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Article

Co3O4/g-C3N4 Hybrids for Gas-Phase Hg0 Removal at Low Temperature

by
Zhen Zhang
1,
Jiang Wu
1,* and
Dongjing Liu
2,*
1
College of Energy and Mechanical Engineering, Shanghai University of Electric Power, Shanghai 200090, China
2
School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China
*
Authors to whom correspondence should be addressed.
Processes 2019, 7(5), 279; https://doi.org/10.3390/pr7050279
Submission received: 1 April 2019 / Revised: 3 May 2019 / Accepted: 6 May 2019 / Published: 13 May 2019
(This article belongs to the Section Chemical Processes and Systems)
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Abstract

:
The Co3O4/g-C3N4 hybrids are constructed via the incipient wetness impregnation method by depositing Co3O4 onto the exterior of g-C3N4, and then employed for Hg0 capture within 60–240 °C. The results show that the Co3O4/g-C3N4 hybrid with a Co3O4 content of 12 wt% performs optimally with the highest Hg0 removal efficiency of ~100% at or above 120 °C. The high performances of the Co3O4/g-C3N4 hybrids are probably attributed to the tight interfacial contact between Co3O4 and g-C3N4, with its improved electron transfer, inferring that cobalt oxide and g-C3N4 display a cooperative effect towards Hg0 removal. NO and SO2 shows a significant suppressive influence on the mercury capture performance, plausibly owing to the competing adsorption and side reactions.

1. Introduction

Mercury, as a kind of heavy metal, is a persistent toxic substance of increasing concern worldwide [1,2]. The combustion of coal accounts for the great majority of the global anthropogenic mercury emissions [3]. Coal-fired mercury can be empirically classified into three main forms: elemental mercury (Hg0), oxidized mercury (Hg2+), and particulate mercury (Hg(p)). Hg2+ and Hg(p) can be removed using wet scrubbers and fabric filters, respectively [4]. However, elemental mercury (Hg0), the predominant speciation of coal-derived mercury, is hardly captured by the available pollutant control facilities because it is highly volatile, chemically stable, and water insoluble [5]. Catalytic oxidation and adsorption is considered to be the most effective and viable method for the reduction of Hg0 emission [6]. Activated carbon is frequently used for mercury emission control in power plants [7]. Nevertheless, the high operation cost, low mercury capacity, and negative effect on fly ash quality has restrained its widespread application in coal-fired power plants [8]. Currently, transition-metal oxides, such as CuO [9], MnO2 [10], Fe2O3 [11], and Co3O4 [12], have been recognized as potential sorbents or catalysts for Hg0 removal because of their excellent redox abilities and lower capital cost. CeO2, as a rare-earth oxide, has also been employed as an additive, active ingredient, and carrier in the synthesis of composites for mercury capture, attributed to the CeO2/Ce2O3 redox couple and structure defect [13].
Metal oxides loading onto the exterior of nano-materials is a type of catalyst with good adsorption as well as redox ability [14]. Thus, these catalysts could be potential candidates for capturing Hg0 from coal combustion flue gas. Graphite-like carbon nitride (g-C3N4), the most stable allotrope of versatile CN structures, has been widely studied in many scientific fields because of its distinct electronic structure, thermal stability, ample source as well as simple synthesis route [15]. Two-dimensional g-C3N4 nanosheet possesses the advantage of providing more anchoring sites for bare metal oxide loading [16]. Besides, the metal oxide/g-C3N4 composites and their intimate interfacial contact, with reduced potential energy barrier and improved charge transfer mobility, are expected to facilitate Hg0 oxidation [17,18]. The Nb2O5/g-C3N4 hybrids outperformed the individual Nb2O5 and g-C3N4 in photocatalytic reactions due to the sufficient interfacial interaction of Nb2O5 and g-C3N4 through a direct Z-scheme [19]. Co3O4 embedded into tubular nanostructures of the g-C3N4 lead to prominent performances for oxygen and hydrogen evolution reactions [20]. Mesoporous Co3O4 anchored onto g-C3N4 would greatly promote the composite conductivity and boost the charge transfer mobility, thereby enhancing the performance of capacitance [21].
Despite the immense applications of the metal oxide/g-C3N4 hybrids, Co3O4/g-C3N4 hybrids so far have not been explored for mercury capture from coal-derived flue gas. In this work, Co3O4/g-C3N4 hybrids are facilely constructed by tuning the content of Co3O4 deposited on g-C3N4 exterior via an incipient wetness impregnation approach and then applied for gas-phase Hg0 removal in an upflow packed-bed quartz type reactor in a temperature range of 60–240 °C. The Co3O4/g-C3N4 hybrids are examined by using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), nitrogen adsorption-desorption, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) techniques. The effects of the Co3O4 loading values, reaction temperature, and flue gas component on the mercury capture performances of the Co3O4/g-C3N4 hybrids are studied. The mechanism of Hg0 oxidation over Co3O4/g-C3N4 hybrids is discussed therewith.

2. Experimental Section

2.1. Preparation of Co3O4/g-C3N4 Hybrids

The g-C3N4 nanosheets (CNNS) were synthesized via a two-step calcination approach as reported in [22,23]. The Co3O4/g-C3N4 hybrids were synthesized using an incipient wetness method. Versatile amounts of Co(NO3)2▪6H2O were dissolved in ultrapure water and mixed with CNNS in a beaker which was then dried at 343 K overnight. Eventually, the formed solids were calcined at 473 K for 2 h [23]. The final specimens are denoted as xCo3O4/CNNS, with x representing the weight content (wt%) of Co3O4 in the hybrids.

2.2. Characterization of Co3O4/g-C3N4 Hybrids

The FESEM images were attained on a Phillips XL-30 FEG/NEW instrument (Eindhoven, The Netherlands). The TEM images were acquired on a Phillips Model CM200 device (Amsterdam, The Netherlands). The XRD patterns were measured on Bruker D8 Advance equipment (Karlsruhe, Germany). The nitrogen isotherms were examined on a Beishide 3H-2000PS4 apparatus (Beijing, China). The FTIR profiles were determined on a FTIR-8400S spectrometer (Shimadzu Corporation, Kyoto, Japan). The XPS analysis were performed on a RBD-upgraded PHI-5000C ESCA system (Perkin Elmer, Waltham, MA, USA) [24].

2.3. Mercury Oxidation

The mercury removal experimental system has been fully expressed in previous papers [23,25]. As depicted in Figure 1, 50 mg of specimens were put into the middle of the reactor and fixed with quartz wool at the tail of the sorbent bed. After heating to the desired temperatures, the Hg0-laden stream, originating from a PSA device (PS Analytical, Kent, UK), was then consecutively charged into the reactor for mercury oxidation tests. The influent and effluent mercury concentration was examined by an online mercury analyzer (Lumex, RA-915-M, St. Petersburg, Russia). Moreover, active carbon was used for the off-gas cleaning. It can adsorb the oxidized mercury as well as elemental mercury in the flue gas.

3. Results and discussion

3.1. Characterization Analysis

The FESEM image and photo of the CNNS are shown in Figure 2. CNNS displays a light-yellow color (the inset in Figure 2a) and it consists of many lamellar structures with lateral sizes of < 1 μm (Figure 2a) and thickness of dozens of nanometers (Figure 2b). The CNNS displays transparent characteristics, indicating the super-thin sheet-like morphology of the CNNS with ~1–10 nm in thickness and ~1–3 μm in size (Figure 3a). The TEM image and the selected area electron diffraction (SAED) analysis of the 12Co3O4/CNNS are displayed in Figure 3b. The lattice fringes of Co3O4 are closely surrounded by g-C3N4. The intimate interfacial contact implies the good interaction between Co3O4 and g-C3N4, which is beneficial for electron transfer and, thus, promoting Hg0 oxidation [26]. The diffraction spots with a distance of 0.205 and 0.429 nm are ascribed to the reflection of the Co3O4 (400) and Co3O4 (111) lattice plane, respectively. The XRD spectra of the pure and Co3O4-modified CNNS are displayed in Figure 4. The intense peaks at ~27.7° detected in all specimens belong to the reflection of the (002) crystal face of g-C3N4 (JCPDS no. 87-1526) [27]. The signals at 31.1°, 36.7°, 44.8°, 59.4°, and 65.1° are assigned to the diffraction of Co3O4 (JCPDS no. 42-1467) [28]. With the increment of the Co3O4 content, its feature peaks gradually increase in intensity, suggesting that Co3O4 has been successfully deposited on g-C3N4 surface.
The nitrogen isotherms of the pure and Co3O4-modified CNNS are presented in Figure 5. Hysteresis loops were observed in all specimens at higher relative pressure. The nitrogen uptakes at lower relative pressures were fairly less, implying that enormous mesoporous structures presented on the catalyst surface [29]. The pure CNNS had a large BET surface area of 109 m2/g and large average pore size of 19 nm (Table 1). After incorporating Co3O4, they all reduced significantly. The BET surface area and average pore size of xCo3O4/CNNS dropped to 27–42 m2/g and 10–12 nm, respectively. The probable reason for this is that parts of the mesoporous structures of the CNNS were filled or blocked by Co3O4 grains [30].
The FTIR profiles of fresh CNNS and xCo3O4/CNNS are displayed in Figure 6a. The two sharp signals at ~892 and ~807 cm−1 are in accord with the feature breathing pattern of the triazine loop units stemming from polymerized C–N heterocycles [31]. The set of peaks between ~1109 and ~1745 cm−1 belong to the aromatic C–N stretch oscillation [32]; the weak bands at ~2143 cm−1 correspond to the cyano C≡N terminal groups [33]. The peaks at ~2352 cm−1 belong to the O=C=O asymmetrical stretch oscillation of the carbon dioxide adhered to the catalyst exterior [34]. The broad bands between 2927 and 3688 cm−1 relate to the N–H stretch oscillation of unpolymerized –NH2 functions and the O–H stretch oscillation of the H2O molecules adhered to the catalyst exterior [35]. As depicted in Figure 6b, the main feature peaks of the g-C3N4 can be distinctly detected in the spent 12Co3O4/CNNS, indicating that the atomic structures of the 12Co3O4/CNNS retain the same during Hg0 removal reactions.
The surface elemental valences of 12Co3O4/CNNS before and after reaction were examined by XPS, as presented in Figure 7. The bands at ~284.6 and ~285.2 eV relate to the sp2-bonded carbon atoms, while the peaks at ~287.6 and ~288.2 eV are associated with the N–C=N structures [36]. The peaks at ~398.0 and ~398.4 eV belong to the hybridized secondary nitrogen (C–N=C). The bands at ~399.0 and ~399.2 eV are related to the hybridized tertiary nitrogen (N–(C)3). The peaks at ~400.0 and ~400.4 eV correspond to the –NH2 groups [37]. What is more, the C 1s and N 1s binding energies of the used 12Co3O4/CNNS are smaller than those of the fresh one. This suggests that Hg0 probably donates electrons to g-C3N4 during mercury removal reactions. With respect to the Co 2p spectrum, the bands at ~782.0 and ~782.8 eV are supposed to be the reflection of the Co3+ in the Co3O4, while the peaks at ~786.4 and ~787.4 eV relate to the signals of the Co2+ in the Co3O4 [38]. The ratio of Co3+/Co2+ for the fresh specimen is ~0.83. It slightly decreases to ~0.75 for the spent specimen. This indicates that a fraction of the Co3+ cations transferred into the Co2+ cations after Hg0 oxidation. The Co3+ cations are involved in the mercury oxidation reactions. The bands at ~531.8 eV belong to the chemisorbed oxygen (Oα) on the catalyst exterior, while the peaks at ~533.2 eV are linked to the oxygen (Oβ) in the –OH group or C–O bond [39]. The percentage of the chemisorbed oxygen increased from ~54.7 to ~61.3% for the 12Co3O4/CNNS before and after the reaction, correspondingly, which infers that the gaseous O2 from the feed gas may replenish the depleted chemisorbed oxygen during the Hg0 oxidation processes. The Hg 4f spectra displays two feature peaks at ~101.4 and ~103.6 eV, which implies the generation of HgO [40]. Moreover, the absence of the feature peak of elemental mercury at 99.9 eV indicates that Hg0 adsorption over 12Co3O4/CNNS is governed by chemisorptions [41].

3.2. Impact of Loading Value

The performances of the pure and Co3O4-modified CNNS with respect to elemental mercury removal at 120 °C in 5% O2/N2 over 90 min are presented in Figure 8. The pristine CNNS shows good Hg0 capture ability, plausibly because of the distinct C–N structure and considerable surface area. The equilibrium Hg0 removal efficiency is ~59.0%. Incorporating Co3O4 with g-C3N4 could remarkably enhance the Hg0 removal ability. The Hg0 sorption rate at the initial stage and the Hg0 removal efficiency both increase after the addition of Co3O4. The mercury conversion rises from ~59.0 to ~86.0% as the Co3O4 loading value elevates from 0 to 8 wt%. The 12Co3O4/CNNS performs the best with mercury conversion reaching ~100%, indicating that the Co3O4 modification contributes to Hg0 removal by the introduction of added reactive sites. Moreover, the tight interfacial interaction of Co3O4 and g-C3N4, with promoted charge transfer mobility, can facilitate redox reactions, which could enhance mercury conversion. Nevertheless, the redox ability would decline with further incremental Co3O4 content. The mercury conversion subtly drops to ~97.5% when loading value exceeds 12 wt%, presumably owing to the significant reduction of the surface area. Thus, the best Co3O4 content for xCo3O4/CNNS is 12 wt%. What is more, bare Co3O4 exhibits a poor performance towards mercury adsorption with a mercury conversion of only ~36.3%, probably owing to its larger grain size and lower surface area. Thus, it can be concluded that Co3O4 and g-C3N4 display a cooperative effect towards Hg0 removal.

3.3. Impact of Reaction Temperature

The mercury conversion of the 12Co3O4/CNNS at 60–240 °C in 5% O2/N2 is presented in Figure 9. It is found that temperature could significantly affect mercury removal performance. Fairly low mercury oxidation activity was exhibited at 60 °C by 12Co3O4/CNNS, with a mercury conversion of only ~33.3%. The performance of mercury oxidation can be remarkably enhanced when temperature goes up to 90 °C, with mercury conversion swiftly rising to ~98.4%. A mercury conversion of 100% can be reached at temperature above or at 120 °C. The lower reaction activity is attributed to the lower reaction rate at lower temperatures, while higher temperature facilitates chemisorption processes because of the decreased activation energy barrier. As displayed in Figure 10, CNNS shows an unstable performance, with mercury conversion gradually dropping from ~77.2 to ~47.3% as time passed likely attributed to the consumption of the active sorption sites. In contrast, 12Co3O4/CNNS performs stably, with mercury conversion rapidly reaching ~100% within ~42 min and then leveling off as time passes, plausibly owing to its prominent redox ability.

3.4. Impact of Flue Gas

Nitrogen monoxide and sulfur dioxide are the predominant acidic gas components in coal-derived flue gas. The influences of NO and SO2 on the mercury conversion of 12Co3O4/CNNS at 120 °C is shown in Figure 11. It is found that NO and SO2 both show detrimental impacts on Hg0 removal processes. The mercury conversion of 12Co3O4/CNNS was significantly reduced to ~44.6 and ~42.7% after adding 800 ppm NO and 1200 ppm SO2 into the feed gas, respectively. This suggests that NO or SO2 molecules and Hg0 vapor could be competitively adsorbed on the surface of 12Co3O4/CNNS [9]. In addition, the adsorbed SO2 may react with Co3O4 to produce thermally-stable metal sulfates, i.e., cobalt sulfate, leading to the decline of the active phases [42]. Thus, the competitive adsorption and side reactions contribute to the remarkable suppressive influences of NO and SO2 towards Hg0 adsorption.

3.5. Mercury Capture Mechanism

In summary, according to the XPS analysis, Co3O4 serves as the main reactive site for elemental mercury capture. The combination of Co3O4 with g-C3N4 nanosheet increased the catalyst surface area as well as enhancing the redox capability of the catalyst, which is beneficial for the oxidation removal of elemental mercury. Additionally, the production of the Co3O4/g-C3N4 hybrids reduced the potential energy barrier and boosted the charge transfer mobility, which facilitates elemental mercury removal. Hence, the Hg0 removal processes using Co3O4/g-C3N4 hybrids could be summarized into three stages. (i) The gas-phase Hg0 and O2 adhere on the catalyst exterior to produce adhered mercury (Hg0ad) and chemisorbed oxygen (Oad). (ii) The Hg0ad is oxidized into Hg2+ by the Co3+ cations in Co3O4 (Hg0ad + 2Co3+ → Hg2+ + 2Co2+). The produced Hg2+ cations react with Oad or the lattice oxygen (Olat) of Co3O4 to generate HgO, which is then captured on the g-C3N4 exterior. (iii) The oxygen in the feed gas can refresh the depleted Oad or Olat, and oxidize Co2+ into Co3+ to rebirth the active phase Co3O4. In addition, since HgO is stable only at temperature below ~300 °C [43], the adsorbed HgO would decompose into elemental mercury again under high-temperature treatment. Therefore, the used Co3O4/g-C3N4 can be facilely regenerated under heating treatment above 300 °C.

4. Conclusions

The pristine g-C3N4 nanosheet has a strong attraction towards Hg0 adsorption at 120 °C, with a Hg0 removal efficiency of ~59.0%. The incorporation of Co3O4 with g-C3N4 nanosheet could strengthen the mercury capture ability, likely due to the cooperative effect of Co3O4 and g-C3N4. The loading value of Co3O4 could affect the Hg0 removal performance. The best performance is displayed by 12Co3O4/CNNS, with a mercury conversion of >98% within 90–240 °C. Nitrogen monoxide and sulfur oxide can both reduce the mercury conversion by more than a half because of the competing adsorption and side reactions. This research could deliver useful insights into the application of metal oxide/g-C3N4 hybrids for the oxidation removal of elemental mercury.

Author Contributions

Z.Z. performed the experiments and characterized the samples. J.W. conceived the project. D.L. designed the experiments, analyzed the data, and wrote the paper.

Funding

This research was subsidized by the Senior Talent Foundation of Jiangsu University (grant no. 18JDG017) and the National Natural Science Foundation of China (grant no. 21237003).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic drawing of mercury removal experimental system.
Figure 1. Schematic drawing of mercury removal experimental system.
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Figure 2. FESEM image and photo of g-C3N4 nanosheets (CNNS).
Figure 2. FESEM image and photo of g-C3N4 nanosheets (CNNS).
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Figure 3. (a) TEM image of CNNS and (b) TEM image and selected area electron diffraction (SAED) analysis of 12Co3O4/CNNS.
Figure 3. (a) TEM image of CNNS and (b) TEM image and selected area electron diffraction (SAED) analysis of 12Co3O4/CNNS.
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Figure 4. XRD patterns of pure and Co3O4-modified CNNS.
Figure 4. XRD patterns of pure and Co3O4-modified CNNS.
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Figure 5. Nitrogen isotherms of pure and Co3O4-modified CNNS.
Figure 5. Nitrogen isotherms of pure and Co3O4-modified CNNS.
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Figure 6. FTIR profiles: (a) fresh CNNS and xCo3O4/CNNS; (b) fresh and spent 12Co3O4/CNNS.
Figure 6. FTIR profiles: (a) fresh CNNS and xCo3O4/CNNS; (b) fresh and spent 12Co3O4/CNNS.
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Figure 7. XPS spectra of 12Co3O4/CNNS before and after the reaction.
Figure 7. XPS spectra of 12Co3O4/CNNS before and after the reaction.
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Figure 8. The Hg0 capture performances of pure and Co3O4-modified CNNS.
Figure 8. The Hg0 capture performances of pure and Co3O4-modified CNNS.
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Figure 9. Impact of reaction temperature on the mercury conversion of 12Co3O4/CNNS.
Figure 9. Impact of reaction temperature on the mercury conversion of 12Co3O4/CNNS.
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Figure 10. The performances of CNNS and 12Co3O4/CNNS towards elemental mercury removal at 120 °C over 600 min on 5% O2/N2 stream.
Figure 10. The performances of CNNS and 12Co3O4/CNNS towards elemental mercury removal at 120 °C over 600 min on 5% O2/N2 stream.
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Figure 11. Impact of NO and SO2 on the mercury conversion of 12Co3O4/CNNS.
Figure 11. Impact of NO and SO2 on the mercury conversion of 12Co3O4/CNNS.
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Table
Table 1. Textural property and average pore diameter of the pure and Co3O4-modified CNNS.
Table 1. Textural property and average pore diameter of the pure and Co3O4-modified CNNS.
SamplesBET Surface Area (m2/g)Total Pore Volume (cm3/g)Micropore Volume (cm3/g)Mesopore Volume (cm3/g)Pore Diameter (nm)
CNNS1090.4560.0110.44519
8Co3O4/CNNS420.2060.0040.20210
12Co3O4/CNNS420.2110.0030.20811
16Co3O4/CNNS330.1750.0030.17212
20Co3O4/CNNS270.1250.0020.12310

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Zhang, Z.; Wu, J.; Liu, D. Co3O4/g-C3N4 Hybrids for Gas-Phase Hg0 Removal at Low Temperature. Processes 2019, 7, 279. https://doi.org/10.3390/pr7050279

AMA Style

Zhang Z, Wu J, Liu D. Co3O4/g-C3N4 Hybrids for Gas-Phase Hg0 Removal at Low Temperature. Processes. 2019; 7(5):279. https://doi.org/10.3390/pr7050279

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Zhang, Zhen, Jiang Wu, and Dongjing Liu. 2019. "Co3O4/g-C3N4 Hybrids for Gas-Phase Hg0 Removal at Low Temperature" Processes 7, no. 5: 279. https://doi.org/10.3390/pr7050279

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