Regenerable CoxMn3−xO4 spinel sorbents for elemental mercury removal from syngas: Experimental and DFT studies

Fuel 266 (2020) 117105

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Regenerable CoxMn3−xO4 spinel sorbents for elemental mercury removal from syngas: Experimental and DFT studies

T

⁎

Zhen Wang, Jing Liu , Yingju Yang, Yingni Yu, Xuchen Yan, Zhen Zhang State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

G R A P H I C A L A B S T R A C T

Hg

Co

Hg

CoMn2O4 sorbent

A R T I C LE I N FO

A B S T R A C T

Keywords: Hg0 removal Syngas CoMn2O4 sorbent Regeneration Density functional theory

The capture of Hg0 in syngas is challenging since the reducing atmosphere is disadvantageous to oxidize Hg0. Spinel CoxMn3−xO4 sorbents synthesized by a low-temperature sol-gel auto-combustion method were employed for the first time to remove Hg0 under simulated syngas. The Hg0 capture performance of CoxMn3−xO4 sorbents increased with Co mole ratio increases. CoMn2O4 showed the highest Hg0 capture performance among the CoxMn3−xO4 sorbents, attained over 95% Hg0 removal efficiency at 40–160 °C. The characterization results indicated that the mobile-electron environment, higher contents of surface Co and chemisorbed oxygen, larger BET surface area of CoMn2O4 sorbent were responsible for its superior performance. Ten repeated adsorptionregeneration cycles demonstrated that the regenerability of CoMn2O4 sorbent is excellent. Density functional theory (DFT) calculations were performed to determine the active sites of CoMn2O4 and to reveal Hg0 adsorption mechanism. The results suggested that Hg0 was chemisorbed on CoMn2O4 with a high adsorption energy (−1.04 eV). The two-fold coordinated surface Co atom was determined as the major active site for Hg0 adsorption. The strong orbital hybridization between Hg and Co atoms resulted in the strong chemisorption of Hg0 on CoMn2O4 surface.

⁎

Corresponding author. E-mail address: [email protected] (J. Liu).

https://doi.org/10.1016/j.fuel.2020.117105 Received 27 September 2019; Received in revised form 21 November 2019; Accepted 13 January 2020 Available online 23 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.

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1. Introduction

of CoxMn3−xO4 sorbents were characterized by using different analysis methods. The influence of reaction temperature on Hg0 removal over CoxMn3−xO4 sorbents was investigated. The Hg0 adsorption capacity and the regenerability of CoMn2O4 sorbent were examined. Moreover, DFT calculations were executed to reveal the active sites and the involved microcosmic mechanism for Hg0 adsorption upon CoMn2O4 surface.

Coal gasification is one of the most potential clean coal technology duo to its high efficiency, abundant by-products, and environmentally friendly relative to traditional combustion technologies [1,2]. However, elemental mercury (Hg0), which is a highly toxic pollutant of worldwide concern and is very difficult to be removed, will be released into syngas during coal gasification process. Worse still, Hg0 is difficult to be oxidized under reducing atmosphere and thus Hg0 concentration in syngas (approximately 80 μg/m3) [3] is far larger than in coal combustion flue gas [4]. In addition, higher concentration of Hg0 would accelerate the damage of aluminum heat exchanger through amalgam corrosion and liquid metal embrittlement mechanisms, which has induced catastrophic industrial accidents in plants [5,6]. Therefore, special attention should be given in the elimination of Hg0 from coal-derived syngas. The capture of Hg0 through adsorption by sorbents is presently the most mature method for reducing Hg0 emission during coal gasification. To date, a variety of sorbents, including carbon-based materials [7–10], metal sulfides [11–14], metal oxides [15–19], noble metals [20,21], and so forth have been studied for Hg0 removal. Carbon-based materials, including activated carbons, porous carbons, and their altered products have been studied for Hg0 removal from syngas [2,7,8]. However, their industrial application is restricted by the poor regeneration performance, high operation cost, and possibility of Hg0 leaching and re-emission. The practical application of metal sulfides is restricted by the possible generation of sulfur-containing pollutants (such as SO2, SO3 and H2S) during sorbent regeneration process [22]. Researchers have found that several metal oxides, including CeO2 [16], Fe2O3 [17], and Co3O4 [23] based sorbents can effectively remove Hg0 under simulated syngas when temperature lower than 150 °C. Nevertheless, to enhance the heat efficiency of the integrated gasification combined cycle, Hg0 elimination in syngas is desired at higher temperature (≥200 °C) [7,21]. Palladium based sorbents can effectively capture Hg0 from syngas at the temperature higher than 200 °C [20,21], but they are too expensive for widespread adoption. Thus, the exploitation of low-cost sorbents with superior activity for Hg0 elimination from syngas at higher temperature is imminently needed. Earth-abundant and inexpensive manganese based oxides have been widely studied as promising materials for Hg0 adsorption and oxidation [3,24,25]. Additionally, it was found that Mn-based sorbents can function as desulfurizers for the removal of H2S [26], which is another toxic pollutant in syngas that have adverse effects on turbine blades and ecosystems [27]. Thus, Mn-based materials will offer a potential for simultaneously removing Hg0 and H2S from syngas in one adsorption reactor. Among various Mn-based materials, spinel type manganese oxides have attracted much attention in the field of gaseous pollutants elimination because of their low cost, strong thermal stability, and unique redox properties [28–31]. The general chemical formula of spinel oxides is AB2O4. Benefiting from the remarkable tunability of A and B cations, spinels can be rationally tuned and equipped with targeted physicochemical characteristics (for example, morphologies, defects, electron mobility, and redox behavior) for enhanced performance [32]. Moreover, the cations at A and B sites can mutually shift, resulting in surface-abundant oxygen defects and mobile-electron environment [33,34]. These properties are conducive for Hg0 adsorption and oxidation. Recently, researchers have developed some spinel-type sorbents for Hg0 elimination from coal-fired flue gas and demonstrated high activity [22,29]. However, no research on the removal of Hg0 from syngas by using spinel-based materials has been reported. The reducing environment of syngas is adverse to Hg0 oxidation. Accordingly, the Hg0 removal performance and reaction mechanism would be different with that of coal-fired flue gas. In this work, a series of CoxMn3−xO4 sorbents synthesized via lowtemperature sol-gel auto-combustion approach were applied for the first time to remove Hg0 from syngas. The physicochemical properties

2. Experimental and calculation section 2.1. Samples preparation A low-temperature sol-gel auto-combustion approach was applied to synthesize the CoxMn3−xO4 sorbents. In a typical preparation, 0.06 mol of citric acid monohydrate and 5 ml alcohol were added in distilled water with continuous stirring. Afterwards, the suitable amount of Co (NO3)2·6H2O and Mn(NO3)2·4H2O were dissolved in the above solution. The mole ratio of metal (Co2+ + Mn2+)/citric acid was maintained at 1. The obtained solution was allowed to react at 60 °C for 1 h for the complexation reaction. Then, the mixture was kept stirring and evaporating under 90 °C to obtain sticky sol-gel. The wet gel was transferred to a drying oven (100 °C) for 12 h. Subsequently, the dry gel was burned in muffle roaster under 400 °C for 4 h. During the calcination, the citric acid was ignited, leading to the formation of loose and porous products. Finally, the solid products were ground and sized to 200 mesh for use. The prepared samples were denoted as CoxMn3−xO4, and × represents the Co mole ratio. Furthermore, a commercial activated carbon (AC) fabricated exclusively for Hg0 removal was used as comparison material. 2.2. Samples characterization The phase composition of the samples were analyzed by X-ray diffraction (XRD-7000, Shimadzu, Japan) using Cu Kα (λ = 1.5406 nm) radiation. The diffraction patterns were recorded from 10° to 80° (2θ) with a scanning rate of 5°/min. The morphology of samples was examined using scanning electron microscopy (SEM, JOEL7100F). A nitrogen adsorption apparatus (ASAP-2020, Micromeritics) based on Brunauer-Emmett-Teller (BET) adsorption model was employed to measure the specific surface area and pore parameter. The samples were degassed at 200 °C for 2 h before BET measurement. The surface chemical characteristics of samples were analyzed by X-ray photoelectron spectroscopy (XPS, VG Multilab 2000), which was equipped with Al Kα (hv = 1486.6 eV) excitation source. The C 1 s peak at 284.6 eV was acted as the reference for binding energy calibration. 2.3. Hg0 removal performance tests The Hg0 adsorption and regeneration performance of Co-Mn spineltype sorbents were examined in a fixed-bed reactor, as presented in Fig. 1. 50 mg sorbent mixed with 1950 mg quartz sand was used in all experiments if not otherwise specified. Quartz sand, an inert material, was used as diluent to mix with CoxMn3−xO4 sorbents. Quartz sand increased the height of heterogeneous reaction region, thus enhancing the residence time of syngas. The simulated syngas (SG) included 20% H2, 20% CO, 400 ppm H2S, 10 ppm HCl, and the balance N2. A stable feed of gaseous Hg0 (approximately 50 μg/m3) was provided by a mercury permeation tube. An online mercury analyzer (Lumex RA915M) was used to detect the Hg0 concentration. The total gas flow rate was set to 1 L/min, which corresponded to a gas volume hourly space velocity (GHSV) of about 50,000 h−1. The accumulation Hg0 capture efficiency and Hg0 adsorption capacity were tested to evaluate the performance of Co-Mn spinels. The accumulation Hg0 removal efficiency (ηa ) was calculated using the following equation: 2

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AC trap

3-way valve

H2S

Sorbent Computer Condenser

CO H2 HCl

Furnace

N2

Online Hg analyzer NaOH solution

Hg0 generator

MFC

Temperature controller

Water bath

Fig. 1. Schematic diagram of the experimental system for Hg0 removal.

O4f

Mn5f

Co2f

bridge

O3f

Fig. 2. Slab models of CoMn2O4(0 0 1) surface. (a) Crystal structure of CoMn2O4. (b) CoMn2O4(0 0 1) surface with five different adsorption sites. Mn5f site denotes 5-fold coordinated Mn. Co2f site denotes 2-fold coordinated Co. O3f and O4f denote 3-fold and 4fold coordinated O, respectively. The bridge sites denote the centers between two adjacent surface atoms.

(a) Crystal structure Co

O

Mn

Co-Mn spinel

(b) CoMn2O4(001) surface

Mn3O4

The Hg0 adsorption capacity (Q) was defined as the following equation:

Co3O4

Q=

Intensity (a.u.)

(e)

(b)

The calculations were conducted by using DFT method as implemented in CASTEP package [35]. The spin-polarized calculation was employed because of the magnetic properties of CoMn2O4. The magnetic moments followed a Neel-type ferrimagnetic arrangement, and all metal atoms were set in the high-spin state [36]. The generalized gradient approximation with Perdew-Burke-Ernzerhof functional (GGAPBE) [37] was used for treating the exchange correlation potential. The one-electron valence states were expanded in a basis of plane waves with a kinetic energy cutoff of 340 eV. The convergence criteria for the force, energy, self-consistent field (SCF), and displacement were set as 0.05 eV/Å, 2.0 × 10−5 eV/atom, 2.0 × 10−6 eV/atom, and 0.002 Å, respectively. Brillouin zones were sampled with 4 × 4 × 3 and 3 × 2 × 1 Monkhorst-Pack [38] k-point meshes for the bulk and surface of CoMn2O4, respectively. CoMn2O4 with space group I41/amd is

t 0

∫ 0

Hg 0in dt

(2)

2.4. Computational methods

∫ Hg 0in dt − ∫ Hg 0out dt t

0 (Hgin0 − Hgout ) × q × dt

(c)

Fig. 3. XRD patterns of CoxMn3–xO4 sorbents. (a) Co0.2Mn2.8O4. (b) Co0.5Mn2.5O4. (c) Co0.8Mn2.2O4. (d) CoMn2O4. (e) Regenerated CoMn2O4.

0

t2

1

(d)

2ș/(°)

ηa =

∫t

where m is the sorbent mass, q is the gas flow rate, t is the adsorption 0 are the instantaneous Hg0 concentration at the time, Hgin0 and Hgout inlet and outlet of the reactor, respectively.

(a)

t

1 m

× 100 (1) 3

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Table 1 BET specific surface area, average pore diameter, total pore volume and surface atomic concentration of CoxMn3−xO4 sorbents. Sorbents

BET surface area (m2/g)

Total pore volume (cm3/g)

Average pore diameter (nm)

Surface atomic concentration (%) Co

Co0.2Mn2.8O4 Co0.5Mn2.5O4 Co0.8Mn2.2O4 CoMn2O4

34.36 54.44 68.71 78.85

19.62 19.04 16.22 15.89

0.17 0.26 0.28 0.31

2.46 5.13 8.44 8.78

Mn

31.96 30.92 28.05 25.85

O Oα

Oβ

44.56 45.84 43.72 42.32

18.32 18.11 19.79 23.06

Fig. 4. SEM images of (a) Co0.2Mn2.8O4. (b) Co0.5Mn2.5O4. (c) Co0.8Mn2.2O4. (d) CoMn2O4.

3. Results and discussion

the spinel tetragonal crystal structure (Fig. 2a). The calculated lattice parameters (a = b = 5.919 Å, c = 8.911 Å) agree well with the experimental data (a = b = 5.775 Å, c = 8.958 Å) [39], demonstrating that the calculations are faithworthy. CoMn2O4(0 0 1) surface, a typical low-index surface, was applied to examine Hg0 adsorption over CoMn2O4 sorbent. Since Hg0 in coal-derived syngas is dilute, the interaction between adjacent Hg atoms is negligible and hence a large surface model is necessary. In this work, CoMn2O4(0 0 1) surface was constructed by periodically repeated p (2 × 1) surface supercell with eight atomic layers, which can result in a minimal Hg-Hg interaction. To prevent the interactions between two periodic slabs, a 16 Å-thick vacuum space was applied to separate the slab models. Fig. 2 shows the slab models of CoMn2O4(0 0 1) surface and the possible active adsorption sites. The adsorption energy (Eads) was calculated by the following equation:

(

Eads = E(CoMn2 O4 − Hg 0) − ECoMn2 O4 + EHg 0

)

3.1. Characterization analysis of sorbents Fig. 3 shows the XRD patterns of the synthesized CoxMn3−xO4 sorbents. For all CoxMn3−xO4 samples, the diffraction peaks indicative of Co-Mn spinel (JCPDS 18-0408), Mn3O4 (JCPDS 24-0734), and Co3O4 (JCPDS 43-1003) were detected, and the Co-Mn spinel was the predominant phase. The diffraction peak intensity of Co-Mn spinel increased and became sharper as the mole ratio of Co increased from 0.2 to 1. Meanwhile, the diffraction peak intensity of Mn3O4 decreased with increasing Co mole ratio. These suggested that Co cation was incorporated into the manganese oxides to form Co-Mn spinel. Table 1 summarizes the BET specific surface area, average pore diameter and total pore volume of CoxMn3−xO4 samples. The BET surface area of the samples ranges from 34.36 to 78.85 m2/g. The total pore volume is ranges from 0.17 to 0.31 cm3/g. CoMn2O4 sorbent exhibits the largest BET surface area of 78.85 m2/g and total pore volume of 0.31 cm3/g. The average pore diameter decreased from 19.62 to 15.89 nm with the Co mole ratio increases. In addition, the pore sizes of mesopores is in the range of 2–50 nm [41]. Therefore, the synthesized CoxMn3−xO4 sorbents are the mesoporous materials. SEM images (Fig. 4) show that the surface morphology of CoxMn3−xO4 samples are full of irregular small grains and macro/

(3)

where E(CoMn2 O4 − Hg 0) , EHg 0 , and ECoMn2 O4 are the gross energies of the CoMn2O4/Hg system, the isolated Hg atom, and the optimized CoMn2O4(0 0 1), respectively. The larger negative Eads value, the stronger adsorption strength of Hg0. In general, the interaction with an adsorption energy lower than −30 kJ/mol is classified as physisorption, while higher than −50 kJ/mol is classified as chemisorption [40]. 4

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Co2+ Co3+

Co2+

3+

Co

Mn2+

Mn3+ Mn4+

OĮ

Oȕ

Co2+

Co3+ Co2+

Mn3+ Co3+

Mn2+

OĮ

Mn4+ Oȕ

OĮ Co2+ Co3+ Co2+

Co3+

Mn2+

Mn

3+

Mn4+ Oȕ

OĮ 2+

Co

Co3+ Co2+

Co3+

Mn3+ Mn2+

Mn4+ Oȕ

Fig. 5. XPS spectra of Co 2p, Mn 2p, and O 1s for CoxMn3−xO4 sorbents. (a-c) CoMn2O4. (d–f) Co0.8Mn2.8O4. (g–i) Co0.5Mn2.5O4. (j–l) Co0.2Mn2.8O4.

mesopores. In addition, the pore size decreased as the Co mole ratio increased from 0.2 to 1, which is in accordance with the BET results. To get an insight into the surface chemical information of CoxMn3−xO4 samples, XPS analysis was conducted. XPS spectra of Co 2p, Mn 2p, and O 1 s in CoxMn3−xO4 sorbents are shown in Fig. 5. The Co 2p spectra were well fitted with two spin–orbit doublets. The fitting peaks at the binding energies of 780.19 ± 0.10 and 795.45 ± 0.07 eV are assigned to tetrahedral Co2+ of Co-Mn spinel, while the peaks at 781.56 ± 0.11 and 796.87 ± 0.10 eV are assigned to octahedral Co3+ of Co-Mn spinel. Consequently, it can be inferred that Co2+ and Co3+ are major Co species on CoxMn3−xO4 sorbents surface. For Mn 2p spectra, three peaks of Mn 2p3/2 at 640.86 ± 0.12, 641.95 ± 0.11, and 643.23 ± 0.12 eV are assigned to Mn2+ in tetrahedral sites, Mn3+ in octahedral sites, and Mn4+ in octahedral sites, respectively. The XPS spectra of O 1 s were fitted by two characteristic peaks. The peak at 530.02 ± 0.03 eV is characteristic of lattice oxygen (Oα) [42] of CoxMn3−xO4 samples. The peak at 531.22 ± 0.10 eV is characteristic of surface chemisorbed oxygen (Oβ). Moreover, the surface atomic concentrations of Co, Mn and O in CoxMn3−xO4 sorbents were calculated and summarized in Table 1. As the increase of Co mole ratio from 0.2 to 1, the concentration of surface Co atom increases from 2.46% to 8.78%. Meanwhile, the surface Mn concentration declines from 31.96%

Fig. 6. Hg0 removal efficiency over CoxMn3−xO4 and AC sorbents under SG as a function of temperature.

5

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(b)

sorbent slightly decreased from 97.2% to 95.6%. Even at 200 °C, CoMn2O4 sorbent still showed a high ηa of 88.2%. It should be noted that the GHSV of 5 × 104 h−1 was larger than that of actual syngas feed [45]. The lower GHSV can lead to a longer duration time for Hg0 adsorption and thus promoted Hg0 elimination. Moreover, we have compared the performance of Hg0 removal from syngas over CoMn2O4 against other reported sorbents, and observed that the ηa of CoMn2O4 at 200 °C was much larger than that of CeO2/TiO2 (< 40% at 200 °C) [16], FexCeyOz/semi-coke (< 50% at 200 °C) [17], CexCoyOz/TiO2 (~60% at 200 °C) [23], and coal-based AC (< 80% at 160 °C) [7]. Therefore, the synthesized CoMn2O4 spinel is a superior sorbent for Hg0 removal from syngas at high temperature. The XPS analyses revealed that Co and Mn atoms on CoMn2O4 sorbent existed in the form of Co2+/3+ and Mn2+/3+/4+. The flexible valence can produce Jahn-Teller distortions (a distinguishing feature), which is able to improve electron mobility [22,46]. The gaseous Hg0 can readily donate electrons to CoMn2O4 sorbent and thus chemisorbs on the surface. Furthermore, CoMn2O4 sorbent exhibited the largest BET surface area, surface Co and Oβ concentrations (Table 1) among the synthesized CoxMn3−xO4 sorbents. Thus, the excellent Hg0 capture performance of CoMn2O4 sorbent was attributed to its mobile-electron environment, larger BET surface area, higher surface Co and chemisorbed oxygen concentrations.

350 ’ Fig. 7. (a) 50%-breakthrough curve of CoMn2O4 under SG; (b) calculated 50%breakthrough (scattered dot) and simulated (solid line) adsorption capacities of CoMn2O4. Gaseous Hg0 concentration: about 150 μg/m3. Sorbent mass = 10 mg.

to 25.85%. In addition, CoMn2O4 sorbent exhibits the highest percentage of surface chemisorbed oxygen among the as-prepared CoxMn3−xO4 sorbents. It has been demonstrated that Oβ is more reactive in Hg0 adsorption and oxidation than Oα [43,44]. Therefore, the highest Oβ concentration of CoMn2O4 sorbent would lead to higher Hg0 removal performance than Co0.8Mn2.2O4, Co0.5Mn2.5O4 and Co0.2Mn2.8O4 sorbents.

3.3. Hg0 adsorption capacity of CoMn2O4 sorbent The Hg0 adsorption capacity over CoMn2O4 sorbent was tested under SG at 160 °C to further evaluate the performance of the sorbent. Fig. 7(a) shows the 50% breakthrough curve for Hg0 removal over CoMn2O4. The 50% breakthrough point was attained after reaction 36 h and the corresponding adsorption capacity was 19.62 mg/g. Furthermore, the 50% breakthrough curve was analyzed by using the pseudofirst-order kinetic model, which has been widely used to estimate the equilibrium Hg0 adsorption capacity of different sorbents [11,47,48]. This kinetic model was expressed as follows [49]

3.2. Hg0 removal performance of CoxMn3−xO4 sorbents Hg0 removal efficiencies (ηa ) over the CoxMn3−xO4 sorbents were examined under SG at 40–200 ℃, and compared with the commercial AC fabricated exclusively for Hg0 removal. As shown in Fig. 6, ηa of CoxMn3−xO4 sorbents are much higher than that of the commercial AC, which suggested that the CoxMn3−xO4 spinels are effective non-carbon sorbents for Hg0 removal from syngas. In addition, it is obvious that the ηa of CoxMn3−xO4 sorbents increased with the Co mole ratio increases, and the CoMn2O4 sorbent showed the highest activity for Hg0 removal. With enhancing reaction temperature from 40 to 160 °C, ηa of CoMn2O4

Q = qe × (1 − e−kt )

(6) 0

where Q and qe are Hg adsorption capacity at time t and at equilibrium, respectively, and k is the rate constant of the kinetic model. The calculated and estimated Hg0 adsorption capacity curves are shown in Fig. 7(b). As presented, the pseudo-first-order kinetic model fitted well with the experimental data with a correlation coefficient (R2) larger than 99.99%, which demonstrated that the kinetic model is very

(a)

(b)

100 nm

(c)

100 nm

Fig. 8. (a) Hg0 removal efficiency of CoMn2O4 sorbent over 10 repeated adsorption-regeneration cycles. (b) SEM image of fresh CoMn2O4 sorbent. (c) SEM image of regenerated CoMn2O4 sorbent. 6

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Fig. 9. (a, b) Adsorption energies, charge transfers, and structural parameters of the optimized configurations for Hg0 adsorption on CoMn2O4(0 0 1) surface. (c) PDOS of Hg and Co atoms in the most stable adsorption configuration.

suitable for predicting Hg0 adsorption on CoMn2O4 sorbent. The equilibrium Hg0 adsorption capacity was estimated to be 39.05 mg/g, which is larger than that of the previously reported representative sorbents, such as sulfur-impregnated AC (2.20 mg/g) [50], [MoS4]2−/CoFe-LDH (16.34 mg/g) [13], and CoMoS/γ-Al2O3 (18.94 mg/g) [11].

in Fig. 9a and b. Fig. 9a shows the most stable adsorption configuration, in which Hg0 adsorbs on surface Co2f site of CoMn2O4(0 0 1) surface with the formation of a Co–Hg (2.719 Å) bond. The corresponding adsorption energy is −1.04 eV and charge transfer is 0.17 e, indicating an intense chemisorption. In Fig. 9b, Hg0 is adsorbed on surface Mn5f site and the equilibrium distance between the Mn and Hg atoms is 3.096 Å. The corresponding adsorption energy is −0.30 eV and the charge transfer is 0.04 e, which are much lower than that of Hg0 adsorption on surface Co2f site. Therefore, Hg0 adsorption on CoMn2O4 sorbent is controlled by a strong chemisorption mechanism and the two-fold coordinated surface Co atom is identified as the most active site. This agrees well with the previous experimental observation that the ηa of CoxMn3−xO4 sorbents increased with the Co mole ratio increases. In order to provide further insight into the atomic-level interaction between gaseous Hg0 and CoMn2O4, the projected density of states (PDOS) of Hg and Co atoms in the most stable structure (Fig. 9a) was analyzed. As presented in Fig. 9c, Hg s-orbital strongly hybridized with Co p- and d-orbitals at −3.22 eV. Hg p-orbital strongly hybridized with Co s- and p-orbitals at 5.23 eV. Meanwhile, Hg d-orbital strongly hybridized with Co p- and d-orbitals at −6.30 eV. The intense orbital hybridization between Hg and Co atoms resulted in the formation Co–Hg bond and the chemical adsorption of Hg0 on CoMn2O4 surface. This can explain why CoMn2O4 sorbent has an excellent Hg0 removal performance.

3.4. Regenerability of CoMn2O4 sorbent Besides Hg0 removal performance, another important characteristic of the CoMn2O4 sorbent is its regenerability, which can cut down the operation cost. The regenerability of the CoMn2O4 sorbent under SG was examined by performing repeated adsorption-regeneration experiments. 50 mg CoMn2O4 was firstly employed to adsorb Hg0 under SG at 160 °C. After that, the spent sorbent was regenerated by thermal treatment at 350 °C under air for 1 h. After the thermal treatment, the adsorbed mercury can be released from CoMn2O4 and the consumed surface oxygens of sorbent can be replenished by gaseous O2 of air. Subsequently, the regenerated sorbent was used for next Hg0 capture cycle. Fig. 8 presents the Hg0 capture efficiencies of CoMn2O4 over 10 repeated adsorption-regeneration cycles. The results exhibited that 10 adsorption-regeneration cycles did not lead to significant decrease of Hg0 removal efficiency over CoMn2O4. After 10 repeated cycles, ηa of CoMn2O4 still maintained at 92.9%. In addition, the CoMn2O4 sorbent after 10 adsorption-regeneration cycles was characterized using XRD and SEM (presented in Figs. 3(e) and 8(c), respectively). Compared to fresh CoMn2O4 sorbent, no obvious variation in the crystalline phase and microcosmic morphology of CoMn2O4 was observed after 10 cycles. Therefore, it can be concluded that the as-synthesized CoMn2O4 sorbent has excellent regenerability for Hg0 removal from syngas.

4. Conclusions Experiments and DFT calculations were conducted to investigate Hg0 removal over Co-Mn spinel-type sorbents. The Hg0 removal performance of CoxMn3−xO4 sorbents increased as the Co mole ratio increased from 0.2 to 1. Above 95% Hg0 removal efficiency was achieved for CoMn2O4 sorbent at 40–160 °C. The equilibrium Hg0 adsorption capacity of CoMn2O4 sorbent was up to 39.05 mg/g. The superior Hg0 removal performance of CoMn2O4 sorbent was closely associated with its mobile-electron environment, larger BET surface area, higher surface Co and chemisorbed oxygen concentrations. The repeated adsorptionregeneration experiments exhibited that the spent CoMn2O4 sorbent could be effectively regenerated by thermal treatment at 350 °C under air. DFT calculation results indicated that chemical adsorption mechanism is responsible for Hg0 adsorption on CoMn2O4 sorbent. The major active site for Hg0 adsorption is the two-fold coordinated surface Co atom. The strong orbital hybridization between Hg and Co atoms led

3.5. Active sites and involved mechanism for Hg0 adsorption on CoMn2O4 sorbent The Hg0 adsorption mechanism and the surface active sites are strongly linked to the excellent Hg0 capture performance of CoMn2O4 sorbent. However, it is difficult to accurately determine the active sites for Hg0 adsorption by experimental investigation. Furthermore, microcosmic adsorption mechanism at atomic-level is very important for understanding the surface reactivity of CoMn2O4 sorbent. Thus, the DFT calculations were performed to study the microcosmic Hg0 adsorption mechanism on CoMn2O4 sorbent. The optimized Hg0 adsorption structures over CoMn2O4(0 0 1) surface and the corresponding adsorption energies, charge transfers and structural parameters are shown 7

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to the formation Co–Hg bond and the chemisorption of Hg0 on CoMn2O4 surface.

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CRediT authorship contribution statement Zhen Wang: Data curation, Writing - original draft. Jing Liu: Supervision. Yingju Yang: Validation. Yingni Yu: Methodology, Software. Xuchen Yan: Investigation. Zhen Zhang: Investigation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by Fundamental Research Funds for the Central Universities (2019kfyRCPY021), Program for HUST Academic Frontier Youth Team (2018QYTD05), and Postdoctoral Science and Technology Activity Foundation of Hubei Province (G63). The authors also thank the Analytical and Testing Center of Huazhong University of Science and Technology for the measurement. References [1] Minchener AJ. Coal gasification for advanced power generation. Fuel 2005;84:2222–35. [2] Shen F, Liu J, Dong Y, Wu D, Gu C, Zhang Z. Elemental mercury removal from syngas by porous carbon-supported CuCl2 sorbents. Fuel 2019;239:138–44. [3] Wang Z, Liu J, Yang Y, Miao S, Shen F. Effect of the mechanism of H2S on elemental mercury removal using the MnO2 sorbent during coal gasification. Energy Fuels 2018;32:4453–60. [4] Lu DY, Granatstein DL, Rose DJ. Study of mercury speciation from simulated coal gasification. Ind Eng Chem Res 2004;43:5400–4. [5] Wilhelm SM. Risk analysis for operation of aluminum heat exchangers contaminated by mercury. Process Saf Prog 2009;28:259–66. [6] Zhang H, Zhang D, Wang J, Xu W, Yang D, Jiao T, et al. Simultaneous removal of Hg° and H2S at a high space velocity by water-resistant SnO2/carbon aerogel. J Hazard Mater 2019;371:123–9. [7] Zhang H, Zhao J, Fang Y, Huang J, Wang Y. Catalytic oxidation and stabilized adsorption of elemental mercury from coal-derived fuel gas. Energy Fuels 2012;26:1629–37. [8] Hong D, Zhou J, Hu C, Zhou Q, Mao J, Qin Q. Mercury removal mechanism of AC prepared by one-step activation with ZnCl2. Fuel 2019;235:326–35. [9] Ling Y, Wu J, Man X, Xu Y, Liu Q, Qi Y, et al. BiOIO3/graphene interfacial heterojunction for enhancing gaseous heavy metal removal. Mater Res Bull 2019;122:110620. [10] Shen F, Liu J, Wu D, Dong Y, Liu F, Huang H. Design of O2/SO2 dual-doped porous carbon as superior sorbent for elemental mercury removal from flue gas. J Hazard Mater 2019;366:321–8. [11] Zhao H, Yang G, Gao X, Pang CH, Kingman SW, Wu T. Hg0 capture over CoMoS/γAl2O3 with MoS2 nanosheets at low temperatures. Environ Sci Technol 2015;50:1056–64. [12] Yang Y, Liu J, Liu F, Wang Z, Miao S. Molecular-level insights into mercury removal mechanism by pyrite. J Hazard Mater 2018;344:104–12. [13] Xu H, Yuan Y, Liao Y, Xie J, Qu Z, Shangguan W, et al. [MoS4]2– cluster bridges in Co–Fe layered double hydroxides for mercury uptake from S-Hg mixed flue gas. Environ Sci Technol 2017;51:10109–16. [14] Li H, Zhu L, Wang J, Li L, Shih K. Development of nano-sulfide sorbent for efficient removal of elemental mercury from coal combustion fuel gas. Environ Sci Technol 2016;50:9551–7. [15] Yang Y, Liu J, Zhang B, Zhao Y, Chen X, Shen F. Experimental and theoretical studies of mercury oxidation over CeO2−WO3/TiO2 catalysts in coal-fired flue gas. Chem Eng J 2017;317:758–65. [16] Zhou J, Hou W, Qi P, Gao X, Luo Z, Cen K. CeO2–TiO2 sorbents for the removal of elemental mercury from syngas. Environ Sci Technol 2019;47:10056–62. [17] Zhang X, Dong Y, Cui L, An D, Feng Y. Removal of elemental mercury from coal pyrolysis gas using Fe–Ce oxides supported on lignite semi-coke modified by the hydrothermal impregnation method. Energy Fuels 2018;32:12861–70. [18] Zhang Z, Wu J, Li B, Xu H, Liu D. Removal of elemental mercury from simulated flue gas by ZSM-5 modified with Mn-Fe mixed oxides. Chem Eng J 2019;375:121946. [19] Wang Z, Liu J, Yang Y, Yu Y, Yan X, Zhang Z. Insights into the catalytic behavior of LaMnO3 perovskite for Hg0 oxidation by HCl. J Hazard Mater

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