Journal of Hazardous Materials 299 (2015) 86–93

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Different crystal-forms of one-dimensional MnO2 nanomaterials for the catalytic oxidation and adsorption of elemental mercury Haomiao Xu, Zan Qu, Songjian Zhao, Jian Mei, Fuquan Quan, Naiqiang Yan ∗ School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

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

• 1D ␣-, ␤- and ␥- MnO2 were synthesized and first used for Hg0 capture.

• The performance for Hg0 removal enhanced in the order of ␤-MnO2 < ␥MnO2 < ␣-MnO2 . • The mechanism for Hg0 removal over different phase of MnO2 was discussed. • Hg-TPD method was used to explain the interaction force between mercury and MnO2 .

a r t i c l e

i n f o

Article history: Received 6 May 2015 Received in revised form 4 June 2015 Accepted 5 June 2015 Available online 9 June 2015 Keywords: One-dimensional Manganese dioxide Elemental mercury Adsorption

a b s t r a c t MnO2 has been found to be a promising material to capture elemental mercury (Hg0 ) from waste gases. To investigate the structure effect on Hg0 uptake, three types of one-dimensional (1D) MnO2 nano-particles, ␣-, ␤- and ␥-MnO2 , were successfully prepared and tested. The structures of ␣-, ␤- and ␥-MnO2 were characterized by XRD, BET, TEM and SEM. The results indicate that ␣-, ␤- and ␥-MnO2 were present in the morphologies of belt-, rod- and spindle-like 1D materials, respectively. These findings demonstrated noticeably different activities in capturing Hg0 , depending on the surface area and crystalline structure. The performance enhancement is in the order of: ␤-MnO2 < ␥-MnO2 < ␣-MnO2 at 150 ◦ C. The mechanism for Hg0 removal using MnO2 was discussed with the help of results from H2 -TPR, XPS and Hg0 removal experiments in the absence of O2 . It was determined that the oxidizability of three forms of MnO2 increased as follows: ␤-MnO2 < ␥-MnO2 < ␣-MnO2 . The mechanism for Hg0 capture was ascribed to the Hg0 catalytic oxidation with the reduction of Mn4+ → Mn3+ → Mn2+ . Furthermore, the interaction forces between mercury and manganese oxide sites are demonstrated to increase in the following order: ␤-MnO2 < ␥-MnO2 < ␣-MnO2 based on the desorption tests. © 2015 Published by Elsevier B.V.

1. Introduction

∗ Corresponding author. Fax: +86 21 54745591. E-mail address: [email protected] (N. Yan). http://dx.doi.org/10.1016/j.jhazmat.2015.06.012 0304-3894/© 2015 Published by Elsevier B.V.

Mercury has long been known for its human and environmental toxicity. Among the mercury compounds in the environment, elemental mercury (Hg0 ) in the atmosphere could increase global

H. Xu et al. / Journal of Hazardous Materials 299 (2015) 86–93

mercury pollution due to its chemical behavior in the environment [1–3]. Coal-fired power plants, cement plants and other non-ferrous industries are considered the main sources of Hg0 emissions [4–6]. Considerable work had been performed to control emissions from these industries [7,8]. However, it is still difficult to meet the strict emissions regulations. Adsorption technology is an effective method for capturing Hg0 . According to previous studies, carbon-based materials and transition metal oxides are two types of potential sorbents [9]. The cost of traditional activated carbon and modified activated carbon materials inhibits their wide application for capturing Hg0 [7,10]. Many reports showed that the transition metal oxides (such as FeOx , MnOx and CuOx ) are potentially suitable materials for the removal of Hg0 [11–17]. Among the transition metal oxides, manganese oxides are considered suitable alternative materials that are abundant, nontoxic and cheap. Additionally, manganese oxides have shown higher Hg0 adsorption capacities compared with other metal oxide sorbents. Qiao et al. reported a modified Mn/␣-Al2 O3 for the catalytic oxidation of Hg0 at low temperature [18]. Mn4+ species were the primary active sites for the oxidation of Hg0 . Based on the heterogeneous reaction between Mn4+ and Hg0 , a series of Fe-MnOx , Ce-MnOx and Sn-MnOx [14,15,19] sorbents were developed for the enhancement of Hg0 capturing. The hydroxyl and lattice oxygens oxidize Hg0 to Hg2+ and reduce Mn4+ to Mn3+ and further to Mn2+ . It is believed that the high valence number of manganese (Mn4+ ) is favorable for Hg0 oxidation. However, mixed-valence species of MnOx decreased the capacity of Hg0 to some extent. Few studies have focused on the valence effect of manganese dioxide (MnO2 ) for Hg0 adsorption. In addition, the activity for Hg0 is related to the surface area, defect structure, reducibility and morphology of a sorbent. MnO2 can form many types of polymorphs such as ␣-, ␤- and ␥-MnO2 [20]. It is generally accepted that the phase structure can significantly influence the catalytic activity of MnO2 . Liang et al. prepared ␣-, ␤-, ␥- and␦-MnO2 as catalysts for CO oxidation; the activities decreased in the order of ␣- ≈ ␦- > ␥- > ␤-MnO2 [21]. Dong et al. synthesized ␣-, ␤- and ␥-MnO2 for catalytic ozonation; the results showed that the phase of MnO2 strongly influenced the catalytic activity [22]. Moreover, the morphologies also make a difference in activity. Wang et al. synthesized MnO2 with rod-, wire-, tube- and flower-like morphologies; one-dimensional (1D) morphological manganese oxides showed higher activity than the corresponding nanoparticles when used for the removal of toluene [20]. Different crystallographic 1D MnO2 materials were also prepared for catalytic phenol degradation [23], and MnO2 exhibited varying activities depending on the surface area and crystalline structure. To gain a better understanding of the effect of structure on Hg0 removal, 1D ␣-, ␤- and ␥-MnO2 samples were prepared in our experiments. In this work, 1D ␣-, ␤- and ␥-MnO2 samples were synthesized using a hydrothermal reaction. The Hg0 removal experiments were performed under a simulated flue gas at 100 ∼ 300 ◦ C containing 0% and 4% O2 . The as-prepared sorbents were characterized by means of XRD, TEM, SEM, H2 -TPR, XPS and BET. The mechanism for the catalytic oxidation and adsorption of Hg0 were also discussed via results from the characterization techniques. Furthermore, the performance of mercury desorption was tested using Hg-TPD experiments.

2. Experimental section 2.1. Material synthesis The different MnO2 samples were prepared according to the hydrothermal method [24,25]. For ␣-MnO2 and ␤-MnO2 prepa-

87

Fig. 1. Process flow diagram for Hg0 adsorption capability assessment system.

ration, the following procedures were used: a mixed solution containing MnSO4 ·H2 O and KMnO4 (at a total amount of 10 mmol) was stirred for 60 min and transferred into a Telfon-lined autoclave. For ␣-MnO2 , the material ratio of n(MnSO4 ·H2 O)/n(KMnO4 ) was 5:5. For ␤-MnO2 , the material ratio of n(MnSO4 ·H2 O)/n(KMnO4 ) was 7:3. The autoclave temperature was then maintained at 160 ◦ C for 12 h, and the obtained precipitates were washed with water and dried at 80 ◦ C for 12 h. For ␥-MnO2 , MnSO4 ·H2 O (10 mmol) and an appropriate amount of (NH4 )2 S2 O8 (1.83 g) were well-mixed and hydrothermally treated at 90 ◦ C for 24 h. Last, the obtained precipitates were also washed with water and dried at 80 ◦ C for 12 h. 2.2. Characterization The XRD patterns of as-prepared samples were recorded on an X-ray diffractometer (APLX-DUO, BRUKER, Germany) with Cu K␣ radiation, and the data were collected in the 2 range 10◦ –80◦ with a scanning velocity of 5◦ /min. The BET (Brunauer–Emmett–Teller) surface areas of the samples were determined using N2 adsorption at −196 ◦ C using a quartz tube (Quantachrome 2200e). The morphologies of the samples were investigated using transmission electronic microscopy (TEM) (JEOLJEM-2010). The surface morphology was studied using a scanning electron microscope (SEM) (Hitachi Corp., Japan) after the surface was coated with platinum. The H2 -TPR experiments were performed on a Chemisorp TPx 290 instrument; the samples were degassed at 200 ◦ C for 3 h under Ar atmosphere before the tests, and the reducing gas was 10% H2 /Ar. Xray photoelectron spectroscopy (XPS) results were recorded with an Ultra DLD (Shimadzu–Kratos) spectrometer with Al K␣ as the excitation source, and the C 1s line at 284.6 eV was used as a reference for the binding energy calibration. 2.3. Hg0 removal test The Hg0 removal tests using the as-prepared 1D ␣-, ␤- and ␥-MnO2 samples were performed in a fixed-bed tubular reactor connected to a cold vapor atomic absorption spectrometer mercury detector (CVASS), which was calibrated by Lumex RA 915+. As illustrated in Fig. 1, the experimental set-up consisted of an Hg0 permeation tube, a fixed-bed reactor with a parallel blank tube, a CVAAS and an online data acquisition system. To investigate the Hg0 removal efficiency, 20 mg of sample was loaded in the quartz reactor (a quartz tube with an inner diameter of 5 mm, and a tube type resistance furnace), and the simulated flue gas was introduced into the reactor. During each test, the total flow rate was kept at 500 ml/min and all balanced in N2 . The inlet concentration of Hg0 was 500 ␮g/m3 . The gas space velocity was about 4.8 × 105 h−1 . The outlet Hg0 concentration was recorded on the CVASS detector.

-MnO2

160

300

002

310

211

541

301

521

310

200

110

A mercury temperature programmed desorption (Hg-TPD) method was developed to investigate the desorption characteristics of the materials. After mercury adsorption at 150 ◦ C in the presence of 4% O2 for 20 min, sorbents were regenerated by heating from 100 ◦ C to 500 ◦ C in pure N2 as the carrier gas. The heating rate was 5 ◦ C/min. The mercury signal was recorded using the CVASS online system.

-MnO2

220

2.4. Desorption test

200

where the Hgin 0 is the inlet concentration of Hg0 , and Hgout 0 is the outlet concentration of Hg0 .

600

(1)

411

,

111

Hg0in

101

Hg0in − Hg0out

110

r =

120

The Hg0 removal efficiency (r ) was calculated according to Eq. (1):

131

H. Xu et al. / Journal of Hazardous Materials 299 (2015) 86–93

211

88

-MnO2

10

20

30

40

50

60

70

2 theta, degree

3. Results and discussion Fig. 2. XRD patterns of the ␣-, ␤- and ␥-MnO2 samples.

3.1. Characterization of MnO2 materials Fig. 2 shows the XRD patterns of as-prepared manganese dioxides samples. The ␣-MnO2 formed double chains of edge-sharing [MnO6 ] octahedra with (2 × 2) and (1 × 1) tunnels, and the lattice constants were in good agreement with ␣-MnO2 (JCPDS 44-0141). For the ␤-MnO2 and ␥-MnO2 samples, the XRD patterns corresponded to the standards of ␤-MnO2 (JCPDS 24-0735) and ␥-MnO2 (JCPDS 14-0644), respectively [26]. The ␤-MnO2 consisted of single strands of edge-sharing [MnO6 ] octahedra with (1 × 1) tunnels. ␥-MnO2 was formed by the random intergrowth of ramsdellite (1 × 2) and pyrolusite with (1 × 1) structure [22,26]. In each pattern, no other peaks could be attributed to manganese oxides of other crystallographic forms indicating the high purity of the asobtained products. Moreover, the strong peaks of the XRD patterns for ␣-MnO2 and ␤-MnO2 showed good crystallinity resulting from thermal treatment at higher temperatures compared with ␥-MnO2 . Table 1 lists the BET surface areas, the pore volumes and pore diameters. The BET surface area of ␣-MnO2 (ca. 31 m2 /g) was similar to the ␥-MnO2 (ca. 35 m2 /g) and much larger than that of ␤-MnO2 (ca. 6 m2 /g). In addition, the pore volume increased in the following order: ␤-MnO2 < ␣-MnO2 < ␥-MnO2 . The average pore diameters of ␣-MnO2 and ␥-MnO2 were approximately 3.6 nm,

Table 1 BET surface areas, pore volume and pore diameter. Samples

BET surface areas (m2 /g)

Pore volume (m3 /g)

Pore diameter (nm)

␣-MnO2 ␤-MnO2 ␥-MnO2

31.013 6.458 35.124

0.066 0.019 0.202

3.693 22.665 3.675

which is noticeably smaller than that of ␤-MnO2 (ca. 22.6 nm). The differences in the phase structures of the manganese dioxides resulted in large differences in the surface areas, pore volumes and average pore sizes, which may cause the differences in the Hg0 adsorption performances. The morphologies of the as-obtained products were observed by TEM and SEM. As indicated from the TEM images (Fig. 3(a) and (c)), ␣-MnO2 (Fig. 3(a)) exhibited belt-like nanostructures with diameters of 25 ∼ 50 nm. ␤-MnO2 (Fig. 3(b)) formed rod-like nanostructures with diameters of 30 ∼ 100 nm and ␥-MnO2 (Fig. 3(c)) appeared as spindle-like structures, of which the morphologies were not as uniform as for ␣-MnO2 and ␤-MnO2 . All three crystallographic MnO2 samples had one-dimensional (1D) morphologies. The TEM results provide further information on the 1D structure

Fig. 3. TEM images of the (a) ␣-MnO2 , (b) ␤- MnO2 and (c) ␥- MnO2 samples, and SEM images of the (d) ␣-MnO2 , (e) ␤- MnO2 and (f) ␥-MnO2 samples.

H. Xu et al. / Journal of Hazardous Materials 299 (2015) 86–93

Mn

4+

Mn 318

100

3+

-MnO2 80

428

-MnO2

323

60

441 Mn

-MnO2

3+

Mn

C/C0

TCD signal

89

2+

40

334

-MnO2

20

-MnO2

361 -MnO2

100

0 0

200

300

400 o

500

600

100

200

700

300

400

Time, min

500

(a)

T, C

100

Fig. 4. H2 -TPR profiles of ␣-, ␤- and ␥-MnO2 sorbents.

3.2. Hg0 removal performance on MnO2 To compare the Hg0 removal performance of different asprepared samples, the experiments were tested in a fixed-bed reactor. As shown in Fig. 5(a), the Hg0 removal efficiency was calculated at 150 ◦ C (4% O2 ). It was obvious that ␣-MnO2 , ␤-MnO2 and ␥-MnO2 revealed different performances for Hg0 capture. ␣MnO2 exhibited the highest Hg0 removal efficiency under these reaction conditions; the Hg0 removal efficiency was higher than 90% after a 600 min adsorption test. For ␥-MnO2 , the Hg0 removal efficiency was approximately 70% after a 600 min adsorption test. However, the ␤-MnO2 showed the lowest activity for Hg0 , reaching the saturation adsorption within 300 min. The Hg0 removal efficiency decreased in the order of ␣-MnO2 > ␥-MnO2 > ␤-MnO2 nanostructures. Obviously, the crystal-forms of MnO2 had remarkable influences on Hg0 removal. Furthermore, the effect of reaction temperature on the activities of the three types of MnO2 was tested, and the results are shown in Fig. 5(b). The reaction conditions were at 150 ◦ C under 4% O2 with a

-MnO2 -MnO2

80

-MnO2 Removal efficiency, %

of the prepared samples. Fig. 3(d) and (e) are belt-like and rod-like MnO2 for ␣-MnO2 and ␤-MnO2 , respectively. The average diameter of ␣-MnO2 was shorter than for ␤-MnO2 . The results were consistent with the TEM study. Fig. 3(f) shows clear spindle-like structures of ␥-MnO2 , and the diameters of ␥-MnO2 were not uniform. Based on the XRD, TEM and SEM results, 1D ␣-, ␤- and ␥-MnO2 samples were successfully synthesized in our study [23–25]. Fig. 4 illustrates the H2 -TPR profiles of the ␣-, ␤- and ␥-MnO2 sorbents. For ␣-MnO2 , there was a strong reduction band at 334 ◦ C, and a weak band at approximately 361 ◦ C. For ␤-MnO2 , two H2 consumption peaks were observed at 323–331 ◦ C. The TPR profile of the ␥-MnO2 was similar to that of ␤-MnO2 , but both reduction peaks shifted to the lower temperatures at 318–428 ◦ C. The lower temperature peak is assigned to the reduction of MnO2 to Mn3 O4 , whereas the higher temperature peak is assigned to the reduction of Mn3 O4 to MnO [21]. It was clear that ␣-MnO2 was the most easily oxidized because nearly all MnO2 was directly reduced to MnO at a lower temperature. The ␥-MnO2 was more easily oxidized than ␤-MnO2 , as indicated by the presentation of the reduction peaks at lower temperatures. Furthermore, the higher temperature peak of ␤-MnO2 was clear, which indicated that the reduction of MnO2 to MnO required two independent steps. According to the H2 -TPR result, the reduction enhancement of the sorbents increased in the order of ␤-MnO2 < ␥-MnO2 < ␣-MnO2 .

600

60

40

20

0 100

150

200

250

300

o

Temperature, C

(b) Fig. 5. (a) Hg0 removal efficiency of ␣-, ␤- and ␥-MnO2 samples at 150 ◦ C and 4% O2 ; (b) Hg0 removal efficiency of the samples at various temperatures ranging from 150 ∼ 300 ◦ C with 4% O2 . The mixed gas was equilibrated with N2 with a total flow rate of 500 ml/min.

total flow rate of 500 ml/min. The reaction time was 600 min. For ␣MnO2 , the removal efficiency increased from 84% to 92% when the temperature increased from 100 ◦ C to 150 ◦ C. Next, a sharp decrease followed when the temperature increased from 150 ◦ C to 300 ◦ C; ␥MnO2 showed the same tendency. The highest removal efficiency resulted at 150 ◦ C, but the Hg0 removal efficiencies did not drop as sharply as with ␣-MnO2 . For ␤-MnO2 , the temperature appeared to have no effect on Hg0 removal, as indicated by a lower Hg0 removal efficiency. The effect of temperature on the Hg0 removal was obvious for ␣-MnO2 and ␥-MnO2 . The effect of temperature on Hg0 removal decreased in the order of ␣-MnO2 > ␥-MnO2 > ␤-MnO2 . Obviously, ␣-MnO2 exhibited the best performance for Hg0 capture, while ␤-MnO2 had the lowest activity for Hg0 . It is believed that both physical and chemical adsorption co-exist during the Hg0 capture process [27]. In general, MnO2 forms a framework of “octahedral molecular sieve” structures [23]. However, the different crystal forms of MnO2 are determined by the way the [MnO6 ] octahedra interlink, which in turn produces the differences in the physical properties. The larger surface areas of ␣-MnO2 and ␥MnO2 enhance the adsorption. For ␤-MnO2 , the lower surface area and pore volume led to a lower capacity for Hg0 , as determined from the BET results. The crystal-form of MnO2 had more signif-

90

H. Xu et al. / Journal of Hazardous Materials 299 (2015) 86–93

Fig. 6. Hg0 adsorption tests for the prepared sorbent at 150 ◦ C under pure N2 or 4% O2 , (1) ␣-MnO2 , (2) ␤- MnO2 and (3) ␥-MnO2 . The mixed gas was equilibrated with N2 with a total flow rate of 500 ml/min.

icant influence on Hg0 removal than the surface areas. However, it is still hard to explain why ␣-MnO2 exhibits a higher activity than ␥-MnO2 . Generally, the chemical adsorption and the physical oxidation co-existed in the Hg0 adsorption process over Mn-based materials [15,28]. The effects of chemical adsorption are discussed in the following section.

3.3. Effect of O2 on the catalytic oxidation of Hg0 To further illustrate the mechanisms for Hg0 adsorption on ␣-MnO2 , ␤-MnO2 and ␥-MnO2 , in-depth experiments were performed. The adsorption was first tested under pure N2 atmosphere for the as-prepared sorbents, and then 4% O2 was added in the mixed gas with an equilibrium flow rate of 500 ml/min. The adsorption curve was compared with the results of 4% O2 . As shown in Fig. 6(1), ␣-MnO2 lost its adsorption capacity gradually in the pure N2 atmosphere condition. When 4% O2 was added, the adsorption curve decreased, which indicated that the adsorption capacity was restored to some extent. For comparison, an adsorption test with 4% O2 + N2 was recorded. It is clear that the removal efficiency in these conditions was higher than for the pure N2 condition. The removal efficiency for the pure N2 condition did not reach as high as with 4% O2 even when 4% O2 was added later in the experiment. Additionally, the same phenomenon was found in the adsorption tests with ␤-MnO2 and ␥-MnO2 . It has been proposed that oxygen plays an important role in the adsorption process [15]. O2 enhanced the catalytic oxidation performance of Hg0 , in which the oxidized mercury could adsorb on the surface of MnO2 with a strong chemical bond. During the chemical adsorption process, the catalytic oxidation performance of the materials enhances Hg0 adsorption. Among the three forms of MnO2 , ␣-MnO2 showed the highest activity. As discussed in the H2 -TPR results, the ability of the materials to be oxidized decreased in the following order: ␣-MnO2 > ␥-MnO2 > ␤-MnO2 . The higher valance of Mn offers the catalytic oxidation performance for Hg0 . The results matched well

with the Hg0 adsorption performance, which further indicates the important role of catalytic oxidation during the adsorption process. 3.4. Mechanism for Hg0 capturing over 1D MnO2 To gain insight into the Hg0 adsorption mechanism, the both as-made ␣-, ␤- and ␥-MnO2 sorbents and sorbents following Hg0 adsorption in the pure N2 atmosphere were selected for XPS analysis. The XPS spectral regions of O 1s, Mn 2p and Hg 4f are shown in Fig. 7. The summary of the results of XPS analysis are shown in Table 2. As shown in Fig. 7(a), for the fresh ␣-, ␤- and ␥-MnO2 sorbents, the XPS spectrum of O 1s spectra exhibited two primary peaks. The peak at 531.5 eV was assigned to adsorption oxygen (Oads ) [20,21]. The peaks at 529.7, 529.4 and 529.8 eV of ␣-, ␤and ␥-MnO2 were assigned to lattice oxygen (Olatt ). The ratios of Oads /Olatt for ␣-, ␤- and ␥-MnO2 were 52.32, 61.03 and 65.34, respectively. The XPS results of Mn 2p for the fresh ␣-, ␤- and ␥MnO2 sorbents are shown in Fig. 7(c). The peaks observed at 642.7, 642.6 and 642.8 eV correspond to Mn4+ for ␣-, ␤- and ␥-MnO2 , respectively [29,30]. In addition, the molar ratio of Mn and O was calculated according to the XPS analysis, as shown in Table 2. The molar ratio of O to Mn was approximately 2.17 for ␣-MnO2 , and approximately 1.88 and 2.06 for ␤-MnO2 and ␥-MnO2 , respectively. These results indicate that the main component of the sorbent was MnO2 . After adsorption, there were no new peaks in the spectra of O 1s, The two prominent peaks of each sorbent were assigned to the adsorption and lattice oxygens. However, the ratios of Oads /Olatt obviously changed. The ratio for ␣-MnO2 increased from 52.32% to 76.62% compared to that of the fresh sorbent, whereas the ratio for ␥-MnO2 increased from 65.34% to 80.50%. However, for ␤-MnO2 , the ratio of after adsorption was a little lower than that of the fresh sorbent. These observations suggest that the lattice oxygen or adsorption oxygen participated in Hg0 oxidation. The increase in the ratio of Oads /Olatt could result from the decrease in lattice oxygen because the adsorption test was performed under pure N2

H. Xu et al. / Journal of Hazardous Materials 299 (2015) 86–93

91

Fig. 7. XPS spectra of the 1D MnO2 : (a) O 1s spectra of the fresh samples; (b) O 1s spectra of the post-adsorption samples; (c) Mn 2p spectra of the fresh samples; (d) Mn 2p spectra of the post-adsorption samples; and (e) Hg 4f spectra of the post-adsorption samples.

Table 2 Summary of the results of the XPS analysis. Sorbent

␣-MnO2 ␤-MnO2 ␥-MnO2

Fresh

After adsorption

Mn, %

O, %

Oads /Olatt , %

Mn2+ /Mn3+ , %

Mn3+ /Mn4+ , %

Oads /Olatt , %

31.50 34.78 32.66

68.50 65.22 67.34

52.32 61.03 65.34

5.98 0 3.31

388.49 42.76 65.12

76.62 57.42 80.50

92

H. Xu et al. / Journal of Hazardous Materials 299 (2015) 86–93

at 150 ◦ C. Furthermore, the spectra of Mn 2p after adsorption are shown in Fig. 7(d); the peaks at 642.9, 642.8 and 643.0 eV were ascribed to Mn4+ for ␣-MnO2 , ␤-MnO2 and ␥-MnO2 , respectively. For ␣-MnO2 , two more peaks were observed at 642.1 and 641.6 eV corresponding to Mn3+ and Mn2+ , respectively. For ␤-MnO2 , a peak at 641.9 eV corresponds to Mn3+ , and no Mn2+ peak was observed [15]. For ␥-MnO2 , the Mn3+ peak was observed at 642.0 eV. In addition, the ratios of Mn2+ /Mn3+ and Mn3+ /Mn4+ were calculated. As shown in Table 2, the ratios of Mn3+ /Mn4+ were approximately 388%, 42% and 65% for ␣-MnO2 , ␤-MnO2 and ␥-MnO2 , respectively, and the ratios of Mn2+ /Mn3+ were approximately 6%, 0 and 3% for ␣-MnO2 , ␤-MnO2 and ␥-MnO2 , respectively. The reduction of Mn4+ to Mn3+ or Mn2+ enhanced the Hg0 oxidation. The ability of the materials to catalytically oxidize Hg0 decreased in the order of ␣-MnO2 > ␥-MnO2 > ␤-MnO2 . During the adsorption process, the change of MnO2 to Mn2 O3 then to MnO likely increases the number of lattice oxygens. Both the reduction of Mn4+ and the oxidation of Hg0 occurred on the surface of the sorbents. Such results also indicate that the concentration of lattice oxygens decreased during the adsorption process. The XPS spectra for Hg 4f, shown in Fig. 7(e), provide further evidence of Hg0 oxidation. The peaks at approximately 104.7 and 100.8 eV correspond to Hg2+ [28]. It is believed that mercury existed in the form of HgO on the surface of ␣-MnO2 , ␤-MnO2 and ␥-MnO2 . Based on the above discussion, it could be speculated that the Hg0 was first adsorbed on the surface of the sorbent, existing as adsorbed mercury ( Hg0 ). The catalytic oxidation of Hg0 occurred together with the reduction of Mn4+ . The high valence of MnO2 transformed to Mn2 O3 or MnO and offered the lattice oxygen ([O]) during the metal transfer process. The [O] contributed to the oxidation of Hg0 , therefore, allowing mercury to exist as HgO on the surface of the sorbent. The mechanism for Hg0 capture is as follows: [9,27] 2MnO2 → Mn2 O3 + [O]

(2)

Mn2 O3 → 2MnO + [O]

(3)

Hg0 →

(4)

Hg0

Hg0 + [O] →

HgO

(5)

In addition, in the presence of O2 in the simulated gas, the Mn2 O3 and MnO can be re-oxidized by O2 . It was believed that O2 provided MnO2 a longer continuous usage time, resulting in a higher Hg0 capacity. The mechanism for the metal oxide regeneration is as follows: 2MnO + 1/2O2 → Mn2 O3

(6)

Mn2 O3 + 1/2O2 → 2MnO2

(7)

3.5. Desorption performance on MnO2 Furthermore, the property of desorption was tested by the HgTPD method. The results of the Hg-TPD curves for the prepared samples as shown in Fig. 8. For ␣-MnO2 , mercury was released from the surface of ␣-MnO2 at approximately 400 ◦ C, and there was a strong desorption peak at 447 ◦ C. For ␤-MnO2 , mercury was released from the surface of ␤-MnO2 beginning at 150 ◦ C, and desorption finished at approximately 450 ◦ C. Interestingly, two desorption peaks were observed at 246 ◦ C and 398 ◦ C. The lower the temperature at which the desorption peak was detected, the weaker the interaction force was between mercury and the manganese oxide sites. Mercury exhibited a lower interaction force with ␤-MnO2 than with ␣-MnO2 . Additionally, for ␥-MnO2 , a small amount of mercury was released at approximately 150 ◦ C, and the rest of the mercury was released at a temperature higher than 400 ◦ C. A weak peak at 234 ◦ C and peaks at 429 ◦ C and higher than

Fig. 8. Hg-TPD curves of the prepared sorbents, (a) ␣-MnO2 , (b) ␤- MnO2 and (c) ␥-MnO2 .

500 ◦ C were recorded. The interaction forces between the mercury and manganese dioxide sites increased for the materials in the order of ␤-MnO2 < ␥-MnO2 < ␣-MnO2 . The results also indicate that the adsorption performance followed the same trend as the interaction force. Moreover, it is a significant that mercury could be release from the surface of the sorbents using a simple thermal regeneration method. 4. Conclusions Various crystallographic phases of MnO2 sorbents were synthesized, and they revealed varying morphologies and Hg0 removal performances. ␣-MnO2 showed the highest Hg0 removal efficiency, and it exhibited a belt-like 1D morphology. The high surface area and oxidizability of ␣-MnO2 increases its potential as a material for use in Hg0 capture applications. ␤-MnO2 had a rod-like 1D morphology and the lowest activity for Hg0 due to its low surface area and stable oxygen reduction. ␥-MnO2 presented lower Hg0 removal efficiency than ␣-MnO2 ; it exhibited a spindle-like morphology and a similar surface area to ␣-MnO2 . Further investigation of the catalytic oxidation of Hg0 indicated that the Hg0 was first oxidized to Hg2+ by the Mn active sites. Then, Hg2+ was adsorbed on the surface of the sorbents existing as HgO. The characteristics of desorption were also tested, and the results show that the interaction force between Hg2+ and Mn active sites increased in the following order: ␤-MnO2 < ␥-MnO2 < ␣-MnO2 . Acknowledgments This study was supported by the Major State Basic Research Development Program of China (973 Program, No. 2013CB430005), and the National Science Foundation of China (No. 21277088, 51478261) National High-Tech R&D Program (863) of China (No. 2013AA065403). References [1] C.-J. Lin, S.O. Pehkonen, The chemistry of atmospheric mercury: a review, Atmos. Environ. 33 (1999) 2067–2079. [2] F. Sprovieri, N. Pirrone, R. Ebinghaus, H. Kock, A. Dommergue, A review of worldwide atmospheric mercury measurements, Atmos. Chem. Phys. 10 (2010) 8245–8265. [3] R. Sun, J. Sonke, L.-E. Heimbürger, H. Belkin, G. Liu, D. Shome, E. Cukrowska, C. Liousse, O. Pokrovski, D.G. Streets, Mercury stable isotope signatures of world coal deposits and historical coal combustion emissions, Environ. Sci. Technol. 48 (2014) 7660–7668. [4] E.G. Pacyna, J. Pacyna, K. Sundseth, J. Munthe, K. Kindbom, S. Wilson, F. Steenhuisen, P. Maxson, Global emission of mercury to the atmosphere from

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