Lu Liu1, Quanjin Deng1, Chenghang Zheng2, Shuang Wang1, Junfeng Wang 1, Xiang Gao2

School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China


Received: April 16, 2019
Revised: August 15, 2019
Accepted: September 20, 2019

Download Citation: ||https://doi.org/10.4209/aaqr.2019.04.0206  


Cite this article:

Liu, L., Deng, Q., Zheng, C., Wang, S., Wang, J. and Gao, X. (2019). An Insight into Electrostatic Field Effects on SO3 Adsorption by CaO with CO2, SO2 and H2O: A DFT Approach. Aerosol Air Qual. Res. 19: 2320-2330. https://doi.org/10.4209/aaqr.2019.04.0206


Highlights

  • DFT was applied to examine the electric field dependence of SO3 adsorption on CaO.
  • Trends of the Eads in a positive or negative electric field present a great difference.
  • A positive electric field is in favor of SO3 adsorption on CaO.
  • Charge transfer of SO42– increases with the increment of adsorption energy.
  • Positive field enhances charge transfer of the surface O and Ca.
 

ABSTRACT


Because of the compulsory installation of electrostatic precipitators in coal-fired power plants, SO3 and particles in flue gas inevitably pass through areas with electric field and electric charge distributed. CaO is the highest alkaline content in fly ash and has strong interactions with SO3. Therefore, it is important to understand the effects of the electric field on the binding energies of SO3 and CaO. Density functional theory calculations were applied to examine the electrostatic field dependence of SO3 adsorption on the CaO(100) surface. The energetic, geometric and electronic nature of the modeled systems was analyzed. Trends of the adsorption energy variation in a positive or negative electrostatic field presented great differences. When the electrostatic field was small, it was observed that the required adsorption energy was reduced regardless of the positive or negative electrostatic field. With increased electrostatic field strength, the adsorption energy showed an increasing trend in a positive field, while the adsorption energy decreased in a negative field. The positive field enhanced the charge density changes of the surface O across the Ca around the OCaO, while the negative field enhanced the change of the charge density of the Ca below the OCaO. The SO42–-like structure accumulated electrons as a unit and the charge transfer of the SO42– increased with the incremental adsorption energy in a linear relationship. On the H pre-adsorbed surface, the total increment was much smaller than in previous cases in a positive field, and the total variation was negligible in a negative field.


Keywords: SO3; CaO; Electrostatic field; Adsorption; Density functional theory.


INTRODUCTION


The coal combustion process has been counted as the most important source of atmospheric pollutant emissions, including particulate matter (PM), SO2, NOx and various heavy metals (Zhao et al., 2010; Czarnowska and Frangopoulos, 2012). To remove those harmful pollutants, NH3 selective catalytic reduction (NH3-SCR), electrostatic precipitators (ESPs) and wet flue gas desulfurization (WFGD) systems have been applied and proven effective (Chang et al., 2015; Hu et al., 2017; Liu et al., 2017; Song et al., 2018). SO3, generated from SO2 oxidation, is more harmful to the environment than SO2 because it can hardly be removed by the above removal equipment and can transform into sulfate aerosols and secondary sulfate particles in WFGD systems and out of the power station flues (Bin et al., 2017; Liu et al., 2018b). When the flue gas has passed through the WFGD, the weight percentage of S and Ca increases significantly, most of which is contained in fine particles. The increase of Ca is ~10%, generated from CaO, CaCO3 and CaSO4 fine particles. For S, the increase can be as high as 30%, resulting from CaSO4 fine particles, acid dew phenomena and physical chemistry interaction between SO3, SO2 and fine particles (Zhou, 2011).

In flue gas, SO3 is generated predominantly through three processes: combustion, catalytic oxidation of SO2 by fly ash or the NH3-SCR system (Wang et al., 2011; Fleig et al., 2012; Du et al., 2018; Wang et al., 2018). By fly ash, SO3 can also be captured and removed via fly ash collection (Spörl et al., 2014; Zhao et al., 2018). CaO, one of the main components of fly ash, has alkaline properties, by which it can adsorb SO3 (Wang et al., 2012). The capture of SO3 by CaO is even better than that of SO2 (Marier and Dibbs, 1974). Wang et al. (2011) have tested the capture efficiency of SO3 by CaO at 250–400°C and found the efficiency can achieve 80% at a proper equivalence ratio and flue gas residence time. Galloway et al. (2015) have found that SO3 interacts with the alkali or alkaline metal oxides through the formation of sulfate structures.

In ESP and other fine-particle removal equipment, such as wet electrostatic precipitators (WESPs) and wet electro-scrubbers (WES), particles in flue gas are in areas with electric field and distributed electric charge. In research of molecular adsorption on metal oxide surfaces, noble metal surfaces or other materials with active surfaces, an externally applied electric field can enhance bonding interaction by dipole moment, defect formulation or electrostatic force increase, etc. (Sun et al., 2010; Lv et al., 2011; Youssef et al., 2017) Also, an externally applied electric field can be a promising means to make changes in adsorption enthalpy. Most studies have explored effects of the electrostatic field by means of theoretical calculations, especially ab initio simulations. In a study of hydrogen storage, the capacity of hydrogen adsorption on mesoporous nickel and magnesium oxides is significantly enhanced by the electrostatic field, 37.5% for nickel oxide and 25% for magnesium oxide (Sun et al., 2010). Wang et al. (2017) have found that interactions in dopamine-graphene systems are sensitive to applied electrostatic field. Applying an electrostatic field in an adsorbate-surface system leads to lengthening of adsorbate bonds and an increase of the electronic charge transferred from the surface to the adsorbate (Koper and Santen, 1999). Ao et al. (2010) have studied the correlation of the applied electrostatic field and CO adsorption/desorption behavior on Al-doped grapheme and they found that an electrostatic field at the normal direction to graphene surfaces strengthens CO adsorption, while adsorption is reduced when an electrostatic field is present at an opposite direction. Yue et al. (2013) have applied a perpendicular electrostatic field when investigating the adsorption of various gas molecules on monolayer MoS2, demonstrating that the application of a perpendicular electrostatic field can consistently modifis the charge transfer between the adsorbed molecule and the MoS2 substrate. The above calculations and simulations indicated that molecule adsorption is sensitive to the applied electrostatic potential.

Experimental study in this area is not that easy to implement, but there are several feasible approaches. Zhang et al. (2013) have investigated the performance of hydrogen adsorption on AC/TiO2 nanoparticles in the presence of an electric potential by grounding the upper end of the sample holder and connecting the lower end to a high voltage DC power supply. The results showed that the adsorption capacity could be enhanced by introducing an electric potential and the enhancement increased with increased amounts of TiO2 nanoparticles. Htwe et al. (2016; 2018) have evaluated the amounts of adsorbed protein as a function of applied surface potential and found that a negative polarization of the metal oxide surface tends to restrain protein adsorption, whereas it is strongly enhanced when the surface potential range is above a certain value.

As SO3 and particles in flue gas will inevitably pass through areas with electric field and electric charge distributed, there might be enhancement or suppression of interactions between SO3 and particle components. In this study, the focus was on CaO, which is the highest alkaline content in fly ash that has strong interactions with SO3. If electric field or surface electric charge could increase the binding energy of SO3 and CaO, new approaches for SO3 control might be proposed. Therefore, the goal was to examine the effects of electrostatic field on SO3 adsorption on CaO through density functional theory (DFT) calculations.


COMPUTATIONAL METHODOLOGY


Plane-wave density functional theory calculations were performed in the Vienna Ab-initio Simulation Package (VASP) version 5.2 (Kresse and Furthmuller, 1996a, b). The electron exchange-correlation functional was treated with the generalized gradient approximation (GGA-PBE) (Perdew and Wang, 1992; Perdew et al., 1992) with the projector-augmented wave (PAW) method (Blöchl, 1994; Kresse and Joubert, 1999). The 2s22p4, 3s23p4 and 3s23p64s2 electrons were included explicitly as the valence for the O, S and Ca atoms, respectively, and the remaining electrons were kept fixed as core states.

The supercell model was built from the cubic CaO unit cell with a lattice constant of a = 4.84 Å, which was obtained by calculation. For the surface model, the (100) surface was modeled using periodic slabs to characterize the SO3-surface interactions, because it is the most common exposed surface of CaO. The slab model was a p (2 × 2) lateral cell with three atomic layers thick and a 15 Å vacuum gap in the direction perpendicular to the surface. The bottom three atomic layers were fixed, while the top two atomic layers were relaxed. Relaxation of the model was carried out until the maximum Hellmann-Feynman force was less than 0.03 eV/Å. The calculations were carried out using the Brillouin zone sampled with (3 × 3 × 1) Monkhorst-Pack mesh k-points grid with a cutoff energy of 520 eV and a Gaussian smearing of 0.1 eV. While perform calculation on unit cell, a Monkhorst-Pack of (9 × 9 × 9) was employed. The external electrostatic field is applied by adding a dipole moment pointing perpendicular to the slab model. The topology of the CaO(100) system with electrostatic field is shown in Fig. 1. The arrows denote the positive direction of the electrostatic field.


Fig. 1. Topology of CaO unit cell and side view of CaO(100) system with electric field.Fig. 1. Topology of CaO unit cell and side view of CaO(100) system with electric field.

The adsorption energies (Eads, kJ/mol) were defined as:

 

where 𝐸slab+SO3 and 𝐸slab are the total energy of the systems with and without SO3 adsorbed, respectively, and 𝐸SO3 is the total energy of a SO3 molecule in the gas phase (Liu et al., 2018a). The minus sign in adsorption energy means it is an exothermic process, and the stability of the formed structure improves with absolute value of the adsorption energy.

Charge transfer between SO3 and the surface is obtained via Bader analysis (Henkelman et al., 2006; Tang et al., 2009). Charge density difference (CDD, Δρ, e/Å3) plots were also used to examine the charge transfer, which was calculated from the electron localization function (Becke and Edgecombe, 1990; Silvi and Savin, 1994) and obtained as:

 

In this equation, ρ(slab+ SO3) represents the charge density for the adsorbed system; while ρslab and ρSO3 indicate the charge density of the non-adsorbed subsystem and the SO3 molecule.


RESULTS AND DISCUSSION



Effect
s of Electrostatic Field on SO3 Adsorption onto CaO

When SO3 adsorbs onto a CaO(100) surface, it forms a structure parallel to the surface with the S binding on Oslab, as shown in Fig. S1. The adsorption energy of SO3 on CaO surface in a zero electrostatic field agrees quite well with earlier DFT-generalized gradient approximation results (Galloway et al., 2015). Fig. 2 shows the adsorption energy of SO3 on the surface of CaO for various electrostatic field conditions. In areas where the electrostatic field is small, the required adsorption energy is observed to be reduced regardless of it being a positive or negative field, from –321.9 kJ mol1 with no field to –318.4 and –317.6 kJ mol1, respectively. Then, with increased electrostatic field with a positive direction, the adsorption energy shows an increasing trend; in the case of negative electrostatic field, the adsorption energy shows a decreasing trend. As the electrostatic field rises from 0.05 V/Å to 0.5 V/Å in a positive direction, the binding energy increase from –318.4 kJ mol1 to –357.6 kJ mol1. In the negative direction, it declines to –287.0 kJ mol1 with a field of 0.5 V/Å. The variation tendency of the adsorption energy with respect to the electrostatic field is in agreement with results in the study of other adsorbate-surface systems (Ao et al., 2010; Yue et al., 2013). The structural parameters for SO3 adsorption on CaO surface are shown in the Table 1, Tables S1 and S2. While the electrostatic field is increasing in a positive direction, the bond length between S and OCaO is becoming shorter; the bond angles of OSO3-S-OSO3 first increases from 113.65° at 0 eV to 113.75° at 0.05 eV, and then decreases to 113.59° at 0.5 eV. The bond angles of OCaO-S-OSO3 exhibits the opposite trend and the bond length is nearly invariable. When the electrostatic field is in a negative direction, there are also corresponding changes in structural parameters with adsorption energy.


Fig. 2. Adsorption energies of SO3 on CaO(100) surface with electric field in or opposite the normal direction of the surface.Fig. 2. Adsorption energies of SO3 on CaO(100) surface with electric field in or opposite the normal direction of the surface.

Fig. 3 presents the local density of states (LDOS) plot of s- and p-orbitals of S, OSO3 and OCaO. Because of the covalent bonding between S and O, the orbital of S and O eventual reunified in several levels, around –10.03, –7.99, –7.28, –4.85, –4.14, –2.63, –2.10 eV, etc. at zero field. Similar DOS plots of S, OSO3 and OCaO indicate that a SO42–-like structure has formed. It can be seen that the existence of the electrostatic field is observed to have no effect on the distribution of the orbital density, but shifted the energy of the orbital. When the adsorption energy increases, the orbit moves to a lower energy. The change is about 0.2 eV for a field of 0.5 V/Å. The charge transfer of the SO42–-like structure in relation to the electrostatic field is illustrated in Fig. 4. It is found that, except in zero field, the charge transfer of the SO42– increases with the electrostatic field intensity in the positive direction, whereas it tends to decrease once a reverse electrostatic field is applied. The variation of the charge transfer vs. electrostatic field was nearly linear. It is easy to conjecture that the charge transfer also increases with the increment of adsorption energy in a linear relationship, as shown in Fig. S2. This result also illustrates that the SO42–-like structure accumulates electrons as a unit.


Fig. 3. LDOS of SO3 on CaO(100) surface with electric field in or opposite the normal direction of the surface.Fig. 3. LDOS of SO3 on CaO(100) surface with electric field in or opposite the normal direction of the surface.


Fig. 4. The variation of charge transfer with respect to the electric field.
Fig. 4. The variation of charge transfer with respect to the electric field.


Systems with CO2 and SO2 Adsorbed on CaO

When CO2 is adsorbed on the surface at site 1 and site 2, the adsorption energy of SO3 decreases to –250.8 kJ mol–1 and –302.0 kJ mol–1 at zero field, respectively. The adsorption of CO2 is much weaker than SO3, and the  variation tendency of the adsorption energy with respect to the electrostatic field is similar to the results of SO3 adsorbed, as shown in Fig. 4(a). When a small electrostatic field is applied in the case with CO2 adsorbed, the extent of reduction in adsorption energy of SO3 is much larger than the case without CO2, about 7.35~9.23 kJ mol1 (Figs. 5(b) and 5(c)). There is a difference of 54.50 kJ mol1 at site 1 and 48.23 kJ mol1 at site 2 between the field of 0.5 and –0.5 V/Å, which is smaller than the case without CO2 adsorbed (73.75 kJ mol1). The variation of the structural parameters is consistent with the adsorption energy.


Fig. 5. Adsorption energies of (a) CO2 on CaO(100) and SO3 on CaO(100) surface with CO2 adsorbed previously on (b) site ① and (c) site ② versus electric field.
Fig. 5. Adsorption energies of (a) CO2 on CaO(100) and SO3 on CaO(100) surface with CO2 adsorbed previously on (b) site ① and (c) site ② versus electric field.
Fig. 5. Adsorption energies of (a) CO2 on CaO(100) and SO3 on CaO(100) surface with CO2 adsorbed previously on (b) site ① and (c) site ② versus electric field.

The adsorption energy of SO3 is –241.7 kJ mol1 on the SO2-adsorbed surface at zero field, which is lower than that on CO2-adsorbed surface. Because SO2 has higher ability to take charge than CO2, charge transfer between SO3 and the slab should be weaker on the SO2-adsorbed surface, and the SO3 adsorption energy should be smaller on SO2 surface than that on CO2-adsorbed surface. In the case when a small electrostatic field is applied, the extent of reduction in adsorption energy of SO3 is also larger than the case without SO2, about 6.94 and 7.43 kJ mol1, as shown in Fig. 6 and Table 1. Then as the electrostatic field continues to increase, the adsorption energy required for the positive and negative electrostatic field gradually increases and decreases linearly, respectively.


Fig. 6. Adsorption energies of (a) SO2 on CaO(100) and (b) SO3 on CaO(100) surface with SO2 adsorbed previously versus electric field.Fig. 6. Adsorption energies of (a) SO2 on CaO(100) and (b) SO3 on CaO(100) surface with SO2 adsorbed previously versus electric field.


Table 1. Summary of the adsorption energies and structural parameters for SO3 adsorption on CaO(100) surface with electric field.


Effect
s of H2O on SO3 Adsorption in an Electrostatic Field

For H2O adsorption on CaO surface, H2O dissociates into surface OH and H atom, and H binds with O on CaO (Zhang et al., 2015; Jiang and Fang, 2016). Fig. 7 shows the adsorption energy of SO3 on the surface of CaO(100) under various electrostatic field conditions with H, OH and H&OH is adsorbed on the surface. In the case with H adsorbed on the surface, it can be clearly seen that the variation of adsorption energy is different with other cases. When the electrostaticfield is positive, the adsorption energy has a slight decrease first and then increases steadily. The total increment is much smaller than the previous cases, from –295.8 kJ mol1 to –305.1 kJ mol1. When the electrostatic field is negative, the adsorption energy would not decrease, but increase as the electrostatic field is higher than 0.05 V/Å, and decrease slightly at 0.4 V/Å. The total variation is negligible. Unlike the effect of CO2 and SO2, pre-adsorbed H&OH is favorable for the binding of SO3 on CaO surface, as shown in Table 1. The variation of adsorption energy with OH and H&OH adsorbed on the surface is similar to the other cases.


Fig. 7. Adsorption energies of SO3 on CaO(100) surface with (a) H, (b) OH and (c) H and OH adsorbed previously versus electric field.
Fig. 7. Adsorption energies of SO3 on CaO(100) surface with (a) H, (b) OH and (c) H and OH adsorbed previously versus electric field.Fig. 7. Adsorption energies of SO3 on CaO(100) surface with (a) H, (b) OH and (c) H and OH adsorbed previously versus electric field.

The charge transfer between the adsorbed SO3 and CaO surface is characterized by CDD plots, which displays a large charge accumulation in the region between S and OCaO and significant charge depletion in the region between the OCaO in the top layer and the Ca ion in the second layer, as shown in Figs. 8 and S3. That confirms the formation of a strong bond between S and OCaO. There is also an obvious charge accumulation around the O ion in the SO42–-like structure, which is match up with the results of the charge transfer from the slab to the SO42–-like structure, as shown in Fig. S4.


Fig. 8. CDD plot of SO3 bound on the CaO(100) surface with an electric field of 0.5 V/Å; (a) and (b) clean CaO, (c) and (d) –H, (e) and (f) –H&OH; (a), (c) and (e) is in a positive field; (b), (d) and (f) is in a negative field; charge accumulation is in yellow and charge depletion is in blue.Fig. 8. CDD plot of SO3 bound on the CaO(100) surface with an electric field of 0.5 V/Å; (a) and (b) clean CaO, (c) and (d) –H, (e) and (f) –H&OH; (a), (c) and (e) is in a positive field; (b), (d) and (f) is in a negative field; charge accumulation is in yellow and charge depletion is in blue.

The positive field is seen to enhance charge density changes of the surface O across the Ca around the OCaO, while the negative field enhances charge density changes of the Ca below the OCaO. There is a small but essential difference in the CDD plots between the systems with adsorbed H and H&OH. When H is previously adsorbed, the SO42–-like structure interactes more with Ca cations around it. However, when on a clean surface or H&OH is previously adsorbed, charge transfer is more between Ca and O on the surface. 


CONCLUSIONS


The effects of electrostatic field on the interactions between SO3 and a CaO surface were predicted from DFT calculations. Adsorption on a clean surface or surface with pre-adsorbed species, including CO2, SO2, OH, H&OH, is sensitive to the existence of an electrostatic field. When the field is small, the required adsorption energy is seen to reduce regardless of a positive or negative electrostatic field. Then, with increased field strength, the adsorption energy showes an increasing trend in a positive electrostatic field, while the adsorption energy decreases in a negative electrostatic field. A positive field enhances charge density changes of the surface O across the Ca around the OCaO, while a negative field enhances charge density changes of the Ca below the OCaO. On an H pre-adsorbed surface, the total increment is much smaller than previous cases in a positive field and the total variation is negligible in a negative field.

From the results of the charge transfer and CDD, it can be concluded that the SO42–-like structure accumulates electrons as a unit. The charge transfer of SO42– is found to increase with the incremental adsorption energy in a linear relationship. There is obvious charge accumulation in the region between S and OCaO and significant charge depletion in the region between the OCaO in the top layer and the Ca ion in the second layer, which confirms the formation of a strong bond between S and OCaO.


ACKNOWLEDGMENTS


This work was supported by the National Key Research and Development Program of China (2017YFB0603205), the National Natural Science Foundation of China (U1609212), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (18KJB470003) and the Postdoctoral Research Funding Plan of Jiangsu Province (1701029A).



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