Revealing the different performance of Li₄SiO₄ and Ca₂SiO₄ for CO₂ adsorption by density functional theory

Abstract

To reveal the difference between Li₄SiO₄ and Ca₂SiO₄ in CO₂ adsorption performance, the CO₂ adsorption on Li₄SiO₄ (010) and Ca₂SiO₄ (100) surfaces was investigated using density functional theory (DFT) calculations. The results indicate that the bent configuration of the adsorbed CO₂ molecule parallel to the surface is the most thermodynamically favorable for both Li₄SiO₄ and Ca₂SiO₄ surfaces. The Li₄SiO₄ (010) surface has greater CO₂ adsorption energy (E_ads = −2.97 eV) than the Ca₂SiO₄ (100) surface (E_ads = −0.31 eV). A stronger covalent bond between the C atom of adsorbed CO₂ and an O_S atom on the Li₄SiO₄ (010) surface is formed, accompanied by more charge transfer from the surface to CO₂. Moreover, the Mulliken charge of O_S atoms on the Li₄SiO₄ (010) surface is more negative, and its p-band center is closer to the E_f, indicating O_S atoms on Li₄SiO₄ (010) are more active and prone to suffering electrophilic attack compared with the Ca₂SiO₄ (100) surface.

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1. Introduction

CO₂ capture, storage and utilization (CCSU) is considered as one of the most promising technologies for reducing anthropogenic CO₂ emission, which can lead to global warming. Solid inorganic sorbents have been proven to efficiently remove CO₂ at high temperatures, and are more economical and effective than low-temperature amine-based materials in CO₂ capture from high temperature exhaust gas.¹ Lithium orthosilicate (Li₄SiO₄) is one of the best CO₂ capture sorbents due to its significant advantages, such as large adsorption capacity, low regeneration temperature, and good adsorption and desorption cycle stability.^2–4 There have been a lot of experimental studies on Li₄SiO₄ as an adsorbent to capture CO₂, including the synthesis method,^5,6 kinetic behavior^7–9 and modification of Li₄SiO₄.^10–12 However, lithium is relatively expensive and not very abundant in the earth's crust. In particular, lithium batteries have been widely used as a source of power or energy for a lot of things from portable electronics to electric vehicles. As a result, the demand for lithium is increased which leads directly to an increase of its price. Accordingly, it is very difficult to apply lithium-based ceramics on a huge scale to capture CO₂ economically and sustainably. Meanwhile there are abundant resources basic silicates all over the world, especially calcium silicates (Ca₂SiO₄) are often found in the industrial by-products named as slags generated during iron and steel production. Furthermore, Ca₂SiO₄, similar to Li₄SiO₄, is thermodynamically favorable for CO₂ capture from room temperature to 572 °C at ambient pressure, and Gibbs free energy changes for the carbonation of Ca₂SiO₄ and Li₄SiO₄ were calculated by HSC Chemistry 6.0 and shown in Fig. 1. However, the slow diffusion and reaction of carbonation between CO₂ and calcium silicates is a common issue even at high temperatures in the case of no participation of water.¹³ It was found that the amount of CO₂ captured with Ca₂SiO₄ is little at the temperature range from room temperature to 572 °C in our previous study. Zhao et al.^14,15 applied Ca₂SiO₄ as the inert material to enhance the sintering resistance and cyclic stability of CaO during multiple sorption/desorption of CO₂. It could be deduced that Ca₂SiO₄ is much inerter for carbonation compared with Li₄SiO₄ at the high temperatures.

Fig. 1 Relationship between ΔG and temperature of carbonation reaction between CO₂ and Li₄SiO₄ and Ca₂SiO₄.

The investigation on the effect of the electronic structure of the silicates on their carbonation reactivity should be very important for understanding deeply the different carbonation behaviors for Li₄SiO₄ and Ca₂SiO₄, and developing new approaches to improve the carbonation activity of silicates.

There are some investigations about CO₂ adsorption on the surface of the oxides and silicates with the first-principles calculations. Kim et al.¹⁶ made an assessment of Li₂O and Na₂O surfaces for CO₂ adsorption based on DFT calculations. They found that the introduction of dopant atoms larger than host metal atoms of the surfaces can negatively increase CO₂ adsorption energies. Kumar et al.¹⁷ studied the CO₂ adsorption on different terminations of Cr₂O₃ surfaces with DFT calculations and found that carboxylate species are formed on O layer terminated-(0001), and carbonate species are formed on O layer terminated-(101̄2) and Cr layer terminated-(011̄2), indicating that the formation of physisorbed and chemisorbed species depends on different surface terminations. Kang et al.¹⁸ thermodynamically evaluated the CO₂ capture potential of Mg₂MO₄ (M = Si, V, and Ge). Their results indicated that the critical temperature at which CO₂ can be absorbed, increased with decreasing Pauling electronegativity of the M site.

There are several investigations for Li₄SiO₄ on its structural, electronic, lattice dynamical and thermodynamic properties. Duan¹⁹ and Tang et al.²⁰ found the covalency properties of Li₄SiO₄ mainly resulting from the overlap of O 2p and Si 3p orbitals. Kong et al.²¹ studied the adsorption mechanism of H₂O on the Li₄SiO₄ (010) surface. It was suggested an interaction between adsorbed H₂O and Li₄SiO₄ (010) surface, including an electrophilic interaction of hydrogen atom in water with oxygen atoms on the surface and a nucleophilic interaction of oxygen atoms in water with Li atoms on the surface.

Ca₂SiO₄, as the industrial cement clinkers, has been investigated by DFT calculations extensively, and many studies focused on the hydration of Ca₂SiO₄ phases. Qi et al.²² investigated H₂O adsorption on low-index surfaces of Ca₂SiO₄, indicating that electron are mainly transferred from surface atoms to H₂O molecule. Wang et al.²³ evaluated H₂O adsorption on β-Ca₂SiO₄ surfaces and found a dual interaction between H₂O and β-Ca₂SiO₄ (100) surface. Wang et al.²⁴ also studied the relationship between reactivity and electronic structure of -, β- and γ-Ca₂SiO₄ for hydration process. They found that the higher hydration reactivity of - and β-Ca₂SiO₄ compared with γ-Ca₂SiO₄ are attributed to the higher charge density and larger local state density of the active oxygen atoms in - and β-Ca₂SiO₄. However, there are few investigations about explanation of the different behaviors of carbonation of Ca₂SiO₄ and Li₄SiO₄ on the base of their structural and electronic properties.

Herein, we have systematically investigated the structural and electronic properties of Li₄SiO₄ and Ca₂SiO₄, and the adsorption of CO₂ on the most stable surfaces of Li₄SiO₄ and Ca₂SiO₄ on the base of density functional theory calculations. We tried to reveal the relationship between the electronic structures of Li₄SiO₄ and Ca₂SiO₄ and their reactivity for CO₂ adsorption on the molecular scale. The results of this investigation could converge to a proposed mechanism of CO₂ capture with the orthosilicates, on which the more reactive silicates for CO₂ capture can be screened out as the candidates for CO₂ capture.

2. Computational details

The calculations were performed based on density functional theory (DFT), using Cambridge Series Total Energy Package (CASTEP) code.²⁵ The exchange-correlation potential was approximated within the generalized gradient approximation (GGA)²⁶ using the Perdew–Burke–Ernzerhof (PBE) functional.²⁷ Dispersion-corrected calculations²⁸ were performed with Grimme's DFT-D3 methodology.²⁹ To model Li₄SiO₄ and Ca₂SiO₄, the unit cell (1 × 1 × 1) was applied for the calculation.

In order to optimize the crystal structures, the plane wave truncation energy and k-points were tested. A cutoff-energy of 650 eV was used for plane wave expansions. The k-points meshes within Monkhorst-Pack³⁰ framework were set as 3 × 6 × 2 and 4 × 3 × 2 for Li₄SiO₄ and Ca₂SiO₄ respectively. The Broyden–Fletcher–Goldfarb–Shenno (BFGS)³¹ minimization algorithm was used to optimize the primitive unit cell. The surfaces of Li₄SiO₄ and Ca₂SiO₄ were cleaved from the optimized bulk structure. All surfaces were kept stoichiometric and neutral to avoid the polarizing electric field. The thicknesses of vacuum layer were set as 15 Å to avoid the interaction between slabs. The convergence criteria were fixed, specifically: the energy change within 1 × 10⁻⁵ eV per atom, the force on the atoms within 0.03 eV Å⁻¹, the stress on the atoms within 0.05 GPa, and the displacement of atoms within 1 × 10⁻³ Å. All the initial crystal structures and date were obtained from the Inorganic Crystal Structure Database (ICSD).³²

3. Results and discussion

3.1. Structural and electronic properties of bulks

The bulk structure of Li₄SiO₄ with the monoclinic phase, which space group is P21/m (no. 11),^33–35 was optimized. The unit cell of Li₄SiO₄ contains 126 atoms, including 14 [SiO₄]⁴⁻ tetrahedra and 56 Li atoms, as shown in Fig. 2a, which are centrally symmetrical. Meanwhile, the bulk structure of Ca₂SiO₄ with the monoclinic phase, which space group is P21/n (no. 14),^36–38 was optimized as well. The unit cell of Ca₂SiO₄ consists of 28 atoms, including 4 [SiO₄]⁴⁻ tetrahedra and 8 Ca atoms, as shown in Fig. 2b. The calculated and experimentally measured lattice parameters of Li₄SiO₄ and Ca₂SiO₄ are presented in Table 1, and they are in good agreement, implying the simulation settings are reliable and give reasonable results.

Fig. 2 Crystal structures of (a) Li₄SiO₄ and (b) Ca₂SiO₄. The Si, O, Li and Ca atoms are shown by blue, red, green and purple spheres, respectively.

Table 1 Comparison of calculated lattice constants of Li₄SiO₄ and Ca₂SiO₄ with experimental lattice constants

	Li4SiO4			Ca2SiO4
	Cal	Expt^19	Δ (%)	Cal	Expt^40	Δ (%)
a (Å)	11.511	11.532	0.18	5.571	5.502	1.25
b (Å)	6.080	6.075	0.08	6.800	6.745	0.82
c (Å)	16.708	16.678	0.18	9.354	9.297	0.61
β (°)	99.15	99.04	0.11	94.295	94.590	0.31

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Fig. 3 shows the total density of states (TDOS) and partial density of states (PDOS) for Li₄SiO₄ and Ca₂SiO₄. Electrons occupying the orbitals below and near the Fermi level (E_f) is of great significance to the activity of the crystal materials for chemical reactions,³⁹ so we focused on the electrons on the orbitals below and near the E_f. For Li₄SiO₄ and Ca₂SiO₄, their TDOS near the E_f is mainly contributed by the p orbitals of O atoms, suggesting that O atoms are more active and more likely serve as the electron donors. Their PDOS in the region between −6.55 ∼ −2.92 eV and −6.25 ∼ −3.02 eV are overlapped with the Si s and p bands, implying orbital hybridization and Si–O binding in the bulks of Li₄SiO₄ and Ca₂SiO₄. However, the states in the region between −3 and 0 eV, near to E_f is about 73% of the total PDOS for O p orbitals for Li₄SiO₄, while that for Ca₂SiO₄ is about 70%. It can be deduced that there are more electronic states for O p orbitals below and near to E_f in Li₄SiO₄ than those in Ca₂SiO_4, Then the electron transfer from O atoms occurs more easier in Li₄SiO₄ than in Ca₂SiO₄.

Fig. 3 DOS analysis for (a) Li₄SiO₄ and (b) Ca₂SiO₄. The black dashed line shows the Fermi level.

Furthermore, the first high peak position⁴¹ in the PDOS for Li s orbital of Li₄SiO₄, is closer to the E_f compared with that for Ca s orbital of Ca₂SiO₄, implying that the outer electron of Li atoms in Li₄SiO₄ can be transferred away easier than Ca atoms in Ca₂SiO₄.

The electron density distribution can show the bonding between atoms and differential charge density can show the accumulation and depletion of electrons. In Fig. 4a, electron density between Si and O atoms is higher than surrounding and electrons accumulate in the middle of Si and O atoms, demonstrating that a covalent interaction of Si–O. And according to the charge population marked in the figure, the Si–O covalent interaction is stronger in Li₄SiO₄. Li/Ca–O have a certain covalent interaction and the strength of interaction of Li–O is weaker than Ca–O.

Fig. 4 Contour maps of electron density distributions and differential charge density of (a) Li₄SiO₄ in the plane (010) and (b) Ca₂SiO₄ in the plane (100).

3.2. Structural and electronic properties of surfaces

In order to find out the most stable surface, the surface energies of seven low Miller index surfaces were calculated. The surface energy (E_surf) can be calculated according to the eqn (1):^42,43

E_surf = (E_slab − nE_bulk)/2A (1)

where E_slab and E_bulk are the total energy of relaxed slab model and unit cell, respectively. n is the number of formula units contained in the slab. A is the area of the slab. According to the calculation results listed in Table 2, the (010) surface and (100) surface were the most stable surface of Li₄SiO₄ and Ca₂SiO₄ respectively, due to their lowest values of surface energy, which is consistent with the previous calculations.^21,22,42 Fig. 5 shows the atomic arrangement of these two surfaces from the top view. The topmost surface layer consists of Li and O atoms for Li₄SiO₄, and Ca and O atoms for Ca₂SiO₄. The atoms of Li, Ca, and O on the topmost surface layer are referred to as Li_S, Ca_S and O_S hereafter, respectively.

Table 2 The surface energies (E_surf in J m⁻²) of low Miller index surfaces of Li₄SiO₄ and Ca₂SiO₄

Surface	(100)	(010)	(001)	(110)	(101)	(011)	(111)
Li4SiO4	1.28	0.78	1.34	0.80	1.28	0.87	0.84
Ca2SiO4	0.63	1.19	0.80	0.86	0.66	0.75	0.82

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Fig. 5 Atomic arrangement of (a) Li₄SiO₄ (010) surface and (b) Ca₂SiO₄ (100) surface from the top view. The lines and dots represent the underlying atoms and tetrahedra respectively.

The electronic properties of the surfaces should differ from those of the bulks due to the dangling bonds or surface reconstruction. The electronic properties of Li₄SiO₄ (010) surface and Ca₂SiO₄ (100) surface were calculated as well. Fig. 6 shows the TDOS and PDOS of the topmost surface layers of Li₄SiO₄ (010) surface and Ca₂SiO₄ (100) surface. Although the surface TDOS resemble the bulk TDOS shown in Fig. 3 for both Li₄SiO₄ and Ca₂SiO₄, the states of O_S p orbitals are shifted up in energy, and the states in the region between −3 and 0 eV are about 79% of the total PDOS for O_S p orbitals for Li₄SiO₄ (010) surface, while that for Ca₂SiO₄ (100) surface is about 77%. Furthermore, p-band center of O_S atoms increased to −1.725 eV from −1.936 eV of O atoms in bulk Li₄SiO₄, and −1.939 eV from −2.103 eV in bulk Ca₂SiO₄. It can be deduced that the reactivity of the surface O_S atoms is enhanced compared with the O atoms in the bulks. Considering the states near to E_f and the p-band center levels, the O_S atoms in Li₄SiO₄ (010) surface are more prone to suffer the electrophilic attacks with respect to Ca₂SiO₄ (100) surface.

Fig. 6 DOS analysis of the topmost surface layers of (a) Li₄SiO₄ (010) surface and (b) Ca₂SiO₄ (100) surface without CO₂ adsorption. The black dashed line shows the Fermi level.

The electron density and differential charge density of the topmost surface layer atoms are shown in Fig. 7. The covalent interaction between Li_S and O_S atoms is weaker than that in bulk, which can be seen from the charge population. While the covalent interaction between Ca_S and O_S atoms is stronger than that in bulk.

Fig. 7 Contour maps of electron density distributions and differential charge density in the cross sections perpendicular to the (001) plane in (a) Li₄SiO₄ (010) surface and (b) Ca₂SiO₄ (100) surface.

Fig. 8 shows the Mulliken charge of atoms on the topmost surface layer. There is a considerable difference of the charge between bulk atoms (listed in ESI†) and atoms on the topmost surface layer. The positive charge of surface Li_S and Ca_S atoms increases, and the negative charge of the O_S atoms increases compared with bulk atoms. And the deviation of Mulliken charge of surface Li_S atoms from the bulk atoms is relatively large, whereas the Mulliken charge of surface Ca_S and O_S atoms differ from their bulk atoms slightly. Furthermore, the O_S of Li₄SiO₄ (010) surface carry more negative charge than Ca₂SiO₄ (100) surface. According Lewis acid/base theory, O_S atoms of Li₄SiO₄ (010) surface are more basic and easier to lose electrons.

Fig. 8 Mulliken charge analysis of atoms on the topmost surface layers of (a) Li₄SiO₄ (010) surface and (b) Ca₂SiO₄ (100) surface.

3.3. CO₂ adsorption on the surfaces

The adsorption energy (E_ads) of a CO₂ molecule on Li₄SiO₄ (010) surface and Ca₂SiO₄ (100) surface is calculated according to the eqn (2):

E_ads = E_slab+CO2 − (E_slab + E_CO2) (2)

where E_slab+CO2is the total energy of the surface with CO₂ adsorption, E_CO2is the total energy of an isolated CO₂ molecule. The lower adsorption energy describes the stronger binding between the adsorbed CO₂ molecule and the surface, which reflects the stability of adsorption.

An isolated CO₂ molecule has linear configuration with the length of C–O bond of 1.18 Å. Three adsorption configurations are presented in this study when a CO₂ molecule is adsorbed on the Li₄SiO₄ (010) surface as shown in Fig. 9. In the first configuration, the adsorbed CO₂ molecule almost remains linear configuration along the normal to the Li₄SiO₄ (010) surface. The distance between O atom in CO₂ and the nearest Li_S atom is 2.17 Å, and adsorption energy is −0.37 eV. In the second configuration shown in Fig. 9b, the adsorbed CO₂ molecule has a linear configuration parallel to the Li₄SiO₄ (010) surface, and the adsorption energy is −0.46 eV. In the third configuration shown in Fig. 9c, the adsorbed CO₂ molecule is lying flat on the surface with the bent configuration. The C atom in CO₂ forms a bond with a surface O_S atom with the O_S–C distance of 1.39 Å, and its two oxygen atoms (O_C) are coordinated to the two surface Li_S atoms, with the O_C1–Li_S1 and O_C2–Li_S2 distances are 2.04 and 2.01 Å, respectively. The adsorbed CO₂ molecule is bent with an O_C1–C–O_C2 angle of 129.20° and the C–O_C length of 1.27 Å. The adsorption energy is −2.97 eV. The third bent configuration of the adsorbed CO₂ on the Li₄SiO₄ (010) surface is energetically favorable over the other two configurations.

Fig. 9 Three configurations of an adsorbed CO₂ molecule on the Li₄SiO₄ (010) surface: (a) along the normal to the surface, (b) parallel to the surface, (c) bent configuration.

There are also three adsorption configurations considered for an adsorbed CO₂ molecule on the Ca₂SiO₄ (100) surface, as shown in Fig. 10. The first configuration, where an almost linearly CO₂ is adsorbed, is in vertical orientation and tilted slightly to the Ca₂SiO₄ (100) surface, and the adsorption energy is −0.14 eV. The absorbed CO₂ molecule is bent a little bit in the second configuration, which is in parallel orientation, and the adsorption energy is −0.28 eV. When the absorbed CO₂ molecule lying on the Ca₂SiO₄ (100) surface in a bent configuration shown in Fig. 10c, the distance of C–O_S is 1.49 Å, and O_C1–C–O_C2 angle is 136.26°. The bent configuration has the lowest value of adsorption energy, −0.31 eV in the third configurations.

Fig. 10 Three configurations of an adsorbed CO₂ molecule on the Ca₂SiO₄ (100) surface: (a) along the normal to the surface, (b) parallel to the surface, (c) bent configuration.

It is found that the bent configuration consisting of a CO₂ molecule absorbed parallel along to the surface is the most thermodynamically stable among the three configurations considered here for both Li₄SiO₄ and Ca₂SiO₄ surfaces. Furthermore, the Li₄SiO₄ (010) surface has greater adsorption to CO₂ than the Ca₂SiO₄ (100) surface due to the stronger bond between C atom in CO₂ and the surface O_S atom for Li₄SiO₄.

3.4. Partial density of state analysis

Considering that the adsorption energy of adsorbed CO₂ in bent configuration is the lowest, the PDOS calculations were only performed for this configuration, as shown in Fig. 11 and 12. It can be seen that the s and p orbitals of C and O_C of adsorbed CO₂ molecule both move towards lower energy level and broaden compared with the isolated CO₂ molecule, indicating that CO₂ molecule becomes more stable after adsorption. C s and p orbitals of adsorbed CO₂ are hybridized with O_S p orbitals, having bonding character between C and O_S atoms. The states in the region between −3 and 0 eV are about 72% for O_S p orbital for Li₄SiO₄ (010) surface after adsorption, which is decreased from 79% before adsorption, demonstrating that PDOS of O_S p orbitals is moved to lower energy level after CO₂ adsorption. Furthermore, the PDOS peak of Li_S s orbital become weaker and broader after adsorption.

Fig. 11 PDOS analysis of (a) C and (b) O_C atoms of CO₂ before adsorption, (c) C and (d) O_C atoms of CO₂ and (e) O_S and (f) Li_S atoms of Li₄SiO₄ (010) surface after adsorption.

Fig. 12 PDOS analysis of (a) C and (b) O_C atoms of CO₂ before adsorption, (c) C and (d) O_C atoms of CO₂ and (e) O_S and (f) Ca_S atoms of Ca₂SiO₄ (100) surface after adsorption.

Fig. 12 shows the PDOS for CO₂ and Ca₂SiO₄ (100) surface after adsorption. Similarly, the s and p orbitals of C and O_C of adsorbed CO₂ move towards lower energy and broaden, but not as far as Li₄SiO₄ (010) surface. It can be deduced that CO₂ adsorption on Li₄SiO₄ (010) surface is more stable. The states in the region between −3 and 0 eV are about 73% for O_S p orbitals for Ca₂SiO₄ (100) surface. The PDOS peaks for Ca_S s orbital become weaker and broader after adsorption, similar to Li_S s orbital.

To better elucidate the different CO₂ absorption behaviors of Li₄SiO₄ (010) and Ca₂SiO₄ (100) surfaces, the p-band centers of C and O_C in CO₂ and O_S on the surfaces with and without CO₂ absorption were calculated, and the results were shown in Fig. 13. Comparing the p-band centers of O_S atoms on two clean surfaces, it can be found that the p-band center of O_S atoms on the Li₄SiO₄ (010) surface is closer to the E_f than that of Ca₂SiO₄ (100) surface, which means the O_S atoms of Li₄SiO₄ (010) surface are more active and easier to transfer electrons to CO₂ absorbed. When CO₂ is adsorbed on the Li₄SiO₄ (010) surface, it is obvious that p-band centers of C, O_C and O_S atoms are farther away from the E_f, indicating that CO₂ absorbed and O_S become stable. On the other hand, energy-down shift of the p-band centers of C, O_C and O_S atoms of Ca₂SiO₄ (100) surface due to CO₂ adsorption is much smaller compared with Li₄SiO₄ (010) surface, probably leading to its higher absorption energy and less CO₂ absorption.

Fig. 13 The p-band centers of C 2p and O 2p orbitals of CO₂, Li₄SiO₄ (010) surface and Ca₂SiO₄ (100) surface before and after adsorption.

3.5. Mulliken charge analysis

To understand the interactions and charge distributions associated with CO₂ adsorbed with the most energetically favorable configurations on Li₄SiO₄ (010) and Ca₂SiO₄ (100) surfaces, a Mulliken charge anlysis was performed. The detail date of Mulliken charges for CO₂ and Li₄SiO₄ (010) and Ca₂SiO₄ (100) surfaces before and after adsorption were shown in Fig. 14. For CO₂ adsorbed on Li₄SiO₄ (010) surface, it is found that the Mulliken charge on a surface O_S atom changes from −1.33e to −0.86e, while the charges on the surface Li_S1 and Li_S2 atoms increase from 0.93e to 0.96e, and from 0.93e to 0.95e, respectively. It can be deduced that CO₂ adsorption induces the net electron loss of Li₄SiO₄ (010) surface. On the other side, the Mulliken charge of C atom in CO₂ adsorbed decreases from 0.98e to 0.76e, and the charges of O_C1 and O_C2 from −0.49e to −0.78e and −0.76e, respectively, which means that CO₂ adsorbed gains the charges from Li₄SiO₄ (010) surface.

Fig. 14 Mulliken charge analysis of adsorbed CO₂ molecule and (a) Li₄SiO₄ (010) surface, and (b) Ca₂SiO₄ (100) surface before and after adsorption.

In the case of CO₂ adsorption on Ca₂SiO₄ (100) surface, it is found that charge gain for C atom in CO₂ adsorbed is similar to that on Li₄SiO₄ (010) surface, but charges gained by O_C1 and O_C2 atoms are fewer. Furthermore, the Mulliken charge on an O_S atom on Ca₂SiO₄ (100) surface changes from −1.12e to −0.88e, and the charge on the surface Ca_S from 1.34e to 1.40e. The net charge transfer from Ca₂SiO₄ (100) surface to CO₂ adsorbed is much less compared to that Li₄SiO₄ (010) surface.

The charge population between C in CO₂ adsorbed and a surface O_S atom was calculated to be 0.62 and 0.45 for Li₄SiO₄ (010) and Ca₂SiO₄ (100) surfaces, respectively. It can be deduced that the C–O_S covalent interaction on Li₄SiO₄ (010) surface is even stronger, which leads to the stronger adsorption of CO₂.

4. Conclusions

A density functional theory calculation was conducted to research the CO₂ adsorption on the Li₄SiO₄ (010) and Ca₂SiO₄ (100) surfaces. The bent configuration consisting of a CO₂ molecule adsorbed parallel along to the surface is the most thermodynamically favorable for Li₄SiO₄ and Ca₂SiO₄ surfaces. And the adsorption energy of Li₄SiO₄ (010) surface is −2.97 eV, more negative than Ca₂SiO₄ (100) surface, −0.31 eV. Li₄SiO₄ (010) surface is more favorable for forming a stronger covalent bond between a surface O_S atom to the C atom of CO₂ adsorbed and transferring more charges to adsorbed CO₂. In addition, it was found that the Mulliken charge of O_S atoms on the Li₄SiO₄ (010) is more negative, and its p-band center is closer to the E_f, which implies O_S atoms of Li₄SiO₄ (010) are more active and more likely serve as the electron donors with respect to Ca₂SiO₄ (100) surface.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (No. 51574163).

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Footnote

† Electronic supplementary information (ESI) available. See https://doi.org/10.1039/d2ra01021f

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