Nature of Active Sites and Oxygen-Assisted Reaction Mechanism for

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Nature of Active Sites and Oxygen-Assisted Reaction Mechanism for Mercury Capture by Spinel-Type CuMn2O4 Sorbent Yingju Yang, Jing Liu, Zhen Wang, Zhen Zhang, Junyan Ding, and Yingni Yu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01696 • Publication Date (Web): 04 Aug 2019 Downloaded from pubs.acs.org on August 7, 2019

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Nature of Active Sites and Oxygen-Assisted Reaction Mechanism for Mercury Capture by Spinel-Type CuMn2O4 Sorbent Yingju Yang, Jing Liu*, Zhen Wang, Zhen Zhang, Junyan Ding, and Yingni Yu

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

ABSTRACT: CuMn2O4 spinel has been experimentally demonstrated to be a kind of promising sorbents for Hg0 capture from flue gas due to its excellent adsorption performance, regenerability, and recyclability. Theoretical studies based on the state-of-the-art density functional theory (DFT) were performed to gain an understanding of several important aspects of Hg0 removal by CuMn2O4 sorbent, including active sites and reaction mechanism. DFT calculation results show that Hg0 and HgO adsorption on CuMn2O4 surface are dominated by chemisorption mechanism. The stronger interaction between Hg0 and CuMn2O4 surface is closely associated with the orbital hybridization between Hg atom and surface metal atoms (Cu and Mn). CuMn2O4 shows excellent O2-activation ability due to the lower energy barrier. O2 dissociation reaction on CuMn2O4 surface is activated by 6.31 kJ/mol, and is exothermic by 101.37 kJ/mol. Chemisorbed oxygen atom produced from molecular O2 dissociation reacts with adsorbed Hg0 to form HgO species. O2* species can also directly react with adsorbed Hg0. The most favorable pathway for gaseous HgO formation is described by a three-step process: Hg0 adsorption, Hg0 oxidation, and HgO desorption. In the comprehensive mercury adsorption-oxidation-desorption process, HgO desorption is predicted to be *

Corresponding author. Tel.: +86 27 87545526; fax: +86 27 87545526. E-mail address: [email protected] (J. Liu). 1

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rate-limiting.

1. INTRODUCTION Mercury (Hg) emitted from natural and anthropogenic sources is a globally pervasive contaminant that causes serious human and ecosystem health problems.1,

2

The Global Mercury

Assessment 2018 of United Nations Environment Programme found that globally anthropogenic mercury emissions increase by 20%.3 To reduce global mercury emissions, the Minamata Convention on Mercury entered into force in August 2017. Stationary combustion of fossil fuels (especially coal combustion) is one of the main sources of mercury emission,3 and accounts for 24% of total anthropogenic mercury emissions.4 Therefore, more international efforts should be put forth to reduce mercury emission from coal combustion. According to the physiochemical properties of different mercury species in flue gas, mercury emission control technology is mainly classified into three groups: sorbent injection,5-8 catalytic oxidation,9-11 and halogen addition.12-14 For the catalytic oxidation and halogen addition technologies, elemental mercury (Hg0) can be oxidized into oxidized mercury (Hg2+) in flue gas. Hg2+ can be effectively captured by the wet flue gas desulfurization (WFGD) system.15-17 However, the secondary pollution caused by Hg2+ reduction in desulfurization solution limits the wide application of the two technologies.18, 19 For the sorbent injection technology, Hg0 and Hg2+ in flue gas are adsorbed on sorbent surface to form particulate bound mercury (Hgp).20-22 Subsequently, Hg-laden sorbents can be efficiently collected by the particulate control devices (such as electrostatic precipitators and fabric filters).23-25 Mercury within used sorbents can be recovered for centralized control during the regeneration process of sorbent.26 Therefore, sorbent injection is regarded as the 2


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most promising technology for mercury removal from flue gas.27-29 Adsorbent material has been considered to be a determining factor for the successful operation of sorbent injection technology. Even though numerous sorbents have been reported to capture mercury from flue gas,27 the poor practicabilities (such as high cost, poor regenerability and recyclability, and adverse effects on fly ash utilization) restrict heavily their engineering applications. Therefore, more cost-efficient non-carbon sorbents with better regenerability and recyclability are highly desirable for the engineering application. Spinel-type CuMn2O4 with distinguishing feature has taken significant role in catalysis field, such as pollutants removal,30 steam reforming,31 fuel cells,32, 33 and chemical looping combustion.34 Binary manganese oxide CuMn2O4 mainly consists of the earth-abundant copper and manganese elements. As a consequence, the cost of CuMn2O4 synthesis can be significantly decreased due to the huge geological reserve. Recently, spinel-type CuxMn3-xO4 particles were synthesized using the low-temperature sol-gel auto-combustion synthesis method in our previous work,35 and were used to capture Hg0 from flue gas. It was found that the stoichiometric CuMn2O4 particles show superior performance for Hg0 removal in a wide temperature window (50–350 °C). Moreover, CuMn2O4 sorbent exhibited excellent recyclable and regenerable properties with the assistance of oxygen. The superior mercury removal performance of CuMn2O4 sorbent is heavily dependent on the availability and structure of active sites. Meanwhile, the excellent regenerability and recyclability is determined by the interaction intensity of mercury species with active sites. Moreover, Hg0 adsorption on CuMn2O4 surface is controlled by the reaction mechanism. However, there is currently a lack of fundamental knowledge available for the nature of active sites. In addition, the oxygen-assisted reaction mechanism of Hg0 adsorption on CuMn2O4 surface has not yet been understood at the 3


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atomic level. In the present study, periodic density functional theory (DFT) calculations were first conducted to uncover the structure of active sites during Hg0 removal by CuMn2O4 sorbent. The nature of the interaction between Hg0 and active sites were understood by the electronic structure analysis. Molecular O2 activation on CuMn2O4 surface was investigated to explore the formation process of chemisorbed oxygen. Finally, an oxygen-assisted reaction mechanism of Hg0 removal by CuMn2O4 sorbent was proposed based on the reaction pathway (including energy barrier and reaction heat) of HgO formation.

2. COMPUTATIONAL DETAILS Based on the spin-polarized DFT method, the theoretical calculations were conducted using DMol3 code.36 The overall description of the electronic subsystem was performed by Perdew-Burke-Ernzerhoff (PBE)37 functional of the generalized gradient approximation (GGA) scheme.38 The double numerical basis set with polarization functions (DNP) was used to expand the molecular orbitals. A soft confinement potential was used to ensure the strict localization of numerical basis set within an orbital cutoff value of 4.7 Å. The orbital cutoff value of calculation system was tested before its utilization. To balance computational expense and accuracy, the orbital cutoff value of 4.7 Å was used in this work. For Cu, Mn and Hg atoms, the relativistic effects became important in the core electrons and were treated using the effective core potentials (ECP) method.39 Spin-polarized calculation was considered for geometry optimization. A Hubbard-like model of the GGA+U method was used to treat the strong 3d electron-electron correlation effect of Mn and Cu atoms. The Hubbard U 4


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parameter of 2.5 eV was used to accurately reproduce the valence spectra of the occupied Mn 3d and Cu 3d levels. To accelerate the convergence of DFT calculations, a thermal smearing value of 0.05 Hartree was applied to define the partial occupancies of electron states. Moreover, the direct inversion of the iterative subspace (DIIS) method was also used to speed up SCF convergence. The dimension of iterative subspace was 8. Geometry optimization and energy calculation were carried out until the atomic forces, maximum displacement, and total energy variation are less than 2.0×10N* Hartree/Å, 5.0×10N* Å, and 1.0×10N? Hartree, respectively. The detailed computational methods have been described in our previous work.40 CuMn2O4 with space group Fd3m is the spinel cubic crystal structure. The tetrahedral and octahedral sites are occupied by divalent Cu2+ and trivalent Mn3+ cations, respectively.40 This indicates that CuMn2O4 sorbent has a normal spinel structure. The calculated unit cell parameters (a = b = c = 8.315 Å) are in good agreement with the experimental values (a = b = c = 8.327 Å),41 suggesting that the DFT calculations in this work are reliable. It is important to establish a precise model for the investigation of mercury adsorption on CuMn2O4 surface. CuMn2O4(100) surface is a typical low-index surface.40 Thus, this low-index surface is used to investigate mercury adsorption over CuMn2O4 surface. The ratio of surface atoms of CuMn2O4 sorbent can be obtained according to our previous XPS analysis results.35 Based on the ratio of surface atoms, the termination of CuMn2O4(100) surface can be further determined. CuMn2O4(100) surface was simulated by the periodic slab with nine atomic layers. A p(2×1) CuMn2O4(100) surface was applied to provide the reaction surface for mercury adsorption and oxidation. The detailed description of CuMn2O4(100) surface has been reported in our previous work.40 The adsorption energy (Eads)42, determined according to the following equation: 5


43

can be

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Eads = E(CuMn2O4–adsorbate) – (ECuMn2O4 + Eadsorbate)

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(1)

where E(CuMn2O4–adsorbate), ECuMn2O4, and Eadsorbate denote the total energies of CuMn2O4/adsorbate system, CuMn2O4 surface, and isolated gaseous adsorbate, respectively. The physisorption energy is less than –30 kJ/mol, whereas the chemisorption energy is higher than –50 kJ/mol.10 Reaction pathway of HgO formation over CuMn2O4 surface involves intermediate (IM), transition state (TS) and final state (FS). The linear synchronous transit/quadratic synchronous transit (LST/QST) method was used to search the transition states of Hg0 oxidation. The nature of the transition states can be confirmed through vibrational frequency calculation. The energy barrier (Ebarrier) of HgO formation is determined using the following equation: Ebarrier = ETS – EIM

(2)

where ETS and EIM represent the total energies of TS and IM, respectively.

3. RESULTS AND DISCUSSION 3.1. Interactions between Mercury Species and CuMn2O4 Surface Hg0 capture by CuMn2O4 spinel is dominated by the interaction between gaseous mercury species (Hg0 and HgO) and sorbent surface. The interactions between different mercury species and CuMn2O4(100) surface were first understood. The stable optimized structures and adsorption energies of Hg0 adsorption on CuMn2O4(100) surface are presented in Figure 1. In 1A, the gas-phase Hg0 is adsorbed on two-fold coordinated surface Cu site to form Cu-Hg amalgam. The bond length of Cu-Hg bond is 2.781 Å. The adsorption energy is N9.)3>) kJ/mol. Our previous experimental results suggested that Cu-Hg amalgam is formed during Hg0 adsorption on CuMn2O4 sorbent

6


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surface.35 It is clear that Cu-Hg amalgam formation can be simultaneously confirmed by experimental and theoretical results. As shown in Figure 1, the most stable configuration of Hg0 adsorption is 1B with the largest adsorption energy of N9*?3+* kJ/mol. The equilibrium distance between Hg atom and surface Mn atom is 3.318 Å. It was reported that the interaction with an adsorption energy higher than N?+ kJ/mol is defined as a chemisportion process.26 Therefore, it can be concluded that Hg0 adsorption on CuMn2O4 surface is dominated by chemisorption mechanism. The experimental results of temperature programmed decomposition desorption (TPDD) indicated that mercury desorption from the spent CuMn2O4 sorbent occurs in the temperature window of 300-500 ºC.35 This behavior is correlated with the chemical adsorption mechanism. Consequently, the DFT calculation results are in good agreement with experimental results. To further validate the Hg0 adsorption capacity of CuMn2O4 sorbent, the Hg0 adsorption energy of CuMn2O4 sorbent is compared with that of other mineral materials such as attapulgite. Attapulgite is a crystalline hydrated magnesium aluminum silicate. The raw attapulgite showed poor Hg0 adsorption capacity,20 indicating that the Hg0 adsorption energy of attapulgite is lower. However, CuMn2O4 sorbent showed excellent Hg0 adsorption capacity,35 which indicates that the Hg0 adsorption energy 5N9*?3+* kJ/mol) of CuMn2O4 sorbent is much higher than that of attapulgite. The partial density of states (PDOS) analysis shows the contribution of each atomic orbital to the interaction between different atoms. In order to further understand the chemical interaction between gas-phase Hg0 and surface atom, the PDOS analysis of surface atoms in the most stable adsorption configuration (1B) was performed to identify the occupation and energy distribution of atomic orbital. Figure 2 shows the PDOS results of Hg, Cu, Mn and O atoms. It can be seen that Hg 7


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d-orbital is hybridized with Cu d- and Mn s-orbitals at N+3.) Hartree. Moreover, the orbital hybridization of Hg atom with Cu and Mn atoms can be further confirmed by the three-dimensional electron density, as shown in Figure 3. The adsorbed Hg atom significantly shares electrons with surface Cu atom (see Figure 3a). Electron sharing between Hg and Mn atoms is also observed in structure 1B (see Figure 3b). The orbital hybridization and electron sharing indicate that Hg atom strongly interacts with surface Cu and Mn atoms during Hg adsorption process. The stronger interaction between Hg and metal (Cu and Mn) atoms is favorable for Hg0 adsorption on CuMn2O4 sorbent. This can explain why CuMn2O4 sorbent exhibits excellent mercury removal efficiency. The stable optimized structures of HgO adsorption on CuMn2O4 surface are presented in Figure 4. The most stable optimized structure is 2B with the largest adsorption energy of N),)389 kJ/mol. HgO molecule is strongly coordinated with surface Cu atom, resulting in the formation of a complex Cu-Hg-O compound. This indicates that HgO molecule can stably exist on CuMn2O4 surface. The bond length of Cu-Hg is 2.651 Å. In 2A, HgO is molecularly adsorbed on surface Mn atom, the equilibrium distance between Hg and Mn atoms is 3.222 Å. The corresponding adsorption energy is N470.54 kJ/mol. The TPDD and XPS results of our previous studies suggested that HgO can be found on the spent sorbent surface.35 Thus, the DFT calculation results are consistent with the TPDD and XPS results.

3.2. O2 Adsorption and Dissociation on CuMn2O4 Surface It was reported that chemisorbed oxygen produced from O2 dissociation plays an important role in HgO formation.44 Moreover, the experimental results suggested that the adsorbed Hg0 can react with chemisorbed oxygen to produce HgO over CuMn2O4 sorbent.35 Therefore, it is necessary to 8


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investigate the adsorption and dissociation processes of O2 on CuMn2O4 surface. The adsorption energies and geometric parameters of O2 adsorption on CuMn2O4(100) surface are shown in Figure 5. The results clearly indicate that the stability of O2 adsorption configurations follows the order of 3B > 3A. In 3A, gas-phase O2 molecule is strongly adsorbed on Mn site, forming a Mn-O covalent bond with a bond length of 2.220 Å. The corresponding adsorption energy is N*+93E, kJ/mol. The most stable adsorption configuration is 3B with an adsorption energy of N*.9388 kJ/mol. In 3B, O2 molecule is simultaneously coordinated with surface Cu and Mn atoms. The bond lengths of resulting Cu-O and Mn-O bonds are 2.063 and 2.153 Å, respectively. In order to further explore the formation process of chemisorbed oxygen, the reaction pathway of O2 molecule dissociation on CuMn2O4 surface was investigated. According to the most stable structure (3B) of O2 adsorption, the intermediate of dissociation reaction pathway can be determined. The reaction pathway, relative energy and optimized structures of O2 dissociation process over CuMn2O4 surface are presented in Figure 6. It can be seen that molecular oxygen dissociates into chemisorbed oxygen (O*) through two reaction pathways (pathway 1 and pathway 2). In pathway 1, the resulting chemisorbed oxygen is adsorbed on Cu and Mn sites (see FS1 structure). In pathway 2, the formed active oxygen atoms only bind to two-fold coordinated Cu atoms (see FS2 structure). In pathway 1, the adsorbed O2 molecule dissociates on CuMn2O4 surface through transition state TS1 to form final state FS1. The total energy of FS1 is much lower than that of 3B, which indicates that O2 dissociation process over CuMn2O4 surface is an exothermic reaction with an exothermicity of N9+93*, kJ/mol. The energy barrier of O2 dissociation via pathway 1 is 6.31 kJ/mol. During this dissociation reaction, the distance between two O atoms of O2 molecule increases gradually: 1.361 Å (3B) T 3.301 Å (TS1) T 5.277Å (FS1). XPS analysis results suggested that 9


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abundant chemisorbed oxygen species exist on CuMn2O4 sorbent surface.35 This can be explained by the lower energy barrier of O2 dissociation. In pathway 2, an O atom migrates from Mn atom to Cu atom, resulting in the breakage of O=O bond. The energy barrier and reaction heat of O2 dissociation via pathway 2 are 13.90 kJ/mol and N,,3,* kJ/mol, respectively. The energy barrier of pathway 2 is comparable to that of pathway 1, which indicates that both pathway 1 and pathway 2 are responsible for O2 dissociation on CuMn2O4 surface. However, the energy 5N,,3,* kJ/mol) released from pathway 2 is lower than the reaction heat 5N9+93*, kJ/mol) of pathway 1. Thus, pathway 1 is much more exothermic than pathway 2. To further understand the O2-activation ability of CuMn2O4, the energy barrier of O2 dissociation on CuMn2O4 surface was compared with that of O2 dissociation over other sorbents (such as MnO2,44 Au/TiO245). MnO2 and Au/TiO2 have been demonstrated to be active for O2 activation.44,

45

The energy barriers of O2 dissociation on MnO2 and Au/TiO2 surfaces are 97.46

kJ/mol and 48.24 kJ/mol, respectively. It is clear that the energy barrier (6.31 and 13.90 kJ/mol) of O2 dissociation on CuMn2O4 surface is much lower than those of O2 dissociation on MnO2 and Au/TiO2 surfaces. Therefore, CuMn2O4 shows excellent O2-activation ability for the production of active oxygen atoms, which are crucial for HgO formation during Hg0 removal. These can further verify the excellent mercury removal performance of CuMn2O4 sorbent. It is found that active oxygen atoms produced from O2 dissociation show higher reactivity for Hg0 oxidation.26, 44 O2-activation ability can be used to evaluate the mercury removal performance of different sorbents. The relationship between mercury removal performance and O2-activation ability is a scaling relation. Thus, the energy barrier of O2 dissociation can be used as a descriptor to predict the tendency of an adsorbent material to promote mercury removal. 10


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3.3. Reaction Pathway of HgO Formation Experimental results found that HgO species are formed during Hg0 removal by CuMn2O4 sorbent.35 Some of adsorbed Hg0 can be oxidized by surface oxygen species into HgO species. To further understand the reaction mechanism governing mercury oxidation, the energy barrier and reaction heat of mercury oxidation by oxygen species on CuMn2O4 surface were calculated. Surface oxygen species include chemisorbed oxygen and lattice oxygen. However, chemisorbed oxygen is much more active for Hg0 oxidation than lattice oxygen.44 Thus, this work mainly focused on the reaction between Hg0 and chemisorbed oxygen over CuMn2O4 sorbent. In the Section 3.2, the active oxygen atoms can be easily formed via pathway 1. Thus, the structure of FS1 (see Figure 6) was used to investigate the reaction between Hg0 and chemisorbed oxygen atom. The reaction pathway, relative energy and optimized structures for mercury oxidation by chemisorbed oxygen atom are shown in Figure 7(a). The formation of gaseous HgO goes through three steps: Hg0 adsorption, Hg0 oxidation and HgO desorption. In the first step, Hg0 is adsorbed on CuMn2O4 surface with chemisorbed oxygen atom (O*), which comes from the dissociation of gas-phase O2 molecule. During this adsorption process, the intermediate IM1 is formed. Hg0 adsorption leads to the breakage of Cu-O bond (see IM1). In IM1, the distance between Hg and O atoms is 2.554 Å. This step is an exothermic process with an exothermicity of N9.?3*) kJ/mol. In the second step (bimolecular surface reaction), the adsorbed mercury is oxidized by chemisorbed oxygen atom into HgO species (see IM2). Hg0 oxidation occurs through transition state TS to form intermediate IM2. Mercury atom strips a chemisorbed oxygen atom from Cu site, leading to the breakage of Cu-O bond of IM1. Meanwhile, the distance between 11


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Hg and O atoms decreases gradually: 2.554 Å (IM1) T 2.414 Å (TS) T 2.233 Å (IM2). In TS, the Hg-O bond is formed, suggesting the formation of HgO species. Our previous XPS and TPDD results found that HgO species are formed during Hg0 capture by CuMn2O4 sorbent.35 Thus, DFT calculation results are consistent with the experimental results. The total energy of IM2 is much higher than that of IM1 (see Figure 7(a)), which indicates that mercury oxidation process is an endothermic reaction with reaction heat of 71.73 kJ/mol. The energy barrier of Hg0 oxidation is 103.79 kJ/mol. In the third step, HgO molecule desorbs from CuMn2O4 surface. The desorption process of HgO is an endothermic reaction with a reaction heat of 52.09 kJ/mol. Moreover, based on the most stable structure of O2 molecule adsorption (see Figure 5), the reactivity of chemisorbed oxygen molecule (O2*) towards mercury oxidation was also investigated. As shown in Figure 7(b), the adsorption energy of Hg0 on O2*-covered surface is N).3.> kJ/mol. The energy barrier of the reaction between Hg0 and O2* species is 42.72 kJ/mol, which is much lower than that (103.79 kJ/mol) of the reaction between Hg0 and O* species. Therefore, O2* species are much more active for Hg0 oxidation than O* species. O2* species can directly react with adsorbed mercury. HgO desorption step of Hg0 oxidation by O2* species is similar to that of Hg0 oxidation by O* species. In the comprehensive mercury adsorption-oxidation-desorption process, Hg0 adsorption is a barrierless and exothermic reaction (see Figure 7), and can occur spontaneously. Hg0 oxidation by chemisorbed oxygen molecule needs to overcome an energy barrier (42.72 kJ/mol). Additionally, the bimolecular surface reaction between adsorbed Hg0 and chemisorbed oxygen molecule is an exothermic process with a reaction heat of N*+3*, kJ/mol. However, HgO desorption is an endothermic process and needs to get a relatively higher external energy (52.09 kJ/mol). Therefore, it 12


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can

be

concluded

that

HgO

desorption

is

the

rate-determining

step

of

mercury

adsorption-oxidation-desorption process. These results indicate that CuMn2O4 spinel can be used as a dual-functional material for Hg0 removal from flue gas. At the lower temperatures, CuMn2O4 spinel mainly serves as a sorbent material for Hg0 removal, because the formed HgO is difficultly desorbed from sorbent surface at low temperatures due to the external energy requirement (52.09 kJ/mol). At the higher temperatures, CuMn2O4 spinel can be regarded as a catalyst material for Hg0 removal, because the reaction heat of HgO desorption can be surmounted at higher temperatures. These can explain the excellent Hg0 removal performance of CuMn2O4 spinel in a wide temperature window.

4. CONCLUSIONS Theoretical studies based on quantum mechanics were performed to gain insight into the active sites and reaction mechanism of Hg0 capture by spinel-type CuMn2O4 sorbent. Hg0 capture by CuMn2O4 sorbent is dominated by chemical adsorption mechanism. Hg0 can bind to two-fold coordinated Cu atom to form Cu-Hg amalgam. The orbital hybridization of Hg d-orbital with Cu dand Mn s-orbitals is responsible for the stronger interaction between Hg0 and CuMn2O4 surface. CuMn2O4 shows excellent O2-activation ability for the production of active oxygen atoms due to the lower energy barrier. O2 dissociation process over CuMn2O4 surface is an exothermic reaction, and provides active oxygen atoms for Hg0 oxidation. HgO molecule produced from Hg0 oxidation can stably exist on CuMn2O4 surface. Gaseous HgO formation on CuMn2O4 surface is controlled by a three-step process: Hg0 adsorption, Hg0 oxidation, and HgO desorption. In the comprehensive mercury adsorption-oxidation-desorption process, HgO desorption is the rate-determining step.

13


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CuMn2O4 spinel is a promising dual-functional material for Hg0 removal due to the delicate energy-mediated adsorption-oxidation-desorption reaction.

AUTHOR INFORMATION Corresponding Author *Tel: +86 27 87545526; fax: +86 27 87545526; e-mail address: [email protected].

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by National Key Research and Development Program of China (2018YFC1901303), Fundamental Research Funds for the Central Universities (2019kfyRCPY021), National Postdoctoral Program for Innovative Talents (BX20180108), China Postdoctoral Science Foundation (2018M640697) and Program for HUST Academic Frontier Youth Team (2018QYTD05).

REFERENCES (1) Houssard, P.; Point, D.; Tremblay-Boyer, L.; Allain, V.; Pethybridge, H.; Masbou, J.; Ferriss, B. E.; Baya, P. A.; Lagane, C.; Menkes, C. E.; Letourneur, Y.; Lorrain, A. A Model of Mercury Distribution in Tuna from the Western and Central Pacific Ocean: Influence of Physiology, Ecology and Environmental Factors. Environ. Sci. Technol. 2019, 53 (3), 9)..N9)*93 (2) Shen, F.; Liu, J.; Zhang, Z.; Dai, J. On-Line Analysis and Kinetic Behavior of Arsenic Release 14


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List of Figures Captions

Figure 1. Optimized structures and adsorption energies of Hg0 adsorption on CuMn2O4(100) surface. Figure 2. PDOS of Hg, Mn, Cu and O atoms in the most stable adsorption configuration (1B). The Fermi level (Ef) is set to 0 eV (dashed line). Figure 3. Three-dimensional electron density isosurface of Hg0 adsorption on CuMn2O4(100) surface: (a) 1A structure; (b) 1B structure. Figure 4. Optimized structures and adsorption energies of HgO adsorption on CuMn2O4(100) surface. Figure 5. Optimized structures and adsorption energies of O2 adsorption on CuMn2O4(100) surface. Figure 6. The reaction pathway, relative energy and optimized structures of O2 dissociation process over CuMn2O4(100) surface. Figure 7. The reaction pathways and relative energies for mercury oxidation by chemisorbed oxygen atom O* (a) and oxygen molecule O2* (b) over CuMn2O4 surface. The optimized structures represent the intermediate, transition state and final state of mercury oxidation.

20


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Page 21 of 27

9300 Hg

Mn

Cu

O

Hg

9240 Adsorption energy (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2.781 Å

3.318 Å

9180 Eads=9135.03 kJ/mol

Eads=9124.84 kJ/mol 9120

960

0 1A

1B

Figure 1. Optimized structures and adsorption energies of Hg0 adsorption on CuMn2O4(100) surface.

21


Energy & Fuels

250 150 PDOS (electrons/Hartree)

-0.24

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

200 100 50 0 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.0

0.2

-0.2

0.0

0.2

-0.2

0.0

0.2

150 Cu-s orbital Cu-p orbital Cu-d orbital

120 90

16 12 8 4 0

60

-0.24 -0.28

-0.24

-0.20

30 0 -0.8

-0.6

-0.4 -0.2 Energy (Hartree)

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

60 PDOS (electrons/Hartree)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

40 20

-0.24

0 -0.8

-0.6

-0.4

80 O-s orbital O-p orbital O-d orbital

60 40 20 0 -0.8

-0.6

-0.4

Energy (Hartree)

Figure 2. PDOS of Hg, Mn, Cu and O atoms in the most stable adsorption configuration (1B). The Fermi level (Ef) is set to 0 eV (dashed line).

22


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Energy & Fuels

(a)

-0.2

0.2

Hg

Cu

Cu

(b)

Hg

Cu

Cu Mn

Figure 3. Three-dimensional electron density isosurface of Hg0 adsorption on CuMn2O4(100) surface: (a) 1A structure; (b) 1B structure.

23


Energy & Fuels

91000 Hg 9800 Adsorption energy (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

Cu

O

Mn

HgO

Cu-Hg-O

2.651 Å

3.222 Å

9600 Eads=9470.54 kJ/mol

Eads=9474.61 kJ/mol

9400

9200

0 2A

2B

Figure 4. Optimized structures and adsorption energies of HgO adsorption on CuMn2O4(100) surface.

24


Page 25 of 27

9750

Mn

Cu 9600 Adsorption energy (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

O2

2.063 Å

O

2.153 Å

2.220 Å

9450 Eads=9321.66 kJ/mol

Eads=9301.97 kJ/mol 9300

9150

0 3A

3B

Figure 5. Optimized structures and adsorption energies of O2 adsorption on CuMn2O4(100) surface.

25


Energy & Fuels

1.907 Å 1.976 Å

2.261 Å

Mn

Cu

TS2

O

13.90

Relative energy (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

TS1

O*

6.161 Å

O*

6.31 3.301 Å

3B 0.00 2.063 Å

1.824 Å

1.929 Å

FS2 77.73

O2* 1.361 Å 2.153 Å

O* 1.851 Å

5.277 Å 1.984 Å

Pathway 2

O* 1.970 Å

FS1 101.37

Pathway 1

Reaction coordinate

Figure 6. The reaction pathway, relative energy and optimized structures of O2 dissociation process over CuMn2O4(100) surface.

26


Page 27 of 27

Hg0 oxidation

Hg0 adsorption

(a)

100

Hg

Relative energy (kJ/mol)

HgO

2.414 Å 3.176 Å

50

0

HgO desorption

0

O*

Surface-2O*+Hg0

Surface-O*

TS 21.55

1.52 IM2 53.61

950

Ea=103.79 kJ/mol

HgO 2.233 Å

9100

2.676 Å

2.554 Å

IM1 125.34

3.150 Å

9150

O

Mn

Cu

Hg Reaction coordinate Hg0 oxidation

Hg0 adsorption

(b) 100

HgO desorption

3.227 Å

Hg0 O2*

HgO

2.803 Å

50 Relative energy (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0

TSC 0.44

Surface-O2*+Hg0

Surface-O* Ea=42.72 kJ/mol

20.56

IM1C 42.28

950

IM2C 72.65

4.253 Å 2.779 Å

HgO

9100

2.230 Å 2.667 Å

9150

Hg

Cu

Mn

O Reaction coordinate

Figure 7. The reaction pathways and relative energies for mercury oxidation by chemisorbed oxygen atom O* (a) and oxygen molecule O2* (b) over CuMn2O4 surface. The optimized structures represent the intermediate, transition state and final state of mercury oxidation.

27


Nature of Active Sites and Oxygen-Assisted Reaction Mechanism for

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