Journal of Hazardous Materials 283 (2015) 252–259
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Elemental mercury oxidation and adsorption on magnesite powder modified by Mn at low temperature Yalin Xu a,b , Qin Zhong a,∗ , Xinya Liu c a
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China Jiangsu Key Laboratory of Atmospheric Environment Monitoring & Pollution Control, School of Environmental Sciences and Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, PR China c Jiangsu HeYiChang Environmental Protection Engineering & Technology Co., Ltd., PR China b
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• It is a new idea to investigate the removal of elemental mercury over Mn/MgO at low temperature. • The amorphous MnO2 and chemisorbed O2 played key roles on the removal of elemental mercury. • Combined in situ FTIR with XPS and TG, the possible oxidation and adsorption mechanism is proposed and MnHgO3 is the product of elemental mercury oxidation over Mn/MgO.
a r t i c l e
i n f o
Article history: Received 9 June 2014 Received in revised form 1 September 2014 Accepted 2 September 2014 Available online 28 September 2014 Keywords: Elemental mercury Adsorbent Amorphous MnO2 Magnesite powder
a b s t r a c t Mn modified the commercial magnesite powder prepared by wet impregnation method has been shown to be effective for gas-phase elemental mercury (Hg0 ) removal at low temperatures. The prepared samples are characterized in detail across multiform techniques: XRF, BET, SEM-EDX, XRD, H2 -TPR, and XPS, and all the results show that the amorphous MnO2 impregnated on magnesite powder improves the removal efficiency of Hg0 . Through further analysis by TG and in situ FTIR, the reasonable removal mechanism is also speculated. The results indicate that chemisorbed oxygen is an important reactant in the heterogeneous reaction, and gas-phase Hg0 is adsorbed and then oxidized to solid MnHgO3 on the surface of the adsorbent. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Mercury (Hg) released from coal combustion is a typical heavy metal contaminant. Following SO2 , NOX , PM10 and PM2.5 , mercury has attracted increasing attention due to its toxicity, mobility, and bioaccumulation in the ecosystem and food chain [1,2]. Mercury emissions and speciation vary from site to site at coalfired power plants. This is mainly due to different boiler configurations, coals types,
∗ Corresponding author. Tel.: +86 25 84315517; fax: +86 25 84315517. E-mail address: [email protected] (Q. Zhong). http://dx.doi.org/10.1016/j.jhazmat.2014.09.034 0304-3894/© 2014 Elsevier B.V. All rights reserved.
and operating conditions. There are three main states of mercury in the flue gas: elemental (Hg0 ), gaseous divalent (Hg2+ ) and particulate-associated (Hgp ) [3,4], which have different physical and chemical properties. Hg2+ and Hgp are relatively easy to remove from flue gas by using conventional air pollution control devices, such as wet flue gas desulfurization (WFGD) systems, electrostatic precipitators (ESPs), or fabric filter (FF). Hg0 is difficult to be controlled by the aforementioned devices due to its high volatility and low solubility in water [5]. Several methods have been proposed for Hg0 removal during the past few decades, such as activated carbon injection (ACI), oxidation–reduction, and photochemical oxidation [6]. ACI can remove 80–98% of elemental mercury from flue gas, however, considering the high operation cost and complex separation progress from ultrafine fly ash, new methods are still required. Recent research has been shown that heterogeneous oxidation played an important
Y. Xu et al. / Journal of Hazardous Materials 283 (2015) 252–259 role on converting Hg0 to Hg2+ catalytically [7,8]. The involved oxidants were mainly chlorine [9]. In China, the chlorine content in feed-coal varies from 63 to 318 mg/kg, which is much lower than the average value of US coals (628 mg/kg) [10]. Because there is an amount of residual O2 in the real flue gas, it is economically practical to use oxygen as the oxidant in the flue gas, compared with Cl2 , HCl, HBr, etc. Furthermore, the oxidized mercury can exist as the solid state on the surface of adsorbents when the temperature is lower than 400 ◦ C. Hence Hg0 will be adsorbed by the adsorbents and then be subsequently removed from the flue gas [11]. A variety of potential catalysts for mercury oxidation, including Co/TiO2 , Fe/ZSM-5, V2 O5 /AC, Fe2 O3 /TiO2 , MnO2 /AC and CeO2 /SiO2 , have been employed in experiments [12–15]. Especially, transition metal oxides deposited on different supports, possess cost-effective oxidation activity. Among them, manganese oxides showed good oxidization capabilities at low temperatures. Granite et al. [11] found the alumina-supported manganese dioxide (MnO2 ) showed high elemental mercury capacity in the temperature range from 60 to 177 ◦ C. It was also reported that TiO2 -supported by MnOX CeO2 was effective for the removal of elemental mercury at low temperatures [16,17]. Considering the good oxidation performance of manganese at low temperature, manganese supported on MgO was chosen to perform the removal of the elemental mercury in simulated flue gas. The reason for this choice was that magnesium oxide acted as both good support and co-catalysts used in the oxidation reaction [18]. This provides an efficient mercury abatement technology, compatible with the existing conventional air pollution control devices. In this study, a series of Mn/MgO were synthesized using a simple wet-impregnation method and characterized by X-ray fluorescence (XRF), Xray diffraction (XRD), N2 adsorption/desorption isotherms, scanning electron microscopy with energy dispersive X-ray analyses (SEM-EDX) and temperatureprogrammed reduction of H2 (H2 -TPR). Following this, a packed-bed reactor system was used to evaluate the elemental mercury removal performance at low temperature (from 80 to 150 ◦ C). This study also focused on the possible mechanism and reaction process for the removal of elemental mercury, and the heterogeneous reaction was speculated in detail by employing X-ray photoelectron spectroscopy (XPS), thermogravimetric (TG) and in situ FTIR spectroscopy analysis.
2. Experimental 2.1. Sample preparation Commercially available magnesite powder purchased from Jiangsu Zhengli Material Co. Ltd. was used as a magnesium oxide (MgO) precursor, which acted as supported material in this study. Mn/MgO adsorbents were prepared through the impregnation method, with the different loading values (, where is the mass ratio of Mn/MgO) varying from 3 to 15 wt%. The magnesite powder was first calcinated at 800 ◦ C for 1 h and MgO was obtained. Then, the Mn(NO3 )2 ·6H2 O was diluted in distilled water and magnesium oxide was directly added into this solution by continuous stirring.
253
The mixture was sonicated for 45 min in a bath sonicator and dried in drying oven overnight at 100 ◦ C, then was calcined at 500 ◦ C for 3 h to obtain absorbents. 2.2. Mercury oxidation and adsorption test The experiments on the Hg0 removal were carried out in a specially designed system (Fig. 1), and the entire setup includes four parts: an elemental mercury permeation tube, a packed-bed reactor, an online cold vapor atomic absorption spectrometer mercury analyzer (CVAAS) and a tail-end absorption equipment. A flow of N2 (200 mL/min) was used as the Hg0 -laden gas stream by passing through the Hg0 permeation tube. The Hg0 permeation tube was placed in a U-shaped glass tube, which was immersed in a water bath with a constant temperature of 50 ◦ C, so that the tube could provide a constant permeation concentration. A temperature control device was employed to keep the reactor at a desired temperature. To guarantee the exact values measured under different experimental conditions, there were two identical quartz tubes in the packed-bed reactor. One was used to confirm the adsorption reaction, and the other was used to measure initial value of elemental mercury. At the same time, a portion of quartz sand was packed in the middle of the empty tube to keep the identical gas hourly space velocity (GHSV) as the reaction. The gas containing elemental mercury first passed through the empty tube, and then entered the QM201H mercury analyzer to determine the baseline prior to each test of samples in the packed-bed reactor. When the mercury analyzer was stable for more than 30 min, the gas was diverted to the catalyst bed for the tests, and then the Hg0 concentration at the outlet was measured. The sample was inserted in the middle of the quartz tube and then packed with quartz wool and bead to support the catalyst layer and avoid its loss. To preliminarily estimate the elemental mercury capture performance, the adsorbent was first tested under air. The total flow rate was maintained at 1 L/min (corresponding to a space velocity of about 27,000 h−1 ), and then the flow was matched with the mercury analyzer. For the entire series of tests, the Hg0 removal efficiency (or Hg0 oxidation efficiency) was defined as (Cin − Cout )/Cin × 100%, Cin and Cout being Hg0 concentration corresponding to the inlet and outlet, respectively.
Fig. 1. Schematic diagram of experimental setup.
254
Y. Xu et al. / Journal of Hazardous Materials 283 (2015) 252–259
2.3. Sample characterization X-ray fluorescence (XRF) spectrometry (ARL9800XP) was used to determine the chemical composition of the commercial magnesite powder (in wt%). X-ray diffraction (XRD) patterns were recorded on a Beijing Purkinjie general instrument XD-3 X-ray diffraction (CuK␣, voltage 35 kV, electrical current 20 mA, 2 from 5◦ to 80◦ ). Specific surface areas of the samples were determined by N2 adsorption–desorption measurements at 77 K by employing the Brunauer–Emmet–Teller (BET) method (Gold App V-sorb 2008). Scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX) was used to examine the morphology and chemical composition of the raw materials and adsorption samples. A HITACHI S-3400N SEM was used with a BRUKER X5010 auto carbon coater EDX. The SEM was operated at an accelerating voltage of 20 kV and a beam current of 0.5 nA. Elemental scans were used to provide chemical concentrations for the surface of samples. This approach provided visual chemical composition data for the samples. Temperature-programmed reduction of H2 (H2 -TPR) was recorded on the automated chemisorption analyzer (Quantachrome Instruments). About 0.1 g of sample was used. The H2 N2 mixer (10% H2 by volume) was switched on at a flowing rate of 70 mL/min and the temperature was increased linearly at a rate of 20 ◦ C/min. X-ray photoelectron spectroscopy (XPS) experiments were carried out on a RBD upgraded PHI-5000C ESCA system (Perkin-Elmer). The thermogravimetric (TG) analysis of samples was performed with a PerkinElmer Pyris 1 DSC instrument. For each test, the sample was heated from 50 to 680 ◦ C at a heating rate of 20 ◦ C/min under a nitrogen atmosphere (>99.99%). The in situ FTIR measurements for desorption of Hg on the adsorbents were performed with Nicolet iZ10 FTIR spectrometers at 4 cm−1 resolution with 32 co-added scans. In the DRIFTS cell, the spent adsorbent was heated by raising the temperature at a rate of 20 ◦ C/min from 50 to 300 ◦ C under a nitrogen atmosphere (>99.99%). 3. Results and discussion 3.1. Samples characteristics 3.1.1. XRF XRF was used in determining the primary and trace elements of raw commercial magnesite power. The result showed that MgO is the main chemical component and the remaining chemical components consisted of Al2 O3 , CaO, Fe2 O3 . The loss weight was attributed to the complete decomposition of magnesium carbonate at the high operation temperatures of XRF instruments (shown in Table S1). 3.1.2. XRD The powder X-ray diffraction patterns of MgO obtained from magnesite power and Mn/MgO samples are shown in Fig. 2. The diffraction peaks of samples located at 36.9◦ , 42.9◦ and 62.3◦ corresponded to the (1 1 1), (2 0 0), (2 2 0) crystal planes of cubic Mg O. The diffraction peaks of MgO could be detected over all of the samples and matched with the standard ICDD data (JCPDS No. 45-0946). Even if the loading amount of Mn over MgO reached 15%, the peaks ascribed to Mn species did not stand out. The phenomenon indicated that manganese oxides were highly dispersed on the surface of MgO and were in an amorphous or poorly crystalline state [19]. However, when the loading amount was up to 40 wt%, it could be seen that peaks of MnO2 (2 = 18.17◦ , 35.6◦ and 56.4◦ ) appeared, which matched with the standard ICDD data (JCPDS No. 44-0141).
Fig. 2. X-ray diffraction patterns of MgO and Mn/MgO with different manganese loading.
In addition, when the loading amount of Mn increased, the diffraction peaks of MgO became weak and the peak at 62.3 shifted to the higher value, which was due to interaction between Mn and MgO. It is likely that MgO was locally distorted by incorporation of dopant Mn ions. 3.1.3. BET The BET surface areas and volumes of MgO and Mn/MgO are listed in Table 1. It could be observed that when the moderate amount of Mn was loaded on MgO, the surface area and the total pore volume increased. This was due to the contribution of MnOX coated on the surface of MgO. The phenomenon also indicated that Mn species were well dispersed on the surface and they were agglomerated in solid blocks hardly, which was in accord with the analysis of XRD. 3.1.4. SEM-EDX Fig. 3 shows the SEM images of raw magnesite power, MgO obtained from the calcined magnesite power and 10 wt% Mn/MgO adsorbent. The EDX analysis was also employed to determine the composition and content on the surface, and the major element percentage are shown in Table S2. Fig. 3a and b show that the magnesite power possessed a layered structure, which was in agreement with Rhombohedral, while MgO obtained from the calcined magnesite exhibited the smooth surface and cubic structure. The EDX spectra in Fig. 3b show that the C element did not appear after calcination, compared with EDX spectra of the raw magnesite power. The EDX analysis further verified that magnesium carbonate is the primary composition of raw magnesite power, and magnesite power decomposed into MgO and CO2 at 800 ◦ C. This result also corroborates itself well with the XRF, in which about 47% loss weights were due to the release of CO2 and decomposition of other impurities. The SEM picture and EDX spectra of 10 wt% Mn/MgO (Fig. 3c) demonstrated that MgO was uniformly coated by MnOX and the Table 1 Specific surface area and volume of the samples. Sample
BET surface area (m2 /g)
Total pore volume (cm3 /g)
MgO 3% MnOX /MgO 5% MnOX /MgO 10% MnOX /MgO 15% MnOX /MgO
48.20 69.25 81.07 56.82 51.29
0.227 0.343 0.380 0.281 0.316
Y. Xu et al. / Journal of Hazardous Materials 283 (2015) 252–259
Fig. 3. SEM-EDX photographs of (a) raw magnesite power, (b) MgO and (c) 10 wt% Mn/MgO.
255
256
Y. Xu et al. / Journal of Hazardous Materials 283 (2015) 252–259
Fig. 4. Temperature programmed reduction (H2 -TPR) patterns of the Mn/MgO with different manganese loading.
content of Mn was 8.33% (shown in Table S2), which indicated that MnOX was well-dispersed and was not aggregated on the surface of MgO. 3.1.5. H2 -TPR To determine the effect of Mn on the reduction profile of the Mn/MgO adsorbents, temperature programmed reduction (H2 TPR) experiments were carried out in Mn/MgO with different Mn loadings. As shown in the Fig. 4, the weak and broad reduction peaks of Mn/MgO with amount loading of 3–10 wt% could be observed at low temperatures from 250 to 500 ◦ C, which were related to the reduction of one kind of high valent Mn species, such as Mn4+ or Mn3+ . The results also indicated that the Mn species were well dispersed and they were amorphous on the surface of adsorbents, which agreed with the results of XRD and SEM-EDX. It could also be observed that these reduction peaks shifted to high temperature and the intensities increased when the loading of Mn increased, which could be attributed to the strong interaction between Mn and the support material. The phenomenon could also be explained as an increase in the activity of Mn on the surface of adsorbent. The reduction peak could be interpreted as a step reduction of MnO2 to Mn2 O3 . A similar trend was observed by Ji et al. [20].
Fig. 5. Removal efficiency of elemental mercury by Mn/MgO with different Mn loading at temperature range (80–150 ◦ C) in the air; carrier and balance gas N2 ; O2 vol% about 8%; inlet elemental mercury concentration = 30–60 ppb; GHSV = 27,000 h−1 .
It could be seen that Mn, when impregnated on MgO, promoted efficiency of Hg0 removal significantly, and Mn species were the main active component. Furthermore, it could be also found that O2 played a significant role on Hg0 removal. 3.3. Mechanism of elemental mercury removal 3.3.1. X-ray photoelectron spectroscopy analysis XPS was employed to analyze the surface of the adsorbent before and after mercury removal tests in the air. Fig. 7 shows typical XPS survey spectra of the fresh and the spent (mercury loaded) adsorbent (10 wt% Mn/MgO), respectively. As shown in Fig. 7a–c, the photoelectron peaks of Mg 2p, Mn 2p and O 1s appear at bind energies of about 50, 642 and 530 eV, respectively. There is a significant difference in the peak binding energies of Mn 2p and O 1s when contrasting the fresh and the spent adsorbent. Core level photoelectron spectra of Mn 2p in fresh and the spent adsorbents are shown in Fig. 7b. The bind energy of Mn 2p peak clearly shifted to the low energy region, indicating an increase in the electron density around Mn atoms on the surface of the spent Mn/MgO [21]. Consistent with XPS results of Mn 2p, the bind energy of the O 1s
3.2. Performance for elemental mercury removal in the air 3.2.1. Effect of Mn loading values and different temperatures on removal of Hg0 The elemental mercury removal efficiencies with different Mn loads are shown in Fig. 5. It can be seen that the removal efficiency was improved with the increase of Mn load. The 10 wt% Mn/MgO showed the best removal efficiency at 120 ◦ C, which reached about 82%, while the removal efficiency of 15 wt% Mn/MgO reached 86% at 150 ◦ C. The poor removal efficiency of the Mn/MgO adsorbents with low loading amounts may be due to the limitation of Mn active components. 3.2.2. Effect of O2 on elemental mercury removal Transient experiments were performed to evaluate the effect of O2 on elemental mercury removal over Mn/MgO (10 and 15 wt%) at 120 ◦ C. The results were shown in Fig. 6. When Mn/MgO adsorbents were performed in the simulate flue gas in the absence of O2 , the effectiveness of elemental mercury removal was weak and the duration time was short. However, after O2 was introduced into the experimental system, the Hg0 removal efficiency increased quickly.
Fig. 6. Effect of O2 (O2 vol% about 6%) on elemental mercury removal efficiency at 120 ◦ C; carrier and balance gas N2 ; inlet elemental mercury concentration = 30–60 ppb; GHSV = 27,000 h−1 .
Y. Xu et al. / Journal of Hazardous Materials 283 (2015) 252–259
257
Fig. 7. XPS spectra of (a) Mg 2p, (b) Mn 2p and (c) O 1s for the fresh and the spent Mn/MgO, XPS spectra of (d) Hg 4f for the spent Mn/MgO.
peak also decreased substantially to the low energy region in the spent adsorbent (shown in Fig. 7c). To better understand the surface oxide phases and their relative intensities, the two fitted Mn 2p 3/2 peaks were also shown in Fig. 6b. The Mn 2p peak of fresh samples was resolved into two individual peaks at 643.9 and 642.5 eV, corresponding to Mn4+ species, such as MnO2 . The XPS spectra data showed that Mn2+ (640.2 eV) did not appear. Combined with the result of XRD and H2 -TPR, it could be confirmed that the active oxide species of Mn over the fresh samples were assigned to the amorphous MnO2 . There was a new Mn 2p peak of the spent sample at 641.9 eV, which was assigned to Mn3+ . It can be concluded that some of Mn4+ in the fresh samples were reduced to Mn3+ after elemental mercury removal testing. The O 1s spectra of the fresh Mn/MgO showed two fitted characteristic peaks (shown in Fig. 7c), including BEs of about 529.5 (mainly attributed to lattice oxygen) and 531.8 (mainly attributed to chemisorbed oxygen). The adsorbed oxygen is due to the respective oxygen vacancies or other defects like boundaries, steps, and kinks [22], which was reported to be highly active and mobile in the oxidation reaction due to its higher mobility than lattice oxygen [23]. Table 2 lists the BEs of Mn 2p 3/2 and O 1s peaks as well as the calculated relative percentages. As shown in Fig. 7c and Table 2, the peak area percentage of chemisorbed oxygen decreased from 67 to 54.5% after the adsorbent was used, which indicated chemisorbed oxygen promotes the removal of Hg0 . In addition, it is interesting to note that lattice oxygen levels in the metal oxide increased after tests. Fig. 7d shows the Hg 4f spectra of the spent adsorbent. The Hg 4f peak is broad in two spectrums, indicating the presence of multiple Hg oxidation states. It has been reported that the peak at higher binding energy is due to mercury oxidation species [24]. Furthermore, the binding energies of Hg 4f 7/2 at 101 eV and the Hg 4f 5/2 at about 105 eV can be attributed to HgO [25]. However, except for
four high binding energy peaks of Hg 4f appearing at 102.2, 106.3, 108.2, and 112.1 eV, there were not any the fingerprint peaks of HgO. It indicated there was almost no HgO on the surface of the spent adsorbent. Because the BE strongly shifted to low value in Mn 2p and O 1s but shifted to high value in Hg 4f after test, it can be hypothesized that the mercury species on the surface were recombination of binary oxide such as MnHgO3 . 3.3.2. TG analysis To further confirm the mechanism of Hg0 removal over 10 wt% Mn/MgO, TG was performed in both the fresh and the spent samples and the results are shown in Fig. 8. The initial mass loss below
Fig. 8. TG curves of the fresh and the spent 10 wt% Mn/MgO.
258
Y. Xu et al. / Journal of Hazardous Materials 283 (2015) 252–259
Table 2 XPS results of the fresh and the spent Mn/MgO. Samples
Fresh Spent
Mn 2p 3/2
O 1s
Mn4+ Position (eV)
%
643.9 642.5 643.3
100 41.2
Mn3+ Position (eV)
%
641.9
58.8
200 ◦ C is generally attributed to the loss of physically (usually below 100 ◦ C) and chemically adsorbed water [26]. The mass loss of the fresh adsorbent in the temperature range of 200–350 ◦ C is believed to be the loss of chemical oxygen. However, there is an obvious mass loss of the spent adsorbent at this range, which was attributed to the decomposition of the product after the removal of Hg0 at about 200 ◦ C, which is in addition to the loss of chemical oxygen. Because the mass loss of the spent adsorbent did not appear at a range of 400–600 ◦ C, which was assigned to the decomposition of HgO, it indicated that the mercury oxide was not HgO. According to the likely result of XPS, the mass loss could be attributed to recombination component of mercury oxides i.e. MnHgO3 . The following evident mass losses are due to the transformation of MnO2 to Mn2 O3 at about 600 ◦ C [27].
Adsorbed oxygen Position (eV)
%
Lattice oxygen Position (eV)
%
531.8
67.0
529.5
33.0
531.1
54.5
529.5
45.5
chemisorbed oxygen work actively during the Hg0 oxidization and can be replenished from the gas phase oxygyen. It is clear that Mn played a critical role on the oxidation of Hg0 in this study. Although the results in Table 2 show the percentage of Mn4+ to total Mn decreased markedly from 100 to 41% after performing the mercury removal test, the Hg0 removal capacity of the adsorbent is maintained for a long time. It may be attributed to chemisorbed oxygen replenished from the gas phase oxygyen, which is helpful to recover Mn3+ into Mn4+ . With respect to possible forms of oxidized mercury species adsorbed on the Mn/MgO surface, the results of XPS, TG and in situ-FTIR indicate that the product of mercury oxide
3.3.3. In situ FTIR analysis A number of in situ FTIR experiments for the adsorbent were performed to gain a better understanding of the molecular behavior of mercury. The desorption characteristic of Hg0 over the spent Mn/MgO was investigated to ensure the mercury species on the surface. The in situ FTIR spectra of mercury desorption followed at temperature ranges from 50 to 300 ◦ C. The results of the spent adsorbent are shown in Fig. 9a. When the temperature reached 200 ◦ C and above, it could be seen that there are two new peaks appearing and one peak vanishing. Therein, two new peaks were at 2850 and 2186 cm−1 , and the peak disappeared was at 1193 cm−1 , respectively. Notably, the peak which disappeared was located in the low frequency region (500–1300 cm−1 ), which is usually assigned to the fingerprint region of metal oxide [28]. In order to eliminate the change on itself, in situ FTIR experimentation on the fresh adsorbent also was performed and the results are shown in Fig. 9b. Evidently, it is an important phenomenon that the peak at 1193 cm−1 disappeared. It indicates that one kind of metal-O band break at about 200 ◦ C. Because the thermal decomposition and phase change happen at high temperatures of metal oxide, it is not possible that the metal-O break which belongs to MnOX , MgO, or even HgO (400–600 ◦ C). According to the results of XPS, TG, and the test for Hg0 removal, it can be deduced that a recombination compound of mercury oxide such as MnHgO3 decomposes at low temperatures (about 200 ◦ C). 3.3.4. Reaction process analysis It was reported that Mn4+ cations act as Lewis acid to take part in catalytic or oxidation reaction [29,30]. As a Lewis base, Hg0 was first physically adsorbed on the Mn4+ cation and then could be oxidized by Mn4+ cation and Mn4+ further reduced to Mn3+ [31]. Therefore, the first step reaction may be described as: Hg(g) 0 + Mn4+ (LewisacidsitesonMnO2 ) → Mn4+ − Hg(ad) 0 →
Mn3+ Hg(ad) +
(1)
From Fig. 6, the certain Hg0 removal efficiencies could be found and decreased quickly in the absence of O2 . After introducing O2 , the Hg0 removal efficiencies increased. This finding indicates that
Fig. 9. In situ-FTIR of (a) the spent and (b) the fresh 10 wt% Mn/MgO.
Y. Xu et al. / Journal of Hazardous Materials 283 (2015) 252–259
existed as MnHgO3 , which was a recombination mercury oxide and decomposed at about 200 ◦ C. Therefore, the removal mechanism of elemental mercury by Mn/MgO under air at low temperature may be described as the following: Mn3+ Hg(ad) + + (1/2)O2 → (3)
Mn4+ Hg(ad) +
+ • O(ad)
Mn4+ Hg(ad) + + • O(ad) →
(2)
Mn4+ Hg2+ O(ad) (MnHgO3 )
4. Conclusion The elemental mercury adsorption experiments were carried out on Mn/MgO with the loading values from 3 to 15 wt% in simulated flue gas at low temperatures. The results show that removal efficiency of Hg0 is obviously improved due to the activity of Mn, and 10 wt% Mn/MgO adsorbent exhibits high removal efficiency of Hg0 , which reached about 82% at l20 ◦ C. The results show amorphous MnO2 and O2 play a crucial role in the removal of Hg0 from the simulated gas. After the comparative analysis between the fresh and the spent adsorbents by means of XPS, TG, in situ FTIR, the reaction process of Hg0 removal can be summarized that the gaseous Hg0 is adsorbed and oxidized on the surface of the adsorbent, and then forms the solid MnHgO3 , which succeed in separating Hg0 from the simulated gas. Acknowledgments This work was financially supported by Scientific Research Project of Environmental Protection Department of Jiangsu Province (201112), Project from Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control of Nanjing University of Information Science and Technology, Jiangsu Province Innovation Platform for Superiority Subject of Environmental Science and Engineering (KHK1208). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.jhazmat.2014.09.034. References [1] Y.H. Li, C.W. Lee, B.K. Gullett, Importance of activated carbon’s oxygen surface functional groups on elemental mercury adsorption, Fuel 82 (2003) 451–457. [2] J.H. Pavlish, L.L. Hamre, Y. Zhuang, Mercury control technologies for coal combustion and gasification systems, Fuel 89 (2010) 838–847. [3] K. Galbreath, C. Zygarlicke, Mercury speciation in coal combustion and gasification flue gases, Environ. Sci. Technol. 30 (1996) 2421–2426. [4] M.H. Kim, S.W. Ham, J.B. Lee, Oxidation of gaseous elemental mercury by hydrochloric acid over CuCl2 /TiO2 -based catalysts in SCR process, Appl. Catal. B 99 (2010) 272–278. [5] S.H. Wu, S.A. Wang, J.H. Gao, Y.Y. Wu, G.Q. Chen, Y.W. Zhu, Interactions between mercury and dry FGD ash in simulated post combustion conditions, J. Hazard. Mater. 188 (2011) 391–398. [6] A.A. Presto, E.J. Granite, Survey of catalysts for oxidation of mercury in flue gas, Environ. Sci. Technol. 40 (2006) 5601–5609.
259
[7] W.J. Lee, G.N. Bae, Removal of elemental mercury (Hg(O)) by nanosized V2 O5 /TiO2 catalysts, Environ. Sci. Technol. 43 (2009) 1522–1527. [8] Y. Liu, Y.J. Wang, H.Q. Wang, Catalytic oxidation of gas-phase mercury over Co/TiO2 catalysts prepared by sol–gel method, Catal. Commun. 12 (2011) 1291–1294. [9] 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, Ind. Eng. Chem. Res. 50 (2011) 9650–9656. [10] S.X. Wang, L. Zhang, G.H. Li, Y. Wu, J.M. Hao, N. Pirrone, F. Sprovieri, M.P. Ancora, Mercury emission and speciation of coal-fired power plants in China, Atmos. Chem. Phys. 10 (2010) 1183–1192. [11] E.J. Granite, H.W. Pennline, R.A. Hargis, Novel sorbents for mercury removal from flue gas, Ind. Eng. Chem. Res. 39 (2000) 1020–1029. [12] R.Q. Long, R.T. Yang, Characterization of Fe-ZSM-5 catalyst for selective catalytic reduction of nitric oxide by ammonia, J. Catal. 194 (2000) 80–90. [13] J.W. Portzer, J.R. Albritton, C.C. Allen, R.P. Gupta, Development of novel sorbents for mercury control at elevated temperatures in coal-derived syngas: results of initial screening of candidate materials, Fuel Process. Technol. 85 (2004) 621–630. [14] Y. Zhao, M.D. Mann, J.H. Pavlish, B.A.F. Mibeck, G.E. Dunham, E. Olson, Environ. Sci. Technol. 40 (2006) 1603–1608. [15] X.Y. Wen, C.T. Li, X.P. Fan, Experimental study of gaseous elemental mercury removal with CeO2 /Al2 O3 , Energy Fuels 25 (2011) 2939–2944. [16] H.L. Li, C.Y. Wu, Y. Li, Role of flue gas components in mercury oxidation over TiO2 supported MnOx CeO2 mixed-oxide at low temperature, J. Hazard. Mater. 243 (2012) 117–123. [17] L.L. Xing, Y.L. Xu, Q. Zhong, Mn and Fe modified fly ash as a superior catalyst for elemental mercury capture under air conditions, Energy Fuels 26 (2012) 4903–4909. [18] Y.L. Hao, B.C. Gates, Activation of dimethyl gold complexes on MgO for CO oxidation: removal of methyl ligands and formation of catalytically active gold clusters, J. Catal. 263 (2009) 83–91. [19] C.N. Liang, J.H. Li, M.F. Ge, R.H. Zhu, Enhanced activity of tungsten modified CeO2 /TiO2 for selective catalytic reduction of NOx with ammonia, Catal. Today 153 (2010) 77–83. [20] L. Ji, P.M. Sreekanth, P.G. Smirniotis, S.W. Thiel, N.G. Pinto, Manganese oxide/titania materials for removal of NOx and elemental mercury from flue gas, Energy Fuels 22 (2008) 2299–2306. [21] M. Narayanappa, V.D.B.C. Dasireddy, H.B. Friedrich, Catalytic oxidation of noctane over cobalt substituted ceria (Ce0.90 Co0.10 O2−␦ ) catalysts, Appl. Catal. A 447 (2012) 135–143. [22] G. Heinz, H. Adarn, The role of oxygen in photo-oxidation of organic molecules on semiconductor particles, J. Phys. Chem. 95 (1991) 5261–5267. [23] X.B. Chen, S. Gao, H. Wang, Y. Liu, Z. Wu, Selective catalytic reduction of NO over carbon nanotubes supported CeO2 , Catal. Commun. 14 (2011) 1–5. [24] J. He, K.R. Gunugunuri, S.W. Thiel, G. Panagiotis, Ceria-modified manganese oxide/titania materials for removal of elemental and oxidized mercury from flue gas, J. Phys. Chem. C 115 (2011) 24300–24309. [25] S.J. Yang, Y.F. Guo, N.Q. Yan, D.Q. Wu, H.P. He, J.K. Xie, Z. Qu, J.P. Jia, Remarkable effect of the incorporation of titanium on the catalytic activity and SO2 poisoning resistance of magnetic Mn Fe spinel for elemental mercury capture, Appl. Catal. B 101 (2011) 698–708. [26] R. Giovanoli, Thermogravimetry of manganese dioxides, Thermochim. Acta 234 (1994) 303–313. [27] Z.M. Wang, H. Kanoh, Calorimetric study on NH3 insertion reaction into microporous manganese oxides with (2 × 2) tunnel and (2 × ∞) layered structures, Thermochim. Acta 379 (2001) 7–14. [28] C.X. Gao, Q.F. Liu, D.S. Xue, Preparation and characterization of amorphous FeOOH nanowire arrays, J. Mater. Sci. Lett. 21 (2002) 1781–1783. [29] X.F. Tang, J.H. Li, L. Sun, J.M. Hao, Origination of N2 O from NO reduction by NH3 over -MnO2 and ␣-Mn2 O3 , Appl. Catal. B 99 (2010) 156–162. [30] J.H. Li, J.J. Chen, R. Ke, C.K. Luo, J.M. Hao, Effects of precursors on the surface Mn species and the activities for NO reduction over MnOx /TiO2 catalysts, Catal. Commun. 8 (2007) 1896–1900. [31] G.E. Dunham, R.A. DeWall, C.L. Senior, Fixed-bed studies of the interactions between mercury and coal combustion fly ash, Fuel Process. Technol. 82 (2003) 197–213.
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Elemental mercury oxidation and adsorption on magnesite powder modified by Mn at low temperature Yalin Xu a,b , Qin Zhong a,∗ , Xinya Liu c a
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China Jiangsu Key Laboratory of Atmospheric Environment Monitoring & Pollution Control, School of Environmental Sciences and Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, PR China c Jiangsu HeYiChang Environmental Protection Engineering & Technology Co., Ltd., PR China b
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• It is a new idea to investigate the removal of elemental mercury over Mn/MgO at low temperature. • The amorphous MnO2 and chemisorbed O2 played key roles on the removal of elemental mercury. • Combined in situ FTIR with XPS and TG, the possible oxidation and adsorption mechanism is proposed and MnHgO3 is the product of elemental mercury oxidation over Mn/MgO.
a r t i c l e
i n f o
Article history: Received 9 June 2014 Received in revised form 1 September 2014 Accepted 2 September 2014 Available online 28 September 2014 Keywords: Elemental mercury Adsorbent Amorphous MnO2 Magnesite powder
a b s t r a c t Mn modified the commercial magnesite powder prepared by wet impregnation method has been shown to be effective for gas-phase elemental mercury (Hg0 ) removal at low temperatures. The prepared samples are characterized in detail across multiform techniques: XRF, BET, SEM-EDX, XRD, H2 -TPR, and XPS, and all the results show that the amorphous MnO2 impregnated on magnesite powder improves the removal efficiency of Hg0 . Through further analysis by TG and in situ FTIR, the reasonable removal mechanism is also speculated. The results indicate that chemisorbed oxygen is an important reactant in the heterogeneous reaction, and gas-phase Hg0 is adsorbed and then oxidized to solid MnHgO3 on the surface of the adsorbent. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Mercury (Hg) released from coal combustion is a typical heavy metal contaminant. Following SO2 , NOX , PM10 and PM2.5 , mercury has attracted increasing attention due to its toxicity, mobility, and bioaccumulation in the ecosystem and food chain [1,2]. Mercury emissions and speciation vary from site to site at coalfired power plants. This is mainly due to different boiler configurations, coals types,
∗ Corresponding author. Tel.: +86 25 84315517; fax: +86 25 84315517. E-mail address: [email protected] (Q. Zhong). http://dx.doi.org/10.1016/j.jhazmat.2014.09.034 0304-3894/© 2014 Elsevier B.V. All rights reserved.
and operating conditions. There are three main states of mercury in the flue gas: elemental (Hg0 ), gaseous divalent (Hg2+ ) and particulate-associated (Hgp ) [3,4], which have different physical and chemical properties. Hg2+ and Hgp are relatively easy to remove from flue gas by using conventional air pollution control devices, such as wet flue gas desulfurization (WFGD) systems, electrostatic precipitators (ESPs), or fabric filter (FF). Hg0 is difficult to be controlled by the aforementioned devices due to its high volatility and low solubility in water [5]. Several methods have been proposed for Hg0 removal during the past few decades, such as activated carbon injection (ACI), oxidation–reduction, and photochemical oxidation [6]. ACI can remove 80–98% of elemental mercury from flue gas, however, considering the high operation cost and complex separation progress from ultrafine fly ash, new methods are still required. Recent research has been shown that heterogeneous oxidation played an important
Y. Xu et al. / Journal of Hazardous Materials 283 (2015) 252–259 role on converting Hg0 to Hg2+ catalytically [7,8]. The involved oxidants were mainly chlorine [9]. In China, the chlorine content in feed-coal varies from 63 to 318 mg/kg, which is much lower than the average value of US coals (628 mg/kg) [10]. Because there is an amount of residual O2 in the real flue gas, it is economically practical to use oxygen as the oxidant in the flue gas, compared with Cl2 , HCl, HBr, etc. Furthermore, the oxidized mercury can exist as the solid state on the surface of adsorbents when the temperature is lower than 400 ◦ C. Hence Hg0 will be adsorbed by the adsorbents and then be subsequently removed from the flue gas [11]. A variety of potential catalysts for mercury oxidation, including Co/TiO2 , Fe/ZSM-5, V2 O5 /AC, Fe2 O3 /TiO2 , MnO2 /AC and CeO2 /SiO2 , have been employed in experiments [12–15]. Especially, transition metal oxides deposited on different supports, possess cost-effective oxidation activity. Among them, manganese oxides showed good oxidization capabilities at low temperatures. Granite et al. [11] found the alumina-supported manganese dioxide (MnO2 ) showed high elemental mercury capacity in the temperature range from 60 to 177 ◦ C. It was also reported that TiO2 -supported by MnOX CeO2 was effective for the removal of elemental mercury at low temperatures [16,17]. Considering the good oxidation performance of manganese at low temperature, manganese supported on MgO was chosen to perform the removal of the elemental mercury in simulated flue gas. The reason for this choice was that magnesium oxide acted as both good support and co-catalysts used in the oxidation reaction [18]. This provides an efficient mercury abatement technology, compatible with the existing conventional air pollution control devices. In this study, a series of Mn/MgO were synthesized using a simple wet-impregnation method and characterized by X-ray fluorescence (XRF), Xray diffraction (XRD), N2 adsorption/desorption isotherms, scanning electron microscopy with energy dispersive X-ray analyses (SEM-EDX) and temperatureprogrammed reduction of H2 (H2 -TPR). Following this, a packed-bed reactor system was used to evaluate the elemental mercury removal performance at low temperature (from 80 to 150 ◦ C). This study also focused on the possible mechanism and reaction process for the removal of elemental mercury, and the heterogeneous reaction was speculated in detail by employing X-ray photoelectron spectroscopy (XPS), thermogravimetric (TG) and in situ FTIR spectroscopy analysis.
2. Experimental 2.1. Sample preparation Commercially available magnesite powder purchased from Jiangsu Zhengli Material Co. Ltd. was used as a magnesium oxide (MgO) precursor, which acted as supported material in this study. Mn/MgO adsorbents were prepared through the impregnation method, with the different loading values (, where is the mass ratio of Mn/MgO) varying from 3 to 15 wt%. The magnesite powder was first calcinated at 800 ◦ C for 1 h and MgO was obtained. Then, the Mn(NO3 )2 ·6H2 O was diluted in distilled water and magnesium oxide was directly added into this solution by continuous stirring.
253
The mixture was sonicated for 45 min in a bath sonicator and dried in drying oven overnight at 100 ◦ C, then was calcined at 500 ◦ C for 3 h to obtain absorbents. 2.2. Mercury oxidation and adsorption test The experiments on the Hg0 removal were carried out in a specially designed system (Fig. 1), and the entire setup includes four parts: an elemental mercury permeation tube, a packed-bed reactor, an online cold vapor atomic absorption spectrometer mercury analyzer (CVAAS) and a tail-end absorption equipment. A flow of N2 (200 mL/min) was used as the Hg0 -laden gas stream by passing through the Hg0 permeation tube. The Hg0 permeation tube was placed in a U-shaped glass tube, which was immersed in a water bath with a constant temperature of 50 ◦ C, so that the tube could provide a constant permeation concentration. A temperature control device was employed to keep the reactor at a desired temperature. To guarantee the exact values measured under different experimental conditions, there were two identical quartz tubes in the packed-bed reactor. One was used to confirm the adsorption reaction, and the other was used to measure initial value of elemental mercury. At the same time, a portion of quartz sand was packed in the middle of the empty tube to keep the identical gas hourly space velocity (GHSV) as the reaction. The gas containing elemental mercury first passed through the empty tube, and then entered the QM201H mercury analyzer to determine the baseline prior to each test of samples in the packed-bed reactor. When the mercury analyzer was stable for more than 30 min, the gas was diverted to the catalyst bed for the tests, and then the Hg0 concentration at the outlet was measured. The sample was inserted in the middle of the quartz tube and then packed with quartz wool and bead to support the catalyst layer and avoid its loss. To preliminarily estimate the elemental mercury capture performance, the adsorbent was first tested under air. The total flow rate was maintained at 1 L/min (corresponding to a space velocity of about 27,000 h−1 ), and then the flow was matched with the mercury analyzer. For the entire series of tests, the Hg0 removal efficiency (or Hg0 oxidation efficiency) was defined as (Cin − Cout )/Cin × 100%, Cin and Cout being Hg0 concentration corresponding to the inlet and outlet, respectively.
Fig. 1. Schematic diagram of experimental setup.
254
Y. Xu et al. / Journal of Hazardous Materials 283 (2015) 252–259
2.3. Sample characterization X-ray fluorescence (XRF) spectrometry (ARL9800XP) was used to determine the chemical composition of the commercial magnesite powder (in wt%). X-ray diffraction (XRD) patterns were recorded on a Beijing Purkinjie general instrument XD-3 X-ray diffraction (CuK␣, voltage 35 kV, electrical current 20 mA, 2 from 5◦ to 80◦ ). Specific surface areas of the samples were determined by N2 adsorption–desorption measurements at 77 K by employing the Brunauer–Emmet–Teller (BET) method (Gold App V-sorb 2008). Scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX) was used to examine the morphology and chemical composition of the raw materials and adsorption samples. A HITACHI S-3400N SEM was used with a BRUKER X5010 auto carbon coater EDX. The SEM was operated at an accelerating voltage of 20 kV and a beam current of 0.5 nA. Elemental scans were used to provide chemical concentrations for the surface of samples. This approach provided visual chemical composition data for the samples. Temperature-programmed reduction of H2 (H2 -TPR) was recorded on the automated chemisorption analyzer (Quantachrome Instruments). About 0.1 g of sample was used. The H2 N2 mixer (10% H2 by volume) was switched on at a flowing rate of 70 mL/min and the temperature was increased linearly at a rate of 20 ◦ C/min. X-ray photoelectron spectroscopy (XPS) experiments were carried out on a RBD upgraded PHI-5000C ESCA system (Perkin-Elmer). The thermogravimetric (TG) analysis of samples was performed with a PerkinElmer Pyris 1 DSC instrument. For each test, the sample was heated from 50 to 680 ◦ C at a heating rate of 20 ◦ C/min under a nitrogen atmosphere (>99.99%). The in situ FTIR measurements for desorption of Hg on the adsorbents were performed with Nicolet iZ10 FTIR spectrometers at 4 cm−1 resolution with 32 co-added scans. In the DRIFTS cell, the spent adsorbent was heated by raising the temperature at a rate of 20 ◦ C/min from 50 to 300 ◦ C under a nitrogen atmosphere (>99.99%). 3. Results and discussion 3.1. Samples characteristics 3.1.1. XRF XRF was used in determining the primary and trace elements of raw commercial magnesite power. The result showed that MgO is the main chemical component and the remaining chemical components consisted of Al2 O3 , CaO, Fe2 O3 . The loss weight was attributed to the complete decomposition of magnesium carbonate at the high operation temperatures of XRF instruments (shown in Table S1). 3.1.2. XRD The powder X-ray diffraction patterns of MgO obtained from magnesite power and Mn/MgO samples are shown in Fig. 2. The diffraction peaks of samples located at 36.9◦ , 42.9◦ and 62.3◦ corresponded to the (1 1 1), (2 0 0), (2 2 0) crystal planes of cubic Mg O. The diffraction peaks of MgO could be detected over all of the samples and matched with the standard ICDD data (JCPDS No. 45-0946). Even if the loading amount of Mn over MgO reached 15%, the peaks ascribed to Mn species did not stand out. The phenomenon indicated that manganese oxides were highly dispersed on the surface of MgO and were in an amorphous or poorly crystalline state [19]. However, when the loading amount was up to 40 wt%, it could be seen that peaks of MnO2 (2 = 18.17◦ , 35.6◦ and 56.4◦ ) appeared, which matched with the standard ICDD data (JCPDS No. 44-0141).
Fig. 2. X-ray diffraction patterns of MgO and Mn/MgO with different manganese loading.
In addition, when the loading amount of Mn increased, the diffraction peaks of MgO became weak and the peak at 62.3 shifted to the higher value, which was due to interaction between Mn and MgO. It is likely that MgO was locally distorted by incorporation of dopant Mn ions. 3.1.3. BET The BET surface areas and volumes of MgO and Mn/MgO are listed in Table 1. It could be observed that when the moderate amount of Mn was loaded on MgO, the surface area and the total pore volume increased. This was due to the contribution of MnOX coated on the surface of MgO. The phenomenon also indicated that Mn species were well dispersed on the surface and they were agglomerated in solid blocks hardly, which was in accord with the analysis of XRD. 3.1.4. SEM-EDX Fig. 3 shows the SEM images of raw magnesite power, MgO obtained from the calcined magnesite power and 10 wt% Mn/MgO adsorbent. The EDX analysis was also employed to determine the composition and content on the surface, and the major element percentage are shown in Table S2. Fig. 3a and b show that the magnesite power possessed a layered structure, which was in agreement with Rhombohedral, while MgO obtained from the calcined magnesite exhibited the smooth surface and cubic structure. The EDX spectra in Fig. 3b show that the C element did not appear after calcination, compared with EDX spectra of the raw magnesite power. The EDX analysis further verified that magnesium carbonate is the primary composition of raw magnesite power, and magnesite power decomposed into MgO and CO2 at 800 ◦ C. This result also corroborates itself well with the XRF, in which about 47% loss weights were due to the release of CO2 and decomposition of other impurities. The SEM picture and EDX spectra of 10 wt% Mn/MgO (Fig. 3c) demonstrated that MgO was uniformly coated by MnOX and the Table 1 Specific surface area and volume of the samples. Sample
BET surface area (m2 /g)
Total pore volume (cm3 /g)
MgO 3% MnOX /MgO 5% MnOX /MgO 10% MnOX /MgO 15% MnOX /MgO
48.20 69.25 81.07 56.82 51.29
0.227 0.343 0.380 0.281 0.316
Y. Xu et al. / Journal of Hazardous Materials 283 (2015) 252–259
Fig. 3. SEM-EDX photographs of (a) raw magnesite power, (b) MgO and (c) 10 wt% Mn/MgO.
255
256
Y. Xu et al. / Journal of Hazardous Materials 283 (2015) 252–259
Fig. 4. Temperature programmed reduction (H2 -TPR) patterns of the Mn/MgO with different manganese loading.
content of Mn was 8.33% (shown in Table S2), which indicated that MnOX was well-dispersed and was not aggregated on the surface of MgO. 3.1.5. H2 -TPR To determine the effect of Mn on the reduction profile of the Mn/MgO adsorbents, temperature programmed reduction (H2 TPR) experiments were carried out in Mn/MgO with different Mn loadings. As shown in the Fig. 4, the weak and broad reduction peaks of Mn/MgO with amount loading of 3–10 wt% could be observed at low temperatures from 250 to 500 ◦ C, which were related to the reduction of one kind of high valent Mn species, such as Mn4+ or Mn3+ . The results also indicated that the Mn species were well dispersed and they were amorphous on the surface of adsorbents, which agreed with the results of XRD and SEM-EDX. It could also be observed that these reduction peaks shifted to high temperature and the intensities increased when the loading of Mn increased, which could be attributed to the strong interaction between Mn and the support material. The phenomenon could also be explained as an increase in the activity of Mn on the surface of adsorbent. The reduction peak could be interpreted as a step reduction of MnO2 to Mn2 O3 . A similar trend was observed by Ji et al. [20].
Fig. 5. Removal efficiency of elemental mercury by Mn/MgO with different Mn loading at temperature range (80–150 ◦ C) in the air; carrier and balance gas N2 ; O2 vol% about 8%; inlet elemental mercury concentration = 30–60 ppb; GHSV = 27,000 h−1 .
It could be seen that Mn, when impregnated on MgO, promoted efficiency of Hg0 removal significantly, and Mn species were the main active component. Furthermore, it could be also found that O2 played a significant role on Hg0 removal. 3.3. Mechanism of elemental mercury removal 3.3.1. X-ray photoelectron spectroscopy analysis XPS was employed to analyze the surface of the adsorbent before and after mercury removal tests in the air. Fig. 7 shows typical XPS survey spectra of the fresh and the spent (mercury loaded) adsorbent (10 wt% Mn/MgO), respectively. As shown in Fig. 7a–c, the photoelectron peaks of Mg 2p, Mn 2p and O 1s appear at bind energies of about 50, 642 and 530 eV, respectively. There is a significant difference in the peak binding energies of Mn 2p and O 1s when contrasting the fresh and the spent adsorbent. Core level photoelectron spectra of Mn 2p in fresh and the spent adsorbents are shown in Fig. 7b. The bind energy of Mn 2p peak clearly shifted to the low energy region, indicating an increase in the electron density around Mn atoms on the surface of the spent Mn/MgO [21]. Consistent with XPS results of Mn 2p, the bind energy of the O 1s
3.2. Performance for elemental mercury removal in the air 3.2.1. Effect of Mn loading values and different temperatures on removal of Hg0 The elemental mercury removal efficiencies with different Mn loads are shown in Fig. 5. It can be seen that the removal efficiency was improved with the increase of Mn load. The 10 wt% Mn/MgO showed the best removal efficiency at 120 ◦ C, which reached about 82%, while the removal efficiency of 15 wt% Mn/MgO reached 86% at 150 ◦ C. The poor removal efficiency of the Mn/MgO adsorbents with low loading amounts may be due to the limitation of Mn active components. 3.2.2. Effect of O2 on elemental mercury removal Transient experiments were performed to evaluate the effect of O2 on elemental mercury removal over Mn/MgO (10 and 15 wt%) at 120 ◦ C. The results were shown in Fig. 6. When Mn/MgO adsorbents were performed in the simulate flue gas in the absence of O2 , the effectiveness of elemental mercury removal was weak and the duration time was short. However, after O2 was introduced into the experimental system, the Hg0 removal efficiency increased quickly.
Fig. 6. Effect of O2 (O2 vol% about 6%) on elemental mercury removal efficiency at 120 ◦ C; carrier and balance gas N2 ; inlet elemental mercury concentration = 30–60 ppb; GHSV = 27,000 h−1 .
Y. Xu et al. / Journal of Hazardous Materials 283 (2015) 252–259
257
Fig. 7. XPS spectra of (a) Mg 2p, (b) Mn 2p and (c) O 1s for the fresh and the spent Mn/MgO, XPS spectra of (d) Hg 4f for the spent Mn/MgO.
peak also decreased substantially to the low energy region in the spent adsorbent (shown in Fig. 7c). To better understand the surface oxide phases and their relative intensities, the two fitted Mn 2p 3/2 peaks were also shown in Fig. 6b. The Mn 2p peak of fresh samples was resolved into two individual peaks at 643.9 and 642.5 eV, corresponding to Mn4+ species, such as MnO2 . The XPS spectra data showed that Mn2+ (640.2 eV) did not appear. Combined with the result of XRD and H2 -TPR, it could be confirmed that the active oxide species of Mn over the fresh samples were assigned to the amorphous MnO2 . There was a new Mn 2p peak of the spent sample at 641.9 eV, which was assigned to Mn3+ . It can be concluded that some of Mn4+ in the fresh samples were reduced to Mn3+ after elemental mercury removal testing. The O 1s spectra of the fresh Mn/MgO showed two fitted characteristic peaks (shown in Fig. 7c), including BEs of about 529.5 (mainly attributed to lattice oxygen) and 531.8 (mainly attributed to chemisorbed oxygen). The adsorbed oxygen is due to the respective oxygen vacancies or other defects like boundaries, steps, and kinks [22], which was reported to be highly active and mobile in the oxidation reaction due to its higher mobility than lattice oxygen [23]. Table 2 lists the BEs of Mn 2p 3/2 and O 1s peaks as well as the calculated relative percentages. As shown in Fig. 7c and Table 2, the peak area percentage of chemisorbed oxygen decreased from 67 to 54.5% after the adsorbent was used, which indicated chemisorbed oxygen promotes the removal of Hg0 . In addition, it is interesting to note that lattice oxygen levels in the metal oxide increased after tests. Fig. 7d shows the Hg 4f spectra of the spent adsorbent. The Hg 4f peak is broad in two spectrums, indicating the presence of multiple Hg oxidation states. It has been reported that the peak at higher binding energy is due to mercury oxidation species [24]. Furthermore, the binding energies of Hg 4f 7/2 at 101 eV and the Hg 4f 5/2 at about 105 eV can be attributed to HgO [25]. However, except for
four high binding energy peaks of Hg 4f appearing at 102.2, 106.3, 108.2, and 112.1 eV, there were not any the fingerprint peaks of HgO. It indicated there was almost no HgO on the surface of the spent adsorbent. Because the BE strongly shifted to low value in Mn 2p and O 1s but shifted to high value in Hg 4f after test, it can be hypothesized that the mercury species on the surface were recombination of binary oxide such as MnHgO3 . 3.3.2. TG analysis To further confirm the mechanism of Hg0 removal over 10 wt% Mn/MgO, TG was performed in both the fresh and the spent samples and the results are shown in Fig. 8. The initial mass loss below
Fig. 8. TG curves of the fresh and the spent 10 wt% Mn/MgO.
258
Y. Xu et al. / Journal of Hazardous Materials 283 (2015) 252–259
Table 2 XPS results of the fresh and the spent Mn/MgO. Samples
Fresh Spent
Mn 2p 3/2
O 1s
Mn4+ Position (eV)
%
643.9 642.5 643.3
100 41.2
Mn3+ Position (eV)
%
641.9
58.8
200 ◦ C is generally attributed to the loss of physically (usually below 100 ◦ C) and chemically adsorbed water [26]. The mass loss of the fresh adsorbent in the temperature range of 200–350 ◦ C is believed to be the loss of chemical oxygen. However, there is an obvious mass loss of the spent adsorbent at this range, which was attributed to the decomposition of the product after the removal of Hg0 at about 200 ◦ C, which is in addition to the loss of chemical oxygen. Because the mass loss of the spent adsorbent did not appear at a range of 400–600 ◦ C, which was assigned to the decomposition of HgO, it indicated that the mercury oxide was not HgO. According to the likely result of XPS, the mass loss could be attributed to recombination component of mercury oxides i.e. MnHgO3 . The following evident mass losses are due to the transformation of MnO2 to Mn2 O3 at about 600 ◦ C [27].
Adsorbed oxygen Position (eV)
%
Lattice oxygen Position (eV)
%
531.8
67.0
529.5
33.0
531.1
54.5
529.5
45.5
chemisorbed oxygen work actively during the Hg0 oxidization and can be replenished from the gas phase oxygyen. It is clear that Mn played a critical role on the oxidation of Hg0 in this study. Although the results in Table 2 show the percentage of Mn4+ to total Mn decreased markedly from 100 to 41% after performing the mercury removal test, the Hg0 removal capacity of the adsorbent is maintained for a long time. It may be attributed to chemisorbed oxygen replenished from the gas phase oxygyen, which is helpful to recover Mn3+ into Mn4+ . With respect to possible forms of oxidized mercury species adsorbed on the Mn/MgO surface, the results of XPS, TG and in situ-FTIR indicate that the product of mercury oxide
3.3.3. In situ FTIR analysis A number of in situ FTIR experiments for the adsorbent were performed to gain a better understanding of the molecular behavior of mercury. The desorption characteristic of Hg0 over the spent Mn/MgO was investigated to ensure the mercury species on the surface. The in situ FTIR spectra of mercury desorption followed at temperature ranges from 50 to 300 ◦ C. The results of the spent adsorbent are shown in Fig. 9a. When the temperature reached 200 ◦ C and above, it could be seen that there are two new peaks appearing and one peak vanishing. Therein, two new peaks were at 2850 and 2186 cm−1 , and the peak disappeared was at 1193 cm−1 , respectively. Notably, the peak which disappeared was located in the low frequency region (500–1300 cm−1 ), which is usually assigned to the fingerprint region of metal oxide [28]. In order to eliminate the change on itself, in situ FTIR experimentation on the fresh adsorbent also was performed and the results are shown in Fig. 9b. Evidently, it is an important phenomenon that the peak at 1193 cm−1 disappeared. It indicates that one kind of metal-O band break at about 200 ◦ C. Because the thermal decomposition and phase change happen at high temperatures of metal oxide, it is not possible that the metal-O break which belongs to MnOX , MgO, or even HgO (400–600 ◦ C). According to the results of XPS, TG, and the test for Hg0 removal, it can be deduced that a recombination compound of mercury oxide such as MnHgO3 decomposes at low temperatures (about 200 ◦ C). 3.3.4. Reaction process analysis It was reported that Mn4+ cations act as Lewis acid to take part in catalytic or oxidation reaction [29,30]. As a Lewis base, Hg0 was first physically adsorbed on the Mn4+ cation and then could be oxidized by Mn4+ cation and Mn4+ further reduced to Mn3+ [31]. Therefore, the first step reaction may be described as: Hg(g) 0 + Mn4+ (LewisacidsitesonMnO2 ) → Mn4+ − Hg(ad) 0 →
Mn3+ Hg(ad) +
(1)
From Fig. 6, the certain Hg0 removal efficiencies could be found and decreased quickly in the absence of O2 . After introducing O2 , the Hg0 removal efficiencies increased. This finding indicates that
Fig. 9. In situ-FTIR of (a) the spent and (b) the fresh 10 wt% Mn/MgO.
Y. Xu et al. / Journal of Hazardous Materials 283 (2015) 252–259
existed as MnHgO3 , which was a recombination mercury oxide and decomposed at about 200 ◦ C. Therefore, the removal mechanism of elemental mercury by Mn/MgO under air at low temperature may be described as the following: Mn3+ Hg(ad) + + (1/2)O2 → (3)
Mn4+ Hg(ad) +
+ • O(ad)
Mn4+ Hg(ad) + + • O(ad) →
(2)
Mn4+ Hg2+ O(ad) (MnHgO3 )
4. Conclusion The elemental mercury adsorption experiments were carried out on Mn/MgO with the loading values from 3 to 15 wt% in simulated flue gas at low temperatures. The results show that removal efficiency of Hg0 is obviously improved due to the activity of Mn, and 10 wt% Mn/MgO adsorbent exhibits high removal efficiency of Hg0 , which reached about 82% at l20 ◦ C. The results show amorphous MnO2 and O2 play a crucial role in the removal of Hg0 from the simulated gas. After the comparative analysis between the fresh and the spent adsorbents by means of XPS, TG, in situ FTIR, the reaction process of Hg0 removal can be summarized that the gaseous Hg0 is adsorbed and oxidized on the surface of the adsorbent, and then forms the solid MnHgO3 , which succeed in separating Hg0 from the simulated gas. Acknowledgments This work was financially supported by Scientific Research Project of Environmental Protection Department of Jiangsu Province (201112), Project from Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control of Nanjing University of Information Science and Technology, Jiangsu Province Innovation Platform for Superiority Subject of Environmental Science and Engineering (KHK1208). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.jhazmat.2014.09.034. References [1] Y.H. Li, C.W. Lee, B.K. Gullett, Importance of activated carbon’s oxygen surface functional groups on elemental mercury adsorption, Fuel 82 (2003) 451–457. [2] J.H. Pavlish, L.L. Hamre, Y. Zhuang, Mercury control technologies for coal combustion and gasification systems, Fuel 89 (2010) 838–847. [3] K. Galbreath, C. Zygarlicke, Mercury speciation in coal combustion and gasification flue gases, Environ. Sci. Technol. 30 (1996) 2421–2426. [4] M.H. Kim, S.W. Ham, J.B. Lee, Oxidation of gaseous elemental mercury by hydrochloric acid over CuCl2 /TiO2 -based catalysts in SCR process, Appl. Catal. B 99 (2010) 272–278. [5] S.H. Wu, S.A. Wang, J.H. Gao, Y.Y. Wu, G.Q. Chen, Y.W. Zhu, Interactions between mercury and dry FGD ash in simulated post combustion conditions, J. Hazard. Mater. 188 (2011) 391–398. [6] A.A. Presto, E.J. Granite, Survey of catalysts for oxidation of mercury in flue gas, Environ. Sci. Technol. 40 (2006) 5601–5609.
259
[7] W.J. Lee, G.N. Bae, Removal of elemental mercury (Hg(O)) by nanosized V2 O5 /TiO2 catalysts, Environ. Sci. Technol. 43 (2009) 1522–1527. [8] Y. Liu, Y.J. Wang, H.Q. Wang, Catalytic oxidation of gas-phase mercury over Co/TiO2 catalysts prepared by sol–gel method, Catal. Commun. 12 (2011) 1291–1294. [9] 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, Ind. Eng. Chem. Res. 50 (2011) 9650–9656. [10] S.X. Wang, L. Zhang, G.H. Li, Y. Wu, J.M. Hao, N. Pirrone, F. Sprovieri, M.P. Ancora, Mercury emission and speciation of coal-fired power plants in China, Atmos. Chem. Phys. 10 (2010) 1183–1192. [11] E.J. Granite, H.W. Pennline, R.A. Hargis, Novel sorbents for mercury removal from flue gas, Ind. Eng. Chem. Res. 39 (2000) 1020–1029. [12] R.Q. Long, R.T. Yang, Characterization of Fe-ZSM-5 catalyst for selective catalytic reduction of nitric oxide by ammonia, J. Catal. 194 (2000) 80–90. [13] J.W. Portzer, J.R. Albritton, C.C. Allen, R.P. Gupta, Development of novel sorbents for mercury control at elevated temperatures in coal-derived syngas: results of initial screening of candidate materials, Fuel Process. Technol. 85 (2004) 621–630. [14] Y. Zhao, M.D. Mann, J.H. Pavlish, B.A.F. Mibeck, G.E. Dunham, E. Olson, Environ. Sci. Technol. 40 (2006) 1603–1608. [15] X.Y. Wen, C.T. Li, X.P. Fan, Experimental study of gaseous elemental mercury removal with CeO2 /Al2 O3 , Energy Fuels 25 (2011) 2939–2944. [16] H.L. Li, C.Y. Wu, Y. Li, Role of flue gas components in mercury oxidation over TiO2 supported MnOx CeO2 mixed-oxide at low temperature, J. Hazard. Mater. 243 (2012) 117–123. [17] L.L. Xing, Y.L. Xu, Q. Zhong, Mn and Fe modified fly ash as a superior catalyst for elemental mercury capture under air conditions, Energy Fuels 26 (2012) 4903–4909. [18] Y.L. Hao, B.C. Gates, Activation of dimethyl gold complexes on MgO for CO oxidation: removal of methyl ligands and formation of catalytically active gold clusters, J. Catal. 263 (2009) 83–91. [19] C.N. Liang, J.H. Li, M.F. Ge, R.H. Zhu, Enhanced activity of tungsten modified CeO2 /TiO2 for selective catalytic reduction of NOx with ammonia, Catal. Today 153 (2010) 77–83. [20] L. Ji, P.M. Sreekanth, P.G. Smirniotis, S.W. Thiel, N.G. Pinto, Manganese oxide/titania materials for removal of NOx and elemental mercury from flue gas, Energy Fuels 22 (2008) 2299–2306. [21] M. Narayanappa, V.D.B.C. Dasireddy, H.B. Friedrich, Catalytic oxidation of noctane over cobalt substituted ceria (Ce0.90 Co0.10 O2−␦ ) catalysts, Appl. Catal. A 447 (2012) 135–143. [22] G. Heinz, H. Adarn, The role of oxygen in photo-oxidation of organic molecules on semiconductor particles, J. Phys. Chem. 95 (1991) 5261–5267. [23] X.B. Chen, S. Gao, H. Wang, Y. Liu, Z. Wu, Selective catalytic reduction of NO over carbon nanotubes supported CeO2 , Catal. Commun. 14 (2011) 1–5. [24] J. He, K.R. Gunugunuri, S.W. Thiel, G. Panagiotis, Ceria-modified manganese oxide/titania materials for removal of elemental and oxidized mercury from flue gas, J. Phys. Chem. C 115 (2011) 24300–24309. [25] S.J. Yang, Y.F. Guo, N.Q. Yan, D.Q. Wu, H.P. He, J.K. Xie, Z. Qu, J.P. Jia, Remarkable effect of the incorporation of titanium on the catalytic activity and SO2 poisoning resistance of magnetic Mn Fe spinel for elemental mercury capture, Appl. Catal. B 101 (2011) 698–708. [26] R. Giovanoli, Thermogravimetry of manganese dioxides, Thermochim. Acta 234 (1994) 303–313. [27] Z.M. Wang, H. Kanoh, Calorimetric study on NH3 insertion reaction into microporous manganese oxides with (2 × 2) tunnel and (2 × ∞) layered structures, Thermochim. Acta 379 (2001) 7–14. [28] C.X. Gao, Q.F. Liu, D.S. Xue, Preparation and characterization of amorphous FeOOH nanowire arrays, J. Mater. Sci. Lett. 21 (2002) 1781–1783. [29] X.F. Tang, J.H. Li, L. Sun, J.M. Hao, Origination of N2 O from NO reduction by NH3 over -MnO2 and ␣-Mn2 O3 , Appl. Catal. B 99 (2010) 156–162. [30] J.H. Li, J.J. Chen, R. Ke, C.K. Luo, J.M. Hao, Effects of precursors on the surface Mn species and the activities for NO reduction over MnOx /TiO2 catalysts, Catal. Commun. 8 (2007) 1896–1900. [31] G.E. Dunham, R.A. DeWall, C.L. Senior, Fixed-bed studies of the interactions between mercury and coal combustion fly ash, Fuel Process. Technol. 82 (2003) 197–213.
