Elemental Mercury Removal by MnO2 Nanoparticle Decorated

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE

Fossil Fuels

Elemental Mercury Removal by MnO2 Nanoparticle Decorated Carbon Nitride Nanosheet Dongjing Liu, Zhen Zhang, and Jiang Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00149 • Publication Date (Web): 30 Mar 2019 Downloaded from http://pubs.acs.org on March 31, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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

Energy & Fuels

Elemental Mercury Removal by MnO2 Nanoparticle Decorated Carbon Nitride Nanosheet

Dongjing Liu 1*, Zhen Zhang 2, Jiang Wu 2*

1

School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013,

China 2

College of Energy and Mechanical Engineering, Shanghai University of Electric

Power, Shanghai 200090, China

1

ACS Paragon Plus Environment

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

ABSTRACT: Carbon nitride nanosheet (CNNS) is facilely synthesized via a two-step thermal oxidative etching approach and then decorated with MnO2 nanoparticles via incipient wetness impregnation method for Hg0 removal at low temperature. Pristine CNNS exhibits good Hg0 adsorption performance associated with the optimal Hg0 removal efficiency of ~65.8% at 150 °C due to its two-dimensional planar structure and big surface area. MnO2-decoration can greatly enhance the Hg0 removal performance of CNNS presumably ascribed to the improved electron transfer at the interface of MnO2 and g-C3N4. 10MnO2/CNNS presents superior Hg0 oxidation ability with Hg0 removal efficiency all above 91% within temperature range of 90-240 °C. The long-term test and XPS analysis disclose that 10MnO2/CNNS is a cost-effective catalyst/sorbent for oxidation removal of elemental mercury. The gaseous Hg0 is oxidized into HgO by the lattice oxygen of MnO2 which is the dominant active site for Hg0 oxidation via redox couple of Mn4+/Mn3+.

INTRODUCTION Mercury emission from anthropogenic sources has raised considerable global concern due to its persistence, bioaccumulation, and detrimental effect on human nervous systems.1 Elemental mercury, the predominant speciation of atmospheric mercury, possesses a long residence time up to two years in the atmosphere and can be transported and deposited to remote palaces even one thousand kilometers away from the sources.2 What is more, elemental mercury can be converted to methylmercury and accumulated in food chains, which poses a potential threat to

2


Page 2 of 31

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

Energy & Fuels

humans' health.3 Coal-fired power plants, especially those equipped with utility boilers, are commonly considered as the largest emission source of anthropogenic mercury emissions worldwide.4 Nowadays, many countries have taken steps to reduce the anthropogenic mercury emissions. In August 2017, the Minamata Convention on Mercury aiming at the protection of human health and the environment from anthropogenic mercury emissions had been executed by fifty countries.5 To meet these global mercury emission standards, efficient mercury control technologies for coal-fired power plants are urgently needed. Catalytic oxidation and adsorption removal of elemental mercury at low temperature by using solid catalysts or sorbents is deemed as the most promising control technology for mercury capture from coal-derived flue gas.6 So far, many carbon-based and non-carbon based materials has been used for mercury removal, for instance, activated carbon,7 biomass-derived chars,8 and natural minerals.9 Recently, numerous novel carbon materials emerged due to the development of nanotechnology and have attracted increasing research interest in the field of mercury emission control. Luo10 synthesized a carbon nanotube-silver (Ag-CNT) composite for Hg0 removal from flue gas. The mercury could be completely captured by Ag-CNT at 150 °C. Xu11 prepared an Ag-modified graphene for Hg0 removal and it could fully capture elemental mercury at 25 °C. The mercury capacity was as high as 4.2 mg/g and it showed no degradation after six cycles. Xu12 fabricated a MnOx-loaded graphene which exhibited better mercury oxidation performance compared to pure MnOx due to the highly dispersed MnOx on graphene nanosheets and the improved electricity

3


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

conductivity of MnOx. Zhao13 employed Mn-Mo co-impregnated carbon nanotube for catalytic oxidation of elemental mercury at low temperature. Mn-Mo/CNT performed effectively for Hg0 oxidation at 150-250 °C. Yang14 prepared a Fe-Mn decorated carbon nanofiber (CNF) for Hg0 removal from flue gas. The magnetic Fe-Mn/CNF exhibited over 90% Hg0 removal efficiency at 150-200 °C and it can be efficiently regenerated by water washing followed by thermal treatment. However, the high capital cost and tedious synthesis process for graphene, carbon nanotube and carbon nanofiber hindered its large-scale application in mercury emission control of coal-fired power plants. Due to its abundant sources, low cost and facile synthesis operation, graphitic carbon nitride (g-C3N4), as a sustainable and environmentally friendly polymer, has attracted extensive research attention.15 Two-dimensional g-C3N4 nanosheet with big surface area and unique structural feature can be facilely attained via thermal exfoliation of bulk g-C3N4. The apparent merits associated with g-C3N4 nanosheet encompass big surface area for exposing ample reactive sites and short bulk diffusion length for facilitating electron transfer,16 which are beneficial for adsorption and catalytic reactions. 17 To date, the g-C3N4 nanosheet (CNNS) had been widely applied in the field of air pollutant control, such as carbon dioxide reduction 18, 19 and nitrogen oxide removal. 20, 21 Therefore, CNNS could be a potential candidate for Hg0 removal as well. In addition, transition-metal oxides, especially MnO2, are proved to be effective for Hg0 oxidation. 22, 23 In this study, CNNS is modified by decorating with MnO2 nanoparticles (NPs) and applied for Hg0 removal at low temperature. The

4


Page 4 of 31

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

Energy & Fuels

effects of MnO2 loading value and reaction temperature on Hg0 removal performance of CNNS and MnO2/CNNS are investigated. The mechanism of Hg0 capture over MnO2/CNNS is elucidated therewith. EXPERIMENTAL SECTION Chemicals. Melamine and manganese nitrate (Mn(NO3)2·4H2O) are purchased from Guoyao Chemical Reaegent Co., Ltd. All chemical reagents are analytic grade and don’t require further purification. Synthesis of g-C3N4 nanosheet. First, 10 g of melamine are put into a crucible with cover and heated in static air to 550 °C for 2 h with a ramp rate of 5 °C/min. The resultant blocky products are milled into powders which are bulk g-C3N4. Second, 5 g of as-synthesized bulk g-C3N4 are placed into a crucible without cover and heated in static air to 550 °C for 3 h at the same heating rate, yielding g-C3N4 nanosheet labeled as CNNS.24 Preparation of xMnO2/CNNS. First, diverse amounts of Mn(NO3)2·4H2O are dissolved in 0.5 g of deionized water. Second, 0.2 g of as-attained CNNS are put into the aforementioned solution and dried at 70 °C for 12 h. The generated mixture is then placed into an open alumina crucible and heated in static air to 200 °C for 2 h with a heating rate of 5 °C/min. The resulting products are named as xMnO2/CNNS; x denotes the mass percentage of MnO2 in the catalyst. Sample characterization. The X-ray diffraction (XRD) patterns, field emission scanning electron microscopy (FESEM) images, transmission electron microscopy (TEM) images, Brunauer-Emmett-Teller (BET) surface areas as well as pore size

5


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

distribution curves, Fourier transform infrared spectroscopy (FTIR) spectra, and X-ray photoelectron spectroscopy (XPS) profiles are performed. The detailed detection approaches and device parameters had been described in previous papers. 25, 26

Mercury capture. The mercury removal experimental system mainly consists of a gas-phase Hg0 generation device, a packed-bed reactor, a mercury detector and an online data acquisition apparatus as illustrated in Figure 1. A tubular furnace is employed to control the reactor sustaining at desired temperatures. The gas flow is precisely controlled by mass flow controllers. A nitrogen stream (carrier gas) with a flow rate of 0.2 L/min is charged into the PSA device (PS Analytical, Kent, U.K) to yield a stable concentration of gaseous Hg0. Prior to each test, the feed gas stream (5 % O2/N2) with a total flow rate of 1.2 L/min firstly bypasses the reactor (open valve 2, close valve 1 and 3) until reaching a stable mercury concentration and the desired temperature. Afterwards, the Hg0-laden (~55 µg/m3) flue gas is diverted into the upflow fixed-bed reactor (open value 1 and 3, close valve 2) for mercury capture tests. The catalyst (50 mg) is loaded at the center of a quartz tube micro-reactor (8 mm inner diameter, 700 mm length), which corresponds to a gas space hourly velocity of 140, 000 h-1. The influent and effluent mercury concentration is examined by an online mercury analyzer (Lumex, RA-915-M, St. Petersburg, Russia). A reducing bottle containing SnCl2 solution is placed prior to the Lumex detector to reduce the oxidized mercury in flue gas into elemental mercury. The mercury conversion can be calculated by the following formula,

6


Page 6 of 31

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

Energy & Fuels

  1  Cout / Cin  100%

(1)

, where η denotes the mercury removal efficiency (%); Cin and Cout denote the inlet and outlet mercury concentration (µg/m3), respectively. The 10MnO2/CNNS after mercury capture tests at 120 °C refer to the spent 10MnO2/CNNS specimen. RESULTS AND DISCUSSION Characterization analysis. The crystal structures of as-attained CNNS and xMnO2/CNNS are determined by XRD patterns as shown in Figure 2. The hexagonal phase of g-C3N4 (JCPDS no. 87-1526) can be verified by two feature peaks. The sharp peak at ~27.5° stems from the stacking of the conjugated aromatic system, which is indexed as the (002) plane of the graphitic materials. Another minor diffraction peak at ~12.9° corresponds to the (001) plane associated with the in-plane periodic unities of tri-s-triazine.27 The feeble diffraction signals at ~37.1 and 56.8° in xMnO2/CNNS specimens are ascribed to MnO2 (JCPDS no. 44-0141), suggesting that MnO2 is present as an amorphous phase on CNNS surface.28 With respect to the pristine CNNS, the diffraction angles of the peaks arising from the repeated stacking of layers shift toward higher values after modifying with MnO2, implying a reduced gallery distance between the basic sheets in xMnO2/CNNS.29 The FESEM and TEM images of CNNS and 10MnO2/CNNS are shown in Figure 3. The pristine CNNS consists of a few curled lamellar structures and a couple of solid agglomerates (Figure 3a). After loading with MnO2, the surface of 10MnO2/CNNS becomes a little bit slippy and tight presumably causing the decline of BET surface area and pore volume (Figure 3b). Pure CNNS displays a transparent 7


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

feature with lateral size of ~3 μm and thickness of few nanometers (Figure 3c), implying the formation of ultrathin two-dimensional nanosheet morphology of g-C3N4 owing to the stripping of bulk g-C3N4. The darker aggregates are ascribed to the overlap of plentiful g-C3N4 layers. Additionally, a number of mesoporous structures with diameter of dozens of nanometers can be observed in CNNS (Figure 3d). As for 10MnO2/CNNS, MnO2 cuboid nanoparticles of ~60-80 nm are produced and well integrated with g-C3N4 (Figure 3e). The lattice fringes with interplanar distance of 0.493 nm are assigned to the diffraction of MnO2 (200) plane (Figure 3f), which implies the formation of MnO2 phase in 10MnO2/CNNS. The nitrogen adsorption-desorption isotherms and pore size distribution curves of CNNS and xMnO2/CNNS are displayed in Figure 4. All specimens exhibit type IV isotherms with distinct hysteresis loops at higher relative pressures, suggesting the existence of mesopores.30 In addition, the nitrogen uptake at lower relative pressure is fairly less, suggesting that mesopores is the dominant porous structures. The pore size distribution curves further confirm the presence of varisized mesopores. All samples exhibit one sharp pore size peak at 4 nm but also a significant portion of pores in broad mesopore ranges. The BET surface area, total pore volume and average pore diameter of pristine CNNS are 109 m2/g, 0.456 cm3/g and 19 nm (Table 1), respectively. After decorating with MnO2, the BET surface area and total pore volume decrease to 25-56 m2/g and 0.107-0.308 cm3/g correspondingly. This suggests that a proportion of the smaller mesopores of CNNS might be covered by MnO2 particles. The FTIR adsorption bands of CNNS and xMnO2/CNNS are shown in Figure 5.

8


Page 8 of 31

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

Energy & Fuels

Pristine CNNS features two bands at ~1634 and 1574 cm-1 corresponding to C-N stretching vibration.29 The four peaks at 1462, 1406, 1327, 1251 cm-1 are attributed to aromatic C-N stretching vibration

31.

The two adsorption peaks at 893 and 816 cm-1

belong to the characteristic breathing mode of triazine ring mode, which relates to condensed C-N heterocycles.32 All the main feature vibrations of FTIR bands stemmed from g-C3N4 can be distinctly detected in CNNS and xMnO2/CNNS, suggesting that the overall crystal structures of g-C3N4 remain intact after thermal oxidation exfoliation process and depositing with MnO2 NPs. Effect of MnO2 content. The Hg0 capture performances of CNNS and xMnO2/CNNS at 120 °C are presented in Figure 6. For comparison, the blank test using only an empty quartz tube without any catalysts or sorbents for mercury capture is also conducted. The results show that the empty quartz tube has no effect on Hg0 removal. The pristine CNNS has a strong attraction to Hg0 probably owing to its unique electronic property, big surface area and ultrathin nanosheet structure.33 The Hg0 removal efficiency of CNNS rapidly rises to ~77.2% within 4 min and then gradually

reduces

to

~57.5%

after

mercury

adsorption

over

100

min.

MnO2-decorating can greatly promote the performance of mercury oxidation. The Hg0 removal efficiency can be increased by > 28.5% after addition of MnO2. When loading value is 5 wt%, the Hg0 removal efficiency rapidly goes up to ~86.6% and then gradually drops to ~74.0% which is much higher than that of pure CNNS. The most active sample is found with 10 wt% MnO2 content. The Hg0 removal efficiency of 10MnO2/CNNS reaches ~97.8%. The probable reason is that MnO2-decoration

9


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

could generate more active sites for Hg0 oxidation. In addition, MnO2/g-C3N4 heterojunction structure is produced due to the intimate interface contact between MnO2 and g-C3N4 resulting in excellent electron transfer mobility at the contact interface of the two ingredients. This can facilitate mercury oxidation reaction, thereby improving the performance of mercury removal. However, further increasing loading value leads to a slight decrease in the activity. When MnO2 loading values are 15 and 20 wt%, the Hg0 removal efficiency slightly decreases to ~93.5 and ~93.4%, respectively. The presumable reason is that excessive Mn contents may cause aggregation and growth of MnO2 particles leading to the decline in Hg0 oxidation activity. Therefore, the optimal MnO2 loading value is 10 wt%.34 Effect of reaction temperature. Figure 7 illustrates the Hg0 removal efficiency of CNNS and 10MnO2/CNNS as a function of reaction temperature between 90 and 240 °C. The Hg0 removal efficiency of CNNS first gradually increases and then decreases with rising temperatures. The Hg0 removal efficiency is less than 58.0% when reaction temperature is below or at 120 °C plausibly attributed to lower Hg0 vapor diffusion rate. It gradually increases to its optimal value ~65.8% when temperature reaches at 150 °C. However, it reduces to ~60.4% when temperature elevates to 180 °C and significantly decreases to ~42.5 and ~43.9% when temperature goes up to 210 and 240 °C correspondingly. This implies that Hg0 adsorption on pure CNNS is a physisorption process, which would be suppressed at elevated temperatures.35 10MnO2/CNNS exhibits excellent Hg0 capture ability which is much better than

10


Page 10 of 31

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

Energy & Fuels

pure CNNS in temperature range of 90-240 °C with Hg0 removal efficiency all above 91.0%, probably owing to the incremental active sites for Hg0 oxidation originated from MnO2-modification and the intimate interface contact between MnO2 NPs and g-C3N4 nanosheet. The Hg0 removal efficiency of 10MnO2/CNNS significantly increases from ~91.7 to ~98.9% with temperature rising from 90 to 120 °C. It waves between ~96.3 and ~98.6% when temperature is between 150 and 240 °C. This suggests that mercury oxidation over 10MnO2/CNNS is dominated by chemisorption reaction. The higher temperature could enhance the chemisorption process due to the reduced activation energy barrier.36 The Hg0 capture performance of CNNS and 10MnO2/CNNS at 120 °C over 10 h on stream is presented in Figure 8. The Hg0 removal efficiency of CNNS first swiftly rises to ~76.8% within only 5 min and then gradually reduces with time elapsed. It retains ~47.5% Hg0 removal efficiency after mercury adsorption over 10 h. However, 10MnO2/CNNS presents prominent Hg0 capture performance. The Hg0 removal efficiency rapidly goes up to as high as ~96.0% within 52 min and stays at this value with time elapsed. Mercury oxidation mechanism. To elucidate the mechanism of Hg0 capture on MnO2 NPs decorated g-C3N4 nanosheet, the binding energies and valences of the surface elements of fresh CNNS, fresh and used 10MnO2/CNNS are determined by XPS technique. The C 1s and N 1s XPS spectra of fresh CNNS and 10MnO2/CNNS are depicted in Figure 9. The high-resolution spectra of C 1s can be fitted into two principal peaks at binding energy of ~287.8-288.2 and ~285.3-285.8 eV for fresh CNNS and 10MnO2/CNNS respectively, indicative of two different chemical

11


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

environments of carbon existing in these two specimens. The bands at ~287.8 and ~288.2 eV relate to the reflection of C-N-C coordination (Cβ), while the bands at ~285.3 and ~285.8 eV are ascribed to sp2 C atoms (Cα) bonded to N in an aromatic ring.37 The high-resolution spectra of N 1s are deconvoluted into three Gaussian-Lorenzian peaks. The bands at ~400.2 and ~400.8 eV accord with amino functions (Nγ). The bands at ~399.0 and ~399.4 eV correspond to graphitic nitrogen (Nβ). The bands at ~398.0 and ~398.6 eV belong to pyridinic nitrogen (Nα).38 The binding energies of C 1s and N 1s spectra of fresh 10MnO2/CNNS shift toward lower values compared to fresh CNNS, indicating that CNNS accept electrons from MnO2, namely, there are electron transfer between CNNS and MnO2. In addition, the two-dimensional nanosheet structure of CNNS shortens the bulk diffusion length, which can promote the charge transfer mobility between MnO2 and g-C3N4. The fast electron transfer mobility at the contact interface of MnO2 and g-C3N4 can promote the Hg0 oxidation reaction, thereby enhancing the Hg0 removal efficiency. The XPS spectra of fresh and used 10MnO2/CNNS are presented in Figure 10. The strong feature signals of C, N, and O elements are both detected in the full spectrum survey of fresh and used 10MnO2/CNNS. The binding energies of C 1s and N 1s spectra of spent10MnO2/CNNS slightly increased after Hg0 adsorption compared to fresh 10MnO2/CNNS. The Cα/Cβ ratios for fresh and spent 10MnO2/CNNS are both 1.18, suggesting that the valences of C atoms keep the same during mercury oxidation reactions. The percentages of Nα, Nβ and Nγ of fresh 10MnO2/CNNS are 30.7, 37.7 and 31.6%, respectively. The contents of Nα and Nβ ascended to 40.7 and 35.4%

12


Page 12 of 31

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

Energy & Fuels

relatively, while the content of Nγ descended to 23.9%. This suggests that partial amino functions transferred to pyridinic nitrogen and graphitic nitrogen during mercury capture processes. The high-resolution spectra of O 1s can be deconvoluted into two dominating peaks at binding energy of ~531.4-531.6 and ~533.2-532.4 eV for fresh and used 10MnO2/CNNS relatively, indicative of two different chemical environments of oxygen existing in these two specimens as well. The bands at ~533.2 and ~532.4 eV correspond to the oxygen (Oβ) in adsorbed carbonates or water and hydroxyl groups, while the bands at ~531.4 and ~531.6 eV relate to the lattice oxygen (Oα) on catalyst surface derived from MnO2.39 The ratio of Oα/Oβ decreased from 1.11 to 0.43 for fresh and used sample correspondingly, indicating that lattice oxygen (Oα) is consumed and involved in mercury oxidation process.40 As for Mn 2p 3/2 spectra, the peaks observed at ~644.2 and ~644.6 eV correspond to Mn4+ ions, while the peaks detected at ~642.6 and ~642.8 eV belong to the reflection of Mn3+ ions.41 The ratio of Mn4+/Mn3+ decreased from 1.94 to 0.91 for fresh and used 10MnO2/CNNS respectively, indicating that the conversion of Mn4+ ions to lower valence Mn3+ ions happened during mercury removal processes. Additionally, other two peaks at ~654.8 and ~655.0 eV for fresh and used 10MnO2/CNNS respectively are the spin-orbit peak of Mn 2p 1/2.42 The characteristic signals at ~105.4 and ~102.6 eV accord with Hg 4f 5/2 and Hg 4f 7/2 correspondingly, suggesting the formation of oxidized mercury.43 Furthermore, the characteristic signals of Hg0 at 99.9 eV is not observed,44 implying that mercury capture by 10MnO2/CNNS is dominated by chemisorption processes. Based on the aforementioned findings, we can speculate that CNNS has a

13


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

Page 14 of 31

strong attraction toward elemental mercury; the Hg0 can be captured on CNNS surface via physisorption processes. MnO2 NPs act as the predominant active site for Hg0 oxidation; MnO2 NPs decorating on CNNS enlarged the surface area of the catalyst and promoted the adsorption of Hg0 and gas-phase O2, which is favorable for Hg0 removal. What is more, CNNS serves as the support of manganese oxides resulting in the formation of MnO2 NPs and production of MnO2/g-C3N4 heterojunction. The electron transfer is enhanced due to the Mott-Schottky effect stemmed from the close interfacial contact between MnO2 and g-C3N4, contributing to the enhancement of Hg0 oxidation activity.45, 46 The Schottky barriers at the interface of MnO2 and g-C3N4 could modify the electron density of the Mn4+ ions and thereby tune the activation energy toward Hg0 oxidation. The CNNS would also be activated by the Schottky barrier and endowed with higher activity for mercury adsorption.47 Thereby, CNNS and MnO2 exhibit a cooperative effect on Hg0 removal. The mechanism and pathways of Hg0 removal over MnO2 decorated g-C3N4 nanosheet can be summarized as follows (illustrated in Figure 11): (i) The gaseous Hg0 and O2 are adsorbed onto the catalyst surface via van de Waals force to form adsorbed mercury (Hg0(ad)) and chemisorbed oxygen (Oad) correspondingly. Hg0(g) + surface → Hg0(ad)

(2)

O2(g) + surface → 2O(ad)

(3)

(ii) A portion of MnO2 releases lattice oxygen ([O]) and generates suboxide Mn2O3.28, 48

2MnO2 → Mn2O3 + [O]

(4)

14


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

Energy & Fuels

(iii) The adsorbed Hg0(ad) reacts with chemisorbed oxygen (O(ad)) and lattice oxygen ([O]) yielding oxidized mercury (HgO(ad)).49 Hg0(ad) + O(ad) → HgO(ad)

(5)

Hg0(ad) + [O] → HgO(ad)

(6)

(iiii) The gas-phase O2 could replenish the consumed chemisorbed oxygen and lattice oxygen and simultaneously oxidize Mn2O3 into MnO2 to regenerate the active ingredient for Hg0 oxidation. 50 Mn2O3 + 1/2 O2(g)→ 2MnO2

(7)

The proposed model can well illuminate the outstanding Hg0 removal performance of MnO2 NPs decorated g-C3N4 nanosheet. CONCLUSIONS The g-C3N4 nanosheet (CNNS) exhibits good Hg0 capture ability at low temperature associated with over ~42.0% Hg0 removal efficiency at 90-240 °C. Decorating CNNS with MnO2 nanoparticles can promote the Hg0 removal performance due to the tight interaction between MnO2 and g-C3N4. The optimal loading amount of MnO2 is 10 wt% yielding over 91.0% Hg0 removal efficiency within temperature range of 90-240 °C. Hg0 capture by pristine CNNS is a physisorption process whereas Hg0 removal over xMnO2/CNNS is predominated by chemisorption reactions. In addition, 10MnO2/CNNS performs stably with respect to Hg0 capture at 120 °C over 10 h on stream. The Hg0 removal efficiency can stay at as high as ~96.0% for a long period.

15


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

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] (D. Liu). *Email: [email protected] (J. Wu). Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work is supported by Fund for Senior Personnel of Jiangsu University (18JDG017) and National Natural Science Foundation of China (21237003). REFERENCES (1) Driscoll, C. T.; Mason, R. P.; Chan, H. M.; Jacob, D. J.; Pirrone, N. Mercury as a global pollutant: Sources, pathways, and effects. Environmental Science & Technology 2013, 47 (10), 4967-4983. (2) Li, P.; Feng, X.; Qiu, G.; Shang, L.; Li, Z. Mercury pollution in Asia: a review of the contaminated sites. Journal of Hazardous Materials 2009, 168, 591-601. (3) Lee, W.; Bae, G. N. Removal of elemental mercury (Hg0) by nanosized V2O5/TiO2 catalysts. Environmental Science & Technology 2009, 43, 1522-1527. (4) Schofield, K. Mercury emission control from coal combustion systems: a modified air preheater solution. Combustion and Flame 2012, 159(4), 1741-1747. (5) Jiang, S.; Liu, X.; Li, H.; Wang, J.; Yang, Z.; Peng, H.; Shih, K. Synergistic effect of HCl and NO in elemental mercury catalytic oxidation over La2O3/TiO2 catalyst. Fuel 2018, 215, 232-238.

16


Page 16 of 31

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

Energy & Fuels

(6) Xu, H.; Qu, Z.; Zong, C.; Quan, F.; Mei, J.; Yan, N. Catalytic oxidation and adsorption of Hg0 over low-temperature NH3-SCR LaMnO3 perovskite oxide from flue gas. Applied Catalysis B-Environmental 2016, 186, 30-40. (7) Sun, P.; Zhang, B.; Zeng, X.; Luo, G.; Li, X.; Yao, H.; Zheng, C. Deep study on effects of activated carbon’s oxygen functional groups. Fuel 2017, 200, 100-106. (8) Shu, T.; Lu, P.; He, N. Mercury adsorption of modified mulberry twig chars in a simulated flue gas. Bioresource Technology 2013, 136(5), 182-187. (9) Ding, F.; Zhao, Y.; Mi, L.; Li, H.; Li, Y.; Zhang, J. Removal of gas-phase elemental mercury in flue gas by Inorganic chemically promoted natural mineral sorbents. Industrial & Engineering Chemistry Research 2012, 51 (7), 3039-3047. (10) Luo, G.; Yao, H.; Xu, M.; Cui, X.; Chen, W.; Gupta, R.; Xu, Z. Carbon nanotube-silver composite for mercury capture and analysis. Energy & Fuels 2010, 24, 419-426 (11) Xu, H.; Qu, Z.; Huang, W.; Mei, J.; Chen, W.; Zhao, S.; Yan, N. Regenerable Ag/graphene sorbent for elemental mercury capture at ambient temperature. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2015, 476, 83-89. (12) Xu, H.; Qu, Z.; Zong, C.; Huang, W.; Quan, F.; Yan, N. MnOx/graphene for the catalytic oxidation and adsorption of elemental mercury. Environmental Science Technology 2015, 49, 6823-6830. (13) Zhao, B.; Liu, X.; Zhou, Z.; Shao, H.; Xu, M. Catalytic oxidation of elemental mercury by Mn-Mo/CNT low temperature. Chemical Engineering Journal 2016, 284(2), 1233-1241.

17


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

Page 18 of 31

(14) Yang, J.; Zhao, Y.; Liang S.; Zhang, S.; Ma, S.; Li, H.; Zhang, J.; Zheng, C. Magnetic

iron-manganese

binary

oxide

supported

on

carbon

nanofiber

(Fe3-xMnxO4/CNF) for efficient removal of Hg0 from coal combustion flue gas. Chemical Engineering Journal 2018, 334, 216-224. (15) Samanta, S.; Martha, S.; Parida, K. Facile synthesis of Au/g-C3N4 nanocomposites: an inorganic/organic hybrid plasmonic photocatalyst with enhanced hydrogen gas evolution under visible-light irradiation. ChemCatChem 2014, 6, 1453-1462. (16) Niu, P.; Zhang, L.; Liu, G.; Cheng, H. Graphene-like carbon nitride nanosheets for improved photocatalytic activities. Advanced Functional Materials 2012, 22, 4763-4770. (17) Xu, J.; Zhang, L.; Shi, R.; Zhu, Y. Chemical exfoliation of graphitic carbon nitride for efficient heterogeneous photocatalysis. Journal of Materials Chemistry A 2013, 1, 14766-14772. (18) Lang, Q.; Hu, W.; Zhou, P.; Huang, T.; Zhong, S.; Yang, L, Chen, J.; Bai, S. Twin defects engineered Pd cocatalyst on C3N4 nanosheets for enhanced photocatalytic performance in CO2 reduction reaction. Nanotechnology 2017, 28(48), 484003. (19) Liu, J.; Shi, H.; Shen, Q.; Guo, C.; Zhao, G. A biomimetic photoelectrocatalyst of co–porphyrin combined with a g-C3N4 nanosheet based on π–π supramolecular interaction for high-efficiency CO2 reduction in water medium. Green Chemistry 2017, 19, 5900-5910.

18


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

Energy & Fuels

(20) Dong, G.; Jacobs, D. L.; Zang, L.; Wang, C. Carbon vacancy regulated photoreduction of NO to N2 over ultrathin g-C3N4 nanosheets. Applied Catalysis B: Environmental 2017, 218, 515-524. (21) Li, Y.; Ho, W.; Lv, K.; Zhu, B.; Lee, S.C. Carbon vacancy-induced enhancement of the visible light-driven photocatalytic oxidation of no over g-C3N4 nanosheets. Applied Surface Science 2017, 430, 380-389. (22) Xu, H.; Jia, J.; Guo, Y.; Qu, Z.; Liao, Y.; Xie, J.; Shangguan, W.; Yan, N. Design of 3D MnO2/Carbon sphere composite for the catalytic oxidation and adsorption of elemental mercury. Journal of Hazardous Materials 2018, 342, 69-76. (23) Xu, H.; Yan, N.; Qu, Z.; Liu, W.; Mei, J.; Huang, W.; Zhao, S. Gaseous heterogeneous catalytic reactions over Mn-based oxides for environmental applications: a critical review. Environmental Science & Technology 2017, 51, 8879-8892. (24) Xiao, J.; Rabeah, J.; Yang, J.; Xie, Y.; Cao, H.; Brückner, A. Fast electron transfer and OH formation: key features for high activity in visible-light-driven ozonation with C3N4 catalyst. ACS Catalysis 2017, 7, 6198-6206. (25) Liu, D. J.; Zhou, W. G.; Wu, J. Effect of Ce and La on the activity of CuO/ZSM-5 and MnOx/ZSM-5 composites for elemental mercury removal at low temperature. Fuel 2017, 194(4), 115-122. (26) Liu, D. J.; Lu, C.; Wu, J. Gaseous mercury capture by copper-activated nanoporous carbon nitride. Energy & Fuels 2018, 32(8), 8287-8295. (27) Ge, L.; Han, C.; Liu, J.; Li, Y. Enhanced visible light photocatalytic activity of

19


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

Page 20 of 31

novel polymeric g-C3N4 loaded with Ag nanoparticles. Applied Catalysis A General 2011, 409-410, 215-222. (28) Wu, Y.; Xu, W.; Yang, Y.; Wang, J.; Zhu, T. Support effect of Mn-based catalysts for gaseous elemental mercury oxidation and adsorption. Catalysis Science & Technology 2018, 8. 297-306. (29) Li, H.; Jing, Y.; Ma, X.; Liu, T.; Yang, L.; Liu, B.; Yin, S.; Wei, Y.; Wang, Y. Construction of a well-dispersed Ag/graphene-like g-C3N4 photocatalyst and enhanced visible light photocatalytic activity. RSC Advances 2017, 7, 8688-8693. (30) Xiao, J.; Xie, Y.; Nawaz, F.; Wang, Y.; Du, P.; Cao, H. Dramatic coupling of visible light with ozone on honeycomb-like porous g-C3N4 towards superior oxidation of water pollutants. Applied Catalysis B Environmental 2016,183, 417-425. (31) Qin, J.; Huo, J.; Zhang, P.; Zeng, J.; Wang, T.; Zeng, H. Improving the photocatalytic

hydrogen

production

of

Ag/g-C3N4

nanocomposites

by

dye-sensitization under visible light irradiation. Nanoscale 2016, 8, 2249-2259. (32) Bing, W.; Chen, Z.; Sun, H.; Shi, P.; Gao, N.; Ren, J.; Qu, X. Visible-light-driven enhanced antibacterial and biofilm elimination activity of graphitic carbon nitride by embedded Ag nanoparticles. Nano Research 2015, 8, 1648-1658. (33) Deifallah, M.; McMillan, P.; Cora, F. Electronic and structural properties of two-dimensional carbon nitride graphenes. Journal of Physical Chemistry C 2008, 112, 5447-5453. (34) Wang, H.; Cui, L.-F.; Yang, Y.; Casalongue, H. S.; Robinson, J. T.; Liang, Y.;

20


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

Energy & Fuels

Cui, Y.; Dai, H. Mn3O4-graphene hybrid as a high capacity anode material for lithium ion batteries. Journal of the American Chemical Society 2010, 132 (40), 13978-13980. (35) Liu, D. J.; Lu, C.; Wu, J. CuO/g-C3N4 nanocomposite for elemental mercury capture at low temperature. Journal of Nanoparticle Research 2018, 20(10), 227. (36) Tan, Z.; Sun, L.; Xiang, J.; Zeng, H.; Liu, Z.; Hu, S.; Qiu, J. Gas-phase elemental mercury removal by novel carbon-based sorbents. Carbon 2012, 50, 362-371. (37) Vinu, A. Two-dimensional hexagonally-ordered mesoporous carbon nitrides with tunable pore diameter, surface area and nitrogen content. Advanced Functional Materials 2008, 18, 816-827. (38) Teng, Z.; Lv, H.; Wang, C.; Xue, H.; Pang, H.; Wang, G. Bandgap engineering of ultrathin graphene-like carbon nitride nanosheets with controllable oxygenous functionalization. Carbon 2017, 113, 63-75. (39) France, L.J.; Yang, Q.; Li, W.; Chen, Z.; Guang, J.; Guo, D.; Wang, L.; Li, X. Ceria

modified

FeMnOx-Enhanced

performance

and

sulphurresistance

for

low-temperature SCR of NOx. Applied Catalysis B: Environmental 2017, 206, 203-215. (40) Xu, W,; Adewuyi, Y.; Liu, Y.; Wang, Q. Removal of elemental mercury from flue gas using CuOx and CeO2 modified rice straw chars enhanced by ultrasound. Fuel Processing Technology 2018, 170, 21-31. (41) Kang, M.; Park, E. D.; Kim, J. M.; Yie, J. E. Manganese oxide catalysts for NOx reduction with NH3 at low temperatures. Applied Catalysis A: General 2007, 327,

21


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

Page 22 of 31

261-269. (42) Sharma, R. K.; Zhai, L. Multiwall carbon nanotube supported poly (3, 4-ethylenedioxythiophene)/manganese

oxide

nano-composite

electrode

for

super-capacitors. Electrochimica Acta 2009, 54, 7148-7155. (43) Sasmaz, E.; Kirchofer, A.; Jew, A. D.; Saha, A.; Abram, D.; Jaramillo, T. F.; Wilcox, J. Mercury chemistry on brominated activated carbon. Fuel 2012, 99, 188-196. (44) Hutson, N. D.; Attwood, B. C.; Scheckel, K. G. XAS and XPS characterization of mercury binding on brominated activated carbon. Environmental Science & Technology 2007, 41, 1747-1752. (45) Su, H.; Zhang, K.; Zhang, B.; Wang, H.; Yu, Q.; Li, X.; Antonietti, M.; Chen, J. Activating cobalt nanoparticles via the Mott-Schottky effect in nitrogen-rich carbon shells for base-free aerobic oxidation of alcohols to esters. Journal of the American Chemistry Society 2017, 139, 811-818. (46) Chen, T.; Guo, S.; Yang, J.; Xu, Y.; Sun, J.; Wei, D.; Chen, Z.; Zhao, B.; Ding, W. In situ activated nitrogen-doped carbon by embeded nickel via Mott-Schottky effect for oxygen reduction reaction. ChemPhysChem 2017, 18, 3454-3461. (47) Xue, Z.; Han, J.; Feng, W.; Yu, Q.; Li, X.; Antonietti, M.; Chen, J. Tuning the adsorption energy of methanol molecules along Ni-N-doped carbon phase boundaries by the Mott-Schottky effect for gas-phase methanol dehydrogenation. Angewandte Chemie International Edition 2018, 57, 2697-2701. (48) Xu, H.; Qu, Z.; Zhao, S.; Mei, J.; Quan, F.; Yan, N. Different crystal-forms of

22


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

Energy & Fuels

one-dimensional MnO2 nanomaterials for the catalytic oxidation and adsorption of elemental mercury. Journal of Hazardous Materials 2015, 299, 86-93. (49) Xu, H.; Qu, Z.; Zhao, S.; Yue, D.; Huang, W.; Yan, N. Enhancement of heterogeneous oxidation and adsorption of Hg0 in a wide temperature window using SnO2 supported LaMnO3 perovskite oxide. Chemical Engineering Journal 2016, 292, 123-129. (50) Yang, W.; Liu, Y.; Wang, Q.; Pan, J. Removal of elemental mercury from flue gas

using

wheat

straw

chars

modified

by

Mn-Ce

mixed

oxides

with

ultrasonic-assisted impregnation. Chemical Engineering Journal 2017, 326, 169-181.

23


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

Figure 1. Sketch diagram of elemental mercury capture testing system

Figure 2. XRD patterns of CNNS and xMnO2/CNNS

24


Page 24 of 31

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

Energy & Fuels

Figure 3. FESEM images: (a) CNNS and (b) 10MnO2/CNNS; TEM images: (c, d) CNNS and (e, f) 10MnO2/CNNS

25


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

Figure 4. (a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution curves of CNNS and xMnO2/CNNS

Figure 5. FTIR profiles of CNNS and xMnO2/CNNS

26


Page 26 of 31

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

Energy & Fuels

Figure 6. Effect of MnO2 content on Hg0 removal performance of xMnO2/CNNS at 120 °C

Figure 7. Effect of temperature on Hg0 removal efficiency of CNNS and 10MnO2/CNNS

27


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

Figure 8. Hg0 capture performance of CNNS and 10MnO2/CNNS at 120 °C over 10 h

Figure 9. C 1s and N 1s XPS spectra of fresh CNNS and 10MnO2/CNNS

28


Page 28 of 31

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

Energy & Fuels

Figure 10. XPS spectra of fresh and spent 10MnO2/CNNS

29


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

Figure 11. Sketch diagram of the mechanism of Hg0 capture over MnO2-decorated g-C3N4 nanosheet

30


Page 30 of 31

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

Energy & Fuels

Table 1 Porosity properties of CNNS and xMnO2/CNNS BET

Total pore

Micropore

Mesopore

Pore

surface

volume

volume

volume

diameter

area (m2/g)

(cm3/g)

(cm3/g)

(cm3/g)

(nm)

CNNS

109

0.456

0.011

0.445

19

5MnO2/CNNS

56

0.308

0.008

0.300

20

10MnO2/CNNS

37

0.107

0.004

0.103

14

15MnO2/CNNS

35

0.203

0.006

0.197

22

20MnO2/CNNS

25

0.188

0.007

0.181

22

samples

31


Elemental Mercury Removal by MnO2 Nanoparticle Decorated

Recommend Documents