TiO2 under

Subscriber access provided by READING UNIV

Article

Adsorption and Catalytic Oxidation of Mercury over MnOx/TiO2 under the Low Temperature Conditions Jinhuan Cheng, Xueqian Wang, Yanan Li, and Ping Ning Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02749 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 20, 2017

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 free 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 accessible to all readers and 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.

Industrial & Engineering Chemistry Research 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 33

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

Industrial & Engineering Chemistry Research

Adsorption and Catalytic Oxidation of Mercury over MnOx/TiO2 under the Low Temperature Conditions Jinhuan Cheng, Xueqian Wang, Yanan Li, Ping Ning* Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, PR China ABSTRACT：：A series of MnOx/TiO2 sorbents were synthesized via a sol-gel method and evaluated for elemental mercury (Hg0) removal under the low temperature conditions. The effects of manganese oxide contents (5-20wt.%), calcination temperatures of the sorbents (450-750°C), reaction temperatures (50-200 °C) and flue gas components, such as H2S (0-5000 ppm), O2 (0-5%) and H2O (0-5%), on Hg0 adsorption and oxidation activity were investigated. The experimental results showed that the introduction of manganese oxide into the pure TiO2 obviously enhanced Hg0 removal ability. Particularly under the condition that 15 wt.% of manganese oxide content, calcination temperature of 550 °C and 1% O2, the adsorption capacity could be as high as 5.12 mg/g at 100 °C. In addition, O2 significantly enhanced the Hg0 removal efficiency. It works by offering the consumed adsorbed oxygen, which was delivered to Hg0 by redox circle between the changes of manganese states. Catalytic oxidation of Hg0 was the dominated way at 100 °C. Both H2S and H2O inhibited the Hg0 removal efficiency, which was ascribed to the competition between the introduced gas and Hg0 on the adsorption and catalytic oxidation activity sites. 1. INTRODUCTION Mercury has been taken the growing concern attention on the account of its serious toxicity, which could cause long-last hurt to both human health and the environment irreversibly.1 In China, approximate 38% mercury discharges from coal-fired power plants, which account for a substantial part among mercury emission source.2,3 Other anthropogenic activities, such as metal plating, mining operations and yellow phosphorus tail off-gas, are also the sources of Hg0.4 In general, mercury exists in three forms: elemental mercury (Hg0), oxidized mercury (Hg2+), and particle-bound mercury (Hgp).5,6 Elemental mercury vapor is 1 / 31

ACS Paragon Plus Environment


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

the main specie in the mentioned sources above. Among them, Hg2+ and Hgp can be scavenged effectively by existing equipment mentioned by other articles, such as wet-flue gas desulfurization (FGD) and electrostatic precipitators (ESPs), respectively.7,8,9 However, Hg0 is easy to escape into the atmosphere because of its properties of high volatile, thermodynamically stable and slight water solubility.10,11One well-established device alone is quite poor for Hg0 abatement, which is usually installed in the downstream to capture the oxidized Hg0. As such, efficient technology for Hg0 control is worth to explore. Currently, Activated carbon (AC) has been selected as the commercially available and effective mercury adsorption material. But it is unpleasant because the carbon based sorbents need to be renewed when they reach the adsorption saturation capacity, which increase the cost greatly. In order to overcome this defect, carbon-based sorbents modifies by noble metal oxides, transition-metal oxides, sulfur, iodine, chlorine, and bromine are widely investigated, which promote the mercury adsorption capacity in the matter of Hg0 chemisorption.12-18 But the poor thermal stability of the mercury complexes formed on the surface of activated carbon limited its wide application. So far, there are about three kinds of catalysts for mercury oxidation, such as noble based metals catalysts, 19, 20 SCR catalysts, 21-23and transitional based metals catalysts.24, 25 The noble metal based catalysts, such as Pt, Pd, RuO2, Au, and Ir loaded on various supports, have been tested for the oxidation of Hg0.26-30 It has been reported that Pd and Au are the most effective catalysts among them, especially under the both the presence of HCl and O2 conditions. Presto and Granite investigated that nearly no Hg0 oxidation over Pd(1wt.%)/Al2O3 catalyst occurred without HCl. But when HCl was introduced, more than 85% and 65% of Hg0 oxidation efficiency was maintained after 3 months and 20 plus months, respectively.29 Zhao et al. reported that the property that the selective adsorption of mercury and chlorine in flue gases over Au based catalysts made it to be the potential catalyst for Hg0 catalytic oxidation.31 From the mentioned above, it 2 / 31


Page 2 of 33

Page 3 of 33

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


seemed that chlorine was essential for Hg0 oxidation on the noble metal based catalysts. Selective catalytic reduction (SCR) catalysts employed in Hg0 oxidation have also been reported in previous work. They concluded that the oxidation efficiency of Hg0 over SCR catalysts was greatly dependent on chloride content that contained in various coals. In the burning bituminous coal, the oxidation efficiency of Hg0 could reach from 30% to 98%, while in the burning sub-bituminous coal that with the relatively lower chloride concentration, it obviously decreased to 0-26%.32,33 In general, based on the reaction temperatures, the SCR catalysts were divided into two types: high-temperature and low-temperature SCR catalysts. The high-temperature SCR catalysts contained V2O5-based SCR catalysts, such as VOx/TiO2,

V2O5-WO3/TiO2,

V2O5-WO3/TiO2

modified

by

RuO2

and

SiO2-TiO2-V2O5,34-37 most of which have been investigated for Hg0 oxidation under different HCl concentrations at the reaction temperatures ranged from 300 to 400 °C. He et al. reported that compared to the condition of HCl+O2 or V2O5/TiO2 SCR catalyst presented only, under which the Hg0 oxidation efficiency just could reach less than 10%, but 64% of that could be obtained with the introduction of both 50 ppmv HCl and 5% O2 through the catalyst.38 The low-temperature SCR catalysts, like MnOx/Al2O3, Mo-MnOx/Al2O3, and MnOx-CeO2/TiO2, at the reaction temperatures from 100 to 250 °C for Hg0 removal have been studied. Qiao et al. made a performance comparison of MnOx/γ-Al2O3 and MnOx/α-Al2O3 on adsorption and catalytic oxidation of Hg0.39 They concluded that under the same operation conditions, the Hg0 adsorption capacity of the two catalysts with 90% break through time at 350 °C were 1.6 and 75 µg/g, respectively. HgO was the main product on the surface of the catalysts. When in the presence of HCl or Cl2, more than 90% amounts of Hg0 could be oxidized. Li et al. observed that MnOx-CeO2/TiO2 catalyst displayed an excellent performance on Hg0 oxidation at low temperatures, on which Hg0 oxidation 3 / 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

efficiency could reach up to 90% at 200-250 °C.40 Transition metal oxides-based catalysts, as a kind of efficient catalyst for Hg0 oxidation, such as CuO nanoparticles, TiO2, MnO2/Al2O3, CuCoO4/Al2O3, CeO2/TiO2, Mn–Fe spinels and CeO2-TiO2, also have been studied. Yan et al. studied the adsorption and catalytic oxidation of mercury over MnOx/Alumina, of which the mercury adsorption capacity was as high as 2.30 mg/g at the temperature range of 60-177 °C. Smirniotis et al. concluded that MnOx/TiO2 adsorbent showed excellent performance on the mercury removal, on which the mercury capture capacity could reach 17.40 mg/g at 200 °C.41 He et al. proposed that Hg0 removal efficiency over 6%Ce-6% MnOx/Ti-PILC catalyst was higher than 90% at 100-350 °C in the absence of HCl.42 According to the results mentioned above, it could be demonstrated that manganese oxides could become to be the potential candidate for Hg0 catalytic oxidation at low temperatures. Some researchers also have researched the individual flue gas components effect on Hg0 removal, such as HCl, SO2, H2S, CO, H2 .etc, which aimed to make clear that what roles they played in Hg0 removal process.43,44 The catalysts for good performance on Hg0 removal by the oxidation way usually need the assistance of HCl above, which obeys the Deacon process.45 In the absence of HCl, manganese oxides is the relative preferred one for Hg0 oxidation among various kinds of catalysts. It was proven that the catalysts have high catalytic activity and unique redox property at low temperatures.46,47,48 Some studies found that Mn4+ as the most active component took part in the Hg0 oxidation.49 So, in this study, MnOx/TiO2 sorbent was selected for Hg0 adsorption and catalytic oxidation under the low temperature conditions. They were prepared by a sol-gel method. The influences of different factors, including reaction temperature, oxygen concentration, manganese oxide content, relative humidity and H2S on Hg0 removal were investigated. In addition, the mechanism of Hg0 removal over MnOx/TiO2 sorbent under the low temperatures was also proposed.

4 / 31


Page 4 of 33

Page 5 of 33

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


2. MATERIALS AND METHODS 2.1. Preparation of MnOx/TiO2 Sorbent. MnOx/TiO2 was synthesized by using a sol-gel method.50,51,52 Manganese acetate (Mn(CH3COOH)2·4H2O, 99%， Aldrich) and titanium isopropoxide (Ti(OC2H5)4, 97%, Aldrich) were selected as precursors of active component of MnOx and carrier of TiO2, respectively. Firstly, appropriate amounts of citric acid (C6H8O7·H2O, citric acid/M =1, mole ratio), titanium isopropoxide and acetic acid were dissolved in absolute alcohol within 0.5 h at 40 °C under magnetic stirring. In sequence drop different molar of manganese acetate solutions into the homogeneous mixture obtained above and stir vigorously at the same time in a few hours. In addition, the pH of the solution was adjusted at 7 by repeating to drop concentrated ammonia and concentrated nitric acid. After that, continue stirring mixture solution under 60 °C for 2 h. Then it was aged by temperature programming at 0.1 °C/min to 100 °C and stayed for 6-7 days until the dry-gel was obtained. Finally, the sample was calcined at 550 °C for 5 h in muffle furnace, which were ground and sieved to 40-60 mesh for activity test. In this process, the amounts of butyl titanate, deionized water, glacial acetic acid, anhydrous ethanol with a volume ratio of 1: 1: 0.5: 5. The total metal mass percentage is calculated by (M/(M+Ti), M =Mn) in each sorbent. Total Mn content was varied from 5 wt.% to 20 wt.%, in which wt.% means the mass percentage. 2.2. Hg0 Adsorption and Catalytic Activity Test. The elemental mercury (Hg0) removal experiment was carried out in the fixed-bed reactor. As shown in Figure 1, the test consists of four parts: the inlet gas system, a mercury permeation device, a fixed-bed reactor and online mercury analyzer. The individual inlet gases, such as O2, N2, H2S, which were controlled by mass flow controller, were mixed before entering the fixed-bed reactor. The inlet gas N2 transported Hg0 vapor into the system by passing through the mercury permeation U-tube, which placed in a constant water bath system. The sorbents (0.20g) loaded in a diameter of 6 mm and length of 60 cm quartz tube, which was a 5 / 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

Page 6 of 33

temperature-controlled tubular furnace. CVAAS (SG921; Jiangfen, China) was used to measure the Hg0 concentrations before and after the reaction. In order to measure the concentration of the total mercury (HgT), which included both elemental mercury and Hg2+ generated from oxidized elemental mercury in reactant gases, the SnCl2 solution (10%) was used to reduce the oxidized mercury (if any formed) to elemental mercury. The 10% NaOH solution was used to removal the H2S of set experiment before the flue gas entered the SnCl2 solution. The heating bands were used to avoid mercury condensation on the silicone pipeline. Water vapor was generated by using a heated water bubbler. In the experiment, the total flow rate was 400mL/min and the space velocity (GHSV) was around 4.8×104 h-1. The simulated gases included 0-1 % O2, 0-5000 ppm H2S and N2 as a balance gas. The initial concentration of mercury was 2.0 mg/m3. The contents of H2S in the inlet gases could be monitored via using the gas chromatograph with a flame photometry detector. In addition, the % of the gas means volume percentage in the whole article. The “breakthrough time” is defined as the time required that the Hg0 outlet concentration reached 50% of the inlet concentration in the determination of breakthrough curves in our study. Hg0 breakthrough point was defined here as the time when the outlet Hg0 concentration is equal to 50% of that in the inlet, t1/2). We used removal efficiency of Hg0 to estimate the sorbents performance of Hg0 removal. The Hg0 removal efficiency (η) was calculated based on eq 1 and the adsorption capacity (X) of Hg0 was calculated by the corresponding integral according to eq 2 under various conditions of the breakthrough curve:

η=

Hg0in -Hg0out Hg0in

=

(1)

Qc 0 t - Q∫ cdt 0 t

X =

(2)

m

Where η is the Hg0 removal efficiency; X is the adsorption capacity, which was deﬁned as both the mass of elemental mercury and the produced HgO deposited over per unit mass 6 / 31


Page 7 of 33

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


sorbent; m is the weight of the samples in the fixed-bed; t is final test times of the breakthrough curves, min; Q is the gas flow in m3/min; c0 and c present the initial and outlet concentration of Hg0 in test times in mg /m3. Mass flow meter H2S

Temperature Controller Surge flask Water bath

N2

Four way value

SG-921 mercury analyzer

O2

SnCl2 impinger Na2CO3 impinger N2

Fixed bed Hg0 permeation

KCl solution

Figure 1. Schematic diagram of the experimental system

2.3 Material Characterizations. 2.3.1 Brunauer−Emmett−Teller (BET). Specific surface area and pore size distribution parameters of the samples were characterized by Mesopore- and macropore-size distributions were calculated by the Barret-Joyner-Halenda (BJH) method through N2 adsorption at 77 K using an automatic physisorption analyzer. The average pore diameter and average pore volume were obtained from the desorption branches of N2 adsorption isotherm and calculated by the Barrett−Joyner−Halenda (BJH) formula. 2.3.2 X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectroscopy (XPS) analysis conducted on an AXIS UItra DLD (Shimadzu−Kratos) spectrometer with Al Ka were used to analyze the surface atomic concentration and characterize the chemical states of the elements on the catalysts for the fresh 7 / 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

Page 8 of 33

and used catalysts. The binding energy was calibrated by the C 1s binding energy value of 284.8 eV. 2.3.3 Scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDX). The surface morphology and metal ions concentrations of the samples were characterized by scanning electron microscope (SEM) obtained with

a

FEG-250

SEM

instrument

(FEI,

idhoven,

Netherlands)

and

energy-dispersive X-ray spectroscopy (EDX) measured by JEM-2100 (20 kV) respectively. 3. Characterizations Analysis of the Sorbent 3.1. BET of Different Calcination Temperatures of the Sorbent. The corresponding parameters of porous structure contained surface area, pore volume and pore diameter were presented in Table 1. It was obvious that with increasing the calcination temperatures from 450 to 750 °C, the surface area and pore volume were in descending orders. So the sample calcined at 450 °C had the biggest surface area and pore volume, which were corresponded to 90.50 m2·g-1 and 0.10 cm3·g-1, respectively. It is well known that the high surface area could promote the number of collisions that occurred between Hg0 and the sorbent, which was helpful to Hg0 adsorption.53 The nitrogen adsorption-desorption isotherms and pore size distributions of sorbents at different calcination temperatures were shown in Figure 2 and Figure 3, From the Figure 2, the amounts of micropores characterized at lower P/P0 existed in sorbents became fewer with increasing the calcination temperatures. The lowest degree of steep of the adsorption-desorption branch belonged to calcination temperature of 550 °C, which implied that a quite narrow distribution in mesopores range as well as uniform ordered pore sizes,54 while others with a variety of pore radius in the different pores indicated that large irregular shaped particles existed based on the medium relative pressure region. The level of the steep part of the adsorption curve at higher pressures denoted that there were a small number of wide pores, which possibly resulted from the space between particles. 8 / 31


Page 9 of 33

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


From the Figure 3, the majority of the pores in these sorbents fell in the range of 2.0-5.0 nm, indicating that all of them had mesoporous structures. In order to understand the effect of different structures of sorbents on the removal of Hg0, some comparisons were made between the fresh and used samples of calcination temperature of 550 °C. After a prolonged time on stream, the surface area and pore volume dramatically decreased from 61.23 to 7.84 m2·g-1 and 0.078 to 0.015 cm3·g-1, respectively. While the pore average diameters increased from 2.56 to 3.59 nm. The result maybe primarily associated to the produced mercury oxide that conglomerated on the surface of the sorbents and destructed the thin pore walls and blocked the internal porosity.55,56,57 Table1. Parameters of Porous Structure of Sorbents at Different Calcination Temperatures Surface area

Pore volume

Pore diameter

( m2·g-1)

( cm3·g-1)

(nm)

MnOX/TiO2-450 °C

90.50

0.100

2.22

MnOX/TiO2-550 °C

61.23

0.078

2.56

MnOX/TiO2-650 °C

36.50

0.067

3.11

MnOX/TiO2-750 °C

24.29

0.050

3.78

Samples

9 / 31



°C °C °C °C

3

Quantity Adsorbed (cm /g STP)

450 550 650 750

0.0

0.2 0.4 0.6 0.8 Relative Pressure (P/P 0 )

1.0

Figure 2. Nitrogen sorption/desorption isotherms of different calcination temperatures of MnOx/TiO2 sorbents

450 °C 550 °C (b) 650 °C 750 °C 550 °C (a)

3

Pore Volume (cm /g )

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

Page 10 of 33

0

20

40

60 80 100 120 Pore Diam eter / nm

140

160

180

Figure 3. Pore size distributions of different calcination temperatures of MnOx/TiO2 sorbents 3.2. XPS Analysis of MnOx/TiO2 Sorbents for Hg0 Removal at Different Reaction Temperatures. In order to make clear that the reaction mechanism of Hg0 removal, the XPS spectra of O 1s, Mn 2p and Hg 4f of the fresh and used 15 wt.% of MnOx/TiO2 sorbents with 1% O2 at different reaction temperatures (50, 10 / 31


Page 11 of 33

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


100 and 150 °C) were evaluated. As shown in Figure 4(a), the O 1s spectra of the samples could be fitted into two regions: one was the peak with the lower binding energy value that located at 529.4-530.1 eV,58 which was as expected for the lattice oxygen from transition metal oxides (denoted as Oα); the other higher one located at 531.3-531.7 eV was assigned to chemisorbed oxygen or OH (denoted as Oβ), which played an important role as an active oxygen in oxidation reaction.59,60 For the fresh 15 wt.% of MnOx/TiO2 sorbent, the peaks lied at 529.7, 530.1 and 531.3 eV, respectively, which illustrated both the Oα and Oβ existed. After the Hg0 adsorption at 50 °C, compared to the fresh sorbent, the Oα/Oβ ratio decreased from 7.26 to 3.34 obviously. This result was supposed that the weakly adsorbed mercury might be bonded with the lattice oxygen. After the Hg0 adsorption at 100 °C, the ratio of Oα/Oβ increased to 5.10, which suggested that the chemisorbed oxygen participated in the Hg0 oxidation. After the Hg0 adsorption at 150 °C, the Oα/Oβ was 4.73, indicating that a few amount of lattice oxygen took part in Hg0 oxidation process. Based on the above mentioned, O2 was an important factor that acted as an oxidant in the Hg0 oxidation reaction. The corresponding Mn 2p XPS spectra results were shown in Figure 4 (b). For the fresh sorbent, the peaks observed at 644.5, 641.7 and 641.4 eV were corresponded to Mn4+, Mn3+ and Mn2+ respectively.61,62,63 After the test at 50 °C, the ratio of Mn3+/Mn4+ increased from 1.00 to 3.31, which suggested that Hg0 oxidation was resulted from the reduction of Mn4+ to Mn3+. After Hg0 oxidation at 100 °C, the peak of Mn2+ appeared and with a Mn3+/Mn2+ of 4.17, the Mn3+/Mn4+ decreased from 3.31 to 1.63, indicating that both the reduction and oxidation reaction occurred: Mn → Mn and Mn → Mn at 100°C. In this process, the reduction of Mn3+ to Mn2+ was contributed to Hg0 oxidation. After Hg0 oxidation at 150 °C, the Mn3+/Mn2+ decreased from 4.17 to 0.76 continually, and Mn3+/Mn4+ increased from 1.63 to 4.77 simultaneously. Both Mn → Mn and Mn → Mn were ascribed to Hg0 oxidation. Hg 4f XPS spectra displayed before and after reaction at 50, 100 and 150 °C 11 / 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

Page 12 of 33

in Figure 4(c) were for comparison. Based on the Hg 4f XPS spectrum ranged from 99 to 104 eV, it indicated that the adsorbed mercury presented in various forms on the sorbents. It could be divided into two peaks: the characteristic peaks at 100.5 and 104.4 eV were corresponding to Hg 4f 7/2 and Hg 4f 5/2, respectively.64 After reaction at 50 °C, the binding energy of the Hg 4f peaks at 99.5 and 102.3 eV were assigned elemental mercury and HgO, respectively, which suggested that both physisorption and chemisorption of Hg0 occurred. While for the spent sorbent at 100 °C and 150 °C, Hg 4f peaks appeared at approximately 101.3 and 104.0 eV, which were both assigned to mercury oxide (HgO). This result implied that catalytic oxidation way for Hg0 removal existed at all the reaction temperatures.

O 1s

Fresh

a

50 °C

Oβ

150°C

100 °C

Oβ

Oβ

Oβ

Oα Oα

Oα

528

530

532

534

536

528

Oα

530

532

534

536

528

530

532

534

536

528

530

Mn 2p

532

534

Mn

b Mn

4+

Mn

3+

Mn

Mn

4+

Mn Mn

Mn

4+

Mn

Mn

Mn

664 656 648 640 632

664 656 648 640 632

A

2+

664 656 648 640 632

K

3+

4+

3+

3+

2+

536

664 656 648 640 632

I

Hg 4f

c Hg

96

100 104 108 112

Hg

0

2+

Hg

2+

Hg

2+

Hg

2+

Hg

96

100 104 108 112

96

100 104 108 112

96

J

Binding energy(eV)

Figure 4. O 1s (a), Mn 2p (b), Hg 4f (c) XPS spectra for the fresh and the used 12 / 31


2+

100 104 108 112

2+

Page 13 of 33

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


MnOx/TiO2 sorbents at the reaction temperatures of 50, 100, and 150 °C. 3.3. XPS of Different Mn Contents of MnOx/TiO2 Sorbent. To make clear the surface atomic concentrations, oxidation states of manganese and the relationship between active components and support on each sorbent, different manganese oxide contents of the samples were investigated by XPS. As shown in Figure 5, with increasing manganese oxide content, the intensity of Mn 2p peak increased obviously, which could be ascribed to the high amounts of manganese oxide covered on the surface of the supports. At manganese oxide lower 15 wt.% content, the XPS peak of Mn 2p presented broadly that it could be divided into the major Mn3+ peak along with minor Mn4+ peak. At manganese oxide contents of 15 wt.% and 20 wt.%, the XPS peak of Mn 2p became broader and the Mn2+ peak appeared, on which Mn2+ and Mn3+ played the major phase, respectively. The relative dispersions of different manganese oxide contents on the surface of the support were evaluated by XPS analysis. The Mn/Ti ratios that could be the feature of the relative dispersion of manganese oxide on the surface of the support for all the four sorbents were shown in Figure 6. From the Figure 6 we could see that as increasing Mn content from 5 to 15 wt.%, the manganese surface atomic concentration increased but that of titanium had almost no obvious change. Further increasing the Mn content of the sample to 20 wt.%, the surface atomic concentration of manganese increased while the amounts of titanium atomic decreased sharply alone, which lead to Mn/Ti ratio increased of obviously. It was due to the formation of microcrystalline manganese oxide species covered on the support surface. Based on the analysis result, it could conclude that the sample with 15 wt.% content of manganese oxide was highly dispersed at certain level that called the monolayer coverage.65

13 / 31



a

Mn

Mn

3+

4+

Intensity (a.u.)

b

c

Mn

2+

d

657

654

651

648

645

642

639

B in d in g e n e rg y (e V )

Figure 5. XPS of different contents Mn 2p peak for (a) 5 wt.% (b) 10 wt.% (c) 15 wt.% (d) 20 wt.% of MnOx/TiO2 sorbent

0.3

0.6

M n/Ti Ti Mn

0.2

0.4 0.1 0.2

0.0

5

10 15 M n wt.% on TiO

20

Surface atomic concentration (a.u)

0.8 Surface Mn2p/Ti 2p atomic ratio (a.u)

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 33

0.0

2

Figure 6. Surface atomic concentration of Mn 2p, Ti 2p and surface atomic ratio between Mn 2p and Ti 2p obtained from XPS analysis 14 / 31


Page 15 of 33

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


3.4. SEM-EDX Analysis of the 15 wt.% of MnOx/TiO2 Sorbent. Figure 7 showed the SEM images of 15 wt.% of Mn/TiO2 sorbent of fresh and after reaction at a magnification of 20000 times, which revealed the morphology and structure of 15 wt.% of MnOx/TiO2. For the fresh sample, the surface of 15 wt.% of MnOx/TiO2 was smooth and the fewer light zones implied MnOx microcrystal structure was clear, without any particles adsorbed. And amounts of holes presented at the same time. After the reaction, largely numbers of holes were blocked and more light zones appeared, which demonstrated a large quantities of new produced substance deposited on the surface of the sample, which may be Hg0 or HgO. The adsorbent surface feature was believed to be a vital factor in mercury adsorption capacity. So the amount of various components and main constituents on surface of fresh and exhausted 15 wt.% of MnOx/TiO2 at the reaction temperature of 100 °C were investigated by EDX analysis.66 The possible elements and their corresponding percentages were listed in the Table 2. Based on the contents for the fresh sample, TiO2 was the main substance, and small amounts of MnOx were also detected but there was no elemental mercury existed. Compared to the fresh sample, on the used sample the content of elemental mercury appeared obviously and the amounts of O decreased, which may due to that the O was covered by the adsorbed Hg0.67 This result proved that the chemisorption reaction occurred in the removal of Hg0. This was in accordance with the XPS analysis above, which concluded that mercury oxide (HgO) appeared after the adsorbent adsorbed the Hg0. Table 2. EDS Analysis of the Surface of 15 wt.% of MnOx/TiO2 Sorbent

Element

C

O

Ti

Mn

Hg

5.09

46.60

40.65

7.66

0

MnOx/TiO2(fresh) wt.%

15 / 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

Page 16 of 33

MnOx/TiO2(used) wt.%

5.00

39.62

41.02

7.75

6.61

a

b

Figure 7. SEM photographs of before (a) and after (b) reaction for Hg0 removal on 15 wt.% of MnOx/TiO2 sorbent 4. Result and Discussion 4.1 Effects of the Calcination Temperatures of MnOx/TiO2 on Hg0 Removal. The effects of the calcination temperatures (450, 550, 650 and 750 °C) of the 15 16 / 31


Page 17 of 33

wt.% of MnOx/TiO2 on Hg0 removal ability were evaluated at 100°C. Based on the consequence that was shown in Figure 8, increasing of the calcination temperature from 450 to 550 °C, Hg0 removal efficiency increased as well. While further increasing the calcination temperatures, the Hg0 removal efficiency decreased sharply, which may be due to the extremely low surface area. It is well known that higher calcination temperature would cause irreversible structure collapse and decrease the active sites of Hg0. Based on the characterization results of BET, calcination temperature of 450 °C of the sample had the largest BET, which was beneficial for physisorption. However, according to the experimental result, the calcination temperature of 550 °C of the sample performed the best Hg0 removal efficiency, which suggested that the surface area had no effect on removal of Hg0. Instead, the primary effect maybe associated to the oxidation sites of Hg0 over the surface of MnOx/TiO2. Moreover, the interaction between MnOx and support, and crystal phase components of metal oxide on the support may have the closer relation to the catalytic activity to Hg0 removal, which would be discussed latter. 100

0

450 C 0 550 C 0 650 C 0 750 C

80

60

0

Hg removal efficiency %

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


40

20

0

200

400

600

800 1000 1200 1400 1600 t/min

Figure 8. Effects of different calcination temperatures of MnOx/TiO2 sorbent on Hg0 break-through curves. (Reaction condition: Hg0=2.0 mg/m3, O2 = 1%, Q=0.4 L/min, T= 100 °C, 15 wt.% of Mn content of the sorbent ). 17 / 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

Page 18 of 33

4.2. Effects of Mn Content of MnOx/TiO2 on Hg0 Rremoval. The effects of TiO2 modified by different Mn contents on Hg0 removal efficiency were investigated at 100 °C. As shown in Figure 9, all of MnOx/TiO2 exhibited the better Hg0 removal ability than the virgin TiO2. With manganese oxide content increasing from 5 wt.% to 10 wt.% and 15 wt.%, the corresponding Hg0 adsorption capacity increased from 2.73 to 3.18 mg/g and 5.12 mg/g, respectively. This result indicated that the increase of manganese oxide content had a significantly positive effect on Hg0 capture efficiency. However, when the Mn content further increased to 20 wt.%, it appeared to a slight decrease (2.30 mg/g) for Hg0 capture ability. Among these sorbents, MnOx/TiO2 with 15 wt.% Mn content

showed

clearly

higher

Hg0 adsorption

capacity

and

longer

half-breakthrough time that more than 21.60 h. Based on the characterization result of XPS, the high activity of 15 wt.% of MnOx/TiO2 was partly due to the excellent dispersion of manganese oxide on the TiO2 support, which was beneficial for the catalytic oxidation of Hg0.68 In addition, the aggregation of manganese oxide occurred on the sample surface that with 20 wt.% Mn content, which resulted in the small surface area and the valid collision inaccessible between the deep layer of MnOx and the higher diffusion resistance of the Hg0 to the sorbent surface and the fewer active sites, thus further decreased the Hg0 adsorption capacity.39 It also implied that both the physisorption and catalytic oxidation of Hg0 were inhibited to some extent when the manganese oxide content consistently exceed the critical point.

18 / 31


Page 19 of 33

100

80

60

40

5 wt.% MnOx/TiO2

0


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


10 wt.% 15 wt.% 20 wt.% Pure TiO2

20

0

0

200

400

600

800 t/min

1000 1200 1400 1600

Figure 9. Effects of different Mn contents on Hg0 break-through curves. (Reaction condition: Hg0=2.0 mg/m3, O2 = 1%, Q=0.4 L/min, T= 100 °C, 550 °C of calcination temperature of the sorbent).

4.3. Effects of the Reaction Temperatures of MnOx/TiO2 on Hg0 Removal. To evaluate the reaction temperature effect on Hg0 removal, the reaction temperature varied from 50 to 200 °C were conducted in the presence of 1 % O2 over the 15 wt.% of MnOx/TiO2 sorbent. The experimental results were shown in Figure 10. It was obvious that Hg0 adsorption capacity of 15 wt.% of MnOx/TiO2 was about 2.5 times higher at 100 °C (5.12 mg/g) than that of 50 °C (1.92 mg/g). The progressive increase of Hg0 adsorption capacity may be due to the reactants could get more kinetic energy as the reaction temperature increasing, which was in favor of Hg0 oxidation.69,70 However, when the reaction temperature continued going up from 100 °C to higher temperatures, Hg0 adsorption capacity dropped significantly (3.87 mg/g at 150 °C and 2.93 mg/g at 200 °C, respectively). The results may because that the higher reaction temperature was disadvantageous to Hg0 adsorption. In addition, based on the XPS analysis of Hg 4f peaks at 100°C discussed above, it demonstrated that the adsorption of Hg0 on the sorbent was mainly by the chemical adsorption process. 19 / 31



100

80 0

50 C 0 100 C 0 150 C 0 200 C

60

40

0


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

20

0

0

200

400

600

800 t/min

1000 1200 1400 1600

Figure 10. Effects of different reaction temperatures on Hg0 break-through curves. (Reaction condition: Hg0=2.0 mg/m3, O2 = 1%, Q=0.4 L/min, 550 °C of calcination temperature and 15 wt.% of Mn content of the sorbent )

4.4 Effect of Individual Flue Gas Component on Hg0 removal Over MnOx/TiO2 Sorbent. 4.4.1. Effects of O2. O2 was already considered by previous studies, suggesting that it had obvious effect on the Hg0 removal.71,72 The comparative experiments of Hg0 removal under pure N2, 0.5 %, 1 %, 2% and 5% O2 conditions were investigated at the reaction temperature of 100 °C. As presented in Figure 11, under pure N2 gas flow, nearly 100% Hg0 removal efficiency maintained about 360 min and the uptake capacity of Hg0 was 1.75 mg/g. In the absence of oxygen, the removal of Hg0 could partly be ascribed to the Mars-Maessen mechanism, which was the mechanism that the lattice oxygen of the sorbent as the oxidant reacted with the adsorbed Hg0 to form HgO.53 Both the substantial stored oxygen on the surface of the sorbent and the low temperature activity of manganese oxide promoted the Hg0 oxidation.41,73 When O2 introduced continuously increased from 0 to 1 % O2, the length of the maintain time of 100% Hg0 removal efficiency and Hg0 uptake capacity were enhanced obviously, which were corresponding to 600 min and 5.12 mg/g, 20 / 31


Page 20 of 33

Page 21 of 33

respectively. The results were 1.6 and 3.0 times larger than these of the oxygen-free condition. It indicated that oxygen exhibited an excellently promotional effect on Hg0 oxidation, which replenished the consumed lattice oxygen and chemisorbed oxygen by the dissociation adsorption way according to the XPS. Further increasing the O2 concentration from 1% to 5%, Hg0 removal efficiency had almost no obvious change, which indicated that O2 concentration of 1% was enough for Hg0 removal and more O2 introduced did not work for it. 100

0


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


80

60 N2 40

N2+0.5% O2 N2+1% O2 N2+2% O2

20

N2+5% O2 0

0

200

400

600 800 t/min

1000 1200 1400 1600

Figure 11. Effects of different O2 concentrations on Hg0 break-through curves. (Reaction condition: Hg0=2.0 mg/m3, T= 100 °C, Q=0.4 L/min, 550 °C of calcination temperature and 15 wt.% of Mn content of the sorbent)

4.4.2. Effects of H2O. Previous studies have reported that owing to the competitive adsorption of H2O, Hg0 removal efficiency would decrease.72 The effect of water vapor on Hg0 capture was also measured here. As shown in Figure 12, when 1% H2O was introduced into the gas flow, Hg0 removal efficiency could only reach to 90% and the maintain time diminished simultaneously. With further exposing to the higher H2O concentration to 5%, which could lead to the acceleration deactivation of sorbent,69 and Hg0 removal efficiency decreased. The result above suggested that the water vapor inhibited the Hg0 removal over 21 / 31



MnOx/TiO2, which could be associated with the completion adsorption occurred between Hg0 and H2O on the active sites that could chemically bond to Hg0.37,72 The higher concentration of H2O resulted to more active sites covered by H2O, which decreased the Hg0 adsorption capacity further. 100

80

60

40

0


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

N2+1% O2 N2+1% O2+1% H2O

20

0

N2+1% O2+ 5% H2O 0

200

400

600 800 t/min

1000 1200 1400 1600

Figure 12. Effecs of different relative humidity on Hg0 break-through curves. (Reaction condition: Hg0=2.0 mg/m3, O2 = 1%, T= 100 °C, Q=0.4 L/min, 550 °C of calcination temperature and 15 wt.% of Mn content of the sorbent)

4.4.3. Effect of H2S. H2S as a most important pollutant was also considered in this study. From the Figure 13, we could see that when 1000 ppm H2S was introduced to the gas flow, Hg0 removal efficiency and uptake capacity were only 70% and 0.56 mg/g, which was much lower than that under the same atmosphere that without H2S. The inhibitory effect was obvious when the H2S concentration further increased to 3000 ppm and 5000 ppm, indicating that H2S extremely suppressed Hg0 removal on MnOx/TiO2 sorbent. It might be assigned to H2S occupied competitively the active sites on the sorbent surface and interacted strongly with manganese oxide to form manganese sulfide, which consumed the active components.

22 / 31


Page 22 of 33

Page 23 of 33

100

80

60

N2+ 1% O2+1000ppm H2S N2+ 1% O2+3000ppm H2S

0


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


N2+ 1% O2+5000ppm H2S

40

N2+ 1% O2 20

0

200

400

600 800 1000 1200 1400 1600 t/min

Figure 13. Effects of different concentrations of H2S on Hg0 break-through curves. (Reaction condition: Hg0=2.0 mg/m3, O2 = 1%, T= 100 °C, Q=0.4 L/min, 550 °C of calcination temperature and 15 wt.% of Mn content of the sorbent).

5. DISCUSSION 5.1. Mechanism for Hg0 Oxidation over MnOx/TiO2 Sorbent. On the basis of the previous studies and characterization results of XPS, the removal mechanism for oxidation reaction of Hg0 could be supposed. For the first period of the reaction, the physisorption of Hg0 occurred. As the reaction temperature increasing, the Hg0 oxidation ability of MnOx/TiO2 sorbent enhanced. Both hydroxyl oxygen and lattice oxygen contributed to the Hg0 oxidation. At 50 °C, the lattice oxygen ([O]) was from the valence state changes of manganese oxides, which can be described as follows: 2MnO2 → Mn2O3 + [O]

(3)

[O] + Hg → HgO

(4)

In addition, Hg0 oxidation could be carried out in the absence of O2 was due to the lattice oxygen (eq4) provided by the transformation of metal valence state. In the presence of O2, O2 supplied O to metal oxides and regenerated the consumed [O] to keep Hg0 oxidation continued. The transformation of O2 to lattice oxygen was as 23 / 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

Page 24 of 33

follows: Mn2O3 + 1/2O2 →2MnO2

(5)

At 100 and 150 °C, the hydroxyl oxygen (OH) took part in the Hg0 catalytic oxidation. The reaction process was as follows: OH + Hg → HgO + H

(6)

H + 1/2O2 → OH

(7)

6. CONCLUSIONS A series of MnOx supported on TiO2 sorbents were synthesized via sol-gel method and evaluated for Hg0 removal under the low temperature conditions. 15 wt.% manganese content of MnOx/TiO2 sorbent showed the best performance on Hg0 removal. Its adsorption capacity for elemental mercury reached 5.12 mg/g at 100 °C. Both physisorption and catalytic oxidation attributed to the Hg0 removal, between which catalytic oxidation of Hg0 was the main way. H2S and H2O inhibited the adsorption of mercury on the sorbent because of their competitive adsorption on active sites.

■ AUTHOR INFORMATION Corresponding Author *Tel.: 13888183303 *E-mail: [email protected] (Ping Ning) Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (2017YFC0210503), National Natural Science Foundation of China (no. 51368026, no. 51568027) and Candidates of the Young and Middle Aged Academic Leaders of Yunnan Province (2015HB012).

24 / 31


Page 25 of 33

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


■ REFERENCES (1) chounwou, P. B.; Ayensu, W. K.; Ninashvili, N.; Sutton, D. Environmental exposure to mercury and its toxico pathologic implications for public health.

Environ Toxicol . 2003, 18, 149. (2) Streets, D. G.; Hao, J.; Wu, Y.; Jiang, J.; Chan, M.; Tian, H.; Feng, X. Anthropogenic mercury emissions in China. Atmos. Environ 2005, 39, 7789. (3) Mackey, T. K.; Contreras, J. T.; Liang, B. A. The convention on mercury: attempting to address the global controversy of dental amalgam use and mercury waste disposal. Sci. Total Environ. 2014, 472, 125. (4) Bailey, S. E.; Olin T. J.; Bricka, R. M.; Adrian, D. D. A review of potentially low-cost sorbents for heavy metals. Water Res. 1999, 11, 2469. (5) Slemr, F.; Seiler, W. J.; Schuster, G. Distribution speciation and budget of atmospheric mercury. J. Atmos. Chem. 1985, 3, 407. (6) Lindqvist, O.; Johansson, K.; Aastrup, M.; Anderson, A.; Timm, B. Mercury in the swedish environment-recent research on causes, consequences, and corrective methods. Water Air Soil Pollut. 1991, 55, 1. (7) Carey, T.; Hargrove, O.; Seeger, D.; Richardson, C.; Rhudy, R.; Meserole, F. Effect of mercury speciation on removal across wet FGD processes. AIChEJ. New York, 1996. (8) Srivastava, R. K.; Hutson, N.; Martin, B.; Princiotta, F.; Staudt, J. Control of mercury emissions from coal-fired electric utility boilers. Environ. Sci. Technol. 2006, 40, 1385. (9) Presto, A. A.; Granite, E. J. Survey of catalysts for oxidation of mercury in flue gas. Environ. Sci. Technol. 2006, 40, 5601. (10) Global Mercury Assessment 2013: Sources, Emissions, Releases and Environmental Transport. UNEP: UNEP Chemicals Branch, Geneva, Switzerland, 2013. (11) Driscoll, C. T.; Mason, R. P.; Chan, H. M.; Jacob, D. J.; Pirrone, N. Mercury as a global pollutant: sources, pathways, and effects. Environ. Sci. Technol. 2013, 47, 4967. 25 / 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

(12) Liu, W.; Vidic, R. D.; Brown, T. D. Optimization of sulfur impregnation protocol for fixed-bed application of activated carbon-based sorbents for gas-phase mercury removal. Environ. Sci. Technol. 1998, 32, 531. (13) Lee, S. J.; Seo, Y. C.; Jurng, J.; Lee, T. G. Removal of gas-phase elemental mercury by iodine-and chlorine-impregnated activated carbons. Atmos. Environ. 2004, 38, 4887. (14) Zeng, H.; Jin, F.; Guo, J. Removal of elemental mercury from coal combustion flue gas by chloride-impregnated activated carbon. Fuel. 2004, 83, 143. (15) Lee, S. H.; Rhim, Y. J.; Cho, SP.; Baek, J. I. Carbon-based novel sorbent for removing gas-phase mercury. Fuel. 2006, 85, 219. (16) Hutson, N. D.; Attwood, B. C.; Scheckel, K. G. XAS and XPS characterization of mercury binding on brominated activated carbon. Environ. Sci.

Technol. 2007, 41, 1747. (17) Morency, J. Zeolite sorbent that effectively removes mercury from flue gases.

Filtr. Sep. 2002, 39, 24. (18) Hassett, D. J.; Eylands, K. E. Mercury capture on coal combustion fly ash.

Fuel. 1999, 78, 243. (19) Poulston, S.; Granite, J. E.; Pennline, W. H.; Myers, R.C.; Stanko, P. D.; Ilkenhans, T.; Chu, W. Metal sorbents for high temperature mercury capture from fuel gas. Fuel. 2007, 86, 2201. (20) Tao, F. M. A new approach to the efficient basis set for accurate molecular calculations: Applications to diatomic molecules. J. Chem. Phys. 1994, 100, 3645. (21) Cao, Y.; Chen, B.; Wu, J.; Cui, H.; Pan, P.W. Study of mercury oxidation by a selective catalytic reduction catalyst in a pilot-scale slipstream reactor at a utility boiler burning bituminous coal. Energy Fuels. 2007, 21, 145. (22) Zhuang, Y.; Laumb, J.; Liggett, R.; Holmes, M.; Pavlish, J. Impacts of acid gases on mercury oxidation across SCR catalyst. Fuel Process. Technol. 2007, 88, 929. (23) Straube, S.; Hahn, T.; Koeser, H. Adsorption and oxidation of mercury in 26 / 31


Page 26 of 33

Page 27 of 33

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


tail-end

SCR-DeNOx

plants-Bench

scale

investigations

and

speciation

experiments. App. Catal. B Environ. 2008, 79, 286. (24) Yang, S.; Guo, Y.; Yan, N.; Wu, D.; He, H.; Qu, Z.; Yang, C.; Zhou, Q.; Jia, J. Nanosized cation-deficient Fe-Ti spinel: A novel magnetic sorbent for elemental mercury capture from flue gas. Appl. Mater. Interfaces. 2011, 3, 209. (25) Yang, S.; Yan, N.; Guo, Y.; Wu, D.; Jia, J. Gaseous elemental mercury capture from flue gas using magnetic nanosized (Fe3−xMnx)1-δO4. Environ. Sci. Technol. 2011, 45, 1540. (26) Schofield,

K. Fuel-mercury combustion emissions: An important

heterogeneous mechanism and an overall review of its implications. Environ. Sci.

Technol. 2008, 42, 9014. (27) Yan, N. Q.; Chen, W.; Chen, J.; Qu, Z.; Guo, Y.; Yang, S. Significance of RuO2 modified SCR catalysts for elemental mercury oxidation in coal-fired flue gas. Environ. Sci. Technol. 2011, 45, 5725. (28) Hrdlicka, J. A.; Seames, W. S.; Mann, M. D.; Muggli, D. S.; Horabik, C. A. Mercury oxidation in flue gas using gold and palladium catalysts on fabric filters.

Environ. Sci. Technol. 2008, 42, 6677. (29) Presto, A. A.; Granite, E. J. Noble metal catalysts for mercury oxidation in utility flue gas. Plat in. Met. Rev. 2008, 52, 144. (30) Granite, J. E.; Pennline, H. W. Catalysts for oxidation of mercury in flue gases. U.S. Patent 7776, 780 B1, 17 August 2010. (31) Zhao, Y. X.; Mann, M. D.; Pavlish, J. H.; Mibeck, B.A.F.; Dunham, G. E.; Olson, E. S. Application of gold catalyst for mercury oxidation by chlorine.

Environ. Sci. Technol. 2006, 40 (5), 1603. (32) Chu, P.; Laudal, D. L.; Brickett, L.; Lee, C. W. Power plant evaluation of the effect of SCR technology on mercury. In Proceedings of the Combined Power Plant Air Pollutant Control Symposium-The Mega Symposium. Washington, DC, 2003. (33) Cao, Y.; Gao, Z.; Zhu, J.; Wang, Q.; Huang, Y.; Chiu, C.; Parker, B.; Chu, P.; 27 / 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

Pan, W. P. Impacts of halogen additions on mercury oxidation, in a slipstream selective catalyst reduction (SCR), reactor when burning sub-bituminous coal.

Environ. Sci. Technol. 2008, 42, 256. (34) Kamata, H.; Ueno, S.; Naito, T.; Yamaguchi, A.; Ito, S. Mercury oxidation by hydrochloric acid over a VOx/TiO2 catalyst. Catal. Commun. 2008, 9 (14), 2441. (35) Wan, Q.; Duan, L.; Li, J.; Chen, L.; He, K.; Hao, J. Deactivation performance and mechanism of alkali (earth) metals on V2O5-WO3/TiO2 catalyst for oxidation of gaseous elemental mercury in simulated coal-fired flue gas. Catal. Today. 2011, 175 (1), 189. (36) Yan, N. Q.; Liu, S. H.; Chang, S. G.; Miller, C. Method for the study of gaseous oxidants for the oxidation of mercury gas. Ind. Eng. Chem. Res. 2005, 44 (15), 5567. (37) Li, H.; Li, Y.; Wu, C. Y.; Zhang, J. Oxidation and capture of elemental mercury over SiO2-TiO2-V2O5 catalysts in simulated low rank coal combustion flue gas. Chem. Eng. J. 2011, 169 (1−3), 186. (38) He, S.; Zhou, J.; Zhu, Y.; Luo, Z.; Ni, M.; Cen, K. Mercury oxidation over a vanadia-based selective catalytic reduction catalyst. Energy Fuels. 2009, 23 (1), 253. (39) Qiao, S.; Chen, J.; Li, J.; Qu, Z.; Liu, P.; Yan, N.; Jia, J. Adsorption and catalytic oxidation of gaseous elemental mercury in flue gas over MnOx/alumina.

Ind. Eng. Chem. Res. 2009, 48 (7), 3317. (40) Li, H.; Wu, C. Y.; Li, Y.; Zhang, J. Superior activity of MnOx-CeO2/TiO2 catalyst for catalytic oxidation of elemental mercury at low flue gas temperatures.

Appl. Catal. B. 2012, 111, 381. (41) Ji, L.; Sreekanth, P. M.; Smirniotis, P. G. Manganese oxide/titania materials for removal of NOx and elemental mercury from flue gas. Energy Fuels. 2008, 22, 2299. (42) He, C.; Shen, B. X.; Chen, J. H.; Ca, J. Adsorption and oxidation of elemental mercury over Ce-MnOx/Ti-PILCs. Environ. Sci. Technol. 2014, 48, 7891. 28 / 31


Page 28 of 33

Page 29 of 33

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


(43) Li, H.; Wu, CY.; Li, Y.; Li, L.;

Zhao, Y.; Zhang, J. Impact of SO2 on

elemental mercury oxidation over CeO2-TiO2 catalyst. Chem. Eng. J. 2013, 219, 319. (44) Zhang, H.; Zhao, J. T.; Fang, Y. T.; Huang, J. J.; Wang, Y. Catalytic oxidation and stabilized adsorption of elemental mercury from coal-derived fuel gas. Energy Fuels. 2012, 26, 629. (45) Pan, H. Y.; Minet, R. G.; Benson, S. W.; Tsotsis, T. T. Process for converting hydrogen chloride to chlorine. Ind. Eng. Chem. Res. 1994, 33 (12), 2996. (46) Delimaris, D.; Ioannides, T. VOC oxidation over MnOx-CeO2 catalysts prepared by a combustion method. Appl. Catal. B. 2008, 84, 303. (47) Tang, X.; Chen, J.; Huang, X.; Xu, Y.; Shen, W. Pt/MnOx-CeO2 catalysts for the complete oxidation of formaldehyde at ambient temperature. Appl. Catal. B. 2008, 81, 115. (48) Tang, X.; Li, Y.; Huang, X.; Xu, Y.; Zhu, Wang, H. J.; Shen, W. MnOx-CeO2 mixed oxide catalysts for complete oxidation of formaldehyde: Effect of preparation method and calcination temperature. Appl. Catal. B. 2006, 62, 265. (49) Li, H.; Wu, CY.; Li, Y.; Li, L.; Zhao, Y.; Zhang, J. Role of flue gas components in mercury oxidation over TiO2 supported MnOx-CeO2 mixed-oxide at low temperature. J. Hazard. Mater. 2012, 243, 117. (50) Yuzhakova, T.; Rakic, V.; Guimon, C.; Auroux, A. Preparation and characterization of Me2O3-CeO2 (Me=B, Al, Ga, In) mixed-oxide catalysts. Chem.

Mater. 2007, 19, 2970. (51) Sun, X. H.; Qu, R. J.; Sun, C. M.; Zhang, Y.; Sun, SY.; Ji, C. N.; Yin, P. Sol-Gel preparation and Hg (II) adsorption properties of silica-gel supported low generation polyamidoamine dendrimers polymer adsorbents. Ind. Eng. Chem. Res. 2014, 53, 2878. (52) Jung, Y. S.; Kim, D. W.; Kim, Y. S.; Park, E. K.; Baeck, S. H. Synthesis of alumina-titania solid solution by sol-gel method. J Phys Chem Solid. 2008, 69, 1464. 29 / 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

(53) Granite, J. E.; Pennline, W. H.; Hargis, A. R. Novel sorbents for mercury removal from flue gas. Ind. Eng. Chem. Res. 2000, 39, 1020. (54) Fang, J.; Bao, H. Z.; He, B.; Wang, F.; Si, D. J.; Jiang, Z. Q.; Pan, Z. Y. Interfacial and surface structures of CeO2-TiO2 mixed oxides. J. Phys. Chem. C 2007, 111, 19078. (55) Kuang, M.; Yang, G. H.; Chen, W. J.; Zhang, Z. X. Study on mercury desorption from silver-loaded activated carbon fibre and activated fibre. Fuel

Chem. Technol. 2008, 36, 468. (56) Lu, P.; Li, C. T.; Zeng, G. M.; He, L. J.; Peng, D. L.; Cui, H. F. Low temperature selective catalytic reduction of NO by activated carbon fiber loading lanthanum oxide and ceria. Appl. Catal. B. 2010, 96, 157. (57) Li, Y. H.; Lee, C. W.; Gullett, B. K. Importance of activated carbon's oxygen surface functional groups on elemental mercury adsorption. Fuel. 2003, 82, 451. (58) Qu, L.; Li, C.; Zeng, G.; Zhang, M.; Fu, M. Support modification for improving the performance of MnOx−CeOy/γ-Al2O3 in selective catalytic reduction of NO by NH3. Chem. Eng. J. 2014, 242, 76. (59) Deng, S.; Liu, H.; Zhou, W.; Huang, J.; Yu, G. Mn-Ce oxide as a high-capacity adsorbent for fluoride removal from water. J. Hazard. Mater. 2011, 186 (2-3), 1360. (60) Liu, C. X.; Chen, L.; Li, J. H.; Ma, L.; Arandiyan H.; Du, Y. Enhancement of activity and sulfur resistance of CeO2 supported on TiO2-SiO2 for the Selective catalytic reduction of NO by NH3. Environ. Sci. Technol. 2012, 46 (11), 6182. (61) Wu, Z. B.; Jiang, B. Q.; Liu, Y. Effect of transition metals addition on the catalyst of manganese/titania for low-temperature selective catalytic reduction of nitric oxide with ammonia. Appl. Catal. B. 2008, 79, 347. (62) Machocki, A.; Ioannides, T.; Stasinska, B.; Gac, W. Manganese-lanthanum oxides modified with silver for the catalytic combustion of methane. J. Catal. 2004, 227, 282. (63) Smirniotis, P. G.; Sreekanth, P. M.; Penã, D. A.; Jenkins, R. G. Manganese oxide catalysts supported on TiO2, Al2O3, and SiO2: A comparison for 30 / 31


Page 30 of 33

Page 31 of 33

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


low-temperature SCR of NO with NH3. Ind. Eng. Chem. Res. 2006, 45, 6436. (64) Xu, H. M.; Qu Z.; Zong, C. X.; Huang, W. J.; Yan, N. Q. MnOx/Graphene for the catalytic oxidation and adsorption of elemental mercury. Environ. Sci.

Technol. 2015, 49, 6823. (65) Padmanabha, R. E.; Neeraja, E.; Sergey, M.; Punit, B.; Panagiotis, G. S. Surface characterization studies of TiO2 supported manganese oxide catalysts for low temperature SCR of NO with NH3. Appl. Catal. B: Environ. 2007, 76, 123. (66) Suresh Kumar Reddy, K.; Prabhu, A.; Shoaibi, A. A.; Srinivasakannan, C. Application of sulfonated carbons for mercury removal in gas processing. Energy

Fuels. 2016, 30, 3227. (67) Kamata, H.; Ueno, S. I.; Naito, T.; Yukimura, A. Mercury oxidation over the V2O5 (WO3)/TiO2 commercial SCR catalyst. Ind. Eng. Chem. Res. 2008, 47, 813. (68) Zhang, R. D.; Yang, W.; Luo, N.; Li, P. X.; Lei, Z. G.; Chen, B. H. Low-temperature NH3-SCR of NO by lanthanum manganite perovskites: Effect of A-/B-site substitution and TiO2/CeO2 support. Appl. Catal. B: Environ. 2014, 146, 94. (69) Zhang, A. C.; Xiang, J.; Sun, L. S.; Hu, S. Preparation, characterization, and application of modified chitos an sorbents for elemental mercury removal. Ind.

Eng. Chem. Res. 2009, 48, 4980. (70) Mochida, I.; Korai, Y.; Shirahama, M. Removal of SOx and NOx over activated carbon fibers. Carbon. 2000, 38, 227. (71) Li, H. L.; Wu, C. Y.; Li, Y. CeO2-TiO2 catalysts for catalytic oxidation of elemental mercury in low-rank coal combustion flue gas. Environ. Sci. Technol. 2011, 45, 7394. (72) Li, Y.; Murphy, P. D.; Wu, C. Y.; Powers, K. W.; Bonzongo, J. C. Development of silica/vanadia/titania catalysts for removal of elemental mercury from coal-combustion flue gas. Environ. Sci. Technol. 2008, 42, 5304. (73) Li, F.; Yan, N. Q.; Qu, Z.; Qiao, S. H.; Jia, J. P. Catalytic oxidation of elemental mercury over the modified catalyst Mn/alpha-Al2O3 at lower temperatures. Environ. Sci. Technol. 2010, 44, 426. 31 / 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

32 / 32


Page 32 of 33

Page 33 of 33

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


Adsorption and catalytic oxidation of mercury over MnOx/TiO2 under the low temperature conditions Jinhuan Cheng, Xueqian Wang, Yanan Li, Ping Ning* Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, China

Mn4+

Mn3+

Lattice Oxygen ([O])

Hgads 3+

Mn

2+

Mn

Hydroxyl (OH)

Oxygen

On the basis of the previous studies and characterization results of XPS, Both physisorption and catalytic oxidation of Hg0 were occurred. As the reaction temperature increasing, the Hg0 oxidation ability enhanced. In addition, hydroxyl oxygen and lattice oxygen contributed to the Hg0 oxidation at different reaction temperatures. The O2 supplied O to metal oxides to regenerate the consumed hydroxyl oxygen and lattice oxygen to keep Hg0 oxidation continued.


Hg0 O

TiO2 under

Recommend Documents