Co3O4 Nanorods with a Great Amount of Oxygen Vacancies for Highly

Jun 2, 2019 - Co3O4 Nanorods with a Great Amount of Oxygen Vacancies for Highly Efficient Hg Oxidation from Coal Combustion Flue Gas ...
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Cite This: Energy Fuels 2019, 33, 6552−6561

Co3O4 Nanorods with a Great Amount of Oxygen Vacancies for Highly Efficient Hg0 Oxidation from Coal Combustion Flue Gas Xiaopeng Zhang, Hang Zhang, Hongda Zhu, Chengfeng Li, Ning Zhang, Junjiang Bao,* and Gaohong He* School of Petroleum and Chemical Engineering, State Key Laboratory of Fine Chemicals, Dalian University of Technology, Panjin 124221, China

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ABSTRACT: Oxidizing elemental mercury (Hg0) to Hg2+ is an effective way to remove Hg0 from flue gas. Surface-active oxygen species are considered to be important active sites in Hg0 oxidation process. The concentration enhancement of surfaceactive oxygen species is a primary challenge for this technology. Oxygen vacancies can easily capture and activate gaseous oxygen, forming more surface-active oxygen species, which may lead to a better Hg0 oxidation efficiency. Co3+ in Co3O4 can generate oxygen vacancies through the reduction of Co3+ to Co2+, and the oxygen vacancies formation process is controlled by Co2+/Co3+ ratio. Inspired by this, Co3O4 nanorods exposing (220) facet with a high Co3+/Co2+ ratio were successfully synthesized. Raman and X-ray photoelectron spectroscopy (XPS) results show that the high concentration of Co3+ leads to more oxygen vacancies. It results in a better catalytic performance for Co3O4 nanorods whose Hg0 oxidation efficiency remains above 90% at 180 000 h− in the temperature range of 100−300 °C. After 2880 min reaction, the Hg0 oxidation efficiency of Co3O4 nanorods reduces to about 72%, and it recovers to the original level after in situ thermal treatment at 550 °C, suggesting a great renewable property. Furthermore, XPS results of Co3O4 nanorods before and after the reaction show that the concentrations of Co3+ and surface-active oxygen decrease after the reaction. The reaction mechanism was revealed based on these results. Hg0 reacts with surface-active oxygen forming HgO, and the consumed oxygen is replenished by gaseous O2. Co3+/Co2+ redox couple can improve the electron-transfer activity to enhance the Hg0 oxidation efficiency in the presence of O2. The effects of flue gas components on the Hg0 oxidation efficiency are also investigated. O2 and NO have positive effects, while H2O and SO2 have negative effects on the Hg0 removal process. However, Co3O4 nanorods still have an efficiency of 75% even in the presence of 8% H2O and 200 ppm SO2. facilitate the Hg0 oxidation process by Hg0 (ad) + O* (surfaceactive oxygen) → HgO. Hence, increasing the amount of surface-active oxygen will further improve the catalytic efficiency for Hg0 effectively. Metal iron doping is a general method to increase the amount of surface-active oxygen.24 Zhang et al. reported that Ce-doped Co3O4 could obtain 50% more surface-active oxygen when compared to single Co3O4,18 resulting in a 30% higher Hg0 oxidation efficiency below 150 o C. In the meanwhile, decreasing catalyst particle size can increase the number of low-coordination atoms located at the edges and corners,25,26 which will capture and activate more gaseous oxygen leading to a better Hg0 oxidation performance. Esswein et al. researched Co3O4 nanoparticles with different particle sizes contrastively and found the small-sized Co3O4 exhibits better performance.27 However, the catalyst synthesis process cannot be controlled accurately by doping metal iron or decreasing the particle size, and the surface-active oxygen is need to be further enhanced.28,29 Oxygen vacancies as one type of special microstructures can easily adsorb gaseous oxygen and form more surface-active oxygen.30 Co3+ in Co3O4 can generate oxygen vacancies through the reduction of Co3+ to Co2+. Therefore, the proper

1. INTRODUCTION Mercury as a major pollutant in coal-fired flue gas has got worldwide attention due to its extreme toxicity, high volatility, persistence, and bioaccumulation in the environment.1−3 Mercury is present in coal combustion flue gas in three different forms, particle-bound mercury (Hgp), oxidized mercury (Hg2+), and elemental mercury (Hg0).4,5 Among these forms, Hg0 is the most difficult to remove through traditional environmental protection equipment because of its high volatility and insolubility.6 It has been proven that converting Hg0 to an easily removed form of Hg2+ is a feasible way to control the Hg0 emission from flue gas.7,8 Various transition-metal oxides, such as VO x , 9,10 WOx,11,1211,12 MnOx,13,1413,14 ZrOx,15 RuO2,1616 CoOx,17,18 and CeOx,19,2019,20 have been widely used as catalysts for Hg0 oxidation. CoOx with the favorable features of low cost, earth abundance, and good stability has been considered to be a promising catalyst.21 Co3O4 with a unique redox couple Co2+/ Co3+ is one of the most efficient active constituents for Hg0 oxidation22 because the active electron transfer between Co2+/ Co3+ can lead to a good redox property. Mei et al.23 synthesized spinel Co3O4 for Hg0 removal, and it showed a relatively good Hg0 removal efficiency of 70% at high temperature. In recent years, some research studies have pointed out that surface-chemisorbed oxygen as an important active site in Hg0 catalytic oxidation reaction can greatly © 2019 American Chemical Society

Received: March 13, 2019 Revised: May 31, 2019 Published: June 2, 2019 6552

DOI: 10.1021/acs.energyfuels.9b00765 Energy Fuels 2019, 33, 6552−6561

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Figure 1. Schematic diagram of the fix-bed experiment system.

control of the Co2+/Co3+ ratio can control the formation of oxygen vacancies.29,31 As the ion valence is different in different lattice planes of Co3O4, synthesizing and regulating Co3O4 nanocatalysts exposing different lattice planes is an attractive method to control the Co3+ amount on catalysts’ surfaces, which will then control the formation of oxygen vacancies and the Hg0 oxidation ability of the catalysts. It has been reported by Xie et al.32 that the Co3+ cations are only present on the {110} plane of Co3O4. Thus, the greater exposure of the {110} plane will lead to a higher Co3+/Co2+ ratio, which may give more oxygen vacancies, resulting in good Hg0 oxidation efficiency. In the present work, Co3O4 catalysts exposing more {110} plane were synthesized for Hg0 oxidation and to make a contrastive investigation with general Co3O4 nanoparticles. Brunauer−Emmett−Teller (BET), X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analysis were performed to characterize the physicochemical properties of the catalysts.

ethanol. Then, the specimen was prepared by dropping it on a clean copper grid and drying it in air. N2 adsorption was characterized at −196 °C on an Autosorb-iQ-C automated gas sorption system (Quantachrome Instruments). The specific surface areas of the catalysts were calculated by multipoint BET analysis of the N2 adsorption isotherm. All of the samples were degassed at 300 °C for 2 h prior to the measurements. Raman spectra were recorded on a Renishaw inVia spectrometer with a 532 nm emission line. The ion valence and atomic concentration of metal species on the catalyst surface were measured by X-ray photoelectron spectroscopy (XPS) using an ESCALAB250 (Thermo Fisher Scientific Corporation) with a monochromatic Al Kα radiation. The charging effects of the measurement were eliminated by correcting the observed spectra with the C 1s binding energy value of 284.6 eV. Hydrogen temperature-programmed reduction (H2-TPR) was performed to analyze the redox properties on an Autosorb-iQ-C automated gas sorption system (Quantachrome Instruments). Before the measurement, 50 mg of the samples was treated in He atmosphere at 300 °C for 2 h to clean the surface. Then, the samples were cooled down to 50 °C in the same atmosphere. Subsequently, the samples were heated and temperature-programmed reduced under 10 vol % H2/Ar (30 mL min−1) under the heating rate of 10 °C min−1 and the temperature of 50−900 °C. The H2 consumption amount was quantitatively measured by a thermal conductivity detector. 2.3. Catalytic Activity Measurement. Hg0 removal efficiency was measured in a fixed-bed flow reactor (Figure 1). A mercury permeation tube (VICI, Metronics Inc.) as a Hg0 source was placed in a U-shaped glass tube, which was immersed in a water bath at constant temperature (40 °C). A simulated flue gas consisting 50 μg m−3 Hg0, 5 vol % O2, NO (when used), H2O (when used), SO2 (when used), and balance N2 was used. NO and SO2 were controlled by mass flow controllers and water vapor was generated by a heated water bubbler. The total gas flux was 600 mL min−1. The catalyst usage amount is 0.2 mL (about 0.22 g), and the gas hourly space velocity (GHSV) is about 180 000 h−1. All the feed gases were mixed and preheated in a gas-mixing chamber before entering the reactor. The balance N2 was divided into two branches: one branch converged with individual streams of O2 to form the main gas flow and the other branch (200 mL min−1) passed through the U-tube to introduce Hg0 vapor into the reactor system. To avoid mercury condensation, silicone pipelines were warmed at 100 °C by heating belts. An online mercury analyzer (VM-3000, Mercury Instruments Analytical Technologies, Germany) was employed to measure the Hg 0 concentrations of the inlet and outlet of the reactor (denoted as Hg0in and Hg0out). To eliminate the interference of Hg0 adsorption, the catalysts reached an adsorption equilibrium in the Hg0 balance N2

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. Co3O4 nanorods (denoted as Co3O4-nanorods) were prepared by the method of ethylene glycol precipitation. Cobalt acetate tetrahydrate (4.98 g) was dissolved in 60 mL of ethylene glycol, and the obtained solution was heated to 160 o C under vigorous stirring and a continuous flow of nitrogen. Then, 200 mL aqueous Na2CO3 solution (0.2 mol L−1) was added dropwise into the cobalt acetate solution. The mixtures of liquid and sediments were stirred for 1 h under the N2 flow. Blue powders were obtained by vacuum filtration and washing. Finally, the obtained products were dried at 60 °C overnight and calcined at 450 °C for 4 h. Co3O4 nanoparticles (denoted as Co3O4-nanoparticles) were synthesized by the same method and the same reactant except for the N2 flow. The whole process of synthesizing Co3O4-nanoparticles is carried out in the air atmosphere. 2.2. Characterization of Catalysts. The crystalline phases and crystallinity of the catalysts were measured by the powder X-ray diffraction (XRD) with XRD-7000 S system using Cu Kα radiation (40 kV, 100 mA) (SHIMADZU Corporation, Japan). Scanning velocity is 5° min−1 with a step size of 0.02°. TEM and high-resolution TEM (HR-TEM) were performed on a Hitachi HT7700 transmission electron microscope (TEM) operating at 100 kV to investigate the particle sizes, morphologies, and lattice planes of the catalysts. The samples were ultrasonically dispersed in 6553

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Figure 2. (A) TEM and (A′, A″) HR-TEM of Co3O4-nanorods, (B) TEM and (B′, B″) HR-TEM of Co3O4-nanoparticles. atmosphere. The Hg0 oxidation efficiency (Eoxi) can be defined as follows

Eoxi =

0 Hg 0in − Hg out

Hg 0in

× 100%

3. RESULTS AND DISCUSSION 3.1. Catalysts Characterization. 3.1.1. TEM and HRTEM. TEM and HR-TEM measurements were performed to investigate the microstructures and exposed the crystal facets of the two catalysts. As shown in Figure 2A,B, the morphologies of the two catalysts are definitely confirmed as nanorod and nanoparticle, respectively, and their grain sizes are in the same order of magnitude. The lattice fringes of Co3O4nanorods and Co3O4-nanoparticles can be noticed in the HRTEM images (shown in Figure 2A′,B′), and the crystalline interplanar spacing measured and calculated by fast Fourier transform of the HR-TEM images are marked in those two images. According to the crystalline interplanar spacing in Figure 2A′,B′, the most lattice fringes of Co3O4-nanorods can be ascribed to the (220) plane (belonging to the {110} plane) and lattice fringes of Co3O4-nanoparticles mainly belong to the (111) and (311) planes. This phenomenon proves that two different Co3O4 structures of nanorods and nanoparticles were successfully synthesized. As can be seen in Figure 2A″,B″, Co3O4-nanoparticles have clear lattice fringes, while some parts of the lattice images became blurred for Co3O4-nanorods, indicating that Co3O4-nanorods have more oxygen vacancies.33,34 3.1.2. XRD. The XRD patterns of Co3O4-nanorods and Co3 O4 -nanoparticles are shown in Figure 3. Obvious diffraction peaks of Co3O4 can be detected in both the catalysts. The peaks at 2θ of 31.27, 36.85, 44.81, 59.35, and 65.23° are, respectively, indexed to the (220), (311), (400), (511), and (440) crystal planes of Co3O4 (PDF# 43−1003). Scherrer particle sizes corresponding to different facets are the sample sizes perpendicular to these facets. It gives direct information of the growth orientation and the vertical facet exposed degrees of the samples. As shown in Table 1, Scherrer particle sizes corresponding to the (220) and (440) facets of Co3O4-nanorods are higher than those of Co3O4-nanoparticles,

Figure 3. XRD patterns of the catalysts.

but the Scherrer particle sizes corresponding to the other three facets of Co3O4-nanorods are lower than those of Co3O4nanoparticles. It suggests that Co3O4-nanorods grow in the direction vertical to the (220) plane and expose more (220) plane. These results are consistent with the results of TEM and HR-TEM. 3.1.3. N2 Adsorption and Desorption. The specific surface areas of the catalysts were determined by N2 adsorption and desorption. As shown in Table 2, Co3O4-nanorods have a much larger surface area than Co3O4-nanoparticles. The larger surface area usually corresponds to more available surfaceactive sites that will adsorb more reaction components, resulting in a higher catalytic activity of heterogeneous catalysis.35 3.1.4. Raman. Raman patterns of the two catalysts are shown in Figure 4. Four peaks around 475, 517, 614, and 680 cm−1 are observed in both the catalysts, which correspond to the Eg, 1F2g, 2F2g, and A1g modes of the Co3O4 spinel structure.36,37 As has been reported, the peak at 687 cm−1 is due to the sublattice with highest valency cations36 and any differences in this peak can be ascribed to the lattice distortion.38 The Raman peak of Co3O4-nanorods at 687 cm−1 (full width at half-maximum (FWHM) is 19.07) is 6554

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Energy & Fuels Table 1. Scherrer Particle Sizes of Co3O4-Nanorods and Co3O4-Nanoparticles crystalline grain sizes of the diffraction peaks (nm) catalysts

(220)

(311)

(400)

(511)

(440)

Co3O4-nanorods Co3O4-nanoparticles

18.283 16.660

13.992 17.406

15.831 19.326

98.097 104.228

33.970 32.105

give more O vacancies through the reduction of Co3+ to Co2+ during the synthesis process. 3.1.5. XPS. XPS was carried out to analyze the concentration and chemical state of surface elements. The XPS patterns of Co 2p and O 1s are shown in Figure 5, and the surface atomic concentrations are listed in Table 3. As shown in Figure 5A,

Table 2. Structural Parameters of Catalysts Measured by N2 Adsorption catalysts

surface area (m2 g−1)

pore volume (cm3 g−1)

Co3O4-nanorods Co3O4-nanoparticles

115.80 70.01

0.678 0.411

Table 3. Surface Atomic Concentration and the Ratios of Different Chemical States catalysts

Co3+/Co2+

(Oα + Oγ)/Oβ

Co3O4-nanorods Co3O4-nanoparticles

0.90 0.63

3.58 0.57

the two main peaks in the Co 2p spectra can be split into four overlapping peaks. The two peaks around 780 and 795 eV are attributed to Co3+, while the other two peaks around 781 and 796 eV belong to Co2+.40,4140,41 In Figure 5B, two peaks around 529.8 and 531.5 eV, which are ascribed to lattice oxygen (Oβ) and chemisorbed oxygen (Oα), respectively,42,43 can be detected in both the catalysts. However, for Co3O4nanorods, a new intense peak at 532.8 eV shows up, which is attributed to the high binding energy peak from the surface oxygen defect species (Oγ).44,45 It suggests that Co3O4nanorods have more oxygen vacancies. These results are in great agreement with the Raman results. As shown in Table 3, the Co3+/Co2+ ratio of Co3O4nanorods is much higher than that of Co3O4-nanoparticles, which will generate more anionic defects to adsorb and activate oxygen in the gas phase forming the surface-active oxygen. This deduction can be confirmed by the concentration of Oα and Oγ, which were much higher on Co3O4-nanorods than those on Co3O4-nanoparticles. Chemisorbed oxygen as long as oxygen defect species were generally considered as important active sites for the oxidation of Hg0,46,4746,47 thus, the higher

Figure 4. Raman spectra of the catalysts.

broader when compared to that of Co3O4-nanoparticles (FWHM is 17.98), which is ascribed to lattice distortion.39 These results suggest that Co3O4-nanorods exposing the (220) facet contain more O vacancies. Xie et al.32 have pointed that Co3+ cations are only present on the {110} plane of Co3O4, suggesting that Co3O4-nanorods exposing the (220) plane have a higher concentration of surface Co3+ (it will be confirmed by XPS results in Section 3.1.5), which, in turn, can

Figure 5. XPS spectra of Co 2p and O 1s for the catalysts. 6555

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much higher catalytic activity than Co3O4-nanoparticles. In the temperature range of 100−300 °C, the Hg0 oxidation efficiency of Co3O4-nanoparticles is below 30% and that of Co3O4nanorods is higher than 90%, with a maximum of 98% at 200 °C. To investigate the effects of morphology on Hg0 oxidation efficiency, Co3O4-nanosheets and Co3O4-nanocubes were synthesized to make a comparison with Co3O4-nanorods. TEM and HR-TEM of these two catalysts are shown in Figure 1s. It can be seen that Co3O4-nanosheets and Co3O4nanocubes are successfully synthesized, and both catalysts mainly expose the (220) facet. Figure 7B shows the Hg0 oxidation efficiencies of the three catalysts with different morphologies. All of the three catalysts have similar Hg0 oxidation efficiencies. This phenomenon reveals that the exposing facet and not the morphology of the catalyst plays a key role in Hg0 oxidation process. The Hg0 catalytic removal performance of common Mn/Ce/Ti and W/V/Ti catalysts from 21 references has been summarized to make a comparison with Co3O4-nanorods, and the results are shown in Table 1S. After a comprehensive consideration of GHSV and reaction temperature window, it can be found that Co3O4nanorods have a higher Hg0 removal efficiency at a higher GHSV in a wider temperature range. The performance of a catalyst is related to its physicochemical properties. Characterization results have shown that Co3O4-nanorods mainly expose the (220) facet with a high Co3+/Co2+ ratio, which results in more oxygen vacancies. The oxygen vacancies can easily capture and activate the gas phase oxygen to form active oxygen species, and it plays an important role in the Hg0 oxidation process. Furthermore, Co3O4nanorods have a larger BET surface area, leading to more available surface-active sites. 3.2.2. Effects of Flue Gas Components on Hg0 Oxidation Efficiency. Flue gas components usually play an important role in Hg0 oxidation process. Therefore, the effects of individual flue gas components on Hg0 oxidation efficiency over Co3O4nanorods were investigated and the results are shown in Figure 8. O2 shows a promotive effect on Hg0 oxidation efficiency. It has been pointed that gaseous O2 can be adsorbed on a catalyst surface to form active oxygen species (O*). O* will react with Hg 0 through O* + Hg 0 → HgO. 50 The higher O 2

chemisorbed Oα and Oγ concentrations on Co3O4-nanorods are the key factor for the higher Hg0 oxidation activity. 3.1.6. H2-TPR. H2-TPR was performed to study the redox ability of Co3O4-nanorods and Co3O4-nanoparticles, and the reduction profiles are shown in Figure 6. Both catalysts have

Figure 6. H2-TPR profiles of the catalysts.

two main reduction peaks due to the two-step reduction of Co3O4. The temperatures of Co3O4-nanorods reduction peaks are around 297 and 549 °C and those of Co3O4-nanoparticles are around 312 and 436 °C. The peak at lower temperature is attributed to the reduction of Co3O4 to CoO, while another one at higher temperature is produced by the reduction of CoO to Co.48,49 Compared with Co3O4-nanoparticles, Co3O4nanorods have a lower starting reduction temperature (188 °C). In addition, the reduction peak at low temperature of Co3O4-nanorods shifts to lower temperature. It can be due to the surface oxygen vacancies, which promote the Co3+/Co2+ redox and oxygen mobility, leading to a higher reducibility of the catalyst.48 3.2. Catalytic Performance. 3.2.1. Hg0 Oxidation Efficiency at Different Temperatures. Figure 7A presents the Hg0 oxidation efficiency of Co3O4-nanorods and Co3O4nanoparticles. It can be noted that Co3O4-nanorods exhibit a

Figure 7. Hg0 oxidation efficiency of the catalysts with different morphologies. 6556

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Figure 9. SO2 effects on Hg0 adsorption and oxidation.

effect of Hg0 adsorption, Hg0 oxidation efficiency is obtained after the catalyst reaches the Hg0 adsorption equilibrium. However, in Figure 9, Hg0 adsorption on the catalyst surface is unsaturated, and there are some amounts of surface chemisorbed oxygen species for SO2 giving SO3 which can improve Hg0 oxidation process through SO3 + O* + Hg0 → HgSO4. Therefore, the higher Hg0 oxidation efficiency in Figure 9 is due to Hg0 adsorption. 3.2.3. Hg0 Oxidation Efficiency in a Relatively Long-Term Test. Hg0 oxidation efficiency of Co3O4-nanorods during 2880 min test and in situ thermal recovery was investigated, and the results are shown in Figure 10. The catalytic performance is

Figure 8. Effects of individual flue gas components on Hg oxidation efficiency over Co3O4-nanorods. 0

concentration results in more surface-active oxygen species, which accelerates the reaction between O* and Hg0. NO slightly enhances the Hg0 oxidation efficiency. After 100 and 200 ppm NO addition, the Hg0 oxidation efficiency increases from 92.4 to 95.2 and 96.2%, respectively. According to previous works, NO can react with O2 to form NO2 with a certain oxidation ability that can improve the Hg0 oxidation process through NO2 + O* + Hg0 → HgNO3.51,52 H2O has a prohibitive effect on the Hg0 oxidation process, which is mainly due to the strong adsorption competition between H2O and Hg0. The prohibitive effect of H2O can be eliminated after H2O is cut off.46,47 SO2 has serious inhibitory effects on the Hg0 oxidation process. When 100 ppm of SO2 is added into the flue gas, its Hg0 oxidation efficiency declines from 92.4 to 83.2%, and it declines further from 83.2 to 75.2% when 200 ppm of SO2 is added. The inhibitory effects of SO2 are mainly due to the SO2 poisoning catalyst surface and the competition between SO2 and Hg0. To better understand its inhibitory effects, SO2 is introduced into the flue gas without O2 and then into the flue gas with O2. This experiment can give the details of the inhibitory effects of SO2 on the Hg0 adsorption process and Hg0 oxidation process, and the results are shown in Figure 9. When the flue gas is without O2, 200 ppm of SO2 can deactivate the catalyst in about 20 min. This is because SO2 can react with Co oxides to form Co sulfates, and it will cut off the reaction path way of O* + Hg0 → HgO, leading to the total loss of Hg0 adsorption capacity of Co3O4.5353 After O2 is added, the Hg0 removal efficiency sharply increases to 85% in 20 min. SO2 can give another reaction path way in the presence of O2. SO2 first reacts with O* to form SO3, which, as a new adsorption site, in turn can absorb O* and then react with Hg0 to form HgSO4.54,5554,55

Figure 10. Hg0 oxidation efficiency of Co3O4-nanorods for 2880 min and in situ thermal recovery.

stable in 800 min and then decreases slowly during the following test. The Hg0 oxidation efficiency reduces to about 72% after 2880 min. After in situ thermal recovery, the catalytic activity of Co3O4-nanorods recovers to the original level, which suggests that Co3O4-nanorods have a good renewable property. 3.3. Reaction Mechanism. To understand the reaction mechanism, XPS was performed to analyze the surface atomic concentration and ratios of Co3O4-nanorods after 2880 min reaction and regeneration. As shown in Figure 11 a, the peak around 102.5 eV corresponds to SiO2, which may be attributed

SO2 + O* → SO3 SO3 + O* + Hg 0 → HgSO4

It should be noted that the Hg0 oxidation efficiency (the removal efficiency after the addition of 85% O2) in Figure 9 is more than that (75%) in Figure 8. In Figure 8, to eliminate the 6557

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improve the Hg0 oxidation efficiency in the presence of O2. Based on the above analysis, a probable mechanism of Hg0 oxidation could be deduced. Hg0 was first adsorbed on the surface-active sites to form Hg0 (ad) bonded with surfaceactive oxygen and then Hg0 (ad) was oxidized to form HgO. Finally, the consumed surface-active oxygen was replenished by gaseous O2. The reaction mechanism could be summarized by the following reaction equations Hg 0 + Oα → Hg(ad) − O Hg 0 + Oγ → Hg(ad) − O Hg(ad) − O → HgO

1/2O2 (gaseous) + *(adsorption sites) → Oα Co3O3 − □(oxygen vacancy) + 1/2O2 (gaseous) Figure 11. Hg 4f XPS spectra of Co3O4-nanorods for 2880 min reaction and regeneration.

→ Co3O3 − Oγ Co3O3 − Oγ → Co3O3 − Oβ → Co3O4

to impurities from Na2CO3 with SiO2 concentration of about 0.6%. The two peaks at around 100.3 and 104.6 eV can be detected in Co3O4-nanorods after 2880 min of reaction, which can be ascribed to HgO.56,57 The intensity of those two peaks sharply gets weaker after regeneration. Hg0 was oxidized to HgO during the oxidation process.58−60 It can accumulate on the catalyst surface, leading to less available surface-active sites and a lower Hg0 oxidation efficiency. After in situ thermal regeneration at 550 °C, most of the HgO is decomposed and the Hg0 oxidation efficiency is recovered to the original level. Figure 12 shows the Co 2p and O 1s XPS spectra of Co3O4nanorods for 2880 min reaction and regeneration, and Table 4 shows the surface atomic ratios. The Co3+/Co2+ and (Oα + Oγ)/Oβ ratios of Co3O4-nanorods for 2880 min reaction were much lower than those of fresh Co3O4-nanorods. It suggests that Co3+, Oα, and Oγ take part in the Hg0 oxidation process. Based on previous works, chemisorbed oxygen and oxygen defects were considered to be active sites for Hg0 oxidation, and these can react with Hg0 to form HgO.61 During this process, Co3+ was reduced to Co2+. Then, Co2+ can be reoxidized to Co3+ by gaseous O2 to finish the reaction cycle.46,62 Therefore, Co3+/Co2+ redox system can greatly

After regeneration, the peaks ascribed to Oα increase in intensity and the Oα/Oβ ratios of Co3+/Co2+ increases. This suggests that the decomposition of HgO gives more available oxygen vacancies, which can capture gaseous oxygen to form more surface chemisorbed oxygen species.

4. CONCLUSIONS Co3O4-nanorods that expose the (220) facet were successfully synthesized. Compared to Co3O4-nanoparticles, Co3O4-nanorods have a larger BET surface area, leading to more surfaceactive sites. Raman and XPS show that Co3O4-nanorods have a much higher Co3+/Co2+ ratio, resulting in higher chemisorbed oxygen and oxygen defect species, which play an important role in the Hg0 oxidation process. Therefore, Co3O4-nanorods have excellent Hg0 oxidation efficiency, which is more than 90% in the temperature range of 100−300 °C. After 2880 min of reaction, the Hg0 oxidation efficiency of Co3O4-nanorods reduces to about 72% but recovers to the original level after in situ thermal treatment at 550 °C, suggesting a good renewable property. A probable reaction pathway was deduced based on the XPS analysis of Co3O4-nanorods before and after the

Figure 12. XPS spectra of Co3O4-nanorods for 2880 min of reaction and regeneration: (A) Co 2p and (B) O 1s. 6558

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Energy & Fuels Table 4. Surface Atomic Ratios of the Catalysts catalysts

Co3+/Co2+

(Oα + Oγ)/Oβ

Co3O4-nanorods for 2880 min reaction Co3O4-nanorods for 2880 min reaction regeneration

0.42 0.54

2.33 3.01

reaction. Hg0 first reacts with chemisorbed oxygen to form HgO and the consumed chemisorbed oxygen is replenished by gaseous O2. The Co3+/Co2+ redox system can improve the electron shift to enhance the Hg0 oxidation efficiency in the presence of O2. The effects of flue gas components O2, NO, H2 O, and SO 2 on Hg 0 oxidation efficiency are also investigated. O2 and NO have positive effects, while H2O and SO2 have negative effects on the Hg0 removal process. However, Co3O4-nanorods still have an efficiency of 75% even in the presence of 8% H2O and 200 ppm SO2.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.9b00765. Preparation of Co3O4-nanocubes and Co3O4-nanosheets; TEM and HR-TEM of Co3O4-nanocubes and Co3O4-nanosheets; reaction conditions and Hg0 removal performance of common catalysts (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86427-2631803 (J.B.). *E-mail: [email protected]. Tel: +86427-2631916 (G.H.). ORCID

Xiaopeng Zhang: 0000-0002-6267-9851 Gaohong He: 0000-0002-6674-8279 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial supports from the National Natural Science Foundation of China (51408098), Liaoning Provincial Natural Science (20180510054), Foundation of China the Program for Changjiang Scholars (T2012049), Education Department of the Liaoning Province of China (LT2015007), and the Fundamental Research Funds for the Central Universities (DUT18JC45).

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