Simultaneous Removal of NO and Hg0 from Flue ... - ACS Publications

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Simultaneous Removal of NO and Hg0 from Flue Gas over Mn−Ce/TiPILCs Yinyin Wang,† Boxiong Shen,*,‡ Chuan He,† Shiji Yue,† and Fumei Wang† †

College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China School of Energy & Environmental Engineering, Hebei University of Technology, Tianjin 056038, China

‡

S Supporting Information *

ABSTRACT: A series of Mn−Ce/Ti-PILCs (PILCs, pillared interlayered clays) catalysts were prepared via impregnation method in simultaneous removal of NO and elemental mercury in simulated flue gas. The physicochemical properties of these catalysts have been examined by some characterization methods, such as H2-TPR, nitrogen adsorption, XRD and XPS. Mn(6%)−Ce(6%)/Ti-PILCs exhibited superior NO conversion (>95%) and Hg0 removal efficiency (>90%) at low temperature (250 °C). The results indicated that the elemental mercury had little impact on NO removal efficiency, while the presence of NH3 and NO in SCR system inhibited the Hg0 removal. NO and Hg0 removal activity was strongly affected by the transform between surface adsorbed oxygen and lattice oxygen. The species ratio of Mn4+/Mn3+ and Ce4+/Ce3+ on the catalyst surface contributed to the NO conversions and Hg0 removal. Mn−Ce/Ti-PILCs displayed a broad prospect for controlling the emission of NO and mercury. On the basis of the results obtained, a mechanism for the simultaneous removal of NO and Hg0 was proposed for the Mn−Ce/Ti-PILCs catalysts: −NH2 + NO → N2 + H2O, −OH + 1/2 Hg(ad) →1/2 HgO + 1/2 H2O. from stationary sources.8,9 As for elemental mercury, the technology of activated carbon injection (ACI) is considered as a common method to remove mercury at present.10 The current control technology is used alone to remove NO and Hg0, respectively, which results in the low efficiency of apparatus, large investments, and high operating costs.11 Hence, the simultaneous removal of air pollutants will be very interesting. Yuan et al.4 investigated the simultaneous removal of SO2, NO, and mercury using TiO2-aluminum silicate fiber by photocatalysis. Xu et al.8 studied the simultaneous oxidation of NO, SO2, and Hg0 from flue gas by pulsed corona discharge. Recent studies indicated that the selective catalytic reduction (SCR) catalysts were also effective in oxidizing mercury.12,13 MnOx-based catalyst has been investigated extensively in low temperature SCR because of its high catalytic activities.14−16 Ji

1. INTRODUCTION Coal-fired power plants, which produce various air pollutants, are one of the major anthropogenic emission sources for the environment. NOx and mercury are considered as significant atmospheric contaminants and have attracted broad attention in recent years. NOx in flue gas consists of over 90−95% of NO.1 It is known that NOx can cause a lot of environmental problems such as acid rain and photochemical smog.2 Mercury is one of the most hazardous environmental toxins and can be a threat to both human health and the environment due to the extreme toxicity, persistence, and the bioaccumulation.3,4 Mercury usually exists in three forms in flue gas, i.e., elemental mercury (Hg0), particle bound mercury (Hgp), and oxidized mercury (Hg2+).5,6 In general, Hg0 is hardly removed due to its high volatility and insolubility in water; however, Hgp can be captured by electrostatic precipitators or fabric filters and Hg2+ is water-soluble, which can be removed by wet flue gas desulfurization devices (WFGD).7 Therefore, how to convert NO and Hg0 to N2 and Hg2+ has become the key to control NO and mercury emission. Selective catalytic reduction (SCR) has been proven to be an effective approach for NO reduction © XXXX American Chemical Society

Received: March 20, 2015 Revised: June 29, 2015 Accepted: July 8, 2015

A

DOI: 10.1021/acs.est.5b01435 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

Figure 1. Simultaneous removal efficiency of different catalysts: (a) NO removal, and (b) total Hg0 removal (Reaction conditions: NO = NH3 = 500 ppm, Hg0 = 50 μg/m3, O2 = 5%, Cat. = 0.5 g, GHSV = 50 000/h, N2 as balance).

et al.17 found that MnOx/TiO2 catalyst could achieve 97% of NO conversion and about 90% of mercury removal in lowtemperature. Additionally, CeO2, which has large oxygen storage capacity and oxygen conversion ability, has been used as catalyst.18,19 In our previous study, Ce as an additive was added to the support for SCR catalysts and showed high activity.20 Qi et al.21 found that there was a strong interaction between CeO2 and MnOx, which resulted in high SCR activity. In addition to the active ingredient, catalyst support is also crucial to catalysis performance.20,22 Jin et al.23 compared the catalyst activity of Mn−Ce supported on TiO2 and Al2O3 and found that the supports can provide different acid sites which is beneficial to NO conversion. Pillared interlayered clays (PILCs) are the kind of molecular sieve materials with a special two-dimensional structure, and they have been widely used in the field of catalytic, adsorption, and separation processes due to controllable pore structure, high thermal stability and low-cost.24−27 Chae et al.28 synthesized a novel SCR catalyst by using Ti-PILCs as the support and found that it exhibited superior performance compared to conventional catalysts. The modified PILC was regarded as a kind of potential catalyst support. In this study, the Mn−Ce/Ti-PILCs catalysts prepared via the impregnating method were investigated with respect to their simultaneous NO and Hg0 removal performance in the simulated flue gas. Meanwhile, the essential analysis and characterization of catalysts have been conducted to reveal the mechanism of NO and Hg0 removal over the catalysts.

activity of NOx and Hg0, a 1 mL (0.5 g) sample was loaded in the quartz reactor, and simulated flue gas (SFG) was introduced into the reactor with the inlet flow rate at 850 mL min−1 and a gas hourly space velocity (GHSV) of approximately 50 000 h−1. The basic composition of SFG included 500 ppm of NO, 500 ppm of NH3, 5% O2, and pure N2 as the balance. N2 was divided into two branches. One branch converged with NO, NH3, and O2 to form the main gas flow, the other one passed through a mercury permeation device (Metronics, U.S.) to transport Hg0 (∼50 μg m−3) vapor into the system. All of the components of the reaction mixture were introduced directly from the cylinders, and their flow rates were controlled by the mass flow-meters. The feed gases were mixed and preheated in a chamber (100 °C) before entering the reactor. The NO and Hg0 concentrations at the inlet and outlet of the reactor were monitored respectively by a Flue Gas Analyzer (KM940, Kane international limited, U.K.) and an online mercury analyzer (H11-QM201H, Qing’an Co. LTD, CHN). NH3, NO2, and N2O were analyzed using a Fourier Transform Infrared Spectrometer (WQF-510A, Beijing analytical instrument factory). During the measurements, sampling tests and analyses were performed for 3 times to reduce the error and uncertainty. The exhaust gas from the reactor passed through the carbon trap and then expelled into the atmosphere. The removal efficiency of NO and Hg0 are respectively defined in eq 1 and eq 2:

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The Mn−Ce/Ti-PILCs catalysts employed in this study are prepared by mixing the supports and active components using an impregnation method. The details of the synthetic procedure are available in the Supporting Information (SI) (Part 1). The catalysts of Mn(12%)/Ti-PILCs, Mn(6%)−Ce(6%)/TiPILCs and Ce(12%)/Ti-PILCs were obtained (the value in brackets represents the mass percentage), and they were abbreviated as Mn/TP, MnCe/TP, and Ce/TP, respectively. All of the materials were from Tianjin Guangfu Reagent Co. Ltd. 2.2. Catalytic Activity Test. A schematic diagram of the experimental setup is shown in Figure S1 (Part 2 in the SI). The activities of catalysts for NO (NH3−SCR) and the elemental mercury (Hg0) removal were determined using a fixed-bed system. To investigate the simultaneous removal

⎛ [NO]out ⎞ ReNO = ⎜1 − ⎟ × 100% [NO]in ⎠ ⎝

(1)

⎛ [Hg 0]out ⎞ ⎟ × 100% ReHg = ⎜1 − [Hg 0]in ⎠ ⎝

(2)

where, the subscript “in” denotes the inlet gas concentration, and the subscript “out” denotes the outlet gas concentration. 2.3. Catalyst Characterization. The H2-TPR experiments were performed on a TP-5080 automated chemisorption analyzer with 0.1 g catalysts to analyze the redox properties of the catalyst. The samples were pretreated in pure N2 at 500 °C for 1 h in order to eliminate the adsorbed volatile impurity of its pore structure and then cooled to room temperature, performed in a flow of H2 (5%) in N2 (30 mL min−1) from room temperature to 850 °C with a heating rate of 10 °C min−1. The H2 was quantified by a thermal conductivity detector (TCD). B


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Environmental Science & Technology The Brunauer−Emmett−Teller specific areas (BET) of the samples were determined by N2 adsorption at −196 °C on a NOVA2000 automated gas sorption system. X-ray diffraction (XRD) spectra were performed on a Rigaku D/Max 2500 system for obtaining the crystal structure by using Cu Ka(40 kV, 100 mA) radiation (Rigaku Corporation, JPN) with a scanning range of 3 to 80° (2θ). X-ray photoelectron spectroscopy (XPS) was performed in a Kratos Axis Ultra DLD spectrometer operating at 10−9 Pa with an Al radiation (1486.6 eV) to analyze the surface atomic concentration and characterize the chemical states of the catalysts. The observed spectra were corrected by the C 1s binding energy value of 284.6 eV (Kratos Analytical Ltd. Co., U.K.).

3. RESULTS AND DISCUSSION 3.1. Comparisons of the NO and Hg0 Removal over Different Catalysts. To determine the simultaneous removal efficiency of the catalysts, a comparison of the removal efficiency for Mn/TP, MnCe/TP, and Ce/TP catalysts in the temperature range of 100−350 °C were shown in Figure 1. The NO removal efficiency (ReNO) of all the catalysts increased with the increase of reaction temperature from 100 to 300 °C. However, for the ReNO of Mn-based catalysts (Mn/TP, MnCe/ TP), different trends emerged with further increase of the reaction temperature (up to 350 °C, Figure 1(a)). The activities of Mn-based catalysts declined as the temperature rose to a certain degree. Park and his group29 proved that NH3 would be oxidized to NO and/or NO2 at sufficiently high temperatures in the presence of O2. Therefore, the slight decrease of ReNO over Mn-based catalysts may be attributed to the NH3 oxidation at higher temperature (350 °C). According to Park and his partners’ research,29 “the oxidation rate of NH3 certainly increased with increasing reaction temperature, and eventually, all of the NH3 disappeared above 250 °C. The main reaction for the oxidation of ammonia is 4NH3 + 3O2 = 2N2 + 6H2O. The reaction forming NO and NO2 could be negligible over the temperature range 150−300 °C. However, the formation of NO and NO2 according to the mechanism (4NH3 + 5O2 = 4NO + 6H2O; 4NH3 + 7O2 = 4NO2 + 6H2O;) rapidly increased above 325 °C.” This explained the high potential of Mn-based catalysts for use in SCR at low temperature. However, the ReNO over Ce-based catalyst (Ce/TP) was still on the rise when the temperature increased to 350 °C. It was assumed that CeO2 could partly extend the catalyst activity at higher temperatures.30−32 At all of the reaction temperatures, the Mn-based catalysts (Mn/TP, MnCe/TP) exhibited a higher Hg0 removal efficiency (ReHg) compared to Ce catalyst (Ce/ TP), especially at low temperatures (i.e., 100, 150, 200 °C) (Figure 1(b)). The Mn-based catalysts (Mn/TP, MnCe/TP) demonstrated a ReHg rising at first, dropping afterward, and then rising at the higher temperature again. Although the highest ReHg was still lower than 50%, Ce/TP catalyst exhibited a rising ReHg in the whole temperature range. The results indicated that MnOx afforded a larger promotion to Hg0 removal than CeOx. In most cases, the MnCe/TP catalyst maintained better activity for NO and Hg0 removal, which might be due to the interaction of Mn and Ce in the catalyst. On the basis of the above results, the MnCe/TP catalyst was used to investigate the catalytic reaction in the following study because of the preferable simultaneous removal efficiency. 3.2. Effects of Hg0 on SCR de-NOx Activity. As shown in Figure 2, the effect of Hg0 on the NO removal efficiency was

Figure 2. Effects of Hg0 on NO removal efficiency.

obtained. It was obvious that the ReNO increased with the increase of reaction temperature from 100 to 250 °C then a slight drop occurred for 300 and 350 °C in the two systems (single SCR system and SCR+Hg0 system). Although a little fluctuation was observed in low temperature, the trend of ReNO for the two systems was similar. This indicated that the addition of Hg0 had little impact on ReNO. The possible reason was that the Hg0 concentration was extremely small (∼50 μg m−3) compared to NO concentration (500 ppm). The N2 selectivity over MnCe/TP catalyst in SCR and SCR +Hg0 system was shown in SI Figure S2 (from 100 to 350 °C) (Part 2 in SI). MnCe/TP catalyst showed an excellent N2 selectivity below 200 °C (>90%) both in SCR and SCR+Hg0. However, N2 selectivity obviously decreased with the further increase of reaction temperature. It indicated that the amount of N2O formed increased with the increase of temperature, which decreased the removal of NO and mercury. 3.3. Effects of SCR on Hg0 Removal. The effects of SCR system on Hg0 removal efficiency were investigated in Figure 3. The mercury removal efficiency without SCR was significantly higher than that with SCR at all temperatures (Figure 3(a)). In the absence of SCR, the ReHg of the MnCe/TP catalyst remains above 90%, independently of temperature. In the presence of SCR, the ReHg presented an uptrend as the temperature increased, with the exception point of 300 °C. It demonstrated that the presence of NH3 and NO would inhibit Hg0 removal. To understand the possible mechanisms involved in the deactivation of Hg0 removal by NH3, 200 ppm of NH3 and 500 ppm of NH3 was added to the gas flow containing 5% O2, and balanced with N2. It was clearly shown in Figure 3(b) that the high concentration of NH3 greatly inhibited the mercury removal. Obviously, the ReHg decreased sharply when the reaction temperature exceeded 250 °C, and sank to 38.7% at the highest temperature and concentration. This suggested that the high concentration of NH3 rapidly occupied the active sites and generated stronger adsorption, which weakened the interaction between Hg0 and catalyst surface active sites. Other literature also indicated that the competitive adsorption on the catalyst surface should be responsible for the decreasing of ReHg.33−35 The adsorbed NO could be oxidized by O2 on the surface of metal oxide catalysts, producing some species such as NO+ and NO2, which were probably responsible for Hg0 oxidation.7 However, the effect of NO on Hg0 removal over C


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Figure 3. Effects of SCR gas component on Hg0 removal efficiency (Reaction conditions: Hg0 = 50 μg/m3, O2 = 5%, N2 as balance, (a) NO = NH3 = 500 ppm, (b) NO = 0 ppm, NH3 = 0, 200, 500 ppm, and (c) NH3 = 0 ppm, NO = 0, 200, 500 ppm).

nonactive species like nitrite, and nitrate species might cover the active sites, inhibited the Hg0 oxidation. 3.4. Effects of O2 on NO and Hg0 Removal Efficiency over MnCe/TP. Oxygen is an important factor in flue gas, which can affect the transformation of NO and mercury and the formation of oxide species. By studying the effect of O2 in the flue gas on the ReNO and ReHg, the removal mechanism of NO and Hg0 can be elucidated. As shown in Figure S4(a) (Part 2 in the SI), the ReNO of MnCe/TP in the absence of O2 (0% O2) was obviously lower than the presence of O2 (i.e., 5% O2, 10% O2). It could be seen that oxygen accelerated the SCR reaction. However, excessive O2 concentration (10%) resulted in a small decrease of ReNO. This decrease of ReNO was thought to be the oxidation of NH3: 4NH3 + 5O2 → 4NO + 6H2O. It could be explained that excess oxygen would react with NH3, which weakened the reaction of NH3 and NO. SI Figure S4(b) showed the effect of O2 concentrations on mercury removal. If the simulated flue gas contained no oxygen, then the ReHg was no more than 20% at all temperatures. Xu et al. demonstrated the role of oxygen in promoting Hg0 removal in the simulate flue gas.38 In the absence of O2, Hg0 removal was weak. However, the Hg0 removal efficiency increased quickly after introducing O2.With the introduction of oxygen, the ReHg increased significantly. Many studies also revealed that the addition of O2 could facilitate Hg0 oxidation.7,36,39 That was consistent with our results. As shown in SI Figure S4(b), in the absence of O2, Hg0 removal was low (less than 20%). When the temperature was 300 °C, the ReHg was much lower (almost

the MnCe/TP catalyst was still observed to be inhibitory, but it was not as obvious as that of NH3. As shown in Figure 3(c), for the gas flow containing 200 ppm of NO, ReHg was almost unchanged. Further increase in NO concentration to 500 ppm resulted in a little lower ReHg. It indicated that a fraction of adsorption NO could react with the surface oxygen to form limited NO2, nitrite, and nitrate species.23 Although the active species like NO2 could promote the Hg0 oxidation,7 many other nonactive species like nitrite, and nitrate species might cover the active sites thus inhibiting the Hg0 oxidation. The results showed that NO still slightly inhibited Hg0 removal when compared to the gas flow without NO. It was necessary to explain that the lower ReHg was not caused by competitive adsorption between NO and Hg0. The competitive adsorption of NO and Hg0 on the MnCe/TP catalyst was demonstrated by a desorption experiment, the results are shown in Figure S3 (Part 2 in the SI). MnCe/TP was first pretreated at 250 °C under a flow of 50 μg m−3 Hg0 balanced in N2 for several hours in this test. At the beginning of the desorption experiment, only little Hg0 was repelled from catalyst surface. After 40 min, no obvious desorption of Hg0 was observed when NO was added. This result indicated that NO can hardly exclude Hg0 from the Mn−Ce/Ti catalyst. This result was consistent with Li et al.36 According to He’s studies,37 20 wt % MnOx/CeO2−TiO2 revealed very good mercury removal capacity, which was not influenced by NO in flue gas. In our study, it was known that MnCe/TP catalyst demonstrated higher Hg0 removal efficiency in the absence of NO. So it was therefore hypothesized that the D


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Figure 4. XPS spectra of MnCe/TP over the spectral regions of (a) O 1s, (b) Mn 2p, (c) Ce 3d, and (d) Hg 4f.

support showed a higher specific area (163.30 m2 g−1) than that of other catalysts. While the load of metal oxide gave a decrease of BET specific area as follows: TP > Mn/TP > Ce/TP > MnCe/TP. The result may be due to a partial blockage of the catalyst surface by the metal oxides.40,41 In general, the large

zero). This might be attributed to the higher volatility of Hg0 at higher temperature. Our study further demonstrated that O2 as the oxidizer could promote Hg0 oxidation in SCR system. 3.5. Characterization of Materials. The BET of the catalysts is listed in Table S1 (Part 3 in the SI). The Ti-PILCs E


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Environmental Science & Technology BET specific area offers more active sites,42 which gives rise to a high activity. However, the MnCe/TP displayed a higher catalyst activity (Figure 1), while it had a lower surface area (93.46 m2 g−1). Thereby, the BET specific area is a crucial factor, but not the only factor for catalytic activity. So, it is necessary to carry out other characterization methods for further study of the removal mechanisms of NO and Hg0. To discuss the effect of Mn and Ce on ReNO and ReHg, the reduction/oxidation (redox) properties of catalysts were obtained by H2-TPR, and the Gaussian fitting results are shown in Figure S5 (Part 2 in the SI). Ti-PILC was difficult to reduce by H2 at temperatures less than 900 °C, and only a small peak was observed at 610 °C. After CeOx was supported on TP, the reduction peak of the catalyst was obviously broadened. This suggested that the reducible cerium species existed in mixed valence, such as CeO2 and Ce2O3, which was consistent with the results of XRD (Figure S6 (Part 2 in the SI)) and XPS (Figure 4). Two peaks shown at 510 and 730 °C, respectively, over Ce/TP which were attributed to the reduction of CeO2.43 The TPR profile of Mn/TP exhibited three reduction peaks. The reduction peak at low temperature was attributed to the reduction of MnO2 to Mn2O3, and the reduction peak around 450 °C was regarded as the reduction of Mn2O3 to Mn3O4.44−46 The reduction peak around 480−600 °C should be assigned to the reduction of Mn3O4 to MnO.44−47 For MnCe/TP, there were two reduction peaks in the range of 100−900 °C, the low temperature peak was attributed to the reduction of MnO2 to Mn3O4, whereas the higher temperature peak represented the reduction of Mn2O3 to MnO and the reducible of CeO2 to Ce2O3.44,45,48 This showed that the addition of Ce in the catalysts induced the reduction of MnO2 to MnO from 3 steps to 2 steps, which indicated the interaction of Ce with Mn in the catalysts. SI Figure S6 showed the XRD patterns of TP support and other three catalysts. The peaks of anatase titanium dioxide were at 25.6° and 54.7° in TP support, and the peaks of other crystallite phases TiO2 were observed at 27.54°, 36.13°, and 62.5° in the patterns of all catalysts, which corresponded to the rutile titanium dioxide. This indicated that the obtained catalysts were anatase and rutile mixed phase under 500 °C calcination temperature. As observed, the XRD pattern of Mn/ TP contained four different peaks corresponding to the diffraction angles at 28.6°, 37.2°, 42.63°, and 56.6°, which indicated the presence of crystal manganeseoxide (MnO2). In addition, the characteristic peaks of Ce/TP catalyst at 28.7°, 33.1°, 47.6°, and 56.5° were found and corresponded to the cubic fluorite structure of CeO2.20 However, no obvious manganese and cerium structural peaks were detected in MnCe/TP catalyst during XRD analysis, which indicated that MnOx and CeO2 existed as an amorphous phase or highly dispersed on the TP support surface. It has been reported that the addition of Ce could promote the dispersion of Mn particles,49 which was consistent with our results. To reveal the removal mechanisms by MnCe/TP, the XPS spectra of O, Mn, Ce, and Hg over the fresh and used MnCe/ TP catalysts were investigated. The results shown in Figure 4 to identify the surface characteristics, the oxidation state and surface atomic concentration were summarized in Table S2 (Part 3 in the SI). For the fresh MnCe/TP catalyst, the O 1s XPS spectrum in Figure 4(a) was divided into two peaks at higher binding energy (BE) of 532.1 eV and the lower BE of 529.8 eV, which corresponded to the weakly adsorbed surface oxygen (Oα) and the lattice oxygen (Oβ), respectively.20 On the

contrary, it could be seen that there was a new peak for O 1s appeared at higher BE value of 534.4 eV for the used MnCe/ TP catalyst, which was ascribed to the surface oxygen by hydroxyl species and/or adsorbed water species presser as contaminants on the surface (Oc).33,50 He et al.51 attributed this new peak to H2O and announced that H2O was formed on the catalysts surface. As shown in Figure 4(a) and SI Table S2, the peaks of Oα and Oβ both decreased over the used catalyst, which demonstrated that both adsorbed surface oxygen and lattice oxygen participated in the oxidation reaction. In general, O2 took part in SCR reaction via occupying the oxygen vacancies of catalyst surface.52,51 Figure 4(b) displayed the Mn 2p XPS spectra, which consisted of double peaks (Mn 2p1/2 and 2p3/2). It was clear that there was one group of Mn 2p3/2 fitting peaks in fresh catalyst, which might be due to the presence of different oxidation states of Mn. The main peaks of the Mn 2p3/2 at lower BE value of 641.2 and 642.5 eV were ascribed to the Mn3+ species and the Mn4+ species, respectively.30 Kapteijn et al.53 determined that the NO conversion decreased for different MnOx catalysts in the following order: MnO2 > Mn5O8 > Mn2O3 > Mn3O4. A larger Mn4+/Mn3+ ratio indicated that there was more MnO2 in the MnCe/TP catalyst. MnO2 was considered as a major phase existing in the fresh catalysts due to the higher ratio of Mn4+/ Mn3+ on the surfaces. It was interesting to note that the atomic ratio of Mn4+/(Mn3+/Mn4+) in the fresh catalyst was 0.78, while the atomic ratio of Mn4+ in the used catalyst was 0 (SI Table S2). It indicated that the electron transfer occurred, accompanied by the formation of oxygen vacancies. This was consistent with XRD results. Besides, the peaks at the higher BE value of 644.4 eV was attributed to the undecomposed manganese nitrate.30 However, the used catalysts showed a different set of Mn 2p3/2 fitting peak. The lower BE value peak was shifted to 641.7 eV, which was attributed to the Mn3+ species,54,55 and the ratio of Mn3+ species increased from 0.54% to 1.92%. In addition, the ratio of manganese nitrate increased from 1.16% to 1.61%, and no Mn4+ peak was observed. Li et al.56 presented that the Mn4+/ Mn3+ ratio in Mn−Fe/AS(573) decreased from 3.24 to 1.10, suggesting that the Mn species mainly existed in the form of MnO2 and Mn2O3. Additionally, our previous study confirmed that the coexistence of MnO2 and Mn2O3 could promote the oxidation of NO to NO2, and improve the NO conversion in the low-temperature SCR of NO by NH3.20 Our previous study confirmed that the coexistence of MnO2 and Mn2O3 could promote the oxidation of NO to NO2. The reduction of Mn4+ to Mn3+ was because the Mn4+ adsorbed electrons in oxygen vacancies to form Mn3+ (i.e., Mn4+ + e−→ Mn3+), which probably contributed to the oxidation of NO and Hg0. The Ce 3d spectra of the catalysts were complicated, as shown in Figure 4(c), which consisted of eight components in each pattern. The u′ and v′ represented the two characteristic peaks of Ce3+, and the other six characteristic peaks attributed to Ce4+.21,57 The results illuminated that both Ce3+ and Ce4+ coexisted in MnCe/TP catalyst surface, and the Ce4+ was the predominant oxidation state. Ce4+ was considered to be beneficial for Hg0 oxidation as the main valence state.58 The change of ratio of Ce4+/Ce3+ before (Ce4+/Ce3+ = 4.68) and after (Ce4+/Ce3+ = 3) reaction indicated there was a redox reaction occurred between Ce4+ and Ce3+ (i.e., 2CeO2 → Ce2O3 + [O], Ce2O3 + 1/2O2 → 2CeO2). In addition, the reduction from CeO2 to Ce2O3 was observed in the catalyst surface by XPS. It was known that CeO2 has been studied F


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Environmental Science & Technology extensively due to its ability of storing and releasing oxygen via the redox shift between Ce4+ and Ce3+. Due to its redox properties, CeO2 could facilitate the oxidation of NO to generate NO2, a favorable process observed in the lowtemperature SCR of NO by NH3.59,60 Considering the results from our studies, the ratio of MnO2/ Mn2O3 and CeO2/Ce2O3 on the surface of the catalysts may play an important role in determining the activity. Many researchers have proposed that MnOx−CeO2 mixed oxides have much higher catalytic activity than that of pure MnOx and CeO2 due to the oxidation states of Ce and Mn and synergetic effects between them. Wu et al.61 reported that the increase of the Mnx+ oxidation state and the decrease of the Cex+ oxidation state occurred simultaneously, which implied an electron interaction between MnOx and CeO2. The results indicated that the formation of Ce3+ could impel the formation of oxygen vacancies and thereby increase the amount of the adsorbed oxygen species. Meanwhile, the oxidation state of Mn tended to increase, while the ionic valence of cerium decreased in the mixed oxides, accompanied by the formation of oxygen vacancies. Additionally, Imamura62 revealed that the form of Mn was Mn2O3, and CeO2 was the amorphous state in the Mn/Ce catalyst. He also found that Ce helped maintain the higher valence state of Mn when Mn/Ce was calcined below 500 °C, that is, Ce provided oxygen to Mn and thereby increased the catalytic oxidation ability. In our study, we have deduced the same mechanism based on the proof of the experimental results and the XPS characterization as discussed in the text. The lattice oxygen ([O]) came from the valence state changes of Ce and Mn, which can be described as 2CeO2 → Ce2O3+[O]; 2MnO2 → Mn2O3+[O]; The transfer of O2 from adsorbed oxygen (O*) to lattice oxygen in metal oxides is O2 → 2O*; Ce2O3+O* → CeO2; Mn2O3+ O* → MnO2. Figure 4(d) displays the Hg 4f XPS spectra. For the fresh catalyst, the peak of BE at 102.9 eV corresponded to characteristic peak of Si 2p. While in the used catalyst, there was a new peak appeared at about 104.5 eV, which was ascribed to HgO. This result showed that Hg0 oxidation reaction occurred on catalyst surface. In addition, no obvious adsorbed Hg0 was detected on the catalyst surface, which might be due to the strong volatility and the weak physical adsorption of elemental mercury at high temperature. 3.6. Removal Mechanism for the NO and Hg0 over MnCe/TP Catalyst. On the basis of the above studies and characterization results, there were two possible removal mechanisms coexisted in the catalytic reaction of NO and Hg0. 3.6.1. The Role of Different Forms of Oxygen. As shown in the XPS result of O 1s, both adsorbed surface oxygen and lattice oxygen participated in the oxidation reaction. The adsorbed surface oxygen (O*) might come from the gas phase of O2 eq 1), and the lattice oxygen ([O]) was from the valence state changes of Mn/Ce (eqs 2, 3, and 5, the additional oxygen generated by the reaction between Mn2O3 and CeO2 (eq 4). All of the oxygen was advantageous to the oxidation reaction. The reaction equations (eq ) were described as follows: O2 → 2O*

(1)

2MnO2 → Mn2O3 + [O]

(2)

Mn2O3 → 2MnO + [O]

(3)

Mn2O3 + 2CeO2 → 2MnO2 + Ce2O3

(4)

2CeO2 → Ce2O3 + [O]

(5)

Ce2O3 + 1/2O2 → 2CeO2

(6)

Hg(g) → Hg(ad)

(7)

Hg(ad) + O* → HgO

(8)

Hg(ad) + [O] → HgO

(9)

NH3(g) → NH3(ad)

(10)

4NH3(ad) + 4NO + O2 → 4N2 + 6H 2O

(11)

NO + O* → NO2

(12)

6NO2 + 8NH3 → 7N2 + 12H 2O

(13)

For Hg removal, the gaseous elemental mercury first formed Hg(ad) on the catalyst surface (eq 7), then reacted with the adsorbed surface oxygen and lattice oxygen to generate HgO (eqs 7, and 9. The oxygen storage and release of Ce eqs 5, 6 was considered to be effective for improving the SCR activity.20,21 The SCR reaction was mainly carried out according to eq 10. While other research63 pointed out that the NO in simulated flue gas would react with active oxygen to generate a small amount of NO2, which facilitated SCR activity. The reaction equation was shown in eq 13, which is named the “fast SCR reaction”. 3.6.2. The Role of Manganese Metal Oxide. Manganese metal oxide could achieve significant promotional effect on NO and Hg0 oxidation. The MnO bond of manganese oxide could react with NH3 and then formed the activities species of manganese surface amino (MnONH2) and manganese surface hydroxyl (MnOH), which promoted SCR reaction.16 On the basis of the results from our research, the promotion mechanisms of simultaneous removal NO and Hg0 might be explained as follows: 0

−NH 2 + NO → N2 + H 2O

(13)

−OH + 1/2Hg(ad) → 1/2HgO + 1/2H 2O

(14)

NO + O* → NO2

(12)

Hg(ad) + NO2 ↔ HgO + NO

(15)

Hg(ad) + 2NO2 + 2O* → Hg(NO3)2

(16)

4. CONCLUSIONS From the above studies, it is known that the MnCe/Ti-PILC catalyst exhibited optimal performance in simultaneous NO and Hg0 removal. This result indicated that the addition of Hg0 had no impact on ReNO due to the smaller Hg0 concentration. However, SCR gas components (NH3 and NO) showed an inhibition to the mercury removal, especially the ammonia. In addition, the potential simultaneous removal mechanisms were proposed involving the role of different forms of oxygen and the role of manganese metal oxide. In addition, the potential simultaneous removal mechanisms were proposed involving the role of different forms of oxygen and the role of manganese metal oxide. Both adsorbed surface oxygen and lattice oxygen participated in the oxidation reaction. Some intermediates such G


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as MnONH2 and MnOH on the surfaces of the catalysts were proven to be responsible for the high SCR activity and Hg0 capture capacity.

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ASSOCIATED CONTENT

* Supporting Information S

Catalyst preparation; (Figure S1) schematic diagram of the experimental setup; (Figure S2) N2 selectivity over MnCe/TP catalyst in SCR and SCR+Hg0 system; (Figure S3) Hg0 desorption experiment over MnCe/TP catalyst by NO; (Figure S4) Effects of O2 concentration on simultaneous removal efficiency; (Figure S5) H2-TPR profiles of the catalysts; (Figure S6) XRD patterns of TP, MnCe/TP, Mn/TP, and Ce/TP; (Table S1) BET specific areas of catalysts; and (Table S2) surface atomic concentrations of fresh and used MnCe/TP by XPS. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b01435.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: 0086-022-60435784; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This project was financially supported by the National Natural Science Foundation of China (No. 51176077), Key Project of Natural Science Foundation of Tianjin (No. 12JCZDJC29300), and the Marine Science and Technology project from Tianjin Marine Bureau (No. KJXH2013-05).

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