Effect of molybdenum incorporation on the activity of magnetic Fe-Mn

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Effect of molybdenum incorporation on the activity of magnetic Fe-Mn sorbent for the capture of elemental mercury Qiang Zhou, Xin Tao, Yu Lei, Yibo Liu, Ping Lu, and Yunjun Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04391 • Publication Date (Web): 24 Feb 2019 Downloaded from http://pubs.acs.org on February 25, 2019

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Effect of molybdenum incorporation on the activity of magnetic Fe-Mn sorbent for the capture of elemental mercury Qiang Zhou*, Xin Tao, Yu Lei, Yibo Liu, Ping Lu, Yunjun Wang Engineering Laboratory for Energy System Process Conversion & Emission Control Technology of Jiangsu Province, School of Energy and Mechanical Engineering, Nanjing Normal University, Nanjing 210042, China

ABSTRACT: A molybdenum (Mo) incorporated magnetic Fe-Mn sorbent (Fe-Mn-Mo) was developed to capture elemental mercury from flue gas. The effect of Mo incorporation on the mercury capture performance and physicochemical properties of Fe-Mn sorbent was investigated. The mechanism for the functions of Mo on enhancing mercury capture activity was deeply studied. The fixed-bed test results display that a small amount of Mo incorporation (Mo/Mn=0.1) improves mercury capture performance, but excessive Mo plays an opposite role. The developed Fe-Mn-Mo-0.1 sorbent can keep good mercury capture activity at relatively low temperature as well as under the atmosphere without O2 existence. The sorbent characterization result shows that the developed Fe-Mn-Mo-0.1 sorbent owns good magnetization property, which makes it feasible to be separated from fly ash with addition of an external magnetic field. Mo incorporation (Mo/Mn=0.1) slightly increases the BET area, and reduces the agglomeration and crystallization degree of Fe-Mn particles. Mo incorporation is in the form of Mo6+, which promotes the generation of more high activity Mn4+ and lattice oxygen, and increases the total oxygen storage of Fe-Mn sorbent. Mo incorporation does not change the mechanism of mercury capture, and the mercury capture process can be described by the Mars–Maessen mechanism with the adsorption product of mercurous oxide. KEY WORDS: Mo incorporation; Magnetic sorbent; Mercury capture; Function mechanism

*Correspondence author: Qiang Zhou ([email protected]) ACS Paragon Plus Environment

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1. Introduction On August 16th, 2017, the “Minamata Convention on Mercury” entered into force and there are 128 countries all over the word signing the convention for reducing global mercury emission [1,2]. It is well known that excessive anthropogenic mercury emission is threatening the environment and human health [3,4]. At present, coal-fired power plant is believed to be one of the largest anthropogenic mercury sources [5-7]. All of mercury emitted from boiler furnace is elemental mercury (Hg0). As flue gas temperature reduces, part of Hg0 is converted to oxidized mercury (Hg2+) and particle-bound mercury (HgP) [8-11]. Most of water-soluble Hg2+ is adsorbed by wet flue gas desulfurization (WFGD) [12,13]. HgP in fly ash is removed by electrostatic precipitators (ESPs) or fabric filters (FFs) [14,15]. However, Hg0 cannot be removed by existing pollution control devices and directly discharges to the atmosphere [16-19]. Activated carbon injection (ACI) technology has been considered to be the most effective method for in-duct Hg0 removal from coal-fired power plant [20,21]. The injected activated carbon powder can adsorb gas phase Hg0 and Hg2+ to form Hgp within a short residence time. The ACI technology can effectively reduce the gas-phase mercury emission to the atmosphere [22], but the adsorbed gas-phase mercury is moved to coal-fired fly ash, which is an important resource for cement production. In the reuse process of fly ash, high temperature and leaching cause the re-release of mercury into the environment. Therefore, in order to reduce the secondary emission of mercury from fly ash, development of magnetic mercury sorbents that can be separated from fly ash has received widespread attention. Currently, various magnetic mercury sorbents have been synthesized, including magnetic zeolitesilver nanoparticles (MagZAg0) [23], FeMnOx [24], MnOx/γ-Fe2O3 [25], Mn-Fe spinel [26], and Fe-TiMn spinel [27] etc. Among these sorbents, low-cost magnetic Fe-Mn sorbent has attracted wide attention

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[28,29]. Zeng et al [24] synthesized a magnetic FeMnOx sorbent treated by non-thermal plasma, and they found that the plasma treated FeMnOx sorbent was an excellent recyclable sorbent for mercury removal. Yang et al [25] synthesized a series of magnetic Mn/γ-Fe2O3 sorbents and found that the 15%-Mn/γFe2O3-400 sorbent obtained a mercury removal rate of 49% during 10 h. It can be clearly seen that magnetic Fe-Mn sorbent is a promising mercury sorbent, but some existing problems restrict the practical application, including limited mercury capture activity, low sulfur resistance and lacking of efficient regeneration method. This is because the active component Mn4+ in Fe-Mn sorbent is not a high oxygen storage material [30] and easy to be poisoned by SO2 [31]. In order to improve the mercury capture activity and enhance the sulfur resistance of Fe-Mn sorbent, it is feasible to incorporate another transition metal into Fe-Mn sorbent [32]. Studies have shown that transition metal molybdenum (Mo) can effectively improve the oxidative activity and sulfur tolerance of catalyst [33,34]. Li et al [33] found that Mo incorporation can significantly enhance the Hg0 catalytic conversion and sulfur resistance of Mn/γ-Al2O3 catalyst, because Mo modified the dispersion of MnOx particles and formed a compound with stronger sulfur resistance. Guo et al [34] found that Mo incorporation on the catalyst can obviously improve the Hg0 removal efficiency and sulfur-tolerance, because Mo owned a larger affinity with sulfur compared to Mn, which decreased the deactivation of Mn-base active sites and diminished the competitive adsorption between Hg0 and SO2 on Mn. Currently, the related literatures regarding to Mo incorporated magnetic Fe-Mn sorbent has not been reported, so the effect of Mo incorporation on the activity of magnetic Fe-Mn sorbent for the capture of elemental mercury is worthy of in-depth study. Therefore, in presented work, a Mo incorporated magnetic Fe-Mn sorbent (Fe-Mn-Mo) was developed to capture elemental mercury from flue gas. The effect of Mo incorporation on the mercury capture


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performance and physicochemical properties of Fe-Mn sorbent was investigated. The mechanism for the functions of Mo on enhancing mercury capture activity was deeply studied. The effect of Mo incorporation on sulfur resistance of magnetic Fe-Mn sorbent will be further explored in another work. 2. Experimental part 2.1 Sorbent synthesis Magnetic Fe-Mn-Mo sorbents were synthesized by using co-precipitation method. Firstly, a mixture solution including ferrous sulfate, ferric chloride, and manganese sulfate was prepared. The concentration of Fe2+, Fe3+, and Mn2+ in the mixture solution is 0.10 molL-1, respectively. Secondly, ammonium molydate was weighed and added into the mixture solution. After the complete dissolution of ammonium molydate, the solution was poured into NH3H2O solution and stirred with a magnetic stirrer. The rotating speed of the magnetic stirrer was kept at 800 rpm for 2 h until precipitate completely formed. The generated precipitate was separated from the turbid solution by filtration. Deionized water was used to wash the precipitate for three times and then the precipitate was dried at 105 ℃ in an oven (12 h). The calcination (3 h) of the precipitate was carried out in a tube furnace under air atmosphere where the temperature was kept at 500 ℃. After calcination and crushing, the magnetic Fe-Mn-Mo-X sorbents were synthesized. The Mo incorporation amount was presented by the X in Fe-Mn-Mo-X, in which X stood for the molar ratio of Mo to Mn. In order to screen suitable Mo incorporation amount, FeMn-Mo-0.1, Fe-Mn-Mo-0.3, and Fe-Mn-Mo-0.5 were prepared. Due to the very weak magnetism feature, the Fe-Mn-Mo-0.5 was not investigated in this paper. The preparation procedure of magnetic Fe-Mn sorbent was consistent with that of Fe-Mn-Mo, except the addition of ammonium molydate. 2.2 Mercury capture test apparatus The tests of elemental mercury capture by the synthesized sorbents were carried out in a fixed-bed


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system (Figure 1). The volume flow rate of the simulated flue gas was fixed at 2.0 L/min, which was composed by Hg0 vapor, 6% O2, and balance N2. An Hg0 permeation device (VICI Metronics Inc, USA) was utilized to generate stable Hg0 vapor. Pure N2 was used as Hg0 carrier gas and kept at 300 ml/min with the inlet Hg0 vapor concentration stabled at 30.0±0.5 μg/m3. The gas cylinders were applied to generate stable pure N2 and O2, and mass flow controllers were used to control the volume flow rate. The flue gas mixing and preheating was completed before flowing through the adsorption column. The mercury adsorption column was enclosed in a vertical tube furnace with temperature controlled by a thermocouple. The mercury adsorption column was made up of a quartz glass tube and whose internal diameter was designed as 20 mm. During each test, 200mg of sorbent was placed evenly on the quartz wool in the mercury adsorption column. The Hg0 concentration in flue gas was detected online by the VM3000 mercury analyzer. The exhaust gas was treated by using activated carbon. The mercury capture performance of the synthesized sorbent was evaluated by using Hg0 breakthrough rate as well as Hg0 uptake, which has been defined in our previous work [35]. 2.3 Sorbent characterization In order to further explore the mechanism for the functions of Mo, the physicochemical properties of Fe-Mn and Fe-Mn-Mo were characterized as a comparison. A vibrating sample magnetometer was used to detect the magnetization characteristic of sorbent. N2 adsorption and desorption tests of sorbents were conducted by using the ASAP 2020 analyzer (Micromeritics) to obtain the BET specific surface area and pore parameters. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) pattern analysis were carried out at the same time by using an electron microscope (JEOL 2100) to obtain the microscopic morphology and crystal structure, respectively. X-ray diffraction (XRD) analysis was conducted by using the D8 Advance analyzer to test out the crystalline substance on sorbent. X-ray


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photoelectron spectroscopy (XPS) analysis was carried out by using the ESCALAB 250xi analyzer to obtain the chemical valence change of Fe 2p, Mn 2p, Mo 3d, O 1s, and Hg 4f on sorbent. 3. Results and discussion 3.1 Mercury capture performance 3.1.1 Effect of Mo incorporation amount The effect of Mo incorporation amount on mercury capture performance was studied in the fixed-bed system. The flue gas temperature was stabled at 150 ℃, and the flue gas component concluded 6% O2 and balance N2. As Figure 2(a) shows, the Fe-Mn-Mo-0.1 displays the best mercury capture performance compared to the Fe-Mn and Fe-Mn-Mo-0.3. Within 150 mins, the Hg0 breakthrough rate of the Fe-MnMo-0.1 keeps lower than 20%, and the Fe-Mn-Mo-0.3 obtains the worst mercury capture performance. In Figure 2(b), the Hg0 uptake of the Fe-Mn-Mo-0.1 within 150 min reaches 38.5 μg/g and is slightly higher than that of raw Fe-Mn, whose Hg0 uptake is 35.1μg/g. However, the Fe-Mn-Mo-0.3 only achieves 25.5μg/g Hg0 uptake, which is far lower than that of the Fe-Mn-Mo-0.1. Therefore, it can be concluded that a small amount of Mo incorporation (Mo/Mn=0.1) can improve the mercury capture performance, but excessive Mo cannot modify mercury capture activity of Fe-Mn sorbent. 3.1.2 Effect of flue gas temperature From above experiment, it is clear that the Fe-Mn-Mo-0.1 sorbent owns the best mercury capture performance at 150 ℃, so in this section, the mercury capture activity of the Fe-Mn-Mo-0.1 at other temperatures was further studied. Figure 3(a) shows that at 100 ℃, the Fe-Mn-Mo-0.1 obtains very similar Hg0 breakthrough rate with that at 150 ℃, and at 200 ℃, the Hg0 breakthrough rate almost keeps at approximately 30% within 150 min, which is worse than that at 150 ℃. Figure 3(b) shows that the maximum Hg0 uptake appears at 100 ℃ reaching 39.7μg/g, and with the rise of temperature, the Hg0


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uptake continuously decreases. It illustrates that the developed Fe-Mn-Mo-0.1 sorbent displays good mercury capture capability at relatively low temperature. Li et al [33] has obtained similar result that the addition of Mo significantly improved the Hg adsorption and the catalytic performance of Mn/γ-Al2O3 at lower temperatures. 3.1.3 Effect of O2 The role of 6% O2 existing in flue gas on mercury capture by the Fe-Mn-Mo-0.1 was explored through comparative experiment. A simulated flue gas without 6% O2 was generated at the temperature of 150 ℃. As shown in Figure 4(a), within first 120 mins, the Hg0 breakthrough rate under pure N2 atmosphere is very close to that under the atmosphere with 6% O2 existence, but in the last 30 mins, the Hg0 breakthrough rate slightly increases. In Figure 4(b), without the existence of 6% O2, the Hg0 uptake of the Fe-Mn-Mo-0.1 within 150 mins is similar to that under the atmosphere with 6% O2, whose Hg0 uptake reaches 38.2 μg/g. It can clearly be seen that the 6% O2 in flue gas has very little effect on the mercury capture by the Fe-Mn-Mo-0.1 within 120 min. With the continuous increase of test time, O2 in flue gas somewhat enhances the mercury capture, and this may be because O2 can supplement the consumed active oxygen. The obtained experimental result is different from that of Guo et al [34] and he found that O2 played an important role on the Hg0 removal by Mo-Ru-Mn. 3.2 VSM analysis From above fixed-bed test results, it can be found that the Fe-Mn-Mo-0.1 displays the best mercury capture activity, so the Fe-Mn-Mo-0.1 sorbent was selected to be characterized. Magnetism is one of the most important properties of the synthesized Fe-Mn-Mo-0.1. From Figure 5, it can be found that the FeMn-Mo-0.1 owns good magnetization property with the existence of an external magnetic field. The magnetic field intensity increasing from 0 to 20000 Oe leads to the magnetization of the Fe-Mn-Mo-0.1


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increasing from 0 to 2.65 emu/g. Therefore, From Figure 5, it can be concluded that it is feasible to separate the Fe-Mn-Mo-0.1 from fly ash with addition of an external magnetic field, and after the external magnetic field disappears, the Fe-Mn-Mo-0.1 will not be permanently magnetized which makes the sorbent re-disperse without aggregation. 3.3 Pore structure The N2 adsorption/desorption curves of sorbents are given in Figure 6 with the abscissa of relative pressure (P/P0) and ordinate of volume. From Figure 6, it can be seen that the synthesized Fe-Mn, FeMn-Mo-0.1, and Fe-Mn-Mo-0.3 sorbents own similar N2 adsorption/desorption curves. The gently growth of volume at small P/P0 displays the sorbents owning very few micropores (50 nm) [36]. According to Figure 6, the Fe-Mn-Mo-0.1 has more mesopores or macropores compared to the Fe-Mn and Fe-Mn-Mo-0.3. Figure 7 shows that the pore diameter of the sorbents mainly centers between 1~3 nm as well as 7~40nm. The Mo incorporation reduces the volume of pore diameter at 1~3 nm and there is a small amount of macropores (50~100 nm) appearing in the surface of the Fe-Mn-Mo0.1. From Table 1, it can be found that the BET area of raw Fe-Mn is 32.948 m2/g, and Mo incorporation slightly increases the BET area. The BET area of the Fe-Mn-Mo-0.1 reaches 42.406 m2/g, which is larger than that of the Fe-Mn-Mo-0.3. The pore volume and mean pore diameter of raw Fe-Mn sorbent are 0.198 cm3/g and 24.08 nm, respectively. A small amount of Mo incorporation (Mo/Mn=0.1) increases the pore volume and mean pore diameter, but excessive Mo incorporation (Mo/Mn=0.3) reduces the pore volume and mean pore diameter. 3.4 TEM analysis Figure 8 shows that the synthesized Fe-Mn-Mo-0.1 is constituted by a large number of extremely small


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nanoparticles and the particle sizes mainly center at 20~40 nm. The Fe-Mn-Mo-0.1 sorbent agglomerates to form many irregularly shaped particles and some of which own a core/shell structure. From Figure 9, it can be found that the TEM spectrograms of raw Fe-Mn are similar with those of the Fe-Mn-Mo-0.1, but the raw Fe-Mn displays a greater degree of agglomeration, which indicates that Mo incorporation somewhat mitigates the agglomeration of Fe-Mn particles. The SAED spectrum in Figure 9 shows that the raw Fe-Mn displays obvious single crystal diffraction spots indicating that there is only one kind of crystal structure existence. However, the SAED pattern of the Fe-Mn-Mo-0.1 (in Figure 8) shows that part of diffraction ring disappears, which indicates the missing of crystal structure on the surface of the Fe-Mn-Mo-0.1. 3.5 XRD analysis Figure 10 displays that the raw Fe-Mn owns multiple diffraction peaks between 20◦ and 80◦. The peaks at 2θ=30.1°, 35.5°, 43.1°, 53.4°, 57.0°, and 62.6° can correspond very well to the (220), (311), (400), (422), (511), and (440) diffraction peaks of Fe3O4. However, in the diffraction scan, the diffraction peak of crystalline manganese oxide (MnOx) is not found, which indicates that MnOx is well dispersed as an amorphous phase in the raw Fe-Mn. The XRD pattern verifies that the raw Fe-Mn presents in a single crystal structure. Figure 10 also shows that almost no FeOx peak is observed in the XRD spectrum of the Fe-Mn-Mo-0.1, which indicates that Fe possibly exists in an amorphous or highly disperse phase on the surface of the Fe-Mn-Mo-0.1 after Mo incorporation. It is visible that the XRD analysis result is in line with the above SAED analysis. Therefore, it can be concluded that Mo incorporation can promote the dispersion of Fe-Mn particle and reduce the degree of crystallization. 3.6 XPS analysis Figure 11(a) shows that at binding energy of 709.9 eV and 711.1 eV, there are two Fe spectrum peaks


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appearing in the raw Fe-Mn, which are attributed to Fe 2p3/2 and Fe 2p1/2, respectively, representing the existence of Fe3+. The presence of Fe2+ can also be demonstrated by the broad and asymmetrical distribution of Fe 2p3/2 peak [37]. As Figure 11(c) shows, the Fe spectrum peaks of the Fe-Mn-Mo-0.1 are similar with those of raw Fe-Mn, which illustrates that Mo incorporation has almost no effect on the valence state of Fe 2p. As Figure 11(b) and Figure 11(d) show, the spent Fe-Mn and Fe-Mn-Mo-0.1 display the same spectral regions of Fe 2p compared to the fresh Fe-Mn and Fe-Mn-Mo-0.1, which indicates that Fe is not involved in the reaction of mercury capture. Figure 11(e) shows that at binding energy of 642.6 eV and 641.2 eV, there are two Mn spectrum peaks existing in the Fe-Mn sorbent. The spectrum peak at binding energy of 642.6 eV indicates the presence of Mn4+, and the other one at binding energy of 641.2 eV displays the existence of Mn3+. As Figure 11(g) shows, after Mo incorporation, Mn4+ (643.1 eV) and Mn3+ (641.6 eV) are still the main existence form in the Fe-Mn-Mo-0.1. The missing of characteristic peak at 640.4 eV illustrates that Mn2+ does not exist in the fresh Fe-Mn and Fe-Mn-Mo-0.1. The concentration ratio of Mn4+/Mn3+ on sorbents is calculated from the peak area ratio and shown in Table 2. The Mn4+/Mn3+ concentration ratio of the fresh Fe-Mn is 1.614 and after Mo incorporation, the Mn4+/Mn3+ concentration ratio of fresh Fe-Mn-Mo-0.1 increases to 2.267, which indicates that Mo incorporation on the Fe-Mn sorbent promotes more high activity Mn4+ generation. As shown in Figure 11(f) and Figure 11(h), there is no clear change appearing on the position of Mn 2p characteristic peaks on the spent Fe-Mn and Fe-Mn-Mo-0.1. However, from Table 2, it can be found that the Mn4+/Mn3+concentration ratios of the spent Fe-Mn and Fe-Mn-Mo-0.1 decrease to 0.807 and 1.049, respectively, which indicates that during the process of mercury capture, part of Mn4+ converts to Mn3+ without Mn2+ cations generation.


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As shown in Figure 11(i), there are two spectrum peaks of Mo 3d existing in the fresh Fe-Mn-Mo-0.1 including Mo 3d3/2 (232.7 eV) and Mo 3d5/2 (235.9 eV), which indicates the existence of Mo6+. In Figure 11(j), the spent Fe-Mn-Mo-0.1 displays very similar spectrum regions of Mo 3d with fresh Fe-Mn-Mo0.1, which indicates that after mercury capture, there is no valence state change on the Mo element. As shown in Figure 11(k), there are two spectrum peaks of O 1s appearing in the fresh Fe-Mn. One spectrum peak at binding energy of 530.1 eV is attributed to the characteristic peak of lattice oxygen and the other one at binding energy of 532.2 eV belongs to the characteristic peak of adsorption oxygen. Figure 11(m) shows that the fresh Fe-Mn-Mo-0.1 also owns lattice oxygen (530.3 eV) and adsorption oxygen (532.0 eV), and whose concentration can be determined from the area of spectrum peak. From Table 2, it can be seen that the total oxygen area of the fresh Fe-Mn is 15293.9, in which lattice oxygen and adsorbed oxygen area are 9251.3 and 6042.6, respectively. After Mo incorporation, the total oxygen area of the fresh Fe-Mn-Mo-0.1 reaches 15519.7, which is slightly larger than that of the fresh Fe-Mn. The lattice oxygen area of the fresh Fe-Mn-Mo-0.1 increases to 13929.5 and the adsorption oxygen area reduces to 1590.2. It indicates that Mo incorporation increases the total oxygen storage and promotes more lattice oxygen generation. Compared to the fresh Fe-Mn and Fe-Mn-Mo-0.1 sorbents, as Figure 11(l) and Figure 11(n) show, the position of O 1s spectrum peaks on the spent Fe-Mn and Fe-Mn-Mo-0.1 displays no obvious change. However, as can be seen from Table 2, the total oxygen area of the spent Fe-Mn and Fe-Mn-Mo-0.1 reduces to 11612.8 and 13689.6, respectively. The decrease amount of total oxygen area of the spent FeMn-Mo-0.1 is 1830.1 which is half of that of the spent Fe-Mn. It indicates that during the process of mercury capture on the Fe-Mn-Mo-0.1, part of consumed oxygen was supplemented by the O2 in flue gas, which can be verified by the significant increase of adsorbed oxygen area (from 1590.2 to 2604.0)


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on the spent Fe-Mn-Mo-0.1. From Table 2, it can also be found that in the process of mercury capture on the Fe-Mn, the consumed oxygen is adsorbed oxygen (from 6042.6 to 2061.8), but during mercury capture on the Fe-Mn-Mo-0.1, lattice oxygen is consumed (from 13929.5 to 11085.5). 3.7 Mechanism analysis As can be seen from Figure 12(a), there are two spectrum peaks of Hg 4f centering at 94.1 eV and 100.4 eV appearing in the spent Fe-Mn. As Figure 12(b) shows, the Hg 4f spectra peak of the Fe-MnMo-0.1 is similar with that of spent Fe-Mn, centering at 94.3 ev and 100.9 ev, respectively. Due to the absence of Hg 4f5/2 at binding energy of 105 eV, mercury adsorbed by the Fe-Mn and Fe-Mn-Mo-0.1 is believed to be Hg2O [26]. Therefore, it can be concluded that Mo incorporation does not change the mechanism of mercury capture, and the mercury capture process can be described by the Mars–Maessen mechanism [38]. Firstly, Hg0 transfers to the sorbent surface, and is adsorbed physically by the cation vacancies. Then, the physically adsorbed Hg0 reacts with Mn4+ cation on the surface of Fe-Mn and FeMn-Mo-0.1 to form Hg1+, which combines with lattice oxygen or adsorption oxygen to form Hg2O. The mechanism for the functions of Mo on enhancing mercury capture performance of Fe-Mn sorbent can be summarized as follows: in the preparation process of the Fe-Mn-Mo-0.1, the Mo incorporation modifies the dispersion of Fe-Mn particles, and promotes more high activity Mn4+ and lattice oxygen generation. In the process of mercury capture, Mo6+ in the Fe-Mn-Mo-0.1 sorbent can still oxidize part of reduced Mn3+ to re-generate Mn4+. The reaction process can be described as follows [33]. Reaction (1) illustrates that in the processes of both sorbent preparation and mercury capture, part of Mn3+ in the FeMn-Mo-0.1 sorbent can be oxidized by Mo6+ to form Mn4+ with the Mo6+ reduced to Mo4+. Reaction (2) indicates that the Mo4+ can be further oxidized by the O2 in flue gas to re-generated Mo6+. Mo6+ + Mn3+  Mn4+ + Mo5+


(1)

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2Mo5+ + 0.5O2  2Mo6+ + O2-

(2)

4. Conclusion In presented work, a magnetic Fe-Mn-Mo-0.1 sorbent was developed to capture elemental mercury from flue gas. The effect of Mo incorporation on the mercury capture performance and physicochemical properties of Fe-Mn sorbent was investigated. The mechanism for the functions of Mo on enhancing mercury capture performance of Fe-Mn sorbent was deeply studied. The obtained results are summarized as follows: 1) A small amount of Mo incorporation (Mo/Mn=0.1) improves mercury capture activity, but excessive Mo plays an opposite role. The developed Fe-Mn-Mo-0.1 sorbent can keep good mercury capture capability at relatively low temperature as well as under the atmosphere without O2 existence. 2) The developed Fe-Mn-Mo-0.1 sorbent owns good magnetization property, which makes it feasible to be separated from fly ash with addition of an external magnetic field. Mo incorporation (Mo/Mn=0.1) slightly increases the BET area, and reduces the agglomeration and crystallization degree of Fe-Mn particles. 3) Mo incorporation is in the form of Mo6+, which promotes more high activity Mn4+ and lattice oxygen generation, and increases the total oxygen storage. Mo incorporation does not change the mechanism of mercury capture, and the mercury capture process can be described by the Mars– Maessen mechanism with the adsorption product of mercurous oxide. Acknowledgements


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This study is supported by the National Nature Science Foundation of China (51476079), China Postdoctoral Science Foundation (2017M621780), and Natural Science Foundation of the Jiangsu Higher Education Institutions of China (17KJD470003). References [1]

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S. Zhao, D. Chen, H. Xu, J. Mei, Z. Qu, P. Liu, Y. Cui, N. Yan, Combined effects of Ag and UiO66 for removal of elemental mercury from flue gas, Chemosphere 197 (2018) 65-72.

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[7]

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[8]

H. Li, Y. Wang, S. Wang, X. Wang, J. Hu, H. Li, Y. Wang, S. Wang, X. Wang, J. Hu, Removal of elemental mercury in flue gas at lower temperatures over Mn-Ce based materials prepared by coprecipitation, Fuel 208 (2017) 576-586.

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coal

fired

flue

gas

and

the

mechanism

investigation,

Environmental Science & Technology 50 (2016) 2564-2572. [10] H. Wang, Y.F. Duan, Y.N. Li, Y. Xue, M. Liu, Investigation of mercury emission and its speciation from an oxy-fuel circulating fluidized bed combustor with recycled warm flue gas, Chemical Engineering Journal 300 (2016) 230-235. [11] D. Liu, W. Zhou, J. Wu, Effect of Ce and La on the activity of CuO/ZSM-5 and MnOx/ZSM-5 composites for elemental mercury removal at low temperature, Fuel 194 (2017) 115-122. [12] C.S. Zhou, L.S. Sun, J. Xiang, S. Hu, S. Su, A.C. Zhang, The experimental and mechanism study of novel heterogeneous Fenton-like reactions using Fe3-xTixO4 catalysts for Hg0 adsorption, Proceedings of the Combustion Institute 3 (2015) 2875-2882. [13] S. Su, L. Liu, L. Wang, S. A. Syed-Hassan, F. Kong, S. Hu, Y. Wang, L. Jiang, K. Xu, A. Zhang, J. Xiang, Mass flow analysis of mercury transformation and effect of seawater flue gas desulfurization on mercury removal in a full scale coal-fired power plant, Energy Fuels 31 (2017) 11109-11116. [14] B. Zhao, H. Yi, X. Tang, Q. Li, D. Liu, F. Gao, Copper modified activated coke for mercury removal from coal-fired flue gas, Chemical Engineering Journal 286 (2016) 585-593.


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Energy & Fuels

[15] Y. Ma, B. Mu, D. Yuan, H. Zhang, H. Xu, Design of MnO2/CeO2-MnO2 hierarchical binary oxides for elemental mercury removal from coal-fired flue gas, Journal of Hazardous Materials 333 (2017) 186-193. [16] X. Li, Q. Huang, C. Luo, D. Zhou, Y. Xu, L. Zhang, C. Zheng, Effect of acid gases on elemental mercury removal in an oxy-fuel CO2 compression process, Energy Fuels 32 (2018) 4334-4340. [17] P. Hu, Y. Duan, W. Ding, J. Zhang, L. Bai, N. Li, H. Wei, Enhancement of mercury removal efficiency by activated carbon treated with nonthermal plasma in different atmospheres, Energy Fuels 31 (2017) 13852-13858. [18] F.H. Shen, J. Liu, Y.C. Dong, D.W. Wu, Mercury removal by biomass-derived porous carbon: Experimental and theoretical insights into the effect of H2S, Chemical Engineering Journal 348 (2018) 409-415. [19] L.C. Zhong, W.H. Li, Y.S. Zhang, P. Norris, Y. Cao, W.P. Pan, Kinetic studies of mercury adsorption in activated carbon modified by iodine steam vapor deposition method, Fuel 188 (2017) 343-351. [20] Y. Xiao, D. Pudasainee, R. Gupta, Z. Xu, Y. Diao, Bromination of petroleum coke for elemental mercury capture, Journal of Hazardous Materials 336 (2017) 232-239. [21] B. Shen, L. Tian, F. Li, X. Zhang, H. Xu, S. Singh, Elemental mercury removal by the modified bio-char from waste tea, Fuel 187 (2017) 189-196. [22] Y. Cao, Q.H. Wang, J. Li, J.C. Cheng, C.C. Chan, M. Cohron, W.P. Pan, Enhancement of mercury capture by the simultaneous addition of hydrogen bromide (HBr) and fly ashes in a slipstream facility, Environmental Science & Technology 43 (2009) 2812-2817. [23] J. Dong, Z.H. Xu, S.M. Kuznicki, Mercury removal from flue gases by novel regenerable magnetic nanocomposite sorbents, Environmental Science & Technology 43 (2009) 3266-3271. [24] X.B. Zeng, Y. Xu, B. Zhang, G.G. Luo, P. Sun, R.J. Zou, H. Yao, Elemental mercury adsorption and regeneration performance of sorbents FeMnOx enhanced via non-thermal plasma, Chemical Engineering Journal 309 (2017) 503-512. [25] S.J. Yang, Y.F. Guo, N.Q. Yan, Z. Qu, J.K. Xie, C. Yang, J.P. Jia, Capture of gaseous elemental mercury from flue gas using a magnetic and sulfur poisoning resistant sorbent Mn/γ-Fe2O3 at lower temperatures, Journal of Hazardous Materials 186 (2011) 508-515. [26] S.J. Yang, Y.F. Guo, N.Q. Yan, D.Q. Wu, H.P. He, Z. Qu, J.P. Jia, Elemental mercury capture from flue gas by magnetic Mn-Fe spinel: effect of chemical heterogeneity, Industrial & Engineering Chemistry Research 50 (2011) 9650-9656. [27] Y. Liao, S.C. Xiong, H. Dang, X. Xiao, S.J. Yang, P.K. Wong, The centralized control of elemental mercury emission from the flue gas by a magnetic regenerable Fe–Ti–Mn spinel, Journal of Hazardous Materials 299 (2015) 740-746. [28] Y.J. Yang, J. Liu, B.K. Zhang, F. Liu, Density functional theory study on the heterogeneous reaction between Hg0 and HCl over spinel-type MnFe2O4, Chemical Engineering Journal 308 (2017) 897903. [29] Y.J. Yang, J. Liu, B.K. Zhang, F. Liu, Mechanistic studies of mercury adsorption and oxidation by oxygen over spinel-type MnFe2O4, Journal of Hazardous Materials 321 (2017) 154-161. [30] Z.J. Zhou, X.W. Liu, Z.Q. Liao, H.Z. Shao, C. Lv, Y. C. Hu, M. H. Xu, Manganese doped CeO2ZrO2 catalyst for elemental mercury oxidation at low temperature, Fuel Processing Technology 152 (2016) 285-293. [31] H.Z. Chang, X.Y. Chen, J.H. Li, L. Ma, C.Z. Wang, C.X. Liu, J.W. Schwank, J.M. Hao,


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Improvement of activity and SO2 tolerance of Sn-modified MnOx-CeO2 catalysts for NH3-SCR at low temperatures, Environmental Science & Technology 47 (2013) 5294-5301. [32] A.C. Zhang, W.B. Xing, Z.H. Zhang, F.M. Meng, Z.C. Liu, J. Xiang, L.S. Sun, Promotional effect of SO2 on CeO2-TiO2 material for elemental mercury removal at low temperature, Atmospheric Pollution Research 7 (2016) 895-902. [33] J.F. Li, N.Q. Yan, Z. Qu, S.H. Qiao, S.J. Yang, Y.F. Guo, P. Liu, J.P. Jia, Catalytic oxidation of elemental

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Environmental Science & Technology 1 (2010) 426-431. [34] Y.F. Guo, N.Q. Yan, S.J. Yang, P. Liu, J. Wang, Zan. Qu, J.P. Jia, Conversion of elemental mercury with a novel membrane catalytic system at low temperature, Journal of Hazardous Materials 213214 (2012) 62-70. [35] Q. Zhou, Y. Lei, Y. Liu, X. Tao, P. Lu, Y. Duan, Y. Wang, Gaseous elemental mercury removal by magnetic Fe-Mn-Ce sorbent in simulated flue gas, Energy Fuels XXX (XXXX) XXX-XXX. (10.1021/acs.energyfuels.8b03445). [36] Q. Zhou, Y.F. Duan, Y.G. Hong, C. Zhu, M. She, J. Zhang, H.Q. Wei, Experimental and kinetic studies of gas-phase mercury adsorption by raw and bromine modified activated carbon, Fuel Processing Technology 134 (2015) 325-332. [37] Q. Xu, R.G. Su, L. Cao, Y.Q. Li, C.Y. Yang, Y. Luo, J. Street, P.C. Jiao, L.L. Cai, Facile preparation of high-performance Fe-doped Ce-Mn/TiO2 catalysts for the low-temperature selective catalytic reduction of NOx with NH3, Rsc Advances 7 (2017) 48785-48792. [38] A.A. Presto, E.J. Granite, Survey of catalysts for oxidation of mercury in flue gas, Environmental Science & Technology 40 (2006) 5601-5609.


Page 17 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

List of figures and tables Figure 1 Experimental system of mercury capture in a fixed-bed Figure 2 Effect of Mo incorporation amount on Hg0 capture: (a) Hg0 breakthrough rate; (b) Hg0 uptake Figure 3 Effect of temperature on Hg0 capture of Fe-Mn-Mo-0.1: (a) Hg0 breakthrough rate; (b) Hg0 uptake Figure 4 Effect of 6% O2 on Hg0 capture of Fe-Mn-Mo-0.1: (a) Hg0 breakthrough rate; (b) Hg0 uptake Figure 5 Magnetization feature of Fe-Mn-Mo-0.1 Figure 6 N2 adsorption/desorption isotherm curves of Fe-Mn with different Mo incorporation amount Figure 7 Pore size distribution of Fe-Mn with different Mo incorporation amount Figure 8 TEM and SAED spectrogram of Fe-Mn-Mo-0.1 Figure 9 TEM and SAED spectrogram of raw Fe-Mn Figure 10 XRD spectrogram of Fe-Mn and Fe-Mn-Mo-0.1 Figure 11 XPS spectra regions of Fe 2p, Mn 2p, Mo 3d, and O 1s on fresh and spent sorbents Figure 12 XPS spectra regions of Hg 4f: (a) Fe-Mn; (b) Fe-Mn-Mo-0.1 Table 1 BET area and pore parameters of Fe-Mn with different Mo incorporation amount Table 2 Concentration change of Mn 2p and O 1s on the fresh and spent sorbents


Energy & Fuels

Computer

O2

N2

Electrical Heating Belts

VM3000

Mercury Adsorption Column T

Activated Carbon Trap T

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 32

Mass Flow Controller

Hg0 Vapor Generator

Quartz Wool

Vertical Tube Furnace

N2 Temperature Controller

Figure 1 Experimental system of mercury capture in a fixed-bed


Page 19 of 32

50

80

Fe-Mn Fe-Mn-Mo-0.1 Fe-Mn-Mo-0.3

60

38.5

40

35.1

Hg0 uptake (μg/g)

Hg0 breakthrough rate (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

40

20

30

25.5

20

10

0

0

40

80

120

160

0

Fe-Mn

Time (min)

Fe-Mn-Mo-0.1

Fe-Mn-Mo-0.3

Sorbent

Figure 2 Effect of Mo incorporation amount on Hg0 capture: (a) Hg0 breakthrough rate; (b) Hg0 uptake


Energy & Fuels

50

50

100 ℃ 150 ℃ 200 ℃

40

40 Hg0 uptake (μg/g)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 32

30

20

10

39.7

38.5 32.6

30

20

10

0 0

40

80

120

0

160

100

Time (min)

150

200

Temperature (℃ )

Figure 3 Effect of temperature on Hg0 capture of Fe-Mn-Mo-0.1: (a) Hg0 breakthrough rate; (b) Hg0 uptake


Page 21 of 32

50

50

N2+6% O2 N2

40

40 Hg0 uptake (μg/g)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

30

20

10

38.5

38.2

30

20

10

0 0

40

80

120

160

0

Time (min)

N2

Conditions

N2 +6% O2

Figure 4 Effect of 6% O2 on Hg0 capture of Fe-Mn-Mo-0.1: (a) Hg0 breakthrough rate; (b) Hg0 uptake


Energy & Fuels

4 3

Magnetization (emu/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 32

2 1 0 -1 -2 -3 -4 -24000

-16000

-8000

0

8000

16000

24000

Magnetic field (Oe)

Figure 5 Magnetization feature of Fe-Mn-Mo-0.1


Page 23 of 32

200


160

Volume,stp (cm3/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

120

80

40

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

Figure 6 N2 adsorption/desorption isotherm curves of Fe-Mn with different Mo incorporation amount


Energy & Fuels

0.010

Incremental pore volume (cm3/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 32


0.008 0.006 0.004 0.002 0.000 1

10

100

Pore diameter (nm)

Figure 7 Pore size distribution of Fe-Mn with different Mo incorporation amount


Page 25 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 8 TEM and SAED spectrogram of Fe-Mn-Mo-0.1


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9 TEM and SAED spectrogram of raw Fe-Mn


Page 26 of 32

Page 27 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Fe-Mn-Mo-0.1

℃ 311℃

Fe-Mn

℃ 311℃

℃ 220℃

20

℃ 400℃

℃ 511℃℃ 440℃ ℃ 422℃

40

60

80

2θ/degree Figure 10 XRD spectrogram of Fe-Mn and Fe-Mn-Mo-0.1


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a) Fresh Fe-Mn 724.0 Fe 2p

(b) Spent Fe-Mn 725.0 Fe 2p

711.1 719.0

738

732

726

Page 28 of 32

713.0

714

708

738

732

726

Binding Energy/ev

726

711.4

656

652

720

714

708

732

726

720

(f) Spent Fe-Mn Mn 2p

708

640

641.6 643.1

636

656

652

Binding Energy/ev

(g) Fresh Fe-Mn-Mo-0.1 Mn 2p

714

Binding Energy/ev

641.2

644

711.4 710.3

712.8

719.4

642.6

648

708

710.2

Binding Energy/ev

(e) Fresh Fe-Mn Mn 2p

714

(d) Spent Fe-Mn-Mo-0.1 Fe 2p 724.6

718.4 713.2

732

720

Binding Energy/ev

(c) Fresh Fe-Mn-Mo-0.1 724.9 Fe 2p

738

713.7

718.8

720

710.5

711.8

709.9

648

644

640

636

Binding Energy/ev (h) Spent Fe-Mn-Mo-0.1 Mn 2p

642.7

643.0

641.6

641.4

656

652

648

644

Binding Energy/ev

640

636

656

652

648

644

Binding Energy/ev


640

636

Page 29 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

232.7

(i) Fresh Fe-Mn-Mo-0.1 Mo 3d

(j) Spent Fe-Mn-Mo-0.1 Mo 3d

235.9

238

232.9

236.0

236

234

232

230

238

236

Binding Energy/ev 530.1

(k) Fresh Fe-Mn O 1s

234

232

230

Binding Energy/ev 530.3

(l) Spent Fe-Mn O 1s

532.2 531.7

536

534

532

530

528

526

534

532

Binding Energy/ev (m) Fresh Fe-Mn-Mo-0.1 O 1s

530.3

(n) Spent Fe-Mn-Mo-0.1 O 1s

528

530.4

532

532.0

534

530

Binding Energy/ev

532

530

Binding Energy/ev

528

534

532

530

528

Binding Energy/ev

Figure 11 XPS spectra regions of Fe 2p, Mn 2p, Mo 3d, and O 1s on fresh and spent sorbents


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

94.1

(a)

Page 30 of 32

(b)

Hg 4f

Hg 4f

100.4

108

104

94.3

100.9

100

Binding Energy/ev

96

92

108

104

100

96

Binding Energy/ev

Figure 12 XPS spectra regions of Hg 4f: (a) Fe-Mn; (b) Fe-Mn-Mo-0.1


92

Page 31 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 1 BET area and pore parameters of Fe-Mn with different Mo incorporation amount Sample

Specific surface area

Pore volume

Mean pore diameter

(m2/g)

(cm3/g)

(nm)

Fe-Mn

32.948

0.198

24.08

Fe-Mn-Mo-0.1

42.406

0.268

25.31

Fe-Mn-Mo-0.3

39.155

0.187

19.12


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 32

Table 2 Concentration change of Mn 2p and O 1s on the fresh and spent sorbents Samples

Mn4+/Mn3+

Total oxygen

Lattice oxygen

Adsorbed oxygen

area

area

area

Fresh Fe-Mn

1.614

15293.9

9251.3

6042.6

Spent Fe-Mn

0.807

11612.8

9551.0

2061.8

Fresh Fe-Mn-Mo-

2.267

15519.7

13929.5

1590.2

1.049

13689.6

11085.5

2604.0

0.1 Spent Fe-Mn-Mo0.1


Effect of molybdenum incorporation on the activity of magnetic Fe-Mn

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