Selective Reduction of SO2 in Smelter Off-Gas with Coal Gas to Sulfur

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Selective Reduction of SO2 in Smelter Off-Gas with Coal Gas to Sulfur over Metal Sulfide Supported Catalysts Tingting Ge,† Cuncun Zuo,‡,§ Junping Zhang,‡,§ Lubin Wei,*,† and Chunshan Li*,‡,§ †

School of Chemical and Environmental Engineering, China University of Mining & Technology-Beijing, Beijing 100083, P. R. China Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, The National Key Laboratory of Clean and Efficient Coking Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China § College of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100190, P. R. China ‡

S Supporting Information *

ABSTRACT: A series of metal sulfide supported catalysts were first developed for the SO2 reduction to sulfur by coal gas, and different supports and active components were selected to prepare catalysts through the wet impregnation method. This reduction technology is also a promising desulfurization process of smelter off-gas. Before the calcined supported catalyst was evaluated in a fixed-bed reactor, the presulfidation was performed to get the stronger stability and catalyst ability. The catalysts were characterized by X-ray diffraction, XPS, BET, and SEM analyses. Changes in the catalyst structure and active phase were explored. The acid−base properties of the supports were detected by CO2-TPD, and the reduction ability of the catalyst was evaluated via CO-TPR. The long life of catalyst was evaluated, and the probable catalytic mechanism was proposed.

1. INTRODUCTION Sulfur dioxide (SO2) emissions from smelters are the culprit of serious air pollution problems. The resulting acid rain can cause serious damage to people’s health, crops, and forests.1−3 The content of SO2 in the smelter off-gas is very high, ranging from 10 to 20 wt %. Scholars have focused on developing a flue gas desulfurization technology.4−8 Most SO2 exhaust gases are used to produce sulfuric acid. On the one hand, sulfuric acid is inexpensive and has low added value, but the cost of sulfuric acid transportation is very high. On the other hand, sulfuric acid is produced with surplus, and the market has been saturated. Excessive production of sulfuric acid seriously affects the planning and development in the smelter industry. Sulfur has occupied a pivotal position in the national economic production, such as in the dye, rubber, paper-making, military, and other industries.9−15 In response to the lack of sulfur deposits, China has annually spent huge amounts of money to import of sulfur. In this case, reducing SO2 for sulfur production is of practical significance.16−21 SO2 can be selectively reduced to S by different reducing agents. Accordingly, the methods can be divided into the H2 reduction method, carbon reduction method, hydrocarbon (mainly CH4) reduction method, CO reduction method, and NH3 reduction method. At present, the high temperature methane reduction © XXXX American Chemical Society

method has achieved industrialization. However, the consumption of methane and oxygen is huge, and the reaction temperature is too high. The CO reduction method has gained increasing attention because of its convenience, ease of operation, and high sulfur purity. The reduction of sulfur oxide to sulfur has been studied in the early 1930s by using reducing agents such as carbonaceous material,22,23 carbon monoxide,24−30 methane,31 and hydrogen.32−34 Considerable effort has been devoted to developing efficient catalysts and reductants. Paik et al. have reported the selective reduction of SO2 by H2 and obtained good results,15,32 but the high reaction temperature and the problems of H2 prices, sources, and transportation are still limiting factors. Moody et al.35 successfully studied Ru-supported catalysts for the reduction of SO2, but the cost of catalyst was high. Kim et al.36 successfully developed Co3O4−TiO2 catalysts for the reduction of SO2 by CO to elemental sulfur. Using coal gas as reductant, the SnO2−ZrO2 catalysts were used for the reduction of SO2 to sulfur,37 but the Received: January 17, 2018 Revised: March 7, 2018 Accepted: March 9, 2018

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DOI: 10.1021/acs.iecr.8b00213 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Fixed-bed reactor.

catalytic activity was relatively low. Mulligan et al.38 studied some sulfide crystals and their supported catalysts for the SO2 reduction to sulfur by the CH4 reduction method. Presently, the high-content SO2 reduction by coal gas has not achieved satisfactory results. Ordinary coal gas is used as reductant for the SO2 reduction, which is provided with significant economic attractiveness, wide application range, and environmental acceptability. In the present study, a new catalytic system was reported for the SO2 reduction by coal gas. Four supports and five active components were selected to prepare the supported catalysts by impregnation. SO2 conversion and sulfur selectivity were used as the standards for catalyst evaluation. The catalyst preparation conditions and reaction conditions were optimized. Some characterizations were performed to investigate the changes of catalyst structure and active phase. Two-component supported catalysts were developed, and the catalyst life was investigated.

above solution to begin to impregnate. The wet-impregnated catalyst precursor was then put aside for 0.5 h. After being dried at 120 °C for 24 h, the precursor was calcined in a muffle furnace at 500 °C for 10 h. Then the catalyst was presulfided at 420 °C for 2 h in a flow of 10% v/v H2S/H2. 2.2. Catalyst Characterization. The X-ray diffraction (XRD) patterns of the supported catalysts were recorded with a Shimadzu diffractometer model XRD 6000 operated at an accelerating voltage of 40 kV using Cu Kα radiation, 4/min, and the scanned angle (2θ) ranged from 5° to 90°. The deconvolution analysis of a specific XPS peak can be used to estimate the metal element content in relative amount on the surface of the supported catalysts used for SO2 reduction to sulfur. The BET specific surface area and the pore size of the supported catalysts synsthesized were obtained from a Quanta Chrome Instrument NOVA 2000. Total acidity and basicity of different supports were determined by temperature-programmed desorption of CO2 using a Autochem II 2920 apparatus from Micromeritics. COTPR experiments of supported catalysts were also carried out on the apparatus using 5 wt % mixture of CO in Ar for the CO pretreatment. The morphology of active components supported on the catalyst and the sulfur products obtained were observed in a SU8020 SEM unit manufactured by Hitachi, Ltd., headquartered in Tokyo, Japan. 2.3. Catalyst Evaluation. A fixed-bed reactor was used to determine the catalyst activity for SO2 reduction (Figure 1).

2. EXPERIMENTAL SECTION 2.1. Catalysts Preparation. The wet impregnation method was employed for preparing all catalysts involved in the present study. First, four supports (γ-Al2O3, ZSM-5, HZSM-5, and SBA-15) dried in a vacuum oven at 120 °C for at least 12 h then were cooled down to room temperature. Before the catalyst was prepared, the support was tested for water absorption. Then Fe(NO3)3·9H2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O, (NH4)6Mo7O24·4H2O, and Cu(NO3)2·3H2O were dissolved in the corresponding amount of distilled water. After dissolving completely, the support was poured into the B


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Industrial & Engineering Chemistry Research About 1.0 g of catalyst sample (30−50 mesh) was placed in the middle of the reactor. Before the temperature rose, the N2 gas was fed to the reactor for protection. When the expected reaction temperature was obtained, the homemade coal gas and smelter off-gas containing SO2 with a constant molar ratio of (CO + H2):SO2 as 2:1 were simultaneously fed to the reactor. For the measurement of sulfur obtained, a sulfur collection tube was installed. The gaseous products were analyzed by a gas chromatograph equipped with a thermal conductivity detector. An external standard method was developed to quantify accurately CO, CO2, COS, H2S, and SO2. The conversion of SO2 (X) and the sulfur selectivity (S) are defined as follows: X=

S=

[SO2 ]in − [SO2 ]out × 100% v/v [SO2 ]in

(1)

[SO2 ]in − [SO2 ]out − [H 2S]out − [COS]out × 100% v/v [SO2 ]in − [SO2 ]out (2) Figure 3. XPS curves of 14 wt % Co−4 wt % Cu/γ-Al2O3 catalyst with different presulfidation time.

where [SO2]in was the inlet concentration of SO2; [SO2]out, [COS]out, and [H2S]out were the effluent concentrations.

3. RESULTS AND DISCUSSION 3.1. Characterization of Catalysts. 3.1.1. XRD. Figure 2 shows the XRD patterns of γ-Al2O3 support and 14 wt % Co−4

Figure 4. CO-TPR profiles of Co−Cu supported γ-Al2O3 catalyst with different Cu loadings. Figure 2. XRD patterns of 14 wt % Co−4 wt % Cu/γ-Al2O3 supported catalyst with different presulfidation time.

wt % Cu/γ-Al2O3 catalyst with different presulfidation time. Compared to the XRD pattern of γ-Al2O3 support, the XRD spectra of 14 wt % Co−4 wt % Cu/γ-Al2O3 without presulfidation treatment show distinct differences. After calcination, the Co and Cu active component was converted to the cobalt and copper composite oxide crystal phase ((Cu0.3Co0.7)Co2O4). However, there were no obvious differences between the XRD patterns of the presulfided 14 wt % Co−4 wt % Cu/γ-Al2O3 catalyst and that of the single γ-Al2O3 support, proving that the active component crystalline form may be in the form of microcrystalline on the support surface. With the further increase in the presulfidation time, the XRD pattern of the presulfided 14 wt % Co−4 wt % Cu/γ-Al2O3 catalyst did not change. 3.1.2. XPS. XPS characterization showed that the presulfidation treatment had a significant influence on the distribution of Co compounds in different valence states. Figure 3 shows XPS

Figure 5. Surface areas for presulfided catalysts with different active components loading.

C


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Figure 6. SEM image of the presulfided 14 wt % Co−4 wt % Cu/γ-Al2O3 catalyst.

Table 1. Optimization of the Supports and Active Components support γ-Al2O3

ZSM-5

HZSM-5

SBA-15

loading based on support (wt %)

conversion of SO2 (%)

selectivity of sulfur (%)

0 14Fe 14Cu 14Ni 14Mo 14Co 0 14Fe 14Cu 14Ni 14Mo 14Co 0 14Fe 14Cu 14Ni 14Mo 14Co 0 14Fe 14Cu 14Ni 14Mo 14Co

30.4 92.3 91.1 87.7 88.3 92.5 8.6 30.1 32.4 25.9 23.8 32.0 6.6 37.3 38.4 39.6 33.8 37.5 7.9 20.3 22.4 23.7 24.2 25.0

60.2 92.5 92.2 89.5 89.7 93.8 60.8 71.3 75.2 63.1 60.4 76.6 77.3 82.1 84.9 83.4 80.0 86.5 58.0 63.5 65.1 65.6 61.9 70.5

Figure 7. CO2-TPD profiles of different supports.

observed in the XPS curve of the presulfided 14 wt % Co−4 wt % Cu/γ-Al2O3 catalyst. The bonding energy of 777.9 eV confirmed the existence of the elemental cobalt (2p2/3). The divalent cobalt was verified by the bonding energy of 780.0 eV (cobalt sulfide). The bonding energy of 787.3 eV indicated there was trivalent cobalt (cobalt sulfide) in the presulfided 14 wt % Co−4 wt % Cu/γ-Al2O3 catalyst. Therefore, the catalyst after presulfidation is a mixed crystal phase including three valence state cobalt. 3.1.3. CO-TPR. Figure 4 demonstrates CO-TPR profiles of Co−Cu supported γ-Al2O3 catalyst with different Cu loadings. When the second active component Cu was introduced to the Co supported catalyst, the CO-TPR curves of the catalysts changed significantly. The locations of the two reduction peaks

curves of the presulfided 14 wt % Co−4 wt % Cu/γ-Al2O3 catalyst in the presence and absence of the presulfidation treatment. The catalyst without presulfidation presented only divalent and trivalent cobalt peaks (mainly cobalt oxide or composite oxide). However, the peaks of elemental cobalt were D


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Table 2. Effect of Presulfuration Time on the Catalytic Activity of 14 wt % Co−4 wt % Cu/γ-Al2O3 Catalyst presulfidation time (h)

SO2 conversion (% v/v)

selectivity of sulfur (% v/v)

0 1 2 6

62.8 91.5 94.8 95.0

84.5 92.7 96.5 96.9

Figure 8. Effect of Co loading on the SO2 conversion and sulfur selectivity.

Figure 11. Effect of reaction temperature on the catalytic performance of 14 wt % Co−4 wt % Cu/γ-Al2O3 catalyst.

Figure 9. Effect of the addition of second active component on the SO2 conversion and sulfur selectivity.

Figure 12. Effect of GHSV on the catalytic performance of 14 wt % Co−4 wt % Cu/γ-Al2O3 catalysts.

of Co−Cu two-component supported catalysts are the same as those of single-supported catalysts (Supporting Information), but the desorption peaks become more intense. We recorded the CO-TPR curves for the Co−Cu/γ-Al2O3 supported catalyst with the Cu loading of 2, 3, 4, and 5 wt %, as shown in Figure 4. For the CO-TPR curve of the 4 wt % Cu loading, the reduction temperature of the high temperature reduction peak is the lowest and the integration area of the reduction peak is the largest compared to the profiles of the other three loadings catalysts. These results indicated that the 14 wt % Co−4 wt % Cu/γ-Al2O3 supported catalyst has the highest reduction ability for the selective reduction of SO2 to sulfur.

Figure 10. Effect of Cu loading (the second active component) on the SO2 conversion and sulfur selectivity.

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Figure 13. SEM spectra and EDS of the sulfur products.

Figure 15. Modified redox mechanism of the SO2 reduction to sulfur by coal gas over the presulfided 14 wt % Co−4 wt % Cu/γ-Al2O3 catalyst.

Figure 14. SO2 conversion and selectivity of sulfur over the presulfided 14 wt % Co−4 wt % Cu/γ-Al2O3 catalyst.

3.1.4. BET. The BET data clearly reveal changes in specific adsorption isotherm of the γ-Al2O3 supported catalyst with different active components loading (Figure 5). More than a certain amount of cobalt loading (14 wt % Co), the specific surface area decreased with the increase of the cobalt active components loading but remained at approximately 200 m2/g. Although the introduction of copper active material further reduces the specific surface area, the two-component supported catalyst exhibits better catalytic activity, which may be attributed to the interaction of the two active components. These results indicate that the specific surface area is not an important factor affecting the effect of the catalyst. 3.1.5. SEM. SEM characterization was performed to observe the distribution of the active components of the catalyst on the support γ-Al2O3. Figure 6 shows that many microcrystals are distributed on the support body structure. Combining with the EDS spectra, it was concluded that these microcrystals on the

support are the active components Co and Cu, consistent with the previous XRD analysis. Only the diffraction peaks of the γAl2O3 support were present in the XRD spectra of the presulfided catalyst, while the active component did not produce diffraction peaks. 3.2. Selective Reduction of SO2 to Sulfur. 3.2.1. Effect of Supports and Active Components. The data of SO2 conversion and sulfur selectivity were collected as the standards for catalyst evaluation. The effects of supports and active components were investigated with the reaction temperature of 400 °C, [reductant]/[SO2] molar ratio of 2, gas hourly space velocity (GHSV) of 5000 h−1, and a fixed mass fraction of the active component oxide at 14 wt %. As shown in Table 1, the Co/γ-Al2O3 showed the best catalytic effect compared with other active components supported catalysts, and the blank γAl2O3 support presented almost no catalytic activity at the reaction temperature of 400 °C. Meanwhile, the other F


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Figure 16. Action mechanism of the 14 wt % Co−4 wt % Cu/γ-Al2O3 catalyst using H2 as the reducing gas.

Figure 17. Action mechanism of the 14 wt % Co−4 wt % Cu/γ-Al2O3 catalyst using CO as the reducing gas.

supported catalysts prepared form the other three supports did not show catalytic activity for the SO2 reduction. These results indicate the cobalt active component favored the reaction and that a strong mutual synergy between cobalt active component and γ-Al2O3 support was verified, which played an important role in the reaction. Over 14 wt % Co/γ-Al2O3 supported catalyst, the SO2 conversion of more than 92 wt % and the sulfur selectivity of 93 wt % were obtained for the selective reduction of SO2 to sulfur. Therefore, Co/γ-Al2O3 supported catalyst was selected for the further study. The CO2-TPD characterization was performed to investigate the acid−base properties of different supports used to demonstrate the outstanding properties of γ-Al2O3 for the reaction. The CO2TPD profiles of different supports are illustrated in Figure 7. Compared with the other three supported catalysts, the γ-Al2O3 supported catalyst has a strong desorption peak at 250−300 °C. Another strong desorption peak is observed between 400 and 550 °C, indicating that the γ-Al2O3 support has stronger adsorption performance and adsorption capacity. 3.2.2. Effect of Active Component Loading. The reaction was repeated to investigate the influence of different active component loadings including 2, 4, 6, 10, 12, 14, and 16 wt % on the catalytic performance of Co/γ-Al2O3 at 400 °C, with [reductant]/[SO2] molar ratio of 2 and GHSV of 5000 h−1. The results are illustrated in Figure 8. At the beginning, the amount of the active components supported on the support channel and surface gradually increased with the increase of the loading, promoting an improved catalytic effect. When the Co loading was below 14 wt %, the SO2 conversion and the selectivity of sulfur both increased rapidly with the Co loading, and a maximum of both was obtained at 14 wt % loading. However, when a certain load amount was exceeded, the excessive load of active material might plug the catalyst pore, resulting in the decrease in the catalytic effect. The SO2

conversion and the sulfur selectivity showed a downward trend when the Co loading further increased from 14 to 16 wt %. 3.2.3. Study on Two-Component Supported Catalysts. We introduced the second active component to the 14 wt % Co/γAl2O3 supported catalyst to further improve the catalytic performance. The two-component supported catalysts were also prepared by the wet impregnation method with two active components supported on γ-Al2O3 at the same time. The Co loading was fixed at 14 wt %, and certain amounts of Fe(NO3)3· 9H2O, Cu(NO3)2·3H2O, Ni(NO3)2·6H2O, and (NH4)6Mo7O24·4H2O were selected to synthesize twocomponent supported catalysts. The results (Figure 9) demonstrated only the addition of Cu-active component significantly enhanced the SO2 conversion and sulfur selectivity. Therefore, the two-component supported catalyst Co−Cu/γAl2O3 the determined. Next, we studied the effect of Cu loading on the catalytic activity of two-component supported catalysts. The amount of Cu added varied from 1 to 6 wt %, and the results are illustrated in Figure 10. When the Cu loading increased from 1 to 4 wt %, the SO2 conversion and sulfur selectivity significantly rose. However, when the Cu amount increased from 4 to 6 wt %, the excessive active component material might clog the catalyst channel, resulting in a poor catalytic effect. Finally, the 14 wt % Co−4 wt % Cu/γ-Al2O3 turned out to be the optimized catalyst for the SO2 reduction to sulfur by the coal gas. 3.2.4. Effect of Presulfidation Conditions. Generally, the catalyst used in a hydrodesulfurization (HDS) process is presulfided with 10% v/v H2S in hydrogen before reaction. Four different presulfidation conditions were selected for the 14 wt % Co−4 wt % Cu/γ-Al2O3 supported catalysts: (a) without presulfidation; (b) presulfided with 10% v/v H2S/H2 gas at 420 °C for 1 h; (c) presulfided with 10% v/v H2S/H2 gas at 420 °C G


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test was performed at a reaction temperature of 400 °C, a GHSV of 5000 h−1, and a [CO + H2]/SO2 molar ratio of 2. The SO2 conversion decreases initially but reaches a steady state with about 90 wt % of the initial conversion in 100 h. The sulfur selectivity also showed the same trend with the SO2 conversion. The catalyst seems to be very stable, not showing any appreciable decrease in both the SO2 conversion and sulfur selectivity after a long run of 100 h. 3.5. Modified Redox Mechanism. In the course of the reaction, H2S and COS were always detected when the catalyst exhibits catalytic activity, which is consistent with the intermediate (H2S and COS) mechanism of the reduction of SO2 to sulfur. The catalytic reaction mechanism of the intermediate product is argued as the following equations:

for 2 h; (d) presulfided with 10% v/v H2S/H2 gas at 420 °C for 6 h. The results are summarized in Table 2. The catalyst without presulfidation shows very low catalytic performance for the reduction of SO2 to sulfur. Compared with the catalyst presulfided for 1 h, the presulfided catalyst for 2 h exhibited higher SO2 conversion and sulfur selectivity. With longer presulfidation time, the catalytic effect did not change obviously. 3.2.5. Effect of Reaction Temperature. Among all the factors, the influence of reaction temperature is particularly important. Using 14 wt % Co−4 wt % Cu/γ-Al2O3 catalyst and coal gas, the fixed reaction conditions were the [reductant]/ [SO2] molar ratio of 2 and GHSV of 5000 h−1. The reaction temperature varied from 300 to 500 °C. Figure 11 shows the changes in the catalytic activity with the reaction temperature including SO2 conversion and sulfur selectivity. When the reaction temperature was 300 °C, the catalyst did not exhibit catalytic activity, which was attributed to the fact that the activation temperature of the catalyst had not been reached. When the reaction temperature rose to 350 °C, the catalyst began to exhibit catalyst activity. The SO2 conversion and sulfur selectivity increased significantly when the reaction temperature reached 400 °C. However, when the reaction temperature was over 400 °C, there was no visible increase. Consequently, the optimum reaction temperature of 400 °C was determined. 3.2.6. Effect of GHSV. Figure 12 shows the SO2 conversion and the sulfur selectivity with the GHSV in the SO2 reduction by the coal gas over 14 wt % Co−4 wt % Cu/γ-Al2O3 catalyst. The reaction temperature and [CO + H2]/[SO2] molar ratio were fixed at 400 °C and 2.0, respectively, and GHSV varied from 3000 to 15 000 h−1. The SO2 conversion decreased from 95 to 88 wt % as GHSV increased from 3000 to 15 000 h−1. This result might be associated with the decrease of the contact time between the reactant and the catalyst surface. The trend in change of the sulfur selectivity with GHSV was different from the SO2 conversion. Below 5000 h−1, the sulfur selectivity was increased from 93 to 96 wt % as the GHSV was increased from 3000 to 5000 h−1. Above 5000 h−1, the sulfur selectivity was decreased from 96 to 90 wt % when the GHSV further increased. In conclusion, when the optimum GHSV was 5000 h−1, the SO2 conversion of 95 wt % and sulfur selectivity of 96 wt % were obtained. 3.3. Identification of Sulfur Products. Figure 13 clearly demonstrates several characterizations of the resulting sulfur products. The XRD spectra of the cooled yellow products obtained were fully consistent with the sulfur XRD standard card, indicating that the product is elemental sulfur, and the sulfur is in the form of orthorhombic sulfur-S8. The sulfur obtained from the condensation at the rear end of the reactor was ground into powder for cold field SEM/EDS characterization. Figure 13 also shows the SEM photo (a magnification of 60.0K) of the sulfur sample obtained from the SO2 reduction in the present study. SEM/EDS analysis demonstrates that these substances are elemental sulfur. The elemental sulfur particles seem to be an aggregation of many small spherical particles from the outer surface of the sulfur sample, and the spherical particles possessed a diameter of less than 500 nm. On the basis of EDS analysis, the sulfur content of the crystal proved to be close to 100 wt %. 3.4. Long Life Evaluation of the Presulfided 14 wt % Co−4 wt % Cu/γ-Al2O3 Catalyst. The stability of the presulfided 14 wt % Co−4 wt % Cu/γ-Al2O3 catalyst was evaluated, and the results are shown in Figure 14. The long run

SO2 + ■ → SO2 ■

(3)

CO + ★ → CO★

(4)

H 2 + ● → H 2●

(5)

CO★ + MSn → MSn − 1 + COS★

(6)

H 2● + MSn → MSn − 1 + H 2S●

(7)

COS★/H 2S● + SO2 ■ → S + CO2 /H 2O

(8)

where ■, ★, and ● represent adsorption sites of SO2, CO, and H2, respectively; MSn is the metal sulfide with catalytic activity. The reaction control steps are eqs 6 and 7. Besides, Figure 15 vividly illustrates the catalytic mechanism of the presulfided 14 wt % Co−4 wt % Cu/γ-Al2O3 catalyst used for the SO2 reduction for sulfur production. For the adsorption of SO2 on the surface of the catalyst, alkaline support might play an important role. Combining with SEM, EDS, XPS, and XRD analysis, the effective active phase turned out to be the CuCoSx microcrystals after presulfidation. First, the high-content acidic SO2 in metallurgical flue gas is adsorbed to the alkaline support γ-Al2O3. H2 and CO gases in the reduced gas (coal gas) are adsorbed onto the metal sulfide active component supported on the γ-Al2O3 support, resulting in the formation of the active reaction intermediates (H2S and CO). Finally, the active intermediate reacts with SO2 to obtain elemental sulfur, and SO2 in metallurgical flue gas is successfully reduced to sulfur product. The more detailed mechanism of catalytic mechanism is shown in Figures 16 and 17. The reduction of SO2 by H2 and CO over the supported catalyst belongs to the free radical mechanism. The production of free radicals is a crucial step in promoting this reaction. At the reaction temperature, the γAl2O3 support contributes to the adsorption of SO2, and the active component sulfides promotes the generation of free radicals. Finally, using the presulfided 14 wt % Co−4 wt % Cu/ γ-Al2O3 catalyst, the SO2 gas was successfully reduced to sulfur by the coal gas.

4. CONCLUSION The presulfided 14 wt % Co−4 wt % Cu/γ-Al2O3 catalyst exhibited outstanding catalytic activity and selectivity for the reduction of high-content SO2 using coal gas as reductant. With the (CO + H2)/SO2 ratio of 2:1 and the GSHV of 5000 h−1, the SO2 conversion of 95% v/v and sulfur selectivity of 96% v/v at 400 °C were obtained. Various applied characterizations indicated active phases of the chosen catalyst, and the analysis proved that the active component was microcrystalline on the H


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support surface and its channel. After long run of 100 h, the values of SO2 conversion and sulfur selectivity were still more than 90 wt %. Therefore, the presulfided 14 wt % Co−4 wt % Cu/γ-Al2O3 was a promising catalyst for the selective reduction of the high-content SO2 in smelter off-gas to sulfur using coal gas as the reducing gas.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b00213. Deoxygenation; CO-TPR profiles of the presulfided Co/ Al2O3 catalysts and the Co−Cu/Al2O3 catalysts without presulfuration; XPS characterization of the presulfided 14 wt % Co/Al2O3 catalyst; pore size distribution of the presulfided Co−Cu/Al2O3 catalysts; molecular simulation of sulfur crystal shape; results with different CO/H2 molar ratios (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(L.W.) Tel/Fax +86-13520040309. E-mail [email protected]. cn. *(C.L.) Tel/Fax +86-10-825445800. E-mail [email protected]. ORCID

Chunshan Li: 0000-0003-2460-8697 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge National Science Fund for Excellent Young Scholars (No. 21422607), Key Program of National Natural Science Foundation of China (No. 91434203), and Research Supported by the CAS/SAFEA International Partnership Program for Creative Research Teams.

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REFERENCES

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Selective Reduction of SO2 in Smelter Off-Gas with Coal Gas to Sulfur

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