Tingting Ge,Cuncun Zuo,Lubin Wei,*,Chunshan Li,*

1School of Chemical and Environmental Engineering,China University of Mining&Technology-Beijing,Beijing 100083,China

2Beijing 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,China

3College of Chemistry and Chemical Engineering,University of Chinese Academy of Sciences,Beijing 100190,China

Keywords:Iron-based catalysts Sulfur production CO–H2gas mixture Reaction mechanism

A B S T R A C T A series of modified γ-Al2O3supported iron-based catalysts(M-Fe/γ-Al2O3)was developed to reduce SO2in actual smelter off-gases using CO–H2gas mixture as reducing agent for sulfur production.Used as modifiers,three metal additives— Ni,Co,and Ce were added to Fe/γ-Al2O3catalysts.Changes in catalyst structure and active phase were characterized with X-ray diffraction,XPS,SEM,and EDS.The reduction ability of catalysts was exhibited via CO-TPR.The prepared catalysts only need to be pre-reacted for a period of time,eliminating the need for presulfidation treatment.Reaction conditions were optimized in a fixed bed reactor to achieve high SO2conversion and sulfur selectivity. XRD characterization was carried out to verify the resulting sulfur products.Combining in situ infrared characterization and catalyst evaluation of support and active component,the reaction mechanism was investigated and proposed.

1.Introduction

Sulfur dioxide(SO2)is one of the universally accepted gaseous pollutants,and the technique of reducing SO2emissions as well as its recycle process has always been the hot topic in the recent years[1–3].SO2mostly comes from pyrometallurgical processes of sulfide ore,such as copper,lead,zinc,nickel,cobalt,mercury,molybdenum and other non-ferrous metal minerals.The high-content SO2in smelter off-gas is typically oxidized to sulfuric acid.Pyrometallurgy is the refining of metal or its compounds from the ore extraction at high temperatures.Most of the non-ferrous metal minerals are in the form of sulfide,such as copper sulfide,iron sulfide,and nickel sulfide.Metal sulfides can be used as raw materials for the production of metal oxides,mainly copper,zinc and lead minerals.However,the production process is accompanied by a large amount of harmful gases containing SO2.In recent decades,SO2has been mainly enriched to produce H2SO4.Through the contact process,the smelter off-gas with ψ(SO2)greater than 3%is used for the preparation of sulfuric acid.This technology is already very mature and widespread[4–6].Sulfuric acid is an important chemical raw material,but the transport problem is the inherent shortcomings of sulfuric acid.As a highly corrosive liquid,sulfuric acid is not suitable for long-distance transport,and its reasonable transport radius is only 300 km.On the one hand,the price of H2SO4is relatively cheap,and it is provided with low added value.On the other hand,the H2SO4production has been surplus,and the market has been saturated,little room for development.Sulfuric acid sales have become a serious constraint to the development of the metallurgical industry[7].

As raw materials,the sulfur in the form of elemental sulfur occupies a very important position in the dye,rubber,paper,military and other industries.The selective reduction of SO2to elemental sulfur will be provided with a broad market prospect[8–10].Besides,it is a good gas desulfurization method suitable for changeable environment.At present,the main sulfur production technology is the Claus process of sulfur with the acidic hydrogen sulfide gas as raw material,but it was not directly applied to the SO2reduction in smelter off-gas[11–14].

According to different reducing agents, the direct reduction of SO2to sulfur can be divided into H2reduction method,carbon reduction method,CH4reduction method,CO reduction method,and NH3reduction process [15–30]. During the direct reduction process, solid catalysts are commonly used,including supported or oxide catalysts.Solid-supported catalysts have important applications in desulfurization processes such as hydrodesulfurization[31]and desulfurization[32].Using different reductants,Fe-ZSM-5 and Co-ZSM-5 catalysts show good catalytic performance for the reduction of nitrogen oxide[33–35].Fe/γ-Al2O3catalysts were firstly developed for hydrogenation of sulfur dioxide to hydrogen sulfide[36].Supported iron catalysts were successfully used for catalytic reduction of SO2with CO,but the concentrations of SO2were very low[37,38].Zhao et al.[39]proposed the selective reduction of SO2to elemental sulfur over sulfided CoMo/γ-Al2O3catalysts using CO as the reducing agent.Kim et al.[40]developed Co3O4–TiO2catalysts for the reduction of SO2by CO to elemental sulfur.Feng et al. carried out experiments and thermodynamic equilibrium calculations on a H2–SO2system in an activated carbon bed[26].Paik et al.studied the catalytic reduction of SO2to elemental sulfur using a stoichiometric amount of H2or CO,sulfided Co/Al2O3or Co/TiO2catalysts,indicating the bifunctional properties of the supported catalyst and the reaction intermediate mechanism[23].Mulligan et al.[41]studied some sulfide crystals and their supported catalysts for the SO2reduction to sulfur by the CH4reduction method.However,the problems of H2prices,sources,transportation and storage weakened the advantages of the reduction process.Besides,the CH4reduction method requires high methane consumption and reaction temperature.So far,no good results have been reported about the catalytic reduction of the high-content SO2in smelter off-gas for sulfur production.

In this study,a new iron-based catalytic system is developed to achieve the reduction of high-content SO2in the smelter off-gas for sulfur production.The supported catalyst was prepared using γ-Al2O3as the efficient support.Fe was selected as the main active component,with a small amount of Ce/Ni/Co as modifiers.The crystal phase of catalysts and active components involved in the reaction were characterized by XRD/XPS/SEM/EDS.The reducing property of catalysts was characterized by CO-TPR and correlated with the actual reactivity.On a fixed bed micro-evaluation device,catalysts were evaluated to optimize catalyst compositions and reaction conditions.The life evaluation was performed for the optimum catalyst,and the detailed reaction mechanism was proposed.

2.Experimental

2.1.Catalyst preparation

Spherical γ-Al2O3(99.5%purity)was purchased from a ceramic company in Zibo city of Shandong province.550–270 μm(30–50 mesh)particles were got after sieving and being calcined in a muffle furnace at 600°C for 6 h to remove adsorbed water and organic matter.In the present study,all catalysts involved were prepared via incipient-wetness impregnation method.Before preparing the catalyst,water-absorbing capacity of γ-Al2O3was tested.The main active components(Fe)and other modified metal centers were employed from Iron(III)nitrate nonahydrate(Fe(NO3)3·9H2O)and water-soluble metal nitrates such as Nickel(II)nitrate hexahydrate(Ni(NO3)2·6H2O),Cobalt nitrate hexahydrate(Co(NO3)2·6H2O),and Ceric ammonium nitrate(Ce(NH4)2(NO3)6).All of above reagents were ordered from Sinopharm Chemical Reagent Co.,Ltd.and listed under analytical reagent without any purification treatment.Catalysts were calcined in a muffle furnace for a period of time and then placed in a dry environment.

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 Kαradiation 4 min-1.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 SO2reduction to sulfur.

CO-TPR experiments of supported catalysts were also carried out on the apparatus using 5 wt%mixture of CO in Ar for the CO pretreatment.

The prepared catalysts were evaluated in a fixed-bed device and the sulfur products obtained were observed in an SU8020 SEM unit.

2.3.Catalyst evaluation

The prepared catalysts were evaluated in a fixed-bed device and the reactor was made by inside diameter of 3 mm stainless steel pipe.Around 1.0 g of catalyst sample(550–270 μm)was placed in the middle of the reactor.Without presulfidation,the prepared catalysts need only be pre-reacted in the reactor under the reaction atmosphere for a certain time.The CO/H2gas mixture and smelter off-gas containing high-content SO2with a constant molar ratio of(CO+H2):SO2as 2:1 were simultaneously fed to the reactor.For the measurement of sulfur products,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 selectivity to sulfur(S)are defined as follows:

where[SO2]inwas the inlet concentration of SO2,and[SO2]out,[COS]outand[H2S]outwere the effluent concentrations.

3.Results and Discussion

3.1.Selective reduction of SO2to sulfur by CO

3.1.1.Effect of active components

Four active components including Fe(NO3)3·9H2O,Cu(NO3)2·3H2O,Ni(NO3)2·6H2O,and(NH4)6Mo7O24·2H2O were selected to prepare supported catalysts using the impregnation method to investigate the catalyst performance.γ-Al2O3was chosen as the support to prepare supported catalysts involved in the study.The data of SO2conversion and sulfur selectivity were collected as standards for catalyst evaluation.The effect of different active components were examined at the reaction temperature of 400°C with[CO]/[SO2]molar ratio of 2 and gas hourly space velocity(GHSV)of 5000 h-1,and a fixed mass fraction of active components of 14 wt%.The synthesized supported catalyst was directly used in the reaction and not subjected to pre-sulfidation treatment.After the catalyst ran for a certain period of time in the reactor(pre-reaction for about 2 h),we collected the data for analysis when the steady state was reached.As shown in Table 1,the Fe-supported γ-Al2O3catalyst showed the best catalytic effect,and the blank γ-Al2O3catalyst presented poor catalytic activity.These results indicate that the Fe activematerial favored the reaction, and the interaction between Fe active material and γ-Al2O3support played an important role in the reaction.Over 14 wt%Fe/γ-Al2O3catalyst,the conversion of SO2was more than 96%and the selectivity of sulfur reached 95%.Therefore,Fe/γ-Al2O3catalyst was selected for the further study.

Table 1Optimization of active components

3.1.2.Effect of Fe loading

For the reduction of SO2to sulfur,hydrogen reduction and carbon monoxide reduction methods are commonly used.In this study,we chose separate CO or H2as the reducing gas to investigate the effect of Fe loading on the catalytic activity.Fig.1 illustrates the evaluation results using CO as the reducing gas.The minimum Fe loading was 6 wt%,and the SO2conversion significantly increased with the Fe loading.The best catalytic effect was obtained with the Fe loading of 14 wt%.However,the catalytic effect showed a downward trend when the Fe loading was further increased.When hydrogen was used as the reducing gas,the catalytic performance was much worse under the same reaction conditions,and the corresponding data were listed in Supporting Information.

Fig.1.Effect of the Fe loading on the catalytic effect using CO as reducing gas.

3.1.3.Study on two-component supported catalysts

We introduced the second active component to the 14 wt%Fe/γ-Al2O3supported catalyst to further improve the catalytic performance.The two-component supported catalysts were also prepared by impregnation method with two active components supported on γ-Al2O3at the same time.The Fe loading was fixed at 14 wt%,and certain amounts of Co(NO3)2·6H2O,Ni(NO3)2·6H2O,and Ce(NO3)3·6H2O were selected to synthesize two-component supported catalysts.For the synthesized two-component supported catalysts,the separate H2or CO was used as reductant to study their catalytic performance in the reduction of SO2to sulfur.The catalyst evaluation results using CO as the reducing gas were clearly illustrated by Fig.2.The reaction temperature was set to400°C and GHSV was 5000 h-1.Using CO as the reducing gas,the excellent catalytic effect can be obtained.However,when H2was used as reductant,the catalyst was very ineffective,indicating hydrogen activation requires a higher reaction temperature for this reaction(Supporting Information).Among the Fe–Ce,Fe–Co,and Fe–Ni/γ-Al2O3two-component supported catalysts,the Fe–Co/γ-Al2O3supported catalysts showed the highest reactivity.Next,we studied the effect of Co loading on the catalytic activity of two-component supported catalysts(Fig.3).The amount of Co added varied from 1 wt%to 4 wt%.When the Co loading increased from1 wt%to2 wt%,the SO2conversion and sulfur selectivity significantly rose.However,when the Co amount increased from 2 wt%to 4 wt%,the catalytic activity showed a downward trend.Finally,14 wt%Fe–2 wt%Co/γ-Al2O3turned out to be the optimized catalyst and was used for the further study.

Fig.2.SO2conversion and sulfur selectivity using 14 wt%Fe(a),14 wt%Fe-2 wt%Ce/γ-Al2O3(b),4 wt%Fe-2 wt%Co/γ-Al2O3(c),and 14 wt%Fe-2 wt%Ni/γ-Al2O3(d)catalyst with CO as the reducing gas.

Fig.3.Effect of Co loading (the second active component) on the SO2conversion and sulfur selectivity.

3.1.4.Effect of reaction temperature

The impact of reaction temperature is particularly important in all the factors that affect the reduction of SO2to elemental sulfur.Using 14 wt%Fe–2 wt%Co/γ-Al2O3catalyst,the reaction conditions were listed as follows:the molar ratio of[reducing agent]/[SO2]was 2,and the GHSV was 5000 h-1.The reaction temperature varied from 350 to 500°C to investigate the influence of reaction temperature on the catalytic activity. The catalyst showed no catalytic activity when the reaction temperature was 300°C,indicating that the catalyst did not lower the activation energy of the reaction on such a large scale.As can be seen from Fig.4,when the reaction temperature increased to 350°C,the catalyst started to show catalyst activity.When the reaction temperature reached 400°C,the SO2conversion and sulfur selectivity increase sharply.However,there was no visible increase in both the SO2conversion and sulfur selectivity when the reaction temperature further increased.Therefore,the optimized reaction temperature of 400°C was determined.

Fig.4.Effect of the reaction temperature on the catalytic effect using CO as reducing gas over 14 wt%Fe–2 wt%Co/γ-Al2O3catalyst.

3.1.5.Effect of GHSV

The impact of gas hourly space velocity(GHSV)can't be ignored in all the factors that affect the reduction of SO2to elemental sulfur.Using 14 wt%Fe–2 wt%Co/γ-Al2O3catalyst,reaction conditions were listed as follows:the molar ratio of[reducing agent]/[SO2]was 2,and the reaction temperature was set to 400°C.The GHSV varied from 2500 to 15000 h-1to investigate the influence of GHSV on the catalytic activity.As can be seen from Fig.5,there was a slight increase in the SO2conversion and sulfur selectivity when the GHSV increased from 2500 to 5000 h-1.However,when the GHSV further increased from 5000 to 15000 h-1,both the SO2conversion and sulfur selectivity show a declining trend,which may be due to the absence of the reaction of reactant gases with the catalyst due to the shorter residence time.Therefore,the optimized GHSV of 5000 h-1was determined.

Fig.5.Effect of GHSV on the catalytic performance of 14 wt%Fe–2 wt%Co/γ-Al2O3catalyst.

3.2.Catalytic reduction of SO2by gas mixture of CO and H2

In the previous section,we have studied the catalytic effect of the separate H2or CO reduction gas using the 14 wt%Fe–2 wt%Co/γ-Al2O3,and the evaluation parameters included SO2conversion and sulfur selectivity.The evaluation results showed that the catalytic effect was better when using CO as the reducing gas,and the catalytic performance was relatively poor when H2was used as the reducing gas.Table 2 lists the evaluation results of the catalyst when the mixing gases of CO and H2with different proportions were used as the reducing gas.In all the catalyst evaluation experiments,the molar ratio of the total amount of the reducing gas to the amount of SO2was kept to 2.It can be seen from the results, the higher the proportion of CO in the mixture,the better the catalytic effect;the higher the proportion of H2in the mixture,the worse the catalytic activity.It can be speculated that CO and H2play a role in the reduction separately,and they are completely independent of each other during the reaction process,without competition or synergies.

Table 2Evaluation results of the 14 wt%Fe–2 wt%Co/γ-Al2O3supported catalyst for the SO2reduction by the mixing gas of CO and H2

3.3.Characterization

3.3.1.XRD

Fig.6shows XRD patterns of the fresh and used14 wt%Fe–2 wt%Co/γ-Al2O3supported catalyst.It should be noted that in this study,all Fe based supported catalysts prepared were not subjected to the presulfuration treatment.As clearly shown in Fig.6,the diffraction peak of Fe2O3mainly appears in the XRD spectra of the fresh supported catalyst,that is,the fresh catalyst is composed of oxide.When the catalyst was run in a fixed bed for a certain time, the XRD spectra of the used catalyst were collected for analysis. Unlike the fresh catalyst, the diffraction peaks of FeS2were detected in the XRD spectra of the used catalyst,demonstrating that the composition of the active component of the catalyst has changed after the reaction.No diffraction peaks of cobalt-related species were detected,probably due to the small addition of cobalt or the presence of crystallites.

Fig.6.XRD patterns of the fresh and used14 wt%Fe–2 wt%Co/γ-Al2O3supported catalyst.

3.3.2.XPS

XPS characterization demonstrated that the catalyst had a significant change in the form of the main active element before and after the reaction. Fig. 7 clearly shows the discrepancy of the XPS curves of Fe element existing in the fresh catalyst and the used catalyst after evaluation.The Fe element in the fresh 14 wt%Fe–2 wt%Co/γ-Al2O3supported catalyst was nearly a pure crystal phase Fe2O3,which can be identified by the peaks appearing at the 710.7 and 725.0 eV.When the catalysts ran in the fixed-bed reactor for a certain time,the catalyst was a mixed crystal phase of Fe2O3and FeS2.The peaks detected at the 706.5 and 720.0 eV indicated the presence of FeS2phase.In addition,the sulfur element analysis for the used catalyst also demonstrated the sulfur element was in the form of disulfide(peaks at 162.1 and 163.4 eV).

Fig.7.XPS curves of the fresh 14 wt%Fe–2 wt%Co/γ-Al2O3catalyst and the used catalyst after evaluation.

3.3.3.CO-TPR

Fig.8 demonstrates CO-TPR profiles of the fresh single-component supported catalyst 14 wt%Fe/γ-Al2O3and two-component supported catalyst 14 wt%Fe–2 wt%Co/γ-Al2O3.When the second active component Co was introduced to the Fe/γ-Al2O3supported catalyst,the COTPR curve of the catalyst changes significantly.The CO-TPR of the Fe-Co supported catalyst shows the reduction peak of the cobalt active component compared to that of the Fe-supported catalyst.The CO depletion peak located at 300–350 °C corresponds to the reduction of Co3O4to CoO.These results indicate that the addition of Co modifier shows a visible change in the reduction performance of the 14 wt%Fe/Al2O3supported catalyst,which leads to a significant increase in both SO2conversion and sulfur selectivity,proved by the catalyst evaluation results.

3.3.4.SEM

SEM characterization was carried out to investigate the distribution of the active components on the support.Fig.9 shows that manymicrocrystals are distributed on theγ-Al2O3support body structure.Combining with the EDS spectra,these microcrystals on the support turned out to be composed by active components Fe,Co,and S elements.The previous XRD analysis also proves the presence of FeS2crystal phase,and the diffraction peaks of Co species were not detected in the XRD spectra because of the small proportion in the catalyst.It can be seen from the EDS image that the Co species are uniformly distributed,so that then can improve the catalytic activity in cooperation with the main Fe active species.

3.4.Identification and purity test of sulfur products

In theory,we can successfully obtain sulfur products in the sulfur collection tank in the evaluation device.For further validation,we collected the yellow material collected in the tank for XRD analysis.Fig.10 shows the XRD spectra of the cooled yellow products obtained were fully consistent with the sulfur XRD standard card,indicating that the product is our objective elemental sulfur.Therefore,the reduction of high-content SO2in metallurgical flue gas is successfully achieved by CO/H2gas mixture as reducing gas over the 14 wt%Fe–2 wt%Co/γ-Al2O3supported catalyst.

GB/T2449-2006(sulfur for industrial use)was selected for our test experiments.The specific steps:

(1)Weigh out 5 parts of sulfur sample(about 2 g)in five crucibles,and the sample was dissolved with CS2.Then the mixture was filtrated to obtain the solid residue,which was placed again to the dry crucible and continued to be rinsed with CS2.

(2)The total mass of crucible and sulfur was accurately weighed before and after each dissolution(accuracy of 0.0001 g).The proportion of soluble sulfur was determined until two measurements were the same.

Sulfur purity test results are shown in Table 3.

3.5.Long life evaluation of the 14 wt%Fe–2 wt%Co/γ-Al2O3catalyst

The stability of the 14 wt%Fe–2 wt%Co/γ-Al2O3catalyst was investigated,and the results were shown in Fig. 11. The long run test was performed at a reaction temperature of 400°C,a GHSV of 5000 h-1,a CO/SO2molar ratio of 2.The SO2conversion showed a slight downward trend,and the sulfur selectivity also showed the same trend.In general,the catalyst seems to be very stable,not showing any appreciable decrease in both the SO2conversion and sulfur selectivity after a long run of 200 h.

3.6.Redox reaction mechanism

In the catalytic reduction of SO2to elemental sulfur,the 14 wt%Fe–2 wt%Co/γ-Al2O3supported catalyst had bifunctional properties;sulfide generates H2S(with H2as reductant)or COS(with CO as reductant)as reaction intermediate and then production of sulfur can be achieved by the reaction between the reaction intermediate(H2S or COS)and SO2over the support Al2O3.The catalytic reaction mechanism of the reaction intermediate can be shown as the following equations:

Fig.9.SEM image of the 14 wt%Fe–2 wt%Co/γ-Al2O3catalyst.

Fig.10.XRD spectra of the sulfur product obtained.

Table 3Soluble sulfur ratios for five sulfur samples

Fig.11.SO2conversion and sulfur selectivity over the 14 wt%Fe–2 wt%Co/γ-Al2O3 catalyst using CO as the reducing gas.

The adsorption of SO2on the14 wt%Fe–2 wt%Co/γ-Al2O3supported catalyst has been verified by in-situ infrared characterization,as shown in Fig.12.The absorption peaks appearing near the 1360 cm-1proves that the catalyst has been completely immersed in the SO2atmosphere,while the absorption peaks appearing at the 1250 cm-1indicate the presence of the active reaction intermediate(S=O bond)on the catalyst,which proved that SO2had been successfully activated and then reacted with reactants.

Fig.12.In situ IR spectra of the 14 wt%Fe–2 wt%Co/γ-Al2O3supported catalyst at different temperatures in SO2+CO atmosphere.

To study the respective role of the active ingredient and the support as well as their possible synergistic effect during the whole reaction,we conducted experimental study using Al2O3,active ingredient and the physical mixture of both as catalysts,respectively.The results were clearly shown in Table 4.In order to obtain the pure sulfide active component,we have studied only the Fe active component catalyst,without considering two active component catalyst.Fe(NO3)3·9H2O used for the preparation of supported catalyst was directly placed into a muffle furnace for calcination under the same calcination conditions as the supported catalyst.The evaluation results show that the poor catalytic performance was obtained using the bare Al2O3support as the catalysts.When the active component was used as a catalyst for the reaction,SO2is completely converted into H2S and COS,without the generation of sulfur.In addition,we collected the XRD spectra of the active component catalyst running in the reactor for a certain period of time,illustrated by Fig.13.It can be seen from the figure that the main component of the active component is FeS2,which is consistent with the XRD pattern of the supported catalyst.In combination with the evaluation results in Table 4,we can conclude that the main role of the active component is the promotion of the generation of active free radicals and intermediates(H2S and COS).Next,we continue to analyze the action mechanism of the supported catalyst as a whole.

Table 4The conversion of SO2and selectivity of sulfur over different catalysts at the reaction temperature of 400°C with the GHSV of 5000 h-1

Fig.13.XRD patterns of the bare active component catalyst after running for a certain period of time in the fixed-bed reactor.

Fig. 14 vividly shows the action mechanism of the supported catalyst as a whole in the reduction of SO2to sulfur.The in-situ infrared characterization confirmed that SO2can be adsorbed and activated by the support Al2O3.The main role of metal sulfide active components is the promotion of the generation of active free radicals and intermediates(H2S and COS).Finally,the sulfur synthesis is achieved by the reaction of active free radicals with the combination product of SO2and Al2O3support.

4.Conclusions

The 14 wt%Fe–2 wt%Co/γ-Al2O3supported catalyst exhibited outstanding catalytic effect for the high-content SO2reduction in metallurgical flue gas to sulfur.With the CO reducing gas and the GSHV of 5000 h-1,the catalyst shows 99%SO2conversion and 99%sulfur selectivity at the reaction temperature of 400°C.The catalyst eliminates the need for complicated step of presulfidation and requires only a short period of pre-reaction in the reactor to achieve high catalytic activity.Several characterizations(XRD,XPS,CO-TPR,BET,and SEM)were carried out to investigate active phases in the 14 wt%Fe–2 wt%Co/γ-Al2O3supported catalyst.When H2was used as the reducing gas,the catalytic effect was particularly poor.Additionally,the mixing gases of H2and CO with different proportions were used for the reducing gases to investigate the catalytic activity,and the evaluation results demonstrated H2and CO play a role in the reduction separately,and they are completely independent of each other during the reaction process,without competition or synergies.After a long run of 200 h,the values of SO2conversion and sulfur selectivity did not show a visible reduction.Lastly,the simulated redox mechanism of the reduction of SO2to sulfur was proposed and verified.In conclusion,the 14 wt%Fe–2 wt%Co/γ-Al2O3supported catalyst is a promising catalyst for the sulfur production by the reduction of the high-content SO2in smelter off-gas.

Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2018.03.031.

Fig.14.Simulated redox mechanism of the SO2reduction to sulfur by H2or CO over the 14 wt%Fe–2 wt%Co/γ-Al2O3supported catalyst.