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Insights into the catalytic behavior of LaMnO3 perovskite for Hg0 oxidation by HCl
Journal of Hazardous Materials 383 (2020) 121156
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Insights into the catalytic behavior of LaMnO3 perovskite for Hg0 oxidation by HCl
T
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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
A R T I C LE I N FO
A B S T R A C T
Editor: Danmeng Shuai
LaMnO3-based catalysts with perovskite structure have gained increasing interest for Hg0 oxidation owing to their excellent catalytic activity, high thermal stability and unique redox behavior. Understanding the Hg0 oxidation behavior on LaMnO3 will broaden the application of LaMnO3-based perovskites in Hg0 removal field. Density functional theory (DFT) calculations were conducted to examine the catalytic mechanism of Hg0 oxidation by HCl on LaMnO3 surface. The results indicate that Mn-terminated LaMnO3(010) surface is more active and stable than La-terminated surface. Hg0 and HgCl2 are chemisorbed on LaMnO3(010) surface. HgCl can be molecularly chemisorbed on LaMnO3(010) and serve as an intermediate in Hg0 oxidation reaction. HCl dissociatively adsorbs on LaMnO3(010) and generates surface active chlorine complexes. Langmuir–Hinshelwood mechanism, where the chemisorbed Hg0 reacts with the dissociatively adsorbed HCl, is responsible for Hg0 oxidation by HCl on LaMnO3(010). Catalytic Hg0 oxidation over the surface contains four-steps: Hg0 → Hg(ads) → HgCl(ads) → HgCl2(ads) → HgCl2, and the second step (Hg(ads) → HgCl(ads)) is the rate-determining step because of its relatively larger energy barrier (0.74 eV).
Keywords: Mercury removal LaMnO3 Catalytic oxidation mechanism HCl Density functional theory
1. Introduction Mercury, a highly toxic pollutant that can cause severe health effects on wildlife, domestic animals, and humans alike, has attracted worldwide concern (McNutt, 2013). The Minamata Convention on Mercury came into effect in August 2017, marking a new phase in worldwide efforts to reduce mercury pollution. The power plant is identified as the largest one among all anthropogenic mercury emission sources in china (Pacyna et al., 2010). The control of mercury emission is closely associated with mercury speciation (Yang et al., 2017a). Mercury in flue gas mainly exist in three forms: elemental mercury (Hg0), oxidized mercury (Hg2+), and particulate mercury (Hgp), among which Hg0 is the predominant species of mercury emitted into environment owing to its chemical inertness and water insolubility (Wilcox et al., 2012; Yang et al., 2016; Shen et al., 2015). Thus, the removal of Hg0 is the main challenge for reducing mercury emission. Several technologies, such as catalytic oxidation (Li et al., 2012; Zhang et al., 2015a, 2018) and sorbent injection (Xu et al., 2018; Shen et al., 2019a; Liu et al., 2020; Shen et al., 2019b), have been developed for removing Hg0 from flue gas. Even through activated carbon injection (ACI) is a mature technology for removing Hg0, it suffers from disadvantages include high operation cost, limited regenerability and negative influence on the reuse of fly ash. In contrast, catalytic ⁎
oxidation of Hg0 by catalysts to its oxidized form, Hg2+, accompanied by the removal of Hg2+ in wet flue gas desulfurization (WFGD) system is considered to be a promising method for Hg0 elimination. V2O5-based SCR catalysts have been commercially employed in coal-fired power plants for many years (Lee and Bae, 2009; Wang et al., 2016). Nevertheless, the Hg0 oxidation activity of commercial SCR catalysts is relatively low and highly dependent on HCl content in flue gas (Zhang et al., 2015b; Pudasainee et al., 2010). Worse still, the higher and limited operation temperature window (300–400 °C) of the commercial SCR catalysts means that the SCR unit must be located upstream of particulate control devices (ESP/FF) and WFGD, where the high concentration of dust and SO2 will speed up the catalyst deactivation. It is therefore expected to develop a low-temperature (100–250 °C) SCR catalyst with excellent catalytic activity for simultaneous removal of Hg0 and NOx. Recently, Mn-based perovskite oxides have been increasingly investigated as hopeful low-temperature catalysts for NOx reduction and Hg0 oxidation due to their good catalytic performance, unique redox behavior, exceptional thermal stability and low cost (Yang et al., 2018a; Zhou et al., 2016; Xu et al., 2016; Joo et al., 2010). The general formula of perovskites is ABO3, in which A is a typical rare/alkaline-earth metal ion while B is always the transition metal ion. Perovskites can be rationally tuned with specific physicochemical characteristics, such as
Corresponding author. E-mail address: [email protected] (J. Liu).
https://doi.org/10.1016/j.jhazmat.2019.121156 Received 19 May 2019; Received in revised form 3 September 2019; Accepted 3 September 2019 Available online 04 September 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 383 (2020) 121156
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Eads = E(LaMnO3-adsorbate) − (ELaMnO3 + Eadsorbate)
redox behavior, oxygen mobility, and ionic conductivity for improved catalysis by substitution of the A or B site with other cations (Zhu et al., 2015). LaMnO3 is one of the most general Mn-based perovskite oxides, which has been studied as a low-temperature catalyst for NOx reduction and Hg0 oxidation, and exhibits good activity (Zhou et al., 2016; Xu et al., 2016). The employ of LaMnO3 perovskite materials is promising to simultaneous remove NOx and Hg0 into one SCR reactor. HCl in coalfired flue gas plays the most important role in the catalytic oxidation of Hg0. The LaMnO3 perovskite oxide was identified as a Hg0 sorbent with superior Hg0 adsorption capacity in the absence of HCl (Xu et al., 2016). Meanwhile, Zhou et al. (Zhou et al., 2016) studied Hg0 oxidation on LaMnO3 oxide and found that approximately 75% of Hg0 was oxidized to HgCl2 with the existence of 10 ppm HCl in the temperature window of 100–150 °C. Although the removal of Hg0 by applying LaMnO3 perovskite materials has been studied experimentally, the detailed catalytic mechanism (especially at molecular-level) of Hg0 oxidation by HCl over LaMnO3 catalyst are still unknown. The Hg0 catalytic oxidation mechanism on LaMnO3 is strongly linked to the Hg0 oxidation performance. Furthermore, the molecular-level understanding of the detailed Hg0 catalytic oxidation mechanism on LaMnO3 surface will serve as an important guidance for the rational design of perovskite oxides with superior Hg0 oxidation activity. Consequently, deeper mechanistic investigations of Hg0 adsorption and catalytic oxidation over LaMnO3 catalyst are essential. Density functional theory (DFT) calculations are able to comprehend catalytic reaction at molecular level. DFT calculations have been extensively employed to investigate Hg0 adsorption and oxidation mechanisms on different sorbents (Wang et al., 2018a; Wilcox et al., 2011; Yang et al., 2017b; Shen et al., 2019c) and catalysts (Suarez Negreira and Wilcox, 2013; Wang et al., 2018b; Lim and Wilcox, 2013). Herein, DFT calculations were carried out with the aims of elucidating the binding mechanism of Hg complexes (Hg0, HgCl, and HgCl2) and the catalytic behavior of Hg0 oxidation by HCl over LaMnO3. Moreover, the transition states and activation energy barriers of Hg0 oxidation by HCl, which are critical to understand catalytic oxidation mechanism, were determined. As far as we know, this is the first theoretical study concerning Hg0 adsorption and catalytic oxidation mechanism on perovskite oxides.
(1)
where E(LaMnO3-adsorbate) , Eadsorbate , and ELaMnO3 on behalf of the gross energies of the LaMnO3-adsorbate system, the gaseous adsorbate molecules, and the optimized LaMnO3 slab model, respectively. Based on Eq. (1), the larger negative Eads value corresponding the stronger adsorption strength. Normally, the interaction with an adsorption energy lower than −0.31 eV is classified as physisorption, while higher than −0.52 eV is classified as chemisorption (Shen et al., 2018; Yang et al., 2019). Linear synchronous transit/quadratic synchronous transit (LST/ QST) tools (Halgren and Lipscomb, 1977), which have been widely recognized to obtain transition state structure and reaction energy barrier, were used to determine the lowest energy channel for the catalytic oxidation of Hg0 by HCl over LaMnO3 surface. The correct transition state is certified when (1) the forces on the atoms disappear and (2) the energy is a maximum along the reaction coordinate but a minimum with respect to all remaining degrees of freedom. The energy barrier (Ebarrier) was determined by using the equation: Ebarrier = ETS - EIM
(2)
where ETS and EIM on behalf of the gross energies of transition state and intermediate, respectively. 2.2. Catalyst models
2. Computational methods and models
LaMnO3(010) surface, the most thermodynamically stable surface (Gavin and Watson, 2017), was employed to examine Hg0 catalytic oxidation mechanism over LaMnO3 catalyst. There are two terminations of LaMnO3(010) surface: the Mn- and La-terminated surfaces. Fig. 1b shows the Mn-terminated LaMnO3(010) surface, which exposes four possible active sites: Mn5f, O2f, hollow, and bridge. La-terminated LaMnO3(010) surface also includes four possible active sites: La, O1f, bridge, and hollow, as shown in Fig. 1c. Since Hg0 in coal-fired flue gas is very dilute, the interaction between adjacent Hg atoms can be ignored and thus a large unit cell is necessary (Yang et al., 2017b). In this study, LaMnO3(010) surfaces were constructed by periodically repeated p(2 × 2) slab with nine atom layers, which can result in a minimal HgHg interaction. The bottom four layers were fixed while the top five layers were allowed to relax. To avert the interference between two periodic slabs, the slabs were divided using a 16 Å-thick vacuum space.
2.1. Theoretical methods
3. Results and discussion
All calculations in this work were carried out using the spin-polarized DFT method as conducted by the CASTEP program (Segall et al., 2002). The generalized gradient approximation (GGA) (Perdew et al., 1996a) within the Perdew-Burke-Ernzerhoff (PBE) approach (Perdew et al., 1996b) was used for treating electron exchange–correlation interaction. The ion cores were modeled using Vanderbilt Ultrasoft pseudopotentials (Vanderbilt, 1990). The one-electron valence states were enlarged in terms of plane waves with kinetic energies up to 340 eV. Brillouin zones were sampled by the Monkhorst-Pack scheme (Monkhorst and Pack, 1976) with 4 × 3 × 4 and 1 × 2 × 2 k-point meshes for the bulk and surface of LaMnO3. The structures were optimized with the following convergence criterions: 0.03 eV/Å for force, 1.0 × 10−5 eV/atom for total energy, 0.001 Å for displacement, and 1.0 × 10−6 eV/atom for self-consistent field (SCF). Under SCR reaction condition, LaMnO3 is crystallizes in an orthorhombic unit cell with the Pnma space group, as illustrated in Fig. 1a. The calculated lattice parameters (a =5.707 Å, b =7.763 Å, and c =5.568 Å) agree well with the experimental results (a =5.724 Å, b =7.69 Å, and c =5.534 Å for the X-ray study (Granado et al., 2000)). The relative deviations are within 0.29%, 0.86%, and 0.61%, respectively, indicating that the calculations are trustworthy. The adsorption energy (Eads) is defined as follows:
3.1. Hg0 adsorption on LaMnO3 surface Revealing the interaction of gas phase reactants (Hg0, HgCl, HgCl2, and HCl) with LaMnO3 is the first procedure toward Hg0 catalytic oxidation mechanism. Two terminated LaMnO3(010) surfaces (Mn and La termination) and every potential adsorption sites were considered for Hg0 adsorption. The optimized structures of Hg0 adsorption on LaMnO3(010) are presented in Fig. 2. The adsorption energies, structural parameters and Mulliken charges are summarized in Table 1. The adsorption strength of Hg0 on LaMnO3(010) surface increases in the following order: 1C < 1B < 1A. The most steady binding configuration is 1A, in which Hg0 is bonded to Mn5f atom of Mn-terminated LaMnO3(010) surface and forms a Hg–Mn bond. The binding energy of 1A is −0.58 eV with 0.29 e Mulliken charge transfer, suggesting a chemical adsorption. In 1B, Hg0 adsorbs on O2f site of Mn-terminated LaMnO3(010) surface with a relatively smaller binding energy of −0.36 eV. Therefore, compared to surface O atoms, Hg0 prefers to bond with Mn atoms of LaMnO3 surface. For Hg0 adsorption on La-terminated LaMnO3(010) surface, only one stable adsorption configuration (1C) is obtained. In 1C, Hg0 is weakly binding to O1f atom with a long equilibrium distance of 3.276 Å between Hg0 and O1f atoms. The corresponding binding energy is −0.10 eV, suggesting a weak 2
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Fig. 1. Slab models of LaMnO3(010) surface. (a) Crystal structure of LaMnO3. (b) Mn-terminated LaMnO3(010) surface. (c) La-terminated LaMnO3(010) surface.
physisorption. In order to further comprehend Hg0 binding mechanism over LaMnO3(010) surface, we analyzed the projected density of states (PDOS) of the stablest adsorption structure (1A). Fig. 3 displays the PDOS of mercury and Mn5f atoms in 1A. It can be seen that the Mn s-, pand d-states strongly hybridize with Hg s- and p-states at approximately –3.2 and 1.5 eV, respectively. The intense state hybridization between mercury and Mn5f atoms stabilizes the adsorption of Hg0 on LaMnO3, resulting in Mn–Hg bond formation and chemical interaction between Hg0 and the catalyst surface. Compared to La-terminated LaMnO3(010) surface, Mn-terminated surface exhibits higher activity for Hg0 adsorption because the surface Mn atoms can strengthen the stability of bonded Hg on the catalyst surface. Moreover, the previous thermodynamic study showed that the Mn-terminated LaMnO3 surface is more steady than La-terminated surface at the temperature below 927 ℃ (Piskunov et al., 2008). Thus, LaMnO3 catalyst mainly exposes Mn-terminated surface in actual application. According to the above analyses, it can be inferred that the binding of Hg0 on LaMnO3 catalyst belongs to chemisorption. Experimental results exhibit that the Hg0 capture efficiency of LaMnO3 catalyst increases as the reaction temperature evaluates from 100 to 150 ℃ and the characteristic is commonly considered as chemical adsorption mechanism (Yang et al., 2018a; Zhou et al., 2016; Xu et al., 2016). Thus, our DFT calculations agree well with the experimental observations.
Table 1 The adsorption energies (Eads, eV), bond lengths (R, Å) and Mulliken charges (Q, e) for Hg0 adsorption on Mn- (1A and 1B) and La- (1C) terminated LaMnO3(010) surface.
1A 1B 1C
Eads
RHg-X
QHg
−0.58 −0.36 −0.10
3.037 3.171 3.276
0.29 0.13 0.07
Subscript “X” denotes surface Mn or O atom.
Fig. 3. PDOS of Hg and Mn atoms in adsorption configuration 1A.
3.2. HgCl adsorption on LaMnO3 surface to its high stability and activity. Every potential active sites and binding directions containing parallel and perpendicular were examined. Three different stable structures for HgCl adsorption are obtained and presented in Fig. 4. The corresponding initial configurations are shown in Fig. S1. Table 2 lists the relevant binding energies, structural
Previous studies suggest that HgCl may serve as the intermediate of Hg oxidation reaction over catalysts (Yang et al., 2017c). Consequently, HgCl interaction with LaMnO3 was investigated to deeper comprehend Hg0 catalytic oxidation reaction. Only Mn-terminated LaMnO3(010) surface was considered in the following calculations due 0
Fig. 2. Adsorption configurations of Hg0 on Mn- (1A and 1B) and La- (1C) terminated LaMnO3(010) surface. 3
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Fig. 4. Adsorption configurations of HgCl on LaMnO3(010) surface. Table 2 The adsorption energies (Eads, eV), bond lengths (R, Å) and Mulliken charges (Q, e) for HgCl adsorption on LaMnO3(010) surface.
2A 2B 2C
Eads
RHg-Mn
RCl-Mn
RHg-Cl
Q
−1.75 −1.46 −1.65
2.946 2.636 −
2.276 − 2.284
3.584 2.469 3.252
0.43 0.34 0.37
Table 3 The adsorption energies (Eads, eV), bond lengths (R, Å), bond angles (θ, °) and Mulliken charges (Q, e) for HgCl2 adsorption on LaMnO3(010) surface.
3A 3B
Eads
RHg-X
RCl-X
RHg-Cl
θCl-Hg-Cl
Q
−0.62 −0.56
− 2.448
2.703/2.614 2.815/2.873
2.461/2.472 2.367/2.362
138.7 165.8
0.24 0.17
Subscript “X” denotes surface Mn or O atom.
parameters and Mulliken charges. The adsorption strength of HgCl on LaMnO3(010) surface declines in the following order: 2A > 2C > 2B. The stablest configuration is 2A with the binding energy of −1.75 eV and charge transfer of 0.43 e. In 2A, HgCl dissociates over LaMnO3(010) and forms a Mn–Hg (2.946 Å) bond and a Mn–Cl (2.276 Å) bond. Similar to 2A, HgCl in configuration 2C is also dissociated on LaMnO3(010), and the binding energy is −1.65 eV. In 2B, HgCl adsorbs on LaMnO3(010) in a molecular manner with a binding energy of −1.46 eV, which indicates that HgCl can molecularly exist on LaMnO3 catalyst and serve as an intermediate link in Hg0 catalytic oxidation reaction. This agrees well with the experimental observations that Hg0 could interact with chlorine complexes to generate HgCl on LaMnO3 surface (Zhou et al., 2016).
180° (free HgCl2) to 138.7° and the Hg–Cl bond distance extends from 2.302 Å (free HgCl2) to 2.461 and 2.472 Å. In addition, the two Cl atoms of HgCl2 react with two different surface Mn5f atoms, leading to the formation of two Mn–Cl bonds. Similar to 3A, HgCl2 in 3B is also molecularly adsorbed on LaMnO3(010) in parallel direction. This configuration yields a binding energy of −0.56 eV, which is slightly lower than that of 3A. Therefore, it can be inferred that HgCl2 can be formed and adsorbed on LaMnO3 catalyst surface during Hg0 oxidation reaction, which is in good agreement with the X-ray photoelectron spectroscopy (XPS) experimental findings that the HgCl2 peak was observed in the used LaMnO3-based catalyst (Zhou et al., 2016). Moreover, the result here is different from HgCl2 adsorption on MnFe2O4 sorbent surface, in which HgCl2 undergoes dissociative adsorption and thus can not stably exist on MnFe2O4 surface (Yang et al., 2017c).
3.3. HgCl2 adsorption on LaMnO3 surface 3.4. HCl adsorption on LaMnO3 surface HgCl2 is well known as an important oxidized mercury compounds in flue gas. Therefore, it is essential to investigate HgCl2 adsorption mechanism over LaMnO3 catalyst. Fig. 5 and Fig. S2 show the optimized and the initial structures of HgCl2 adsorption on LaMnO3(010), respectively. Table 3 lists the corresponding binding energies, structural parameters and Mulliken charges. The stablest configuration for HgCl2 adsorption is 3A with the binding energy of −0.62 eV and charge transfer of 0.24 e, suggesting a chemisorption. In 3A, HgCl2 adsorbs on LaMnO3(010) in parallel direction. The HgCl2 bond angle reduces from
The existence of HCl in coal-fired flue gas plays an important function for Hg0 catalytic oxidation. Therefore, HCl interaction with LaMnO3(010) surface was studied to determine the catalyst activity and the overall Hg0 catalytic reaction. Fig. 6 and Fig. S3 present the optimized and the initial structures of HCl adsorption on LaMnO3(010), respectively. The corresponding binding energies, structural parameters and Mulliken charges are summarized in Table 4. The stablest structure is 4A, in which HCl dissociates on LaMnO3(010) with the formation of
Fig. 5. Adsorption configurations of HgCl2 on LaMnO3(010) surface. 4
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Fig. 6. Adsorption configurations of HCl on LaMnO3(010) surface. Table 4 The adsorption energies (Eads, eV), bond lengths (R, Å) and Mulliken charges (Q, e) for HCl adsorption on LaMnO3(010) surface.
4A 4B
Eads
RH-X
RCl-X
RH-Cl
Q
−0.89 −0.24
0.998 1.939
2.340 −
2.214 1.299
0.45 0.06
Subscript “X” denotes surface Mn or O atom.
MneCl bond and surface hydroxyl group. The binding energy and charge transfer of this configuration are −0.89 eV and 0.45 e, respectively, suggesting a chemical adsorption. In configuration 4B, HCl adsorbs on O2f site of LaMnO3(010) surface in molecular manner. The binding energy and charge transfer of 4B are −0.24 eV and 0.06 e, which are much less than that of 4A. Thus, HCl prefers to dissociatively adsorb on LaMnO3(010) surface. Furthermore, the PDOS analysis of surface atoms in the most stable structure (4A) was conducted to further comprehend the interaction mechanism between HCl and LaMnO3 surface. Fig. 7 shows the PDOS of H and Cl atoms of HCl before and after adsorption on LaMnO3(010) surface are shown in Fig. 7. For gaseous HCl, the s- and p-states of H and Cl atoms intensely hybridize with each other, indicating a powerful interaction between H and Cl. For adsorbed HCl, all states of H and Cl atoms move to low energy level. In addition, the state hybridization between H and Cl vanishes, implying the breakage of H–Cl bond. The PDOS of H, O2f, Cl, and Mn5f atoms in configuration 4A are presented in Fig. 8. It can be seen that the H s-state hybridizes with the O2f s- and pstates at –19.9 and –7.5 eV, suggesting the formation of surface hydroxyl group. In addition, the Mn5f p- and d-states hybridize with Cl sstate at –13.9 eV. And the Cl p-state hybridize with Mn5f s-, p- and dstates at –3.2 eV. The state hybridization between Cl and Mn5f atoms implies the formation of Mn–Cl bond.
Fig. 8. PDOS of H, O, Cl, and Mn atoms over surface system after HCl adsorption on LaMnO3(010) surface.
Based on the above analyses, HCl adsorbs on LaMnO3(010) in a dissociative manner with the formation of surface hydroxyl and manganese–chlorine complexes. This is consistent with the experimental studies (Zhou et al., 2016; Yang et al., 2018b), which found that LaMnO3-based catalysts could adsorb HCl to generate surface chlorine groups for Hg0 oxidation. 3.5. Hg0 catalytic oxidation by HCl on LaMnO3 surface Hg0 catalytic oxidation by HCl on SCR catalysts generally follows either the Eley-Rideal (E–R) mechanism (Yang et al., 2017a; Zhang et al., 2015b) or the Langmuir-Hinshelwood (L-H) mechanism (Yang et al., 2017c, b). For E–R mechanism, one reagents (Hg0 or HCl) adsorbs on catalyst surface firstly, and then interacts with another gas-phase (or weakly adsorbed) reagent to produce HgCl2. For L-H mechanism, two reagents (Hg0 and HCl) adsorb on catalyst surface and then react to form HgCl2. According to the above results, Hg0 chemisorbs and HCl dissociates on LaMnO3(010), L-H mechanism is proposed for Hg0 oxidation by HCl on LaMnO3 catalyst. This conclusion is in accord with the HCl pretreatment experimental findings (Zhou et al., 2016; Yang et al., 2018b). Furthermore, the specific process of Hg0 catalytic oxidation on LaMnO3 catalyst was examined. On the basis of the results of HgCl and HgCl2 adsorption, HgCl can be both dissociatively and molecularly adsorbed, while HgCl2 can be formed and adsorbed on LaMnO3(010). Therefore, two pathways for Hg0 catalytic oxidation on LaMnO3(010) were taken into account. The path 1 contains four elementary steps: (1) Hg0 and HCl co-adsorption, (2) Hg(ads) → HgCl(ads), (3) HgCl(ads) → HgCl2(ads) and (4) HgCl2 desorption. The path 2 includes three steps: (1) Hg0 and HCl co-adsorption, (2) Hg(ads) →HgCl2(ads) and (3) HgCl2 desorption. Fig. 9 presents the energy diagram of Hg0 oxidation on LaMnO3(010) and the associated stable configurations of intermediate, transition state, and final state. For pathway 1, Hg0 catalytic oxidation goes through reactants →
Fig. 7. PDOS of H and Cl atoms for isolated HCl molecule and adsorbed HCl on LaMnO3(010) surface. 5
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Fig. 9. The energy and geometrical diagram of Hg0 oxidation by HCl on LaMnO3(010) surface.
IM1 → TS1 → IM2 → TS2 → IM3 → TS3→ FS + HgCl2. The step one is the co-adsorption of Hg0 and HCl on LaMnO3(010) surface to form IM1. This step is barrierless and exothermic by 2.44 eV. In IM1, Hg0 adsorbs on surface Mn5f site with the formation of Mn–Hg bond. Two HCl molecules dissociate on the surface to produce two active Cl atoms. The second step is the reaction of adsorbed Hg with a surface active Cl atom via IM1 → TS1 → IM2. Meanwhile, the interval of Hg and Cl atoms lessens step by step: 3.724 Å (IM1) → 2.884 Å (TS1) → 2.475 Å (IM2), suggesting HgCl formation. This step is an endothermic course with reaction heat of 0.66 eV, and the corresponding energy barrier is 0.74 eV. The third step (HgCl(ads) → HgCl2(ads)) is endothermic by 0.54 eV and activated by 0.66 eV. With the interval of Cl and Hg atoms reduces (3.553/2.475 Å (IM2) → 2.581/2.366 Å (TS2) → 2.318/ 2.317 Å (IM3)), the adsorbed Hg is eventually oxidized to surfacebonded HgCl2. The last step is the desorption of HgCl2 from the surface via IM3 → TS3 → FS + HgCl2. The energy barrier of the desorption process is as low as 0.36 eV, implying this step is ready to occur. The overall pathway 1 is exothermic by 0.98 eV, and the formation of HgCl is the rate-determining step because of its relatively larger energy barrier. Furthermore, after Hg0 and HCl co-adsorption, the relative energy raises along the reaction coordinate, which indicates that the oxidation reaction (IM1 → FS+HgCl2) is an endothermic process. It is well-known that Hg0 oxidation in coal-fired flue gas is controlled by kinetics rather than thermodynamics (Yang et al., 2016). Thus, this oxidation reaction will occur to form HgCl2 if enough external energy is provided for this reaction system. For pathway 2, the process of Hg0 oxidation goes through reactants → IM1 → TS4 → IM3 → TS3→ FS + HgCl2. The first (i.e., Hg0 and HCl co-adsorption) and last (i.e., HgCl2 desorption) steps of pathway 2 are same to that of pathway 1. In pathway 2, the surface-bonded HgCl2 is formed through a one-step reaction (Hg(ads) → HgCl2(ads)) instead of a two-step reaction (Hg(ads) → HgCl(ads) → HgCl2(ads)). For the second step (IM1→ TS4→ IM3), two surface active Cl atoms approach to the adsorbed Hg, and overcome a 1.65 eV energy barrier to generate HgCl2 on LaMnO3(010). The relatively higher energy barrier of this step suggests that the second step is the rate-determining step. Comparing the two reaction pathways, pathway 1 is kinetically more beneficial because the energy barrier of rate-determining step for pathway 1 (0.74 eV) is much lower than that of pathway 2 (1.65 eV). Thus, Hg0 catalytic oxidation by HCl on LaMnO3 catalyst prefers Hg0 → Hg(ads) → HgCl(ads) → HgCl2(ads) → HgCl2 rather than Hg0 → Hg (ads) → HgCl2(ads) → HgCl2. The previous HCl-pretreatment experiments and XPS analysis also found that the reaction process of Hg0
oxidation over LaMnO3-based catalysts occurs through a path involving HgCl (Zhou et al., 2016). Consequently, the DFT calculations are in agreement with the experimental results. In addition, the rate-determining step is the second step, i.e., HgCl formation. Moreover, compared the energy barrier of the rate-determining step for Hg0 oxidation on LaMnO3(010) with that on V2O5/TiO2(001) (Zhang et al., 2015b) and MnFe2O4(100) (Yang et al., 2017c), the energy barrier of Hg0 oxidation on LaMnO3(010) (0.74 eV) is less than that on V2O5/ TiO2(001) (0.95 eV) and MnFe2O4(100) (2.20 eV). This indicates that Hg0 catalytic oxidation on LaMnO3-based catalysts is easier than that over V2O5/TiO2 catalyst and MnFe2O4 sorbent. Therefore, LaMnO3 material is a potential candidate for Hg0 catalytic oxidation.
4. Conclusions Hg0 catalytic oxidation by HCl on LaMnO3 surface was studied by using the quantum chemistry calculations. Mn-terminated LaMnO3(010) surface exhibits higher activity than La-terminated surface for Hg0 adsorption. Hg0 prefers to adsorb on Mn5f site of LaMnO3(010) surface with chemical adsorption. The intense state hybridization between Hg and Mn5f atoms causes the chemisorption of Hg0 on the surface. Both HgCl and HgCl2 can be molecularly chemisorbed on LaMnO3(010). HCl adsorbs dissociatively on LaMnO3(010) and turned into surface chlorine complexes for Hg0 oxidation. The catalytic oxidation of Hg0 by HCl over LaMnO3 occurs through L-H mechanism where adsorbed Hg0 reacts with surface chlorine complexes to generate HgCl2. The reaction pathway of Hg0 catalytic oxidation to gaseous HgCl2 includes four elementary steps: Hg0 → Hg(ads) → HgCl (ads) → HgCl2(ads) → HgCl2, and the second step (Hg(ads) → HgCl (ads)) is the rate-determining step. After HgCl2 desorption, the adsorbed hydrogens will react with surface oxygen to form H2O and create oxygen vacancies. Subsequently, the oxygen-vacancy surface will be refilled by gaseous O2 to recover LaMnO3 catalyst. In the future study, the recovery process of the catalyst will be examined in detail.
Acknowledgments This work was supported by National Key R&D Program of China (2018YFC1901303), Fundamental Research Funds for the Central Universities (2019kfyRCPY021), and Program for HUST Academic Frontier Youth Team (2018QYTD05). 6
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Appendix A. Supplementary data
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Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.121156. References Gavin, A.L., Watson, G.W., 2017. Modelling oxygen defects in orthorhombic LaMnO3 and its low index surfaces. Phys. Chem. Chem. Phys. 19, 24636–24646. Granado, E., Sanjurjo, J.A., Rettori, C., Neumeier, J., Oseroff, S., 2000. Order-disorder in the Jahn-Teller transition of LaMnO3: a raman scattering study. Phys. Rev. B 62, 11304. Halgren, T.A., Lipscomb, W.N., 1977. The synchronous-transit method for determining reaction pathways and locating molecular transition states. Chem. Phys. Lett. 49, 225–232. Joo, S.W., Lee, S.Y., Liu, J., Qian, S., 2010. Diffusiophoresis of an elongated cylindrical nanoparticle along the axis of a nanopore. ChemPhysChem 11, 3281–3290. Lee, W., Bae, G.N., 2009. Removal of elemental mercury (Hg(0)) by nanosized V2O5/TiO2 catalysts. Environ. Sci. Technol. 43, 1522–1527. Li, H., Wu, C.Y., Li, Y., Zhang, J., 2012. Superior activity of MnOx-CeO2/TiO2 catalyst for catalytic oxidation of elemental mercury at low flue gas temperatures. Appl. Catal. B: Environ. 111−112, 381–388. Lim, D.H., Wilcox, J., 2013. Heterogeneous mercury oxidation on Au(111) from first principles. Environ. Sci. Technol. 47, 8515–8522. Liu, H., Chang, L., Liu, W., Xiong, Z., Zhao, Y., Zhang, J., 2020. Advances in mercury removal from coal-fired flue gas by mineral adsorbents. Chem. Eng. J. 379, 122263. McNutt, M., 2013. Mercury and health. Science 341, 1430. Monkhorst, H.J., Pack, J.D., 1976. Special points for brillouin-zone integrations. Phys. Rev. B 13, 5188. Pacyna, E.G., Pacyna, J.M., Sundseth, K., Munthe, J., Kindbom, K., Wilson, S., Steenhuisen, F., Maxson, P., 2010. Global emission of mercury to the atmosphere from anthropogenic sources in 2005 and projections to 2020. Atmos. Environ. 44, 2487–2499. Perdew, J.P., Burke, K., Wang, Y., 1996a. Generalized gradient approximation for the exchange-correlation hole of a many-electron system. Phys. Rev., B Condens. Matter 54, 16533–16539. Perdew, J.P., Burke, K., Ernzerhof, M., 1996b. Generalized gradient approximation made simple. Phys. Rev. Lett. 77, 3865. Piskunov, S., Heifets, E., Jacob, T., Kotomin, E.A., Ellis, D.E., Spohr, E., 2008. Electronic structure and thermodynamic stability of LaMnO3 and La1-xSrxMnO3(001) surfaces: ab initio calculations. Phys. Rev. B 78, 121406. Pudasainee, D., Lee, S.J., Lee, S.H., Kim, J.H., Jang, H.N., Cho, S.J., Seo, Y.C., 2010. Effect of selective catalytic reactor on oxidation and enhanced removal of mercury in coalfired power plants. Fuel 89, 804–809. Segall, M., Lindan, P.J., Probert, M.A., Pickard, C., Hasnip, P., Clark, S., Payne, M., 2002. First-principles simulation: ideas, illustrations and the CASTEP code. J. Phys. Condens. Matter 14, 2717. Shen, F., Liu, J., Zhang, Z., Dai, J., 2015. On-line analysis and kinetic behavior of arsenic release during coal combustion and pyrolysis. Environ. Sci. Technol. 49, 13716–13723. Shen, F., Liu, J., Zhang, Z., Dong, Y., Gu, C., 2018. Density functional study of hydrogen sulfide adsorption mechanism on activated carbon. Fuel Process. Technol. 171, 258–264. Shen, F., Liu, J., Wu, D., Dong, Y., Zhang, Z., 2019a. Development of O2 and NO co-doped porous carbon as a high-capacity mercury sorbent. Environ. Sci. Technol. 53, 1725–1731. Shen, F., Liu, J., Dong, Y., Wu, D., Gu, C., Zhang, Z., 2019b. Elemental mercury removal from syngas by porous carbon-supported CuCl2 sorbents. Fuel 239, 138–144.
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Insights into the catalytic behavior of LaMnO3 perovskite for Hg0 oxidation by HCl
T
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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
A R T I C LE I N FO
A B S T R A C T
Editor: Danmeng Shuai
LaMnO3-based catalysts with perovskite structure have gained increasing interest for Hg0 oxidation owing to their excellent catalytic activity, high thermal stability and unique redox behavior. Understanding the Hg0 oxidation behavior on LaMnO3 will broaden the application of LaMnO3-based perovskites in Hg0 removal field. Density functional theory (DFT) calculations were conducted to examine the catalytic mechanism of Hg0 oxidation by HCl on LaMnO3 surface. The results indicate that Mn-terminated LaMnO3(010) surface is more active and stable than La-terminated surface. Hg0 and HgCl2 are chemisorbed on LaMnO3(010) surface. HgCl can be molecularly chemisorbed on LaMnO3(010) and serve as an intermediate in Hg0 oxidation reaction. HCl dissociatively adsorbs on LaMnO3(010) and generates surface active chlorine complexes. Langmuir–Hinshelwood mechanism, where the chemisorbed Hg0 reacts with the dissociatively adsorbed HCl, is responsible for Hg0 oxidation by HCl on LaMnO3(010). Catalytic Hg0 oxidation over the surface contains four-steps: Hg0 → Hg(ads) → HgCl(ads) → HgCl2(ads) → HgCl2, and the second step (Hg(ads) → HgCl(ads)) is the rate-determining step because of its relatively larger energy barrier (0.74 eV).
Keywords: Mercury removal LaMnO3 Catalytic oxidation mechanism HCl Density functional theory
1. Introduction Mercury, a highly toxic pollutant that can cause severe health effects on wildlife, domestic animals, and humans alike, has attracted worldwide concern (McNutt, 2013). The Minamata Convention on Mercury came into effect in August 2017, marking a new phase in worldwide efforts to reduce mercury pollution. The power plant is identified as the largest one among all anthropogenic mercury emission sources in china (Pacyna et al., 2010). The control of mercury emission is closely associated with mercury speciation (Yang et al., 2017a). Mercury in flue gas mainly exist in three forms: elemental mercury (Hg0), oxidized mercury (Hg2+), and particulate mercury (Hgp), among which Hg0 is the predominant species of mercury emitted into environment owing to its chemical inertness and water insolubility (Wilcox et al., 2012; Yang et al., 2016; Shen et al., 2015). Thus, the removal of Hg0 is the main challenge for reducing mercury emission. Several technologies, such as catalytic oxidation (Li et al., 2012; Zhang et al., 2015a, 2018) and sorbent injection (Xu et al., 2018; Shen et al., 2019a; Liu et al., 2020; Shen et al., 2019b), have been developed for removing Hg0 from flue gas. Even through activated carbon injection (ACI) is a mature technology for removing Hg0, it suffers from disadvantages include high operation cost, limited regenerability and negative influence on the reuse of fly ash. In contrast, catalytic ⁎
oxidation of Hg0 by catalysts to its oxidized form, Hg2+, accompanied by the removal of Hg2+ in wet flue gas desulfurization (WFGD) system is considered to be a promising method for Hg0 elimination. V2O5-based SCR catalysts have been commercially employed in coal-fired power plants for many years (Lee and Bae, 2009; Wang et al., 2016). Nevertheless, the Hg0 oxidation activity of commercial SCR catalysts is relatively low and highly dependent on HCl content in flue gas (Zhang et al., 2015b; Pudasainee et al., 2010). Worse still, the higher and limited operation temperature window (300–400 °C) of the commercial SCR catalysts means that the SCR unit must be located upstream of particulate control devices (ESP/FF) and WFGD, where the high concentration of dust and SO2 will speed up the catalyst deactivation. It is therefore expected to develop a low-temperature (100–250 °C) SCR catalyst with excellent catalytic activity for simultaneous removal of Hg0 and NOx. Recently, Mn-based perovskite oxides have been increasingly investigated as hopeful low-temperature catalysts for NOx reduction and Hg0 oxidation due to their good catalytic performance, unique redox behavior, exceptional thermal stability and low cost (Yang et al., 2018a; Zhou et al., 2016; Xu et al., 2016; Joo et al., 2010). The general formula of perovskites is ABO3, in which A is a typical rare/alkaline-earth metal ion while B is always the transition metal ion. Perovskites can be rationally tuned with specific physicochemical characteristics, such as
Corresponding author. E-mail address: [email protected] (J. Liu).
https://doi.org/10.1016/j.jhazmat.2019.121156 Received 19 May 2019; Received in revised form 3 September 2019; Accepted 3 September 2019 Available online 04 September 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.
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Eads = E(LaMnO3-adsorbate) − (ELaMnO3 + Eadsorbate)
redox behavior, oxygen mobility, and ionic conductivity for improved catalysis by substitution of the A or B site with other cations (Zhu et al., 2015). LaMnO3 is one of the most general Mn-based perovskite oxides, which has been studied as a low-temperature catalyst for NOx reduction and Hg0 oxidation, and exhibits good activity (Zhou et al., 2016; Xu et al., 2016). The employ of LaMnO3 perovskite materials is promising to simultaneous remove NOx and Hg0 into one SCR reactor. HCl in coalfired flue gas plays the most important role in the catalytic oxidation of Hg0. The LaMnO3 perovskite oxide was identified as a Hg0 sorbent with superior Hg0 adsorption capacity in the absence of HCl (Xu et al., 2016). Meanwhile, Zhou et al. (Zhou et al., 2016) studied Hg0 oxidation on LaMnO3 oxide and found that approximately 75% of Hg0 was oxidized to HgCl2 with the existence of 10 ppm HCl in the temperature window of 100–150 °C. Although the removal of Hg0 by applying LaMnO3 perovskite materials has been studied experimentally, the detailed catalytic mechanism (especially at molecular-level) of Hg0 oxidation by HCl over LaMnO3 catalyst are still unknown. The Hg0 catalytic oxidation mechanism on LaMnO3 is strongly linked to the Hg0 oxidation performance. Furthermore, the molecular-level understanding of the detailed Hg0 catalytic oxidation mechanism on LaMnO3 surface will serve as an important guidance for the rational design of perovskite oxides with superior Hg0 oxidation activity. Consequently, deeper mechanistic investigations of Hg0 adsorption and catalytic oxidation over LaMnO3 catalyst are essential. Density functional theory (DFT) calculations are able to comprehend catalytic reaction at molecular level. DFT calculations have been extensively employed to investigate Hg0 adsorption and oxidation mechanisms on different sorbents (Wang et al., 2018a; Wilcox et al., 2011; Yang et al., 2017b; Shen et al., 2019c) and catalysts (Suarez Negreira and Wilcox, 2013; Wang et al., 2018b; Lim and Wilcox, 2013). Herein, DFT calculations were carried out with the aims of elucidating the binding mechanism of Hg complexes (Hg0, HgCl, and HgCl2) and the catalytic behavior of Hg0 oxidation by HCl over LaMnO3. Moreover, the transition states and activation energy barriers of Hg0 oxidation by HCl, which are critical to understand catalytic oxidation mechanism, were determined. As far as we know, this is the first theoretical study concerning Hg0 adsorption and catalytic oxidation mechanism on perovskite oxides.
(1)
where E(LaMnO3-adsorbate) , Eadsorbate , and ELaMnO3 on behalf of the gross energies of the LaMnO3-adsorbate system, the gaseous adsorbate molecules, and the optimized LaMnO3 slab model, respectively. Based on Eq. (1), the larger negative Eads value corresponding the stronger adsorption strength. Normally, the interaction with an adsorption energy lower than −0.31 eV is classified as physisorption, while higher than −0.52 eV is classified as chemisorption (Shen et al., 2018; Yang et al., 2019). Linear synchronous transit/quadratic synchronous transit (LST/ QST) tools (Halgren and Lipscomb, 1977), which have been widely recognized to obtain transition state structure and reaction energy barrier, were used to determine the lowest energy channel for the catalytic oxidation of Hg0 by HCl over LaMnO3 surface. The correct transition state is certified when (1) the forces on the atoms disappear and (2) the energy is a maximum along the reaction coordinate but a minimum with respect to all remaining degrees of freedom. The energy barrier (Ebarrier) was determined by using the equation: Ebarrier = ETS - EIM
(2)
where ETS and EIM on behalf of the gross energies of transition state and intermediate, respectively. 2.2. Catalyst models
2. Computational methods and models
LaMnO3(010) surface, the most thermodynamically stable surface (Gavin and Watson, 2017), was employed to examine Hg0 catalytic oxidation mechanism over LaMnO3 catalyst. There are two terminations of LaMnO3(010) surface: the Mn- and La-terminated surfaces. Fig. 1b shows the Mn-terminated LaMnO3(010) surface, which exposes four possible active sites: Mn5f, O2f, hollow, and bridge. La-terminated LaMnO3(010) surface also includes four possible active sites: La, O1f, bridge, and hollow, as shown in Fig. 1c. Since Hg0 in coal-fired flue gas is very dilute, the interaction between adjacent Hg atoms can be ignored and thus a large unit cell is necessary (Yang et al., 2017b). In this study, LaMnO3(010) surfaces were constructed by periodically repeated p(2 × 2) slab with nine atom layers, which can result in a minimal HgHg interaction. The bottom four layers were fixed while the top five layers were allowed to relax. To avert the interference between two periodic slabs, the slabs were divided using a 16 Å-thick vacuum space.
2.1. Theoretical methods
3. Results and discussion
All calculations in this work were carried out using the spin-polarized DFT method as conducted by the CASTEP program (Segall et al., 2002). The generalized gradient approximation (GGA) (Perdew et al., 1996a) within the Perdew-Burke-Ernzerhoff (PBE) approach (Perdew et al., 1996b) was used for treating electron exchange–correlation interaction. The ion cores were modeled using Vanderbilt Ultrasoft pseudopotentials (Vanderbilt, 1990). The one-electron valence states were enlarged in terms of plane waves with kinetic energies up to 340 eV. Brillouin zones were sampled by the Monkhorst-Pack scheme (Monkhorst and Pack, 1976) with 4 × 3 × 4 and 1 × 2 × 2 k-point meshes for the bulk and surface of LaMnO3. The structures were optimized with the following convergence criterions: 0.03 eV/Å for force, 1.0 × 10−5 eV/atom for total energy, 0.001 Å for displacement, and 1.0 × 10−6 eV/atom for self-consistent field (SCF). Under SCR reaction condition, LaMnO3 is crystallizes in an orthorhombic unit cell with the Pnma space group, as illustrated in Fig. 1a. The calculated lattice parameters (a =5.707 Å, b =7.763 Å, and c =5.568 Å) agree well with the experimental results (a =5.724 Å, b =7.69 Å, and c =5.534 Å for the X-ray study (Granado et al., 2000)). The relative deviations are within 0.29%, 0.86%, and 0.61%, respectively, indicating that the calculations are trustworthy. The adsorption energy (Eads) is defined as follows:
3.1. Hg0 adsorption on LaMnO3 surface Revealing the interaction of gas phase reactants (Hg0, HgCl, HgCl2, and HCl) with LaMnO3 is the first procedure toward Hg0 catalytic oxidation mechanism. Two terminated LaMnO3(010) surfaces (Mn and La termination) and every potential adsorption sites were considered for Hg0 adsorption. The optimized structures of Hg0 adsorption on LaMnO3(010) are presented in Fig. 2. The adsorption energies, structural parameters and Mulliken charges are summarized in Table 1. The adsorption strength of Hg0 on LaMnO3(010) surface increases in the following order: 1C < 1B < 1A. The most steady binding configuration is 1A, in which Hg0 is bonded to Mn5f atom of Mn-terminated LaMnO3(010) surface and forms a Hg–Mn bond. The binding energy of 1A is −0.58 eV with 0.29 e Mulliken charge transfer, suggesting a chemical adsorption. In 1B, Hg0 adsorbs on O2f site of Mn-terminated LaMnO3(010) surface with a relatively smaller binding energy of −0.36 eV. Therefore, compared to surface O atoms, Hg0 prefers to bond with Mn atoms of LaMnO3 surface. For Hg0 adsorption on La-terminated LaMnO3(010) surface, only one stable adsorption configuration (1C) is obtained. In 1C, Hg0 is weakly binding to O1f atom with a long equilibrium distance of 3.276 Å between Hg0 and O1f atoms. The corresponding binding energy is −0.10 eV, suggesting a weak 2
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Fig. 1. Slab models of LaMnO3(010) surface. (a) Crystal structure of LaMnO3. (b) Mn-terminated LaMnO3(010) surface. (c) La-terminated LaMnO3(010) surface.
physisorption. In order to further comprehend Hg0 binding mechanism over LaMnO3(010) surface, we analyzed the projected density of states (PDOS) of the stablest adsorption structure (1A). Fig. 3 displays the PDOS of mercury and Mn5f atoms in 1A. It can be seen that the Mn s-, pand d-states strongly hybridize with Hg s- and p-states at approximately –3.2 and 1.5 eV, respectively. The intense state hybridization between mercury and Mn5f atoms stabilizes the adsorption of Hg0 on LaMnO3, resulting in Mn–Hg bond formation and chemical interaction between Hg0 and the catalyst surface. Compared to La-terminated LaMnO3(010) surface, Mn-terminated surface exhibits higher activity for Hg0 adsorption because the surface Mn atoms can strengthen the stability of bonded Hg on the catalyst surface. Moreover, the previous thermodynamic study showed that the Mn-terminated LaMnO3 surface is more steady than La-terminated surface at the temperature below 927 ℃ (Piskunov et al., 2008). Thus, LaMnO3 catalyst mainly exposes Mn-terminated surface in actual application. According to the above analyses, it can be inferred that the binding of Hg0 on LaMnO3 catalyst belongs to chemisorption. Experimental results exhibit that the Hg0 capture efficiency of LaMnO3 catalyst increases as the reaction temperature evaluates from 100 to 150 ℃ and the characteristic is commonly considered as chemical adsorption mechanism (Yang et al., 2018a; Zhou et al., 2016; Xu et al., 2016). Thus, our DFT calculations agree well with the experimental observations.
Table 1 The adsorption energies (Eads, eV), bond lengths (R, Å) and Mulliken charges (Q, e) for Hg0 adsorption on Mn- (1A and 1B) and La- (1C) terminated LaMnO3(010) surface.
1A 1B 1C
Eads
RHg-X
QHg
−0.58 −0.36 −0.10
3.037 3.171 3.276
0.29 0.13 0.07
Subscript “X” denotes surface Mn or O atom.
Fig. 3. PDOS of Hg and Mn atoms in adsorption configuration 1A.
3.2. HgCl adsorption on LaMnO3 surface to its high stability and activity. Every potential active sites and binding directions containing parallel and perpendicular were examined. Three different stable structures for HgCl adsorption are obtained and presented in Fig. 4. The corresponding initial configurations are shown in Fig. S1. Table 2 lists the relevant binding energies, structural
Previous studies suggest that HgCl may serve as the intermediate of Hg oxidation reaction over catalysts (Yang et al., 2017c). Consequently, HgCl interaction with LaMnO3 was investigated to deeper comprehend Hg0 catalytic oxidation reaction. Only Mn-terminated LaMnO3(010) surface was considered in the following calculations due 0
Fig. 2. Adsorption configurations of Hg0 on Mn- (1A and 1B) and La- (1C) terminated LaMnO3(010) surface. 3
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Fig. 4. Adsorption configurations of HgCl on LaMnO3(010) surface. Table 2 The adsorption energies (Eads, eV), bond lengths (R, Å) and Mulliken charges (Q, e) for HgCl adsorption on LaMnO3(010) surface.
2A 2B 2C
Eads
RHg-Mn
RCl-Mn
RHg-Cl
Q
−1.75 −1.46 −1.65
2.946 2.636 −
2.276 − 2.284
3.584 2.469 3.252
0.43 0.34 0.37
Table 3 The adsorption energies (Eads, eV), bond lengths (R, Å), bond angles (θ, °) and Mulliken charges (Q, e) for HgCl2 adsorption on LaMnO3(010) surface.
3A 3B
Eads
RHg-X
RCl-X
RHg-Cl
θCl-Hg-Cl
Q
−0.62 −0.56
− 2.448
2.703/2.614 2.815/2.873
2.461/2.472 2.367/2.362
138.7 165.8
0.24 0.17
Subscript “X” denotes surface Mn or O atom.
parameters and Mulliken charges. The adsorption strength of HgCl on LaMnO3(010) surface declines in the following order: 2A > 2C > 2B. The stablest configuration is 2A with the binding energy of −1.75 eV and charge transfer of 0.43 e. In 2A, HgCl dissociates over LaMnO3(010) and forms a Mn–Hg (2.946 Å) bond and a Mn–Cl (2.276 Å) bond. Similar to 2A, HgCl in configuration 2C is also dissociated on LaMnO3(010), and the binding energy is −1.65 eV. In 2B, HgCl adsorbs on LaMnO3(010) in a molecular manner with a binding energy of −1.46 eV, which indicates that HgCl can molecularly exist on LaMnO3 catalyst and serve as an intermediate link in Hg0 catalytic oxidation reaction. This agrees well with the experimental observations that Hg0 could interact with chlorine complexes to generate HgCl on LaMnO3 surface (Zhou et al., 2016).
180° (free HgCl2) to 138.7° and the Hg–Cl bond distance extends from 2.302 Å (free HgCl2) to 2.461 and 2.472 Å. In addition, the two Cl atoms of HgCl2 react with two different surface Mn5f atoms, leading to the formation of two Mn–Cl bonds. Similar to 3A, HgCl2 in 3B is also molecularly adsorbed on LaMnO3(010) in parallel direction. This configuration yields a binding energy of −0.56 eV, which is slightly lower than that of 3A. Therefore, it can be inferred that HgCl2 can be formed and adsorbed on LaMnO3 catalyst surface during Hg0 oxidation reaction, which is in good agreement with the X-ray photoelectron spectroscopy (XPS) experimental findings that the HgCl2 peak was observed in the used LaMnO3-based catalyst (Zhou et al., 2016). Moreover, the result here is different from HgCl2 adsorption on MnFe2O4 sorbent surface, in which HgCl2 undergoes dissociative adsorption and thus can not stably exist on MnFe2O4 surface (Yang et al., 2017c).
3.3. HgCl2 adsorption on LaMnO3 surface 3.4. HCl adsorption on LaMnO3 surface HgCl2 is well known as an important oxidized mercury compounds in flue gas. Therefore, it is essential to investigate HgCl2 adsorption mechanism over LaMnO3 catalyst. Fig. 5 and Fig. S2 show the optimized and the initial structures of HgCl2 adsorption on LaMnO3(010), respectively. Table 3 lists the corresponding binding energies, structural parameters and Mulliken charges. The stablest configuration for HgCl2 adsorption is 3A with the binding energy of −0.62 eV and charge transfer of 0.24 e, suggesting a chemisorption. In 3A, HgCl2 adsorbs on LaMnO3(010) in parallel direction. The HgCl2 bond angle reduces from
The existence of HCl in coal-fired flue gas plays an important function for Hg0 catalytic oxidation. Therefore, HCl interaction with LaMnO3(010) surface was studied to determine the catalyst activity and the overall Hg0 catalytic reaction. Fig. 6 and Fig. S3 present the optimized and the initial structures of HCl adsorption on LaMnO3(010), respectively. The corresponding binding energies, structural parameters and Mulliken charges are summarized in Table 4. The stablest structure is 4A, in which HCl dissociates on LaMnO3(010) with the formation of
Fig. 5. Adsorption configurations of HgCl2 on LaMnO3(010) surface. 4
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Fig. 6. Adsorption configurations of HCl on LaMnO3(010) surface. Table 4 The adsorption energies (Eads, eV), bond lengths (R, Å) and Mulliken charges (Q, e) for HCl adsorption on LaMnO3(010) surface.
4A 4B
Eads
RH-X
RCl-X
RH-Cl
Q
−0.89 −0.24
0.998 1.939
2.340 −
2.214 1.299
0.45 0.06
Subscript “X” denotes surface Mn or O atom.
MneCl bond and surface hydroxyl group. The binding energy and charge transfer of this configuration are −0.89 eV and 0.45 e, respectively, suggesting a chemical adsorption. In configuration 4B, HCl adsorbs on O2f site of LaMnO3(010) surface in molecular manner. The binding energy and charge transfer of 4B are −0.24 eV and 0.06 e, which are much less than that of 4A. Thus, HCl prefers to dissociatively adsorb on LaMnO3(010) surface. Furthermore, the PDOS analysis of surface atoms in the most stable structure (4A) was conducted to further comprehend the interaction mechanism between HCl and LaMnO3 surface. Fig. 7 shows the PDOS of H and Cl atoms of HCl before and after adsorption on LaMnO3(010) surface are shown in Fig. 7. For gaseous HCl, the s- and p-states of H and Cl atoms intensely hybridize with each other, indicating a powerful interaction between H and Cl. For adsorbed HCl, all states of H and Cl atoms move to low energy level. In addition, the state hybridization between H and Cl vanishes, implying the breakage of H–Cl bond. The PDOS of H, O2f, Cl, and Mn5f atoms in configuration 4A are presented in Fig. 8. It can be seen that the H s-state hybridizes with the O2f s- and pstates at –19.9 and –7.5 eV, suggesting the formation of surface hydroxyl group. In addition, the Mn5f p- and d-states hybridize with Cl sstate at –13.9 eV. And the Cl p-state hybridize with Mn5f s-, p- and dstates at –3.2 eV. The state hybridization between Cl and Mn5f atoms implies the formation of Mn–Cl bond.
Fig. 8. PDOS of H, O, Cl, and Mn atoms over surface system after HCl adsorption on LaMnO3(010) surface.
Based on the above analyses, HCl adsorbs on LaMnO3(010) in a dissociative manner with the formation of surface hydroxyl and manganese–chlorine complexes. This is consistent with the experimental studies (Zhou et al., 2016; Yang et al., 2018b), which found that LaMnO3-based catalysts could adsorb HCl to generate surface chlorine groups for Hg0 oxidation. 3.5. Hg0 catalytic oxidation by HCl on LaMnO3 surface Hg0 catalytic oxidation by HCl on SCR catalysts generally follows either the Eley-Rideal (E–R) mechanism (Yang et al., 2017a; Zhang et al., 2015b) or the Langmuir-Hinshelwood (L-H) mechanism (Yang et al., 2017c, b). For E–R mechanism, one reagents (Hg0 or HCl) adsorbs on catalyst surface firstly, and then interacts with another gas-phase (or weakly adsorbed) reagent to produce HgCl2. For L-H mechanism, two reagents (Hg0 and HCl) adsorb on catalyst surface and then react to form HgCl2. According to the above results, Hg0 chemisorbs and HCl dissociates on LaMnO3(010), L-H mechanism is proposed for Hg0 oxidation by HCl on LaMnO3 catalyst. This conclusion is in accord with the HCl pretreatment experimental findings (Zhou et al., 2016; Yang et al., 2018b). Furthermore, the specific process of Hg0 catalytic oxidation on LaMnO3 catalyst was examined. On the basis of the results of HgCl and HgCl2 adsorption, HgCl can be both dissociatively and molecularly adsorbed, while HgCl2 can be formed and adsorbed on LaMnO3(010). Therefore, two pathways for Hg0 catalytic oxidation on LaMnO3(010) were taken into account. The path 1 contains four elementary steps: (1) Hg0 and HCl co-adsorption, (2) Hg(ads) → HgCl(ads), (3) HgCl(ads) → HgCl2(ads) and (4) HgCl2 desorption. The path 2 includes three steps: (1) Hg0 and HCl co-adsorption, (2) Hg(ads) →HgCl2(ads) and (3) HgCl2 desorption. Fig. 9 presents the energy diagram of Hg0 oxidation on LaMnO3(010) and the associated stable configurations of intermediate, transition state, and final state. For pathway 1, Hg0 catalytic oxidation goes through reactants →
Fig. 7. PDOS of H and Cl atoms for isolated HCl molecule and adsorbed HCl on LaMnO3(010) surface. 5
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Fig. 9. The energy and geometrical diagram of Hg0 oxidation by HCl on LaMnO3(010) surface.
IM1 → TS1 → IM2 → TS2 → IM3 → TS3→ FS + HgCl2. The step one is the co-adsorption of Hg0 and HCl on LaMnO3(010) surface to form IM1. This step is barrierless and exothermic by 2.44 eV. In IM1, Hg0 adsorbs on surface Mn5f site with the formation of Mn–Hg bond. Two HCl molecules dissociate on the surface to produce two active Cl atoms. The second step is the reaction of adsorbed Hg with a surface active Cl atom via IM1 → TS1 → IM2. Meanwhile, the interval of Hg and Cl atoms lessens step by step: 3.724 Å (IM1) → 2.884 Å (TS1) → 2.475 Å (IM2), suggesting HgCl formation. This step is an endothermic course with reaction heat of 0.66 eV, and the corresponding energy barrier is 0.74 eV. The third step (HgCl(ads) → HgCl2(ads)) is endothermic by 0.54 eV and activated by 0.66 eV. With the interval of Cl and Hg atoms reduces (3.553/2.475 Å (IM2) → 2.581/2.366 Å (TS2) → 2.318/ 2.317 Å (IM3)), the adsorbed Hg is eventually oxidized to surfacebonded HgCl2. The last step is the desorption of HgCl2 from the surface via IM3 → TS3 → FS + HgCl2. The energy barrier of the desorption process is as low as 0.36 eV, implying this step is ready to occur. The overall pathway 1 is exothermic by 0.98 eV, and the formation of HgCl is the rate-determining step because of its relatively larger energy barrier. Furthermore, after Hg0 and HCl co-adsorption, the relative energy raises along the reaction coordinate, which indicates that the oxidation reaction (IM1 → FS+HgCl2) is an endothermic process. It is well-known that Hg0 oxidation in coal-fired flue gas is controlled by kinetics rather than thermodynamics (Yang et al., 2016). Thus, this oxidation reaction will occur to form HgCl2 if enough external energy is provided for this reaction system. For pathway 2, the process of Hg0 oxidation goes through reactants → IM1 → TS4 → IM3 → TS3→ FS + HgCl2. The first (i.e., Hg0 and HCl co-adsorption) and last (i.e., HgCl2 desorption) steps of pathway 2 are same to that of pathway 1. In pathway 2, the surface-bonded HgCl2 is formed through a one-step reaction (Hg(ads) → HgCl2(ads)) instead of a two-step reaction (Hg(ads) → HgCl(ads) → HgCl2(ads)). For the second step (IM1→ TS4→ IM3), two surface active Cl atoms approach to the adsorbed Hg, and overcome a 1.65 eV energy barrier to generate HgCl2 on LaMnO3(010). The relatively higher energy barrier of this step suggests that the second step is the rate-determining step. Comparing the two reaction pathways, pathway 1 is kinetically more beneficial because the energy barrier of rate-determining step for pathway 1 (0.74 eV) is much lower than that of pathway 2 (1.65 eV). Thus, Hg0 catalytic oxidation by HCl on LaMnO3 catalyst prefers Hg0 → Hg(ads) → HgCl(ads) → HgCl2(ads) → HgCl2 rather than Hg0 → Hg (ads) → HgCl2(ads) → HgCl2. The previous HCl-pretreatment experiments and XPS analysis also found that the reaction process of Hg0
oxidation over LaMnO3-based catalysts occurs through a path involving HgCl (Zhou et al., 2016). Consequently, the DFT calculations are in agreement with the experimental results. In addition, the rate-determining step is the second step, i.e., HgCl formation. Moreover, compared the energy barrier of the rate-determining step for Hg0 oxidation on LaMnO3(010) with that on V2O5/TiO2(001) (Zhang et al., 2015b) and MnFe2O4(100) (Yang et al., 2017c), the energy barrier of Hg0 oxidation on LaMnO3(010) (0.74 eV) is less than that on V2O5/ TiO2(001) (0.95 eV) and MnFe2O4(100) (2.20 eV). This indicates that Hg0 catalytic oxidation on LaMnO3-based catalysts is easier than that over V2O5/TiO2 catalyst and MnFe2O4 sorbent. Therefore, LaMnO3 material is a potential candidate for Hg0 catalytic oxidation.
4. Conclusions Hg0 catalytic oxidation by HCl on LaMnO3 surface was studied by using the quantum chemistry calculations. Mn-terminated LaMnO3(010) surface exhibits higher activity than La-terminated surface for Hg0 adsorption. Hg0 prefers to adsorb on Mn5f site of LaMnO3(010) surface with chemical adsorption. The intense state hybridization between Hg and Mn5f atoms causes the chemisorption of Hg0 on the surface. Both HgCl and HgCl2 can be molecularly chemisorbed on LaMnO3(010). HCl adsorbs dissociatively on LaMnO3(010) and turned into surface chlorine complexes for Hg0 oxidation. The catalytic oxidation of Hg0 by HCl over LaMnO3 occurs through L-H mechanism where adsorbed Hg0 reacts with surface chlorine complexes to generate HgCl2. The reaction pathway of Hg0 catalytic oxidation to gaseous HgCl2 includes four elementary steps: Hg0 → Hg(ads) → HgCl (ads) → HgCl2(ads) → HgCl2, and the second step (Hg(ads) → HgCl (ads)) is the rate-determining step. After HgCl2 desorption, the adsorbed hydrogens will react with surface oxygen to form H2O and create oxygen vacancies. Subsequently, the oxygen-vacancy surface will be refilled by gaseous O2 to recover LaMnO3 catalyst. In the future study, the recovery process of the catalyst will be examined in detail.
Acknowledgments This work was supported by National Key R&D Program of China (2018YFC1901303), Fundamental Research Funds for the Central Universities (2019kfyRCPY021), and Program for HUST Academic Frontier Youth Team (2018QYTD05). 6
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Appendix A. Supplementary data
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