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Article

Highly Dispersed Mn–Ce Binary Metal Oxides Supported on Carbon Nanofibers for Hg0 Removal from Coal-Fired Flue Gas

1
State Grid Jiangxi Electric Power Science Research Institute, Nanchang 330096, China
2
State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science & Technology, Wuhan 430074, China
3
Chongqing Sanfeng Environment Group Co., Ltd., Chongqing 400084, China
4
Nanchang Institute of Technology, Nanchang 330099, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2018, 8(12), 2501; https://doi.org/10.3390/app8122501
Submission received: 30 October 2018 / Revised: 21 November 2018 / Accepted: 4 December 2018 / Published: 5 December 2018
(This article belongs to the Special Issue The Adsorption of Emerging Contaminants in Aqueous Environment)

Abstract

:
Highly dispersed Mn–Ce binary metal oxides supported on carbon nanofibers (MnOx–CeO2/CNFs(OX)) were prepared for Hg0 removal from coal-fired flue gas. The loading value of the well-dispersed MnOx–CeO2 was much higher than those of many other reported supports, indicating that more active sites were loaded on the carbon nanofibers. In the present study, 30 wt % metal oxides (15 wt % MnOx and 15 wt % CeO2) were successfully deposited on the carbon nanofibers, and the sorbent yielded the highest Hg0 removal efficiency (>90%) within 120–220 °C under a N2/O2 atmosphere. An increase in the amount of highly dispersed metal oxides provided abundant active species for efficient Hg0 removal, such as active oxygen species and Mn4+ cations. Meanwhile, the carbon nanofiber framework provides the pathway for charge transfer during the heterogeneous Hg0 capture reaction processes. Under a N2+6%O2 atmosphere, a majority of Hg0 was removed via chemisorption reactions. The effects of flue gas composition were also investigated. O2 replenished the active oxygen species on the surface and thus greatly promoted the Hg0 removal efficiency. SO2 had an inhibitory effect on Hg0 removal, but NO facilitated Hg0 capture performance. Overall, carbon nanofibers seems to be a good candidate for the support and MnOx–CeO2/CNFs(OX) may be promising for Hg0 removal from coal-fired flue gas.

Graphical Abstract

1. Introduction

Mercury (Hg) pollution has attracted worldwide concern because Hg and its compounds cause serious damage to human health and the environment [1]. Coal-fired power plants account for 24% of global anthropogenic mercury emissions, but mercury emitted from power plants has not been controlled efficiently until recent years [2,3]. In coal-fired flue gas, Hg always exists in three forms: oxidized (Hg2+), particulate-bounded (HgP), and elemental (Hg0) mercury [4,5]. Among them, HgP could be captured along with the fly ash by dust removal devices (electrostatic precipitators or fabric filters), and water-soluble Hg2+ could be removed by wet flue gas desulfurization devices [6,7]. However, Hg0 in the flue gas is difficult to control; thus, Hg0 is the dominant species emitted to the atmosphere. To remove Hg0, adsorption and catalytic oxidation technologies have been proposed. Many materials, such as activated carbon [8,9], have been used for Hg0 adsorption, but the poor selectivity and high cost have limited large-scale application. In recent years, much attention has been paid to sorbents based on transition metal oxides. The transition metal oxides exhibit excellent performances for Hg0 capture due to their extraordinary properties of multiple valence states and high chemical stability. VOx [10,11], CoOx [12], CuOx [13], and FeOx [14], etc., have been investigated for Hg0 removal in simulated coal-fired flue gas. However, the Hg0 removal efficiencies of these transition metal oxides greatly rely on the concentration of HCl in the flue gas [5,14]. When there is little HCl in the flue gas, the Hg0 removal activity of the above sorbents will be limited [15]. Therefore, it is essential to develop transition-metal-oxide-based sorbents for efficient Hg0 removal in flue gas with a low HCl concentration or in the absence of HCl.
MnOx-supported sorbents may be a promising candidate for Hg0 removal even in the absence of HCl [16]. In our previous study, we found that chemically adsorbed oxygen species were mainly responsible for the low-temperature activity of MnOx in the absence of HCl [17]. O2 in the flue gas is an important component to promote the Hg0 removal activity because it can replenish the active oxygen species of the sorbents which are consumed in the Hg0 adsorption reactions [18]. In addition, the O2 concentration in flue gas is much higher than that of HCl, so making full use of O2 to remove Hg0 is of significance. Hence, CeO2 has been widely used to modify the metal oxides to enhance their Hg0 removal activity due to its large oxygen storage capacity [18]. Ce3+ cations could yield charge imbalance, vacancies, and unsaturated chemical bonds, which lead to an increase of the chemisorbed oxygen concentration on its surface [19,20]. In the Ce4+/Ce3+ redox couple, CeO2 loses the active oxygen to oxidize the cations with low valence and/or Hg0, which then turn into Ce2O3. Meanwhile, Ce2O3 adsorbs O2 and turns into CeO2, which achieves the use of O2 in flue gas [19].
Up to today, MnOx–CeO2 binary metal oxides have been used for Hg0 removal in a few investigations. As far as is known, MnOx–CeO2 mixed oxides supported on TiO2 [19,21], Ti-PILCs [16], activated coke [22], and γ-Al2O3 [23] for Hg0 removal have been reported. These sorbents exhibited good Hg0 removal performance. However, the results also showed that with increasing active metal oxide loading, the metal oxides tend to agglomerate, leading to an insufficient use of active metal oxides. In addition, the conductivity of MnOx particles on these supports is poor, thus limiting their efficiency. Therefore, a novel highly dispersed MnOx–CeO2-based sorbent still needs to be investigated.
The use of carbon-based materials as sorbent supports could enhance the conductivity of the metal oxides. Carbon nanofibers are graphene-like nanomaterials with large specific surface area and surface nano-effect, so the metal oxides are easily loaded and dispersed. The conductivity of carbon nanofibers is very good because of their high graphitization degree [24]. During the Hg0 oxidation process, Hg0 loses two electrons, and a lot of electrons shift between the Mn4+/Mn3+ and Ce4+/Ce3+ electronic couples [25]. The metal oxides are highly dispersed on the nanofibers and the Hg0 will be captured by the well-dispersed active species. In this process, carbon nanofibers will act well as charge transfer channels. Besides this, oxidation treatment of the carbon nanofibers would increase the amount of oxygen-containing functional groups, which contribute to the dispersion and anchoring of active metals. Therefore, oxidized carbon nanofibers may be an ideal support to load more highly dispersed MnOx–CeO2 composites.
In this work, carbon nanofibers were firstly pretreated using HNO3. Then, the MnOx and CeO2 were loaded on the treated carbon nanofibers by a wet chemistry method. The influences of loading values, reaction temperatures, and individual flue gas components were investigated. The Hg0 removal mechanism of the novel sorbent was also studied.

2. Material and Methods

2.1. Sorbent Preparation and Characterization

The material preparation and characterization are described in the Supporting Information (Sections 1 and 2). The synthesized sorbents were named αMnOx–αCeO2/CNFs(OX), where α represents the mass percentage of MnO2 and CeO2 in the sorbents. The mass ratio of Mn/Ce was set to 1:1. Five sorbents with different loading values (α = 6%, 9%, 12%, 15%, and 18%) were prepared.

2.2. Experimental

The Hg0 removal activity was tested on a lab-scale fixed-bed reactor [17]. A mercury permeation tube was used to generate constant Hg0. The sorbent was packed in a 10 mm inner diameter quartz tube reactor. The Hg0 concentration was monitored by an RA-915M mercury analyzer (LUMEX, St. Petersburg, Russia). In each test, the total gas flow rate was 1000 mL/min, and the mass of the sorbent was 0.2 g. The Hg0 concentration in the simulative flue gas was ~95 μg·m−3. The detailed experimental conditions are shown in Table S1.
In this study, the Q/M (Q represents flow rate of the simulative flue gas; M represents sorbent mass) value was 300,000 mL·h−1·g−1. The Hg0 removal efficiency in this study was defined using Equation (1):
η = ( Hg i n 0 Hg o u t 0 ) / Hg i n 0 × 100 %
in which Hg i n 0 and Hg o u t 0 represent the Hg0 concentrations upstream and downstream of the sorbent bed, respectively, in μg·m−3.

3. Results and Discussion

3.1. Sorbent Characterization

The surface physical characteristics of the samples are shown in Table 1. The BET specific surface area of CNFs(OX) was higher than that of the raw commercial nanofibers (89.512 m2/g). This suggested that the ultrasonic oxidation treatment of the commercial nanofibers resulted in an increase of the specific surface area. This may be owing to the removal of ash on the commercial nanofibers’ surface during the ultrasonic oxidation treatment process. Meanwhile, the treatment process may also enrich surface defects. As shown in Table 1, the BET surface area and average pore volume decreased continuously as the loading value increased from 6% to 18%. This phenomenon indicates that metals entered the pore channels of CNFs(OX).
The Hg0 removal process over the transition metal oxides consists of typical heterogeneous redox reactions, so the details of the surface properties of the sorbents are very significant. The surface atomic concentrations of the different samples are summarized in Table 2. The surface oxygen concentration of CNFs was 2.59%. As shown in Table 2, the surface oxygen concentration increased to 3.24% after ultrasonic oxidation treatment. Six different oxygen species were fitted to O 1s XPS spectra, as shown in Figure 1a–c. As seen in Figure 1a,b, the proportion of oxygen-containing functional groups C=O or –OH in esters and anhydrides (532.3–532.8 eV) [26] increased from 34.01% to 44.15% after the ultrasonic oxidation treatment of the CNFs. Meanwhile, the proportion of –COOH (534.3–535.4 eV) [26] increased from 4.46% to 4.80%. The increase in oxygen-containing functional groups, especially–COOH, would facilitate the dispersion and anchoring of active metals. Besides this, as seen from Table 2, the surface oxygen atom concentration increased constantly with increasing MnOx and CeO2 loading values, which was assigned to oxygen species in the metal oxides.
The surface Mn and Ce atomic concentrations also increased constantly with increasing Mn and Ce loading values. As shown in Figure 1d, two orbits of the Mn 2p XPS spectra belong to Mn 2p1/2 (~653.8 eV) and Mn 2p3/2 (~642.2 eV). Mn 2p3/2 can be fitted to three subpeaks: Mn3+ (~641.8 eV), Mn4+ (~643.5 eV) [16], and satellites (~646.1 eV) [27]. Mn4+ species are recognized as the most important active component for Hg0 oxidation. Although the ratio of Mn4+/Mn3+ decreased slightly with increasing loading value, the absolute amount of Mn4+ ions still increased with increasing loading value except for 18%MnOx–18%CeO2/CNFs(OX). Figure 1e shows the Ce 3d XPS spectra of fresh 15%MnOx–CeO2/CNFs(OX). The Ce 3d XPS spectra could be fitted into eight subpeaks [28]. Among them, v/u, v2/u2, and v3/u3 belong to Ce4+, while v1/u1 belongs to Ce3+. Ce3+ species are considered to be important for the catalytic oxidation and adsorption of Hg0 [29], because Ce3+ could lead to a charge imbalance, which facilitates Hg0 oxidation process as more active oxygen species yield. As shown in Table 3, the absolute amount of Ce3+ cations also increased with increasing CeO2 loading value. From the XPS characterization results, with the increase of the highly dispersed Mn and Ce loading, MnOx–CeO2/CNFs(OX) sorbents offered abundant active oxygen, Mn4+, and Ce3+ species. They play important roles in promoting the Hg0 removal activity. In addition, the electron pairs of Mn4+/Mn3+ and Ce3+/Ce4+ would also increase with increasing Mn and Ce loading value. A cloud of electrons would shift between the redox cycles of Mn4+ Mn3+ and Ce3+ Ce4+ through the CNFs carbon-based framework, which benefits the Hg0 oxidation process.
The XRD patterns of the different sorbents are shown in Figure 2. The raw CNFs contained two kinds of carbon structures [30]: graphite 2H (2θ = 25.91°) and common carbon (2θ = 42.49°), of which the graphite 2H was formed by the convolution of two layers of graphene. Graphene has a high degree of graphitization, which facilitates electron transferal through the CNF framework. In addition, Ni crystallization peaks were observed in raw CNFs due to residual Ni catalyst during CNF production. However, the Hg0 removal efficiency of raw CNFs was only 16.02% (seen from Figure 3), which indicated that the residual Ni has a very limited impact on the Hg0 removal efficiency. Hence, the impacts of residual Ni on the Hg0 removal performance can be ignored in this study. It is worth noting that the XRD peak intensity changed significantly after ultrasonic oxidation treatment. The peak intensity of graphite 2H of the CNFs(OX) was much higher than that of raw CNFs, which was owing to the removal of ash content on the raw CNFs’ surface during the ultrasonic purification treatment process and the exposure of the graphene phase. For the metal-oxide-loaded samples, the crystal phase ascribed to MnOx or CeOx was not observed when Mn and Ce loading increased from 6% to 15%, suggesting that MnOx and CeO2 species were well dispersed on the CNFs(OX) surface in amorphous states [22]. A slight crystal phase ascribed to MnO2 was observed from 18%MnOx–18%CeO2/CNFs(OX), indicating the aggregation of MnOx. The results also demonstrated that the highly dispersed threshold value of the metal oxides on the CNFs(OX) support was about 30%, which is higher than those on other supports. The specific surface area, oxygen-containing functional groups, and the intensity of graphite 2H peak were all enhanced after the ultrasonic oxidation treatment of CNFs, which also facilitated the dispersion and anchoring of active metals.
The micromorphology of the CNFs and the dispersion of MnOx–CeO2 on the surface of the support were evaluated, as shown in Figure 3. As shown in Figure 3a,b, the SEM image of the raw CNFs and 15%MnOx–15%CeO2/CNFs(OX) showed a homogeneous morphology of cross-linked nanofibers of about 100 nm in diameter. It is clearly observed that the MnOx and CeO2 have been homogeneously dispersed on the support, as shown in Figure 3c.

3.2. Influence of Loading Value and Reaction Temperature on Hg0 Removal Performance

Figure 4 shows the experimental results of Set I testing. The Hg0 removal efficiency of the raw CNFs was only 16.02%. Therefore, the effect of residual Ni in the raw CNFs on their Hg0 removal efficiency could be ignored. The Hg0 removal efficiency of CNFs(OX) slightly increased to 18.73%. The increase in the Hg0 removal efficiency of CNFs(OX) was probably owing to the increase of the specific surface area and the content of surface oxygen-containing functional groups, which enhanced the physicochemical adsorption of Hg0. Meanwhile, the Hg0 removal efficiency increased consistently as the loading value increased from 6% to 15%; however, the efficiency of 18%MnOx–18%CeO2/CNFs(OX) slightly decreased. This suggested that the optimal loading value for the MnOx–CeO2/CNFs(OX) sorbent was 15%. This is consistent with the XRD results. The decrease in Hg0 removal efficiency for 18%MnOx–18%CeO2/CNFs(OX) was ascribed to the agglomeration of excessive MnO2, which resulted in insufficient use of Mn active components.
The Hg0 removal efficiencies of individual Mn or Ce supported on CNFs(OX) were also tested. As shown in Figure 4, the Hg0 removal efficiency of 30%Mn/CNFs(OX) was 80.64%, while it was 64.69% for 30%Ce/CNFs(OX). Both of these results were lower than that of 15%MnOx–15%CeO2/CNFs(OX). The results further demonstrated that Mn and Ce have synergistic effects, consistent with a previous study [16,19,22]. Meanwhile, the Hg0 removal performance of the oxidized-activated-carbon-supported Mn and Ce sorbent (15%MnOx–15%CeO2/AC(OX)) was also investigated. It was apparent that CNFs(OX) was a better support of MnOx–CeO2 for Hg0 removal. The mercury capacities of the sorbents (α = 12%, 15% and 18%) are listed in Table 3. In the present study, the Hg0 capacity was defined as the amount of Hg0 adsorbed per mass of sorbent when Hg0 breakthrough reached only 1%.
Therefore, subsequent studies were carried out with 15%MnOx–15%CeO2/CNFs(OX). As shown in Figure 5, the influence of reaction temperature on the Hg0 removal performance was investigated. The observation indicated that the optimal reaction temperature for 15%MnOx–15%CeO2/CNFs(OX) sorbent was 120 °C. It should be noted that over 90% of the Hg0 was removed by 15%MnOx–15%CeO2/CNFs(OX) sorbent in the relatively broad temperature window of 120–220 °C.
In order to evaluate the performance of our sorbents, it was compared with other Mn-based or Ce-based sorbents in some of the published literature [16,19,22,23]. All the results were selected under similar experimental conditions and tested under a N2 + O2 atmosphere. The ratios of total gas flow rate to sorbent mass in different studies were converted into Q/M. As seen from a general view in Table S2, under similar N2 + O2 atmosphere and Q/M experimental conditions, the sorbent in the present study exhibited relatively superior activity.

3.3. Hg0 Removal Mechanism Analysis

Figure 6 shows the analysis of the Hg0 removal process under a N2 + 6%O2 atmosphere by the 15%MnOx–15%CeO2/CNFs(OX) sorbent. The Hg0 in flue gas was almost totally removed by the sorbent during the 9 h reaction. Inset picture (a) displays the Hg-TPD profile of the spent sorbent. Two main desorption peaks were observed in the temperature range of 200–300 °C, indicating that Hg0 was mainly captured on the sorbent surface via chemical adsorption [12,31]. By integrating the Hg-TPD profile, ~81% of total Hg0 was adsorbed on the sorbent, while the rest of the Hg0 was removed by catalytic oxidation and released to the flue gas, as shown in inset picture (b).
Figure 7 shows the XPS analysis of O 1s, Mn 2p, and Ce 3d with spent 15%MnOx–15%CeO2/CNFs(OX). For spent 15%MnOx–15%CeO2/CNFs(OX), the ratio of Mn4+/Mn3+ was 0.3633, and the ratio of Ce3+/Ce4+ was 0.2371. Both of these values are lower than those of the fresh sorbent, so it can be inferred that Mn4+ and Ce3+ participated in the catalytic adsorption and oxidation of Hg0.
In summary, for 15%MnOx–15%CeO2/CNFs(OX), two approaches of adsorption and catalytic oxidation existed for Hg0 removal, of which the chemical adsorption approach occupied the main proportion. The active components such as active oxygen species, Mn4+ species, and Ce3+ species greatly benefited the chemical adsorption and oxidation of Hg0. The relatively high loading capacity of the highly dispersed Mn and Ce provided more active components for Hg0 removal.

3.4. Influence of Individual Flue Gas Components on Hg0 Removal Performance

Figure 8 shows the influences of CO2, O2, SO2, NO, and H2O on the Hg0 removal performance of 15%MnOx–15%CeO2/CNFs(OX) at 120 °C. The Hg0 removal efficiency was 53.48% under a pure N2 atmosphere, while it was 46.08% under a N2 + 12%CO2 atmosphere. This indicated that CO2 had a slight inhibitory effect on Hg0 removal performance, which may be caused by competitive adsorption of Hg0 and CO2 on the sorbent. The Hg0 removal efficiency increased to 81.94% when the O2 concentration was 3% (balanced N2), while it was 99.6% when the O2 concentration increased to 6%. The observation indicated that O2 played an important role in the Hg0 removal process. In this study, a large amount of Ce was well dispersed on the support, which greatly facilitated the transport of O2 in the flue gas to the sorbent surface. Moreover, the active oxygen content in CeO2 favored the transformation of Mn3+ to Mn4+ [22], and the increase of the Mn4+/Mn3+ ratio was beneficial to the removal of Hg0 [32].
When SO2 was added into the N2 flow, the Hg0 removal efficiency decreased significantly. However, the negative influence of SO2 on Hg0 removal performance was relieved in the presence of O2. As shown in Figure 7, the Hg0 removal efficiency was 60.52% under the atmosphere of N2 + 6%O2 + 500 ppm SO2. When the SO2 concentration further increased to 1000 ppm and 1500 ppm, the Hg0 removal efficiency decreased to 50.95% and 46.73%, respectively. The influence of SO2 observed in the present study was consistent with that observed in previous studies [28]. The consumption of active oxygen and the reaction of Mn with SO2 on the sorbent surface may account for the inhibition effects. The Hg0 removal efficiency sharply reached 90.37% when 100 ppm NO was added into the N2 atmosphere. Besides this, when the NO concentration was further increased to 200 ppm and 300 ppm, the Hg0 removal efficiency increased to 97.33% and 98.17%, respectively. Furthermore, NO could promote the Hg0 removal efficiency in the absence and presence of O2. We believe that active oxygen on the sorbent surface promoted the conversion of NO to active nitrites, which were beneficial for Hg0 oxidation [33]. Water vapor (H2O) exhibited only a slight inhibition effect on the Hg0 removal performance, suggesting that the sorbent could be used in low-temperature coal-fired flue gas, which always contains H2O.

3.5. Possible Application Mode of the Sorbent

To reduce anthropogenic mercury emissions into the environment, more than 128 nations have approved the Minamata Convention on Mercury until 4 November 2018. As one of the most important anthropogenic sources, coal-fired power plants should make efforts to reduce their Hg0 emissions. The existing air pollution control devices cannot efficiently remove the Hg0, so some new devices are needed to control the Hg0 emissions. A possible application mode of the sorbent for Hg0 control from coal-fired flue gas is shown in Figure 9. One option is to inject the mercury sorbent into the flue gas downstream of the electrostatic precipitator (ESP) and upstream of a small fabric filter (FF), as shown in Figure 9a. In this way, a injector and an FF should be introduced to the flue gas clean system. The sorbent will capture Hg0 in the flue gas and will be then collected by the FF so as to achieve separation from the fly ash. In this way, the spent sorbent will probably be regenerated. In addition, the FF can efficiently remove fine particulate matter (PM2.5) simultaneously. The other option is to make use of a fixed adsorption bed, as shown in Figure 9b. The sorbent could be shaped into a honeycomb or plate, like the selective catalytic reduction (SCR) catalysts.

4. Conclusions

Hg0 removal performance experiments were carried out on MnOx–CeO2-modified CNF sorbents in this study. The results showed that the Hg0 removal efficiency increased continuously as the loading value increased from 6% to 15%. Moreover, the sorbents exhibited superior performance in a wide temperature range (120–220 °C). Compared with those of some other supports, the well-dispersed Mn–Ce mixed oxide loading amount on CNFs was improved. The Hg0 removal performance of this sorbent was clearly enhanced by O2 and NO in the flue gas. SO2 inhibited the removal activity, and resistance to SO2 should be investigated in future work. Overall, this study showed that CNFs could serve as a good sorbent support for MnOx–CeO2 mixed oxides.

Supplementary Materials

The followings are available online at https://www.mdpi.com/2076-3417/8/12/2501/s1. The sorbent preparation and characterization are described in the Supplementary Materials. Two tables are also presented in the file. Table S1. Experimental conditions. Table S2. A comparison of the Hg0 removal efficiency in this study and those of other Mn–Ce-based composites reported in the literature.

Author Contributions

Y.X., Z.L. and Y.Z. performed the experiments; Z.Z. wrote the paper.

Funding

This research was funded by the National Key R&D Program of China [2018YFB0605200], the China Postdoctoral Science Foundation funded project [2018T110762] and Jiangxi Key R&D Program [20171ACG70012]. And The APC was funded by [20171ACG70012].

Acknowledgments

We appreciate the support from the Analytical and Testing Center at Huazhong University of Science & Technology.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. O 1s XPS spectra of (a) carbon nanofibers (CNFs) and (b) CNFs(OX). XPS spectra of (c) O 1s, (d) Mn 2p, and (e) Ce 3d for 15%MnOx–15%CeO2/CNFs(OX).
Figure 1. O 1s XPS spectra of (a) carbon nanofibers (CNFs) and (b) CNFs(OX). XPS spectra of (c) O 1s, (d) Mn 2p, and (e) Ce 3d for 15%MnOx–15%CeO2/CNFs(OX).
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Figure 2. XRD patterns of the samples. (a) CNFs; (b) CNFs(OX); (c) 6%MnOx–6%CeO2/CNFs(OX); (d) 9%MnOx–9%CeO2/CNFs(OX); (e) 12%MnOx–12%CeO2/CNFs(OX); (f) 15%MnOx–15%CeO2/CNFs(OX); (g) 18%MnOx–18%CeO2/CNFs(OX).
Figure 2. XRD patterns of the samples. (a) CNFs; (b) CNFs(OX); (c) 6%MnOx–6%CeO2/CNFs(OX); (d) 9%MnOx–9%CeO2/CNFs(OX); (e) 12%MnOx–12%CeO2/CNFs(OX); (f) 15%MnOx–15%CeO2/CNFs(OX); (g) 18%MnOx–18%CeO2/CNFs(OX).
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Figure 3. Micromorphology of the (a) CNFs and (b) 15%MnOx–15%CeO2/CNFs(OX), and the EDS mapping of the elements of 15%MnOx–15%CeO2/CNFs(OX).
Figure 3. Micromorphology of the (a) CNFs and (b) 15%MnOx–15%CeO2/CNFs(OX), and the EDS mapping of the elements of 15%MnOx–15%CeO2/CNFs(OX).
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Figure 4. Hg0 removal efficiency over different samples under a N2 + 6%O2 atmosphere at 170 °C. Reaction time for each test was 9 h.
Figure 4. Hg0 removal efficiency over different samples under a N2 + 6%O2 atmosphere at 170 °C. Reaction time for each test was 9 h.
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Figure 5. Hg0 removal efficiency of 15%MnOx–15%CeO2/CNFs(OX) under a N2 + 6%O2 atmosphere at different temperatures. Reaction time for each test was 9 h.
Figure 5. Hg0 removal efficiency of 15%MnOx–15%CeO2/CNFs(OX) under a N2 + 6%O2 atmosphere at different temperatures. Reaction time for each test was 9 h.
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Figure 6. Hg0 concentration versus reaction time for 15%MnOx–15%CeO2/CNFs(OX). Inset pictures: (a) Hg-TPD analysis of spent 15%MnOx–15%CeO2/CNFs(OX); (b) Balance analysis of 15%MnOx–15%CeO2/CNFs(OX).
Figure 6. Hg0 concentration versus reaction time for 15%MnOx–15%CeO2/CNFs(OX). Inset pictures: (a) Hg-TPD analysis of spent 15%MnOx–15%CeO2/CNFs(OX); (b) Balance analysis of 15%MnOx–15%CeO2/CNFs(OX).
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Figure 7. The (a) O 1s, (b) Mn 2p, (c) Ce 3d, and (d) Hg 4f XPS spectra of the spent 15%MnOx–15%CeO2/CNFs(OX).
Figure 7. The (a) O 1s, (b) Mn 2p, (c) Ce 3d, and (d) Hg 4f XPS spectra of the spent 15%MnOx–15%CeO2/CNFs(OX).
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Figure 8. Hg0 removal efficiency of 15%MnOx–15%CeO2/CNFs(OX) under different individual flue gas components at 120 °C.
Figure 8. Hg0 removal efficiency of 15%MnOx–15%CeO2/CNFs(OX) under different individual flue gas components at 120 °C.
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Figure 9. Possible application mode of the sorbent: (a) powder adsorbent injection; (b) fixed adsorbent bed.
Figure 9. Possible application mode of the sorbent: (a) powder adsorbent injection; (b) fixed adsorbent bed.
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Table 1. The surface physical characteristics of different adsorbents.
Table 1. The surface physical characteristics of different adsorbents.
αSpecific Surface Area (m2/g)Pore Volume (cm3/g)
096.5320.461
6%97.0200.350
9%93.8530.299
12%88.9480.307
15%87.1580.293
18%86.7290.286
Table 2. Surface atomic concentration of the adsorbents based on XPS results.
Table 2. Surface atomic concentration of the adsorbents based on XPS results.
αSurface Atomic Concentration (%)
C 1sO 1sMn 2pMn4+/Mn3+Ce 3dCe3+/Ce4+
096.763.24
6%96.312.980.480.42770.230.5523
9%91.286.281.610.42120.830.3174
12%90.246.941.880.41710.940.2900
15%85.989.742.850.41401.430.2859
18%79.9513.123.640.32181.850.2734
Table 3. Mercury capacity of the sorbents (Q/M = 300,000 mL·h−1·g−1).
Table 3. Mercury capacity of the sorbents (Q/M = 300,000 mL·h−1·g−1).
αCapacity (μg·g−1)
12%74.8 ± 1.7
15%118.4 ± 2.3
18%54.1 ± 1.2

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MDPI and ACS Style

Xia, Y.; Liao, Z.; Zheng, Y.; Zhou, Z. Highly Dispersed Mn–Ce Binary Metal Oxides Supported on Carbon Nanofibers for Hg0 Removal from Coal-Fired Flue Gas. Appl. Sci. 2018, 8, 2501. https://doi.org/10.3390/app8122501

AMA Style

Xia Y, Liao Z, Zheng Y, Zhou Z. Highly Dispersed Mn–Ce Binary Metal Oxides Supported on Carbon Nanofibers for Hg0 Removal from Coal-Fired Flue Gas. Applied Sciences. 2018; 8(12):2501. https://doi.org/10.3390/app8122501

Chicago/Turabian Style

Xia, Yongjun, Zhiqiang Liao, Yan Zheng, and Zijian Zhou. 2018. "Highly Dispersed Mn–Ce Binary Metal Oxides Supported on Carbon Nanofibers for Hg0 Removal from Coal-Fired Flue Gas" Applied Sciences 8, no. 12: 2501. https://doi.org/10.3390/app8122501

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