Density Functional Theory Study of Arsenic Adsorption on the

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Density Functional Theory Study of Arsenic Adsorption on the Fe2O3(001) Surface Yue Zhang, and Ji Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04155 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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Energy & Fuels

Density Functional Theory Study of Arsenic Adsorption on the Fe2O3(001) Surface Yue Zhang1*a, Ji Liu *b a

School of Energy and Power Engineering, North China Electric Power University, China

a

National Engineering Laboratory for Biomass Power Generation Equipment, North China Electric Power University, China

Abstract Iron-based sorbent addition is a promising method for arsenic removal from coalfired flue gas, but the adsorption process and surface active site that responsible for arsenic adsorption remains unclear. In this work, quantum chemistry methods base on the density functional theory are carried out to explore the mechanism of As2O3 adsorption on Fe2O3(001) Surface. The results indicate that O-top and O-hollow site served as the active site for As2O3 adsorption on α-Fe2O3(001) surface, among these, the activity of O-top is higher. The critical step of As2O3 adsorption lies in the bond breaking of As-O bond of As2O3 molecule, which is confirmed by comparing binding energy of different adsorption sites. The previous experimental studies have proved that O2 and SO2 have a significant impact on arsenic adsorption, and herein, deep insights into arsenic adsorption in the presence of the above gas components are also included. Under the influence of oxygen, the converting of original Fe-top site into O site results in chemisorption between arsenic and α-Fe2O3(001) surface, which is the primary cause for the promoting action of O2. In the presence of SO2, the adsorption activity of the original Fe-top site is enhanced by the new-formed Sads-top site. In addition, the As adsorption capacity of original O-top site had been also promoted because of the SO2 adsorption. Key Words: As2O3 adsorption, Fe2O3, Mechanism, Density functional theory

*Corresponding author. E-mail address: [email protected] and [email protected] 1

ACS Paragon Plus Environment

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1. Introduction Coal-fired power plant is believed as an important gaseous anthropogenic source of semi-volatile trace elements, such as arsenic, to the environment. Like many other heavy metals, arsenic and its compounds, vaporized at elevated temperatures, and as temperatures drop, only a fraction of the vaporized metal condenses 1. The arsenic emission is of significance because of its potential environmental impacts on human and ecosystem health. Gaseous and particulate forms of arsenic have the characteristic of long distance transmission, therefore, the pollution caused by arsenic in flue gas is not only limited to the power station locally. On the other hand, arsenic vapor in coalfired fuel gas is one of the important reasons that causes the deactivation of SCR catalyst, finally increase the cost of denitration system. In 1990, the Clean Air Act Amendment(CAAA) introduced by American government stated that a variety of trace elements, including arsenic, should be effectively controlled and their annual emission should be limited under 10 tons 2. In the year of 1998, the discharge of arsenic and other toxic substance from power station were required to be predicted and reported by the US environmental protection agency (EPA) 3. In Australia, arsenic and its compounds are included in National Pollution Inventory in the year of 2001. The emission standard of gas-phase arsenic in flue gas was clearly defined for the first time in 2011 1, in which the limitation for existing coal-fired EGUs (electric utility steam generating unites) is 2.0E-2 lb/GWh, and for new EGUs, the number is 3.0 E-3 lb/GWh. All these actions will result in more restrictive waste disposal discharge regulations. As far as coal-fired power station is concerned, there is a demand for arsenic emission reduction in response to these more stringent environmental regulations. Though thermodynamic equilibrium calculations, elemental arsenic (As) and arsenic trioxide (As2O3) are suggested to be the most likely forms of arsenic in an oxidizing flue gas 4. Considering the fact that arsenic oxide is much more volatile and toxic than elemental arsenic, As2O3 in flue gas deserves further attention. Du et al.

5

reveals that trivalent arsenic molecules are thermodynamically stable at high temperature flue gas by the means of DFT calculations. Winter et al. 6 also observes that As2O3 is the only species through the experiment of injection of aqueous As2O3 into simulated combustion environment.

2


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The application of sorbents in gaseous arsenic removal from flue gas is supposed to be a promising method. In this area, many different sorbents, including activated carbons7, fly ash 8, and metal oxides9-12, have been tested for gaseous arsenic adsorption. In our previous studies10, 13, a kind of iron oxide based sorbent be of sulfur/vaporresistance property was prepared and its high adsorption performance was verified through a serial of experiments. The mechanism of arsenic adsorption speculated from experiment and characterization results indicates that O2 and SO2 in flue gas could supple active sites that response for arsenic adsorption. However, limited by detection method, it is impossible to detect active site because it usually presents in atomic-level. In addition, the complex experimental conditions, especially the low concentration of arsenic, make it difficult to explore the intrinsic mechanisms between arsenic and sorbents. Quantum chemistry methods provide an approach to explore the reaction mechanism at the molecule level and fill the gap between the experimental observation and theoretical analysis. Due to the accuracy of first-principle calculations based on density function theory (DFT), Guo et.al 14 explored the roles of γ- Fe2O3infly ash for removing mercury from flue gases. On this basis, Liu et.al 15, 16 took the impact of O2 and H2S into account, and found that the results could be explained well by Eley-Rideal mechanism. In regard to the catalytic ability of ferric oxide, Xiao et.al

17

studied the

effect of Fe2O3 on SO2 catalysis with the density functional theory(DFT). The results indicated the reaction energy barrier of SO3 generation rate with Fe2O3 catalyze is far less than heterogeneous generation in gas phase. Therefore, Fe2O3 has good catalytic effect on SO2 oxidation. Zhang et.al 18 focused on studying the adsorption mechanisms of elemental arsenic on CaO(001) surface. They believed that the O-top site of CaO(001) surface is responsible for As atom adsorption. The above research concentrates on the adsorption of As atom, however, arsenic in flue gas exists mainly in the form of oxidation state4, 6. To the best of our knowledge, the nature of arsenic reaction with iron-based adsorption substrate and the impact of other flue gas components remain unclear. To better understand the arsenic adsorption process on iron-based sorbent, and to identify the surface active site responsible for the arsenic adsorption, the computational studies of interaction between arsenic and Fe2O3 surface are carried out in this work. As a continuation of previous experimental studies, this work focuses on the adsorption behaviors of As2O3 on Fe2O3 surface, and also, the impact of SO2 as well 3


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as O2, which showing a significant impact on As2O3 adsorption10, 13, are also included. 2.

Models and computational methods

Pyrite and siderite are the main sources of iron in fly ash, oxidized to Fe3O4 and γFe2O3 during combustion, and ultimately, translate into α-Fe2O3 in high temperature19. Since α-Fe2O3 is the ultimate form of iron oxide, and also, it is the most stable oxide form of Fe(III), this paper selects α-Fe2O3 as objects. The structure of α-Fe2O3 is an R3c space group with the lattice constants of a=b=5.035 Å, c=13.747Å, and, α=β=90°, γ=120°20. In this work, the surface terminations of -Fe-O3-Fe-, which is the most stable surface termination for almost all oxygen concentrations20, is selected for calculation. In view of the interaction between different atomic layers, the α-Fe2O3(001) is modeled using periodic nine-layer slab, and each slab is separated by 12Å vacuum layer to minimize interactions between the slab, shown in figure 1(a). During the geometry optimization, the bottom three layers are fixed, and the topmost six layers are allowed to relax. Different adsorption sites of α-Fe2O3(001) surface, including Fe-top, O-top, O-hollow and bridge are taken into consideration for calculations, shown in figure 1(b).

Figure 1. Periodic flat panel model of (a) α-Fe2O3(001) and (b) four different adsorption sites on the surface of α-Fe2O3(001). All calculations are performed by the CASTEP plane-wave code21. The exchange and correlation interactions are modeled using GGA (generalized gradient approximation)

22

and the PDE (Perdew–Burke–Ernzerhof)23 functional. The plane-

wave basis set is applied for expansion of electronic wave functions and the ultra-soft pseudopotential is used to describe the interactions between electron and ions. In 4


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consideration of the efficiency and accuracy, the cut-off energy of 360 eV and a smaller k-point mesh size of 3×3×1, which are ascertained by the energy convergence test and the k-point convergence test, are used in calculation. The convergence criteria for structure optimization and energy calculation are set as follows: (a) the SCF tolerance is1.0×10-6 eV/atom, (b) the energy tolerance is 2.0×10-5 eV/atom, (c) the maximum displacement tolerance is 0.002 Å, (d) the maximum force tolerance is 0.05 eV/ Å. The adsorption energy is calculated according to the equation 1. Ead  Esys  ( Eads +Esur )

(1)

where Ead is the adsorption energy, Esys is the total energy of system after adsorption,

Eads is the energy of adsorbate, and Esur is the energy of the surface. 3.

Results and discussion

3.1 As2O3 adsorption on α-Fe2O3(001) surface The As2O3 molecule is optimized in a cubic crystal cell of 10 Å to get a reasonable adsorbate molecule structure, shown in figure 2. The As-O bond length after geometry optimization is 1.858 Å and the As-O-As bond angle is 75.615°, which are close to the reference value of 1.86 Å and 73.5°24, indicating the optimize results are reliable.

Figure 2. The optimized configuration of As2O3 The HOMO (highest occupied molecular orbital) and LUMO(lowest unoccupied molecular orbital) graphs are used for analyzing electronic property of As2O3, presented in figure 3. As can be seen the As2O3 HOMO and LUMO are contributed by O atoms and As atoms, illustrating that these orbitals prone to donate or accept electron during chemical process. Therefore, the As-end and O-end of As2O3 molecular are taken into 5


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consideration for possible adsorption sites.

HOMO

LUMO

Figure 3. The HOMO and LUMO of As2O3 Possible adsorption configurations are investigated including the adsorption of As2O3 with its As-end and O-end on Fe-top site, O-top site, O-hollow site and bridge sites of α-Fe2O3(001) surface. The stable adsorption configurations are shown in the figure 4, and the corresponding adsorption energy as well as the charge transfer from As2O3 to adsorbate surface are listed in the table 1.

Figure 4. The stable adsorption configurations of As2O3 on the Fe2O3(001) surface Table 1. Stable adsorption energy and charge transfer 6


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No.

Adsorption Configuration

A B C D E F G H

O-end on Fe-top As-end on Fe-top As-end on O-top O-end on O-top O-end on Bridge As-end on Bridge As-end on O-hollow O-end on O-hollow

Ead, eV -0.33 -0.10 -2.87 -2.23 -1.86 -1.88 -2.56 -1.84

q, e -0.18 0.09 0.15 0.01 0.09 0.09 0.14 0.16

It can be seen that the chemisorption could be obtained when the As2O3 molecule adsorbed on O-top of Fe2O3(001) surface with its As-end (adsorption configuration C). The chemisorption with adsorption configuration C releases 2.87 eV energy, which is the highest adsorption energy among all the 8 schemes. The As2O3 molecule provides 0.15 e to the surface after the adsorption according to Mulliken population analysis, which break the three As-O bonds of As2O3. In adsorption configuration G, it can be found that the chemisorption energy of 2.56 eV and electron transfer of 0.14 e could be obtained when As2O3 molecule adsorbed on the O-hollow site with its As-end. Similar to the scheme G, the average distance between As and three O atoms in As2O3molecular increases from 1.858 Å to 2.404 Å after adsorption, which lead to the two of As-O bonds breaking. When O-end of As2O3 molecule adsorbed on the O-top site of Fe2O3(001) surface, shown in adsorption configuration D, the adsorption energy is 2.23 eV and the average bond length stretched to 2.314 Å. In scheme D, although it is O-end that portrayed on the O-top site in the initial adsorption scheme, the As-end to O-top site scheme is obtained after geometric optimization. At this point, only one of the AsO bond in As2O3 molecule is broken and the rest two As-O bonds still hold together. The adsorption energy of scheme E (O-end on Bridge) and F (As-end on Bridge) are 1.859 eV and 1.880 eV, respectively, with average As-O bond stretch to 1.878 Å and 1.934 Å. All the three As-O bonds hold together in both E and F schemes. The adsorption energies of scheme A (O-end on Fe-top) and B (As-end on Fe-top) are relatively weak, inferring the adsorbing type is physisorption. From the perspective of As-O bond length variation, the average length of As-O bond of scheme A and scheme B are 1.901 Å and 1.962 Å, respectively, which are pretty close to the As-O free state bond length of 1.858 Å. Given the above, O-top and O-hollow of Fe2O3(001) surface could be the active 7


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sites in As2O3 adsorption. Among them, the adsorption activity of O-top is higher than the O-hollow. Chemisorption will be obtained on O-top and O-hollow sites no matter As2O3 molecule approach to with its As-end or O-end. Lattice oxygen of CaO, Fe2O3 and Al2O3 could also be the active site for other trace elements and gas adsorption, such as mercury, CO and As atom, 18, 25-27. The results obtained from the DFT study agree with the adsorption experimental phenomenon presented in our previous researches10, which suggested that the lattice oxygen of Fe2O3/γ-Al2O3 sorbent surface serves as the active site for gas-phase arsenic adsorption and the electron transferred from the As in arsenic molecule to O in sorbent. By comparing the chemisorption energy and the AsO bonding situation, it can be inferred that the crucial process of As2O3 adsorption on Fe2O3(001) surface lies on the bond breaking of As-O bond of As2O3 molecule. Partial density of states (PDOS) analysis is carried out to study the electron states of the surface O in substrate as well as As and O in Al2O3 before and after the adsorption with the most stable adsorption scheme (scheme C in figure 4). To distinguish O of substrate and O of adsorbate, the O atom of Fe2O3(001) surface is named as Osorb and the O atom of As2O3 is named as Oad. The PDOS of As, Osorb and Oad according to adsorption scheme C before and after the adsorption are shown in the figure 5. 3.5

As s Before adsorption As s After adsorption

1.6 1.4

Density of States (electrons/eV)


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1.2 1.0 0.8 0.6 0.4

As p Before adsorption As p After adsorption

3.0 2.5 2.0 1.5 1.0 0.5

0.2 0.0 -25

-20

-15

-10

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Energy(eV)

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-15

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0

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Oad p Before adsorption Oad p After adsorption

3.0



3.5

Oad s Before adsorption Oad s After adsorption

2.5

2.0

1.5

1.0

0.5

2.5 2.0 1.5 1.0 0.5 0.0

0.0 -25

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-15

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Energy (eV)

Osorb s Before adsorption Osorb s After adsorption

-5

0

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15

2.0

1.5

1.0

0.5

Osorb p Before adsorption Osorb p After adsorption

1.4


2.5

-10

Energy (eV)

1.6


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1.2 1.0 0.8 0.6 0.4 0.2 0.0

0.0 -25

-20

-15

-10

-5

0

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-25

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-20

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Energy (eV)

Energy (eV)

Figure 5. PDOS of As, Oad, and Osorb before and after As2O3 adsorption on Fe2O3(001) surface according to adsorption structure C. As can be seen in the figure 5, the PODS of As, Oad, and Osorb are broadened and lowered after adsorption, indicating the bonding abilities of these atoms are enhanced and interactions between As2O3 and Fe2O3are strengthened. The resonance at -20 eV between s orbital of As and p orbital of Oad disappear and only minor overlap of PDOS at -10 eV, -7.5 eV and -3.7 eV could be observed after adsorption. Simultaneously, it can be observed that the resonance at -20 eV and -16.7 eV between p orbital of As and p orbital of Oad disappear, and the resonance at -3.7 eV and at Fermi level is weakened markedly. It denoted that the As-O covalence bond in As2O3 has been activated, which is in good agreement with the discussion of the adsorption scheme geometry. 3.2 As atom adsorption on α-Fe2O3(001) surface When As2O3 adsorbed on the O-top site (adsorption scheme C), the interaction of orbitals could be occurred not only between As and Osorb, but also occurred between As and Oad as well as Fe and Oad. Therefore, it’s difficult to obtain a clear information of 9


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PDOS correlation during the adsorption process. On the basis of the above analysis, and conclusions from the previous section, which state that the key step of adsorption lies in the bond breaking of As-O bond of As2O3 molecule, a simplified adsorption scheme between As atom and Fe2O3(001) surface is established based on the adsorption configuration C. The optimized configuration is shown in figure 6.

Figure 6 The stable adsorption scheme of As adsorption on O top site of αFe2O3(001) surface The adsorption energy of -4.84 eV and As-O bond of 1.896 Å are obtained in above configuration after geometry optimization. During the adsorption, the surface accepts 0.35 e from As atom according to Mulliken population analysis, which build the As-O bond. O2- anion was proved to exhibit catalytic activity in previous studies including mercury adsorption on CaO, Al2O3 and Fe2O3. In this work, the density functional theory study of arsenic adsorption on Fe2O3 indicates that arsenic-oxygen interaction is favorable either. As s Before adsorption As s After adsorption

As p Before adsorption As p After adsorption

12

Density of states (electrons/eV）

4


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3

2

1

10 8 6 4 2 0

0 -25

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-15

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Osorb s Before adsorption

1.8

Osorb p Before adsorption

Osorb s After adsorption

1.6

Osorb p After adsorption

Density of states (electrons/eV)

3.0


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2.5 2.0 1.5 1.0

1.4 1.2 1.0 0.8 0.6 0.4

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Energy,(eV)

Figure 7. PDOS of As and Osorb before and after As adsorption on Fe2O3(001) surface In addition, as shown in figure 7, partial density of states (PODS) analyses are further investigated to study the electron states of the adsorbed adatom and the surface after adsorption. The new PDOS peaks of s and p orbital of Osorb are formed located at -10.6 eV after adsorption, overlapping well with the s orbital of As at -10.6 eV, indicating that bond is formed between the As atom and the surface. The adsorption of As atom on the surface O site can be regarded as chemisorption, which is in good agreement with the discussion of the Mulliken charge. Meanwhile, the s and p orbitals of As atom overlap well with the s orbital of Osorb at -20.5 eV. The above analyses indicate that As-O covalent bond can be formed when chemisorption taken place at the O top site of Fe2O3(001) surface, which match with the conclusion drawn by Zhang et.al 18 when they carried out adsorption study between As atom and CaO(001) surface. 3.3 The effect of O2 on As adsorption on α-Fe2O3(001) surface Our previous experimental studies have indicated that O2 presented in flue gas can greatly promote the adsorption efficiency of arsenic on iron-base sorbent10, 13. The reason could be largely related to the lattice oxygen on the sorbent surface, however, limited by the detection means, intuitive understanding of this process is relatively lacked. Herein, the DFT method is applied to investigate the oxygen impact on arsenic adsorption on Fe2O3. Considering the fact that arsenic concentration is much smaller than oxygen during the adsorption process, therefore, this work investigates As adsorption on O2 embedded α-Fe2O3(001) surface to clarify the effect of O2. 3.3.1 O2 adsorption on α-Fe2O3(001) surface 11


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Firstly, the O2 molecule is optimized in a cubic crystal cell of 10 Å to get a reasonable structure, shown in figure 7. The total energy of O2 molecule is -868.18 eV and the O-O bond length is 1.241 Å, which is very close to the experimental value of 1.239 Å28, indicating that the optimize results are reliable. Eight initial adsorption schemes are taken into consideration, including a) O2 adsorbed vertically to O-top of surface with its O-end, b) O2 adsorbed vertically to Fe-top of surface with its O-end, c) O2 adsorbed vertically to bridge of surface with its O-end, d) O2 adsorbed vertically to O-hollow of surface with its O-end, e) O2 adsorbed parallel to O-top of surface, f) O2 adsorbed parallel to Fe-top of surface, g) O2 adsorbed parallel to bridge of surface, h) O2 adsorbed parallel to O-hollow of surface. Two stable configurations are obtained after geometry optimization (similar or same adsorption schemes are not all given), shown in figure 8. The optimized parameters of O2 adsorption on Fe2O3(001) surface are included in table 2.

Figure 8. The stable adsorption configuration of O2 adsorption on Fe2O3(001) surface Table 2. The optimized parameters of O2 adsorption on Fe2O3(001) surface Eads (eV)

RO-O, Å

RO-x, Å

Scheme A

-0.29

1.296

--

Scheme B

-2.34

1.343

1.737

The adsorption scheme A is obtained by initial adsorption schemes d and h. After adsorption, the O-O bond in O2 stretches to 1.296 Å and the adsorption energy is 0.29 eV, indicating that the adsorption type is physisorption. The rest initial adsorption schemes will form configuration B or scheme much similar to configuration B. Among 12


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them, the maximum adsorption energy of 2.34 eV could be obtained by initial adsorption scheme f (O2 adsorbed parallel to Fe-top of surface), and forming the adsorption configuration B. In configuration B, the O-O band is elongated from 1.241 Å to 1.343 Å, indicating the cleavage of bond

16, 29.

One O atom dissociated from O2

separates from the surface, and the other O atom bond on the Fe top site of Fe2O3(001) surface, forming the Fe-O bond of 1.737 Å. It should be noted that the Fe top site of Fe2O3(001) surface has turned into the O terminal (named as O/Fe2O3(001) surface), which is partly similar to the O top site and the adsorption activity of this site has been already proved in last section of this paper. The O terminal is believed the dominant interaction for redox reaction30 and having chemical activities15. 3.3.2 As adsorption on O/Fe2O3(001) surface In this section, the oxidized Fe2O3(001) surface (named as O/Fe2O3(001)), which is the most stable scheme after oxidation, is chosen to be the substrate. To maintain the consistency with results, parameter setting is consistent in previous of this work. The optimized configuration of O/Fe2O3(001) surface is shown in figure 9.

Oads-top O-top

Figure 9. The optimized configuration of O/Fe2O3(001) surface. As illustrated in figure 9, the new O terminal resulted from oxidation is named as Oads-top and the original O terminal of Fe2O3(001) surface is named as O-top. Compared with the original Fe2O3(001) surface, the oxidized Fe2O3(001) surface has a lower energy of -9552.36 eV. Herein, the main emphasis is put in comparing the adsorption difference of As between Oads-top site and O-top site. Two stable configurations are obtained, shown in figure 10. Table 3 shows the optimized parameters. 13


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Figure10. The stable adsorption configuration of As adsorption on oxidized Fe2O3(001) surface Table 3. The adsorption energy and structure parameter of As adsorption on O-top site and Oads-top site Eads (eV)

RAs-O, Å

Q (e)

Scheme A

-4.703

1.705

0.87

Scheme B

-3.767

1.868

0.48

Configuration A and B are derived from As adsorption on O-top and Oads-top, respectively. Combine the adsorption schemes with the optimized parameters, it can be found that chemisorption takes place on both schemes, and the As adsorption was greatly promoted due to the surface oxidation. In model A, the adsorption energy of 4.703 eV is obtained when gaseous As adsorbed on the O-top site of oxidized surface, which is basically the same with the adsorption energy when As adsorbed on the O-top site of the original surface. In configuration B, the gaseous As on Oads-top site interacts with the O atom forming the As-O bond with adsorption energy of 3.767 eV, which is slightly lower than As adsorbed on O-top site. However, it should be noted that the original Fe-top site transformed to a O site due to the oxidation of Fe2O3(001) surface. Accordingly, the physisorption of As on Fe-top site has turned into the chemisorption on the Oads-top site. It indicates that promotion effect of oxygen is in the increase of adsorption activity sites. 3.4 The effect of SO2 on As adsorption on α-Fe2O3(001) surface SO2 is the main kind of pollutant in coal-fired flue gas, at the same time, gas-phase arsenic and SO2 always existed simultaneously in the flue gas. Therefore, understanding 14


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the nature of SO2’s influence on arsenic adsorption is of great significance for practical application. Considering the fact that arsenic concentration is far smaller than SO2 during the adsorption process, this work investigates As adsorption on SO2 embedded surface to clarify the effect of SO2. 3.4.1 SO2 adsorption on α-Fe2O3(001) surface Xue31 found that two main types of α-Fe2O3(001) surface could be obtained after SO2 adsorbed on α-Fe2O3(001) surface. One is when SO2 molecular adsorbed on the Fe top site with its S end, subsequently, the original Fe-top site will change into the S-site (named as Sads-top) with a new S-Fe bond formed. And the other type of surface is formed by O atom, which is dissociated form SO2, adsorbed on Fe-top site. The second surface type is the same with the oxidized surface in section 3.3.1, therefore, the study herein will focus on the Sads-top site of vulcanized surface. To get a vulcanized α-Fe2O3(001) surface, the S atom is placed at different adsorption sites, including Fe-top, O-top, O-hollow and bridge site, and then geometry optimizations are carried out. It could be found that all the adsorption models have the same final configuration, in which the S atom tend to bond at the Fe site, shown in figure 11. The Fe-S bond length of 1.979 Å is obtained in this scheme, which is the pretty close to the reference value of 2.04 Å 31, indicating that the calculation results are reliable.

Figure 11. The optimized configuration of S/Fe2O3(001) surface. 3.4.2 As adsorption on S/Fe2O3(001) surface For the vulcanized Fe2O3(001) surface (shown in figure 11), there are four main 15


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adsorption sites: Sads-top, O-top, O-hollow and bridge site. After geometry optimization, it’s found that two main final configurations are obtained, as indicated in figure 12. Calculated optimized parameters are listed in table 4.

A

B

Figure 12. The stable adsorption configuration of As adsorption on S/Fe2O3(001) surface Table 4. The adsorption parameter of As adsorption on Sads-top site and O top site of S/Fe2O3(001) surface Eads (eV)

RAs-x, Å

Q (e)

Scheme A

-3.78

1.980

0.42

Scheme B

-4.98

1.898

0.55

a b

In scheme A, RAs-x represent the distance between As and S atoms. In scheme B, RAs-x represent the distance between As and O atoms.

In configuration A, the As atom interacts with the surface S atom and forms an As-S bond with length of 1.980 Å. The adsorption energy of this scheme is -3.78 eV, indicating the adsorption type is chemisorption. During the adsorption process, 0.42 e transfer from the As to the surface. It can be inferred that the new-formed Sads-top site enhanced the adsorption activity of the original Fe-top site, made the physisorption on Fe-top site became to the chemisorption on Sads-top site. In configuration B, As atom bonded with both O and Fe atom of the surface. The adsorption energy is 4.98 eV and there are 0.55 e transfer to the surface. Compared with the adsorption energy of O-top site of the clear surface, the adsorption energy of O-top site of vulcanization surface has increased by 0.14 eV. On the other hand, not only As-O 16


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bond, but also the As-Fe bond could be built, and this configuration is more stable. It can be concluded that the vulcanization of α-Fe2O3(001) surface doesn’t inhibition its adsorption property, on the contrary, the adsorption capacity of original O-top site has been promoted because of SO2 adsorption. This calculation results coincide with experimental results in our previous study 10. 4.

Conclusions

In this present study, the DFT study is carried out to investigate the micromechanism of arsenic adsorption on α-Fe2O3(001) surface and the impact of O2 as well as SO2 are also included. The critical step of As2O3 adsorption on α-Fe2O3(001) surface is obtained by comparing binding energy of different adsorption sites, based on which, the adsorption model has been simplified. The main conclusions can be drawn as follow: (1) The O-top and O-hollow site are the active site for As2O3 adsorption on αFe2O3(001) surface, among these, the activity of O-top is higher. By comparing the chemisorption energy and the As-O bond breaking situation, it can be inferred that the key process of As2O3 adsorption on Fe2O3(001) surface lies in the bond breaking of AsO bond of As2O3 molecule. (2) The original Fe-top site can be transformed to an O site due to the oxidation of Fe2O3(001) surface. Accordingly, the physisorption of As on Fe-top site has turned into the chemisorption on the Oads-top site, which indicates that promotion effect of oxygen is in the increase of adsorption activity site. (3) Under the influence of SO2, the new-formed Sads-top site enhanced the adsorption activity of the original Fe-top site. In addition, the As adsorption capacity of original O-top site had been promoted because of the SO2 adsorption.

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Density Functional Theory Study of Arsenic Adsorption on the

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