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Ind. Eng. Chem. Res. 2008, 47, 4658–4664
Direct Reduction of Sulfur Dioxide to Elemental Sulfur with Hydrogen over Sn-Zr-Based Catalysts Gi Bo Han,† No-Kuk Park,† Suk Hoon Yoon,† Tae Jin Lee,*,† and Gui Young Han‡ National Research Laboratory, School of Chemical Engineering & Technology, Yeungnam UniVersity, 214-1 Dae-dong, Gyeongsan-si, Gyeongsangbuk-do 712-749, Republic of Korea and Department of Chemical Engineering, Sungkyunkwan UniVersity, Suwon 400-746, Republic of Korea
We conducted the SO2 reduction with H2 over Sn-Zr-based catalysts for the direct sulfur recovery process. The reaction temperature was varied from 250 to 550 °C while using SnO2-only, ZrO2-only, and SnO2-ZrO2 (Sn/Zr ) 2/1) catalysts. The highest reactivity was obtained using the SnO2-ZrO2 (Sn/Zr ) 2/1) catalyst at 550 °C, for which the SO2 conversion and sulfur selectivity were 98 and 55%, respectively. Also, the following mechanistic pathway was suggested: (1) The elemental sulfur is produced by the direct conversion of SO2 according to the redox mechanism (SO2 + 2H2 f S + 2H2O). (2) The produced sulfur is partially converted into H2S with the hydrogenation (H2 + S f H2S). (3) Finally, the Claus reaction proceeds through Lewis and Bro¨nsted acidic sites (SO2 + 2H2S f 3S + 2H2O). It was estimated that the lattice oxygen vacancies might be active sites for the redox mechanism and the Lewis and Bro¨nsted acidic sites might be related to the pathway of the Claus reaction. Introduction IGCC (integrated gasification combined cycle) systems can produce various chemicals and energy resources such as dimethyl ether, synthetic gas, and electric power by gasifying coal. Therefore, these systems will contribute to the development and production of highly efficient and environmentally friendly energy resources. In IGCC systems, sulfur compounds such as H2S and COS can be produced, due to the sulfur component in coal, and then the H2S can be treated using the HGD (hot gas desulfurization) process with a sorbent. An SO2 removal process is necessary for IGCC systems, because SO2 is produced in the regeneration process of the sulfided sorbent. Also, SO2 can be removed by recovering the elemental sulfur produced in the SO2 reduction process using a reducing agent. Various reducing agents such as H2, CO, CH4, and carbonaceous materials have been used for the reduction of SO2.1–23 Especially, mechanistic studies for the reduction of SO2 using H2 have been conducted for dry SO2 treatment. For the SO2 reduction using H2, the reaction pathway involving hydrogenation and the Claus reaction havse been suggested as the conventional mechanism, as follows:1–6,24–27 First, the hydrogenation proceeds, leading to the formation of H2S used in the Claus reaction. Second, the Claus reaction proceeds over a catalyst having Lewis and Bro¨nsted acidic sites (SO2 + 2H2S f 3S + 2H2O). Additionally, it was reported that the transformation of the metal oxide into the metal sulfide takes place, and then the metal sulfide plays the role of the active sites for the formation of H2S during the hydrogenation.2–6 Simultaneously, γ-Al2O3 having Lewis and Bro¨nsted acidic sites acts as a catalyst, having the active sites required for producing sulfur by the Claus reaction.6 In our previous study, the Sn-Zr-based catalysts have been developed for the SO2 reduction with a coal-derived gas and the reactivity has been investigated for the sulfur recovery in IGCC system.10 Then, it was known that the reactivity for the SO2 reduction was decreased when a reducing agent containing * To whom correspondence should be addressed. E-mail: tjlee@ ynu.ac.kr. Tel.: +82 (0)53 810 2519. Fax: +82 (0)53 810 4631. † Yeungnam University. ‡ Sungkyunkwan University.
more H2 than CO was used. In this study, the reaction characteristics for the reduction of SO2 by H2 over Sn-Zr-based catalysts was investigated by the various performance tests and characterization methods more in detail, and it was established why the reactivity was decreased when a reducing agent containing more H2 than CO was used. Experimental Section Preparation of the Sn-Zr-Based Catalysts. SnO2-only, ZrO2-only, and SnO2-ZrO2 (Sn/Zr ) 2/1) catalysts were prepared by the precipitation and coprecipitation methods. The appropriate amount of the precursors to obtain the desired Sn/ Zr molar ratio was dissolved in distilled water. The precipitate was formed by adding NH3 · H2O to the precursor solution until the pH was in the range of 9-10. The solution including the precipitate was warmed to 80 °C in a water bath. The gelated slurry was dried at 110 °C in an electronic oven. The solid product was annealed at 600 °C in an electronic furnace and crushed. After the particle size of the product was sieved to 0.075-0.150 mm, the selected particles of the Sn-Zr-based catalysts were used for the SO2 reduction by H2. Characterization of the Fresh and Used Sn-Zr-Based Catalysts. The crystalline forms of the fresh and used Sn-Zrbased catalysts were analyzed using an X-ray diffractometer (XRD, Rigaku, D/MAX-2500, Ni-filtered Cu KR radiation). The compositions of the fresh and used Sn-Zr-based catalysts were investigated using an energy-dispersive X-ray spectrometer (EDS, KEVEX SIGMA, FISONS Co.). The binding energies of the sulfur components contained in the fresh and used Sn-Zr-based catalysts were analyzed using X-ray photoelectron spectroscopy (XPS). The XPS spectrometer (ESCALAB 250) was fabricated by VG Scientifics and equipped with a focused (spot size 100 A) and monochromatized Al KR anode (hν ) 1486.6 eV). The binding energies corresponding to the C1s peak at 284.6 eV are given with an accuracy of (0.2 eV. The samples were obtained and delivered to the XPS instrument after pretreating the used Sn-Zr-based catalysts in an N2 atmosphere and decreasing the temperature gradually to room temperature.
10.1021/ie800058v CCC: $40.75 2008 American Chemical Society Published on Web 06/11/2008
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The degree of reduction associated with the redox properties of the Sn-Zr-based catalysts was analyzed by temperatureprogrammed reduction (TPR). H2-TPR was conducted using a thermal conductivity detector (TCD, AUTOSORB-1, Quantachrome Co.) for measuring the degree of reduction. After pretreating 0.5 g of the Sn-Zr-based catalysts in an N2 atmosphere for 2 h at 250 °C, the temperature was decreased to room temperature in order to remove the impurities such as H2O. H2-TPR was started by injecting 5 vol % H2, and then the temperature was raised from 60 to 550 °C at a ramping rate of 5 °C min-1. Performance Tests of the Sn-Zr-Based Catalysts. The reduction of SO2 was performed as follows: A gas mixture with an [H2 (or H2S)]/[SO2] molar ratio (20000 ppmv SO2 in N2 balance) of 2.0 and space velocity (GHSV) of 10000 h-1 was fed into the reactor. When a steady state was attained, the outlet concentrations of SO2, H2S, and the other byproduct were determined by a gas chromatograph (Shimadzu-8A) equipped with a thermal conductivity detector. In the SO2 reduction by H2 (SO2 + 2H2 f S + 2H2O), the SO2 conversion and the selectivity values of sulfur and H2S were calculated by the following equations: SO2 conversion (%) ) Sulfur selectivity (%) )
[SO2]in - [SO2]out × 100 [SO2]in
(1)
[SO2]in - [SO2]out - [H2S]out × 100 [SO2]in (2)
H2S selectivity (%) )
[H2S]out × 100 [SO2]in
(3)
In the Claus reaction (SO2 + 2H2S f 3S + 2H2O), the conversion values of SO2 and H2S and the sulfur selectivity were calculated by the following equations: SO2 conversion (%) )
[SO2]in - [SO2]out × 100 [SO2]in
(4)
H2S conversion (%) )
[H2S]in - [H2S]out × 100 [H2S]in
(5)
Sulfur selectivity (%) ) [SO2]in + [H2S]in - [SO2]out - [H2S]out × 100 [SO2]in + [H2S]in
(6)
Results and Discussion XRD Analysis of the Fresh and Used Sn-Zr-Based Catalysts. Figure 1 shows the XRD patterns of the fresh and used Sn-Zr-based catalysts. As shown in Figure 1a, the fresh SnO2-only catalyst has a crystalline form corresponding to the tetragonal structure through the JCPDS card. The fresh ZrO2only catalyst of Figure 1b has two crystalline forms with the monoclinic and tetragonal structures, respectively. In Figure 1c, it was found that the SnO2-ZrO2 (Sn/Zr ) 2/1) catalyst consists of the crystalline forms of the solid solution between SnO2 and ZrO2 because the peaks of the SnO2-only and ZrO2-only were very slightly shifted and their patterns were different from the original peaks. The formation of solid solution of SnO2-ZrO2 has been reported in a previous study.28 It was found that SnO2 was partially sulfided into Sn2S3 having an orthorhombic structure because the peaks for tin sulfide were observed in the XRD patterns of the used SnO2-only and SnO2-ZrO2 (Sn/Zr ) 2/1) catalysts in parts d and f of Figure 1, respectively. It
Figure 1. XRD patterns of the fresh and used Sn-Zr-based catalysts [(a) fresh SnO2-only, (b) used SnO2-only, (c) fresh ZrO2-only, (d) used ZrO2only, (e) fresh SnO2-ZrO2 (Sn/Zr ) 2/1), (f) used SnO2-ZrO2 (Sn/Zr ) 2/1): 1 ) SnO2 (tetragonal structure), 2 ) ZrO2 (monoclinic structure), 3 ) ZrO2 (tetragonal structure), 4 ) SnO2-ZrO2 (Sn/Zr ) 2/1), A ) Sn2S3, B ) Sn2S3 (orthorhombic structure)].
has been reported that the sulfidation process of the metal oxide into the metal sulfide is observed in the SO2 reduction with H2, and that the metal sulfide then plays the role of improving the formation of H2S in the hydrogenation reaction.2–6 However, the crystalline form of the used ZrO2-only catalyst was similar to that of the fresh ZrO2-only catalyst in Figure 1e. Therefore, the SO2 reduction by H2 over Sn-based oxide catalysts was accompanied by the sulfidation process of SnO2, and thus it was found that the SO2 reduction by H2 over Sn-based oxide catalysts proceeds according to the pathway involving the Claus reaction. EDX Analysis of the Fresh and Used Sn-Zr-Based Catalysts. In order to investigate the sulfidation process of each catalyst, the relative concentrations of tin, zirconium, and sulfur were measured by EDX analysis, as shown in Table 1. The fresh Sn-Zr-based catalysts and the used ZrO2-only catalyst do not have any sulfur component, but the sulfur component was observed in the used SnO2-only catalyst (S ) 32 at %) and SnO2-ZrO2 (Sn/Zr ) 2/1) catalyst (S ) 5 at %). Therefore, the SnO2 and SnO2-ZrO2 (Sn/Zr ) 2/1) catalysts were partially changed into Sn2S3, and thus it was found that the SO2 reduction by H2 over the SnO2 and SnO2-ZrO2 (Sn/Zr ) 2/1) catalysts proceeds according to the pathway involving the Claus reaction. H2-TPR Analysis for the Redox Property of the Sn-ZrBased Catalysts. The degree of reduction of the Sn-Zr-based catalysts has been investigated and described in our previous study.29 The temperature was raised from 100 to 575 °C. Figure 2 shows the H2-TPR results of the Sn-Zr-based catalysts as a function of temperature and has been presented in previous report.29 The degree of reduction is related to the redox properties involving the storage capacity and mobility of lattice oxygen. In Figure 2, the starting temperature of the reduction Table 1. EDX Analysis of the Fresh and Used Sn-Zr-Based Catalysts Relative composition excluding oxygen (at %) fresh
used
catalyst
Sn
Zr
S
Sn
Zr
S
SnO2-only ZrO2-only SnO2-ZrO2 (Sn/Zr ) 2/1)
100 0 65
0 100 35
0 0 0
68 0 63
0 100 32
32 0 5
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Figure 2. H2-TPR profiles of Sn-Zr-based catalysts.29
Figure 3. XPS spectra of the fresh and used Sn-Zr-based catalysts.
and integrated area of the H2-TPR curve are related to the mobility and storage capacity of lattice oxygen, respectively. The reduction of the SnO2-only and SnO2-ZrO2 (Sn/Zr ) 2/1) catalysts was started at 180 and 160 °C, respectively, indicating that the reduction temperature of the SnO2-ZrO2 (Sn/Zr ) 2/1) catalyst was slightly lower than that of the SnO2-only catalyst. The starting temperature for the reduction of the ZrO2-only catalyst was 270 °C, which is much higher than that of the other catalysts. Also, the order of the integrated area of the Sn-Zrbased catalysts was SnO2-ZrO2 (Sn/Zr ) 2/1) > SnO2-only > ZrO2-only. Therefore, the order of the storage capacity and mobility of the lattice oxygen was SnO2-ZrO2 (Sn/Zr ) 2/1) > SnO2-only > ZrO2-only. It has been reported that SnO2-ZrO2 catalysts have a crystalline form consisting of a solid solution between SnO2 and ZrO2 having different atomic sizes.28 Also, it has been reported that the storage capacity and mobility can be improved by the distortion of the crystal structure of the metal oxide when the solid solution is formed in the case of mixed metal oxides.30 In addition, the redox properties of the Sn-Zr-based catalysts have been investigated, and it was confirmed that the Sn-Zr-based catalysts have the redox properties through the reduction and reoxidation processes in our previous study.29 Therefore, the storage capacity and mobility of the lattice oxygen of SnO2-ZrO2 (Sn/Zr ) 2/1) might be improved because the crystal structure of the SnO2-ZrO2 catalyst became distorted as the solid solution formed through the addition of ZrO2 to SnO2. XPS Analysis of the Sn-Zr-Based Catalysts. Figure 3 shows the XPS analysis results of the fresh and used Sn-Zrbased catalysts. No peaks were observed for the fresh Sn-Zrbased catalyst in Figure 3a. In the case of the used SnO2-only catalyst, a weak peak was observed at around 169 eV involving the sulfate group (-SO42-) and a strong peak was observed at around 162 eV involving the sulfide group (-S). It was reported that the sulfate group can be formed when the SO2 reduction proceeds according to the redox mechanism as follows: (1) Cat[O] f Cat-O and (2) 2Cat-O + SO2 f Cat-SO4 (Cat-[O] ) surface capping oxygen and Cat-O ) lattice oxygen).31 Therefore, the SO2 reduction by H2 over the SnO2-only catalyst partially proceeds according to the redox mechanism. In a previous study, it was reported that the sulfate group is formed in the Claus reaction.25 In addition, it has been discovered that the sulfide group can be formed because the metal oxide is partially converted into metal sulfide when the metal oxide catalyst is used in the Claus reaction.2–6 In this study, the peak corresponding to the sulfide group was observed because SnO2-
only catalyst might be partially converted into Sn2S3 along with the pathway involving the Claus reaction, and this result is consistent with the XRD result. Therefore, it was estimated that the SO2 reduction by H2 over the SnO2-only catalyst proceeds along with both the redox mechanism and the pathway involving the Claus reaction. In the case of the used SnO2-ZrO2 (Sn/Zr ) 2/1) catalyst, two kinds of peaks corresponding to the sulfide and sulfate groups were observed, and the values of the binding energy were around 169 and 162 eV, respectively. Therefore, the SO2 reduction by H2 over the SnO2-ZrO2 (Sn/Zr ) 2/1) catalyst follows both the redox mechanism and the pathway involving the Claus reaction. In the case of the ZrO2-only catalyst, only one peak corresponding to the sulfate group was observed at around 169 eV, and the peak corresponding to the sulfide group was not observed, unlike with SnO2-only and SnO2-ZrO2 (Sn/Zr ) 2/1) catalysts. Therefore, it was estimated that the SO2 reduction using the ZrO2-only catalyst proceeds along with the redox mechanism. SO2 Reduction by H2 over Sn-Zr-Based Catalysts. Figure 4 shows the effect of the temperature on the SO2 reduction by H2 over the various Sn-Zr-based catalysts. When the SnO2only and SnO2-ZrO2 (Sn/Zr ) 2/1) catalysts were used for the SO2 reduction by H2, the conversion of SO2 was started at 275 °C. As the temperature increased from 250 to 550 °C, both SO2 conversion and H2S selectivity were directly proportional to the temperature. Elemental sulfur can be produced by the two reactions of both SO2 + 2H2 f S + H2O and SO2 + 2H2S f 3S + 2H2,O and H2S can be formed by the hydrogenation of elemental sulfur (S + H2 f H2S). The increase in the H2S selectivity with increasing reaction temperature might be attributed to the decrease of the Claus reaction (SO2 + 2H2S f 3S + 2H2O). The reactivity for the Claus reaction is in inverse proportion to the temperature because it is very exothermal. The reaction characteristics for the Claus reaction will be described more in detail in the next section. It can be estimated that the sulfur yield with temperature can be decided by the reactivity deference between the production of elemental sulfur and the hydrogenation the produced sulfur. The increasing rates of the two reactions of SO2 + 2H2 f S + H2O and SO2 + 2H2S f 3S + 2H2O for SO2 conversion were higher than that of H2S selectivity, though both SO2 conversion and H2S selectivity were simultaneously increased along with the reaction for the SO2 conversion and the hydrogenation of sulfur as the temperature increased. Because the increasing rate of SO2 conversion was higher than that of H2S selectivity, the yield of elemental sulfur formed was increased with the significant
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Figure 4. Effect of the reaction temperature on the SO2 reduction by H2 over Sn-Zr-based catalysts.
increase in the SO2 conversion in spite of the decrease of sulfur selectivity. When the SnO2-only and SnO2-ZrO2 (Sn/Zr ) 2/1) catalysts were used, the maximum values of the SO2 conversion were about 98 and 89% and those of the sulfur yield were about 54 and 51% at 550 °C, respectively. In the case of the ZrO2-only catalyst, the starting temperature was around 525 °C. The reactivity of the ZrO2-only catalyst was the lowest among the Sn-Zr-based catalysts. At 550 °C, its SO2 conversion and sulfur selectivity were about 18 and 97%, respectively. In the case of the SnO2-ZrO2 (Sn/Zr ) 2/1) catalyst, the starting temperature was around 250 °C. The SO2 conversion and H2S selectivity were proportional to the reaction temperature in the temperature range from 250 to 550 °C, but the sulfur selectivity decreased with increasing temperature from 300 to 550 °C. The sulfur yield was the highest at 550 °C because the increasing rate of the SO2 conversion is higher than the decreasing rate of the sulfur selectivity. The maximum values of the reactivity were an SO2 conversion of about 98%, a sulfur
selectivity of 55%, and an H2S selectivity of 45% at 550 °C. As compared with redox properties of the Sn-Zr-based catalyst, it was estimated that the reactivity is directly related to the redox property of each catalyst involving the initialization for the SO2 reduction by H2. Claus Reaction over Sn-Zr-Based Catalysts. It was supposed that the Claus reaction in which H2S is a reducing agent in SO2 reduction has an effect on the reaction of the SO2 reduction by H2 over Sn-Zr-based catalysts. To figure out the relationship between SO2 reduction by H2 and Claus reaction, the reaction characteristics of the Claus reaction were investigated in this section. Figure 5 shows the effect of temperature on the Claus reaction using the SnO2-only, ZrO2-only, and SnO2-ZrO2 (Sn/Zr ) 2/1) catalysts. In the case of the SnO2only catalyst, the conversion values of SO2 and H2S and the sulfur selectivity were increased as the temperature increased from 200 to 325 °C but were decreased when the temperature was further increased. The optimum temperature for the highest efficient sulfur recovery was 325 °C, at which the maximized
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Figure 5. Effect of the reaction temperature on the Claus reaction over Sn-Zr-based catalysts.
conversion values of SO2 and H2S and sulfur selectivity were about 66, 68, and 67%, respectively. In the case of the ZrO2only catalyst, the conversion values of SO2 and H2S and the sulfur selectivity increased as the temperature increased from 200 to 275 °C but decreased when the temperature was further increased above 275 °C. At 275 °C, the maximized conversion values of SO2 and H2S and sulfur selectivity were about 91, 88, and 89%, respectively. In the case of the SnO2-ZrO2 (Sn/ Zr ) 2/1) catalyst, the conversion values of SO2 and H2S and the sulfur selectivity increased as the temperature increased from 200 to 225 °C but decreased when the temperature was further increased above 225 °C. At 225 °C, the highest reactivity was obtained, and the conversion values of SO2 and H2S and the sulfur selectivity were about 96, 96, and 96%, respectively. From these results, it was found that the reactivity was in the order of SnO2-ZrO2 (Sn/Zr ) 2/1) > ZrO2-only > SnO2-only. It has been reported that the active sites for the Claus reaction are Lewis and Bro¨nsted acidic sites and that the Sn-Zr-based catalysts have the Lewis and Bro¨nsted acid sites.6,28 Also, it were found that the quantitative order of the Lewis and Bro¨nsted acidic sites is SnO2-ZrO2 (Sn/Zr ) 2/1) > ZrO2-only > SnO2-
only, and that the strength of these sites is in the order of SnO2only > ZrO2-only > SnO2-ZrO2 (Sn/Zr ) 2/1).28 The orders of quantity and strength of these acidic sites are related to the reactivity and reaction temperature, respectively. The separated roles of the Lewis and Bro¨nsted acidic sites must be investigated through the kinetic studies and the characterization of the catalytic properties more in detail; however, it was estimated as follows on the basis of the results and previous reports as mentioned above. Also, it was known that the Lewis acid site is the active site for the adsorption of H2S and the Bro¨nsted acid site is the active site for the adsorption of SO2 in the Claus reaction. The reaction characteristics of the Claus reaction using Sn-Zr-based catalysts might be attributed to the Lewis and Bro¨nsted acidic sites and be related to the reaction pathway for the SO2 reduction by H2. The low reactivity of the Claus reaction achieved at high temperature might have an effect on the selective sulfur recovery in SO2 reduction by H2 and might be attributed to its exothermal characteristics. It was estimated that this relationship between Claus reaction and SO2 reduction with H2 is contributed to the low reactivity of the SO2 reduction with
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Figure 6. H2S formation by the hydrogenation of S-cat. + H2 f H2S at 550 °C [(a) schematic diagram of the layer arrangement of elemental sulfur and Sn-Zr-based catalysts and the gas stream for the H2S formation and (b) H2S concentration].
a coal-derived gas containing H2 over Sn-Zr-based catalysts in our previous work.10 H2S Formation by the Hydrogenation of Elemental Sulfur. In order to establish that the SO2 reduction by H2 over Sn-Zr-based catalysts is related to the reaction pathway involving the Claus reaction, the H2S formation was investigated because the produced H2S might play an important role as an intermediate and be used for the Claus reaction. In this section, the formation of H2S was investigated by the hydrogenation reaction between a solid phase of elemental sulfur and a gas phase of H2, and the variation of the H2S concentration was also observed. Figure 6 shows the schematic diagram of the arrayed layers of elemental sulfur and Sn-Zr-based catalysts and the gas stream and the variation of the H2S concentration. The reaction temperature and the space velocity were 550 °C and 10000 h-1, respectively. Four vol % H2 (N2 balance) was fed into the reactor packed with elemental sulfur (top layer, 2.0 g) and Sn-Zr-based catalysts (bottom layer, 0.5 g). The H2S concentration values of SnO2-ZrO2 (Sn/Zr ) 2/1) catalyst, SnO2, and ZrO2 was, respectively, increased up to a maximum of less than 3000, 3500, and 4600 ppmv, and the duration time of the H2S production was about 1 h regardless the sort of the catalyst. It was known that the H2S concentration values were different according to the time stream and the sort of Sn-Zrbased catalyst. However, H2S was never observed in the noncatalytic H2S production system, and then it was indicated that the hydrogenation of S + H2 f H2S was a catalytic reaction over Sn-Zr-based catalysts. It was confirmed that H2S can be produced by the reaction of S + H2 f H2S over Sn-Zr-based catalysts. Relationship between the Activity and the Catalytic Properties. In the performance tests for the SO2 reduction with H2 and the Claus reaction using the Sn-Zr-based catalysts, we
concluded that the reactivity values of the Sn-Zr-based catalysts are in the order of SnO2-ZrO2 (Sn/Zr ) 2/1) > SnO2-only > ZrO2-only. The difference of the catalytic activity of these catalysts might be decided by the two catalytic properties such as the redox property and the Lewis and Bro¨nsted acidic sites. The redox properties of the Sn-Zr-based catalysts might be related to the starting temperature and the initiation step for the SO2 conversion into elemental sulfur first because the SO2 conversion was dependent on the degrees of reduction of the Sn-Zr-based catalysts with the temperature. Additionally, it has been reported that the redox property of Sn-Zr-based catalysts is in the order of SnO2-ZrO2 (Sn/Zr ) 2/1) > SnO2-only > ZrO2-only in the H2-TPR analysis results.29 Second, the reactivity for the SO2 reduction with H2 might be related to that of Claus reaction, which is related to the quantity and strength of the Lewis and Bro¨nsted acidic sites of the Sn-Zr-based catalysts. Also, it has been previously reported that the acidic properties for the Lewis and Bro¨nsted acidic sites of Sn-Zrbased catalysts are in the order of SnO2-ZrO2 > SnO2-only > ZrO2-only.28 In addition, it was estimated that the SO2 reduction by H2 might be dependent on the Claus reaction, because it has been reported that the Claus reaction proceeds over catalysts having Lewis and Bro¨nsted acidic sites.1–6 Moreover, the H2S selectivity of the SO2 reduction with H2 was increased and the H2S conversion of the Claus reaction was decreased with increasing temperature, and these tendencies might be attributed to the exothermal characteristics of the Claus reaction. These reaction characteristics might be influenced on the SO2 reduction using a coal-derived gas containing both CO and H2. Therefore, it was estimated that the reactivity was decreased as compared with using only CO because a coal-derived gas containing both CO and H2 was used as the reducing agent in the SO2 reduction over Sn-Zr-based catalysts in our previous work.10 We concluded that the SO2 reduction with H2 over the Sn-Zr-based catalysts might proceed according to the redox mechanism and the pathway involving the Claus reaction as follows: (1) The elemental sulfur is produced by the direct conversion of SO2 according to the redox mechanism (SO2 + 2H2 f S + 2H2O). (2) H2S is produced by the hydrogenation of the produced sulfur (S + H2 f H2S). (3) The Claus reaction actively proceeds over the Sn-Zr-based catalysts having the Lewis and Bro¨nsted acidic sites, and abundant elemental sulfur is produced (SO2 + 2H2S f 3S + 2H2S). Conclusion We investigated the reaction characteristics and the catalytic properties of the Sn-Zr-based catalysts in order to investigate the mechanistic pathway of the SO2 reduction with H2 over Sn-Zr-based catalysts in detail. The best catalyst was SnO2-ZrO2 (Sn/Zr ) 2/1), for which the SO2 conversion and sulfur selectivity were about 98 and 55%, respectively, at 550 °C. It was concluded that the SO2 reduction with H2 over Sn-Zr-based catalysts follows both the redox mechanism and the pathway involving the Claus reaction. In addition, the SO2 reduction with H2 over Sn-Zr-based catalysts proceeds according to the following pathway: (1) Elemental sulfur is produced by the reaction of SO2 + 2H2 f S + 2H2O involving the redox mechanism. (2) H2S is produced by the reaction of H2 + S f H2S over the Sn-Zr-based catalyst. (3) Finally, the Claus reaction actively proceeds over the Sn-Zr-based catalysts having Lewis and Bro¨nsted acidic sites, and more abundant elemental sulfur is produced. In addition, the relatively low reactivity was achieved when a simulated coal-derived gas was used for the selective sulfur recovery in our previous work.10
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We can conclude that the reactivity is decreased as compared with using only CO because the mechanistic pathway involving the Claus reaction is directly related to the selective sulfur recovery and its reactivity is very low when a coal-derived gas containing both H2 and CO was used as a reducing agent in the SO2 reduction over Sn-Zr-based catalyst. Acknowledgment This work was supported by “National R & D Organization for Hydrogen and Fuel Cells” in New & Renewable Energy R & D Program (2004-N-HY12-P-03-0000) under the Ministry of Commerce, Industry and Energy (MOCIE) of Korea. Literature Cited (1) Ishiguro, A.; Liu, Y.; Nakajima, T.; Wakatsuki, Y. Efficient Reduction of Sulfur Dioxide with Hydrogen over TiO2-Supported Catalysts Derived from Ruthenium Salts and Ruthenium Cluster Complexes. J. Catal. 2002, 206, 159. (2) Chung, J. S.; Paik, S. C.; Kim, H. S.; Lee, D. S.; Nam, I. S. Removal of H2S and/or SO2 by Catalytic Conversion Technologies. Catal. Today 1997, 35, 37. (3) Paik, S. C.; Chung, J. S. Selective Catalytic Reduction of Sulfur Dioxide with Hydrogen to Elemental Sulfur over Co-Mo/Al2O3. Appl. Catal., B 1995, 5, 233. (4) Paik, S. C.; Chung, J. S. Selective Hydrogenation of SO2 to Elemental sulfur over Transition Metal Sulfides Supported on Al2O3. Appl. Catal., B 1996, 8, 267. (5) Chen, C.-L.; Wang, C.-H.; Weng, H.-S. Supported Transition-Metal Oxide Catalysts for Reduction of Sulfur Dioxide with Hydrogen to Elemental Sulfur. Chemosphere 2004, 56, 425. (6) Paik, S. C.; Kim, H.; Chung, J. S. The Catalytic Reduction of SO2 to Elemental Sulfur with H2 or CO. Catal. Today 1997, 38, 193. (7) Hibbert, D. B.; Tseun, A. C. C. Catalyst Participation in the Reduction of Sulphur Dioxide by Carbon Monoxide in the Presence of Water and Oxygen. J. Chem. Soc., Faraday Trans. 1 1978, 74, 1981. (8) Liu, W.; Wadia, C.; Flytzani-Stephanopoulos, M. Transition Metal/ Fluorite-type Oxides as Active Catalysts for Reduction of Sulfur Dioxide to Elemental Sulfur by Carbon Monoxide. Catal. Today 1996, 28, 391. (9) Lau, N. T.; Fang, M.; Chan, C. K. Reduction of SO2 by CO and COS over La2O2SsA Mechanistic Study. J. Mol. Catal., A 2003, 203, 221. (10) Han, G. B.; Park, N.-K.; Lee, J. D.; Ryu, S. O.; Lee, T. J. A Study on the Characteristics of the SO2 Reduction Using Coal Gas over SnO2ZrO2 Catalysts. Catal. Today 2006, 111, 205. (11) Khalafalla, S. E.; Haas, L. A. The Role of Metallic Component in the Iron-Alumina Bifunctional Catalyst for the Reduction of SO2 with CO. J. Catal. 1972, 24, 121. (12) Khalafalla, S. E.; Haas, L. A. Active Sites for Catalytic Reduction of SO2 with CO on Alumina. J. Catal. 1972, 24, 115. (13) Kim, B.-S.; Lee, J.-D.; Park, N.-K.; Ryu, S.-O.; Lee, T.-J.; Kim, J.-C. A Study of Ce1-xZrxO2 Catalytic Reaction for the Recovery of Elemental Sulfur from SO2. Hwahak Konghak 2003, 41, 572. (14) Han, G. B.; Shin, B.-Y.; Lee, T. J. SO2 Reduction with CO over SnO2-ZrO2 (Sn/Zr ) 2/1) Catalyst for Direct Sulfur Recovery Process with
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ReceiVed for reView January 14, 2008 ReVised manuscript receiVed April 16, 2008 Accepted April 16, 2008 IE800058V
