Density functional theory approach to CO₂ adsorption on a spinel mineral: determination of binding coordination

Abstract

The mechanism of the adsorption of CO₂ onto various sites of MgAl₂O₄ (100), in particular with regards to binding coordination, was investigated using density functional theory (DFT) calculations. Of the available sites, CO₂ binding was calculated to be strongly adsorbed to oxygen atoms on the octahedral Al³⁺ and tetrahedral Mg²⁺ sites, with adsorption energy values of −1.60 eV and −1.86 eV, respectively, which was attributed to the small band gap of the CO₂–MgAl₂O₄ system. It is clearly found that strongly adsorbed CO₂ molecules bound to MgAl₂O₄ using polydentate (e.g., bidentate and tridentate) bonds. We also simulated the adsorption of multiple CO₂ molecules on MgAl₂O₄, and found three of eight CO₂ molecules to be strongly adsorbed using tridentate bonds onto the MgAl₂O₄ surface, with an interaction energy of −0.61 eV. The other five CO₂ molecules were also adsorbed, but weakly, i.e., using physical interactions with a modest binding energy of <0.10 eV and at a relatively long distance from the MgAl₂O₄ surface.

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Introduction

Anthropogenic emissions from the combustion of fossil fuels account for a significant proportion of the global emissions of CO₂, which is the main factor driving global warming.¹ To stem such emissions from power plants, the generated CO₂ must be separated from the flue gas and sequestered,² and such CO₂ capture and storage is a promising approach for slowing down the rate of increase of CO₂ levels in the atmosphere.³ Of the various CO₂ separation technologies, the application of solid adsorbents holds particular promise because this process requires relatively little energy to operate and has negligible corrosion problems with good recyclability and a wide operating temperature range, and has led to the development of economical and effective CO₂ capture materials.^3,4 Simple alkali-earth metal oxide minerals (e.g., MgO, CaO, and BaO) have been used as CO₂ adsorbents due to their simplicity and low cost.⁵ In the carbonation process, these alkali-earth metal oxides react with CO₂ exothermically, and the basicity of the metal oxides determines the reactivity with CO₂.⁶ Furthermore, stable carbonates are produced by the carbonation process and can be utilized in environmentally friendly ways such as mine reclamation and soil amendments.⁷ However, the poor stability of carbonates at high generation temperatures has limited their practical applications.⁸ To improve their stability, other metal oxides such as Al₂O₃, SiO₂, TiO₂ and ZrO have been commonly added to alkali-earth metal adsorbents.⁹ We recently introduced the Mg-based spinel mineral magnesium aluminate (MgAl₂O₄) for CO₂ adsorption on account of its high basicity,¹⁰ but the mechanism by which CO₂ adsorbs onto MgAl₂O₄ has not yet been fully investigated using a theoretical approach. MgAl₂O₄ is a well-known refractory oxide used as a structural ceramic that shows high resistance to most acids and alkalis with a low electrical loss. These properties allow MgAl₂O₄ to be used in a wide range of applications in structural, chemical, optical and electrical industries.

For research involving interaction energetics at the atomic level, quantum chemical density functional theory (DFT) calculations have been widely performed with high accuracy to determine intrinsic energetic properties such as adsorption energy with optimized geometry.¹¹ Herein, we investigated the mechanism of CO₂ adsorption on MgAl₂O₄ (100) by analyzing the adsorption energy of this process and the optimized geometry using DFT calculations. To obtain a fundamental understanding of such CO₂ adsorption, we carried out the calculations for various orientations of CO₂ bound at different sites and for the adsorption of multiple CO₂ molecules on the MgAl₂O₄ (100) surface. These calculations can help us understand the binding coordination and accompanying characteristics of the most stable number of bound CO₂ molecules due to charge redistribution between CO₂ and MgAl₂O₄, because the interaction of CO₂ molecules with the metal-oxide surface influences the adsorptive characteristics of the metal oxide.

Experimental

Computational details

Our calculations were performed using the DMol³ module of Materials Studio from Biovia with the generalized gradient approximation (GGA) and Perdew–Burke–Ernzerhof (PBE) functional.¹² We used all-electron Kohn–Sham wave functions and DNP numerical basis with DFT-D3 correction¹³ for all DFT calculations. The GGA-PBE functional has been widely used to describe the interactions between molecules¹⁴ and minerals.¹⁵ For obtaining the optimized unit structures, we fully relaxed the unit structure of MgAl₂O₄, and four atomic layers were cleaved along the (100) crystallographic direction with the constraint of the fourth layer. Three-dimensional periodic slab models were employed to simulate infinite surfaces. On the supercell, a sorbent surface was exposed as a slab, providing absorption sites. Vacuum separation was set to 20 Å along the z direction to avoid interactions beyond the periodic boundary condition. The k-point sampling for the Brillouin zone was performed using a 4 × 4 × 1 Monkhorst–Pack k-point mesh.¹⁶ A self-consistent field (SCF) convergence set to 1 × 10⁻⁵ Ha. Adsorption energy, E_ads, between the adsorbate and the adsorbent slab was determined by three individual energy calculations: (i) energy from energy minimized adsorbate, E_CO2, (ii) energy of the optimized adsorbent slab without adsorbates, E_MgAl2O4, and (iii) energy from geometry optimization of the adsorbent slab interacting with adsorbates, E_MgAl2O4+CO2.where E_MgAl2O4+CO2 is the total energy of the MgAl₂O₄ slab that adsorbed CO₂, E_MgAl2O4 and E_CO2 are the total energy values of the isolated MgAl₂O₄ slab and of CO₂, respectively, and n represents the number of CO₂ molecules adsorbed onto the slab. Electronic charge distributions play a key role in accounting for electrostatic interaction between the gas molecules and surfaces. Mulliken charge analysis was carried out to determine the electronic charge distribution of each surface.

Results and discussion

Properties of the spinel MgAl₂O₄ structure

MgAl₂O₄ spinel forms a cubic (isometric) close-packed (face-centered cubic, fcc) crystal phase with Al³⁺ and Mg²⁺ occupying octahedral and tetrahedral sites in the lattice. Fig. 1a shows the unit cell of the optimized structure, which yielded a cubic lattice length of 8.08 Å. This calculated lattice length is in good agreement with the 8.05 Å experimentally determined length.¹⁷ The Mulliken charge distribution analysis of MgAl₂O₄ indicated a (negative) charge of −1.33e on each O, and (positive) charges of 1.41e and 1.95e on each Mg and Al, respectively, which can provide sufficiently high basicity for promoting adsorption of CO₂. Compared to that in primitive cubic structures such as MgO (atomic packing factor of 0.52), more tightly packed arrangement of the atoms in the fcc structure (atomic packing factor of 0.74) also leads to a greater attraction between the atoms since a crystal field stabilization energy of MgAl₂O₄ is generated by the σ-type interactions between the metal cations and the oxide anions in the solids.¹⁸ This description provides an explanation for the high adsorptive affinity of MgAl₂O₄ for introduced CO₂ gas.

Fig. 1 Optimized crystal structure of MgAl₂O₄. (a) Structure of the unit cell of MgAl₂O₄. (b) The structure of a periodic slab of MgAl₂O₄; (c) top view of the slab structure.

Using the optimized unit cells, three-dimensional periodic slab models of MgAl₂O₄ were developed to simulate “infinite” surfaces, as shown in Fig. 1b and c. Note that the experimental X-ray diffraction (XRD) patterns of MgAl₂O₄ (JCPDS no. 21-11052) indicate that, of the experimentally observed (400), (511), and (440) crystalline plains, the (100) plain has a readily available surface for CO₂ adsorption since the (400) plain is observed clearly with a high-intensity first-order peak. Thus, four atomic layers were cleaved along the MgAl₂O₄ (100) crystallographic direction with the constraint of the fourth layers.

Adsorption of a single CO₂ molecule on MgAl₂O₄

Eight oxygen sites are available on the MgAl₂O₄ (100) surface for the adsorption of a single CO₂ molecule (Fig. 2a and b). Since spinel MgAl₂O₄ is symmetric structure, we focused on two differently oriented oxygen sites, with “site 1” corresponding to oxygen bound to an octahedral Al³⁺ site and “site 2” to a tetrahedral Mg²⁺ site. We first optimized the geometry of the CO₂ adsorbed on MgAl₂O₄ (Fig. 2c and d). The geometry of the adsorbed CO₂ molecule was found to be significantly different from that of the free CO₂ molecule. Specifically, the O_CO2–C_CO2–O_CO2 bond angle contracted from 180° to 134.1° when adsorbed onto site 1, and to 125.3° on site 2, while the C_CO2–O_CO2 bond length, whether for CO₂ bound to site 1 or to site 2, increased from 1.18 Å to 1.26 Å, as shown in Fig. 2c and d, and Table 1. This result is consistent with the high adsorption energy at site 2 (−1.88 eV) compared to that at site 1 (−1.60 eV). Furthermore, the bond lengths between the adsorbed CO₂ and O_MgAl2O4 (i.e., the adsorption site on the MgAl₂O₄ surface) were calculated to be 1.99 Å and 2.16 Å at site 1 and 1.41 Å at site 2. Interestingly, at O site 1, CO₂ was adsorbed using the bidentate bond type and at site 2 with the tridentate bond type. These results show that, as the CO₂ molecule approached the oxygen atom of the MgAl₂O₄ surface, the linear molecular structure of free CO₂ became bent into a V shape. The elongated C_CO2–O_CO2 bond length is also in agreement with a hybridization change from sp¹ to sp², which is consistent with a loss of a shared pair of electrons from each of the chemical bonds. In addition, the adsorption energy was calculated to be quite high (>−1.60 eV) for each site, which suggests the formation of a new chemical bond between CO₂ and the MgAl₂O₄ surface and hence a strong chemisorption. The charge reorganization of the MgAl₂O₄ (100) surface and the adsorbed CO₂ molecule was quantified as shown in Fig. 2. It is found that 0.64e and 0.67e charge was transferred from the MgAl₂O₄ (100) surface to single CO₂ molecule for site 1 and site 2, respectively.

Fig. 2 Available oxygen sites on the MgAl₂O₄ (100) surface: (a) top view, and (b) tilted view. Optimized geometry of CO₂ adsorption on (c) site 1 and (d) site 2. C.T. denotes the charge transfer (e) from the MgAl₂O₄ (100) surface to CO₂ molecule.

Table 1 Geometry and energy of CO₂ adsorbed on the MgAl₂O₄ (100) surface

	Bond length (CCO2–OMgAl2O4) (Å)	Bond length (OCO2–CCO2) (Å)	Bond angle (OCO2–CCO2–OCO2) (°)	Adsorption energy (eV)	Bond type
Site 1	1.99, 2.16	1.26	134.1	−1.60	Reverse bidentate
Site 2	1.41, 1.98	1.26	125.3	−1.88	Tridentate

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The strong chemical bond is attributed to the sharing of electrons between the adsorbed CO₂ molecule and the MgAl₂O₄ surface. To determine the mechanism of this favorable adsorption, we analyzed energy trends on the basis of molecular orbitals. Fig. 2 also shows the energetics of the bands and the localization of the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) in the CO₂–MgAl₂O₄ system. These features indicate the orbital energies and the size of the HOMO–LUMO gap in MgAl₂O₄ with CO₂ adsorbed. The stable pristine MgAl₂O₄ showed a large band gap without CO₂ adsorption. Adsorption of CO₂ onto octahedral Al³⁺ (site 1) and tetrahedral Mg²⁺ (site 2) caused, according to the calculations, different changes in the molecular orbitals: there was a reduction of the band gap at the tetrahedral Mg²⁺ site to 0.12 eV, attributed to a change in the energy levels due to overlapping p orbitals and increasing electron delocalization compared to that of the octahedral Al³⁺ site, for which the band gap was calculated to be 0.36 eV. The low calculated band gap values suggest that relatively little energy is required to excite the molecule, and hence promote the interaction of CO₂ with MgAl₂O₄.

Note that the orientation of the adsorbed CO₂ molecule relative to the MgAl₂O₄ surface depends on which oxygen the CO₂ is being adsorbed onto since the geometries of the exposed surfaces of the octahedral or tetrahedral sites for CO₂ adsorption differ from one another.

In addition, we determined an energy profile of the corresponding optimized geometry as a function of reaction coordinate for the adsorption of CO₂ to the tetrahedral Mg²⁺ site of MgAl₂O₄ (Fig. 3). To determine the energy profile of the adsorption of CO₂, we optimized the structure with fixed carbon atom of CO₂ at each reaction coordination. Note that the relative energy was calculated as the difference between the adsorption energy of a CO₂ molecule located 4.0 Å from the surface and the adsorption energy at the evaluated distance from the surface. In the calculations, the CO₂ molecule began to form appreciable attractive interactions with MgAl₂O₄, i.e., with a relative energy of −0.29 eV, at a distance of 2.60 Å from the MgAl₂O₄ surface. A re-hybridized CO₂ formed at 1.42 Å from the MgAl₂O₄ surface with a relative energy of −1.78 eV.

Fig. 3 The energy profile of the adsorption of CO₂ as it is moved from a vacuum to the MgAl₂O₄ (100) surface concurrent. The optimized geometries of this adsorption are shown at three locations on this profile.

Adsorption of multiple CO₂ molecules on MgAl₂O₄

Since many oxygen sites on the MgAl₂O₄ (100) surface are available for the adsorption of multiple CO₂ molecules, we designed a structure containing eight adsorption sites and then optimized the structure with eight CO₂ molecules in order to study the mechanism by which multiple CO₂ molecules adsorb onto MgAl₂O₄. As shown in Fig. 4, only three of the eight CO₂ molecules were determined according to our calculations to be chemically adsorbed onto the MgAl₂O₄ surface (black circle). The rest of the CO₂ molecules were relatively far from the surface at distances of 3.8–7.8 Å, and interacted with it weakly with average adsorption energy of ∼0.09 eV per molecule. Fig. 4 also shows the charge reorganization of the MgAl₂O₄ (100) surface and CO₂ molecules. In average, 0.54e charge per CO₂ molecule was transferred from MgAl₂O₄ (100) surface to CO₂ molecules. This result shows that these five CO₂ molecules are physically rather than chemically bound to the surface, as confirmed from their linear molecular shape. The electrostatic repulsion between the CO₂ molecules destabilizes the adsorption, but three CO₂ molecules are still strongly interacted with the MgAl₂O₄ surface at the same time. It is worth noting that a considerable number of CO₂ molecules can be adsorbed onto MgAl₂O₄ (100) surface after the chemisorption.

Fig. 4 Optimized geometry of the adsorption of multiple CO₂ molecules on the MgAl₂O₄ (100) surface: (a) tilted view, (b) top view. C.T. denotes the charge transfer (e) from the MgAl₂O₄ (100) surface to CO₂ molecules.

It should be noted that the chemisorbed gas molecules cannot be fully reversible and are required high energy for regeneration. For chemisorption, high temperature under constant pressure condition can regenerate the adsorbent for additional reaction of adsorption. In this study, we focused on the evaluation of the inherent properties of the MgAl₂O₄ (100) surface for CO₂ adsorptions without considering the temperature effect using DFT. We will pursue further study on CO₂ adsorption and desorption mechanisms with thermal effect.

Conclusions

We investigated CO₂ adsorption characteristics on the Mg-based spinel mineral, magnesium aluminate (MgAl₂O₄) (100), surface by analyzing adsorption energy and configuration using density functional theory (DFT) calculations. The optimized spinel structure of MgAl₂O₄ is well characterized by DFT calculation with a good agreement with experimental study. It turns out that CO₂ molecules were here adsorbed on the sites of two oxygens belonging to octahedral Al³⁺ and tetrahedral Mg²⁺ sites with reverse bidentate (with adsorption energy of −1.60 eV) and tridentate (−1.88 eV) bonds, respectively. The MgAl₂O₄ surface was also found to readily allow both CO₂ physisorption and chemisorption for multiple CO₂ molecules. This study has also provided a basic understanding of CO₂ adsorption mechanism of spinel mineral through computational approach, including procedure and analysis, that may be used to evaluate other spinel group materials such as (Mg, Fe)Al₂O₄, FeAl₂O₄, and MnFe₂O₄.

Acknowledgements

This work was supported by KCRC through the NRF funded by Ministry of Science, ICT, and Future Planning (NRF-2015M1A8A1048902). This work was also supported by the Korea Ministry of Environment as Eco-Innovation project (2014000140003).

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Footnote

† These authors contributed equally to this work.

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