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Enhancing the catalytic oxidation of elemental mercury and suppressing sulfur-toxic adsorption sites from SO2-containing gas in Mn-SnS2
Journal of Hazardous Materials 392 (2020) 122230
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Enhancing the catalytic oxidation of elemental mercury and suppressing sulfur-toxic adsorption sites from SO2-containing gas in Mn-SnS2
T
Haomiao Xua,c, Yongpeng Maa,b, Bailong Mub, Wenjun Huanga, Qinyuan Honga, Yong Liaoa, Zan Qua,c, Naiqiang Yana,c,* a
School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Henan Collaborative Innovation Center of Environmental Pollution Control and Ecological Restoration, School of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou, 450001, China c Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China b
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Editor: Danmeng Shuai
It is difficult to stabilize gaseous elemental mercury (Hg°) on a sorbent from SO2-containing industrial flue gas. Enhancing Hg° oxidation and activating surface-active sulfur (S*) can benefit the chemical mercury adsorption process. A Mn-SnS2 composite was prepared using the Mn modification of SnS2 nanosheets to expose more Mn oxidation and sulfur adsorption sites. The results indicate that Mn-Sn2 exhibits better Hg° removal performances at a wide temperature range of 100–250 °C. A sufficient amount of surface Mn with a valance state of Mn4+ is favorable for Hg° oxidation, while the electron transfer properties of Sn can accelerate this oxidation process. Oxidized mercury primary exists as HgS with surface S*. A larger surface area, stable crystal structure, and active valance state of each element are favorable for Hg° oxidation and adsorption. The Mn-SnS2 exhibits an excellent SO2 resistance when the SO2 concentration is lower than 1500 ppm. The effects of H2O and O2 were also evaluated. The results show that O2 has no influence, while H2O and SO2 coexisting in the flue gas have a toxic effect on the Hg° removal performance. The Mn-SnS2 has a great potential for the Hg° removal from SO2containing flue gas such as non-ferrous smelting gas.
Keywords: Gaseous mercury Metal sulfide sulfur dioxide Surface activation Adsorption
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Corresponding author. E-mail address: [email protected] (N. Yan).
https://doi.org/10.1016/j.jhazmat.2020.122230 Received 9 December 2019; Received in revised form 30 January 2020; Accepted 3 February 2020 Available online 08 February 2020 0304-3894/ © 2020 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 392 (2020) 122230
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1. Introduction Manganese-based oxides (MnOx) have good catalytic oxidation and adsorption performances suitable for the removal of gaseous elemental mercury (Hg0) from industrial flue gas (Xu et al., 2017a; Li et al., 2012, 2009). During the catalytic oxidation process, the adsorbed Hg0 can be catalytically oxidized to Hg2+ along with the reduction of higher-valance Mn (Mn4+ or Mn3+) to lower-valance Mn (Mn3+ or Mn2+). The oxidized Hg2+ can then form bonds with the surface oxygen (HgO). The keys for this Hg0 removal process are the utilization of the oxidation activity of Mn and sufficient amount of surface oxygen for mercury stabilization (Li et al., 2009; Xu et al., 2017b). However, Mn-based oxides often suffer from several limitations when used for gaseous heterogeneous reactions: 1) particle aggregation: pure MnOx particles easily aggregate; thus, not all surface MnOx active sites can be fully used. Supported MnOx hybrids are often designed to obtain a high dispersion of MnOx nanoparticles. As catalytic carriers of MnOx particles, TiO2 (Wu et al., 2010; Gu et al., 2013), Ce–Zr solid solution (Shen et al., 2014; Liu et al., 2010), and carbon-based materials (Su et al., 2013; Huang et al., 2017; Fang et al., 2018) were used. In our previous studies, we observed that graphene nanosheets can confine the MnOx particles, which then grow larger under hydrothermal synthesis conditions, enhancing the Hg° removal performances; (Xu et al., 2015) and 2) low activity at high temperature: although MnOx has been widely used for the catalytic combustion of volatile organic compounds (VOCs) (Chen et al., 2018; Bai et al., 2016; Hao et al., 2020) and low temperature NH3-selective catalytic reduction (NH3-SCR) for NOx removal, (Xu et al., 2017a; Li et al., 2011; Tian et al., 2012) the catalytic activities sharply decrease at high temperature (> 200 °C). At such a high temperature, electron transfer in MnOx is not favorable. The use of good electron conductors, such as carbon-based materials and SnO2, can widen the reaction temperature window (Xu et al., 2015; Xie et al., 2014). However, the most severe problem is limitation 3), that is, the toxic effect of SO2 in the gas: MnOx has a high selectivity of SO2 to SO3 when it is exposed to SO2-containing flue gas. The surface adsorbed SO2 can easily oxidize to SO3 and form surface sulfate, occupying the surface active sites (Chang et al., 2012; YAN et al., 2016; Zhang et al., 2017). Such a toxic effect limits the utilization of several SO2-containing flue gases. In the past, MnOx-based materials were often used for coal-fired types of flue gas (Xu et al., 2017a). The concentration of SO2 in the coalfired flue gas is not very high and provides an optimum reaction temperature (∼150 °C in dust removal devices) (Ma et al., 2017). However, many recent studies have focused on non-ferrous smelting gas, which contains high concentrations of Hg° and SO2. Non-ferrous smelting industries are also regarded as the primary mercury emission sources (Zhang et al., 2012; Pirrone et al., 2010; Pacyna et al., 2010). In contrast to coal-type flue gas, the raw materials for non-ferrous smelting often exist as sulfides and coexist with mercury in ores. During the smelting process at high temperature, SO2 and gaseous mercury are generated in the smelting gas. In the past, the Boliden absorption method, which is based on the use of HgCl2 as absorbent for Hg°, was utilized for mercury recycling (Ma et al., 2014). However, HgCl2 is also a contaminant and the “Minamata convention” requires the replacement of the mercury product in industrial applications (Selin, 2014). In addition, although Hg2Cl2 is one of the useful products of this absorption method, the removal of Hg2Cl2 from the absorption system is a challenge (Ma et al., 2014; Hao et al., 2019). The key for the control of mercury from this SO2-containing flue gas is the development of novel removal methods for Hg° in the gas phase. They will benefit the quality of the product (H2SO4) but also the decrement of complexity acid wastewater (which is generated in a wet scrubbing system). Several sulfur-based materials have been developed to capture mercury in SO2-containing flue gas. Polysulfur powders, transition sulfides (e.g., CuS, FeS, FeS2, and ZnS), and sulfur clusters reportedly have good gaseous Hg0 removal performances (Li et al., 2018; Yang
Fig. 1. XRD analysis of the prepared MnOx, SnS2, and Mn-SnS2.
et al., 2018; Zhao et al., 2018; Liao et al., 2019; Xu et al., 2017c). Li et al. reported a series of studies on nanosized transition metal sulfides, such as nanosized ZnS, CuS, and FeS, which can be used to capture gaseous Hg0 (Li et al., 2018; Yang et al., 2018; Zhao et al., 2018). Surface Hg-S species form on the surface of these nanomaterials. The HgS is one stable mercury species in the environment. The red HgS can transform into black HgS at a temperature of 344 °C, while it decomposes at 584 °C (Sedlar et al., 2015). The formation of HgS on the material surface benefits mercury stabilization. In our previous studies, we prepared [MoS4]2− clusters to capture mercury from S-Hg mixed flue gas (Xu et al., 2017c). Such a composite exhibits a good SO2 resistance and Hg-S is the primary mercury bonding state. The H2S modification method was developed to capture mercury from the flue gas; H2S-modified Fe–Ti spinel oxide exhibits an excellent Hg0 capture performance (Zou et al., 2017). The activated surface generates more sulfur bonding sites, enhancing the Hg0 removal performance. Based on these studies, surface-active S* species, such as polysulfur and active S2−, are the primary mercury adsorption sites. Oxidized mercury (Hg2+) forms during this chemical adsorption process. Accelerating the surface Hg° oxidation process will generate more surface Hg2+, which might be favorable for the bonding of mercury with surface S*. As discussed above, Mn is a good choice because of its high oxidation–reduction potential. However, pure MnSx has a very low activity when it is used for Hg° capture. The poor electron transfer performance of Mn in sulfide lead to an inactivation. Therefore, it is better to build a bridge between Mn and S. The SnS2, one of the semiconductors, has excellent electron transfer properties. It is one of the n-type semiconductors with a bandgap of 2.18–2.44 eV (Zhang et al., 2010; Oh et al., 2011). The SnS2 exhibits good oxidative and thermal stabilities in air. Chang et al. used Sn to modify the selective catalysis reduction (SCR) catalyst. The results indicated that Sn enhances the Lewis acidity and improves the NO oxidation performance (Chang et al., 2013). In addition, Sn-modified materials exhibit excellent SO2 resistances at a wide range of temperatures. The absence of surface adsorption oxygen can also reduce the SO2 poisoning. Therefore, we attempted to design a material with a new structure that enhances the roles of Mn, Sn, and S. Based on the above-mentioned discussion about the limitations of MnOx, the new structure should yield a high dispersion of Mn. Two-dimensional SnS2 nanosheets exhibit a huge surface area for surface Mn loading (Seo et al., 2008; Li et al., 2017). Using the SnS2 as the core of the composite and Mn as the surface modification might be favorable for the catalytic oxidation process. In this study, a novel Mn-SnS2 composite was prepared for the capture of gaseous Hg0 under SO2 conditions. The reactions were conducted in a fixed-bed reactor. The Hg0 removal performances and 2
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Fig. 2. TEM images of (a) and (b): SnS2, (c) and (d): MnOx, and (e) and (f): Mn-SnS2.
rate of 5°/min and angle range of 5°–80°. The BET surface area was calculated based on the nitrogen adsorption–desorption method. The SEM and TEM images were obtained using high-resolution electron microscopy. The EDS mapping was selected for the elemental analysis. For the surface analysis, XPS profiles were measured on the fresh and spent samples. The mercury temperature-programmed desorption (HgTPD) method was used to determine the mercury bonding state of the prepared materials.
Table 1 BET surface areas, pore volumes and pore sizes of the synthesized Mn-SnS2, MnOx and SnS2 samples. Samples
Surface area (m2·g−1)
Pore volume (cm3·g−1)
Pore size (nm)
Mn-SnS2 MnOx SnS2
15.2 5.4 25.7
3.50 0.03 5.90
20.84 4.34 27.23
2.3. Catalytic oxidation and adsorption performance studies effects of the temperature, SO2, and H2O on the performances were evaluated. The Hg0 removal mechanism was also discussed.
The catalytic oxidation and adsorption performances of gaseous Hg° were evaluated using a fixed-bed reactor (Xu et al., 2017c). In this reaction system, the gas generation unit provides the required concentration of O2, SO2, H2O, and Hg° and all gas components are carried by pure N2. For the Hg° removal experiments, 300 μg/m3 was selected. The concentration of SO2 used for the investigation of the effects of SO2 ranged from 500 to 1500 ppm. A total of 2 % of H2O and 5 % of O2 were chosen for the experiments. The reaction temperature ranged from 100 to 300 °C. To avoid the effects of SO2, Lumex RA915+ was used as mercury detector. The Hg° removal efficiencies were calculated based on the inlet and outlet concentrations of Hg°.
2. Experimental section 2.1. Preparation of Mn-SnS2 composites The Mn-SnS2 composite was prepared using a solvent-thermal method (Khan et al., 2017). First, 3.506 g of SnCl4·5H2O and 1.5026 g of thioacetamide (TAA) were dissolved in 50 mL polyethylene glycol (PEG-600) and the mixture was constantly stirred to obtain a transparent solution. Subsequently, a suitable concentration of Mn (NO3)2·4H2O was added to the solution and the mixture was stirred for 3 h. The mixture was then transferred into a Teflon-lined stainless steel autoclave and maintained at 180 °C for 12 h. Finally, the product was collected until the autoclave cooled down to air temperature, washed with ethanol and deionized water several times, and dried at 60 °C for 12 h. For comparison, pure SnS2 was prepared using the same method but without the addition of Mn(NO3)2·4H2O to the mixed solution. For MnOx preparation, a chemical precipitation method was selected. The Mn(NO3)2·4H2O was firstly dissolved in deionized water and then NH3·H2O was added to the solution until the precipitate formed. The precipitate was then collected using filtration and calcined under 500 °C in a muffle furnace. For the Hg° removal experiments, all prepared materials were ground to a 40–60 mesh particle size.
3. Results and discussion 3.1. Structure analysis of the prepared materials Powder XRD was used for the structure analysis of the prepared materials. Fig. 1 shows the MnOx peaks, which can be ascribed to the (211), (222), (400), (332), (440), and (622) phases. These peaks are primary peaks and in accordance with the standard PDF card #71-0636 for manganese oxide (Zhang et al., 2014). The peaks detected for the prepared SnS2 agree with those listed on the PDF card #23-0677 (Zai et al., 2012). After Mn modification, the primary peaks of SnS2 still existed at the same peak positions. Several newly generated peaks were ascribed to manganese sulfides (Tang et al., 2015). The peak intensities of the (110) and (111) reflections weakened. The reason could be that Mn primarily occupied these phases. Notably, the addition of Mn during the synthesis did not change the crystal structure of SnS2. However, some manganese sulfides were generated during the synthesis.
2.2. Material characterization For the crystal structure analysis, powder XRD was used at a scan 3
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Fig. 3. XPS analysis of the Mn-SnS2 composite; (a) Mn 2p, (b) Sn 3d, and (c) S 2 s; (d) nanostructure proposed for Mn-SnS2. Fig. 4. (a) Hg° removal of the prepared MnOx, SnS2, and Mn-SnS2 samples, temperature: 150 °C; and (b) Effects of different temperatures on the Hg° removal performance of the prepared materials, reaction time: 60 min. Reaction conditions: total flow rate: 350 mL/ min; 300 μg/m3 of Hg°, 5 % of O2; GHSV = 493,827h−1; catalyst mass =10 mg.
size is uniform (∼5 nm). As shown in Fig. 2f, the lattice spacing of the exposed facets changes to 0.33 nm on the surface due to the generation of Mn sulfides. After Mn modification, the particle size is smaller, which is beneficial for the exposure of more surface-active sites during catalytic reactions of gases. We analyzed the surface areas and pore characteristics of the prepared samples. The N2 adsorption–desorption isotherms are illustrated in Fig. S2. The BET surface areas are provided in Table 1. The BET surface areas of pure MnOx and SnS2 are 5.4 and 25.7 m2 · g−1, respectively. The surface area of Mn-SnS2 is 15.2 m2 · g−1, that is, three times larger than that of pure MnOx. However, it is smaller than that of SnS2. The pore volume and average pore size were calculated using the BJH method. The pore volume of MnOx is only 0.03 cm3 · g−1 and the pore size is 4.34 nm. The TEM images show that the bulk particles are aggregated. Therefore, the pore size and pore volume are small. However, the flower-like SnS2 exhibits a large layered structure and thus a
Furthermore, we obtained TEM images of the prepared Mn-SnS2 sample; the images of SnS2 and MnOx were also collected for comparison. As shown in Fig. 2a, SnS2 has large and thin 2D layers and the length of the thin nanosheets is very large. The morphology of such SnS2 was further indicated by the SEM images, as shown in Fig. S1. Flower-like SnS2 was prepared using the hydrothermal method, resulting in these thin layers. The lattice spacing of 0.59 nm shown in Fig. 2b confirms that the (001) facets represent the exposed surface nanosheets (Seo et al., 2008). These results agree with the XRD results; the (001) facets exhibit a high intensity. The bulk particles of MnOx can be observed in the TEM images, as shown in Fig. 2c. The particle sizes of these bulk particles are not uniform but range from 30 to 100 nm. Fig. 2d shows the lattice fringes, indicating a spacing of ∼0.25 nm, which corresponds with the (222) facets of Mn3O4. In the Mn-SnS2 composite, the larger nanosheet layers are thicker and several small particles are generated on the surface, as shown in Fig. 2e. The particle 4
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3.2. Hg0 removal over Mn modified SnS2 3.2.1. Hg0 removal performance of the prepared materials The Hg° removal performances of the prepared samples were evaluated in a fixed-bed reactor. Based on Fig. 4a, pure SnS2 shows almost no Hg° removal efficiency. A 40 % removal efficiency is obtained in the first several minutes of the experiment. Subsequently, SnS2 gradually loses its activity; after 60 min of reaction, the activity has ceased. Pure MnOx, which reportedly has a good Hg° removal performance, also exhibits a good Hg° removal performance in our study. The total Hg° removal efficiency is 80 % during 60 min of reaction. After Mn modification, Mn-SnS2 exhibits a better Hg° removal performance than pure MnOx. The total Hg° removal efficiency increases to ∼90 % during the experiment. The Hg° removal performances were tested at wide range of reaction temperatures. The Hg° removal efficiencies were calculated based on the total 60 min of reaction. Based on the results shown in Fig. 4b, SnS2 shows low Hg° removal efficiencies in the temperature range from 100 to 300 °C; the Hg° removal efficiencies are lower than 10 %. The MnOx shows the best Hg° removal efficiency at 150 °C. With the increases in the temperature from 150 to 300 °C, the Hg° removal efficiencies gradually decrease from 88 % to 37 %. The Mn-SnS2 composite exhibits high Hg° removal efficiencies at a wide range of temperatures. The removal efficiencies are higher than 87 % when the reaction temperatures are lower than or equal to 250 °C. At 300 °C, a Hg° removal efficiency of 61 % is obtained. Notably, when using Mn-modified SnS2, the reaction temperature window increased. Based on these results, the Mn modification enhances the Hg° removal efficiencies and enlarges the reaction temperature window. The Mn participates in the Hg° removal. Furthermore, we investigated the effects of the Mn loading concentration on the Hg° removal. The Mn mass ratio based on the results of the experiment is 10 % of the total mass. Fig. S4 shows that the Hg° removal efficiency remains higher than 90 %, even after 120 min of reaction. When 20 % of Mn is loaded on the surface of SnS2, the Hg° removal efficiency decreases. After 120 min of reaction, the Hg° removal efficiency is lower than 40 %. One could speculate that excess surface Mn on the surface might block the sulfur adsorption sites. When the surface sulfur sites are fully occupied, the composite gradually loses its Hg° removal efficiency. In addition, when the surface Mn mass ratio decreases to 5 %, the Hg° removal efficiency also decreases compared with that at a Mn mass ration of 10 %. The Hg° removal performance is ∼50 % after 120 min. An insufficient amount of surface-active sites for Hg° also results in a decrease in the activity.
Fig. 5. Hg-TPD profiles of Mn-SnS2, MnOx, and SnS2 at a heating rate of 5 °C/ min.
larger pore volume (∼5.9 cm3 · g−1) and pore size (∼27.23 nm). When Mn was used for surface modification, the surface structure was partly destroyed, resulting in the decrease in the surface area (∼15.2 m2 · g−1). The pore volume and average pore size changed to 3.50 m3 · g−1 and 20.84 nm, respectively. We further analyzed the SEM image of the Mn-SnS2 composite, which exhibits a different morphology than pure SnS2, as shown in Fig. S3. The flower-like morphology was destroyed because of the surface Mn modification. The new morphology is similar to an aerogel structure; many small pores were generated on the surface. The EDS maps for S, Mn, and Sn show that the surface is primarily characterized by Mn; S and Sn are inside the composite, especially Sn. For the surface element analysis, XPS profiles were obtained, which can be used to detect the elements within several nanometers of depth and the valance state of each element. The results are shown in Fig. 3. Fig. 3a shows the profile of Mn 2p. The peaks centered at 644.3 and 641.1 eV can be ascribed to surface Mn4+ and Mn3+, respectively (Xu et al., 2015). The Sn 3d profile in Fig. 3b shows two peaks at 494.8 and 486.4 eV, which are associated with Sn 3d3/2 and Sn 2d5/2, respectively (Tu et al., 2016). The splitting energy of these two peaks is 8.4 eV, which is representative for Sn4+ in SnS2 (Khan et al., 2017). The S 2 s profile in Fig. 3c shows two peaks centered at 162.6 and 161.3 eV, which can be ascribed to surface-active S* and lattice sulfur (S2−), respectively (Tu et al., 2016). The ratio of S* to S2- ranges from 28 % to 72 %. Most of the sulfur elements are used for the bonding of transition metal elements. Based on this discussion, the structure proposed for Mn-SnS2 is shown in Fig. 3d. The Mn covers the surface of this composite and Sn primary exists within the structure. Such a structure is conducive for the catalytic performance of Mn.
3.2.2. Mercury bonding analysis for Mn-SnS2 The Hg-TPD profiles of Mn-SnS2, MnOx, and SnS2 were measured at a desorption heating rate of 5 °C/min. Based on the results shown in Fig. 5, MnOx has a wide desorption peak centered at 345 °C, which is due to HgO (Xu et al., 2015). The intensity of the mercury desorption signal of SnS2 is weak and the peak is centered at 262 °C. This desorption peak is much lower than due to Hg-O bonding, indicating that
Scheme 1. Reaction pathway for the catalytic oxidation of gaseous Hg0 and chemical adsorption of Mn-SnS2. 5
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Fig. 6. (a) Effects of SO2 and O2 on the Hg0 removal efficiencies of the Mn-SnS2 composite; (b) Effects of H2O and SO2 on the Hg0 removal efficiencies of the Mn-SnS2 composite. Reaction conditions: total flow rate: 350 mL/min, temperature: 150 °C, 300 μg/m3 of Hg°, GHSV = 493,827 h−1, catalyst mass =10 mg.
500 and 1000 ppm SO2 are added to the simulated flue gas, high Hg° removal efficiencies are obtained. The Hg° removal efficiencies are higher than 80 % after 120 min of reaction. However, when 1500 ppm SO2 is added to the simulated flue gas, the Hg° removal efficiency is gradually lost, which could explain that parts of the high-valance Mn (Mn4+ or Mn3+) are reduced to a lower valance (Mn3+ or Mn2+) in the reducing atmosphere. Therefore, the adsorbed mercury cannot easily oxidize, resulting in the decrease in the activity. The effects of H2O based on the addition of 2 % H2O vapor to the simulated flue gas were also evaluated. The results are shown in Fig. 6b. When 2 % H2O is added to the simulated flue gas, the Hg° removal efficiency decreases to ∼20 % of the total removal efficiency compared with that at 5 % O2. Moreover, the coexistence of 2 % H2O and 500 ppm SO2 in the simulated flue gas has a toxic effect on the Hg° removal performance of Mn-SnS2. Notably, the H2O vapor has a toxic effect on the Hg° removal efficiency, especially when SO2 and H2O coexist in the flue gas. Based on these results, Mn generates more small surface pores and primary exists at the surface of the Mn-SnS2 composite after Mn modification. The Sn is covered by surface Mn layers and S acts as a bridge between Mn and Sn. The Sn accelerates the Hg° oxidation through the electron transfer of Mn. Mercury primary exists as HgS on the surface of Mn-SnS2. It can be used at a very wide temperature range. In addition, it has a good SO2 resistance when the SO2 concentration is lower than 1500 ppm. However, when SO2 and H2O coexist in the flue gas, the flue gas has a toxic effect on Mn-SnS2.
mercury is weakly bonded on the surface of SnS2. However, mercury desorbs from the surface of Mn-SnS2 and the associated peak is centered at 312 °C, that is, at a lower temperature than that of MnOx but at a higher temperature than that of SnS2. In addition, only one regular high-intensity peak can be observed, indicating that a high amount of mercury is adsorbed on the surface of Mn-SnS2. 3.2.3. Hg0 removal mechanism of Mn-SnS2 After Hg° adsorption, we further investigated the surface elements of the spent Mn-SnS2. Fig. S5 shows the profiles of Mn 2p, Sn 3d, S 2 s, and Hg 4f. The profiles in Fig. S5a show two primary peaks at 645.2 and 641.2 eV, which can be attributed to Mn4+ and Mn3+. This means that the profile did not change during the reaction. Similarly, the profile of Sn 3d (Fig. S5b) shows the two primary Sn4+peaks. The S 2 s profile in Fig. S5c shows two peaks at 162.6 and 161.3 eV, which can also be ascribed to S* and S2−, respectively. However, the ratio of S* to S2− changed from 23 % to 77 %, indicating that parts of the S* was lost during the reaction process. This active S* can combine with the surface mercury. Fig. S5d shows the profile of Hg 4f. One mercury peak can be observed at ∼102.1 eV, which can be assigned to surface Hg2+. The oxidized mercury can only form bonds with surface sulfur species. Note that mercury exists as HgS on the surface of the spent material. As shown in Scheme 1, during the reaction (i, ii) gaseous Hg0 is firstly adsorbed in surface vacancies or sulfur defects ([]); (iii, iv) the adsorbed mercury is then oxidized by Mn and higher-valance Mn (Mn4+) is reduced to its lower state (Mn3+). During this step, the electron transfer performance of Sn4+ ↔ Sn2+ is favorable for the reduction of Mn4+ → Mn3+, accelerating the oxidation process of Hg0 oxidation; (v, vi) after Hg0 is oxidized to Hg2+, surface-active S* combines with Hg2+ and forms HgS. The reduced Mn3+ returns to its original state after losing one electron to Sn; and (vii, viii) the SnS generated by the material is intermediate and can be attributed to the dominant tin vacancy associated with the Sn(II) (Whittles et al., 2016). However, SnS2 displays an n-type conductivity, which is due to the dominant sulfur vacancies associated with the Sn(IV) oxidation state. When Hg2+ occupies the sulfur vacancy, the sulfur in SnS could replenish such sites, resulting in the transition of Sn(II) to its original state. After these reactions, gaseous Hg0 is captured by Mn-SnS2 and exists as stable HgS on the surface of the composite.
4. Conclusion In this study, we prepared the Mn-SnS2 composite for the capture of gaseous Hg0 under SO2 condition. Such composite exhibit better performance for Hg0 adsorption due to the active S* species. The absence of surface oxygen guaranteed that the surface active adsorption sites cannot be destroyed by high concentration of SO2. During the reaction, Mn element was the primary oxidation site for gaseous Hg0, Sn played a role of promoter, while S was the bonding site for surface mercury. Based on this strategy, some related materials can be further designed to capture Hg0 under SO2-rich condition. Declaration of competing interest
3.3. Effects of SO2 and H2O on the Hg0 removal performance of Mn-SnS2
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The SO2 is a gaseous component in the flue gas and has a severe toxic effect on many transition metal oxides. In this study, we investigated the effects of SO2 and O2 on the Hg° removal performances of a Mn-SnS2 composite. The results are shown in Fig. 6a. The Hg° removal efficiencies under N2 + 5 % O2 and pure N2 conditions are almost the same. The O2 does not play an important role in the reactions. When
CRediT authorship contribution statement Haomiao 6
Xu:
Conceptualization,
Writing
-
original
draft.
Journal of Hazardous Materials 392 (2020) 122230
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Yongpeng Ma: Writing - original draft. Bailong Mu: Validation, Formal analysis. Wenjun Huang: Validation, Formal analysis. Qinyuan Hong: Formal analysis. Yong Liao: Formal analysis. Zan Qu: Supervision. Naiqiang Yan: Supervision, Writing - review & editing.
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Acknowledgments This study was supported by the National Key R&D Program of China (2017YFC0210500) and National Natural Science Foundation of China (Nos 21806105 and 21677096). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2020.122230. References Bai, B., Qiao, Q., Arandiyan, H., Li, J., Hao, J., 2016. Three-dimensional ordered mesoporous MnO2-supported Ag nanoparticles for catalytic removal of formaldehyde. Environ. Sci. Technol. 50, 2635–2640. Chang, H., Li, J., Chen, X., Ma, L., Yang, S., Schwank, J.W., Hao, J., 2012. Effect of Sn on MnOx–CeO2 catalyst for SCR of NOx by ammonia: enhancement of activity and remarkable resistance to SO2. Catal. Commun. 27, 54–57. Chang, H., Chen, X., Li, J., Ma, L., Wang, C., Liu, C., Schwank, J.W., Hao, J., 2013. Improvement of activity and SO2 tolerance of Sn-modified MnO x–CeO2 catalysts for NH3-SCR at low temperatures. Environ. Sci. Technol. 47, 5294–5301. Chen, J., Chen, X., Xu, W., Xu, Z., Jia, H., Chen, J., 2018. Homogeneous introduction of CeOy into MnOx-based catalyst for oxidation of aromatic VOCs. Appl. Catal. B 224, 825–835. Fang, R., Huang, H., Ji, J., He, M., Feng, Q., Zhan, Y., Leung, D.Y., 2018. Efficient MnOx supported on coconut shell activated carbon for catalytic oxidation of indoor formaldehyde at room temperature. Chem. Eng. J. 334, 2050–2057. Gu, T., Jin, R., Liu, Y., Liu, H., Weng, X., Wu, Z., 2013. Promoting effect of calcium doping on the performances of MnOx/TiO2 catalysts for NO reduction with NH3 at low temperature. Appl. Catal. B 129, 30–38. Hao, R., Dong, X., Wang, Z., Fu, L., Han, Y., Yuan, B., Gong, Y., Zhao, Y., 2019. Elemental mercury removal by a method of ultraviolet-heat synergistically catalysis of H2O2halide complex. Environ. Sci. Technol. Hao, R., Wang, Z., Gong, Y., Ma, Z., Qian, Z., Luo, Y., Yuan, B., Zhao, Y., 2020. Photocatalytic removal of NO and Hg0 using microwave induced ultraviolet irradiating H2O/O2 mixture. J. Hazard. Mater. 383, 121135. Huang, Y., Sun, Y., Xu, Z., Luo, M., Zhu, C., Li, L., 2017. Removal of aqueous oxalic acid by heterogeneous catalytic ozonation with MnOx/sewage sludge-derived activated carbon as catalysts. Sci. Total Environ. 575, 50–57. Khan, Z., Parveen, N., Ansari, S.A., Senthilkumar, S., Park, S., Kim, Y., Cho, M.H., Ko, H., 2017. Three-dimensional SnS2 nanopetals for hybrid sodium-air batteries. Electrochim. Acta 257, 328–334. Li, J., Yan, N., Qu, Z., Qiao, S., Yang, S., Guo, Y., Liu, P., Jia, J., 2009. Catalytic oxidation of elemental mercury over the modified catalyst Mn/α-Al2O3 at lower temperatures. Environ. Sci. Technol. 44, 426–431. Li, J., Chang, H., Ma, L., Hao, J., Yang, R.T., 2011. Low-temperature selective catalytic reduction of NOx with NH3 over metal oxide and zeolite catalysts—a review. Catal. Today 175, 147–156. Li, H., Wu, C.-Y., Li, Y., Zhang, J., 2012. Superior activity of MnOx-CeO2/TiO2 catalyst for catalytic oxidation of elemental mercury at low flue gas temperatures. Appl. Catal. B 111, 381–388. Li, S., Liu, X., Liu, Y., Liu, F., Luo, J., Pan, F., 2017. Optimized hetero-interfaces by tuning 2D SnS2 thickness in Bi2Te2. 7Se0. 3/SnS2 nanocomposites to enhance thermoelectric performance. Nano Energy 39, 297–305. Li, H., Zhu, W., Yang, J., Zhang, M., Zhao, J., Qu, W., 2018. Sulfur abundant S/FeS 2 for efficient removal of mercury from coal-fired power plants. Fuel 232, 476–484. Liao, Y., Xu, H., Liu, W., Ni, H., Zhang, X., Zhai, A., Quan, Z., Qu, Z., Yan, N., 2019. One step interface activation of ZnS using cupric ions for mercury recovery from nonferrous smelting flue gas. Environ. Sci. Technol. Liu, L., Yu, Q., Zhu, J., Wan, H., Sun, K., Liu, B., Zhu, H., Gao, F., Dong, L., Chen, Y., 2010. Effect of MnOx modification on the activity and adsorption of CuO/Ce0. 67Zr0. 33O2 catalyst for NO reduction. J. Colloid Interface Sci. 349, 246–255. Ma, Y., Qu, Z., Xu, H., Wang, W., Yan, N., 2014. Investigation on mercury removal method from flue gas in the presence of sulfur dioxide. J. Hazard. Mater. 279, 289–295. Ma, Y., Mu, B., Yuan, D., Zhang, H., Xu, H., 2017. Design of MnO2/CeO2-MnO2 hierarchical binary oxides for elemental mercury removal from coal-fired flue gas. J. Hazard. Mater. 333, 186–193.
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Enhancing the catalytic oxidation of elemental mercury and suppressing sulfur-toxic adsorption sites from SO2-containing gas in Mn-SnS2
T
Haomiao Xua,c, Yongpeng Maa,b, Bailong Mub, Wenjun Huanga, Qinyuan Honga, Yong Liaoa, Zan Qua,c, Naiqiang Yana,c,* a
School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Henan Collaborative Innovation Center of Environmental Pollution Control and Ecological Restoration, School of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou, 450001, China c Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China b
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Editor: Danmeng Shuai
It is difficult to stabilize gaseous elemental mercury (Hg°) on a sorbent from SO2-containing industrial flue gas. Enhancing Hg° oxidation and activating surface-active sulfur (S*) can benefit the chemical mercury adsorption process. A Mn-SnS2 composite was prepared using the Mn modification of SnS2 nanosheets to expose more Mn oxidation and sulfur adsorption sites. The results indicate that Mn-Sn2 exhibits better Hg° removal performances at a wide temperature range of 100–250 °C. A sufficient amount of surface Mn with a valance state of Mn4+ is favorable for Hg° oxidation, while the electron transfer properties of Sn can accelerate this oxidation process. Oxidized mercury primary exists as HgS with surface S*. A larger surface area, stable crystal structure, and active valance state of each element are favorable for Hg° oxidation and adsorption. The Mn-SnS2 exhibits an excellent SO2 resistance when the SO2 concentration is lower than 1500 ppm. The effects of H2O and O2 were also evaluated. The results show that O2 has no influence, while H2O and SO2 coexisting in the flue gas have a toxic effect on the Hg° removal performance. The Mn-SnS2 has a great potential for the Hg° removal from SO2containing flue gas such as non-ferrous smelting gas.
Keywords: Gaseous mercury Metal sulfide sulfur dioxide Surface activation Adsorption
⁎
Corresponding author. E-mail address: [email protected] (N. Yan).
https://doi.org/10.1016/j.jhazmat.2020.122230 Received 9 December 2019; Received in revised form 30 January 2020; Accepted 3 February 2020 Available online 08 February 2020 0304-3894/ © 2020 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 392 (2020) 122230
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1. Introduction Manganese-based oxides (MnOx) have good catalytic oxidation and adsorption performances suitable for the removal of gaseous elemental mercury (Hg0) from industrial flue gas (Xu et al., 2017a; Li et al., 2012, 2009). During the catalytic oxidation process, the adsorbed Hg0 can be catalytically oxidized to Hg2+ along with the reduction of higher-valance Mn (Mn4+ or Mn3+) to lower-valance Mn (Mn3+ or Mn2+). The oxidized Hg2+ can then form bonds with the surface oxygen (HgO). The keys for this Hg0 removal process are the utilization of the oxidation activity of Mn and sufficient amount of surface oxygen for mercury stabilization (Li et al., 2009; Xu et al., 2017b). However, Mn-based oxides often suffer from several limitations when used for gaseous heterogeneous reactions: 1) particle aggregation: pure MnOx particles easily aggregate; thus, not all surface MnOx active sites can be fully used. Supported MnOx hybrids are often designed to obtain a high dispersion of MnOx nanoparticles. As catalytic carriers of MnOx particles, TiO2 (Wu et al., 2010; Gu et al., 2013), Ce–Zr solid solution (Shen et al., 2014; Liu et al., 2010), and carbon-based materials (Su et al., 2013; Huang et al., 2017; Fang et al., 2018) were used. In our previous studies, we observed that graphene nanosheets can confine the MnOx particles, which then grow larger under hydrothermal synthesis conditions, enhancing the Hg° removal performances; (Xu et al., 2015) and 2) low activity at high temperature: although MnOx has been widely used for the catalytic combustion of volatile organic compounds (VOCs) (Chen et al., 2018; Bai et al., 2016; Hao et al., 2020) and low temperature NH3-selective catalytic reduction (NH3-SCR) for NOx removal, (Xu et al., 2017a; Li et al., 2011; Tian et al., 2012) the catalytic activities sharply decrease at high temperature (> 200 °C). At such a high temperature, electron transfer in MnOx is not favorable. The use of good electron conductors, such as carbon-based materials and SnO2, can widen the reaction temperature window (Xu et al., 2015; Xie et al., 2014). However, the most severe problem is limitation 3), that is, the toxic effect of SO2 in the gas: MnOx has a high selectivity of SO2 to SO3 when it is exposed to SO2-containing flue gas. The surface adsorbed SO2 can easily oxidize to SO3 and form surface sulfate, occupying the surface active sites (Chang et al., 2012; YAN et al., 2016; Zhang et al., 2017). Such a toxic effect limits the utilization of several SO2-containing flue gases. In the past, MnOx-based materials were often used for coal-fired types of flue gas (Xu et al., 2017a). The concentration of SO2 in the coalfired flue gas is not very high and provides an optimum reaction temperature (∼150 °C in dust removal devices) (Ma et al., 2017). However, many recent studies have focused on non-ferrous smelting gas, which contains high concentrations of Hg° and SO2. Non-ferrous smelting industries are also regarded as the primary mercury emission sources (Zhang et al., 2012; Pirrone et al., 2010; Pacyna et al., 2010). In contrast to coal-type flue gas, the raw materials for non-ferrous smelting often exist as sulfides and coexist with mercury in ores. During the smelting process at high temperature, SO2 and gaseous mercury are generated in the smelting gas. In the past, the Boliden absorption method, which is based on the use of HgCl2 as absorbent for Hg°, was utilized for mercury recycling (Ma et al., 2014). However, HgCl2 is also a contaminant and the “Minamata convention” requires the replacement of the mercury product in industrial applications (Selin, 2014). In addition, although Hg2Cl2 is one of the useful products of this absorption method, the removal of Hg2Cl2 from the absorption system is a challenge (Ma et al., 2014; Hao et al., 2019). The key for the control of mercury from this SO2-containing flue gas is the development of novel removal methods for Hg° in the gas phase. They will benefit the quality of the product (H2SO4) but also the decrement of complexity acid wastewater (which is generated in a wet scrubbing system). Several sulfur-based materials have been developed to capture mercury in SO2-containing flue gas. Polysulfur powders, transition sulfides (e.g., CuS, FeS, FeS2, and ZnS), and sulfur clusters reportedly have good gaseous Hg0 removal performances (Li et al., 2018; Yang
Fig. 1. XRD analysis of the prepared MnOx, SnS2, and Mn-SnS2.
et al., 2018; Zhao et al., 2018; Liao et al., 2019; Xu et al., 2017c). Li et al. reported a series of studies on nanosized transition metal sulfides, such as nanosized ZnS, CuS, and FeS, which can be used to capture gaseous Hg0 (Li et al., 2018; Yang et al., 2018; Zhao et al., 2018). Surface Hg-S species form on the surface of these nanomaterials. The HgS is one stable mercury species in the environment. The red HgS can transform into black HgS at a temperature of 344 °C, while it decomposes at 584 °C (Sedlar et al., 2015). The formation of HgS on the material surface benefits mercury stabilization. In our previous studies, we prepared [MoS4]2− clusters to capture mercury from S-Hg mixed flue gas (Xu et al., 2017c). Such a composite exhibits a good SO2 resistance and Hg-S is the primary mercury bonding state. The H2S modification method was developed to capture mercury from the flue gas; H2S-modified Fe–Ti spinel oxide exhibits an excellent Hg0 capture performance (Zou et al., 2017). The activated surface generates more sulfur bonding sites, enhancing the Hg0 removal performance. Based on these studies, surface-active S* species, such as polysulfur and active S2−, are the primary mercury adsorption sites. Oxidized mercury (Hg2+) forms during this chemical adsorption process. Accelerating the surface Hg° oxidation process will generate more surface Hg2+, which might be favorable for the bonding of mercury with surface S*. As discussed above, Mn is a good choice because of its high oxidation–reduction potential. However, pure MnSx has a very low activity when it is used for Hg° capture. The poor electron transfer performance of Mn in sulfide lead to an inactivation. Therefore, it is better to build a bridge between Mn and S. The SnS2, one of the semiconductors, has excellent electron transfer properties. It is one of the n-type semiconductors with a bandgap of 2.18–2.44 eV (Zhang et al., 2010; Oh et al., 2011). The SnS2 exhibits good oxidative and thermal stabilities in air. Chang et al. used Sn to modify the selective catalysis reduction (SCR) catalyst. The results indicated that Sn enhances the Lewis acidity and improves the NO oxidation performance (Chang et al., 2013). In addition, Sn-modified materials exhibit excellent SO2 resistances at a wide range of temperatures. The absence of surface adsorption oxygen can also reduce the SO2 poisoning. Therefore, we attempted to design a material with a new structure that enhances the roles of Mn, Sn, and S. Based on the above-mentioned discussion about the limitations of MnOx, the new structure should yield a high dispersion of Mn. Two-dimensional SnS2 nanosheets exhibit a huge surface area for surface Mn loading (Seo et al., 2008; Li et al., 2017). Using the SnS2 as the core of the composite and Mn as the surface modification might be favorable for the catalytic oxidation process. In this study, a novel Mn-SnS2 composite was prepared for the capture of gaseous Hg0 under SO2 conditions. The reactions were conducted in a fixed-bed reactor. The Hg0 removal performances and 2
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Fig. 2. TEM images of (a) and (b): SnS2, (c) and (d): MnOx, and (e) and (f): Mn-SnS2.
rate of 5°/min and angle range of 5°–80°. The BET surface area was calculated based on the nitrogen adsorption–desorption method. The SEM and TEM images were obtained using high-resolution electron microscopy. The EDS mapping was selected for the elemental analysis. For the surface analysis, XPS profiles were measured on the fresh and spent samples. The mercury temperature-programmed desorption (HgTPD) method was used to determine the mercury bonding state of the prepared materials.
Table 1 BET surface areas, pore volumes and pore sizes of the synthesized Mn-SnS2, MnOx and SnS2 samples. Samples
Surface area (m2·g−1)
Pore volume (cm3·g−1)
Pore size (nm)
Mn-SnS2 MnOx SnS2
15.2 5.4 25.7
3.50 0.03 5.90
20.84 4.34 27.23
2.3. Catalytic oxidation and adsorption performance studies effects of the temperature, SO2, and H2O on the performances were evaluated. The Hg0 removal mechanism was also discussed.
The catalytic oxidation and adsorption performances of gaseous Hg° were evaluated using a fixed-bed reactor (Xu et al., 2017c). In this reaction system, the gas generation unit provides the required concentration of O2, SO2, H2O, and Hg° and all gas components are carried by pure N2. For the Hg° removal experiments, 300 μg/m3 was selected. The concentration of SO2 used for the investigation of the effects of SO2 ranged from 500 to 1500 ppm. A total of 2 % of H2O and 5 % of O2 were chosen for the experiments. The reaction temperature ranged from 100 to 300 °C. To avoid the effects of SO2, Lumex RA915+ was used as mercury detector. The Hg° removal efficiencies were calculated based on the inlet and outlet concentrations of Hg°.
2. Experimental section 2.1. Preparation of Mn-SnS2 composites The Mn-SnS2 composite was prepared using a solvent-thermal method (Khan et al., 2017). First, 3.506 g of SnCl4·5H2O and 1.5026 g of thioacetamide (TAA) were dissolved in 50 mL polyethylene glycol (PEG-600) and the mixture was constantly stirred to obtain a transparent solution. Subsequently, a suitable concentration of Mn (NO3)2·4H2O was added to the solution and the mixture was stirred for 3 h. The mixture was then transferred into a Teflon-lined stainless steel autoclave and maintained at 180 °C for 12 h. Finally, the product was collected until the autoclave cooled down to air temperature, washed with ethanol and deionized water several times, and dried at 60 °C for 12 h. For comparison, pure SnS2 was prepared using the same method but without the addition of Mn(NO3)2·4H2O to the mixed solution. For MnOx preparation, a chemical precipitation method was selected. The Mn(NO3)2·4H2O was firstly dissolved in deionized water and then NH3·H2O was added to the solution until the precipitate formed. The precipitate was then collected using filtration and calcined under 500 °C in a muffle furnace. For the Hg° removal experiments, all prepared materials were ground to a 40–60 mesh particle size.
3. Results and discussion 3.1. Structure analysis of the prepared materials Powder XRD was used for the structure analysis of the prepared materials. Fig. 1 shows the MnOx peaks, which can be ascribed to the (211), (222), (400), (332), (440), and (622) phases. These peaks are primary peaks and in accordance with the standard PDF card #71-0636 for manganese oxide (Zhang et al., 2014). The peaks detected for the prepared SnS2 agree with those listed on the PDF card #23-0677 (Zai et al., 2012). After Mn modification, the primary peaks of SnS2 still existed at the same peak positions. Several newly generated peaks were ascribed to manganese sulfides (Tang et al., 2015). The peak intensities of the (110) and (111) reflections weakened. The reason could be that Mn primarily occupied these phases. Notably, the addition of Mn during the synthesis did not change the crystal structure of SnS2. However, some manganese sulfides were generated during the synthesis.
2.2. Material characterization For the crystal structure analysis, powder XRD was used at a scan 3
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Fig. 3. XPS analysis of the Mn-SnS2 composite; (a) Mn 2p, (b) Sn 3d, and (c) S 2 s; (d) nanostructure proposed for Mn-SnS2. Fig. 4. (a) Hg° removal of the prepared MnOx, SnS2, and Mn-SnS2 samples, temperature: 150 °C; and (b) Effects of different temperatures on the Hg° removal performance of the prepared materials, reaction time: 60 min. Reaction conditions: total flow rate: 350 mL/ min; 300 μg/m3 of Hg°, 5 % of O2; GHSV = 493,827h−1; catalyst mass =10 mg.
size is uniform (∼5 nm). As shown in Fig. 2f, the lattice spacing of the exposed facets changes to 0.33 nm on the surface due to the generation of Mn sulfides. After Mn modification, the particle size is smaller, which is beneficial for the exposure of more surface-active sites during catalytic reactions of gases. We analyzed the surface areas and pore characteristics of the prepared samples. The N2 adsorption–desorption isotherms are illustrated in Fig. S2. The BET surface areas are provided in Table 1. The BET surface areas of pure MnOx and SnS2 are 5.4 and 25.7 m2 · g−1, respectively. The surface area of Mn-SnS2 is 15.2 m2 · g−1, that is, three times larger than that of pure MnOx. However, it is smaller than that of SnS2. The pore volume and average pore size were calculated using the BJH method. The pore volume of MnOx is only 0.03 cm3 · g−1 and the pore size is 4.34 nm. The TEM images show that the bulk particles are aggregated. Therefore, the pore size and pore volume are small. However, the flower-like SnS2 exhibits a large layered structure and thus a
Furthermore, we obtained TEM images of the prepared Mn-SnS2 sample; the images of SnS2 and MnOx were also collected for comparison. As shown in Fig. 2a, SnS2 has large and thin 2D layers and the length of the thin nanosheets is very large. The morphology of such SnS2 was further indicated by the SEM images, as shown in Fig. S1. Flower-like SnS2 was prepared using the hydrothermal method, resulting in these thin layers. The lattice spacing of 0.59 nm shown in Fig. 2b confirms that the (001) facets represent the exposed surface nanosheets (Seo et al., 2008). These results agree with the XRD results; the (001) facets exhibit a high intensity. The bulk particles of MnOx can be observed in the TEM images, as shown in Fig. 2c. The particle sizes of these bulk particles are not uniform but range from 30 to 100 nm. Fig. 2d shows the lattice fringes, indicating a spacing of ∼0.25 nm, which corresponds with the (222) facets of Mn3O4. In the Mn-SnS2 composite, the larger nanosheet layers are thicker and several small particles are generated on the surface, as shown in Fig. 2e. The particle 4
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3.2. Hg0 removal over Mn modified SnS2 3.2.1. Hg0 removal performance of the prepared materials The Hg° removal performances of the prepared samples were evaluated in a fixed-bed reactor. Based on Fig. 4a, pure SnS2 shows almost no Hg° removal efficiency. A 40 % removal efficiency is obtained in the first several minutes of the experiment. Subsequently, SnS2 gradually loses its activity; after 60 min of reaction, the activity has ceased. Pure MnOx, which reportedly has a good Hg° removal performance, also exhibits a good Hg° removal performance in our study. The total Hg° removal efficiency is 80 % during 60 min of reaction. After Mn modification, Mn-SnS2 exhibits a better Hg° removal performance than pure MnOx. The total Hg° removal efficiency increases to ∼90 % during the experiment. The Hg° removal performances were tested at wide range of reaction temperatures. The Hg° removal efficiencies were calculated based on the total 60 min of reaction. Based on the results shown in Fig. 4b, SnS2 shows low Hg° removal efficiencies in the temperature range from 100 to 300 °C; the Hg° removal efficiencies are lower than 10 %. The MnOx shows the best Hg° removal efficiency at 150 °C. With the increases in the temperature from 150 to 300 °C, the Hg° removal efficiencies gradually decrease from 88 % to 37 %. The Mn-SnS2 composite exhibits high Hg° removal efficiencies at a wide range of temperatures. The removal efficiencies are higher than 87 % when the reaction temperatures are lower than or equal to 250 °C. At 300 °C, a Hg° removal efficiency of 61 % is obtained. Notably, when using Mn-modified SnS2, the reaction temperature window increased. Based on these results, the Mn modification enhances the Hg° removal efficiencies and enlarges the reaction temperature window. The Mn participates in the Hg° removal. Furthermore, we investigated the effects of the Mn loading concentration on the Hg° removal. The Mn mass ratio based on the results of the experiment is 10 % of the total mass. Fig. S4 shows that the Hg° removal efficiency remains higher than 90 %, even after 120 min of reaction. When 20 % of Mn is loaded on the surface of SnS2, the Hg° removal efficiency decreases. After 120 min of reaction, the Hg° removal efficiency is lower than 40 %. One could speculate that excess surface Mn on the surface might block the sulfur adsorption sites. When the surface sulfur sites are fully occupied, the composite gradually loses its Hg° removal efficiency. In addition, when the surface Mn mass ratio decreases to 5 %, the Hg° removal efficiency also decreases compared with that at a Mn mass ration of 10 %. The Hg° removal performance is ∼50 % after 120 min. An insufficient amount of surface-active sites for Hg° also results in a decrease in the activity.
Fig. 5. Hg-TPD profiles of Mn-SnS2, MnOx, and SnS2 at a heating rate of 5 °C/ min.
larger pore volume (∼5.9 cm3 · g−1) and pore size (∼27.23 nm). When Mn was used for surface modification, the surface structure was partly destroyed, resulting in the decrease in the surface area (∼15.2 m2 · g−1). The pore volume and average pore size changed to 3.50 m3 · g−1 and 20.84 nm, respectively. We further analyzed the SEM image of the Mn-SnS2 composite, which exhibits a different morphology than pure SnS2, as shown in Fig. S3. The flower-like morphology was destroyed because of the surface Mn modification. The new morphology is similar to an aerogel structure; many small pores were generated on the surface. The EDS maps for S, Mn, and Sn show that the surface is primarily characterized by Mn; S and Sn are inside the composite, especially Sn. For the surface element analysis, XPS profiles were obtained, which can be used to detect the elements within several nanometers of depth and the valance state of each element. The results are shown in Fig. 3. Fig. 3a shows the profile of Mn 2p. The peaks centered at 644.3 and 641.1 eV can be ascribed to surface Mn4+ and Mn3+, respectively (Xu et al., 2015). The Sn 3d profile in Fig. 3b shows two peaks at 494.8 and 486.4 eV, which are associated with Sn 3d3/2 and Sn 2d5/2, respectively (Tu et al., 2016). The splitting energy of these two peaks is 8.4 eV, which is representative for Sn4+ in SnS2 (Khan et al., 2017). The S 2 s profile in Fig. 3c shows two peaks centered at 162.6 and 161.3 eV, which can be ascribed to surface-active S* and lattice sulfur (S2−), respectively (Tu et al., 2016). The ratio of S* to S2- ranges from 28 % to 72 %. Most of the sulfur elements are used for the bonding of transition metal elements. Based on this discussion, the structure proposed for Mn-SnS2 is shown in Fig. 3d. The Mn covers the surface of this composite and Sn primary exists within the structure. Such a structure is conducive for the catalytic performance of Mn.
3.2.2. Mercury bonding analysis for Mn-SnS2 The Hg-TPD profiles of Mn-SnS2, MnOx, and SnS2 were measured at a desorption heating rate of 5 °C/min. Based on the results shown in Fig. 5, MnOx has a wide desorption peak centered at 345 °C, which is due to HgO (Xu et al., 2015). The intensity of the mercury desorption signal of SnS2 is weak and the peak is centered at 262 °C. This desorption peak is much lower than due to Hg-O bonding, indicating that
Scheme 1. Reaction pathway for the catalytic oxidation of gaseous Hg0 and chemical adsorption of Mn-SnS2. 5
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Fig. 6. (a) Effects of SO2 and O2 on the Hg0 removal efficiencies of the Mn-SnS2 composite; (b) Effects of H2O and SO2 on the Hg0 removal efficiencies of the Mn-SnS2 composite. Reaction conditions: total flow rate: 350 mL/min, temperature: 150 °C, 300 μg/m3 of Hg°, GHSV = 493,827 h−1, catalyst mass =10 mg.
500 and 1000 ppm SO2 are added to the simulated flue gas, high Hg° removal efficiencies are obtained. The Hg° removal efficiencies are higher than 80 % after 120 min of reaction. However, when 1500 ppm SO2 is added to the simulated flue gas, the Hg° removal efficiency is gradually lost, which could explain that parts of the high-valance Mn (Mn4+ or Mn3+) are reduced to a lower valance (Mn3+ or Mn2+) in the reducing atmosphere. Therefore, the adsorbed mercury cannot easily oxidize, resulting in the decrease in the activity. The effects of H2O based on the addition of 2 % H2O vapor to the simulated flue gas were also evaluated. The results are shown in Fig. 6b. When 2 % H2O is added to the simulated flue gas, the Hg° removal efficiency decreases to ∼20 % of the total removal efficiency compared with that at 5 % O2. Moreover, the coexistence of 2 % H2O and 500 ppm SO2 in the simulated flue gas has a toxic effect on the Hg° removal performance of Mn-SnS2. Notably, the H2O vapor has a toxic effect on the Hg° removal efficiency, especially when SO2 and H2O coexist in the flue gas. Based on these results, Mn generates more small surface pores and primary exists at the surface of the Mn-SnS2 composite after Mn modification. The Sn is covered by surface Mn layers and S acts as a bridge between Mn and Sn. The Sn accelerates the Hg° oxidation through the electron transfer of Mn. Mercury primary exists as HgS on the surface of Mn-SnS2. It can be used at a very wide temperature range. In addition, it has a good SO2 resistance when the SO2 concentration is lower than 1500 ppm. However, when SO2 and H2O coexist in the flue gas, the flue gas has a toxic effect on Mn-SnS2.
mercury is weakly bonded on the surface of SnS2. However, mercury desorbs from the surface of Mn-SnS2 and the associated peak is centered at 312 °C, that is, at a lower temperature than that of MnOx but at a higher temperature than that of SnS2. In addition, only one regular high-intensity peak can be observed, indicating that a high amount of mercury is adsorbed on the surface of Mn-SnS2. 3.2.3. Hg0 removal mechanism of Mn-SnS2 After Hg° adsorption, we further investigated the surface elements of the spent Mn-SnS2. Fig. S5 shows the profiles of Mn 2p, Sn 3d, S 2 s, and Hg 4f. The profiles in Fig. S5a show two primary peaks at 645.2 and 641.2 eV, which can be attributed to Mn4+ and Mn3+. This means that the profile did not change during the reaction. Similarly, the profile of Sn 3d (Fig. S5b) shows the two primary Sn4+peaks. The S 2 s profile in Fig. S5c shows two peaks at 162.6 and 161.3 eV, which can also be ascribed to S* and S2−, respectively. However, the ratio of S* to S2− changed from 23 % to 77 %, indicating that parts of the S* was lost during the reaction process. This active S* can combine with the surface mercury. Fig. S5d shows the profile of Hg 4f. One mercury peak can be observed at ∼102.1 eV, which can be assigned to surface Hg2+. The oxidized mercury can only form bonds with surface sulfur species. Note that mercury exists as HgS on the surface of the spent material. As shown in Scheme 1, during the reaction (i, ii) gaseous Hg0 is firstly adsorbed in surface vacancies or sulfur defects ([]); (iii, iv) the adsorbed mercury is then oxidized by Mn and higher-valance Mn (Mn4+) is reduced to its lower state (Mn3+). During this step, the electron transfer performance of Sn4+ ↔ Sn2+ is favorable for the reduction of Mn4+ → Mn3+, accelerating the oxidation process of Hg0 oxidation; (v, vi) after Hg0 is oxidized to Hg2+, surface-active S* combines with Hg2+ and forms HgS. The reduced Mn3+ returns to its original state after losing one electron to Sn; and (vii, viii) the SnS generated by the material is intermediate and can be attributed to the dominant tin vacancy associated with the Sn(II) (Whittles et al., 2016). However, SnS2 displays an n-type conductivity, which is due to the dominant sulfur vacancies associated with the Sn(IV) oxidation state. When Hg2+ occupies the sulfur vacancy, the sulfur in SnS could replenish such sites, resulting in the transition of Sn(II) to its original state. After these reactions, gaseous Hg0 is captured by Mn-SnS2 and exists as stable HgS on the surface of the composite.
4. Conclusion In this study, we prepared the Mn-SnS2 composite for the capture of gaseous Hg0 under SO2 condition. Such composite exhibit better performance for Hg0 adsorption due to the active S* species. The absence of surface oxygen guaranteed that the surface active adsorption sites cannot be destroyed by high concentration of SO2. During the reaction, Mn element was the primary oxidation site for gaseous Hg0, Sn played a role of promoter, while S was the bonding site for surface mercury. Based on this strategy, some related materials can be further designed to capture Hg0 under SO2-rich condition. Declaration of competing interest
3.3. Effects of SO2 and H2O on the Hg0 removal performance of Mn-SnS2
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The SO2 is a gaseous component in the flue gas and has a severe toxic effect on many transition metal oxides. In this study, we investigated the effects of SO2 and O2 on the Hg° removal performances of a Mn-SnS2 composite. The results are shown in Fig. 6a. The Hg° removal efficiencies under N2 + 5 % O2 and pure N2 conditions are almost the same. The O2 does not play an important role in the reactions. When
CRediT authorship contribution statement Haomiao 6
Xu:
Conceptualization,
Writing
-
original
draft.
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Yongpeng Ma: Writing - original draft. Bailong Mu: Validation, Formal analysis. Wenjun Huang: Validation, Formal analysis. Qinyuan Hong: Formal analysis. Yong Liao: Formal analysis. Zan Qu: Supervision. Naiqiang Yan: Supervision, Writing - review & editing.
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