dation, which are known as the main causes of catalytic deactivation in sulfur recovery process. Catalyst deactivation caused by deposition of sulfur on the catalyst surface resulted in a decrease in the conversion of H2S, not in the selectivity to sulfur, at low reaction temperatures. With an increase in the O2/H2S ratio from 0.5 to 4, the conversion was slightly increased, but the selectivity to elemental sulfur was remarkably decreased. The presence of water vapor decreased both the activity and selectivity. Temperature programmed desorption with NH3 and CH3COOH and reaction tests after doping K2O and B2O3 to TiO2/SiO2 revealed that the selective oxidation of H2S occurred on acidic sites and the reverse Claus reaction proceeded on basic sites. # 1998 Elsevier Science B.V. Keywords: Hydrogen sul®de; Sulfur; Catalytic oxidation; TiO2/SiO2
1. Introduction In these days, air pollution is a serious global problem and legal regulation has been tightened. Especially, the emission limits for SOx become stringent since sulfur oxides are considered responsible for acid rain. Recently, a large amount of hydrogen sul®de is released from crude oil, natural gas re®neries and metal smelting process in steel production. The coal *Corresponding author. Tel: +82 51 510 2399; fax: +82 51 512 8563; e-mail: [email protected] 0926-3373/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-3373(97)00078-7
liquefaction process is considered to be the major source of H2S emissions in the near future. Hydrogen sul®de is usually removed by the wellknown Claus process [1,2]. The Claus process consists of two steps: thermal oxidation and catalytic reaction. Step I 2H2 S 3O2 ! 2SO2 2H2 O thermal oxidation 3 Step II 2H2 S SO2 $ Sn 2H2 O n catalytic reaction
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In the thermal oxidation step, one-third of the hydrogen sul®de is ®rst burned with air to sulfur dioxide in a waste heat furnace. In the subsequent catalytic reaction step, SO2 is reacted with unconverted H2S to elemental sulfur over an Al2O3 catalyst. However, due to thermodynamic limitations, typically 3±5% of H2S is not converted to sulfur. As the environmental protection law becomes more strict, it is necessary to further treat the residual gas of the Claus installation, the so-called tail gas. Various commercial tail gas treating (TGT) processes have been developed. Conventional Claus TGT processes involve a hydrogen sul®de absorption step, in which a tail gas containing unreacted hydrogen sul®de is introduced in an alkaline bath. The most attractive process that has been recently developed is MODOP (mobil direct oxidation) process [3±5] or Super-Claus process [6±9], both of which are based on the direct catalytic oxidation of H2S to elemental sulfur. 2 2H2 S O2 ! Sn 2H2 O n
direct oxidation
High conversion can be obtained in direct oxidation of H2S to elemental sulfur because it is not a thermodynamic reversible reaction. In the MODOP process, H2S is oxidized to elemental sulfur with a stoichiometric amount of oxygen on a TiO2 catalyst. Drawback of the MODOP process is removal of water from the tail gas before the reaction owing to the deactivation of the catalyst by the presence of water. In the Super-Claus process, tail gas can be treated without dehydration step since Fe2O3/SiO2 catalyst is not deactivated even in the presence of 30 vol% of water vapor. However, the Super-Claus process cannot treat high concentrations of H2S above 2 vol% because it is necessary to supply excess oxygen (usually 10 times the stoichiometric amount) to overcome catalytic deactivation caused by the water. To treat a high concentration of H2S using a stoichiometric amount of oxygen, TiO2/SiO2 catalyst was selected in this study. Although the MODOP process is a well-known commercial process [3±5], there is no information available about the role of TiO2/SiO2 catalyst. Therefore the primary object of this paper is to test the stability of TiO2/SiO2 catalyst and to understand the effects of O2/H2S ratio and water vapor
on the direct oxidation of hydrogen sul®de to elemental sulfur. Kinetic studies on the effects of surface acidity are also carried out to elucidate the reaction path. 2. Experimental 2.1. Catalyst preparation Catalysts were prepared by an incipient wetness method in a glove chamber under ¯owing inert gas (N2), to avoid hydrolysis of precursor. TiO2/SiO2 catalysts were prepared by impregnating a solution of Ti(OCH(CH3)2)4 (titanium isopropoxide; Aldrich) dissolved in anhydrous toluene (Junsei) onto an SiO2 support (JRC-SIO-5), followed by drying for 12 h in an oven at 1208C. After impregnation, toluene was evaporated with a rotary vacuum evaporator at 608C. Then the catalysts were dried in a vacuum oven at 1308C for 12 h and calcined in ¯owing air at 5008C for 5 h. The K2O/[TiO2/SiO2] and B2O3/ [TiO2/SiO2] catalysts were prepared by impregnating aqueous solutions of KNO3 (potassium nitrate; Katayama) and HBO3 (boric acid; Junsei), respectively, on the previously calcined 10 wt% TiO2/SiO2 catalyst. 2.2. Characterization of catalysts The speci®c surface areas of the catalysts were determined by the continuous ¯ow technique using a BET apparatus (Quantachrom, Autosorb-1). The phase analysis was performed by X-ray diffraction crystallography with Cu K radiation (Rigaku, DMAX 2400). The morphology of the catalysts was monitored from scanning electron microscopy (JSM, JEOL 5400). In order to check acid/base site distribution, temperature programmed desorption (TPD) experiments using NH3 and CH3COOH were carried out with a thermal conductivity detector. 2.3. Reaction test The reaction test apparatus is illustrated in Fig. 1. A vertical continuous ¯ow ®xed bed reactor made of
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Conversion of H2 S %
237
H2 Sinlet ÿH2 Soutlet 100 H2 Sinlet
Selectivity to sulfur %
H2 Sinlet ÿ H2 Soutlet ÿ SO2 outlet 100 H2 Sinlet ÿ H2 Soutlet
3. Results and discussion 3.1. The activity test of TiO2 =SiO2 catalysts
Fig. 1. Schematic diagram of fixed bed reactor system: 1, moisture trap; 2, oxygen trap; 3, MFC; 4, cut-off valve; 5, three-way valve; 6, bubble flowmeter; 7, sulfur condenser; 8, reactor heater; 9, syringe pump; 10, sulfur removal filter; 11, six-port valve; 12, exhaust gas trap.
Pyrex tube (i.d.1 in.) was used for the reaction test at atmospheric pressure. The gas ¯ow rate was controlled by mass ¯ow controllers (Brooks MFC, 5850E). A mixture of H2S and O2 diluted with helium was used as reactant. A sulfur condenser was attached at the ef¯uent side of the reactor, and its temperature was constantly maintained at 1108C to condense only sulfur vapor. A line ®lter was installed after the condenser to trap any sulfur mist which had not been captured by the condenser. From the condenser up to gas chromatograph, all the lines and ®ttings were heated above 1208C to prevent condensation of water vapor. In a typical experiment, the reactant composition consisted of 5 vol% H2S, 2.5 vol% O2 and balance He. The ratio of O2/H2S was varied between 0.5 and 4, and the gas hourly space velocity (GHSV) was ®xed at 3000 hÿ1. Water vapor was introduced to the reactant stream using a steam evaporator ®lled with small glass beads, and its amount was controlled by a syringe pump (Sage Instruments, 341A). The O2, H2S and SO2 content of ef¯uent gas were analyzed by a gas chromatography (HP 5890) equipped with a thermal conductivity detector and a 6 ft Porapak T column (80±100 mesh) at 1008C. The exit gas from the analyzer was passed through a trap containing a concentrated NaOH solution and vented out to a hood. The conversion of H2S and the selectivity to sulfur are de®ned as follows:
TiO2/SiO2 catalysts prepared with different loadings of TiO2 (2.5, 5, 10, 30 wt%) showed about the same value in the BET area; 194 m2/g (2.5 wt%), 192 m2/g (5 wt%) 195 m2/g (10 wt%), and 195 m2/g (30 wt%), respectively. Therefore, one can see that TiO2 is dispersed as very small particles, and it has been con®rmed from mapping images of TiO2/SiO2 catalyst by a SEM/EDX. The results of the reaction test at 2758C under typical reaction condition (5 vol% H2S, 2.5 vol% O2, 92.5 vol% He) is shown in Fig. 2. The SiO2 support showed very low conversion of H2S. The conversion of H2S increased from 82% to 94% with an increase in TiO2 loading amount from 2.5 to 30 wt%, and bulk TiO2 showed 95% of H2S conversion. However, the selectivity to elemental sulfur remained almost constant around 98±99% for all the catalysts tested.
Fig. 2. Conversion and selectivity of various catalysts at 2758C: (a) SiO2; (b) 2.5 wt% TiO2/SiO2; (c) 5 wt% TiO2/SiO2; (d) 10 wt% TiO2/SiO2; (e) 30 wt% TiO2/SiO2; (f) bulk TiO2.
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Fig. 3. Conversion and selectivity with time on stream for TiS2 at 2758C (5 vol% H2S, 2.5 vol% O2, 92.5 vol% He, GHSV 3000 hÿ1).
Catalysts for sulfur treating process (Claus reaction, COS hydrolysis, H2S oxidation) are generally known to be deactivated mainly by sulfation or sul®dation [10±12]. In order to test the stability of TiO2/SiO2 catalyst, we examined TiS2 and Ti(SO4)2 in the reaction system. These two sulfur compounds are more easily available than other compounds like TiS, TiS3, Ti2S, Ti2S3, Ti(SO4)2, TiOSO4 or TiOSO42H2O. The reaction test results on TiS2 (titanium disul®de) for 15 h are shown in Fig. 3. The conversion of H2S increased continuously from 22% to 87% up to 7 h and remained almost constant. Concerning the selectivity to elemental sulfur, it is interesting to observe that the selectivity was maintained at 100% even though the conversion is varied over a wide range. XRD patterns of fresh and used TiS2 catalyst shown in Fig. 4 indicate that the TiS2 structure was changed to anatase type of TiO2 after reaction. However, TiO2 catalysts before reaction and after 11 h reaction showed the same XRD patterns. It was previously reported that TiS2 could be decomposed into H2S and TiO2 upon heating in moist air [13]. Hence, the structural change of TiS2 to TiO2 during the oxidation of H2S may be proceeded by the reaction of TiS2 with unreacted oxygen or H2O formed at the reaction condition. The gradual phase change of TiS2 to active TiO2 results in the continuous increase of conversion during 7 h from the beginning of the reaction. On the other hand, Ti(SO4)2 (titanium sulfate) was completely decomposed when preconditioning of the
Fig. 4. X-ray diffraction patterns of catalysts: (a) fresh TiS2; (b) TiS2 after 15 h reaction; (c) fresh and used TiO2.
catalyst was accomplished at 2758C under a helium ¯ow for 2 h. The results are in agreement with the report that Ti(SO4)2 is readily decomposed to titanyl sulfate TiOSO4 and then TiO2 by heating [13]. Saur et al. [14] studied the structural stability of sulfated alumina and titania, and reported that Ti(SO4)2 was less thermally stable, and more easily reduced by hydrogen than Al2(SO4)3. However, SiO2 unlike Al2O3, MgO or Fe2O3 could not be easily sulfated by heating in the presence of H2S or SO2 with excess oxygen [15]. The TiO2, employed as an active component in this research, was known to have a good resistance to sulfation and sul®dation during hydrolysis of carbonyl sul®de [16±18]. Consequently one can suppose that catalytic deactivation by sulfation or sul®dation is not serious in the TiO2/SiO2 catalyst. A series of cyclic sweeping of temperature was also carried out to con®rm the structural durability of TiO2. The temperature of the catalyst bed was increased stepwise (208C) from 1608C to 3608C, and then decreased back down to 1608C. When the temperature arrived at a preset value, catalytic performance was measured at each step after waiting for 12 min. This cycle was repeated three times for total 11 h. Fig. 5 shows the catalytic performance of the fresh bulk TiO2 with repeated cycles. At temperatures above
S.W. Chun et al. / Applied Catalysis B: Environmental 16 (1998) 235±243
Fig. 5. Catalytic performance of bulk TiO2 in cyclic temperature operations (5 vol% H2S, 2.5 vol% O2, 92.5 vol% He, and GHSV3000 hÿ1).
2608C, the conversion did not change even though the cycle was repeated. Below 2608C, there was a hysteresis in the H2S conversion between the temperaturedown cycle (Run 2 and Run 4) and the subsequent temperature-up cycle (Run 3 and Run 5); the former always exhibited higher conversion than the latter. However, the conversion of H2S at Run 2 and Run 4 (temperature-down cycle) was almost equal over all the ranges of temperature. The same phenomenon was also observed at Run 3 and Run 5 (temperature-up cycle). Accordingly, one can see that the difference in conversion between temperature-up and temperaturedown cycle does not result from irreversible catalytic deactivation. We suggest that the hysteresis is caused by deposition of elemental sulfur on the catalyst during the reaction. Assuming that the sulfur deposition rate is very slow at low temperatures and the deposition becomes reversible only at temperatures higher than 2608C, the catalyst at the temperature-down cycle must have shorter period of experiencing the irreversible deposition of sulfur than the catalyst at the next temperature-up cycle which experiences longer period
239
for the irreversible sulfur deposition. Compared to used catalysts after Run 1, the ®rst run with fresh catalyst as starting material has shorter period of experiencing the irreversible sulfur deposition, yielding much higher conversion especially at the beginning stage of the ®rst cycle. The same conversion between Run 2 and Run 4 (or Run 3 and Run 5) means that irreversible deactivation of the catalyst by structural deformation did not occur in spite of long-term operation at high temperature. To verify the physical deposition of sulfur on the catalyst surface, continuous reaction was carried out as a function of reaction time at 1808C. The conversion decreased continuously with time. During the reaction, the color of the catalyst was changed from white to yellow, indicating the deposition of elemental sulfur. The catalytic activity was readily recovered by simply heating the used catalyst at 2758C under inert gas (He) condition. However, XRD patterns of TiO2 before and after use showed that the phase and particle size of TiO2 had not changed after the reaction. From the above results, it is concluded that TiO2 was not deactivated irreversibly by sulfation or sul®dation but by reversible sulfur deposition especially at low reaction temperatures. 3.2. Effect of O2/H2S ratio We have studied the in¯uence of the partial pressure of oxygen by varying the oxygen concentration in the feed from 2.5 to 20 vol% while maintaining the concentration of H2S to 5 vol%. Fig. 6 shows the conversion of H2S and selectivity of sulfur as a function of the O2/H2S ratio. For 10 wt% TiO2/SiO2 catalyst, the conversion increased from 93.3% to 99.6% by increasing the ratio of O2/H2S from 0.5 to 2. Bulk TiO2 showed 94.7% of the conversion at O2/H2S 0.5 and 98.9% at O2/H2S1. The selectivity was drastically decreased with the increase of O2/H2S ratio and reached to about 0 when the ratio of O2/H2S was 4. One can propose that the overall reaction is composed of four individual reactions: selective oxidation (1), oxidation of sulfur (2), deep oxidation (3), and the Claus reaction (4). Reactions (1)±(3) are known as irreversible, while reaction (4) is reversible [19]. H2S is mainly converted to elemental sulfur by reaction (1)
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Fig. 7. NH3 TPD profiles of modified catalysts with B2O3 or K2O: (a) 2 wt% B2O3/[TiO2/SiO2]; (b) 1 wt% B2O3/[TiO2/SiO2]; (c) 10 wt% TiO2/SiO2; (d) 1 wt% K2O/[TiO2/SiO2]; (e) 2 wt% K2O/ [TiO2/SiO2].
Fig. 6. Effect of O2/H2S ratio on conversion and selectivity with 10 wt% TiO2/SiO2 catalyst at 2758C.
where the conversion remains constant. This indicates that the elemental sulfur produced by reaction (1) is converted to SO2 by the consecutive reaction (2), and that reactions (1) and (3) are proceeded competitively.
and partly to SO2 by reaction (3).
3.3. Addition of B2O3/K2O and effect of water vapor
2 2H2 S O2 ! Sn 2H2 O n 1 Sn O2 ! SO2 n 2H2 S 3O2 ! 2SO2 2H2 O 3 2H2 S SO2 $ Sn 2H2 O n
(1) (2) (3) (4)
SO2, which is produced by reactions (2) and (3), can be converted into elemental sulfur by Claus reaction (4). The high selectivity can be achieved by suppressing reactions leading to SO2. An excess of O2 can lead to the formation of SO2 according to either the consecutive reaction (2) or parallel reaction (3). In addition, the hydrolysis of the sulfur (reverse Claus reaction) can also produce SO2. It might be expected that in the presence of excess oxygen, reactions (2) and (3) could be promoted. It is interesting to observe that, for the bulk TiO2, there exists a drastic decrease in the selectivity even in the region of O2/H2S ratio
To investigate the in¯uence of acidity on the reaction path, B2O3 and K2O was added to 10 wt% TiO2/ SiO2 catalyst. The acid and base amount of catalysts were measured by temperature programmed desorption of NH3 and CH3COOH, in which NH3 was adsorbed at 508C and CH3COOH at 1508C. As shown in the NH3 TPD pro®le (Fig. 7), the amount of acid sites decreased with increasing the addition of K2O, compared to the standard catalyst (10 wt% TiO2/ SiO2). TPD pro®les of CH3COOH (Fig. 8) indicated that the amount of basic sites decreased by addition of B2O3. It was also observed for the catalysts modi®ed with K2O that Tmax (temperature at peak maximum) in the TPD pro®les of CH3COOH was shifted toward lower temperatures, about 308C. The surface area determined by the BET method and amount of acid and base measured by NH3 and CH3COOH TPD are listed in Table 1. It shows that the surface area of the catalysts had only a little change. One can also see that the acid amount of K2O modi®ed catalysts are
S.W. Chun et al. / Applied Catalysis B: Environmental 16 (1998) 235±243
Fig. 8. CH3COOH TPD profiles of modified catalysts with B2O3 or K2O: (a) 2 wt% B2O3/[TiO2/SiO2]; (b) 1 wt% B2O3/[TiO2/ SiO2]; (c) 10 wt% TiO2/SiO2; (d) 1 wt% K2O/[TiO2/SiO2]; (e) 2 wt% K2O/[TiO2/SiO2].
decreased with increasing K2O, and the base amount of B2O3 modi®ed catalysts are also decreased with increasing B2O3. The meaning of `relative acid/base ratio' in Table 1 is the relative value after setting the acid/base ratio of 10 wt% TiO2/SiO2 as unity. Fig. 9 shows the effect of modifying acid/base properties on the catalytic activity. The conversion was nearly independent upon addition of B2O3 but the selectivity decreased from 98% (TiO2/SiO2 catalyst) to 92.7% (2 wt% B2O3/[TiO2/SiO2] catalyst). In the case of 2 wt% K2O/[TiO2/SiO2] catalyst, the conversion was drastically decreased while maintaining the selectivity very high at about 100%. These results con®rm that the selective oxidation to elemental sulfur is proceeded on acidic sites and that reaction (1) is dominant over reaction (2).
241
Fig. 9. Conversion and selectivity of modified catalysts with K2O or B2O3 as a function of the relative acid/base ratio indicated in Table 1 (5 vol% H2S, 2.5 vol% O2, 92.5 vol% He, GHSV 3000 hÿ1).
Water vapor was added into the reactant stream to study the effects of water on the reverse Claus reaction. For the unmodi®ed catalyst (10 wt% TiO2/SiO2) in Fig. 10, increased amount of water vapor from 5 to 10 vol% decreased the conversion and selectivity. When 10 vol% of water was added, both the conversion and selectivity were decreased simultaneously for all the catalysts of different acid amount. This is different from the general trend, that the lower the conversion the higher the selectivity. The simultaneous decrease of both the conversion and selectivity may result from the reverse Claus reaction that is promoted by adding water vapor. It means that the reaction path is by parallel and not consecutive. In the presence of water vapor, 1 wt% B2O3/[TiO2/ SiO2] and TiO2/SiO2 catalyst showed about the same degree of decrease in both the conversion and selectivity, therefore one can suggest that an increase of acidic site does not affect the reverse Claus reaction.
Table 1 The surface area and amount of acid and base site of TiO2/SiO2 catalysts modified with B2O3 or K2O Catayst
Surface area (m2/g)
Acid amounta (mmol/g)
Base amountb (mmol/g)
Relative acid/base ratio
TiO2/SiO2 (10% TiO2) 1 wt% B2O3/[TiO2/SiO2] 2 wt% B2O3/[TiO2/SiO2] 1 wt% K2O/[TiO2/SiO2] 2 wt% K2O/[TiO2/SiO2]
195.3 198.3 191.7 196.0 182.2
0.217 0.222 0.251 0.175 0.163
0.119 0.101 0.091 0.122 0.141
1.00 1.21 1.53 0.75 0.64
a
Measured by NH3 TPD experiment. Measured by CH3COOH TPD experiment.
b
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Fig. 10. Effects of water vapor on conversion and selectivity of modified catalysts with K2O or B2O3 as a function of the relative acid/base ratio indicated in Table 1.
cyclic sweeping experiments of temperature revealed that they had good resistance to sul®dation and sulfation, both of which are known as main causes of catalytic deactivation in sulfur recovery process. Kinetic experiments with TiS2 and Ti(SO4)2 also con®rmed that the catalytic deactivation was not serious in TiO2/SiO2 catalysts. The conversion of H2S increased with increasing the molar ratio of O2/H2S, but the selectivity to sulfur was remarkably decreased. Therefore, the O2/H2S ratio has to be maintained in stoichiometry in order to decrease SO2 emission effectively. By decreasing the acidic sites of TiO2/SiO2 catalyst with K2O, the conversion of H2S was drastically decreased, whereas adding B2O3 did not affect the conversion signi®cantly. In the presence of 10 vol% water to the reactant stream, the catalyst modi®ed by K2O showed remarkable decreases in both the conversion and selectivity, compared with the unmodi®ed TiO2/SiO2 and the catalyst modi®ed with B2O3. From these results, it can be concluded that selective oxidation of H2S occurs on acidic sites and the reverse Claus reaction proceeds on basic sites. Acknowledgements
Meanwhile, 1 wt% K2O/[TiO2/SiO2] catalyst showed a more drastic decrease in both the conversion and selectivity, compared to 1 wt% B2O3/[TiO2/SiO2] and TiO2/SiO2. The remarkable decrease of both conversion and selectivity with alkali promoted catalyst (1 wt% K2O/[TiO2/SiO2]) allows us to suppose that the reverse Claus reaction may be activated on basic sites. It is known in the MODOP process that, after hydration, the concentration of water vapor is less than 5% in the feed stream [4]. Therefore, in view of the development of commercial catalyst one can conclude that the 10 wt% TiO2/SiO2 catalyst can be a good candidate for the selective oxidation of H2S to elemental sulfur at the MODOP process condition because it shows around 85% of H2S conversion and more than 88% of sulfur selectivity even in the presence of 10 vol% of water vapor. 4. Conclusions TiO2/SiO2 catalysts showed good activity in the selective oxidation of H2S to elemental sulfur, and
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