Investigation of Catalytic Reduction of Sulfur Dioxide with Carbon

characteristics: (1) A relatively high reaction temperature, (2) near-zero COS .... Figure 1 Effect of the reaction temperature on SO2 reduction b...
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Ind. Eng. Chem. Res. 2008, 47, 1427-1434

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Investigation of Catalytic Reduction of Sulfur Dioxide with Carbon Monoxide over Zirconium Dioxide Catalyst for Selective Sulfur Recovery Gi Bo Han, No-Kuk Park, Suk Hoon Yoon, and Tae Jin Lee* National Research Laboratory, School of Chemical Engineering & Technology, Yeungnam UniVersity, 214-1 Dae-dong, Gyeongsan-si, Gyeongsangbuk-do 712-749, Republic of Korea

In this work, a ZrO2 catalyst was used to reduce SO2 using CO for the direct sulfur recovery process (DSRP), and a mechanistic investigation was performed. ZrO2 catalyst was prepared by a precipitation method. It was supposed that ZrO2 catalysts exhibit high activity in the SO2 reduction by CO at relatively high temperature because of their Lewis acidic sites and Bro¨nsted acidic sites. In addition, the following mechanistic pathway could be suggested: (1) In the first step initialized by the redox mechanism, the ZrO2 catalyst was reduced by CO and then sulfate groups, which have the effect of improving the Lewis acidic sites and Bro¨nsted acidic sites, were formed on the surface. (2) In the second step, elemental sulfur was produced by the movement of lattice oxygen between SO2 and the lattice oxygen vacancies of the ZrO2 catalyst having redox catalytic properties. (3) In the third step, COS was formed by the reaction of S + CO f COS. (4) In the fourth step, SO2 and COS were adsorbed and reacted on the surface of the ZrO2 catalyst having Lewis acidic and Bro¨nsted acidic sites, and then the abundant amount of elemental sulfur was produced. Consequently, we would like to suggest the mechanistic pathway corresponding to the modified COS intermediate mechanism involving the redox mechanism. Introduction Today’s researchers are increasingly focusing on the development of clean technologies because of the current interest in environmental problems. SO2 is a toxic and corrosive sulfur compound, which damages the health, corrodes equipment, generates acid rain, and pollutes and acidifies the soil. As environmental regulations are becoming increasingly more restricted, there is a large range of opinion on the development of clean-up technologies for the control of SOx emissions. SO2 can be treated by throwaway processes, such as the lime or limestone scrubbing processes, which are the prevailing methods of flue gas desulfurization. DSRP, in which SO2 is converted into elemental sulfur with a reducing agent over a catalyst, has been proposed as another process for SO2 treatment.1-5 The various reducing agents such as carbonaceous materials, carbon monoxide, natural gas, and hydrogen have been used in the DSRP as follows:6-12

2Cat - O + 2CO f 2Cat - + 2CO2

(5)

SO2 + Cat- f SOcat + Cat - O

(6)

SOcat + Cat- f Scat + Cat - O

(7)

1 Scat S Sx + cat x

(8)

SO2 + C f CO2 + S

(1)

The redox mechanism has the following reaction and catalytic characteristics: (1) A relatively high reaction temperature, (2) near-zero COS formation, (3) high storage capacity and mobility of lattice oxygen of applicable catalysts such as CeO2-based catalysts, fluorite-type oxide catalysts, and LaTiO3, (4) involves active sites consisting of lattice oxygen vacancies, and (5) sulfate group formation on the surface of the catalyst in the reaction process.5-6,10,15 It is known that the COS intermediate mechanism involves the surface reaction between SO2 and COS on the Lewis acid sites and Bro¨nsted acid sites of catalyst surface, as indicated below:

SO2 + 2H2 f S + 2H2O

(2)

2M - S + 2CO f 2COS + 2M-S

(9)

SO2 + 2CO f 2CO2 + S

(3)

SO2 + 2COS S 3S + 2CO2

(10)

2SO2 + CH4 f 2S + CO2 + 2H2O

(4)

2M - S + 2S f 2M - S

(11)

Also, various catalysts such as Ce-Zr, Sn-Zr, Co-Mo, transition metals, and alumina are used in the DSRP.13-14 Depending on the type of catalyst that is employed, the redox mechanism and the COS intermediate mechanism have generally been suggested for the SO2 reduction by CO. The mechanistic pathway for the redox mechanism is correlated with the mobility of the lattice oxygen in the crystal structure of the metal oxide catalyst, as indicated below: * To whom correspondence should be addressed. Tel.: +82 (0)53 810 2519. Fax: +82 (0)53 810 4631. E-mail: [email protected].

The COS intermediate mechanism has the following reaction and catalytic characteristics: (1) A relatively low reaction temperature, (2) high COS formation, (3) the relatively low mobility of the lattice oxygen of the applicable catalysts, such as iron/alumina and La2O2S, (4) involves Lewis and Bro¨nsted acidic sites, (5) partial transformation of metal oxide catalyst into the metal sulfide, and (6) high activity of an applicable catalyst in the reaction between SO2 and COS.5,7-8,11 A mechanistic study was conducted for the SO2 reduction by CO using ZrO2 catalyst, which is known as a solid acidic catalyst having Lewis and Bro¨nsted acid sites.16-21 SO2 reduction using a ZrO2 catalyst was investigated by comparing its

10.1021/ie0709483 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/06/2008

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reaction characteristics with those of conventional mechanisms such as the redox and COS intermediate mechanisms. In addition, fresh and used ZrO2 catalysts were characterized by various analysis methods. Experimental Section Preparation of the Catalysts and the Reactivity Test. ZrO2 catalyst was prepared by a precipitation method as follows. Zirconyl nitrate hydrate (ZrO(NO3)2‚H2O, Aldrich Chem.) was dissolved as the precursor in deionized water with vigorous stirring. Ammonium hydroxide (NH4OH, Domestic) was added dropwise to the aqueous solution of the zirconium precursor until the pH reached the range of 9-10, whereupon a white precipitate was formed. This precipitated slurry was heated in a water bath at 80 °C for 4 h. After drying the viscous liquid slurry at 110 °C overnight, the solid product was calcined at 600 °C for 6 h using an electric furnace. After crushing the solid product, those particles with a diameter of 0.075-0.150 mm were selectively used in the performance test of SO2 reduction by CO. The reactivity test for the reduction of SO2 was performed as follows: ZrO2 catalyst of 0.5 g was packed inside a tubular reactor with a diameter of 0.5 in. A gas mixture with a molar ratio of [CO]/[SO2] (40 000 ppmv CO and 20 000 ppmv SO2 in N2 balance) of 2.0 and a space velocity (GHSV) of 10 000 mL/g-cat.‚h was fed into the fixed bed reactor with the reactant placed downstream. The flow rate of the reactant gases was controlled by a mass flow controller (Brooks MFC 5850E). When a steady state was attained, the concentration of SO2, COS, and the other byproducts in the outlet gas streams were determined by a gas chromatograph (Shimadzu-8A) equipped with a thermal conductivity detector and a column consisting of Hayesep Q (8 ft) and Porapak T (2 ft). In this experimental process, the other byproduct was only CO2, and then the CO2 peak area was measured and used as a result involving the production of the elemental sulfur because the CO2 concentration could not exactly be calibrated by the CO2 peak area. To capture the elemental sulfur and prevent the trouble of the GC instrument, the sulfur trap using ceramic wool was equipped between the reactor and the GC instrument. In addition, the performance of the ZrO2 catalyst in the SO2 reduction by COS was conducted using the same procedure under the same conditions excluding the reactant concentration ([COS]/[SO2] ) 2.0, a space velocity (GHSV) ) 10 000 mL/g-cat.‚h, SO2 concentration ) 4500 ppmv, and COS concentration ) 9000 ppmv, amount of ZrO2 catalyst ) 0.5 g). In the performance test, the test results of the SO2 reduction by CO were calculated in the form of the SO2 conversion, sulfur yield, and the selectivity of sulfur and COS as follows:

SO2 conversion (%) )

[SO2]in - [SO2]out [SO2]in

× 100

Sulfur selectivity (%) ) [SO2]in - [SO2]out - [COS]out [SO2]in - [SO2]out COS selectivity (%) )

[COS]out [SO2]in - [SO2]out

× 100

× 100

Sulfur yield (%) ) SO2 conversion × Sulfur selectivity

Also, the results of the SO2 reduction by COS were calculated in the form of the SO2 conversion, sulfur yield, and the selectivity of sulfur and COS as follows:

Sulfur yield (%) ) [SO2]in + [COS]in - [SO2]out - [COS]out [SO2]in + [COS]in SO2 conversion (%) ) COS conversion (%) )

[SO2]in - [SO2]out [SO2]in

× 100

[COS]in - [COS]out [COS]in

× 100

× 100

Characterization of the Catalysts. The fresh and used ZrO2 catalysts were characterized by the various analysis methods to investigate their physical and chemical properties as described below. The crystalline of the fresh and used ZrO2 catalysts was analyzed by X-ray diffraction (XRD; Rigaku, D/MAX-2500) with nickel-filtered Cu KR radiation. The characterization of the ZrO2 catalyst was conducted by infrared (IR) spectroscopy (Jasco 7300 FTIR with an MCT detector at a resolution of 4 cm-1 with 64 scans). For the further analysis of the ZrO2 catalyst, X-ray photoelectron spectroscopy (XPS) was performed using a XPS spectrometer (ESCALAB 250) fabricated by VG Scientifics and equipped with a focused (spot size 10 nm) monochromatized Al KR anode (hν ) 1486.6 eV). The X-ray source power was kept at around 150 W. The binding energies corresponding to the carbon 1s peak at 284.6 eV and are given with an accuracy of (0.2 eV. To investigate the reduction degree of the ZrO2 catalyst, COTPR (temperature programmed reduction) was conducted. After pretreating the ZrO2 catalyst of 0.5 g in flowing N2 for 2 h at 250 °C for the removal of the impurities like H2O, whereupon cooling it, CO-TPR was initiated with the injection of 5 vol % CO (balanced by N2). At the same time, the sample was heated from 60 to 570 °C at a ramping rate of 5 °C/min. The CO-TPR profile of the ZrO2 catalyst with temperature was obtained as TCD signals analyzed with a thermal conductivity detector (TCD) of Autosorb-1 (Quantachrome Co.). Therefore, the label of y axis means the reduction degree of ZrO2 catalyst in Figure 7. Results and Discussion Effect of Reaction Temperature on SO2 Reduction by CO over ZrO2 Catalyst. The effect of temperature on the SO2 reduction by CO over the ZrO2 catalyst was investigated. It was confirmed that the SO2 reduction did not take place under any temperature range of 100-800 °C without the ZrO2 catalyst. However, the blank test result of SO2 reduction was not presented. Figure 1 shows the variations of the SO2 conversion, sulfur yield, and selectivities of sulfur and COS with the temperature. The temperature was varied from 350 to 800 °C at a fixed space velocity of 10 000 mL/g-cat.‚h and [CO]/[SO2] molar ratio of 2.0. The starting temperature of SO2 conversion was around 450 °C, and then the SO2 conversion of about 5.0% and the sulfur selectivity of about 97% were observed, whereas COS was not produced. After 40 min of the reaction time, the reactivity was stabilized and remained constant. The reactivity increased with raising the temperature. Especially, as the temperature was increased from 485 to 490 °C, the SO2

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Figure 1. Effect of the reaction temperature on SO2 reduction by CO over the ZrO2 catalyst.

conversion abruptly increased, whereas the selectivity of the products varied very little. Generally, it is known that the selectivity of the products is either decreased or increased as the conversion is increased. However, in this experiment, no variation of the selectivity of sulfur or COS was observed in spite of the considerable increase of the SO2 conversion with increasing temperature. This result might be due to the increase of sulfur selectivity with raising the temperature because the reaction of SO2 + 2COS f 3S + 2CO2 occurred much more

selectively than other reactions for SO2 conversion. Therefore, it could be known that SO2 conversion is a more effective parameter than the selectivity of the various products to obtain a high sulfur yield in the SO2 reduction by CO over the ZrO2 catalyst. The highest reactivity was observed at 650 °C, and then the SO2 conversion and sulfur yield reached about 100 and 95%, respectively. Part e of Figure 1 shows the sulfur yield and CO2 peak area involving CO2, which is the essential byproduct with temperature. In part e of Figure 1, it was known

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Figure 2. Effect of the space velocity (GHSV) on SO2 reduction by CO over the ZrO2 catalyst.

that the sulfur yield has a tendency to that of CO2 formation, and then it was estimated that the amount of the produced sulfur was proportional to the amount of the produced CO2. Effect of Space Velocity on SO2 Reduction by CO over ZrO2 Catalyst. Figure 2 shows the effect of the space velocity on the SO2 reduction by CO over ZrO2 catalyst. At the fixed conditions of the temperature of 490 °C and a [CO]/[SO2] molar ratio of 2.0, the space velocity was varied between 5000 and 20 000 mL/g-cat.‚h. The optimal space velocity was 10 000 mL/ g-cat.‚h, at which point SO2 conversion and the selectivity of sulfur and COS reached about 100, 86, and 14%, respectively. The sulfur selectivity increased and COS selectivity decreased with raising space velocity, respectively. However, the tendency of the SO2 conversion was different according to the range of space velocity. Below 10 000 mL/g-cat.‚h, the SO2 conversion increased as the space velocity decreased and reached almost 100%, because the contact time between the catalyst surface and reactants might be extended. Although the SO2 conversion reached about 100%, the sulfur yield decreased because COS was more easily formed than elemental sulfur at lower space velocities. On the other hand, at a space velocity above 10 000 mL/g-cat.‚h, the SO2 conversion decreased because the contact time between the catalyst surface and the reactant became shorter as the space velocity increased. In this case, the sulfur yield increased probably because the reaction producing elemental sulfur proceeded more readily than that producing COS. Effect of [CO]/[SO2] Molar Ratio on SO2 Reduction by CO over ZrO2 Catalyst. The [CO]/[SO2] molar ratio varied from 1.0 to 4.0 to investigate the effect of the reactant

Figure 3. Effect of the [CO]/[SO2] molar ratio on SO2 reduction by CO over the ZrO2 catalyst.

Figure 4. XRD patterns of the fresh and used ZrO2 catalysts (0, monoclinic ZrO2 (JCPDS card number 86-1451; b, tetragonal ZrO2 (JCPDS card number 88-1007)).

composition on the SO2 reduction by CO over the ZrO2 catalyst. The fixed conditions were the temperature of 490 °C and space velocity of 10 000 mL/g-cat.‚h. Figure 3 shows the variation of the SO2 conversion, sulfur yield, and the selectivity of sulfur and COS with the [CO]/[SO2] molar ratio. To obtain a high sulfur yield, the optimal [CO]/[SO2] molar ratio was 2.0, corresponding to the stoichiometric [CO]/[SO2] ratio, at which point the SO2 conversion and sulfur yield were about 100 and 87%, respectively. As the [CO]/[SO2] molar ratio increased, the selectivity of sulfur and COS decreased and increased, respectively, because COS might be formed more easily than elemental

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Figure 5. X-ray photoelectron spectra of the S2p peak for the fresh and used ZrO2 catalysts.

Figure 6. IR spectrum of the fresh and used ZrO2 catalysts.

Figure 7. CO-TPR profile of ZrO2 catalyst.

sulfur through the reaction between sulfur and CO as the content of CO increased. However, the SO2 conversion has two tendencies in accordance with the range of the [CO]/[SO2] molar ratio. In the case of a [CO]/[SO2] molar ratio of below 2.0, the SO2 conversion was decreased because the CO concentration was insufficient as compared with the stoichiometric [CO]/[SO2]

molar ratio of 2.0, and therefore the SO2 conversion became increasingly limited as the [CO]/[SO2] molar ratio decreased. In addition, the sulfur selectivity was much higher than the COS selectivity because the reaction leading to the COS formation became increasingly restricted as the CO concentration decreased. On the other hand, in the case of a [CO]/[SO2] molar ratio of above 2.0, the COS selectivity was higher than the sulfur selectivity except at a [CO]/[SO2] molar ratio of 2.5, because COS was produced more easily than elemental sulfur at the high concentration of CO, at which the COS formation is favorable. In addition, the SO2 conversion reached around 100% because the CO concentration was sufficient as compared with the stoichiometric [CO]/[SO2] molar ratio. At the optimal [CO]/ [SO2] molar ratio of 2.0, the sulfur selectivity was about 87%. XRD Analysis of the ZrO2 Catalyst. The analysis of the crystalline of the ZrO2 catalysts was conducted by XRD. Figure 4 shows the XRD patterns of the fresh and used ZrO2 catalysts in the SO2 reduction by CO. From a JCPDS data base card, it was confirmed that both the fresh and used ZrO2 catalysts have simultaneously two kinds of crystalline structures, viz. the monoclinic and tetragonal crystal structures. In addition, it was found that the ZrO2 catalyst has a stable crystal structure because the XRD patterns of the fresh and used ZrO2 catalysts were similar in spite of its catalytic role in the SO2 reduction by CO. The phases of the fresh and used ZrO2 catalysts were not changed because the simultaneous oxidation and reduction processes might momentarily occur over the ZrO2 catalyst. Also, the ZrO2 catalyst might be reoxidized by the oxygen species as soon as the ZrO2 catalyst reduced partially after the SO2 reduction was exposed into the atmosphere. The artificial sulfidation of the ZrO2 catalyst was performed using 1 vol % COS and 1 vol % SO2, but it was confirmed that the phase of the ZrO2 catalyst was not sulfided and changed. The XRD patterns of the sulfided ZrO2 catalyst were not presented because the phases of the ZrO2 catalysts sulfided by 1 vol % COS and 1 vol % SO2 were similar to the fresh ZrO2 catalyst. Therefore, COS might not be formed by the reaction between the metal sulfide and the unreacted CO but might be formed by the reaction between an elemental sulfur and the unreacted CO. The COS formation with the reaction between an elemental sulfur and CO will be described in a later section. IR and XPS Analysis of the Fresh and Used ZrO2 Catalysts. The fresh and used ZrO2 catalysts were characterized by IR spectrometry and XPS. Figures 5 and 6 show the results of the XPS and IR analysis, respectively. In the result of the XPS analysis, unlike the fresh ZrO2, the binding energy peak of 167.7 eV corresponding to the formation of sulfate groups was observed in the case of the ZrO2 catalysts used in the SO2 reduction using CO or COS. Moreover, the formation of sulfate groups corresponding to a wavelength of 1040 cm-1 was also observed in the IR spectrum of the used ZrO2 catalyst. It was already reported that the formation of sulfate groups provides evidence that the SO2 reduction by CO proceeds by the following reaction pathway involving the redox mechanism with the lattice oxygen mobility: (1) Cat-[O] T Cat-O and (2) 2CatO+SO2 T Cat-SO4 (Cat-[O]: surface capping oxygen and CatO: lattice oxygen).22 In this study, the redox properties of the ZrO2 catalyst for the SO2 reduction by CO will be described in section of CO-TPR analysis of the ZrO2 catalyst. From the results of the IR and XPS analyses, it was inferred that the SO2 reduction by CO over the ZrO2 catalyst proceeds partially by the reaction pathway involving the redox mechanism. Additionally, in the previous research results, it was reported by Noda et al. that the sulfated ZrO2 catalyst has more effective Lewis

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and Bro¨nsted acid sites than the unsulfated one.23-28 It was also reported that the Lewis and Bro¨nsted acid sites function as the active sites when the SO2 reduction using CO over a solid acidic catalyst proceeds on the pathway involving the COS intermediate mechanism.7,29-31 It could be expected that the SO2 reduction by CO over the ZrO2 catalyst might proceed on the pathway involving the COS intermediate mechanism. Especially, the Lewis and Bro¨nsted acid sites of the ZrO2 catalyst might be improved by the sulfate group, which was formed during the process of SO2 reduction by CO. Therefore, it was estimated that the sulfated ZrO2 catalyst has high activity for the SO2 reduction by CO due to the improved catalytic properties of the Lewis and Bro¨nsted acid sites. CO-TPR Analysis of ZrO2 Catalyst. The reducibility associated with the redox properties of the ZrO2 catalyst was investigated in this section. Figure 7 shows the results of the CO-TPR analysis using 5 vol % CO. The starting temperature for the reduction of the ZrO2 catalyst was around 100 °C. It was estimated that ZrO2 catalyst has stepwise reducibility according to the temperature range because the reduction degree tendency of the ZrO2 catalyst could be classified into two regions in the ranges of above and below 250 °C. The lattice oxygen species of the metal oxide catalyst can be classified into those that were contained in the surface and the bulk. In the COTPR profile, the area is directly related to the movable amount of the lattice oxygen, and the peak position is related to the temperature for the reduction of the ZrO2 catalyst. Therefore, it was known that the less amount of surface lattice oxygen might be more easily moved than the bulk lattice oxygen at a low temperature of below 250 °C in Figure 7, whereas the bulk lattice oxygen species would be moved in the high-temperature range of around 400 °C. In the CO-TPR profile, the point having the highest reduction degree and integrated curve area was observed at 490 °C, and the SO2 reduction by CO might be started by the highest reducibility at 490 °C. From these results, it is suggested that the SO2 reduction by CO over the ZrO2 catalyst followed the redox mechanism because the SO2 reduction by CO might be progressed by the high mobility of the bulk lattice oxygen above 400 °C. This pathway for the mechanism redox may be described as follows: (1) 2CatO+2CO f 2Cat-+2CO2, (2) SO2+2Cat- f Scat+2Cat-[O], and (3) 2Cat-[O]+2CO f 2Cat-+2CO2. In addition, it is supposed that the production of the elemental sulfur involving the redox mechanism is necessary as an elementary reaction step. The reason might be as follows: It had been reported that redox properties of the ZrO2 catalyst were relatively low in comparison with other metal oxides having redox properties and ZrO2 that has Lewis and Bro¨nsted acid sites.14.16-21,32 Therefore, the active sites of the ZrO2 catalyst for SO2 reduction might be the Lewis and Bro¨nsted acid sites. COS should be produced in theCOS intermediate involving Lewis and Bro¨nsted acid sites. COS can be produced by the reaction of metal sulfide and unreacted CO or the elemental sulfur and unreacted CO. In this study, COS might be produced by only the reaction of the elemental sulfur and unreacted CO. Therefore, it was estimated that the production step of the elemental sulfur with the redox mechanism is the initialization step and the most important among the various elemental steps. Effect of Reaction Temperature on SO2 Reduction by COS over ZrO2 and γ-Al2O3 Catalysts. The effect of temperature on the SO2 reduction by CO over ZrO2 was investigated in previous section, where it was said that the selectivity of each product was maintained in spite of the steep increase of the SO2 conversion with increasing temperature. The

Figure 8. Effect of the reaction temperature on SO2 reduction by COS or CO.

SO2 reduction by CO might be partially correlated with the redox properties of the ZrO2 catalyst because SO2 reduction by CO occurred above 400 °C and not below 400 °C in spite of the reducibility of the ZrO2 catalyst. It has been suggested that the ZrO2 catalyst has other catalytic properties of its Lewis and Bro¨nsted acid sites, apart from its redox properties. As mentioned in section on IR and XPS analysis of the fresh and used ZrO2 catalysts, it was found that the sulfate group was formed on the surface of the ZrO2 catalyst. In previous works, it was reported that ZrO2 catalysts have Lewis and Bro¨nsted sites, which are improved by the sulfate group.23-28 In addition, it was found that the mechanistic pathway is connected with the COS intermediate mechanism in the SO2 reduction by CO over a solid acidic catalyst such as γ-Al2O3 having Lewis and Bro¨nsted acid sites and that the reaction between SO2 and COS proceeds very easily.7,29-31 From these previous research results, it can be suggested that the ZrO2 catalyst plays the catalytic role, having Lewis and Bro¨nsted acid sites involving the COS intermediate. Accordingly, the reaction between SO2 and COS (SO2 + 2COS f 3S + 2CO2) was investigated using the ZrO2 catalyst having Lewis and Bro¨nsted acid sites this section. The effect of temperature on the SO2 reduction by COS over the ZrO2 catalyst was investigated. Part a of Figure 8 shows the variation of the sulfur yield and the conversion of SO2 and COS with temperature in the SO2 reduction by COS over the ZrO2 and the γ-Al2O3 catalyst. In the SO2 reduction by COS over the ZrO2 catalyst, the reaction temperature was varied from 225

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to 425 °C and then the space velocity and [COS]/[SO2] molar ratio were fixed at 10 000 mL/g-cat.‚h (catalyst amount ) 0.5 g) and 2.0, respectively. The concentrations of SO2 and COS were 4500 and 9000 ppmv, respectively, which correspond to the stoichiometric [COS]/[SO2] molar ratio. The starting temperature for the SO2 conversion was around 250 °C and was much lower than 450 °C, which is the starting temperature for the SO2 reduction by CO. At 250 °C, the sulfur yield was about 8.7%. It was observed that the conversions of SO2 and COS corresponding to the sulfur yield were increased as the temperature increased. SO2 and COS were completely converted into elemental sulfur at 425 °C and then the sulfur yield was about 100%. It has been previously reported that γ-Al2O3 catalysts can be used as solid acidic catalysts having Lewis and Bro¨nsted acid sites and that consequently they show high reactivity in the SO2 reduction by COS.7,29-31 Therefore, the reaction characteristics of the SO2 reduction by CO over a γ-Al2O3 catalyst were investigated with varying the temperature to compare with using the ZrO2 catalyst. The temperature was varied from 175 to 400 °C while keeping the other reaction conditions the same as those used in the case of the ZrO2 catalyst. In the SO2 reduction by COS over the γ-Al2O3 catalyst, the starting temperature for the conversion of SO2 was around 200 °C, and at this point the sulfur yield was about 36.3%. This temperature was only slightly different from the starting temperature in the SO2 reduction by COS over the ZrO2 catalyst. However, it was lower than the starting temperature in the SO2 reduction by CO over the ZrO2 catalyst. In the SO2 reduction by COS, the sulfur yield increased as the temperature increased. In addition, the tendency of the sulfur yield with temperature in the reaction using the γ-Al2O3 catalyst was similar to that in the reaction using the ZrO2 catalyst. SO2 and COS were completely converted into the elemental sulfur at 400 °C. The highest sulfur yield of about 100% was obtained at 400 °C. At 400 °C, the reactivity for the SO2 reduction using COS over the γ-Al2O3 catalyst was similar to that using COS over the ZrO2 catalyst but was much higher than that using CO over the ZrO2 catalyst. Part b of Figure 8 shows the SO2 reduction by CO over the ZrO2 catalyst using 4500 ppmv SO2 and 9000 ppmv CO. The starting temperature for the SO2 reduction by CO was 490 °C, and the maximum of the sulfur yield was about 94% at 600 °C. On comparing parts a and b of Figure 8, the reactivity of the SO2 reduction by COS was higher at a lower temperature than in the SO2 reduction by CO. It was inferred from this result that COS is a more effective reducing agent than CO in the SO2 reduction, and this result agrees well with previous reports.33-36 According to the stated results above, it was estimated that the SO2 reduction by CO over the ZrO2 catalyst might proceed mainly by the reaction pathway involving the COS intermediate mechanism. The formation of COS might be essential for the pathway involving the COS intermediate mechanism in the SO2 reduction by CO over the ZrO2 catalyst. Therefore, it was suggested that COS was produced by the reaction of S + CO f COS. The experiment to investigate the reaction of S + CO f COS will be described in the section on COS Formation. COS Formation. Parts a and b of Figures 9 show the schematic diagram of the layer arrangement, which was packed with the ZrO2 catalyst and elemental sulfur in the downstream reactor and the concentration of COS, which was produced by the reaction of S + CO f COS. To observe the formation of COS, the experimental procedure was conducted as follows: In the layer arrangement inside the reactor, the amounts of

Figure 9. COS formation by the reaction between elemental sulfur and 4 vol % CO over ZrO2 catalyst ((a) COS concentration, (b) gas flow and arrangement of elemental sulfur and ZrO2 catalyst).

elemental sulfur (top layer) and ZrO2 catalyst (bottom layer) were about 1.2 and 0.5 g, respectively. CO (4 vol %, diluted by N2) was fed downstream into the packed reactor. The temperature and space velocity were 550 °C and 10 000 mL/ g-cat.‚h, respectively. The formation of COS was observed as soon as the feeding of 4 vol % CO was started. The COS concentration increased up to 3100 ppmv during the first 20 min but then continuously decreased thereafter. Consequently, it was estimated that COS can be formed by the reaction of S + CO f COS. Conclusion In this work, a mechanistic study was conducted to investigate the pathway of the SO2 reduction by CO over the ZrO2 catalyst. Especially, it was determined that the ZrO2 catalyst was sulfated in the process of SO2 reduction by CO and that the Lewis and Bro¨nsted acid sites were improved by the formation of the sulfate group. From the high activity of the ZrO2 catalyst in the SO2 reduction by COS, it was inferred that the ZrO2 catalyst also having Lewis and Bro¨nsted acid sites plays a similar role to the γ-Al2O3 catalyst, which is known to have Lewis and Bro¨nsted acid sites for the COS intermediate mechanism. After elemental sulfur was produced by the pathway involving the redox mechanism for initializing the SO2 reduction by CO, COS must be formed by the reaction of S + CO f COS. The redox properties of the ZrO2 catalyst were confirmed by CO-TPR. Consequently, it was concluded that the SO2 reduction by CO over the ZrO2 catalyst proceeds simultaneously by the pathway involving both redox and COS intermediate mechanisms, as follows: (1) In the first step initialized by the redox mechanism, the ZrO2 catalyst is reduced by CO, and then the sulfate group, which plays the role of improving the Lewis acidic and Bro¨nsted acidic sites, is formed on the surface. (2) In the second step, elemental sulfur is produced by the movement of lattice oxygen between SO2 and the lattice oxygen vacancies of ZrO2 having redox catalytic properties. (3) In the third step, COS is formed by the reaction of S+CO f COS. (4) Finally, in the fourth step, SO2 and COS are adsorbed and react on the surface of the ZrO2 catalyst having Lewis acidic and Bro¨nsted acidic sites, resulting in the production of abundant amounts of elemental sulfur. Literature Cited (1) Lee, T. J.; Park, N.-K.; Kim, J. H.; Kim, K. S.; Park, Y. W.; Yi, K. K. Removal of H2S by Zinc-Based Sorbents form High Temperature CoalDerived Gases. Hwahak Konghak 1996, 34, 435.

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ReceiVed for reView July 12, 2007 ReVised manuscript receiVed November 11, 2007 Accepted December 19, 2007 IE0709483