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Catalytic reduction of SO2 over supported transition-metal oxide catalysts with C2H4 as a reducing agent
Applied Catalysis B: Environmental 40 (2003) 331–345
Catalytic reduction of SO2 over supported transition-metal oxide catalysts with C2 H4 as a reducing agent Ching-Huei Wang a , Shiow-Shyung Lin b , Pei-Chang Sung c , Hung-Shan Weng c,∗ b
a Department of Chemical Engineering, Kao Yuan Institute of Technology, Kaohsyung 821, Taiwan Department of Environmental Engineering and Health, Chia-Nan University of Pharmacy and Science, Tainan 717, Taiwan c Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan
Received 11 April 2002; received in revised form 1 July 2002; accepted 2 July 2002
Abstract This work investigates performances of supported transition-metal oxide catalysts for the catalytic reduction of SO2 with C2 H4 as a reducing agent. Experimental results indicate that the active species, the support, the feed ratio of C2 H4 /SO2 , and pretreatment are all important factors affecting catalyst activity. Fe2 O3 /␥-Al2 O3 was found to be the most active catalyst among six ␥-Al2 O3 -supported metal oxide catalysts tested. With Fe2 O3 as the active species, of the supports tested, CeO2 is the most suitable one. Using this Fe2 O3 /CeO2 catalyst, we found that the optimal Fe content is 10 wt.%, the optimal feed ratio of C2 H4 /SO2 is 1:1, and the catalyst presulfidized by H2 + H2 S exhibits a higher performance than those pretreated with H2 or He. Although the feed concentrations of C2 H4 :SO2 being 3000:3000 ppm provide a higher conversion of SO2 , the sulfur yield decreases drastically at temperatures above 300 ◦ C. With higher feed concentrations, maximum yield appears at higher temperatures. The C2 H4 temperature-programmed desorption (C2 H4 -TPD) and SO2 -TPD desorption patterns illustrate that Fe2 O3 /CeO2 can adsorb and desorb C2 H4 and SO2 more easily than can Fe2 O3 /␥-Al2 O3 . Moreover, the SO2 -TPD patterns further show that Fe2 O3 /␥-Al2 O3 is more seriously inhibited by SO2 . These findings may properly explain why Fe2 O3 /CeO2 has a higher activity for the reduction of SO2 . © 2002 Elsevier Science B.V. All rights reserved. Keywords: Reduction of SO2 ; Conversion of SO2 ; Yield of elemental sulfur; Fe2 O3 /CeO2 ; TPD
1. Introduction Since the 1930s, various commercial flue gas desulfurization (FGD) processes, including throwaway, adsorption, absorption, and catalytic reduction, have been developed. These technologies have recently attracted increasing attention as the regulations for the emission of SOx have become more stringent, ∗ Corresponding author. Tel.: +886-6-2757575x62637; fax: +886-6-2344496. E-mail address: [email protected] (H.-S. Weng).
and can now deliver desulfurization rates above 90% [1–3]. Among the FGD processes, the throwaway method has the drawback of requiring subsequent waste disposal [4–7]. Meanwhile, with both adsorption and absorption it is necessary to devise a process to treat SO2 released from the sorption process. This leaves catalytic reduction, in which SO2 is reduced to elemental sulfur, as the method of choice. This method has the advantages of feasible recovery of sulfur and no waste disposal problem. Reducing agents for the catalytic reduction of SO2 include carbonaceous material, CH4 ,
0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 1 5 9 - 5
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CO, and H2 . By using coke as a reducing agent, Lepsoe [8] obtained 80% conversion of SO2 at 850 ◦ C. Salis and Berk [9] and Mulligan and Berk [10] reported only slightly higher conversion rates, using CH4 as a reducing agent, over activated alumina or transition-metal sulfide catalysts, whilst Zhu et al. [3] obtained complete conversion of SO2 over ceria-based catalysts at 750 ◦ C, also using CH4 . Other reports [11–21] indicate that when using CO as a reducing agent, lower reaction temperatures (320–800 ◦ C) can be used. Another intensively investigated reducing agent, H2 , has also been shown to effectively reduce SO2 to elemental sulfur [2,22–27]. While CH4 is widely known as a reducing agent, the use of other gaseous hydrocarbons has rarely been reported. The use of ethylene, the most important raw material in the chemical industry, has been reported once to be used to reduce NO to N2 over platinum ion-exchanged MFI zeolites [28]. Though it is somewhat costlier to use than other reducing agents, it is more easily available, and therefore has potential as a reducing agent for SO2 . Specifically, for those plants that produce sulfur emissions and already have easy access to ethylene, the combination of efficiency and safety considerations may in fact make C2 H4 a better choice of reducing agent. Within this category are electric utility plants, natural gas producing plants and petroleum refining plants [29]. Our previous study [20] demonstrated the superior performance of supported transition-metal oxide catalysts in the catalytic reduction of SO2 with CO as a reducing agent. The results showed that SO2 was completely converted using an Fe2 O3 /CeO2 catalyst at 320 ◦ C, producing an elemental sulfur yield above 90%. In the current work, by employing C2 H4 as a reducing agent, the SO2 reduction activity of several transition-metal oxides supported on ␥-Al2 O3 , and subsequently the activity of the most active species (in this study, Fe2 O3 ) on various supports, were tested to find an optimal one. Superior activity of the optimal catalyst was further verified by C2 H4 temperature-programmed desorption (C2 H4 -TPD) and SO2 -TPD. Moreover, the effect of C2 H4 /SO2 ratio, feed concentrations of C2 H4 and SO2 , content of Fe species on the CeO2 , and pretreatment conditions were also investigated. Finally, the results of using C2 H4 , CO and H2 as the reducing gases for the reduction of SO2 are compared.
2. Experimental 2.1. Catalyst preparation The incipient impregnation method was employed for preparing all catalysts tested in this study. In the preparation, the support (␥-Al2 O3 , SiO2 , TiO2 , CeO2 , La2 O3 , V2 O5 or Zeolite-Y, except TiO2 was supplied by Nihon Shiyaku Co., others were purchased from Merck Co.) was firstly dried in a vacuum oven at 120 ◦ C for at least 12 h. It was then cooled to room temperature, after which a known amount of metal (Fe, Ni, Mn, Mo, Cr or Co) salt in deionized water (7 ml) was added into it. The sources of metals were nitrates except that of Mo was (NH4 )6 Mo7 O24 ·4H2 O (all these salts were the products of Merck Co.). The supports and salts used were of reagent grade and were not purified further. Nevertheless, considering the supported Fe catalyst was the best catalyst in this study, which will be shown later, herein the composition of ferric nitrate (Fe(NO3 )3 ·9H2 O) is shown: assay > 98%; Cl < 0.01%; SO4 < 0.01%; Cu < 0.005%; Mn < 0.02%; Pb < 0.005%; Zn < 0.002%. Subsequently, well-impregnated catalyst precursors were put aside for 0.5 h, dried in air at 120 ◦ C for 24 h and, after being ground to particle sizes between 140 and 220 mesh, calcined in an oven with the air supply at 500 ◦ C for 8 h. Except the tests for the influence of Fe content, where the loading of Fe species was varied between 4 and 16 wt.%, metal loading was fixed at 10 wt.%. 2.2. Measurement of activity A packed-bed reactor (8 mm i.d.; Pyrex) was used to measure catalyst activity. An amount of 0.5 g of catalyst powder (140–220 mesh) was placed in the reactor and was pretreated by being purged in a He flow (50 ml/min) at 500 ◦ C for 80 min, then presulfied at the same temperature for 2 h in a flow of He gas containing H2 (50,000 ppm) and H2 S (5000 ppm). In order to compare the effect of presulfidization with H2 + H2 S, H2 and He were also used for the pretreatment of the catalyst. After pretreatment, the catalyst was cooled to 260 ◦ C and the gas mixture (5000 ppm SO2 and 5000 ppm C2 H4 in He balance) was fed into the reactor as reactants. When a steady state was attained (about 2 h), the inlet and outlet gases were analyzed. Then the temperature was raised stepwise, 20 ◦ C each time,
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until 360 ◦ C (or 400 ◦ C). In each step, the gases were analyzed after a steady state was attained. Concentrations of SO2 in the inlet and outlet gases and those of H2 S, COS and CS2 (reaction by-products) in the effluent gas were determined using a gas chromatograph with a flame photometric detector (FPD). Additionally, concentrations of C2 H4 in the inlet and outlet gases were also determined using a gas chromatograph with a flame ionization detector (FID). Moreover, the concentration of COx (CO + CO2 ) in the effluent gas was measured with a nondispersive IR spectrometer (ADC D/7V/76/S for CO; Telaire/USA 2001 V for CO2 ). The percent conversion (X) of SO2 , the selectivity (YS ) and percent yield (Y) of elemental sulfur are defined as follows: X=
[SO2 ]in − [SO2 ]out × 100% [SO2 ]in
[SO2 ]in −[SO2 ]out −[COS]out −[H2 S]out − 2[CS2 ]out YS = , [SO2 ]in − [SO2 ]out
Y = X × YS
where [SO2 ]in is the inlet concentration of SO2 (ppm) determined at a stable bypass concentration, while [SO2 ]out , [COS]out , [H2 S]out and [CS2 ]out are the effluent concentrations of SO2 , COS, H2 S and CS2 , all in ppm. 2.3. Measurement of temperature-programmed desorption (TPD) The temperature-programmed desorptions of SO2 and C2 H4 were conducted using a Micromeritics 2900 TPD/TPR analyzing instrument. The catalyst sample (0.2 g) was placed in the instrument in a U-shaped quartz tube, which was plugged with quartz wool to prevent the sample from being carried away by the gas flowing through it. After being presulfidized, the sample was cooled to 30 ◦ C, and then either C2 H4 or SO2 (both are 40,000 ppm in the flow of He) adsorption was carried out for 30 min. Thereafter, the temperature was raised to and maintained at 100 ◦ C for 30 min to eliminate the physically adsorbed C2 H4 or SO2 . The sample was then heated to 1000 ◦ C at a constant heating rate of 15 ◦ C/min. Increase in the sample temperature caused desorption of chemically adsorbed C2 H4 or SO2 , which was detected by a thermal conductivity detector (TCD).
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2.4. Characterization of catalyst structure The structure of the catalyst was characterized by an XRD analyzer (Brookhaven Instrumental Corporation Model: APX 63/Powder). Conditions of analysis were as follows: Target: Cu (λ = 1.5405 Å); scanning speed: 3◦ /min; scanning range (2θ ): 20–60◦ . 3. Results and discussion 3.1. Screening for the optimal catalyst In screening for an optimal catalyst, first, catalysts with different metal species (Fe, Ni, Mn, Mo, Cr and Co) supported on ␥-Al2 O3 were assessed. Each metal oxide/␥-Al2 O3 catalyst was presulfied, and then a gas mixture with a constant molar ratio of C2 H4 :SO2 = 1:1 (C2 H4 , 5000 ppm; SO2 , 5000 ppm) was fed into the reactor. Weight hourly space velocity (WHSV) was fixed at 8000 ml/(h g). Percentage SO2 conversion versus temperature, plotted in Fig. 1A, shows that Fe2 O3 /␥-Al2 O3 was the most active catalyst, and NiO/␥-Al2 O3 the second most active. Our previous study [20], in which the reducing agent was CO, yielded the same result. Conversely, using H2 as a reducing agent, Paik and Chung [2] found that the activity of ␥-Al2 O3 -supported Co and Ni was greater than that of Fe2 O3 /␥-Al2 O3 . This may be due to the difference in adsorption capability of H2 , CO and C2 H4 onto these metal species. In our results, there was a maximum elemental sulfur yield with Fe2 O3 /␥-Al2 O3 at 360 ◦ C (Fig. 1B), whereas according to the definition of percent yield of elemental sulfur (Y = XYS ), higher sulfur yield is obtained at a higher SO2 conversion. Existence of a fall in sulfur yield with Fe2 O3 /␥-Al2 O3 despite continued rising SO2 conversion is due to the higher production of by-products (CS2 , COS and H2 S) at higher reaction temperatures. Based on the above observation that Fe2 O3 was the most active species, catalysts of Fe2 O3 impregnated on various supports were further tested to find the optimal one. On measures of both SO2 conversion (Fig. 2A) and elemental sulfur yield (Fig. 2B), we found that Fe2 O3 /CeO2 performed best. This outcome is largely
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Fig. 1. Effect of different supported metals on the reduction of SO2 . Support: ␥-Al2 O3 ; weight of catalyst: 0.50 g; feed: 5000 ppm SO2 , 5000 ppm C2 H4 ; WHSV: 8000 ml/(h g).
due to better adsorption of C2 H4 by Fe2 O3 /CeO2 , as will be verified later in this work. Furthermore, Fig. 2B shows that there is maximum sulfur yield (about 72%) at 320 ◦ C with Fe2 O3 /CeO2 . This is again attributed to a higher conversion of SO2 leading to a higher production of by-products as the reaction temperature is increased. Variations in the yield of elemental sulfur
and major by-products with temperature are shown in Fig. 3. Noticeable yield of by-products was observed at higher reaction temperatures. Via a durability test, the results of which are shown in Fig. 4, we were able to further demonstrate the superior performance of Fe2 O3 /CeO2 to that of Fe2 O3 /␥-Al2 O3 . The catalysts were initially pretreated
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Fig. 2. Effect of different supports on the reduction of SO2 . Active species: Fe2 O3 ; weight of catalyst: 0.50 g; feed: 5000 ppm SO2 , 5000 ppm C2 H4 ; WHSV: 8000 ml/(h g); content of Fe: 10 wt.%.
with H2 S + H2 at 500 ◦ C for 2 h; then cooled to 320 ◦ C to conduct the isothermal reduction of SO2 for 24 h. Fig. 4 shows that the Fe2 O3 /CeO2 catalyst produced a SO2 conversion rate of 100% throughout the entire 24 h reaction time, while Fe2 O3 /␥-Al2 O3 decayed seriously (from about 90% to about 32% conversion) as the reaction proceeded. As for the ele-
mental sulfur yield, we found that with Fe2 O3 /CeO2 it increased from 62 to 78%, before leveling off at about 12 h. With Fe2 O3 /␥-Al2 O3 , although the sulfur yield at the onset was as high as 72%, it decreased significantly during the reaction. Note that when the same catalyst was employed the highest conversions of SO2 in Fig. 4 are higher than the conversions in
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Fig. 3. Variation in yields of elemental sulfur and by-products with temperature. Catalyst: Fe2 O3 /CeO2 ; weight of catalyst: 0.50 g; feed: 5000 ppm SO2 , 5000 ppm C2 H4 ; WHSV: 8000 ml/(h g); content of Fe: 10 wt.%.
Figs. 1A and 2A at 320 ◦ C. This is due to poisoning of the catalyst at lower reaction temperatures as the programmed temperature reaction proceeded. This poisoning effect is also demonstrated by conducting durability tests at different temperatures. 3.2. Effect of reaction temperature on the durability of the Fe2 O3 /CeO2 catalyst Fig. 5 shows the results of isothermal reduction of SO2 at 300 and 320 ◦ C over 24 h. The conversion of SO2 decreased noticeably when the reaction was carried out at 300 ◦ C, but maintained at 100% at 320 ◦ C. As to the sulfur yield, it increased sharply at first 5 h, but decreased rapidly with time thereafter at 300 ◦ C, while it increased gradually and then leveled off at 320 ◦ C. Inhibition of catalyst activity at 300 ◦ C is thus demonstrated by the inferior SO2 reduction performance. The fact that an increase in sulfur yield up to 5 h at 300 ◦ C may be due to catalyst inhibition at this stage having a bigger influence on the rate of
by-product formation than on the rate of elemental sulfur formation, as the formation of by-products is more difficult than the reduction of SO2 . 3.3. Effect of Fe content in Fe2 O3 /CeO2 on SO2 reduction Having concluded that Fe2 O3 /CeO2 is the best choice for catalytic reduction of SO2 , the effect of Fe content in Fe2 O3 /CeO2 on catalytic performance was investigated. Fe2 O3 is the active species of the Fe2 O3 /CeO2 catalyst, and therefore increasing amounts of the supported Fe2 O3 species increase activity of the catalyst, and so the rate of SO2 conversion. However, Fig. 6 shows that the optimal Fe content is 10 wt.%. This is because of a higher level of Fe2 O3 on the support leading to the formation of large Fe2 O3 crystals, which results in a decrease in the amount of active sites, and therefore a decline in SO2 conversion. The aforementioned formation of large Fe2 O3 crystals at higher Fe content was further verified by XRD
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Fig. 4. Durability of Fe2 O3 /CeO2 and Fe2 O3 /␥-Al2 O3 for the reduction of SO2 . Reaction temperature: 320 ◦ C; weight of catalyst: 0.50 g; feed: 5000 ppm SO2 , 5000 ppm C2 H4 ; WHSV: 8000 ml/(h g); content of Fe: 10 wt.%.
Fig. 5. Effect of reaction temperature on the durability of Fe2 O3 /CeO2 for the reduction of SO2 . Weight of catalyst: 0.50 g; feed: 5000 ppm SO2 , 5000 ppm C2 H4 ; WHSV: 8000 ml/(h g); content of Fe: 10 wt.%.
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Fig. 6. Effect of Fe content on the activity of Fe2 O3 /CeO2 for the reduction of SO2 . Weight of catalyst: 0.50 g; feed: 5000 ppm SO2 , 5000 ppm C2 H4 ; WHSV: 8000 ml/(h g).
analysis of Fe2 O3 /CeO2 , results of which are shown in Fig. 7. This figure shows how the peaks characterizing the Fe2 O3 crystal, especially that at 35◦ (2θ ), become more and more intense as the content increases above 10 wt.%. This indicates that large size crystals of Fe2 O3 are formed and the dispersion of Fe2 O3 species is worse at Fe content exceeding 10 wt.%. Based on the results of Figs. 6 and 7, the Fe content was fixed
at 10 wt.% in the preparation of the Fe2 O3 /CeO2 catalyst for further investigation. 3.4. Effect of C2 H4 /SO2 ratio on SO2 reduction Although the stoichiometric ratio of C2 H4 /SO2 is 1/3 for reduction of SO2 by C2 H4 (C2 H4 + 3SO2 → 3S+2CO2 +2H2 O), to get complete reduction of SO2 ,
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Fig. 7. Effect of Fe content on the XRD pattern of Fe2 O3 /CeO2 .
higher ratios were investigated in the search for an optimal one. Reaction conditions were the same as those for screening of catalyst except that the feed ratio of C2 H4 /SO2 was varied from 1:2 to 3:1. Results in Fig. 8 show that both SO2 conversion and sulfur yield were the highest with the feed ratio at 1:1. Existence of an optimal feed ratio for the conversion of SO2 may be due to the competitive adsorption of C2 H4 and SO2 on the active sites of the catalyst. Since catalyst affinity for SO2 is higher than for C2 H4 , overwhelming concentrations of the latter in the gas phase is necessary for a higher rate of C2 H4 adsorption. However, if the ratio is too high, the rate of SO2 adsorption may be too low to proceed the reduction reaction effectively. As for the sulfur yield, excess C2 H4 resulted in a further deep reduction of SO2 . At a higher feed ratio, this also caused the lower sulfur yield in addition to the lower SO2 conversion. Similar results, showing that a higher feed ratio of H2 /SO2 resulted in deep reduction of SO2 were obtained in the studies of Paik and
Chung [27]. Therefore, based on the results shown in Fig. 8, we conclude that the optimal ratio of C2 H4 /SO2 is 1:1. 3.5. Effect of inlet concentration on SO2 reduction Having established that the optimal ratio of C2 H4 /SO2 is 1:1 and that the feed concentrations of C2 H4 and SO2 may affect the efficacy of SO2 reduction, three different feed concentrations, all at the ratio 1:1 (C2 H4 :SO2 = 3000:3000, 5000:5000 and 7000:7000, all in ppm) were tested to assess their influence. At lower reaction temperatures, a feed concentration of 3000:3000 ppm provided the highest rate of SO2 reduction as shown in Fig. 9A. This feed concentration also gave the highest sulfur yield at 300 ◦ C (Fig. 9B). The maximum yield at a feed concentration of 5000:5000 ppm occurred at 320 ◦ C, whilst that at 7000:7000 ppm took place at 340 ◦ C. In general, a higher conversion leads to a higher yield of main
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Fig. 8. Effect of feed ratio of C2 H4 /SO2 on the reduction of SO2 . Weight of catalyst: 0.50 g; WHSV: 8000 ml/(h g); content of Fe: 10 wt.%.
product unless the formation of by-products is noticeable. In this study, the by-product formation rose at higher reaction temperatures, thereby a maximum sulfur yield occurred at an intermediate temperature. Moreover, a higher feed concentration of SO2 and C2 H4 would attain the complete conversion of SO2 at a higher temperature, hence the maximum sulfur yield would occur at a higher temperature.
3.6. Effect of pretreatment on the activity of the catalyst To further identify the efficacy of presulfidization, we also compared the effect of pretreating Fe2 O3 /CeO2 with different gases (H2 + H2 S, H2 , and He). The catalyst was pretreated with the different gases at 500 ◦ C for 2 h, and then cooled to 320 ◦ C
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Fig. 9. Effect of feed concentration on the reduction of SO2 . Catalyst: Fe2 O3 /CeO2 ; weight of catalyst: 0.50 g; WHSV: 8000 ml/(h g).
to conduct the isothermal reduction of SO2 . Results shown in Fig. 10A reveal that when the pretreatment gas is H2 + H2 S, SO2 conversion was 100% throughout the reaction. With H2 or He, SO2 conversion attained 100% only after the initial stage of the reaction (after about 3 h for H2 and 5 h for He). Further inspection on the effect of the different pretreatment gases on the yield of elemental sulfur (Fig. 10B) confirmed the gas mixture of H2 and H2 S to be the most
productive pretreatment. Therefore, we conclude that by using H2 + H2 S as the pretreatment gas the catalyst would exhibit the most stable and active performance. 3.7. Comparison of different reducing agents The reducing agents commonly used for the reduction of SO2 are CO and H2 . Thus, the reduction of SO2 with CO, H2 and C2 H4 as reducing gases over
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Fig. 10. Effect of pretreatment on the reduction of SO2 . Reaction temperature: 310 ◦ C; catalyst: Fe2 O3 /CeO2 ; weight of catalyst: 0.50 g; feed: 5000 ppm SO2 , 5000 ppm C2 H4 ; WHSV: 8000 ml/(h g).
Fe2 O3 /CeO2 were compared. As shown in Fig. 11, the order of efficacy was found to be CO > C2 H4 > H2 . Differences in reducing ability is mainly caused by the difference of adsorbing ability of various agents onto the Fe2 O3 /CeO2 catalyst. Although the use of C2 H4 does not exhibit a better performance as CO does, it can be still considered as a candidate because of its safety and availability.
3.8. Characterization of catalyst by TPD Following the conclusion that Fe2 O3 /CeO2 is the most active catalyst and Fe2 O3 /␥-Al2 O3 is the second most active catalyst for the reduction of SO2 , we further characterized the catalysts by performing C2 H4 -TPD and SO2 -TPD to find the relationship between the surface properties and catalyst activity.
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Fig. 11. Effect of reducing agent on the reduction of SO2 . Catalyst: Fe2 O3 /CeO2 ; weight of catalyst: 0.50 g; feed: 5000 ppm SO2 , 5000 ppm reducing agent; WHSV: 8000 ml/(h g).
3.8.1. C2 H4 -TPD The C2 H4 -TPD patterns of Fe2 O3 /CeO2 and Fe2 O3 /␥-Al2 O3 are shown in Fig. 12A. Both patterns of Fe2 O3 /CeO2 and Fe2 O3 /␥-Al2 O3 exhibit two peaks. In comparison, the first peak of Fe2 O3 /CeO2 is at a far lower temperature (220 ◦ C) than that of Fe2 O3 /␥-Al2 O3 (420 ◦ C), and the amount of C2 H4 desorbed from Fe2 O3 /CeO2 is larger than that from
Fe2 O3 /␥-Al2 O3 . The higher amounts of adsorption and desorption of C2 H4 by the Fe2 O3 /CeO2 catalyst may render this catalyst with higher activity for SO2 reduction. The results shown in Fig. 2 verify this point. 3.8.2. SO2 -TPD The SO2 -TPD experiments were also conducted to elucidate the higher activity of Fe2 O3 /CeO2 for SO2
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Fig. 12. Comparison of C2 H4 -TPD and SO2 -TPD patterns of Fe2 O3 /CeO2 and Fe2 O3 /␥-Al2 O3 .
reduction. The desorption patterns of Fe2 O3 /CeO2 and Fe2 O3 /␥-Al2 O3 are shown in Fig. 12B. There are two peaks in the pattern of Fe2 O3 /CeO2 (225 and 595 ◦ C), while three peaks are in the pattern of Fe2 O3 /␥-Al2 O3 (352, 558 and 616 ◦ C). As was the case in C2 H4 -TPD, the first peak of the SO2 -TPD pattern of Fe2 O3 /CeO2 represents weaker adsorption of SO2 on active sites. Comparing with the SO2 conversion–temperature curves shown in Fig. 3, the adsorption of SO2 on these sites is the key step for
the reduction reaction of SO2 . Again, the peaks at higher temperatures indicate stronger adsorption on active sites. According to our previous work [20], the strong adsorption of SO2 on active sites may cause inhibition of catalyst activity. We found that the area of the first peak in the pattern of Fe2 O3 /CeO2 is much larger than that of Fe2 O3 /␥-Al2 O3 , while the area of peaks at higher temperatures in the pattern of Fe2 O3 /CeO2 is approximately the same as that of Fe2 O3 /␥-Al2 O3 . These two facts can be applied to
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explain that Fe2 O3 /CeO2 has a higher activity for the reduction reaction of SO2 , while Fe2 O3 /␥-Al2 O3 is more easily to be deactivated. 4. Conclusions (1) With ␥-Al2 O3 as the support, Fe2 O3 is the most active species of those transition-metal oxides impregnated for the reduction of SO2 . With Fe2 O3 as the active species, CeO2 is the best support of those tested. Thus, screening of catalysts revealed that Fe2 O3 /CeO2 is the optimal catalyst for the reduction of SO2 . In addition, the most suitable Fe content is 10 wt.%. (2) The most suitable feed ratio of C2 H4 /SO2 is 1:1 and the presulfidization of catalyst is necessary. For catalytic reduction of SO2 , catalyst pretreated with H2 +H2 S exhibits higher stability and activity than that pretreated with H2 or He. (3) The results of C2 H4 -TPD and SO2 -TPD demonstrate that Fe2 O3 /CeO2 can adsorb and desorb C2 H4 and SO2 more effectively than Fe2 O3 /␥Al2 O3 does. SO2 -TPD further reveals that adsorption of SO2 contributes to the inhibition of the Fe2 O3 /␥-Al2 O3 catalyst. These two facts can be used to explain why Fe2 O3 /CeO2 has a higher stability and activity than the Fe2 O3 /␥-Al2 O3 catalyst. (4) As to the reducing agents, although C2 H4 performs far better than H2 does, it is not as good as CO. Further improvement in the performance of catalyst for the reduction of SO2 using C2 H4 as a reducing gas is needed. Acknowledgements The authors are grateful to National Science Council of ROC for financial supports (NSC 89-2214E006-032).
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Catalytic reduction of SO2 over supported transition-metal oxide catalysts with C2 H4 as a reducing agent Ching-Huei Wang a , Shiow-Shyung Lin b , Pei-Chang Sung c , Hung-Shan Weng c,∗ b
a Department of Chemical Engineering, Kao Yuan Institute of Technology, Kaohsyung 821, Taiwan Department of Environmental Engineering and Health, Chia-Nan University of Pharmacy and Science, Tainan 717, Taiwan c Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan
Received 11 April 2002; received in revised form 1 July 2002; accepted 2 July 2002
Abstract This work investigates performances of supported transition-metal oxide catalysts for the catalytic reduction of SO2 with C2 H4 as a reducing agent. Experimental results indicate that the active species, the support, the feed ratio of C2 H4 /SO2 , and pretreatment are all important factors affecting catalyst activity. Fe2 O3 /␥-Al2 O3 was found to be the most active catalyst among six ␥-Al2 O3 -supported metal oxide catalysts tested. With Fe2 O3 as the active species, of the supports tested, CeO2 is the most suitable one. Using this Fe2 O3 /CeO2 catalyst, we found that the optimal Fe content is 10 wt.%, the optimal feed ratio of C2 H4 /SO2 is 1:1, and the catalyst presulfidized by H2 + H2 S exhibits a higher performance than those pretreated with H2 or He. Although the feed concentrations of C2 H4 :SO2 being 3000:3000 ppm provide a higher conversion of SO2 , the sulfur yield decreases drastically at temperatures above 300 ◦ C. With higher feed concentrations, maximum yield appears at higher temperatures. The C2 H4 temperature-programmed desorption (C2 H4 -TPD) and SO2 -TPD desorption patterns illustrate that Fe2 O3 /CeO2 can adsorb and desorb C2 H4 and SO2 more easily than can Fe2 O3 /␥-Al2 O3 . Moreover, the SO2 -TPD patterns further show that Fe2 O3 /␥-Al2 O3 is more seriously inhibited by SO2 . These findings may properly explain why Fe2 O3 /CeO2 has a higher activity for the reduction of SO2 . © 2002 Elsevier Science B.V. All rights reserved. Keywords: Reduction of SO2 ; Conversion of SO2 ; Yield of elemental sulfur; Fe2 O3 /CeO2 ; TPD
1. Introduction Since the 1930s, various commercial flue gas desulfurization (FGD) processes, including throwaway, adsorption, absorption, and catalytic reduction, have been developed. These technologies have recently attracted increasing attention as the regulations for the emission of SOx have become more stringent, ∗ Corresponding author. Tel.: +886-6-2757575x62637; fax: +886-6-2344496. E-mail address: [email protected] (H.-S. Weng).
and can now deliver desulfurization rates above 90% [1–3]. Among the FGD processes, the throwaway method has the drawback of requiring subsequent waste disposal [4–7]. Meanwhile, with both adsorption and absorption it is necessary to devise a process to treat SO2 released from the sorption process. This leaves catalytic reduction, in which SO2 is reduced to elemental sulfur, as the method of choice. This method has the advantages of feasible recovery of sulfur and no waste disposal problem. Reducing agents for the catalytic reduction of SO2 include carbonaceous material, CH4 ,
0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 1 5 9 - 5
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CO, and H2 . By using coke as a reducing agent, Lepsoe [8] obtained 80% conversion of SO2 at 850 ◦ C. Salis and Berk [9] and Mulligan and Berk [10] reported only slightly higher conversion rates, using CH4 as a reducing agent, over activated alumina or transition-metal sulfide catalysts, whilst Zhu et al. [3] obtained complete conversion of SO2 over ceria-based catalysts at 750 ◦ C, also using CH4 . Other reports [11–21] indicate that when using CO as a reducing agent, lower reaction temperatures (320–800 ◦ C) can be used. Another intensively investigated reducing agent, H2 , has also been shown to effectively reduce SO2 to elemental sulfur [2,22–27]. While CH4 is widely known as a reducing agent, the use of other gaseous hydrocarbons has rarely been reported. The use of ethylene, the most important raw material in the chemical industry, has been reported once to be used to reduce NO to N2 over platinum ion-exchanged MFI zeolites [28]. Though it is somewhat costlier to use than other reducing agents, it is more easily available, and therefore has potential as a reducing agent for SO2 . Specifically, for those plants that produce sulfur emissions and already have easy access to ethylene, the combination of efficiency and safety considerations may in fact make C2 H4 a better choice of reducing agent. Within this category are electric utility plants, natural gas producing plants and petroleum refining plants [29]. Our previous study [20] demonstrated the superior performance of supported transition-metal oxide catalysts in the catalytic reduction of SO2 with CO as a reducing agent. The results showed that SO2 was completely converted using an Fe2 O3 /CeO2 catalyst at 320 ◦ C, producing an elemental sulfur yield above 90%. In the current work, by employing C2 H4 as a reducing agent, the SO2 reduction activity of several transition-metal oxides supported on ␥-Al2 O3 , and subsequently the activity of the most active species (in this study, Fe2 O3 ) on various supports, were tested to find an optimal one. Superior activity of the optimal catalyst was further verified by C2 H4 temperature-programmed desorption (C2 H4 -TPD) and SO2 -TPD. Moreover, the effect of C2 H4 /SO2 ratio, feed concentrations of C2 H4 and SO2 , content of Fe species on the CeO2 , and pretreatment conditions were also investigated. Finally, the results of using C2 H4 , CO and H2 as the reducing gases for the reduction of SO2 are compared.
2. Experimental 2.1. Catalyst preparation The incipient impregnation method was employed for preparing all catalysts tested in this study. In the preparation, the support (␥-Al2 O3 , SiO2 , TiO2 , CeO2 , La2 O3 , V2 O5 or Zeolite-Y, except TiO2 was supplied by Nihon Shiyaku Co., others were purchased from Merck Co.) was firstly dried in a vacuum oven at 120 ◦ C for at least 12 h. It was then cooled to room temperature, after which a known amount of metal (Fe, Ni, Mn, Mo, Cr or Co) salt in deionized water (7 ml) was added into it. The sources of metals were nitrates except that of Mo was (NH4 )6 Mo7 O24 ·4H2 O (all these salts were the products of Merck Co.). The supports and salts used were of reagent grade and were not purified further. Nevertheless, considering the supported Fe catalyst was the best catalyst in this study, which will be shown later, herein the composition of ferric nitrate (Fe(NO3 )3 ·9H2 O) is shown: assay > 98%; Cl < 0.01%; SO4 < 0.01%; Cu < 0.005%; Mn < 0.02%; Pb < 0.005%; Zn < 0.002%. Subsequently, well-impregnated catalyst precursors were put aside for 0.5 h, dried in air at 120 ◦ C for 24 h and, after being ground to particle sizes between 140 and 220 mesh, calcined in an oven with the air supply at 500 ◦ C for 8 h. Except the tests for the influence of Fe content, where the loading of Fe species was varied between 4 and 16 wt.%, metal loading was fixed at 10 wt.%. 2.2. Measurement of activity A packed-bed reactor (8 mm i.d.; Pyrex) was used to measure catalyst activity. An amount of 0.5 g of catalyst powder (140–220 mesh) was placed in the reactor and was pretreated by being purged in a He flow (50 ml/min) at 500 ◦ C for 80 min, then presulfied at the same temperature for 2 h in a flow of He gas containing H2 (50,000 ppm) and H2 S (5000 ppm). In order to compare the effect of presulfidization with H2 + H2 S, H2 and He were also used for the pretreatment of the catalyst. After pretreatment, the catalyst was cooled to 260 ◦ C and the gas mixture (5000 ppm SO2 and 5000 ppm C2 H4 in He balance) was fed into the reactor as reactants. When a steady state was attained (about 2 h), the inlet and outlet gases were analyzed. Then the temperature was raised stepwise, 20 ◦ C each time,
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until 360 ◦ C (or 400 ◦ C). In each step, the gases were analyzed after a steady state was attained. Concentrations of SO2 in the inlet and outlet gases and those of H2 S, COS and CS2 (reaction by-products) in the effluent gas were determined using a gas chromatograph with a flame photometric detector (FPD). Additionally, concentrations of C2 H4 in the inlet and outlet gases were also determined using a gas chromatograph with a flame ionization detector (FID). Moreover, the concentration of COx (CO + CO2 ) in the effluent gas was measured with a nondispersive IR spectrometer (ADC D/7V/76/S for CO; Telaire/USA 2001 V for CO2 ). The percent conversion (X) of SO2 , the selectivity (YS ) and percent yield (Y) of elemental sulfur are defined as follows: X=
[SO2 ]in − [SO2 ]out × 100% [SO2 ]in
[SO2 ]in −[SO2 ]out −[COS]out −[H2 S]out − 2[CS2 ]out YS = , [SO2 ]in − [SO2 ]out
Y = X × YS
where [SO2 ]in is the inlet concentration of SO2 (ppm) determined at a stable bypass concentration, while [SO2 ]out , [COS]out , [H2 S]out and [CS2 ]out are the effluent concentrations of SO2 , COS, H2 S and CS2 , all in ppm. 2.3. Measurement of temperature-programmed desorption (TPD) The temperature-programmed desorptions of SO2 and C2 H4 were conducted using a Micromeritics 2900 TPD/TPR analyzing instrument. The catalyst sample (0.2 g) was placed in the instrument in a U-shaped quartz tube, which was plugged with quartz wool to prevent the sample from being carried away by the gas flowing through it. After being presulfidized, the sample was cooled to 30 ◦ C, and then either C2 H4 or SO2 (both are 40,000 ppm in the flow of He) adsorption was carried out for 30 min. Thereafter, the temperature was raised to and maintained at 100 ◦ C for 30 min to eliminate the physically adsorbed C2 H4 or SO2 . The sample was then heated to 1000 ◦ C at a constant heating rate of 15 ◦ C/min. Increase in the sample temperature caused desorption of chemically adsorbed C2 H4 or SO2 , which was detected by a thermal conductivity detector (TCD).
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2.4. Characterization of catalyst structure The structure of the catalyst was characterized by an XRD analyzer (Brookhaven Instrumental Corporation Model: APX 63/Powder). Conditions of analysis were as follows: Target: Cu (λ = 1.5405 Å); scanning speed: 3◦ /min; scanning range (2θ ): 20–60◦ . 3. Results and discussion 3.1. Screening for the optimal catalyst In screening for an optimal catalyst, first, catalysts with different metal species (Fe, Ni, Mn, Mo, Cr and Co) supported on ␥-Al2 O3 were assessed. Each metal oxide/␥-Al2 O3 catalyst was presulfied, and then a gas mixture with a constant molar ratio of C2 H4 :SO2 = 1:1 (C2 H4 , 5000 ppm; SO2 , 5000 ppm) was fed into the reactor. Weight hourly space velocity (WHSV) was fixed at 8000 ml/(h g). Percentage SO2 conversion versus temperature, plotted in Fig. 1A, shows that Fe2 O3 /␥-Al2 O3 was the most active catalyst, and NiO/␥-Al2 O3 the second most active. Our previous study [20], in which the reducing agent was CO, yielded the same result. Conversely, using H2 as a reducing agent, Paik and Chung [2] found that the activity of ␥-Al2 O3 -supported Co and Ni was greater than that of Fe2 O3 /␥-Al2 O3 . This may be due to the difference in adsorption capability of H2 , CO and C2 H4 onto these metal species. In our results, there was a maximum elemental sulfur yield with Fe2 O3 /␥-Al2 O3 at 360 ◦ C (Fig. 1B), whereas according to the definition of percent yield of elemental sulfur (Y = XYS ), higher sulfur yield is obtained at a higher SO2 conversion. Existence of a fall in sulfur yield with Fe2 O3 /␥-Al2 O3 despite continued rising SO2 conversion is due to the higher production of by-products (CS2 , COS and H2 S) at higher reaction temperatures. Based on the above observation that Fe2 O3 was the most active species, catalysts of Fe2 O3 impregnated on various supports were further tested to find the optimal one. On measures of both SO2 conversion (Fig. 2A) and elemental sulfur yield (Fig. 2B), we found that Fe2 O3 /CeO2 performed best. This outcome is largely
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Fig. 1. Effect of different supported metals on the reduction of SO2 . Support: ␥-Al2 O3 ; weight of catalyst: 0.50 g; feed: 5000 ppm SO2 , 5000 ppm C2 H4 ; WHSV: 8000 ml/(h g).
due to better adsorption of C2 H4 by Fe2 O3 /CeO2 , as will be verified later in this work. Furthermore, Fig. 2B shows that there is maximum sulfur yield (about 72%) at 320 ◦ C with Fe2 O3 /CeO2 . This is again attributed to a higher conversion of SO2 leading to a higher production of by-products as the reaction temperature is increased. Variations in the yield of elemental sulfur
and major by-products with temperature are shown in Fig. 3. Noticeable yield of by-products was observed at higher reaction temperatures. Via a durability test, the results of which are shown in Fig. 4, we were able to further demonstrate the superior performance of Fe2 O3 /CeO2 to that of Fe2 O3 /␥-Al2 O3 . The catalysts were initially pretreated
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Fig. 2. Effect of different supports on the reduction of SO2 . Active species: Fe2 O3 ; weight of catalyst: 0.50 g; feed: 5000 ppm SO2 , 5000 ppm C2 H4 ; WHSV: 8000 ml/(h g); content of Fe: 10 wt.%.
with H2 S + H2 at 500 ◦ C for 2 h; then cooled to 320 ◦ C to conduct the isothermal reduction of SO2 for 24 h. Fig. 4 shows that the Fe2 O3 /CeO2 catalyst produced a SO2 conversion rate of 100% throughout the entire 24 h reaction time, while Fe2 O3 /␥-Al2 O3 decayed seriously (from about 90% to about 32% conversion) as the reaction proceeded. As for the ele-
mental sulfur yield, we found that with Fe2 O3 /CeO2 it increased from 62 to 78%, before leveling off at about 12 h. With Fe2 O3 /␥-Al2 O3 , although the sulfur yield at the onset was as high as 72%, it decreased significantly during the reaction. Note that when the same catalyst was employed the highest conversions of SO2 in Fig. 4 are higher than the conversions in
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Fig. 3. Variation in yields of elemental sulfur and by-products with temperature. Catalyst: Fe2 O3 /CeO2 ; weight of catalyst: 0.50 g; feed: 5000 ppm SO2 , 5000 ppm C2 H4 ; WHSV: 8000 ml/(h g); content of Fe: 10 wt.%.
Figs. 1A and 2A at 320 ◦ C. This is due to poisoning of the catalyst at lower reaction temperatures as the programmed temperature reaction proceeded. This poisoning effect is also demonstrated by conducting durability tests at different temperatures. 3.2. Effect of reaction temperature on the durability of the Fe2 O3 /CeO2 catalyst Fig. 5 shows the results of isothermal reduction of SO2 at 300 and 320 ◦ C over 24 h. The conversion of SO2 decreased noticeably when the reaction was carried out at 300 ◦ C, but maintained at 100% at 320 ◦ C. As to the sulfur yield, it increased sharply at first 5 h, but decreased rapidly with time thereafter at 300 ◦ C, while it increased gradually and then leveled off at 320 ◦ C. Inhibition of catalyst activity at 300 ◦ C is thus demonstrated by the inferior SO2 reduction performance. The fact that an increase in sulfur yield up to 5 h at 300 ◦ C may be due to catalyst inhibition at this stage having a bigger influence on the rate of
by-product formation than on the rate of elemental sulfur formation, as the formation of by-products is more difficult than the reduction of SO2 . 3.3. Effect of Fe content in Fe2 O3 /CeO2 on SO2 reduction Having concluded that Fe2 O3 /CeO2 is the best choice for catalytic reduction of SO2 , the effect of Fe content in Fe2 O3 /CeO2 on catalytic performance was investigated. Fe2 O3 is the active species of the Fe2 O3 /CeO2 catalyst, and therefore increasing amounts of the supported Fe2 O3 species increase activity of the catalyst, and so the rate of SO2 conversion. However, Fig. 6 shows that the optimal Fe content is 10 wt.%. This is because of a higher level of Fe2 O3 on the support leading to the formation of large Fe2 O3 crystals, which results in a decrease in the amount of active sites, and therefore a decline in SO2 conversion. The aforementioned formation of large Fe2 O3 crystals at higher Fe content was further verified by XRD
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Fig. 4. Durability of Fe2 O3 /CeO2 and Fe2 O3 /␥-Al2 O3 for the reduction of SO2 . Reaction temperature: 320 ◦ C; weight of catalyst: 0.50 g; feed: 5000 ppm SO2 , 5000 ppm C2 H4 ; WHSV: 8000 ml/(h g); content of Fe: 10 wt.%.
Fig. 5. Effect of reaction temperature on the durability of Fe2 O3 /CeO2 for the reduction of SO2 . Weight of catalyst: 0.50 g; feed: 5000 ppm SO2 , 5000 ppm C2 H4 ; WHSV: 8000 ml/(h g); content of Fe: 10 wt.%.
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Fig. 6. Effect of Fe content on the activity of Fe2 O3 /CeO2 for the reduction of SO2 . Weight of catalyst: 0.50 g; feed: 5000 ppm SO2 , 5000 ppm C2 H4 ; WHSV: 8000 ml/(h g).
analysis of Fe2 O3 /CeO2 , results of which are shown in Fig. 7. This figure shows how the peaks characterizing the Fe2 O3 crystal, especially that at 35◦ (2θ ), become more and more intense as the content increases above 10 wt.%. This indicates that large size crystals of Fe2 O3 are formed and the dispersion of Fe2 O3 species is worse at Fe content exceeding 10 wt.%. Based on the results of Figs. 6 and 7, the Fe content was fixed
at 10 wt.% in the preparation of the Fe2 O3 /CeO2 catalyst for further investigation. 3.4. Effect of C2 H4 /SO2 ratio on SO2 reduction Although the stoichiometric ratio of C2 H4 /SO2 is 1/3 for reduction of SO2 by C2 H4 (C2 H4 + 3SO2 → 3S+2CO2 +2H2 O), to get complete reduction of SO2 ,
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Fig. 7. Effect of Fe content on the XRD pattern of Fe2 O3 /CeO2 .
higher ratios were investigated in the search for an optimal one. Reaction conditions were the same as those for screening of catalyst except that the feed ratio of C2 H4 /SO2 was varied from 1:2 to 3:1. Results in Fig. 8 show that both SO2 conversion and sulfur yield were the highest with the feed ratio at 1:1. Existence of an optimal feed ratio for the conversion of SO2 may be due to the competitive adsorption of C2 H4 and SO2 on the active sites of the catalyst. Since catalyst affinity for SO2 is higher than for C2 H4 , overwhelming concentrations of the latter in the gas phase is necessary for a higher rate of C2 H4 adsorption. However, if the ratio is too high, the rate of SO2 adsorption may be too low to proceed the reduction reaction effectively. As for the sulfur yield, excess C2 H4 resulted in a further deep reduction of SO2 . At a higher feed ratio, this also caused the lower sulfur yield in addition to the lower SO2 conversion. Similar results, showing that a higher feed ratio of H2 /SO2 resulted in deep reduction of SO2 were obtained in the studies of Paik and
Chung [27]. Therefore, based on the results shown in Fig. 8, we conclude that the optimal ratio of C2 H4 /SO2 is 1:1. 3.5. Effect of inlet concentration on SO2 reduction Having established that the optimal ratio of C2 H4 /SO2 is 1:1 and that the feed concentrations of C2 H4 and SO2 may affect the efficacy of SO2 reduction, three different feed concentrations, all at the ratio 1:1 (C2 H4 :SO2 = 3000:3000, 5000:5000 and 7000:7000, all in ppm) were tested to assess their influence. At lower reaction temperatures, a feed concentration of 3000:3000 ppm provided the highest rate of SO2 reduction as shown in Fig. 9A. This feed concentration also gave the highest sulfur yield at 300 ◦ C (Fig. 9B). The maximum yield at a feed concentration of 5000:5000 ppm occurred at 320 ◦ C, whilst that at 7000:7000 ppm took place at 340 ◦ C. In general, a higher conversion leads to a higher yield of main
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Fig. 8. Effect of feed ratio of C2 H4 /SO2 on the reduction of SO2 . Weight of catalyst: 0.50 g; WHSV: 8000 ml/(h g); content of Fe: 10 wt.%.
product unless the formation of by-products is noticeable. In this study, the by-product formation rose at higher reaction temperatures, thereby a maximum sulfur yield occurred at an intermediate temperature. Moreover, a higher feed concentration of SO2 and C2 H4 would attain the complete conversion of SO2 at a higher temperature, hence the maximum sulfur yield would occur at a higher temperature.
3.6. Effect of pretreatment on the activity of the catalyst To further identify the efficacy of presulfidization, we also compared the effect of pretreating Fe2 O3 /CeO2 with different gases (H2 + H2 S, H2 , and He). The catalyst was pretreated with the different gases at 500 ◦ C for 2 h, and then cooled to 320 ◦ C
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Fig. 9. Effect of feed concentration on the reduction of SO2 . Catalyst: Fe2 O3 /CeO2 ; weight of catalyst: 0.50 g; WHSV: 8000 ml/(h g).
to conduct the isothermal reduction of SO2 . Results shown in Fig. 10A reveal that when the pretreatment gas is H2 + H2 S, SO2 conversion was 100% throughout the reaction. With H2 or He, SO2 conversion attained 100% only after the initial stage of the reaction (after about 3 h for H2 and 5 h for He). Further inspection on the effect of the different pretreatment gases on the yield of elemental sulfur (Fig. 10B) confirmed the gas mixture of H2 and H2 S to be the most
productive pretreatment. Therefore, we conclude that by using H2 + H2 S as the pretreatment gas the catalyst would exhibit the most stable and active performance. 3.7. Comparison of different reducing agents The reducing agents commonly used for the reduction of SO2 are CO and H2 . Thus, the reduction of SO2 with CO, H2 and C2 H4 as reducing gases over
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Fig. 10. Effect of pretreatment on the reduction of SO2 . Reaction temperature: 310 ◦ C; catalyst: Fe2 O3 /CeO2 ; weight of catalyst: 0.50 g; feed: 5000 ppm SO2 , 5000 ppm C2 H4 ; WHSV: 8000 ml/(h g).
Fe2 O3 /CeO2 were compared. As shown in Fig. 11, the order of efficacy was found to be CO > C2 H4 > H2 . Differences in reducing ability is mainly caused by the difference of adsorbing ability of various agents onto the Fe2 O3 /CeO2 catalyst. Although the use of C2 H4 does not exhibit a better performance as CO does, it can be still considered as a candidate because of its safety and availability.
3.8. Characterization of catalyst by TPD Following the conclusion that Fe2 O3 /CeO2 is the most active catalyst and Fe2 O3 /␥-Al2 O3 is the second most active catalyst for the reduction of SO2 , we further characterized the catalysts by performing C2 H4 -TPD and SO2 -TPD to find the relationship between the surface properties and catalyst activity.
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Fig. 11. Effect of reducing agent on the reduction of SO2 . Catalyst: Fe2 O3 /CeO2 ; weight of catalyst: 0.50 g; feed: 5000 ppm SO2 , 5000 ppm reducing agent; WHSV: 8000 ml/(h g).
3.8.1. C2 H4 -TPD The C2 H4 -TPD patterns of Fe2 O3 /CeO2 and Fe2 O3 /␥-Al2 O3 are shown in Fig. 12A. Both patterns of Fe2 O3 /CeO2 and Fe2 O3 /␥-Al2 O3 exhibit two peaks. In comparison, the first peak of Fe2 O3 /CeO2 is at a far lower temperature (220 ◦ C) than that of Fe2 O3 /␥-Al2 O3 (420 ◦ C), and the amount of C2 H4 desorbed from Fe2 O3 /CeO2 is larger than that from
Fe2 O3 /␥-Al2 O3 . The higher amounts of adsorption and desorption of C2 H4 by the Fe2 O3 /CeO2 catalyst may render this catalyst with higher activity for SO2 reduction. The results shown in Fig. 2 verify this point. 3.8.2. SO2 -TPD The SO2 -TPD experiments were also conducted to elucidate the higher activity of Fe2 O3 /CeO2 for SO2
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Fig. 12. Comparison of C2 H4 -TPD and SO2 -TPD patterns of Fe2 O3 /CeO2 and Fe2 O3 /␥-Al2 O3 .
reduction. The desorption patterns of Fe2 O3 /CeO2 and Fe2 O3 /␥-Al2 O3 are shown in Fig. 12B. There are two peaks in the pattern of Fe2 O3 /CeO2 (225 and 595 ◦ C), while three peaks are in the pattern of Fe2 O3 /␥-Al2 O3 (352, 558 and 616 ◦ C). As was the case in C2 H4 -TPD, the first peak of the SO2 -TPD pattern of Fe2 O3 /CeO2 represents weaker adsorption of SO2 on active sites. Comparing with the SO2 conversion–temperature curves shown in Fig. 3, the adsorption of SO2 on these sites is the key step for
the reduction reaction of SO2 . Again, the peaks at higher temperatures indicate stronger adsorption on active sites. According to our previous work [20], the strong adsorption of SO2 on active sites may cause inhibition of catalyst activity. We found that the area of the first peak in the pattern of Fe2 O3 /CeO2 is much larger than that of Fe2 O3 /␥-Al2 O3 , while the area of peaks at higher temperatures in the pattern of Fe2 O3 /CeO2 is approximately the same as that of Fe2 O3 /␥-Al2 O3 . These two facts can be applied to
C.-H. Wang et al. / Applied Catalysis B: Environmental 40 (2003) 331–345
explain that Fe2 O3 /CeO2 has a higher activity for the reduction reaction of SO2 , while Fe2 O3 /␥-Al2 O3 is more easily to be deactivated. 4. Conclusions (1) With ␥-Al2 O3 as the support, Fe2 O3 is the most active species of those transition-metal oxides impregnated for the reduction of SO2 . With Fe2 O3 as the active species, CeO2 is the best support of those tested. Thus, screening of catalysts revealed that Fe2 O3 /CeO2 is the optimal catalyst for the reduction of SO2 . In addition, the most suitable Fe content is 10 wt.%. (2) The most suitable feed ratio of C2 H4 /SO2 is 1:1 and the presulfidization of catalyst is necessary. For catalytic reduction of SO2 , catalyst pretreated with H2 +H2 S exhibits higher stability and activity than that pretreated with H2 or He. (3) The results of C2 H4 -TPD and SO2 -TPD demonstrate that Fe2 O3 /CeO2 can adsorb and desorb C2 H4 and SO2 more effectively than Fe2 O3 /␥Al2 O3 does. SO2 -TPD further reveals that adsorption of SO2 contributes to the inhibition of the Fe2 O3 /␥-Al2 O3 catalyst. These two facts can be used to explain why Fe2 O3 /CeO2 has a higher stability and activity than the Fe2 O3 /␥-Al2 O3 catalyst. (4) As to the reducing agents, although C2 H4 performs far better than H2 does, it is not as good as CO. Further improvement in the performance of catalyst for the reduction of SO2 using C2 H4 as a reducing gas is needed. Acknowledgements The authors are grateful to National Science Council of ROC for financial supports (NSC 89-2214E006-032).
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