- Email: [email protected]
DFT study of Se and SeO2 adsorbed on CaO (0 0 1) surface: Role of oxygen
Applied Surface Science 510 (2020) 145488
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
DFT study of Se and SeO2 adsorbed on CaO (0 0 1) surface: Role of oxygen ⁎
T
Jiaying Xing, Chunbo Wang , Chan Zou, Yue Zhang Department of Energy Power & Mechanical Engineering, North China Electric Power University, Baoding 071003, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Selenium removal Density functional theory (DFT) Adsorption Pre-adsorbed O2 CaO (0 0 1) surface Reaction energy barrier
The effect of pre-adsorbed O2 on selenium removal by CaO was investigated by density functional theory (DFT) calculations. Adsorption configurations of Se and SeO2 on CaO (0 0 1) surface with pre-adsorbed O2 were optimized. And the conversion of Se to SeO2 on CaO (0 0 1) surface has been studied as well. Results showed that O2/CaO (0 0 1) surface had weaker adsorption ability than CaO (0 0 1) surface for both Se and SeO2, which indicated that pre-adsorbed O2 was not beneficial to selenium capture. When Se atom adsorbed on O2/CaO (0 0 1) surface, two kinds of stable adsorption configurations were formed: O-Se-O group and O-O-Se group on CaO (0 0 1) surface with chemical activities. When SeO2 molecule adsorbed on O2/CaO (0 0 1) surface, some valence electrons in adsorption substrate transferred into the orbits of SeO2 molecule, forming Se-O covalent bond. Additionally, the reaction energy barrier of Se and O2 conversing into SeO2 in heterogeneous was less than that in homogeneous, which indicated that CaO could not only act as an adsorbent, but also promote the conversion of Se to SeO2.
1. Introduction Selenium is a hazardous element existing in coal and mineral, which has been classified as one of the most volatile trace elements (TEs) by Clarke and Sloss [1]. Coal-fired power plant is the main anthropogenic source responsible for selenium emissions [2,3]. During coal combustion, more than 99% selenium originally presented in coal will be converted to gas-phase selenium, and about 52% emitted into atmosphere [4]. Excessive emission of selenium from coal fired power plant is relatively catastrophic, which will contaminate soil and water, and its overabundance in food chains can lead to sever selenosis [5]. The Clean Air Act Amendment (CAAA) in 1990 claimed that selenium compounds had been included as the toxic air pollutants [6]. In 2011, the emission of selenium in coal-fired power station was firstly regulated by US Environmental Protection Agency (EPA), in which the limitation was set as 5.0 × 10−2 lb/MWh [7]. With more discharge regulations in the future, there is a demand for selenium control technologies that can be applied to coal-fired power plant. The chemical state of selenium existed in the flue gas has drawn an intense controversy, and its possible oxidation states are follows: −2, 0, +2, +4 and +64. In the research of Liu et al. [8], the speciation of selenium during coal combustion has been studied by a chemicalthermal dynamic equilibrium analytical method. It was found that selenium dioxide (SeO2) was the most stable specie in oxidation atmosphere. Yan et al. [9] also concluded that SeO2 was the main product of
⁎
selenium compounds in an oxygen-containing flue gas. However, SeO2 could be partially reduced to elemental selenium (Se) by SO2 at low temperature flue gas [10]. Andren et al. [11] postulated that selenium was mainly in the form of Se at the inlet and outlet of electrostatic precipitator, in which the temperature was only about 150 °C. From the results in these studies, we can discuss that Se and SeO2 are most likely species of gaseous selenium in flue gas. Capture of selenium is a difficult task because of its high volatility and existence in the vapor phase for coal combustion. As an alternative method, the sorbent has been employed in selenium emission control. Many different solid sorbents, including activated carbon [12], fly ash [13], calcium-based sorbents [14–16], and metal sorbents [17], have been used in both pulverized coal-fired and integrated gasification combined cycle (IGCC) power plants. Among these sorbents, calcium oxide (CaO) has been proved effective on selenium removal, and it has obvious advantages of low cost and possible desulfurization capability [18,19]. Zhang et al. [20] demonstrated that CaO exhibited high capability for adsorbing gas-phase selenium, and the retention of volatile selenium was about 36.0% at 815 °C. Xiong et al. [21] claimed that CaSeO4 was the main product in selenium removal by CaO under oxygen-combustion atmosphere. Nevertheless, the mechanism of selenium removal by CaO is unclear yet, especially at different atmospheres. In particular, O2 has been researched for a long time as the main composition in the flue gas, which shows an important part in many
Corresponding author. E-mail address: [email protected] (C. Wang).
https://doi.org/10.1016/j.apsusc.2020.145488 Received 20 November 2019; Received in revised form 14 January 2020; Accepted 20 January 2020 Available online 23 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.
Applied Surface Science 510 (2020) 145488
J. Xing, et al.
O(1)
types of heterogeneous catalysis [22]. Fan et al. [23,24] studied the selenium capture by CaO in the presence of O2 using density functional theory (DFT) method. They pointed out that the oxidation of SeO2 on CaO (0 0 1) surface was difficult and the adsorption product was mainly selenite at oxidizing atmosphere. Given the huge volume fraction difference of selenium and oxygen in the flue gas, as well as the excessive adsorbent, the adsorption order of corresponding gases will be affected. In the study of Galbreath et al [25], experiments were conducted to investigate the adsorption of Hg0 (g) on fly ash (25% CaO) in the presence of O2. The result showed that O2 might be adsorbed onto the sorbent surface firstly, forming O2/CaO surface, which indicated that pre-adsorbed O2 showed a great influence on Hg removal. Zhang et al. [26] and Liu et al. [27] studied the effect of pre-adsorbed O2 on gasphase arsenic and mercury removal by Fe2O3, they concluded that O2 was conductive to the adsorption of Hg0 and As2O3. In this paper, the effect of pre-adsorbed O2 on selenium capture by CaO is investigated using DFT method. Different adsorption configurations of Se on CaO surface with pre-adsorbed O2 are optimized firstly. Considering that the presence of O2 will promote Se oxidation, the conversion of Se to SeO2 on CaO (0 0 1) surface is also considered. In addition, the adsorption of SeO2 onto CaO surface with pre-adsorbed O2 are researched as well. The results in this study can provide reference to control selenium emission by adsorbents.
O(2) O(4)
O(5) O(1) bridge O(6)
O(2)
O(3)
Fig. 1. Side and top views of O2/CaO (0 0 1) surface (O and Ca atoms are in red and blue, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
with). For the adsorption on pre-adsorbed O2, three different sites were obtained: O(1)-top site, O(2)-top site and bridge site. Due to the result that O-top site was the active site in CaO (0 0 1) surface from frontier orbital analysis, as well as the same chemical environments of O(4) atom and O(6) atom, we considered the possible adsorption sites of O(3)-top, O(4)-top and O(5)-top when Se and SeO2 adsorbed on the second kind of adsorption sites. The adsorption energy (Ead) is defined as follows:
2. Models and computational methods
Ead = Esys − Eads − Esur 3
All the calculations in this paper were carried out by DMol [28,29] package incorporated in Material Studio Software, which can be used to study the adsorption structure, reaction energies, reaction barriers, molecular orbitals, thermodynamic properties, and vibrational spectra, etc. The GGA (Generalized Gradient Approximation) with PBE [30,31] (Perdew-Burke-Ernzerhof) functional was selected for calculating the exchange and correlation functional. The molecular orbitals were expanded by a double numerical plus d-functions (DND), whose magnitude of basis set super-position error (BSSE) was less than that of Gaussian 6–311+G (3df,2pd) [32,33]. The transition states (TS) were searched with a combination of linear synchronous transit (LST) and quadratic synchronous transit (QST) method [34], which were further calculated to guarantee only one imaginary frequency. And the lattice constant of calculated CaO crystal were as follows: a = b = c = 4.81 Å, and α = β = γ = 90°, which was consistent with the experimental value [35]. Fan et al. [24] concluded that CaO (0 0 1) surface was easily exposed to ambient atmosphere, and it was the most stable and symmetrical surface of CaO powder [36]. Therefore, all of our calculations were based on CaO (0 0 1) surface. In this work, CaO (0 0 1) surface was modeled by a periodically repeated (2 × 2) slab consisting of five atomic layers, and each slab was separated by a 15 Å vacuum layer to minimize interactions between the slabs. The bottom three atomic layers were frozen in the original bulk positions, and the top two atomic layers were fully relaxed during geometric optimization with 3 × 3 × 1 k-point in a Monhorst-pack grid. The convergence criteria of geometric optimization and TS search were as bellows: (a) an optimized energy tolerance of 2.0 × 10−5 Ha; (b) an optimized gradient tolerance of 0.004 Ha/Å; (c) an optimized displacement tolerance of 0.005 Å; (d) a self-consistent field (SCF) density tolerance of 2.0 × 10−5 Ha; Reasonable structures of O2 and SeO2 had been obtained by geometric optimization. The calculated O-O bond of O2 molecule and Se-O bond of SeO2 molecule were 1.23 Å and 1.66 Å, and the angle of O-Se-O is 115.04°, which agreed well with the experimental data of 1.24 Å, 1.61 Å and 113.80° [37,38]. Fig. 1 shows the most stable adsorption structure when O2 molecule adsorbed on the bridge site in CaO (0 0 1) surface parallel [39] (O2/CaO (0 0 1) surface). In Fig. 1, two kinds of adsorption sites in O2/CaO (0 0 1) surface were taken into consideration: the Se and SeO2 adsorbed directly onto the pre-adsorbed O2 or near of it (not in direct contact
(1)
where Esur is energy of the surface before adsorption, Eads is the energy of the adsorbate, Esys is the total energy of the system after adsorption, Ead is the adsorption energy. A negative Ead value represents a stable adsorption system. 3. Results and discussion 3.1. Effect of pre-adsorbed O2 on Se adsorption 3.1.1. Adsorption of Se on O2/CaO (0 0 1) surface Adsorption of Se atom on O2/CaO (0 0 1) surface was firstly investigated with six adsorption schemes, including Se on O(1)-top site, O(2)-top site, O(3)-top site, O(4)-top site, O(5)-top site and bridge site. Four different stable adsorption structures were obtained. The adsorption energies were found to be −1.31 eV, −2.47 eV, −2.63 eV and −2.66 eV, respectively. The optimized configurations are depicted in Fig. 2, and the corresponding adsorption energies as well as the charge transfers of Se atom are listed in Table 1. For Se atom adsorbed directly onto pre-adsorbed O2, as seen from the results, O-Se-O group was formed in adsorption configuration-a. The Se atom provided 0.20 e to the adsorbate surface after adsorption according to Mulliken population analysis, and the reason could be explained as follow: Some valence electrons in Se atom transferred into the orbits of O(1) atom. Which confirmed that O2 could oxidize Se atom under heterogeneous condition. In addition, there was a stronger interaction existed between Se atom and O(2) atom (adsorption configuration-b) with lower chemisorption energy, indicating that this mode was the most stable structure for Se atom adsorbed directly onto peradsorbed O2. Upon adsorption, the Se-O(2) covalent bond was formed and the O(1)-O(2) bond length increased from 1.30 Å to 1.36 Å. The expansion of the bond length meant that O(1)-O(2) bond became easier to break. When the adsorption system reached equilibrium, the Se terminal in O-O-Se group was formed with chemical activity, which had a great possibility conversing into SeO2. Therefore, it was essential to explore the transformation of Se to SeO2 in CaO (0 0 1) surface. From Table 1, the adsorption of Se atom near pre-adsorbed O2 belonged to chemisorption (lower than −1.04 eV [40]). Configuration-c and configuration-d were obtained from Se on O(3) and O(4) top site, respectively. The adsorption energy in configuration-d was −2.66 eV, 2
Applied Surface Science 510 (2020) 145488
J. Xing, et al.
Fig. 2. Optimized configurations of Se on O2/CaO (0 0 1) surface (O, Ca and Se atoms are in red, blue, and yellow, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 1 Adsorption energies and the charge transfers of Se on O2/CaO (0 0 1) surface. Number
Ead/eV
q(Se)/e
a b c d
−1.31 −2.47 −2.63 −2.66
0.20 0.18 0.04 −0.55
which was the lowest adsorption energy among those four schemes. The Se atom obtained 0.55 e from substrate surface during the adsorption process and the Se-O bond was formed between Se atom and O(3) atom. However, the binding energy in configuration-c was higher than that in configuration-d, which indicated that the adsorption ability in O(3) top site was relatively weak, and it would be further investigated in the following parts. 3.1.2. Differences between Se on CaO (0 0 1) surface with and without preadsorbed O2 The binding mechanism of Se on CaO (0 0 1) surface was investigated. The adsorption energy was −2.65 eV and Se atom obtained 0.62 e from the adsorption substrate after adsorption. Compared with the adsorption energies of Se on O2/CaO (0 0 1) surface from 3.1.1, the Se adsorption on CaO (0 0 1) surface was stronger, which indicated that the pre-adsorbed O2 was not conductive to Se adsorption. In addition, the adsorption in configuration-c was the weakest when Se atom adsorbed on O2/CaO (0 0 1) surface near pre-adsorbed O2. To clarify the effect of O2 on the removal of Se atom, the projected density of states (PDOS) of Se atom and O(3) atom were plotted in different configurations (Se on CaO (0 0 1) surface and O2/CaO (0 0 1) surface with the O(3) top site), as shown in Fig. 3. In Fig. 3, the s orbital, p orbital of Se atom and the s orbital, p orbital of O(3) atom were plotted in black, red, blue and grey color, respectively. Comparing the PDOS of Se atom and O(3) atom in different geometries, when Se adsorbed on O2/CaO (0 0 1) surface, the s orbitals showed lower energy states than that on CaO (0 0 1) surface in both two atoms. Due to the additional interaction between Se atom and pre-absorbed O2, the resonances at −18.3 eV and −11.6 eV between the s orbital of Se atom and s orbital of O(3) atom for Se on O2/CaO (0 0 1) surface were lower than that at −17.9 eV and −10.9 eV on CaO (0 0 1)
Fig. 3. PDOS of Se atom and O(3) atom after Se adsorbed on CaO (0 0 1) surface and O2/CaO (0 0 1) surface.
surface. Simultaneously, it could be observed that the similar phenomenon occurred in the resonance at −1.2 eV between the p orbital of Se atom and p orbital of O(3) atom. However, from the respective of orbitals’ overlap, the overlaps of p orbitals between Se atom and O(3) atom were the main electronic participants in chemisorption. The peaks of p orbitals in both Se atom and O(3) atom at −4.2 eV and −3.7 eV split into two existed peaks for Se on CaO (0 0 1) surface and three existed peaks for Se on O2/CaO (0 0 1) surface. The overlaps between the p orbital of Se atom and p orbital of O(3) atom for Se on CaO (0 0 1) surface was a little well than that on O2/CaO (0 0 1) surface. Which indicated that the adsorption of Se atom adsorbed directly onto preadsorbed O2 was the major factor on Se adsorption.
3.2. Conversion of Se to SeO2 on CaO (0 0 1) surface Adsorption is the first step for a gas–solid reaction [41], from 3.1.1, the O-O-Se group and O-Se-O group were formed with chemical 3
Applied Surface Science 510 (2020) 145488
J. Xing, et al.
TS → SeO2, and the changes of the distance between Se and O, O and O, as well as the angle of O-Se-O were as follows: 2.18 Å → 1.90 Å → 1.82 Å → 1.66 Å, 1.23 Å → 1.49 Å → 1.78 Å → 2.80 Å, and 32.78° → 46.25° → 58.40° → 115.04°. Compared with the formation of SeO2 on CaO (0 0 1) surface, the reaction energy barrier of the reaction in homogenous (0.52 eV) was much higher than that in heterogeneous (0.27 eV). Therefore, we could conclude that CaO can not only act as an adsorbent, but also promote the conversion of Se to SeO2. And this was similar to the result from Galbreath et al. [25] on the study of Hg0 adsorption by fly ash particles (25% CaO). And they discussed that the HgO was formed on particle surfaces accounting for the reaction between Hg0 and pre-adsorbed O atom. 3.3. Effect of pre-adsorbed O2 on SeO2 adsorption 3.3.1. Adsorption of SeO2 on O2/CaO (0 0 1) surface In order to make the study more comprehensive, two orientations (perpendicular and parallel) of SeO2 approaching O2/CaO (0 0 1) surface with its Se atom were considered, and six different sites (O(1)-top, O(2)-top, bridge, O(3)-top, O(4)-top and O(5)-top) were examined systematically for each orientation. Five configurations were obtained after optimization. The optimized configurations and parameters are given in Fig. 6 and Table 2, respectively. Configuration-a was the stable structure when SeO2 adsorbed on O(1) top site perpendicular with its Se atom, and SeO2 was parallel to pre-adsorbed O2 concurrently. Configuration-b was the stable structure when SeO2 adsorbed on bridge site perpendicular with its Se atom, and SeO2 was perpendicular to pre-adsorbed O2. As seen from Fig. 6, the SeO bond between Se atom and pre-adsorbed O atom, were formed during the optimization processes among these two adsorption geometries. Upon adsorption, some electrons transferred from Se atom to the substrate O atoms. This was consistent with the conclusion reached by Zhang et al. [20] in the study of the retention of selenium during coal combustion. That is, CaO combined with SeO2 to form CaSeO4, inhibiting the volatilization of selenium. For configuration-c, the adsorption was relatively weak and its adsorption energy was just −0.49 eV, which suggested that the adsorbing type was physisorption. From Table 2, when SeO2 adsorbed on O2/CaO (0 0 1) surface in configuration-b, the most stable structure was obtained with its lowest adsorption energy (−2.03 eV). During the adsorption process, the distance between O(2) atom and CaO (0 0 1) surface increased from 2.73 Å to 3.13 Å, and the length of O(1)-O(2) bond was extended from 1.30 Å to 1.35 Å. These results indicated that the adsorption between Se atom and the O(2) atom was strong, and the O(2) atom gradually broke off the binding from the CaO(0 0 1) surface at the same time. The PDOS analysis was carried out to study the electron states of Se atom and O(2) atom after the adsorption in geometry-b. As shown in Fig. 5, the electron states in 2p orbital of Se atom were overlapped well with the 2p orbital of O(2) atom in the peaks around −14.2 eV, −11.1 eV as well as −7.7 to 0 eV, which proved the formation of Se-O bond between the Se atom of SeO2 and the O atom of substrate surface. Configuration-d and configuration-e were the adsorption geometries when SeO2 adsorbed on O(3) top and O(5) top site parallel with its Se atom respectively. It could be seen from Fig. 7 that the Se-O bond between Se atom of SeO2 and original O atom of O2/CaO (0 0 1) surface was formed in these two stable schemes. We could conjecture that a repulsion existed between SeO2 and pre-absorbed O2 on CaO (0 0 1) surface, accounting for their increasing distance. According to the data in Table 2, there was a conclusion that the O(5) top site was the active site for SeO2 adsorbed on O2/CaO (0 0 1) surface near of pre-adsorbed O2, and its adsorption energy was −1.97 eV (configuration-e). The SeO2 obtained 0.56 e from the adsorption substrate and Se atom provided 0.013 e to O atom adjacently during adsorption. However, configuration-d showed a relatively weak adsorption due to the interaction between SeO2 and pre-absorbed O2 on CaO (0 0 1) surface, and its adsorption energy was just −1.82 eV.
Fig. 4. Reaction potential energy surface of the heterogeneous reactions.
activities. Two possible pathways of O-O-Se group and O-Se-O group conversing into SeO2 on CaO (0 0 1) surface were considered and the reaction potential energy surfaces were shown in Fig. 4. According to the transition state theory, reaction energy barrier is defined as the energy difference between transition and reactant (or intermediate). As shown in Fig. 4, R1 and R2 were the stable configurations of O-O-Se group and O-Se-O group adsorbed on CaO (0 0 1) surface. P1, P2 were corresponding stable structures of SeO2 adsorbed on CaO (0 0 1) surface, respectively. For the reaction possess of O-O-Se group conversing into SeO2 on CaO (0 0 1) surface, the initial adsorption state needed to overcome 0.27 eV energy barrier and release 2.46 eV. The mechanism was as follow: O-O-Se/CaO (0 0 1) → TS → SeO2/CaO (0 0 1). During the reaction, the changes of the distance between Se and O(1), O(1) and O(2), as well as the angle of O(1)-Se-O(2) were as follows: 2.85 Å → 2.79 Å → 1.71 Å, 1.36 Å → 1.33 Å → 2.80 Å, and 24.53° → 25.26° → 110.59°, which meant that the formation of SeO(1) bond and the cleavage of the O(1)-O(2) bond were carried out simultaneously. In terms of O-Se-O group conversing into SeO2 on CaO (0 0 1) surface, the energy barrier was 0.24 eV and it was also an exothermic process releasing 3.30 eV. The mechanism of O-Se-O group to SeO2 on CaO (0 0 1) surface was similar as former. The formation of SeO2 on CaO (0 0 1) surface was one-step reaction and there was no intermediate formed. Fig. 5 described the reaction potential energy surface of Se + O2 → SeO2 in homogenous. The Se atom reacted with O2 molecule to form a stable intermediate M, and then O-O bond in M broke down into the transition state. The mechanism for this reaction was Se + O2 → M →
Fig. 5. Reaction potential energy surface of the homogenous reaction. 4
Applied Surface Science 510 (2020) 145488
J. Xing, et al.
Fig. 6. Optimized configurations of Se O2 on O2/CaO (0 0 1) surface (O, Ca and Se atoms are in red, blue, and yellow, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 2 Adsorption energies and the charge transfers of SeO2 on O2/CaO (0 0 1) surface. Number
Ead/eV
q(Se to O)/e
q(SeO2)/e
a b c d e
−1.38 −2.03 −0.49 −1.82 −1.97
0.088 0.069 0.064 −0.056 −0.013
−0.36 −0.56 0.17 −0.60 −0.56
Fig. 7. PDOS of Se atom and O(2) atom after SeO2 adsorbed on O2/CaO (0 0 1) surface.
3.3.2. Differences between SeO2 on CaO (0 0 1) surface with and without pre-adsorbed O2 For SeO2 adsorbed on CaO (0 0 1) surface, the most stable structure was obtained when SeO2 adsorbed on O top site of CaO (0 0 1) surface with its Se atom [22]. Results showed that the adsorption energy was −2.28 eV and SeO2 molecule obtained 0.61 e from CaO (0 0 1) surface upon adsorption. Compared with the adsorption energy of Se on O2/ CaO (0 0 1) surface, we could discuss that pre-adsorbed O2 was not conducive to the adsorption of SeO2 by CaO. The PDOS of Se atom, Osurf atom, pre-adsorbed O atom and Casurf atom were plotted to explore the effects of pre-adsorbed O2 as shown in Fig. 8. Osurf, Casurf are atoms in CaO (0 0 1) surface nearest from pre-absorbed O atom, Se atom
Fig. 8. PDOS of Se atom, Osurf atom, O atom of SeO2 and Casurf atoms after SeO2 adsorbed on CaO (0 0 1) surface and O2/CaO (0 0 1) surface: (a) PDOS of Se atom and Osurf atom; (b) PDOS of O atom of SeO2 and Casurf atom.
respectively. As can be seen from Fig. 8, the PDOS analysis was further investigated to reflect the differences of SeO2 on CaO (0 0 1) surface in 5
Applied Surface Science 510 (2020) 145488
J. Xing, et al.
configuration-d. From Fig. 8(a), the energy states for SeO2 on CaO (0 0 1) surface were lower than that on O2/CaO (0 0 1) surface. Especially, the resonances at −20.5 eV, −17.8 eV and −3.3 eV for SeO2 on CaO (0 0 1) surface was lower than that at −20.2 eV, −17.6 eV and −3.1 eV on O2/CaO (0 0 1) surface. Therefore, the configuration of SeO2 on CaO (0 0 1) surface was more stable, which confirmed the results obtained by adsorption energy and electron transfer. In addition, there was no obvious difference between the configurations of SeO2 on CaO (0 0 1) surface and O2/CaO (0 0 1) surface. The overlaps of the orbitals between O atom of SeO2 and Casurf atom were almost equal. This was similar to the result from 3.1.1, and the adsorption of SeO2 adsorbed directly onto pre-adsorbed O2 was the major influencing factor on SeO2 adsorption.
[7] U.S. Environmental Protection Agency, Mercury and Air Toxics Standards (MATS), 2017, http://www3.epa.gov/mats/index.html. [8] Y. Liu, C. Zheng, Q. Wang, Speciation of most volatile toxic trace elements during coal combustion, Asia-Pac. J. Chem. Eng. 11 (3–4) (2010) 381–394. [9] R. Yan, D. Gauthier, G. Flamant, G. Peraudeau, J. Lu, C. Zheng, Fate of selenium in coal combustion: volatilization and speciation in the flue gas, Environ. Sci. Technol. 35 (7) (2001) 1406–1410. [10] E.B. Dismukes, Trace element control in electrostatic precipitators and fabric filters, Fuel Process. Technol. 39 (1–3) (1994) 403–416. [11] A.W. Andren, D.H. Klein, Y. Talmi, Selenium in coal-fired steam plant emissions, Environ. Sci. Technol. 9 (9) (1975) 856–858. [12] M. Diaz-Somoano, M.R. Martinez-Tarazona, Retention of arsenic and selenium compounds using limestone in a coal gasification flue gas, Environ. Sci. Technol. 38 (3) (2004) 899–903. [13] M.A. López-Antón, M. Díaz-Somoano, D.A. Spears, M.R. Martínez-Tarazona, Arsenic and selenium capture by fly ashes at low temperature (Article), Environ. Sci. Technol. 40 (12) (2006) 3947–3951. [14] R.O. Sterling, J.J. Helble, Reaction of arsenic vapor species with fly ash compounds: kinetics and speciation of the reaction with calcium silicates, Chemosphere 51 (10) (2003) 1111–1119. [15] M.A. López-Antón, M. Díaz-Somoano, J.L.G. Fierro, M.R. Martínez-Tarazona, Retention of arsenic and selenium compounds present in coal combustion and gasification flue gases using activated carbons, Fuel Process. Technol. 88 (8) (2007) 799–805. [16] Y. Li, H. Tong, Z. Yu, L. Yan, C. X, Simultaneous removal of SO2 and trace As2O3 from flue gas: mechanism, kinetics study, and effect of main gases on arsenic capture, Environ. Sci. Technol. 41 (8) (2007) 2894. [17] M. Diaz-Somoano, M.A. López-Antón, M.R. Martínez-Tarazona, Retention of arsenic and selenium during hot gas desulfurization using metal oxide sorbents, Energy Fuels 18 (5) (2007) 1238–1242. [18] R. Agnihotri, S. Chauk, S. Mahuli, L.S. Fan, Selenium removal using Ca-based sorbents: reaction kinetics, Environ. Sci. Technol. 12 (1998) 1841. [19] Y. Li, H. Tong, Y. Zhuo, C. Chen, X. Xu, Simultaneous removal of SO2 and trace SeO2 from flue gas: effect of product layer on mass transfer, Environ. Sci. Technol. 40 (13) (2006) 4306–4311. [20] J. Zhang, D. Ren, Q. Zhong, F. Xu, Y. Zhang, J. Yin, Retention of selenium volatility using lime in coal combustion, Environ. Sci. 22 (3) (2001) 100–103. [21] Q. Xiong, J. Qiu, C. Xu, Q. Wang, H. Liu, H. Liu, Y. Chen, Z. Xu, Study on the volatilization behavior of Se under oxygen-combustion atmosphere, J. Eng. Thermophys. 2 (2006) 195–198. [22] C. Wang, Z. Yue, H. Liu, Experimental and mechanism study of gas-phase arsenic adsorption over Fe2O3/γ-Al2O3 sorbent in oxy-fuel combustion flue gas, Ind. Eng. Chem. Res. 55 (40) (2016) 10656–10663. [23] Y. Fan, Y. Zhuo, Y. Lou, Z. Zhu, L. Li, SeO2 adsorption on CaO surface: DFT study on the adsorption of a single SeO2 molecule, Appl. Surf. Sci. 413 (2017) 366–371. [24] Y. Fan, Y. Zhuo, L. Li, SeO2 adsorption on CaO surface: DFT and experimental study on the adsorption of multiple SeO2 molecules, Appl. Surf. Sci. 420 (2017) 465–471. [25] K.C. Galbreath, C.J. Zygarlicke, Mercury transformations in coal combustion flue gas, Fuel Process. Technol. 65 (99) (2000) 289–310. [26] Y. Zhang, J. Liu, Density functional theory study of arsenic adsorption on the Fe2O3(001) surface, Energy Fuels 33 (2019) 1414–1421. [27] T. Liu, L. Xue, X. Guo, C. Zheng, DFT study of mercury adsorption on α-Fe2O3 surface: role of oxygen, Fuel 115 (2014) 179–185. [28] B. Delley, An all-electron numerical method for solving the local density functional for polyatomic molecules, J. Chem. Phys. 92 (1) (1990) 508–517. [29] B. Delley, From molecules to solids with the DMol3 approach, J. Chem. Phys. 113 (18) (2000) 7756–7764. [30] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, C. Fiolhais, Atoms, molecules, solids, and surfaces: applications of the generalized gradient approximation for exchange and correlation, Phys. Rev. B 46 (11) (1992) 6671–6687. [31] D. Vanderbilt, Soft self-consistent pseudopotentials in a generalized eigenvalue formalism, Phys. Rev. B: Condens. Matter 41 (11) (1990) 7892–7895. [32] Y. Inada, H. Orita, Efficiency of numerical basis sets for predicting the binding energies of hydrogen bonded complexes: Evidence of small basis set superposition error compared to Gaussian basis sets, J. Comput. Chem. 29 (2) (2008) 225–232. [33] V.A. Basiuk, Interaction of porphine with closed-end zigzag (6,0) single-walled carbon nanotube: the effect of parameters in DMol3 DFT calculations, J. Comput. Theor. Nanosci. 5 (11) (2008) 2114–2118. [34] N. Govind, M. Petersen, G. Fitzgerald, D. King-Smith, J. Andzelm, A generalized synchronous transit method for transition state location, Comput. Mater. Sci. 28 (2) (2003) 250–258. [35] C.H. Shen, R.S. Liu, J.G. Lin, C.Y. Huang, Phase stability study of La1.2Ca1.8Mn2O7, Mater. Res. Bull. 36 (5–6) (2001) 1139–1148. [36] P. Blowers, B.G. Kim, The adsorption of mercury-species on relaxed and rumpled CaO (001) surfaces investigated by density functional theory, J. Mol. Model. 17 (3) (2011) 505–514. [37] M.H. Shah, J. Iqbal, N. Shaheen, N. Khan, M.A. Choudhary, G. Akhter, Assessment of background levels of trace metals in water and soil from a remote region of Himalaya, Environ. Monit. Assess. 184 (3) (2012) 1243–1252. [38] D.A. Otoniel, S. Thierry, H. Philippe, D. Marie-Lise, M. Sarantos, Potential energy surface and rovibrational energy levels of the H2-CS van der Waals complex, J. Chem. Phys. 137 (23) (2012) L53. [39] L. Wang, Adsorption of O2 on the CaO and SrO (100) surfaces: a first-principles study, Appl. Surf. Sci. 257 (13) (2011) 5499–5502. [40] J.K. Nørskov, F. Studt, F. Abild-Pedersen, T. Bligaard, Fundamental Concepts in Heterogeneous Catalysis 33 (3) (2015) 10. [41] L. Wu, Q. Wu, X. Hu, C. Dong, Y. Yang, Mechanism study on the influence of in situ SOx removal on N2O emission in CFB boiler, Appl. Surf. Sci. 333 (2015) 194–200.
4. Conclusions Density functional theory (DFT) was employed to study the effect of pre-adsorbed O2 on the adsorption of Se, SeO2 on CaO (0 0 1) surface. Binding energies and electron transfers in different configurations were calculated to reveal the adsorption mechanism. It was found that when Se atom adsorbed on O2/CaO (0 0 1) surface, two kinds of stable adsorption configurations were formed: O-Se-O group and O-O-Se group on CaO (0 0 1) surface with chemical activities. When SeO2 molecule adsorbed on O2/CaO (0 0 1) surface, some valence electrons in adsorption substrate transferred into the orbits of SeO2 molecule, forming Se-O covalent bond. Moreover, O2/CaO (0 0 1) surface showed weaker adsorption ability than CaO (0 0 1) surface for both Se and SeO2, which indicated that the pre-adsorbed O2 was not beneficial to selenium capture. And the adsorption of Se, SeO2 adsorbed directly onto preadsorbed O2 were the major factors. For the processes of Se and O2 conversing into SeO2, the reaction energy barrier in heterogeneous was less than that in homogeneous, which stated that CaO could not only act as an adsorbent, but also promote the conversion of Se to SeO2. CRediT authorship contribution statement Jiaying Xing: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Chunbo Wang: Validation, Formal analysis, Visualization, Supervision. Chan Zou: Validation, Formal analysis, Visualization, Software. Yue Zhang: Validation, Formal analysis, Visualization, Software. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by the National Natural Science Foundation of China (51976059) and the Fundamental Research Funds for the Central Universities (2018ZD03). References [1] L.B. Clarke, Lesley Sloss, Trace elements emissions from coal combustion and gasification, IEA Coal Res. (1992). [2] J.F. Cheng, H.C. Zeng, Z.H. Zhang, M.H. Xu, The effects of solid absorbents on the emission of trace elements, SO2, and NOx during coal combustion, Int. J. Energy Res. 25 (12) (2001) 1043–1052. [3] L.B. Clarke, The fate of trace elements during coal combustion and gasification: an overview, Fuel 72 (6) (1993) 731–736. [4] A. Ghosh-Dastidar, S. Mahuli, R. Agnihotri, L.S. Fan, Selenium capture using sorbent powders: mechanism of sorption by hydrated lime, Environ. Sci. Technol. 30 (2) (1996) 447–452. [5] S.J. Hamilton, Review of selenium toxicity in the aquatic food chain, Sci. Total Environ. 326 (1–3) (2004) 1–31. [6] W.G. Rosenberg, Clean air act amendments, Science 251 (5001) (1991) 1546–1547.
6
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
DFT study of Se and SeO2 adsorbed on CaO (0 0 1) surface: Role of oxygen ⁎
T
Jiaying Xing, Chunbo Wang , Chan Zou, Yue Zhang Department of Energy Power & Mechanical Engineering, North China Electric Power University, Baoding 071003, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Selenium removal Density functional theory (DFT) Adsorption Pre-adsorbed O2 CaO (0 0 1) surface Reaction energy barrier
The effect of pre-adsorbed O2 on selenium removal by CaO was investigated by density functional theory (DFT) calculations. Adsorption configurations of Se and SeO2 on CaO (0 0 1) surface with pre-adsorbed O2 were optimized. And the conversion of Se to SeO2 on CaO (0 0 1) surface has been studied as well. Results showed that O2/CaO (0 0 1) surface had weaker adsorption ability than CaO (0 0 1) surface for both Se and SeO2, which indicated that pre-adsorbed O2 was not beneficial to selenium capture. When Se atom adsorbed on O2/CaO (0 0 1) surface, two kinds of stable adsorption configurations were formed: O-Se-O group and O-O-Se group on CaO (0 0 1) surface with chemical activities. When SeO2 molecule adsorbed on O2/CaO (0 0 1) surface, some valence electrons in adsorption substrate transferred into the orbits of SeO2 molecule, forming Se-O covalent bond. Additionally, the reaction energy barrier of Se and O2 conversing into SeO2 in heterogeneous was less than that in homogeneous, which indicated that CaO could not only act as an adsorbent, but also promote the conversion of Se to SeO2.
1. Introduction Selenium is a hazardous element existing in coal and mineral, which has been classified as one of the most volatile trace elements (TEs) by Clarke and Sloss [1]. Coal-fired power plant is the main anthropogenic source responsible for selenium emissions [2,3]. During coal combustion, more than 99% selenium originally presented in coal will be converted to gas-phase selenium, and about 52% emitted into atmosphere [4]. Excessive emission of selenium from coal fired power plant is relatively catastrophic, which will contaminate soil and water, and its overabundance in food chains can lead to sever selenosis [5]. The Clean Air Act Amendment (CAAA) in 1990 claimed that selenium compounds had been included as the toxic air pollutants [6]. In 2011, the emission of selenium in coal-fired power station was firstly regulated by US Environmental Protection Agency (EPA), in which the limitation was set as 5.0 × 10−2 lb/MWh [7]. With more discharge regulations in the future, there is a demand for selenium control technologies that can be applied to coal-fired power plant. The chemical state of selenium existed in the flue gas has drawn an intense controversy, and its possible oxidation states are follows: −2, 0, +2, +4 and +64. In the research of Liu et al. [8], the speciation of selenium during coal combustion has been studied by a chemicalthermal dynamic equilibrium analytical method. It was found that selenium dioxide (SeO2) was the most stable specie in oxidation atmosphere. Yan et al. [9] also concluded that SeO2 was the main product of
⁎
selenium compounds in an oxygen-containing flue gas. However, SeO2 could be partially reduced to elemental selenium (Se) by SO2 at low temperature flue gas [10]. Andren et al. [11] postulated that selenium was mainly in the form of Se at the inlet and outlet of electrostatic precipitator, in which the temperature was only about 150 °C. From the results in these studies, we can discuss that Se and SeO2 are most likely species of gaseous selenium in flue gas. Capture of selenium is a difficult task because of its high volatility and existence in the vapor phase for coal combustion. As an alternative method, the sorbent has been employed in selenium emission control. Many different solid sorbents, including activated carbon [12], fly ash [13], calcium-based sorbents [14–16], and metal sorbents [17], have been used in both pulverized coal-fired and integrated gasification combined cycle (IGCC) power plants. Among these sorbents, calcium oxide (CaO) has been proved effective on selenium removal, and it has obvious advantages of low cost and possible desulfurization capability [18,19]. Zhang et al. [20] demonstrated that CaO exhibited high capability for adsorbing gas-phase selenium, and the retention of volatile selenium was about 36.0% at 815 °C. Xiong et al. [21] claimed that CaSeO4 was the main product in selenium removal by CaO under oxygen-combustion atmosphere. Nevertheless, the mechanism of selenium removal by CaO is unclear yet, especially at different atmospheres. In particular, O2 has been researched for a long time as the main composition in the flue gas, which shows an important part in many
Corresponding author. E-mail address: [email protected] (C. Wang).
https://doi.org/10.1016/j.apsusc.2020.145488 Received 20 November 2019; Received in revised form 14 January 2020; Accepted 20 January 2020 Available online 23 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.
Applied Surface Science 510 (2020) 145488
J. Xing, et al.
O(1)
types of heterogeneous catalysis [22]. Fan et al. [23,24] studied the selenium capture by CaO in the presence of O2 using density functional theory (DFT) method. They pointed out that the oxidation of SeO2 on CaO (0 0 1) surface was difficult and the adsorption product was mainly selenite at oxidizing atmosphere. Given the huge volume fraction difference of selenium and oxygen in the flue gas, as well as the excessive adsorbent, the adsorption order of corresponding gases will be affected. In the study of Galbreath et al [25], experiments were conducted to investigate the adsorption of Hg0 (g) on fly ash (25% CaO) in the presence of O2. The result showed that O2 might be adsorbed onto the sorbent surface firstly, forming O2/CaO surface, which indicated that pre-adsorbed O2 showed a great influence on Hg removal. Zhang et al. [26] and Liu et al. [27] studied the effect of pre-adsorbed O2 on gasphase arsenic and mercury removal by Fe2O3, they concluded that O2 was conductive to the adsorption of Hg0 and As2O3. In this paper, the effect of pre-adsorbed O2 on selenium capture by CaO is investigated using DFT method. Different adsorption configurations of Se on CaO surface with pre-adsorbed O2 are optimized firstly. Considering that the presence of O2 will promote Se oxidation, the conversion of Se to SeO2 on CaO (0 0 1) surface is also considered. In addition, the adsorption of SeO2 onto CaO surface with pre-adsorbed O2 are researched as well. The results in this study can provide reference to control selenium emission by adsorbents.
O(2) O(4)
O(5) O(1) bridge O(6)
O(2)
O(3)
Fig. 1. Side and top views of O2/CaO (0 0 1) surface (O and Ca atoms are in red and blue, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
with). For the adsorption on pre-adsorbed O2, three different sites were obtained: O(1)-top site, O(2)-top site and bridge site. Due to the result that O-top site was the active site in CaO (0 0 1) surface from frontier orbital analysis, as well as the same chemical environments of O(4) atom and O(6) atom, we considered the possible adsorption sites of O(3)-top, O(4)-top and O(5)-top when Se and SeO2 adsorbed on the second kind of adsorption sites. The adsorption energy (Ead) is defined as follows:
2. Models and computational methods
Ead = Esys − Eads − Esur 3
All the calculations in this paper were carried out by DMol [28,29] package incorporated in Material Studio Software, which can be used to study the adsorption structure, reaction energies, reaction barriers, molecular orbitals, thermodynamic properties, and vibrational spectra, etc. The GGA (Generalized Gradient Approximation) with PBE [30,31] (Perdew-Burke-Ernzerhof) functional was selected for calculating the exchange and correlation functional. The molecular orbitals were expanded by a double numerical plus d-functions (DND), whose magnitude of basis set super-position error (BSSE) was less than that of Gaussian 6–311+G (3df,2pd) [32,33]. The transition states (TS) were searched with a combination of linear synchronous transit (LST) and quadratic synchronous transit (QST) method [34], which were further calculated to guarantee only one imaginary frequency. And the lattice constant of calculated CaO crystal were as follows: a = b = c = 4.81 Å, and α = β = γ = 90°, which was consistent with the experimental value [35]. Fan et al. [24] concluded that CaO (0 0 1) surface was easily exposed to ambient atmosphere, and it was the most stable and symmetrical surface of CaO powder [36]. Therefore, all of our calculations were based on CaO (0 0 1) surface. In this work, CaO (0 0 1) surface was modeled by a periodically repeated (2 × 2) slab consisting of five atomic layers, and each slab was separated by a 15 Å vacuum layer to minimize interactions between the slabs. The bottom three atomic layers were frozen in the original bulk positions, and the top two atomic layers were fully relaxed during geometric optimization with 3 × 3 × 1 k-point in a Monhorst-pack grid. The convergence criteria of geometric optimization and TS search were as bellows: (a) an optimized energy tolerance of 2.0 × 10−5 Ha; (b) an optimized gradient tolerance of 0.004 Ha/Å; (c) an optimized displacement tolerance of 0.005 Å; (d) a self-consistent field (SCF) density tolerance of 2.0 × 10−5 Ha; Reasonable structures of O2 and SeO2 had been obtained by geometric optimization. The calculated O-O bond of O2 molecule and Se-O bond of SeO2 molecule were 1.23 Å and 1.66 Å, and the angle of O-Se-O is 115.04°, which agreed well with the experimental data of 1.24 Å, 1.61 Å and 113.80° [37,38]. Fig. 1 shows the most stable adsorption structure when O2 molecule adsorbed on the bridge site in CaO (0 0 1) surface parallel [39] (O2/CaO (0 0 1) surface). In Fig. 1, two kinds of adsorption sites in O2/CaO (0 0 1) surface were taken into consideration: the Se and SeO2 adsorbed directly onto the pre-adsorbed O2 or near of it (not in direct contact
(1)
where Esur is energy of the surface before adsorption, Eads is the energy of the adsorbate, Esys is the total energy of the system after adsorption, Ead is the adsorption energy. A negative Ead value represents a stable adsorption system. 3. Results and discussion 3.1. Effect of pre-adsorbed O2 on Se adsorption 3.1.1. Adsorption of Se on O2/CaO (0 0 1) surface Adsorption of Se atom on O2/CaO (0 0 1) surface was firstly investigated with six adsorption schemes, including Se on O(1)-top site, O(2)-top site, O(3)-top site, O(4)-top site, O(5)-top site and bridge site. Four different stable adsorption structures were obtained. The adsorption energies were found to be −1.31 eV, −2.47 eV, −2.63 eV and −2.66 eV, respectively. The optimized configurations are depicted in Fig. 2, and the corresponding adsorption energies as well as the charge transfers of Se atom are listed in Table 1. For Se atom adsorbed directly onto pre-adsorbed O2, as seen from the results, O-Se-O group was formed in adsorption configuration-a. The Se atom provided 0.20 e to the adsorbate surface after adsorption according to Mulliken population analysis, and the reason could be explained as follow: Some valence electrons in Se atom transferred into the orbits of O(1) atom. Which confirmed that O2 could oxidize Se atom under heterogeneous condition. In addition, there was a stronger interaction existed between Se atom and O(2) atom (adsorption configuration-b) with lower chemisorption energy, indicating that this mode was the most stable structure for Se atom adsorbed directly onto peradsorbed O2. Upon adsorption, the Se-O(2) covalent bond was formed and the O(1)-O(2) bond length increased from 1.30 Å to 1.36 Å. The expansion of the bond length meant that O(1)-O(2) bond became easier to break. When the adsorption system reached equilibrium, the Se terminal in O-O-Se group was formed with chemical activity, which had a great possibility conversing into SeO2. Therefore, it was essential to explore the transformation of Se to SeO2 in CaO (0 0 1) surface. From Table 1, the adsorption of Se atom near pre-adsorbed O2 belonged to chemisorption (lower than −1.04 eV [40]). Configuration-c and configuration-d were obtained from Se on O(3) and O(4) top site, respectively. The adsorption energy in configuration-d was −2.66 eV, 2
Applied Surface Science 510 (2020) 145488
J. Xing, et al.
Fig. 2. Optimized configurations of Se on O2/CaO (0 0 1) surface (O, Ca and Se atoms are in red, blue, and yellow, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 1 Adsorption energies and the charge transfers of Se on O2/CaO (0 0 1) surface. Number
Ead/eV
q(Se)/e
a b c d
−1.31 −2.47 −2.63 −2.66
0.20 0.18 0.04 −0.55
which was the lowest adsorption energy among those four schemes. The Se atom obtained 0.55 e from substrate surface during the adsorption process and the Se-O bond was formed between Se atom and O(3) atom. However, the binding energy in configuration-c was higher than that in configuration-d, which indicated that the adsorption ability in O(3) top site was relatively weak, and it would be further investigated in the following parts. 3.1.2. Differences between Se on CaO (0 0 1) surface with and without preadsorbed O2 The binding mechanism of Se on CaO (0 0 1) surface was investigated. The adsorption energy was −2.65 eV and Se atom obtained 0.62 e from the adsorption substrate after adsorption. Compared with the adsorption energies of Se on O2/CaO (0 0 1) surface from 3.1.1, the Se adsorption on CaO (0 0 1) surface was stronger, which indicated that the pre-adsorbed O2 was not conductive to Se adsorption. In addition, the adsorption in configuration-c was the weakest when Se atom adsorbed on O2/CaO (0 0 1) surface near pre-adsorbed O2. To clarify the effect of O2 on the removal of Se atom, the projected density of states (PDOS) of Se atom and O(3) atom were plotted in different configurations (Se on CaO (0 0 1) surface and O2/CaO (0 0 1) surface with the O(3) top site), as shown in Fig. 3. In Fig. 3, the s orbital, p orbital of Se atom and the s orbital, p orbital of O(3) atom were plotted in black, red, blue and grey color, respectively. Comparing the PDOS of Se atom and O(3) atom in different geometries, when Se adsorbed on O2/CaO (0 0 1) surface, the s orbitals showed lower energy states than that on CaO (0 0 1) surface in both two atoms. Due to the additional interaction between Se atom and pre-absorbed O2, the resonances at −18.3 eV and −11.6 eV between the s orbital of Se atom and s orbital of O(3) atom for Se on O2/CaO (0 0 1) surface were lower than that at −17.9 eV and −10.9 eV on CaO (0 0 1)
Fig. 3. PDOS of Se atom and O(3) atom after Se adsorbed on CaO (0 0 1) surface and O2/CaO (0 0 1) surface.
surface. Simultaneously, it could be observed that the similar phenomenon occurred in the resonance at −1.2 eV between the p orbital of Se atom and p orbital of O(3) atom. However, from the respective of orbitals’ overlap, the overlaps of p orbitals between Se atom and O(3) atom were the main electronic participants in chemisorption. The peaks of p orbitals in both Se atom and O(3) atom at −4.2 eV and −3.7 eV split into two existed peaks for Se on CaO (0 0 1) surface and three existed peaks for Se on O2/CaO (0 0 1) surface. The overlaps between the p orbital of Se atom and p orbital of O(3) atom for Se on CaO (0 0 1) surface was a little well than that on O2/CaO (0 0 1) surface. Which indicated that the adsorption of Se atom adsorbed directly onto preadsorbed O2 was the major factor on Se adsorption.
3.2. Conversion of Se to SeO2 on CaO (0 0 1) surface Adsorption is the first step for a gas–solid reaction [41], from 3.1.1, the O-O-Se group and O-Se-O group were formed with chemical 3
Applied Surface Science 510 (2020) 145488
J. Xing, et al.
TS → SeO2, and the changes of the distance between Se and O, O and O, as well as the angle of O-Se-O were as follows: 2.18 Å → 1.90 Å → 1.82 Å → 1.66 Å, 1.23 Å → 1.49 Å → 1.78 Å → 2.80 Å, and 32.78° → 46.25° → 58.40° → 115.04°. Compared with the formation of SeO2 on CaO (0 0 1) surface, the reaction energy barrier of the reaction in homogenous (0.52 eV) was much higher than that in heterogeneous (0.27 eV). Therefore, we could conclude that CaO can not only act as an adsorbent, but also promote the conversion of Se to SeO2. And this was similar to the result from Galbreath et al. [25] on the study of Hg0 adsorption by fly ash particles (25% CaO). And they discussed that the HgO was formed on particle surfaces accounting for the reaction between Hg0 and pre-adsorbed O atom. 3.3. Effect of pre-adsorbed O2 on SeO2 adsorption 3.3.1. Adsorption of SeO2 on O2/CaO (0 0 1) surface In order to make the study more comprehensive, two orientations (perpendicular and parallel) of SeO2 approaching O2/CaO (0 0 1) surface with its Se atom were considered, and six different sites (O(1)-top, O(2)-top, bridge, O(3)-top, O(4)-top and O(5)-top) were examined systematically for each orientation. Five configurations were obtained after optimization. The optimized configurations and parameters are given in Fig. 6 and Table 2, respectively. Configuration-a was the stable structure when SeO2 adsorbed on O(1) top site perpendicular with its Se atom, and SeO2 was parallel to pre-adsorbed O2 concurrently. Configuration-b was the stable structure when SeO2 adsorbed on bridge site perpendicular with its Se atom, and SeO2 was perpendicular to pre-adsorbed O2. As seen from Fig. 6, the SeO bond between Se atom and pre-adsorbed O atom, were formed during the optimization processes among these two adsorption geometries. Upon adsorption, some electrons transferred from Se atom to the substrate O atoms. This was consistent with the conclusion reached by Zhang et al. [20] in the study of the retention of selenium during coal combustion. That is, CaO combined with SeO2 to form CaSeO4, inhibiting the volatilization of selenium. For configuration-c, the adsorption was relatively weak and its adsorption energy was just −0.49 eV, which suggested that the adsorbing type was physisorption. From Table 2, when SeO2 adsorbed on O2/CaO (0 0 1) surface in configuration-b, the most stable structure was obtained with its lowest adsorption energy (−2.03 eV). During the adsorption process, the distance between O(2) atom and CaO (0 0 1) surface increased from 2.73 Å to 3.13 Å, and the length of O(1)-O(2) bond was extended from 1.30 Å to 1.35 Å. These results indicated that the adsorption between Se atom and the O(2) atom was strong, and the O(2) atom gradually broke off the binding from the CaO(0 0 1) surface at the same time. The PDOS analysis was carried out to study the electron states of Se atom and O(2) atom after the adsorption in geometry-b. As shown in Fig. 5, the electron states in 2p orbital of Se atom were overlapped well with the 2p orbital of O(2) atom in the peaks around −14.2 eV, −11.1 eV as well as −7.7 to 0 eV, which proved the formation of Se-O bond between the Se atom of SeO2 and the O atom of substrate surface. Configuration-d and configuration-e were the adsorption geometries when SeO2 adsorbed on O(3) top and O(5) top site parallel with its Se atom respectively. It could be seen from Fig. 7 that the Se-O bond between Se atom of SeO2 and original O atom of O2/CaO (0 0 1) surface was formed in these two stable schemes. We could conjecture that a repulsion existed between SeO2 and pre-absorbed O2 on CaO (0 0 1) surface, accounting for their increasing distance. According to the data in Table 2, there was a conclusion that the O(5) top site was the active site for SeO2 adsorbed on O2/CaO (0 0 1) surface near of pre-adsorbed O2, and its adsorption energy was −1.97 eV (configuration-e). The SeO2 obtained 0.56 e from the adsorption substrate and Se atom provided 0.013 e to O atom adjacently during adsorption. However, configuration-d showed a relatively weak adsorption due to the interaction between SeO2 and pre-absorbed O2 on CaO (0 0 1) surface, and its adsorption energy was just −1.82 eV.
Fig. 4. Reaction potential energy surface of the heterogeneous reactions.
activities. Two possible pathways of O-O-Se group and O-Se-O group conversing into SeO2 on CaO (0 0 1) surface were considered and the reaction potential energy surfaces were shown in Fig. 4. According to the transition state theory, reaction energy barrier is defined as the energy difference between transition and reactant (or intermediate). As shown in Fig. 4, R1 and R2 were the stable configurations of O-O-Se group and O-Se-O group adsorbed on CaO (0 0 1) surface. P1, P2 were corresponding stable structures of SeO2 adsorbed on CaO (0 0 1) surface, respectively. For the reaction possess of O-O-Se group conversing into SeO2 on CaO (0 0 1) surface, the initial adsorption state needed to overcome 0.27 eV energy barrier and release 2.46 eV. The mechanism was as follow: O-O-Se/CaO (0 0 1) → TS → SeO2/CaO (0 0 1). During the reaction, the changes of the distance between Se and O(1), O(1) and O(2), as well as the angle of O(1)-Se-O(2) were as follows: 2.85 Å → 2.79 Å → 1.71 Å, 1.36 Å → 1.33 Å → 2.80 Å, and 24.53° → 25.26° → 110.59°, which meant that the formation of SeO(1) bond and the cleavage of the O(1)-O(2) bond were carried out simultaneously. In terms of O-Se-O group conversing into SeO2 on CaO (0 0 1) surface, the energy barrier was 0.24 eV and it was also an exothermic process releasing 3.30 eV. The mechanism of O-Se-O group to SeO2 on CaO (0 0 1) surface was similar as former. The formation of SeO2 on CaO (0 0 1) surface was one-step reaction and there was no intermediate formed. Fig. 5 described the reaction potential energy surface of Se + O2 → SeO2 in homogenous. The Se atom reacted with O2 molecule to form a stable intermediate M, and then O-O bond in M broke down into the transition state. The mechanism for this reaction was Se + O2 → M →
Fig. 5. Reaction potential energy surface of the homogenous reaction. 4
Applied Surface Science 510 (2020) 145488
J. Xing, et al.
Fig. 6. Optimized configurations of Se O2 on O2/CaO (0 0 1) surface (O, Ca and Se atoms are in red, blue, and yellow, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 2 Adsorption energies and the charge transfers of SeO2 on O2/CaO (0 0 1) surface. Number
Ead/eV
q(Se to O)/e
q(SeO2)/e
a b c d e
−1.38 −2.03 −0.49 −1.82 −1.97
0.088 0.069 0.064 −0.056 −0.013
−0.36 −0.56 0.17 −0.60 −0.56
Fig. 7. PDOS of Se atom and O(2) atom after SeO2 adsorbed on O2/CaO (0 0 1) surface.
3.3.2. Differences between SeO2 on CaO (0 0 1) surface with and without pre-adsorbed O2 For SeO2 adsorbed on CaO (0 0 1) surface, the most stable structure was obtained when SeO2 adsorbed on O top site of CaO (0 0 1) surface with its Se atom [22]. Results showed that the adsorption energy was −2.28 eV and SeO2 molecule obtained 0.61 e from CaO (0 0 1) surface upon adsorption. Compared with the adsorption energy of Se on O2/ CaO (0 0 1) surface, we could discuss that pre-adsorbed O2 was not conducive to the adsorption of SeO2 by CaO. The PDOS of Se atom, Osurf atom, pre-adsorbed O atom and Casurf atom were plotted to explore the effects of pre-adsorbed O2 as shown in Fig. 8. Osurf, Casurf are atoms in CaO (0 0 1) surface nearest from pre-absorbed O atom, Se atom
Fig. 8. PDOS of Se atom, Osurf atom, O atom of SeO2 and Casurf atoms after SeO2 adsorbed on CaO (0 0 1) surface and O2/CaO (0 0 1) surface: (a) PDOS of Se atom and Osurf atom; (b) PDOS of O atom of SeO2 and Casurf atom.
respectively. As can be seen from Fig. 8, the PDOS analysis was further investigated to reflect the differences of SeO2 on CaO (0 0 1) surface in 5
Applied Surface Science 510 (2020) 145488
J. Xing, et al.
configuration-d. From Fig. 8(a), the energy states for SeO2 on CaO (0 0 1) surface were lower than that on O2/CaO (0 0 1) surface. Especially, the resonances at −20.5 eV, −17.8 eV and −3.3 eV for SeO2 on CaO (0 0 1) surface was lower than that at −20.2 eV, −17.6 eV and −3.1 eV on O2/CaO (0 0 1) surface. Therefore, the configuration of SeO2 on CaO (0 0 1) surface was more stable, which confirmed the results obtained by adsorption energy and electron transfer. In addition, there was no obvious difference between the configurations of SeO2 on CaO (0 0 1) surface and O2/CaO (0 0 1) surface. The overlaps of the orbitals between O atom of SeO2 and Casurf atom were almost equal. This was similar to the result from 3.1.1, and the adsorption of SeO2 adsorbed directly onto pre-adsorbed O2 was the major influencing factor on SeO2 adsorption.
[7] U.S. Environmental Protection Agency, Mercury and Air Toxics Standards (MATS), 2017, http://www3.epa.gov/mats/index.html. [8] Y. Liu, C. Zheng, Q. Wang, Speciation of most volatile toxic trace elements during coal combustion, Asia-Pac. J. Chem. Eng. 11 (3–4) (2010) 381–394. [9] R. Yan, D. Gauthier, G. Flamant, G. Peraudeau, J. Lu, C. Zheng, Fate of selenium in coal combustion: volatilization and speciation in the flue gas, Environ. Sci. Technol. 35 (7) (2001) 1406–1410. [10] E.B. Dismukes, Trace element control in electrostatic precipitators and fabric filters, Fuel Process. Technol. 39 (1–3) (1994) 403–416. [11] A.W. Andren, D.H. Klein, Y. Talmi, Selenium in coal-fired steam plant emissions, Environ. Sci. Technol. 9 (9) (1975) 856–858. [12] M. Diaz-Somoano, M.R. Martinez-Tarazona, Retention of arsenic and selenium compounds using limestone in a coal gasification flue gas, Environ. Sci. Technol. 38 (3) (2004) 899–903. [13] M.A. López-Antón, M. Díaz-Somoano, D.A. Spears, M.R. Martínez-Tarazona, Arsenic and selenium capture by fly ashes at low temperature (Article), Environ. Sci. Technol. 40 (12) (2006) 3947–3951. [14] R.O. Sterling, J.J. Helble, Reaction of arsenic vapor species with fly ash compounds: kinetics and speciation of the reaction with calcium silicates, Chemosphere 51 (10) (2003) 1111–1119. [15] M.A. López-Antón, M. Díaz-Somoano, J.L.G. Fierro, M.R. Martínez-Tarazona, Retention of arsenic and selenium compounds present in coal combustion and gasification flue gases using activated carbons, Fuel Process. Technol. 88 (8) (2007) 799–805. [16] Y. Li, H. Tong, Z. Yu, L. Yan, C. X, Simultaneous removal of SO2 and trace As2O3 from flue gas: mechanism, kinetics study, and effect of main gases on arsenic capture, Environ. Sci. Technol. 41 (8) (2007) 2894. [17] M. Diaz-Somoano, M.A. López-Antón, M.R. Martínez-Tarazona, Retention of arsenic and selenium during hot gas desulfurization using metal oxide sorbents, Energy Fuels 18 (5) (2007) 1238–1242. [18] R. Agnihotri, S. Chauk, S. Mahuli, L.S. Fan, Selenium removal using Ca-based sorbents: reaction kinetics, Environ. Sci. Technol. 12 (1998) 1841. [19] Y. Li, H. Tong, Y. Zhuo, C. Chen, X. Xu, Simultaneous removal of SO2 and trace SeO2 from flue gas: effect of product layer on mass transfer, Environ. Sci. Technol. 40 (13) (2006) 4306–4311. [20] J. Zhang, D. Ren, Q. Zhong, F. Xu, Y. Zhang, J. Yin, Retention of selenium volatility using lime in coal combustion, Environ. Sci. 22 (3) (2001) 100–103. [21] Q. Xiong, J. Qiu, C. Xu, Q. Wang, H. Liu, H. Liu, Y. Chen, Z. Xu, Study on the volatilization behavior of Se under oxygen-combustion atmosphere, J. Eng. Thermophys. 2 (2006) 195–198. [22] C. Wang, Z. Yue, H. Liu, Experimental and mechanism study of gas-phase arsenic adsorption over Fe2O3/γ-Al2O3 sorbent in oxy-fuel combustion flue gas, Ind. Eng. Chem. Res. 55 (40) (2016) 10656–10663. [23] Y. Fan, Y. Zhuo, Y. Lou, Z. Zhu, L. Li, SeO2 adsorption on CaO surface: DFT study on the adsorption of a single SeO2 molecule, Appl. Surf. Sci. 413 (2017) 366–371. [24] Y. Fan, Y. Zhuo, L. Li, SeO2 adsorption on CaO surface: DFT and experimental study on the adsorption of multiple SeO2 molecules, Appl. Surf. Sci. 420 (2017) 465–471. [25] K.C. Galbreath, C.J. Zygarlicke, Mercury transformations in coal combustion flue gas, Fuel Process. Technol. 65 (99) (2000) 289–310. [26] Y. Zhang, J. Liu, Density functional theory study of arsenic adsorption on the Fe2O3(001) surface, Energy Fuels 33 (2019) 1414–1421. [27] T. Liu, L. Xue, X. Guo, C. Zheng, DFT study of mercury adsorption on α-Fe2O3 surface: role of oxygen, Fuel 115 (2014) 179–185. [28] B. Delley, An all-electron numerical method for solving the local density functional for polyatomic molecules, J. Chem. Phys. 92 (1) (1990) 508–517. [29] B. Delley, From molecules to solids with the DMol3 approach, J. Chem. Phys. 113 (18) (2000) 7756–7764. [30] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, C. Fiolhais, Atoms, molecules, solids, and surfaces: applications of the generalized gradient approximation for exchange and correlation, Phys. Rev. B 46 (11) (1992) 6671–6687. [31] D. Vanderbilt, Soft self-consistent pseudopotentials in a generalized eigenvalue formalism, Phys. Rev. B: Condens. Matter 41 (11) (1990) 7892–7895. [32] Y. Inada, H. Orita, Efficiency of numerical basis sets for predicting the binding energies of hydrogen bonded complexes: Evidence of small basis set superposition error compared to Gaussian basis sets, J. Comput. Chem. 29 (2) (2008) 225–232. [33] V.A. Basiuk, Interaction of porphine with closed-end zigzag (6,0) single-walled carbon nanotube: the effect of parameters in DMol3 DFT calculations, J. Comput. Theor. Nanosci. 5 (11) (2008) 2114–2118. [34] N. Govind, M. Petersen, G. Fitzgerald, D. King-Smith, J. Andzelm, A generalized synchronous transit method for transition state location, Comput. Mater. Sci. 28 (2) (2003) 250–258. [35] C.H. Shen, R.S. Liu, J.G. Lin, C.Y. Huang, Phase stability study of La1.2Ca1.8Mn2O7, Mater. Res. Bull. 36 (5–6) (2001) 1139–1148. [36] P. Blowers, B.G. Kim, The adsorption of mercury-species on relaxed and rumpled CaO (001) surfaces investigated by density functional theory, J. Mol. Model. 17 (3) (2011) 505–514. [37] M.H. Shah, J. Iqbal, N. Shaheen, N. Khan, M.A. Choudhary, G. Akhter, Assessment of background levels of trace metals in water and soil from a remote region of Himalaya, Environ. Monit. Assess. 184 (3) (2012) 1243–1252. [38] D.A. Otoniel, S. Thierry, H. Philippe, D. Marie-Lise, M. Sarantos, Potential energy surface and rovibrational energy levels of the H2-CS van der Waals complex, J. Chem. Phys. 137 (23) (2012) L53. [39] L. Wang, Adsorption of O2 on the CaO and SrO (100) surfaces: a first-principles study, Appl. Surf. Sci. 257 (13) (2011) 5499–5502. [40] J.K. Nørskov, F. Studt, F. Abild-Pedersen, T. Bligaard, Fundamental Concepts in Heterogeneous Catalysis 33 (3) (2015) 10. [41] L. Wu, Q. Wu, X. Hu, C. Dong, Y. Yang, Mechanism study on the influence of in situ SOx removal on N2O emission in CFB boiler, Appl. Surf. Sci. 333 (2015) 194–200.
4. Conclusions Density functional theory (DFT) was employed to study the effect of pre-adsorbed O2 on the adsorption of Se, SeO2 on CaO (0 0 1) surface. Binding energies and electron transfers in different configurations were calculated to reveal the adsorption mechanism. It was found that when Se atom adsorbed on O2/CaO (0 0 1) surface, two kinds of stable adsorption configurations were formed: O-Se-O group and O-O-Se group on CaO (0 0 1) surface with chemical activities. When SeO2 molecule adsorbed on O2/CaO (0 0 1) surface, some valence electrons in adsorption substrate transferred into the orbits of SeO2 molecule, forming Se-O covalent bond. Moreover, O2/CaO (0 0 1) surface showed weaker adsorption ability than CaO (0 0 1) surface for both Se and SeO2, which indicated that the pre-adsorbed O2 was not beneficial to selenium capture. And the adsorption of Se, SeO2 adsorbed directly onto preadsorbed O2 were the major factors. For the processes of Se and O2 conversing into SeO2, the reaction energy barrier in heterogeneous was less than that in homogeneous, which stated that CaO could not only act as an adsorbent, but also promote the conversion of Se to SeO2. CRediT authorship contribution statement Jiaying Xing: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Chunbo Wang: Validation, Formal analysis, Visualization, Supervision. Chan Zou: Validation, Formal analysis, Visualization, Software. Yue Zhang: Validation, Formal analysis, Visualization, Software. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by the National Natural Science Foundation of China (51976059) and the Fundamental Research Funds for the Central Universities (2018ZD03). References [1] L.B. Clarke, Lesley Sloss, Trace elements emissions from coal combustion and gasification, IEA Coal Res. (1992). [2] J.F. Cheng, H.C. Zeng, Z.H. Zhang, M.H. Xu, The effects of solid absorbents on the emission of trace elements, SO2, and NOx during coal combustion, Int. J. Energy Res. 25 (12) (2001) 1043–1052. [3] L.B. Clarke, The fate of trace elements during coal combustion and gasification: an overview, Fuel 72 (6) (1993) 731–736. [4] A. Ghosh-Dastidar, S. Mahuli, R. Agnihotri, L.S. Fan, Selenium capture using sorbent powders: mechanism of sorption by hydrated lime, Environ. Sci. Technol. 30 (2) (1996) 447–452. [5] S.J. Hamilton, Review of selenium toxicity in the aquatic food chain, Sci. Total Environ. 326 (1–3) (2004) 1–31. [6] W.G. Rosenberg, Clean air act amendments, Science 251 (5001) (1991) 1546–1547.
6