Mechanistic studies of mercury adsorption and oxidation by oxygen over spinel-type MnFe2O4

Journal of Hazardous Materials 321 (2017) 154–161

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Mechanistic studies of mercury adsorption and oxidation by oxygen over spinel-type MnFe2 O4 Yingju Yang a , Jing Liu a,b,∗ , Bingkai Zhang a , Feng Liu a a b

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China Shenzhen Institute of Huazhong University of Science and Technology, Shenzhen 518000, China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Hg adsorption and oxidation mechanisms on MnFe2 O4 were studied using DFT method. • Hg0 adsorption on Mn-terminated MnFe2 O4 (100) surface is a chemisorption process. • HgO shows high chemical reactivity for its adsorption on MnFe2 O4 surface. • The reaction between adsorbed Hg and surface oxygen is the ratedetermining step.

a r t i c l e

i n f o

Article history: Received 14 June 2016 Received in revised form 1 September 2016 Accepted 3 September 2016 Available online 4 September 2016 Keywords: Mercury adsorption Oxidation mechanism MnFe2 O4 Density functional theory

a b s t r a c t MnFe2 O4 has been regarded as a very promising sorbent for mercury emission control in coal-fired power plants because of its high adsorption capacity, magnetic, recyclable and regenerable properties. First-principle calculations based on density functional theory (DFT) were used to elucidate the mercury adsorption and oxidation mechanisms on MnFe2 O4 surface. DFT calculations show that Mn-terminated MnFe2 O4 (1 0 0) surface is much more stable than Fe-terminated surface. Hg0 is physically adsorbed on Fe-terminated MnFe2 O4 (1 0 0) surface. Hg0 adsorption on Mn-terminated MnFe2 O4 (1 0 0) surface is a chemisorption process. The partial density of states (PDOS) analysis indicates that Hg atom interacts strongly with surface Mn atoms through the orbital hybridization. HgO is adsorbed on the MnFe2 O4 surface in a chemical adsorption manner. The small HOMO–LUMO energy gap implies that HgO molecular shows high chemical reactivity for HgO adsorption on MnFe2 O4 surface. The energy barriers of Hg0 oxidation by oxygen on Fe- and Mn-terminated MnFe2 O4 surfaces are 206.37 and 76.07 kJ/mol, respectively. Mn-terminated surface is much more favorable for Hg0 oxidation than Fe-terminated surface. In the whole Hg0 oxidation process, the reaction between adsorbed mercury and surface oxygen is the rate-determining step. © 2016 Published by Elsevier B.V.

∗ Corresponding author at: State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. E-mail address: [email protected] (J. Liu). http://dx.doi.org/10.1016/j.jhazmat.2016.09.007 0304-3894/© 2016 Published by Elsevier B.V.

Y. Yang et al. / Journal of Hazardous Materials 321 (2017) 154–161

1. Introduction Mercury (Hg), a global hazardous atmospheric pollutant with high toxicity and volatility, has attracted considerable public concern due to its harmful effects on human health and ecosystem [1,2]. Coal-fired power plants are regarded as the largest anthropogenic source of mercury emissions in China [3]. Mercury species in coal-fired flue gas includes elementary mercury (Hg0 ), oxidized mercury (Hg2+ ) and particulate bound mercury (Hgp ). Hg0 is the major mercury species released into atmosphere, because it is very difficult to capture Hg0 using the existing air pollution control devices (APCD) due to its water insolubility and chemical inertness [1,4]. Therefore, one of the largest challenges of mercury emission control in coal-fired power plants is the Hg0 capture. In recent years, many researchers engaged in mercury emission control have developed several technologies for Hg0 capture, such as catalytic oxidation [5–7], bromine addition [8–10], sorbent injection [11–13] and so forth. In the catalytic oxidation and bromine addition technologies, Hg0 can be oxidized into Hg2+ and subsequently captured by the wet flue gas desulfurization (WFGD) system. However, Hg2+ in desulfurization solutions can be reduced and then released into atmosphere [14], which causes the secondary pollution. The traditional activated carbon injection technology is restricted by the high operating cost, sorbent recovery and reuse, and secondary pollution caused by the disposition of mercury adsorbed on spent sorbents. To date, the novel magnetic sorbent injection has been considered to be a promising and cost-effective mercury control technology, because the magnetic sorbent can be recovered from fly ash for reuse using magnetic separation [15,16]. In addition, the mercury within spent sorbent can be recovered during the thermal disposition process of sorbent regeneration [17]. Among the magnetic sorbents, spineltype transition-metal oxides with a general formula of AB2 O4 have received considerable attention because of its excellent catalytic properties [15–18]. Spinel ferrite (XFe2 O4 ) is a kind of low-cost magnetic materials and widely used for the removal of gaseous pollutants [15,16,18–21]. Mn is an element with excellent low-temperature catalytic activity [22,23], and can be used to replace the cation X2+ at the tetrahedral site in XFe2 O4 to enhance the catalytic activity of iron spinel. Yang et al. [15,16] synthesized a series of magnetic (Fe3 − x Mnx )1 − ␦ O4 ferrite nanoparticles with spinel structures for gaseous Hg0 capture from simulated flue gas. They found that MnFe spinel showed high mercury capture capacity at 100–300 ◦ C. The mercury adsorption and oxidation mechanisms over Mn-Fe spinel are closely related to the high mercury capture capacity. Moreover, the fundamental understanding of the comprehensive mechanism can serve as a basis for exploring the chemical reaction of mercury oxidation over Mn-Fe spinel. However, to date, limited knowledge is available for the understanding of mercury adsorption and oxidation mechanisms over manganese ferrite surface. Therefore, mechanistic studies on the adsorption and oxidation of mercury over Mn-Fe spinel are necessary. In recent years, density functional theory (DFT) calculation, a computational chemistry method, has been widely used to understand the heterogeneous mechanisms of mercury adsorption and oxidation over different sorbent/catalyst surfaces, such as V2 O5 /TiO2 [24,25], CaO [26], Pd alloys [27,28], MnO2 [22,23], CeO2 [29], and so forth. In this study, DFT calculations were carried out to investigate the mechanisms associated with mercury adsorption and oxidation over spinel-type MnFe2 O4 . The adsorption mechanism of different mercury species on MnFe2 O4 (1 0 0) surface was proposed based on the adsorption energies, Milliken population, molecular orbital and electronic structure analysis. Meanwhile, the energy barrier and reaction heat of Hg0 oxidation by oxygen over MnFe2 O4 (1 0 0) surface were calculated. To the best of the authors’

155

knowledge, this is the first theoretical study for mercury adsorption and oxidation over MnFe2 O4 surface. 2. Computational details 2.1. Computational methods All calculations based on the spin-polarized DFT method were performed using DMol3 code [30]. The exchange-correlation potential was calculated using Perdew-Burke-Ernzerhoff (PBE) [31] functional in the generalized gradient approximation (GGA) scheme [32]. The core electrons of Mn, Fe and Hg were treated using the effective core potentials (ECP) method [33] in which the relativistic effect of core electrons was replaced with a simple potential. The core electrons of O atom were treated in the same manner as valence electrons. A double numerical basis set with a polarization p-function on all hydrogen atoms was confined within a global orbital cutoff value of 4.7 Å during calculations. The Monkhorst-Pack grid [34] k-points (4 × 4 × 4) were used to provide cell parameters. MnFe2 O4 unit cell optimization is performed until the atomic forces, maximum displacement, and total energy variation are less than 2.0 × 10−3 Hartree/Å, 5.0 × 10−3 Å, and 1.0 × 10−5 Hartree, respectively. It was reported that MnFe2 O4 generally crystallizes in the solid phase mainly consisting of normal spinel structure [35]. Therefore, the normal spinel was used to investigate mercury adsorption and oxidation in this study. The spinel cubic crystal structure of MnFe2 O4 is presented in Fig. 1(a). It can be seen that divalent Mn2+ and trivalent Fe3+ cations are located at tetrahedral and octahedral sites, respectively. Mn2+ and Fe3+ cations with unpaired electrons exhibit high-spin state [35]. The spin directions of Mn2+ and Fe3+ cations were set as up and down, respectively. The optimized bulk lattice parameters (a = b = c = 8.550 Å) are in good agreement with the experimentally reported value (a = b = c = 8.511 Å) [35]. The deviation is found to be 0.465%, which indicates that the calculations are reliable. The adsorption energy (Eads ) is defined as follows: E ads = E (MnFe2O4 − adsorbate) − (E MnFe2O4 + E adsorbate )

(1)

where E(MnFe2O4 − adsorbate) , EMnFe2O4 , and Eadsorbate represent the total energy of MnFe2 O4 surface adsorbing adsorbate, the total energy of MnFe2 O4 surface, and the total energy of gas-phase adsorbate, respectively. According to Eq. (1), the more negative the adsorption energy is, the stronger the interaction between adsorbate and MnFe2 O4 surface is. Hg0 oxidation occurs through different reaction pathways including intermediate (IM), transition state (TS) and final state (FS). All transition states along the different mercury oxidation reaction pathways were searched using the linear synchronous transit/quadratic synchronous transit (LST/QST) method [36]. After a successful search for transition state, the obtained transition state was verified using vibrational frequency calculation. A true transition state has an imaginary vibrational frequency with a normal mode corresponding to the reaction coordinate. The energy difference between transition state and intermediate is known as the energy barrier of mercury oxidation process. According, the energy barrier (Ebarrier ) can be calculated according to the following equation: E barrier = E (transitio state) − E intermediate

(2)

where E(transitionstate) and Eintermediate denote the total energy of transition state and intermediate, respectively. 2.2. Model development MnFe2 O4 (1 0 0) surface, a typical low-index surface, was used to investigate the adsorption behavior of different mercury species

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Fig. 1. Slab model of MnFe2 O4 (1 0 0) surface. (a) Crystal structure of MnFe2 O4 . (b) Mn-terminated MnFe2 O4 (1 0 0) surface with seven different adsorption sites. (c) Feterminated MnFe2 O4 (1 0 0) surface. a, b, c, d, e, f, and g represent the adsorption sites of Mn2 , bridge, O4 , hollow, Fe, O3 , and Mn4 , respectively. Mn2 and Mn4 denote 2-fold and 4-fold coordinated Mn, respectively. O4 and O3 denote 4-fold and 3-fold coordinated O, respectively.

on MnFe2 O4 surface. MnFe2 O4 (1 0 0) surface was constructed by cleaving the optimized unit cell. MnFe2 O4 (1 0 0) surface includes Mn-terminated and Fe-terminated surfaces, as shown in Fig. 1. Mn-terminated surface includes eight exposed Fe cations, three exposed Mn cations, and ten lattice O anions. Fe-terminated surface involves four exposed Fe cations, six exposed Mn cations, and twelve lattice O anions. MnFe2 O4 (1 0 0) surface is modeled by a periodically repeated p(2 × 1) slab with nine atomic layers and separated by a 12 Å-thick vacuum region to avoid the interactions between two periodic slabs. The bottom five layers were fixed in their original bulk positions and the top four layers were fully relaxed for structure optimization. Mercury in coal fired flue gas has a lower concentration (typically less than 10 ppbv) [4]. Thus, the interaction between neighboring mercury atoms can be neglected. The energy effect of neighboring Hg atoms was tested by comparing the adsorption energies of Hg on p(2 × 1) and p(2 × 2) surfaces. The surface coverages of p(2 × 1) and p(2 × 2) surfaces are 1/2 and 1/4 ML, respectively. The results indicate that the geometric parameters of Hg adsorption on p(2 × 2) surface are similar to that of Hg adsorption on p(2 × 1) surface. The adsorption energy increases slightly with decreasing the surface coverage from 1/2 to 1/4 ML (difference less than 5%). Therefore, p(2 × 1) surface is used to simulate the MnFe2 O4 surface for mercury adsorption. 3. Results and discussion 3.1. Hg0 adsorption on MnFe2 O4 (1 0 0) surface Hg0 adsorption on the surface is the first step toward heterogeneous mercury oxidation over MnFe2 O4 . Therefore, Hg0 adsorption on MnFe2 O4 (1 0 0) surface was first studied. Two terminated surfaces (Fe and Mn termination) and all possible adsorption sites (Mn2 , Mn4 , bridge, O3 , O4 , Fe, and hollow) were considered for Hg0 adsorption on MnFe2 O4 (1 0 0) surface, as shown in Fig. 1. The calculated total energy of Mn-terminated MnFe2 O4 (1 0 0) surface is lower than that of Fe-terminated surface. Thus, Mn-terminated MnFe2 O4 (1 0 0) surface is much more stable than Fe-terminated surface. The stable configurations of Hg0 adsorption on MnFe2 O4 (1 0 0) surface are shown in Fig. 2. The corresponding adsorption ener-

Table 1 The adsorption energies, geometric parameters and Mulliken charge for Hg0 adsorption on Fe- and Mn-terminated MnFe2 O4 (1 0 0) surfaces. Surfaces

Configurations

Eads (kJ/mol)

RX-Hg (Å)

QHg (e)

Fe termination

1A 1B

−29.72 −15.33

3.029 2.987

0.049 0.063

Mn termination

1C 1D

−55.63 −60.82

3.018 2.955

0.063 0.067

X denotes surface atom.

gies, geometric parameters and Mulliken charge of Hg0 adsorption are summarized in Table 1. All the adsorption energies are negative values, which indicates that Hg0 adsorption on MnFe2 O4 (1 0 0) surface is an exothermic process with exothermicity of −15.33 to −60.82 kJ/mol. Hg0 adsorption on Fe-terminated MnFe2 O4 (1 0 0) surface yields relatively lower adsorption energies, i.e. −29.72 and −15.33 kJ/mol. Meanwhile, the distance between Hg and surface atoms (Fe and O) ranges from 2.987 to 3.029 Å. About 0.049 and 0.063 e charges of Hg atom are transferred to the surface in the configurations of 1A and 1B, respectively. Therefore, Hg0 is physically adsorbed on Fe-terminated MnFe2 O4 (1 0 0) surface. Hg0 adsorption on Mn-terminated MnFe2 O4 (1 0 0) surface was also investigated, as shown in Fig. 2. The calculated mercury adsorption energies of 1C and 1D are −55.63 and −60.82 kJ/mol, respectively. In addition, 0.063 and 0.067 e charges are transferred from Hg0 to Mn-terminated surface. It can be concluded that the interaction between Hg0 and Mn-terminated MnFe2 O4 (1 0 0) surface is a chemisorption behavior. Experimental results indicated that an increase in reaction temperature (100–250 ◦ C) can enhance mercury capture capacity of (Fe3 − x Mnx )1 − ␦ O4 spinel and the chemisorption mechanism is responsible for the mercury adsorption behavior [16]. Therefore, the calculation results are in agreement with experimental results. Compared to Fe-terminated MnFe2 O4 (1 0 0) surface, Hg0 prefers to adsorb on Mn-terminated surface because of the higher adsorption energy. To further investigate the adsorption mechanism of Hg0 on Mn-terminated MnFe2 O4 (1 0 0) surface, the partial density of states (PDOS) analysis of surface atoms in configuration 1D was performed, as shown in Fig. 3. For Hg0 pre-adsorption, the

Y. Yang et al. / Journal of Hazardous Materials 321 (2017) 154–161

157

Fig. 2. Adsorption configurations of Hg0 on Fe- (1A and B) and Mn- (1C and D) terminated MnFe2 O4 (1 0 0) surfaces.

120

(a)

PDOS (electrons/Hartree)

80

Table 2 The adsorption energies and geometric parameters for HgO adsorption on Fe- and Mn-terminated MnFe2 O4 (1 0 0) surfaces.

0.04

-0.13 Mn-s orbital Mn-p orbital Mn-d orbital

40

-0.19

-0.69 0 -0.8 600

-0.6

-0.4

(b)

400

-0.2

0.0

0.4

-0.11

0.6

0.8

0.59

Hg-s orbital Hg-p orbital Hg-d orbital

200

0.2

0.53 -0.6

-0.4

-0.2 0.0 0.2 Energy (Hartree)

0.4

0.6

0.8

120 -0.24

(c) PDOS (electrons/Hartree)

80

0.05

-0.07

Mn-s orbital Mn-p orbital Mn-d orbital

40 0 -0.8 600

-0.6

-0.4

-0.2

0.0

0.2

0.0

0.2

(d)

400

Hg-s orbital Hg-p orbital Hg-d orbital

200 0 -0.8

-0.6

-0.4 -0.2 Energy (Hartree)

Configurations Eads (kJ/mol) RX-Hg (Å)

RO-Hg (Å) RX-O (Å)

Fe termination

2A 2B 2C

−170.74 −422.46 −75.09

3.685 1.979 2.889 2.757 2.913/2.885 2.016

– 1.655 –

Mn termination 2D 2E 2F

−405.13 −388.92 −154.25

2.812 – 2.164

1.690 1.644 –

2.603 2.951 1.947

X denotes surface atom.

0.27

0 -0.8

Surfaces

Fig. 3. PDOS of Hg0 pre-adsorption and post-adsorption on Mn-terminated MnFe2 O4 (1 0 0) surface. (a) PDOS of Mn atom for Hg0 pre-adsorption. (b) PDOS of Hg atom for Hg0 pre-adsorption. (c) PDOS of Mn atom for Hg0 post-adsorption. (d) PDOS of Hg atom for Hg0 post-adsorption. The Fermi level (Ef ) is set to 0 eV (dashed line in figures).

energy bands of Mn2+ are located between −0.3 and 0.1 hartree. Hg s- and p-orbitals show single peaks at 0.53 and 0.27 hartree, respectively. In addition, Hg s-orbital centered at the Fermi level is also occupied. The energy band of Hg d-orbital is located at −0.11 and

0.59 hartree. For Hg0 post-adsorption on MnFe2 O4 (1 0 0) surface, all the orbitals of Hg atom are shifted to the lower energy level. Meanwhile, Hg d-orbital located at 0.59 hartree disappears thoroughly after Hg adsorption. All orbitals of surface Mn atom shows no apparent changes compared to Hg0 pre-adsorption, because the diameter of Mn atom is smaller than that of Hg0 so that the electronic orbitals of Mn are difficult to be polarized. The higher shift of Hg electronic orbitals and the inapparent changes of Mn electronic orbitals indicate that the higher adsorption capacity of MnFe2 O4 can maintain the stability of adsorbed mercury [37]. This is consistent with the previous experimental results in which mercury species stably adsorbed on MnFe2 O4 surface and were detected using X-ray photoelectron spectroscopy [16]. In addition, Hg pand d-orbitals are hybridized with Mn s- and d-orbitals at −0.24, −0.07 and 0.05 hartree. An overlap of Hg s-orbital and Mn d-orbital between −0.17 and −0.08 hartree shows that Hg atom significantly interacts with surface Mn atom. The strong interaction between Hg and Mn atoms is favorable for Hg0 adsorption on MnFe2 O4 surface, which can explain why MnFe2 O4 spinel has a higher mercury adsorption capacity. 3.2. HgO adsorption on MnFe2 O4 (1 0 0) surface HgO can be formed and adsorbed on sorbent/catalyst surface. The adsorption behavior of HgO on MnFe2 O4 was also investigated. All possible adsorption orientations (parallel and perpendicular) were considered for HgO adsorption on MnFe2 O4 (1 0 0) surface. The stable structures of HgO adsorption on MnFe2 O4 (1 0 0) surface are presented in Fig. 4. The adsorption energies of HgO molecule and the corresponding geometric parameters are given in Table 2. For Fe-terminated surface, the most stable configuration is 2B with an adsorption energy of −422.46 kJ/mol. In 2B, HgO is adsorbed on

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Fig. 4. Adsorption configurations of HgO on Fe- (2A–C) and Mn- (2D–F) terminated MnFe2 O4 (1 0 0) surfaces.

the Fe-terminated surface in a dissociative way, resulting in the formation of Fe-Hg bond with a distance of 2.889 Å and Fe-O bond with a distance of 1.655 Å. In addition, HgO can also be molecularly adsorbed on MnFe2 O4 surface, as shown in 2A and 2C. In 2A, HgO molecular is adsorbed on surface O atom, forming a new bridging Hg-O bond with a distance of 2.133 Å. The corresponding adsorption energy is −170.74 kJ/mol. In 2C, HgO is adsorbed on two Fe atoms without dissociation. The Fe-Hg bonds with distance of 2.913 and 2.885 Å are formed. Configuration 2C yields a relatively lower adsorption energy of −75.09 kJ/mol compared to 2A and 2B. Based on the above analysis, it can be concluded that HgO is chemically adsorbed on Fe-terminated MnFe2 O4 (1 0 0) surface. For HgO adsorption on Mn-terminated MnFe2 O4 (1 0 0) surface, the most stable configuration is 2D with an adsorption energy of −405.13 kJ/mol. In 2D, HgO is dissociatively adsorbed on Mnterminated surface by forming Mn-O and Mn-Hg bonds. Hg-O bond is thoroughly broken after HgO adsorption on Mn-terminated surface. In 2E, HgO is adsorbed on MnFe2 O4 surface in a dissociative adsorption manner. The adsorption energy of 2E is −388.92 kJ/mol. In 2F, HgO is molecularly adsorbed on O site with Hg atom toward surface O atom, and the adsorption energy is −154.25 kJ/mol. Mnterminated surface is more favorable for HgO adsorption than Fe-terminated surface. The highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gap can be used as an index for predicting the chemical reactivity and hardness of a molecular [38]. The HOMO–LUMO energy gap of different gas molecules has been predicted using DFT, and used to understand the interaction between gas molecule and solid surface [39,40]. In this study, to further understand the interaction between HgO molecular and MnFe2 O4 surface, the HOMO and LUMO energy levels of HgO were calculated. Fig. 5 presents the calculated molecular energy levels of HgO. The HOMO and LUMO are mainly located on O and Hg atoms, respectively. The energy levels of the HOMO and LUMO are −5.246 and 0.059 eV, respectively. Hg atom in HgO molecular can easily offer electrons to occupy d orbitals of surface metal atoms dur-

Fig. 5. Electronic density distribution of HgO molecular orbital.

ing HgO adsorption, because the HOMO energy level is relatively higher. O atom can accept electrons from MnFe2 O4 surface due to the lower LUMO energy level. In addition, the HOMO–LUMO energy gap of HgO molecular is 5.305 eV. A small HOMO–LUMO energy gap (5.305 eV) implies that HgO molecular shows high chemical reactivity for HgO adsorption on MnFe2 O4 surface, because the high-lying LUMO can energetically extract electrons from the lowlying HOMO. Therefore, HgO can be easily adsorbed on MnFe2 O4 surface, which is consistent with the experimental observations [16] in which the formed oxidized mercury in flue gas was not detected at the exit of reactor and mainly adsorbed on manganese ferrite spinel. 3.3. Hg0 oxidation by oxygen on MnFe2 O4 surface It was reported that Hg0 can be oxidized through the reaction between Hg0 and surface oxygen on catalyst/sorbent surface [5]. The presence of O2 can regenerate and replenish the consumed surface oxygen during mercury oxidation process. The surface oxygen plays a critical role in oxidation reaction over catalyst/sorbent

Y. Yang et al. / Journal of Hazardous Materials 321 (2017) 154–161

Relative energy (kJ/mol)

(a)

200

TS1 178.34

150 FS1

100

96.80 50 0

Hg + O-MnFe2O4 0

Energy barrier (206.37 kJ/mol)

IM1 -28.03

-50 0

Hg adsorption

0

Hg oxidation

Reaction pathway

(b)

0

0 Relative energy (kJ/mol)

surface [41]. Therefore, Hg0 oxidation by oxygen on MnFe2 O4 surface mainly occurs through the reaction between Hg0 and surface oxygen. Two different kinds of terminated surfaces (Fe and Mn termination) were considered for investigating mercury oxidation on MnFe2 O4 . Gas-phase Hg0 is first adsorbed on MnFe2 O4 surface and subsequently reacts with surface oxygen to form HgO. According to the configurations of Hg0 and HgO adsorption on MnFe2 O4 surface, the configurations of intermediate and final state could be determined. Reaction pathways and relative energy for mercury oxidation by surface oxygen over MnFe2 O4 surface are shown in Fig. 6. The corresponding optimized configurations of intermediate (IM), transition state (TS) and final state (FS) are shown in Fig. 7. As shown in Fig. 6(a), Hg0 adsorption on Fe-terminated surface is the first step of Hg0 oxidation and is barrierless for IM1 formation. The first step is an exothermic reaction with exothermicity of −28.03 kJ/mol. In IM1, the surface oxygen is adsorbed on Fe atom with a bond length of 1.654 Å. As mentioned above, Hg atom is physically adsorbed on Fe-terminated MnFe2 O4 surface. The distance between Hg and O atoms is 2.740 Å which is larger than the Hg-O bond length (2.049 Å) of gas-phase HgO molecular. In the second step, the adsorbed mercury is oxidized by surface oxygen into HgO through transition state TS1. The distance between Hg and O atoms decreases gradually: 2.740 Å (IM1) → 2.399 Å (TS1) → 1.979 Å (FS1). In TS1, the Hg-O bond is formed. The energy barrier of this reaction step is 206.37 kJ/mol. In addition, the reaction between adsorbed mercury and surface oxygen is an endothermic process with a reaction heat of 124.83 kJ/mol. In the whole Hg0 oxidation process, the second step (reaction between adsorbed mercury and surface oxygen) is the rate-determining step. Fig. 6(b) presents the energy profile of Hg0 oxidation by oxygen on Mn-terminated surface. Hg0 is first adsorbed on the surface Mn atom, which leads to the formation of IM2. In IM2, the Hg-Mn and OMn bond lengths are 2.866 and 1.678 Å, respectively. The distance between Hg and O atoms is 2.688 Å which is larger than the Hg-O bond length of gas-phase HgO molecular. Hg0 adsorption on Mnterminated surface is an exothermic reaction with an exothermicity of −152.39 kJ/mol. Furthermore, Hg0 adsorption step is barrierless and can occur spontaneously. Subsequently, the oxidation reac-

159

Hg + O-MnFe2O4

-40 FS2

TS2 -76.32

-80

-73.27

-120 Energy barrier (76.07 kJ/mol)

IM2 -160

-152.39 0

Hg adsorption

0

Hg oxidation

Reaction pathway Fig. 6. The energy profile of Hg0 oxidation by oxygen on MnFe2 O4 (1 0 0) surfaces. (a) Fe-terminated surface. (b) Mn-terminated surface.

tion between adsorbed mercury and surface oxygen occurs through TS2. Hg atom strips an oxygen atom from the surface, resulting in a breakage of Mn-O bond. The distance between Hg and O atoms

Fig. 7. The optimized configurations of intermediates, transition states and final states of Hg0 oxidation by oxygen over Fe- (IM1, TS1 and FS1) and Mn- (IM2, TS2 and FS2) terminated MnFe2 O4 (1 0 0) surfaces.

160

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decreases gradually: 2.688 Å (IM2) → 2.153 Å (TS2) → 2.084 Å (FS2), indicating the formation of Hg O bond. In TS2, HgO molecular is formed and adsorbed on surface Mn atom with a Mn-Hg bond length of 2.583 Å. The energy barrier of mercury oxidation reaction on Mn-terminated surface is 76.07 kJ/mol. The corresponding reaction heat of this reaction step is 79.12 kJ/mol. The reaction between adsorbed mercury and surface oxygen is the rate-determining step for mercury oxidation on Mn-terminated surface, because the Hg0 adsorption step is barrierless. This indicates that adsorption is relatively easy compared to oxidation. In addition, it was reported that mercury removal by MnFe2 O4 sorbent mainly occurs in the adsorption manner [16]. Therefore, it is expected that the percentage of adsorption is much higher than that of oxidation. The energy barrier of mercury oxidation by oxygen on Mn-terminated MnFe2 O4 (1 0 0) surface is much lower than that on Fe-terminated MnFe2 O4 (1 0 0) surface. Thus, Mn-terminated surface is much more favorable for Hg0 oxidation than Fe-terminated surface. As mentioned above, Mn-terminated MnFe2 O4 (1 0 0) surface is much more stable than Fe-terminated surface. MnFe2 O4 sorbent easily exposes Mn-terminated surface during the practical application. Therefore, MnFe2 O4 sorbent shows lower energy barrier for mercury oxidation by oxygen. MnFe2 O4 is a promising material for mercury removal in low-rank coal-fired flue gas with low HCl concentration. 4. Conclusions Mercury adsorption and oxidation mechanisms on MnFe2 O4 surface were investigated using density functional theory calculations. Hg0 adsorption on Fe-terminated MnFe2 O4 (1 0 0) surface is attributed to physisorption mechanism. The chemisorption mechanism is responsible for Hg0 adsorption on Mn-terminated MnFe2 O4 (1 0 0) surface. Hg atom significantly interacts with surface Mn atom during Hg0 adsorption on Mn-terminated MnFe2 O4 (1 0 0) surface. HgO is adsorbed on the MnFe2 O4 (1 0 0) surface in a chemical adsorption manner. HgO molecular shows high chemical reactivity for HgO adsorption on MnFe2 O4 surface because of a small HOMOLUMO energy gap of 5.305 eV. Mn-terminated surface is much more favorable for Hg0 oxidation by oxygen than Fe-terminated surface. In the whole Hg0 oxidation process, the reaction between adsorbed mercury and surface oxygen is the rate-determining step. Acknowledgments This work was supported by National Natural Science Foundation of China (51376072), National Basic Research Program of China (2014CB238904), National Key Research and Development Program (2016YFB0600604), and Basic Research Project of Shenzhen (JCYJ20150831202633340). References [1] J. Liu, W. Qu, J. Yuan, S. Wang, J. Qiu, C. Zheng, Theoretical studies of properties and reactions involving mercury species present in combustion flue gases, Energy Fuels 24 (2010) 117–122. [2] J. Liu, S. Abanades, D. Gauthier, G. Flamant, C. Zheng, J. Lu, Determination of kinetic law for toxic metals release during thermal treatment of model waste in a fluid-bed reactor, Environ. Sci. Technol. 39 (2005) 9331–9336. [3] J. Liu, Q. Falcoz, D. Gauthier, G. Flamant, C. Zheng, Volatilization behavior of Cd and Zn based on continuous emission measurement of flue gas from laboratory-scale coal combustion, Chemosphere 80 (2010) 241–247. [4] Y. Yang, J. Liu, F. Shen, L. Zhao, Z. Wang, Y. Long, Kinetic study of heterogeneous mercury oxidation by HCl on fly ash surface in coal-fired flue gas, Combust. Flame 168 (2016) 1–9. [5] H. Li, C.Y. Wu, Y. Li, J. Zhang, CeO2 –TiO2 catalysts for catalytic oxidation of elemental mercury in low-rank coal combustion flue gas, Environ. Sci. Technol. 45 (2011) 7394–7400. [6] B. Zhang, J. Liu, J. Zhang, C. Zheng, M. Chang, Mercury oxidation mechanism on Pd (100) surface from first-principles calculations, Chem. Eng. J. 237 (2014) 344–351.

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