First-principles Study on the Properties of CaO(100)Surface Adsorbing Carbon Dioxide①

LI Ming-Yng LI Ji-Yu WU Mio-Mio② WANG Xio-Lin

a (Department of Materials Science and Engineering, China University of Mining and Technology(Beijing), Beijing100083, China)

b (Center on Nanoenergy Research, Guangxi Key Laboratory of Processing for Nonferous Metal and Featured Materials, Guangxi Key Laboratory for Relativity Astrophysics, School of Physical Science & Technology, Guangxi University, Nanning 530004, China)

c (Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing100084, China)

ABSTRACT The increasing carbon dioxide emissions have a huge impact on the global environment. Carbonation reaction of CaO is regarded as a potential method to capture carbon dioxide. The density functional theory calculations have been performed to investigate the adsorption of CO2 on CaO(100) surface. This paper systematically studied the adsorption of CO2 at different adsorption sites on CaO(100) surface and the influence of adsorption angle on adsorption energy. Based on the studying of adsorption sites, adsorption energy and electronic structure of the CO2/CaO(100) systems, chemical adsorption mainly happens when CO2 molecules are absorbed on the CaO(100) surfaces, but physical adsorption may also happen. The research found that CO2 molecules reacted with surface O atom through C, forming monodentate surface carbonate species and tridentate carbonate. Among them, low-coordinated monodentate ligands have a higher stability than tridentate ligands due to the shorter C–OS bond length of monodentate ligands.

Keywords: density functional theory, CO2 adsorption, surface chemistry, CaO(100)surface, mono-dentate ligands; DOI: 10.14102/j.cnki.0254–5861.2011–3072

1 INTRODUCTION

CO2can be converted into organic matter such as starch by photosynthesis of plants in nature. In the current respect, CO2can be used as a carbon resource. How to make CO2become an effective carbon resource and how to achieve the capture and reuse of CO2remain a research hotspot currently[1-3]. In recent years, facing the global energy crisis and the dramatic increasing carbon dioxide emissions, scientists urgently need to further study how to recycle CO2[4-13].

Nobel laureate Josef Michl published a commentary article in Nature Chemistry, pointing to the direction of CO2photochemical reduction[4].Ekambaram Balaraman et al. reported on the process of CO2catalyzed by noble metals so as to obtain methanol[5]. In the same year, Robert D.Richrdon et al. used an organic amine as a catalyst to photo catalyze the conversion of CO2to formic acid[6]. Related research results are published in Nature Chemistry. Additionally, researchers also reported articles on the conversion of CO2to methane, carbon monoxide, propanol, and oxalic acid[7-13].

Whether it is the previous Carbon Capture and Sequestration (CCS) or Carbon Capture and Recycling (CCR) technology, the economic capture and separation of carbon dioxide are important links[3,10-13]. Scholars in various countries are generally consistent with that CO2is a relatively weak Lewis acid and can be easily adsorbed on a slightly alkaline surface. Calcium oxide possesses advantages of high adsorption capacity, low cost,and strong practicality (Fig.1).

Fig.1. Crystal structure of CaO

Regardless of the above advantages of CaO in previous articles, it also has the following defects:1) Slow adsorption rate. Anthony et al. showed that the CO2absorption efficiency of CaO gradually decreases with the increase of cycles[14];2) Poor cycling stability. Bouquet et al. found that CaO particles are prone to sintering and structurally collapsing at high temperature. CaCO3generated by carbonation adheres to the surface of the adsorbent, thereby decreasing the pore size and surface area of CaO. As the adsorption progresses,CaCO3covers the surface of CaO to prevent CaO from further entering the internal adsorbent for reaction, thus resulting in a sharp drop in CaO adsorption capacity and adsorption rate[15].

Numerous works on improving CaO adsorption capacity and cycling stability through methods such as dope, load, support, immobilization of inert/active components, and solvent modification are published[16-23]. Zhao et al. studied the adsorption characteristics of CO2on the surface of Ni/CaO by DFT simulation. It is proved that CaO doped with Ni caused a change in the electronic structure of the Osurfatom and the 10 wt/% Ni/CaO catalyst is conductive toin situCO2capture in biomass pyrolysis[24]. Sun et al. focus on the doped effects of alkali metals, such as Li, Na, K, Rb and Cs, on the uptake of CO2and SO2by CaO.Adsorption energy calculations indicate that doping Cs on CaO leads to stronger interactions between the adsorbate and the surface due to the structural defects and impurities caused by doping,which affects the stability of CO2[25]. Although great progress has been made in this filed, there is a few literatures about the role of CO2and CaO(100) surfaces.

Pacchioni et al embed the appropriate Madelung field in the model by constructing a cluster model to calculate the adsorption of CO2and SO2molecules on the surfaces of MgO and CaO[16].According to the results, two molecules form a weakly bound surface complex on the surface of MgO, and a strongly acting carbonate species is also formed on the surface of CaO. The reason for the above results refers to that the potential energy in CaO is smaller than that of MgO, resulting in more electron delocalization around the O ion,which can effectively overlap with the orbitals of adsorbed molecules. Elly J.Karlsen employed a similar model to analyze the adsorption of CO2on alkaline earth metal oxides[17]. When the alkalinity of alkaline earth metal oxides increases, the tendency for carbonates to form on the surface becomes more apparent. For CaO～BaO, CO2is distorted to some extent with the C–O bond distance stretched by up to 0.08 Å and the O–C–O angle reduced down to 130.0°, which is close to the ideal carbonate angle of 120°. The adsorption energy is steadily rising in the series MgO < CaO

Jensen et al. compare CO2adsorption at terrace and defect sites on the (100) face of CaO[19]. The terrace and defect (step and corner) of CaO (100)correspond to five-, four-, and three-coordinated-O2-,respectively. When the O2-coordination number decreases, the Madelung potential decreases, making it easier for electrons to flow from the CaO surface to CO2. Kadossov et al. proposed that CO2bonds to the CaO surface O atoms through C atoms, which is the most stable structure of CO2adsorbed on the surface of CaO[20]. CO2molecule does not adsorb to the surface through O atoms, which does not react with Ca site. Voigts et al. investigated the adsorption of CO2on CaO films by employing Electron Spectroscopy (MIES), Ultraviolet Photoelectron Spectroscopy (UPS), and X-ray Photoelectron Spectroscopy(XPS)[21]. The CO2molecule bombards the surface of calcium oxide to form carbonate species. After adsorption, CO2will not decompose until the surface of calcium oxide is covered with carbonate.Additionally, the reaction between carbon dioxide and the surface will also stop. R. Besson investigated the interaction effect between CO2and CaO surface,and gave calcite in the CaO surface to form nucleation mechanism[22]. Zhang Ying et al. argued that when CO2adsorbs the point defect of CaO, an oxygen atom of CO2occupies the defect position. At this time, the surface of the substrate is restored to a complete surface, and the CO2molecule is decomposed into CO[23].

Recently, above content briefly introduced the related research on carbon dioxide adsorption on the surface of calcium oxide. These studies were based on the adsorption of carbon dioxide on the oxygen active site of CaO. However, there are no published articles on the bridge, hollow, and Ca adsorption of calcium dioxide on the surface of calcium oxide. The adsorption mechanism of metal oxides studied by the first principles study proves that CO2tends to maintain linear adsorption on most oxides such as TiO, CrO, VO and MnO[24]. Based on the previous researches, we used the first principles study calculation based on density functional theory by referring to the relevant parameter settings. We presented the adsorption of O-top, bridge, hollow, and Ca-top on the surface of CaO (100) in both the parallel and perpendicular states of carbon dioxide molecules. In the present study, we used charge density, difference charge density, density of states (DOS), and Bader charge analysis methods to characterize the bonding of the adsorbed molecules to the surface. In addition,we also discussed the influence of carbon dioxide surface on the adsorption energy and the bonding due to the different angles and distances.

2 COMPUTATIONAL METHODOLOGY

The calculations of this study are based on DFT and are carried out with the Vienna Ab Initio Simulation Package (VASP)[27]. The interaction between the ions and the valence electrons is described by the projector augmented wave (PAW)method and exchange and correlation treating within the PBE generalized gradient approximation(GGA)[28-30]. A Monkhorst-Pack mesh of 3 × 3 × 1 k-points was used to sample the Brillouin zone for determining the optimized adsorbate structures of CO2molecule on the model surfaces, which was increased to 4 × 4 × 1 for the electronic property calculations. The plane wave cutoff energy was 400 eV. All atoms were relaxed until forces are < 0.05 eV/Å and the convergence criteria for the electronic self-consistent loops is set to 1 × 10-6eV[31-34].

The CaO cubic bulk lattice constant is computed to be 4.812 Å, in good agreement with the experiments(4.81 Å)[35]. A 2 × 2 supercell of the CaO (100)surface with five-layer thick was considered,involving 56 atoms corresponding to five CaO planes.The vacuum region between the slabs was set to 15 Å to avoid interactions between atoms in neighboring cells[36]. Depending on the CO2coverage, the coverage corresponds to 1/9. Therefore, we conclude that Van der Waals between adsorbed molecules can be ignored[37].

During structure optimization, all atoms in the CaO film and the four topmost layers were allowed to relax, while the remaining one layer was frozen at the bulk positions. VESTA 3 software was used to visualize all molecular structures[38]. The five-layer CaO(100) slab model is shown in Fig.2a, and four different adsorption sites on the substrate are considered, as depicted in Fig.2b. They are hollow,bridge, Ca-top and O-top sites, respectively.

Fig.2. (a) Five-layer CaO slab model; (b) Top view of the CaO(100) substrate, where the different points include top, bridge, and hollow sites. Red balls represent O atoms and gray balls are Ca atoms

The adsorption energy of CO2on the CaO surfaces is calculated as follows:

where Esurfrepresents the energy of clear surface,ECO2is the total energy of the optimized gas phase CO2molecule, and Esurf+CO2denotes the energy of the whole system.

3 RESULTS and DISCUSSION

3. 1 CO2 adsorbed on the CaO(100)surface vertically

We calculate the stability of the ordered atomic planes (100), (110) and (111) surfaces of CaO. The atom bonding conditions were analyzed by surface energy, density of states, work function, and charge density. The results showed that (100) surface is more stable than other surfaces because of the Ca–O strong interaction of electron clouds, in agreement with the previous calculations[39,40]. It provided theoretical guidance for understanding the properties of CaO surface and the adsorption mechanism of CO2on the CaO surface. The calculated are listed in Figs.S1～S4.

We have systematically discussed the adsorption of CO2molecules on the surface of CaO(100) in a vertical state. As shown in Fig.3a, b, c, and d, the initial modelling structures of CO2are adsorbed vertically on the CaO surface O-top, Ca-top, hollow and bridge. Fig.3e, f, g and h are the optimized structures calculated by the initial models a, b, c, and d, respectively. Table 2 shows the geometrical properties, adsorption energy, and the vertical height of carbon dioxide from the surface for different adsorption sites for CO2adsorption on the CaO(100)surface.

Fig.3. Adsorption configurations of upright CO2 on the CaO (100) surface; (a-d) Side views of CO2 adsorption on the different points of the CaO (100) substrate, O-top, bridge, hollow and Ca-top, respectively; (e-f) Side view of the relaxed structures

Table 1. Adsorption Properties of Upright CO2 on the CaO (100) Surface

In the initial structure, the C–O bond length of the CO2molecule is 1.177 Å and the ∠OCO angle is 179°. It is found that the bond length and bond angle of CO2remained the same before and after optimization, and the corresponding adsorption energies are 0.067, 0.017, and 0.064 eV, respectively,indicating that the vertical adsorption of CO2on the CaO surface was weak physical adsorption[16,19].

Fig.2a is the initial structure before optimization.CO2is kept vertical at a position of 2.9 Å directly above the O ion on the CaO surface, and the corresponding adsorption energy is +0.02 eV. After adding van der Waals correction, the adsorption energy obtained is –0.06 eV, indicating a weak interaction between CO2molecule and CaO (100)surface[41].

Through adjusting the distance between CO2and the surface and several optimizations, the CO2molecule still had no change in bond length and bond angle. When CO2molecules are vertically adsorbed on the CaO (100) surface, CO2tends to stay away from the CaO surface. Therefore, the interaction between CO2and CaO surface mainly comes from the Van der Waals interaction.

3. 2 CO2 adsorbed parallelly on the CaO(100) surface

As shown in Table 2, O-top site C–OSbond length is 1.389 Å, indicating a partial double bond between the C atom and OS. The bridge site bond length is 1.407 Å, thereby slightly larger and corresponding to a single C–O bond. This value is similar to other existing theoretical calculations. For example, the bond length obtained by Schneider using a PW is 1.384 Å[18]. Pacchioni adopts the cluster model to obtain a bond length of 1.412 Å[14]. Sun et al. employ periodic structure to get a bond length of about 1.39～1.42 Å[36].

Compared with the gas phase CO2molecule, the C–O bond length was increased from 1.177 to 1.271 Å, revealing that the C–O bond is weakened after adsorption, and this value is close to the bond length of the carbonate. In the meantime, the bond angle also declined from the initial 179.9° to 129.1°, which is almost that of carbonate (120°)[34]. As a consequence, CO2combines with oxygen ion on the surface of CaO at the O-top site to form a structure similar to CO32-. Compared with the O-top site, the bridge site has a marginally larger C–OSbond length of 1.407 Å. The C–O length and ∠OCO angle are slightly smaller, which are 1.264 Å and 127.9°,respectively. The adsorption of CO2at the bridge site is unstable, and will eventually generate carbonates with the oxygen ions nearby. At the same time, C and oxygen atoms in CO2are in the same plane as the surface OS, Ca2and Ca4ion.

For the hollow adsorption configuration, the Eais–0.1324 eV, and the CO2molecule has no change in bond length, which is a physical adsorption. The adsorption energy of Ca-top site is greater than zero,indicating that the carbon atoms in the CO2molecule will not bond with the surface Ca ions, nor will physical adsorption occur at this site. As can be seen from Table 2, the Eaat the O-top site is –1.394 and–1.361 eV at the bridge site. It was experimentally found that the adsorption enthalpy varied within the range of 140～200 kJ/mol, close to the adsorption energy calculated in this paper. The Eacalculated by Jensen and Pacchioni using the cluster model is between –60 and –100 kJ/mol[16,19]. And on that basis, Solis calculated the Ea of isolated CO2in a Ca9O9cluster model of 3 × 3 × 1 as 151 kJ/mol by increasing the number of atoms in the model[42]. This result is similar to the adsorption energy (146 kJ/mol)calculated by using a supercell with periodic boundary conditions.

In this paper, our simulation is based on the perfect CaO crystal, and the adsorption energy has certain differences compared with the calculated value because of some differences between the model structure used in our calculation and the limestone in the experiment. In the process of CO2adsorption on the surface of CaO(100), the different coordination environments of oxygen ions on the surface of CaO contribute to different adsorption energies after CO2adsorption[36]. It can be inferred that the experimental reaction heat value of 140～200 kJ/mol is a result of the reactions in different chemical environments, as the structures in nature are more complex and uncontrolled. Through the above analysis of CO2parallel adsorption on the surface of CaO, we conclude that the shorter the C–O bond length, the stronger its bonding and the greater the adsorption energy.

The electron transfers were also calculated for comparison through Bader charge, as shown in Fig.6.The negative charge of O1and O2increased from 7.05 to 7.22 e, and for OSit decreased from 7.49 to 7.30 e. After the adsorption of CO2molecules on the surface of CaO, the negative charge is accumulated to the C atoms in CO2through the OSions on the surface of CaO and eventually flows to the O atoms of CO2, leading to a reduction of CaO surface electrons. The total number of electrons transferred from the surface to the CO2molecules can be obtained from Table 2 as 0.44 |e|. The work function calculated after O-top adsorption was 4.28 eV, which was 0.32 eV higher than the clean CaO surface (3.96 eV). The work function of bridge site is 4.14 eV, less than the value of the work function of O-top. The work function is the lowest energy required for electrons to escape from a solid surface and can therefore be used as a criterion for surface stability.The larger the work function is, the greater the energy required for the electrons to escape from the surface will be. The work function calculation is consistent with the Bader analysis result, which proves that carbonates formed by bonding CO2with oxygen ion at the O-top are the most stable adsorption structures.

Fig.4. Relaxed structures of CO2 on CaO(100) in parallel. (a) and (b) Top and side views of CO2 adsorbed on the O-top;(c) and (d) Top and side views of CO2 adsorbed on the bridge site; (e) and (f) Top and side views of CO2 adsorbed on the hollow; (g) and (h) Top and side views of CO2 adsorbed on the Ca-top

Table 2. Ea and Geometric Properties of CO2 Adsorbed Parallelly on the CaO Surface

3. 2. 1 Density of states

As calculated, no peak showed up on the clean surface of CaO in the range of –7～4 eV, but new peaks emerged in the valence band after CO2adsorption. This is attributed to the hybridization between theporbital of the OSatom on the CaO surface and the C atom orbital in CO2, indicating a strong covalent interaction[41-43].

The delocalization of O2-ion electrons on the surface of CaO moves toward the adsorbed CO2molecules, leading to the activation of the molecules[16]. The bending of CO2causes the 2Πuorbital without electrons to split up into two symmetrical orbitals, C2∞, where one of the orbital energy is relatively lower, which enables the transfer of electrons from the surface to the CO2molecule in order to occupy the orbital[41].

The CO2molecule interacts with five-coordinated-O2-on the surface of CaO (100). The 2pzorbital of oxygen on the surface of CaO will overlap with the emptyπ* MO orbital of CO2to form surface carbonate species[43]. When the negative charge moves from the surface to the CO2orbital, the C–O bond length of CO2likewise changes from 1.177 to 1.271 Å, and the transfer charge can be achieved from Bader charge.

In Fig.5, the PDOS of carbon atom in CO2shows that the orbitals of the bridge are 0.282 eV higher in energy compared to the O-top species. This value, on one hand, can well explain the reasons for the difference of adsorption energy between the O-top bridge and bridge, and on the other hand, it indicates the bonding mechanism of these two configurations on the same level.

Fig.5. DOS of (a) Carbonate CO32- on the O-top; (b) Carbonate CO32- on the bridge

3. 2. 2 Differential charge density

As presented in Fig.6, the bonding between OSand C atoms is further confirmed by differential charge density. Yellow represents the electron-rich areas, whereas blue shows the electron-deficient areas[36,44-46]. The calculated differential charge density of CO2adsorption on the surface of CaO(100)demonstrates clearly the strong bonding among C–O and C–OSions, which is consistent with the CO2bonding in the bridge position. Thereby, the two configurations have comparable binding mechanisms.

By comparing the size of the yellow area around the O ion surface in Fig.6, it can be seen that weak electron transport also happens among other atoms.The delocalized surface charge of calcium ions flows to the O ion. The positional charge transfer of OSions is considerably higher than that of the O2-ions nearby.

Fig.6. Charge density differences of CO2 adsorption on the CaO(100) surface (isodensity plot 0.007 e/Å3); (a) Carbonate CO32- on the O-top; (b) Carbonate CO32- on the bridge

3. 2. 3 Charge density analysis

To further study the bonding among O atoms in the O-top site CO2and Ca ions from the surface of CaO(100), further analysis has been performed on the charge density, as illustrated in Fig.7. Fig.7a displays the cross-section of the O-top charge density map along the surface of Ca1, Ca2, and OS. Blue points out a decrease in charge density, whereas red signifies an increase in surface charge density. As a whole, the charge around the surface O increases while that around Ca decreases, which indicates that electrons migrate from Ca to O. In accordance with the contour line, the colour around OSis darker,which demonstrates that after the negative charge is transferred from OSto C, due to electronegativity,the OSion will attract more electrons from the surrounding coordinated Ca ions to supplement those electrons flowing to CO2, thereby forming bluer regions in Fig.7. Thus, OSions are bluer in Fig.7.

Fig.7. Charge density distribution plots for (a) Surface of CaO (100);(b) CO2 computed in the plane; (c) O1, Ca1 and Ca4 plane

The VESTA software is utilized to measure the distance among OSand the top four-coordinate Ca ions on the surface, finding that the bond length is around 2.52 Å, whereas the bond lengths among other oxygen ions and adjacent Ca2+range from 2.29 to 2.34 Å. The bond length likewise confirmed that the bond between OS–Ca is weaker. In Fig.6, the charge density difference provides a similar outcome as the CO2conducted the Bader analysis, indicating that the surface Ca and O are bonded by an ionic bond.

Fig.7b is a cross-sectional view of the charge density along the plane of the CO2molecule. It can be clearly observed that the charge density of CO2and surface OSions overlap with each other and undergo some deformation, highlighting that C–O and C–OSbonds are covalent. The outcomes are consistent with those received from the PDOS analysis. Fig.7c is a cross-sectional view of the charge density of the plane where the O2atoms in CO2and the surface Ca2and Ca3are located. It is evident from the figure that no bonding exists between O atoms and the surface Ca ions. According to the above analysis, the CO2bonding on the O-top site is quite obvious. CO2tends to bond with the surface oxygen ions to form a monodentate ligand.

Fig.8 exhibits the corresponding charge density map of CO2adsorbed at the bridge site. Fig.8a is a cross-sectional view of the charge density along the surface of CaO. We observed that the resulting charge density map was basically the same as shown in Fig.5b. Compared with the electrons around other O ions, the electron around the OSion is substantially reduced.

Fig.8b presents a cross-section view of the charge density along the plane of the CO2molecule, and it can be observed that the negative charge is transferred from Ca1to the O1atom. A similar situation likewise happens between Ca2and O2,representing that the O atom in CO2is bound with the surface Ca ion through an ionic bond. O1–Ca1and O2–Ca2bond lengths are 2.371 and 2.372 Å,respectively, which are longer than the bulk O–Ca bond. Eventually, CO2generates carbonate with tridentate configuration with O ion by means of the bridge site, and CO2is in the same plane with the surface Ca1, OS, and Ca2[47,48].

Fig.8. Charge density distribution plots for (a) Surface of CaO(100) and (b) Plane of the CO2

Hence, based on the above analysis, the carbonate having monodentate configuration has a shorter C–OScovalent bond compared with the tridentate carbonate. The binding energy released by the key formation is greater and the configuration is more stable. Nevertheless, tridentate carbonates will produce O–Ca ion bonds, thereby complementing the gap formed by covalent bonds. This is a reasonable explanation for the reason why the monodentate and tridentate configurations produced by CO2adsorption at the O-top position have a slight difference in adsorption energy.

4 CONCLUSION

We have studied the adsorption of CO2on the CaO(100) surface by density functional theory method. It was found that CO2tended to maintain linear adsorption on surface O atom through C atom, and no reaction occurred either via the O atom or at the Ca sites. We identified four different CO2adsorption configurations: O-top, bridge,hollow, and Ca-top. The most possible adsorption geometry is that carbon dioxide is adsorbed on the surface of calcium oxide in parallel with the adsorption angle to be 0°. The monodentate ligand produced by adsorption has higher adsorption energy than the multidentate one. The results show that the adsorption energy changes with the change of C–OSbond length. The shorter the C–OSbond length, the greater the adsorption energy. This paper hopes to lay a foundation for subsequent research and provide a reliable theoretical basis for further experimental research and guidance for the industrial design and synthesis of new calcium oxide adsorbents.