CO2 Adsorption and Activation on the (110) Chalcopyrite Surfaces: A Dispersion-Corrected DFT + U Study.

We have used the density functional theory within the plane-wave framework to understand the reconstruction of most stable (110) chalcopyrite surfaces. Reconstructions of the polar surfaces are proposed, and three different possible nonpolar terminations for the (110) surface, namely, I, II, and III, are investigated. A detailed discussion on stabilities of all three surface terminations is carried out. It is generally observed that the (110) chalcopyrite surfaces encounter significant reconstruction in which the metal Fe and Cu cations in the first atomic layer considerably move downward to the surface, while the surface S anions migrate slightly outward toward the surface. We also investigated the adsorption of the CO2 molecule on the three terminations for the (110) surface by exploring various adsorption sites and configurations using density functional theory calculations, in which long-range dispersion interactions are taken into consideration. We show that the CO2 molecule is adsorbed and activated, while spontaneous dissociation of the CO2 molecule is also observed on the (110) surfaces. Structural change from a neutral linear molecule to a negatively charged (CO2 -δ) slightly or considerably bent species with stretched C-O bond distances are highlighted for description of the activation of the CO2 molecule. The results address the potential catalytic activity of the (110) chalcopyrite toward the reduction and conversion of CO2 to the organic molecule, which is appropriate to the production of liquid fuels.

Introduction

Transition-metal sulfides are the main class of Earth materials with an interesting variety of structure types[1] that have been technologically familiarized for different advanced functional material applications. Chalcopyrite (CuFeS2) as one of these structure types reveals a host of industrially applicable electronic, magnetic, and catalytic properties. Chalcopyrite is known as a common mineral of noticeable economic applicability that is commercially the main source of copper, accounting for the majority of copper reserves in the world.[2]

Structurally, in chalcopyrite, each anion (sulfur atom) is bonded to four cations (two copper atoms and iron atoms), forming a tetrahedra.[3] Chalcopyrite is known as an antiferromagnetic (AFM) semiconductor material with spins on any two iron atoms bonded to a common sulfur atom are opposed and directed along the c axis[3] (Figure ). The magnetic and electrical properties of chalcopyrite are still not completely known today. The electrical, structural, and in particular the band gap properties of chalcopyrite have made it as an interesting compound in a range of semiconductor applications such as optical devices,[4] photodiodes,[5] spintronic devices,[6] and thin-film intermediate-band solar cells.[7]

Figure 1

Schematic of the CuFeS2 bulk structure. Red, blue, and yellow spheres represent the Cu, Fe, and S atoms, respectively. The S atoms are repeated in the edges because of the periodic boundary conditions.

Because the properties of nanostructures deviate considerably from their bulk complement, a comprehensive understanding at the molecular level is required to design functional devices. Significant efforts have been performed toward understanding the chemistry and physics of chalcopyrite, and as a result of these studies, a variety of interesting properties have been discovered.[8−16] It is well understood that the catalytic properties are intensely affected by the surface structures. The connection between atomic-scale properties and the macroscopic functionality is an effective factor to modify catalytic surfaces,[17] and the first step is the adsorption of molecules on a catalyst surface through their activation and conversion.

A significant number of theoretical investigations were performed for exploring the capability of different CuFeS2 surfaces on the adsorption of molecules.[8−14] Stirling et al.,[14] for example, analyzed the adsorption of H2O on the (100) surface using the density functional theory (DFT) and plane wave basis set, and the strongest chemisorption energy obtained was almost −54 kJ/mol. In a similar study,[13] they studied the molecular adsorption of H2S on the (100) surface, and it was presented that the adsorption is favorable with an adsorption energy of almost −46 kJ/mol. In addition, the H2O adsorption on the (001) surface was studied by de Lima et al.[15] and they stated that the water adsorption on the iron atom is the preferred mechanism with an adsorption energy of almost −95 kJ/mol. They also studied the adsorption of sulphuric acid and hydrochloric acid on the relaxed CuFeS2 (001) surface.[16] Other surfaces such as (111), (101), (110), and (112) were introduced as relevant surfaces for chalcopyrite and similar structures;[3,18−20] however, the (110) surface was found to be the most stable nonpolar surface.[21−23] Natarajan et al. studied the chemical and flotation behavior of chalcopyrite in the presence of Acidithiobacillus thiooxidans for the selective removal of pyrite from chalcopyrite for the economic extraction of valuable copper. Recently, Mikhlin et al.[24] have studied possible configurations arising at CuFeS2(110) and (012) surfaces because of the removal of adjacent Fe atoms in the top layers by applying DFT calculations and also performed X-ray photoelectron spectroscopy studies of the chalcopyrite samples slightly oxidized in water.[25]

Carbon dioxide is known as the main source of the current climate changes.[26] The adverse influence of greenhouse gas emissions could be alleviated by CO2 utilization. The synthesis product chemicals from CO2 are not only a promising substitute to conventional fossil fuels but also it could improve the current technologies of carbon capture, sequestration, and storage.[27] A considerable number of efforts have been performed so far to realize the efficient ways of reducing CO2 and converting it into organic molecules, which are the origins of fuel and chemical feedstocks.[27,28] The adsorption, activation, and conversion of CO2 have been widely studied on metal–organic frameworks,[29−32] transition-metal complexes,[33,34] and metal-oxide surfaces.[35−40] For example, based on the previously performed theoretical studies, CO2 adsorbs on FeS(001), FeS(011), and FeS(111) surfaces with an energy of −19.3, −0.73, and −83.9 (kJ/mol), respectively. CO2 adsorbs exothermally with a binding energy of −71.4, −76, and −93.2 (kJ/mol) on CuO(111), CuO(1̅11), and CuO(011) surfaces, respectively. The adsorption energy of CO2 on graphene is calculated in a range of 25–30 (kJ/mol) in the physisorption regime. In addition, CO adsorption and oxidation on Fe3O4 and Fe4C surfaces are studied, and the results show that the adsorbed CO is partially negatively charged, indicating the improved electron transfer from the surfaces to CO, and the more the electron transfer, the stronger the CO activation.[41−43] Besides, the studies accomplished so far, to the best of our knowledge, there are no published studies of the adsorption, activation, and conversion of CO2 on the chalcopyrite surfaces.

In the present study, the reactivity of the (110) chalcopyrite surface as the most stable surface appearing in the CuFeS2 has been investigated extensively toward the adsorption and activation of CO2 using DFT to realize the factors that have a great influence on the catalytic activity. To do this, first, a detailed first-principles-based study of different terminations of CuFeS2(110) surfaces has been provided. The reconstruction and stability of the surfaces has been discussed by analyzing their structure properties and surface energies. Second, we have studied the CO2 adsorption and activation on the terminated CuFeS2(110) surfaces by interpreting the structural of different cases of adsorption. Different adsorption sites on the surfaces have been explored, aiming to contribute to understand the role of chalcopyrite for CO2 adsorption and activation at a molecular level. Our results show that different terminated (110) surfaces can activate the CO2 molecule. In addition, the results show the subsequent dissociation of the molecule to form CO and O species, which provide useful information in the expansion of more well-organized CuFeS2 catalysts with reactive surfaces.

Computational Methodology

Spin-polarized periodic DFT calculations have been carried out using the Vienna Ab initio Simulation Package (VASP) with a plane-wave basis set.[44−47] The DFT + U(48) methodology with the generalized gradient approximation using the exchange–correlation functional of Perdew, Burke, and Ernzerhof[49,50] and the formalism of Dudarev et al.[48] has been employed to capture the strong correlation effect of 3d electrons that are required for a more accurate description of the transition metals of Cu and Fe in chalcopyrite.[51,52]

Chalcopyrite crystalizes in the tetragonal group with the space group I4̅2d (D2d12).[51−53] The lattice constants determined by the X-ray diffraction experiment was found to be a = b = 5.289[54] (5.286[55]) Å and c = 10.423[54] (10.410[55]) Å. In our previous study,[52] temperature-dependent properties (structure, mechanics, and thermodynamics) of magnetic CuFeS2 were evaluated from first-principles calculations. The Hubbard correction term (i.e., the Ueff value)[48] was carefully chosen for the localized 3d electrons of Cu and Fe, where Ueff is the difference between the Coulomb U and exchange J parameters. Calculations including a Hubbard term for Fe atoms were executed with Ueff in intervals of 1 eV up to 9 eV with a constant Hubbard term for Cu atoms. Calculations with a different Hubbard term for Cu atoms did not have any significant effect on the relative energies of the considered states and the atomic populations. Different magnetic states of the AFM and FM phases for the supercells were analyzed, and the spin configurations corresponding to the AFM phases were initially assigned to the Fe atoms, as the most stable spin arrangement, in the conventional supercells, as visualized using VESTA[56] and shown in Figure . A conjugate gradient scheme was applied with the residual interatomic forces of 0.01 eV/Å acting on the atoms, and a self-consistent field convergence criterion better than 10–6 eV per unit cell was required. A Fermi smearing of 0.01 was considered in order to induce the total energy to absolute zero more accurately.[57] A plane-wave cutoff energy of 550 eV and a 4 × 4 × 2 Monkhorst–Pack[46]k-point mesh for the Brillouin zone sampling of a 1 × 1 × 1 unit-cell of CuFeS2 were employed. It was suggested that the DFT + U methodology with the Ueff value of 5 eV is the most robust methodology to analyze the CuFeS2 structure, which provides the best match to the experimental geometric properties, magnetic moment, and band gap values of CuFeS2.[52]

In the present study, the DFT calculations have been performed with the same accuracy parameters applied in the previous study[52] and with the same value of plane-wave cutoff energy; however, the number of k-points has been adjusted related to the unit cell size. We have investigated different possible nonpolar terminations of CuFeS2(110). The slab modeled in the present work has lattice parameters as a = 10.418 Å and b = 7.443 Å. We tested different vacuum thicknesses and relaxed layer numbers to achieve the convergence within 1 meV per atom. Then, two layers of atoms at the base of the system were fixed with the same positions as their relaxed bulk positions. The surface were represented by one layer of atoms above these two layers and were permitted to move freely during the optimization. In addition, the vacuum region above the surface was at least 10 Å, which the results show that it is large enough to escape the interactions between the periodic slabs. The (1 × 1) surface cells consisting of 12 Cu, 12 Fe, and 24 S atoms have been sampled with a 3 × 5 × 1 Monkhorst–Pack[46]k-point mesh.

The surface energy of the relax slab has been calculated by considering both calculations of the relaxed and unrelaxed surfaces.[37,40] The top and bottom atomic layers of the surface are not the same after relaxation, and therefore it is required to take the unrelaxed surface energy (γu) into consideration to calculate the final surface energy of the relaxed surface. The unrelaxed surface energy is the surface energy before any surface optimization and is calculated aswhere Eslab,u is the energy of the unrelaxed slab; nEbulk is the energy of an equal number of bulk atoms; and A is the surface area of one side of the slab. We calculated the relaxed surface energy (γr) from the total energy of the relaxed slab as followswhere Eslab,r is the energy of the relaxed slab.

The adsorption energy of CO2 molecule has been calculated by taking van der Waals interactions into account, which is crucial for the precise description of the interaction between CO2 and a surface,[37,40] where the correction according to the Grimme method[58] for the long-range dispersion interactions (DFT-D2) has been applied. We have repeated few calculations to see the effect of dipole correction scheme; however, the results have shown that the difference in energy with and without the dipole correction here is less than 0.03 kJ/mol, which is not significant.

The interactions of the CO2 adsorbate with the (110) CuFeS2 surface slab have been modeled under conditions in which the atoms of the adsorbate and the first atomic layer of the slab have been allowed to relax without any constraint until residual forces on all atoms reached 0.01 eV/Å. In order to avoid periodic interactions between neighboring CO2 molecules, the CO2 adsorption has been calculated on a (1 × 2) supercell consisting of 24 Cu, 24 Fe, and 48 S atoms which has been sampled with a 5 × 3 × 1 Monkhorst–Pack[46]k-point mesh. Symmetry constraints have been excluded in the structural optimization; in particular, the CO2 molecule can freely move in all directions and reorient until reaching the minimum energy adsorption structure. The isolated CO2 molecule was also modeled in the center of a broken symmetry cell with lattice constants of 20 Å and was sampled by the γ-point of the Brillouin zone with the same accuracy parameters described for the surfaces.

The adsorption energy per molecule has been calculated as followswhere Esurf+mol is the total energy of the adsorbate–substrate system; Esurf is the energy of the surface slab; and Emol is the energy of the isolated CO2 molecule. Within this definition, the negative and positive values of adsorption energy would indicate an exothermic and endothermic process, respectively.

In order to elucidate the electronic properties and quantify the charge transfer between the surfaces and CO2 molecule, a Bader charge analysis has been executed using the code developed by Henkelman and co-workers.[59,60]

Results and Discussion

CuFeS2(110) Surface

Tasker[61] proposed a very simple model for surface stability of the ionic covalent compound by considering ionic crystal surfaces as stacks of planes, where three possibilities (type I, II, and III) can exist. In the type I surface, each plane has an overall zero charge because of the stoichiometric ratio of anions and cations that makes the surface nonpolar. Type II surfaces consist of charged layers but without a perpendicular charge because of stacking sequence, while type III surfaces have charged with the perpendicular dipole moment that causes the divergence of the surface energy with the size of the slab. According to Tasker,[61] in type I and II surfaces, the reconstruction of the surface is not needed because there is no net dipole perpendicular to the surface in the repeat unit. This causes the existence of these kinds of surfaces in nature with just small relaxation and reconstruction; however, type III surfaces require significant reconstruction in order to disperse the net charge. It is worth nothing that some surfaces of type I or II can also reconstruct considerably because of the important covalent character of the chalcopyrite. The reconstruction of (001), (100), (111), (112), (101), and (110) chalcopyrite surfaces was analyzed using DFT calculations within the plane-wave framework by de Oliveira et al.[59] In their study, only one termination for the (110) surface was introduced, while, in the present study, we investigate the (110) surface in detail, and three different possible terminations for the (110) surface have been obtained. In this section, these three possible terminations for the (110) surface (i.e., (110)_I, (110)_II, and (110)_III) and their surface energies have been presented. All the relaxed and unrelaxed structure of these three (110) surfaces are shown in Figure .

Figure 2

Unrelaxed and relaxed nonpolar (110) surfaces: (a)-(d) (110)_I, (e)-(h) (110)_II, and (i)-(l) (110)_III. Left panels show the side view of the (110) surface, while the top view of the (110) surface is shown in the right panels. Red, blue, and yellow spheres represent Cu, Fe, and S atoms, respectively. The S atoms are repeated in the edges because of the periodic boundary conditions.

The unrelaxed (110) surface is a stepped surface with atomic layers composed by sulfur–metal–sulfur chains, as illustrated in Figure . The (110)_I and (110)_III CuFeS2 surfaces do not reconstruct with the first atomic layer consisting of four sulfur, two iron, and two copper atoms, as shown in Figure a,b,i,j, respectively. In the present study, one reconstruction has been obtained for the CuFeS2(110) surface, named the (110)_II surface. In the reconstructed (110)_II CuFeS2 surface, half of the ions with the same charge from the first surface layer at the top of the repeated unit are removed and transferred to the bottom of the simulation slab. As a result, the surface obtained is partially vacant either in cations or onions so that the first atomic layer comprises two sulfur, one iron, and one copper atoms, as shown in Figure e,f. This configuration guarantees that the surface does not generate an electrical field within the crystal, and therefore the potential felt at each ion site reaches the constant bulk value.

After the relaxations of the two unreconstructed (110)_I and (110)_III surfaces, the metal Fe and Cu cations in the first atomic layer considerably relocate inward, while the surface S anions displace slightly outward toward the surface, as shown in Figure c,d,k,l. This displacement behavior of the surface atoms is consistent with the previously published computational studies.[21,62] This relaxation improves the metal–metal (Cu–Fe, Cu–Cu, and Fe–Fe) interactions in the first atomic layer and reduces under-coordination of surface cations. The Fe–S bond decreases from 2.280 to 2.261 Å, and the Cu–S bond decreases from 2.275 to around 2.225 Å. However, the relaxation of the reconstructed (110)_II surface which includes the half-vacant plane in the first atomic layer is found very unstable and there is no specific pattern for the surface layers, as shown in Figure g,h.

Once the surface with no dipole moment perpendicular to the surface is generated and relaxed, the surface energy is calculated using the relations 1 and 2. The surface energy of 0.51 J/m2 is calculated for the two unreconstructed (110)_I and _III surfaces, which is in good agreement with the value of 0.58 J/m2 calculated by Chen et al.,[21] while the slightly high value of 0.66 J/m2 surface energy is calculated for the reconstructed (110)_II surface. Details are given in Table .

Table 1

Calculated surface energies of the CuFeS2(110) Surfacesa

	(110)_I	(110)_II	(110)_III
current study	0.51	0.66	0.51
previous theoretical studies	0.58[21]	none	none

The surface energies are in J/m2.

CO2 Adsorption on (110) Chalcopyrite Surfaces

We studied the CO2 adsorption by placing the molecule, parallely and perpendicularly in different possible directions and exploring all possible atomic sites on a (1 × 2) supercell for all the surfaces obtained (Figure ). During free relaxation, the CO2 molecule prefers to move away from the surface, whilst remaining in a linear geometry, when placed parallel (x, y-directions) and perpendicular (z-direction) on the top of the first atomic layer of the surface. CO2 is a linear molecule with two C=O bonds each with a length of 1.176 Å and a O–O bond with a length of 2.352 Å. To further investigate the interaction with atoms in the sublayer, we placed the molecule parallely and perpendicularly close to different atomic sites in the second atomic layer in different possible directions. In contrast to the first atomic layer, the CO2 molecule interacts with the surface and binds both exothermically and endothermally, when placed parallely and perpendicularly to the surface in different adsorption configurations of the sublayer. We surprisingly notified the reactivity of the system with respect to CO2 dissociation to form surface-bond CO and O species in a couple of configurations. CO2 dissociation in all configurations is calculated to be endothermic with the energies of almost 100 kJ/mol. In this section, the results of the exothermal CO2 adsorption onto the surface obtained are explained, and the results of the endothermal CO2 adsorption are provided in the Supporting Information as Figures S1–S3.

Figure 3

Top view of the (1 × 2) supercell of the relaxed (110) surfaces: (a) (110)_I, (b) (110)_II, and (c) (110)_III. Red, blue, and yellow spheres represent Cu, Fe, and S atoms, respectively. The S atoms are repeated in the edges because of the periodic boundary conditions.

CuFeS2(110)_I Surface

The first set of CO2 adsorption calculations was performed on the CuFeS2(110)_I surface. The strongest binding was found, when we placed the molecule on top of the S atom in the subatomic layer bonded with the Fesuf1 atom in the top layer (Figure a), perpendicular to the surface in the z-direction (configuration A). In this configuration, the Fesuf1 atom in the top layer moves up by increasing the bonding with the S atom in the subatomic layer from 2.289 to 2.338 Å, whereas only one of the oxygen atoms, O1, of the molecule binds at a surface Fesuf1 site [d(Fesuf1–O1) = 2.315 Å], as shown in Figure a. As a result, the CO2 molecule tilts relative to the surface normal and retaining a nearly linear structure with α(OCO) changing to 177.3°, whereas the C–O bonds slightly stretched to 1.181 and 1.169 Å for the two oxygen atoms O1 and O2, respectively. There is no other binding between the molecule and the surface, and we note that in this configuration, the CO2 molecule binds exothermally to the CuFeS2(110)_I surface with an adsorption energy of −22.8 kJ/mol.

Figure 4

CO2 molecule adsorbed on the (110)_I surface in (a) configuration A and (b) configuration B in a (1 × 2) supercell. Left panels illustrate the side views, while the top views are illustrated in the right panels. Red, blue, and yellow spheres represent Cu, Fe, and S surface atoms, respectively, while O and C atoms of the CO2 molecule are represented by green and black spheres, respectively. Bond length values are in Angstroms.

The second strongest binding was found by placing the molecule perpendicular to the surface in the z-direction in the sublayer and in the middle of the space of the molecular ring, which contains of Fesuf1, Cusuf2, Cusub1, and three S atoms (configuration B) (Figure a). We found that, similar to configuration A, the Fesuf1 atom moves up while the bond length with the S atom in the second atomic layer decreases from 2.289 to 2.238 Å. Similar to configuration A, only one of the oxygen atoms, O1, of the molecule binds at a surface Fesuf1 site [d(Fesuf1–O1) = 2.255 Å], as shown in Figure b. The CO2 molecule slightly bends to 177.4°, and the C–O bonds slightly stretched to 1.188 and 1.166 Å for the two oxygen atoms O1 and O2, respectively. The CO2 adsorption to the CuFeS2(110)_I surface is found to be −14.2 kJ/mol (exothermic adsorption).

The adsorption energies and representative parameters of these two configuration are summarized in Table . It shows that the most stable adsorption site is the Fe atom in the first atomic layer with an adsorption energy of −22.8 and −14.2 kJ/mol.

Table 2

CO2 Adsorption Energies and Representative Parameters for the CuFeS2(110)_I Surface

adsorption site	bonding type	Eads (kJ/mol)	d(C–O1) (Å)	d(C–O2) (Å)	α(OCO) (deg)	d(Fe–O1) (Å)	figures
Fesuf1	config. A	–22.8	1.181	1.169	177.3	2.315	3a
Fesuf1	config. B	–14.2	1.188	1.166	177.4	2.255	3b

Different atomic sites of the CuFeS2(110)_I surface were explored for possible interaction with the CO2 molecule; however, only the two above configuration found to be exothermal adsorption energy, as presented above. The other configurations leading to endothermal adsorption energy are provided in the Supporting Information as Figure S1a–j.

Moreover, we also notice that for few initial placement of CO2 molecules, the molecule dissociates into the CO and O species on the CuFeS2(110)_I surface. In the first site (configuration C), the bond between the two nearby Fesub1 and S atoms in the first atomic layer breaks, and the dissociated O atom of the CO2 molecule, O1, binds at the surface Fesuf1 site in the first atomic layer [d(Fesuf1–O1) = 1.983 Å] and the Fesub1 site in the second atomic layer [d(Fesub1–O1) = 2.059 Å] (Figure a). This dissociation of the CO2 molecule is found, when the molecule is placed in the sublayer perpendicular to the surface in the z-direction and in an equal distance of the Fesuf1 and Cusuf1 atoms in the first atomic layer and close to the S atom made chain with both Fesuf1 and Cusuf1 atoms (Figure a). In addition, the bond between the two adjacent Cusuf1 and S atoms in the first atomic layer breaks, and the CO moiety with a bond length of 1.195 Å binds via the C atom at the adjacent Cusuf1 and S atoms [d(Cusuf1–C) = 1.995 Å and d(S–C) = 1.719 Å].

Figure 5

Spontaneous dissociation of the CO2 molecule into the CO molecule on the (110)_I surface in (a) configuration C and (b) configuration D in a (1 × 2) supercell. Left panels illustrate the side views, while the top views are illustrated in the right panels. Red, blue, and yellow spheres represent Cu, Fe, and S surface atoms, respectively, while O and C atoms of the CO2 and CO molecules are represented by green and black spheres, respectively.

The second site (configuration D) leading to the CO2 molecule dissociation is, when the CO2 molecule is placed in the sublayer perpendicular to the surface in the z-direction in an equal distance of the Fesuf1 and Cusuf2 atoms in the first atomic layer and close to the S atom bonded with both Fesuf1 and Cusuf2 atoms (Figure a). In this configuration, the dissociated O species, O1, binds at a site between two adjacent surface Fesuf1 site [d(Fesuf1–O1) = 1.869 Å] and the S atom in the second atomic layer [d(S–O1) = 1.594 Å] (Figure b). In addition, similar to configuration C, the bond between the two adjacent Cusuf2 and S atoms in the first atomic layer breaks, and the CO moiety binds via the C atom at the adjacent Cusuf2 and S atoms [d(Cusuf2–C) = 1.986 Å and d(S–C) = 1.763 Å] with a bond length of 1.191 Å. These results for both configuration C and configuration D show that spontaneous dissociation of the CO2 molecule is possible over the CuFeS2(110)_I surface through interaction of the CO2 molecule with sublayer surface atoms.

CuFeS2(110)_II Surface

As described earlier, there is no specific pattern in the reconstructed (110)_II surface (Figure g,h), while different possible sites of the surface are considered to study the CO2 adsorption by placing the molecule on these sites in different directions. The strongest binding is found by placing the CO2 molecule perpendicular to the surface in the z-direction in the sublayer in the middle of the space of the molecular ring obtained, which contains the Fesuf1, Cusuf1, Fesuf2, and three S atoms (configuration A) (Figure b). We note that the bonds between the S atom and two nearby Fesuf2 in the first atomic layer and Fesub1 atoms in the second atomic layer break, whereas the CO2 molecule binds via the O2 atom at the Fesuf2 atom [d(Fesuf2–O2) = 2.008 Å] and also binds via the C atom at the S atom [d(C–S) = 1.771 Å] (Figure a). In addition, the other O atom, O1, binds at both Fesuf1 atom [d(Fesuf1–O1) = 2.198 Å] and Fesub1 [d(Fesub1–O1) = 2.076 Å]. As a result, the molecule loses linearity [α(OCO) = 124.1°], and the two C–O1 and C–O2 bonds are elongated to 1.311 and 1.258 Å, respectively. In this configuration, the CO2 molecule adsorbs exothermally with an adsorption energy of −29.8 kJ/mol.

Figure 6

CO2 molecule adsorbed on the (110)_II surface in (a) configuration A and (b) configuration B in a (1 × 2) supercell. Left panels illustrate the side views, while the top views are illustrated in the right panels. Red, blue, and yellow spheres represent Cu, Fe, and S surface atoms, respectively, while O and C atoms of the CO2 molecule are represented by green and black spheres, respectively. Bond length values are in Angstroms.

The CO2 molecule also binds exothermally to the surface in a different configuration (configuration B) with a binding energy of −0.8 kJ/mol, when placed in the same site as configuration A but parallel to the surface in the y-direction. Here also, the bonds between the S atom and two Fesuf1 and Fesuf2 atoms in the first atomic layer break, whereas the CO2 molecule binds via the O1 atom at the Fesuf1 atom [d(Fesuf1–O1) = 2.006 Å] and also binds through the C atom at the S atom [d(C–S) = 1.756 Å] (Figure b). In addition, the other O atom, O2, binds at both Fesuf2 atom [d(Fesuf2–O2) = 2.055 Å] and Cusuf1 in the first atomic layer [d(Cusuf1–O2) = 2.165 Å]. As a result, the CO2 molecule loses linearity to change α(OCO) to 124.1°, and the two C–O1 and C–O2 bonds are lengthened to 1.261 and 1.308 Å, respectively.

The adsorption energies and representative parameters of these two configuration are summarized in Table . It shows that the most stable adsorption sites for the oxygen atoms are the Fe atom in the first and second atomic layers, similar to the CuFeS2(110)_I, and the S atom is the strongest bonding site for the C atom with an adsorption energy of −29.8 and −0.8 kJ/mol.

Table 3

CO2 Adsorption Energies and Representative Parameters for the CuFeS2(110)_II Surface

adsorption sites	bonding type	Eads (kJ/mol)	d(C–O1) (Å)	d(C–O2) (Å)	α(OCO) (deg)	d(C–S) (Å)	figures
{O1: (Fesuf1; Fesub)} {O2: (Fesuf2)} {C: (S)}	config. A	–29.8	1.311	1.258	124.1	1.771	5a
{O1: (Fesuf1)} {O2: (Fesuf2; Cusuf1)} {C: (S)}	config. B	–0.8	1.261	1.308	124.1	1.756	5b

Here, only the above two configurations are found to be exothermal adsorption energy on the CuFeS2(110)_II surface, and the other configurations leading to endothermal adsorption energy are provided in the Supporting Information as Figure S2a–g.

Similar to the CuFeS2(110)_I surface, the CO2 molecule dissociation to the CO and O species was also observed here on the CuFeS2(110)_II surface but only for one initial configuration among different possible configurations investigated. In this configuration, the bonds between two adjacent Fesuf2 and S atoms as well as Cusuf1 and its nearby S atom in the first atomic layer break, when the CO2 molecule is placed in the same site as configuration A but parallel to the surface in the x-direction (configuration C). The CO moiety binds via the C atom with the Fesuf2 atom [d(Fesuf2–C) = 2.159 Å] having a bond length of 1.144 Å, in which the Fesuf2 atom bond with the adjacent S atom is broken. In addition, the dissociated O species, O1, binds at the bridge site between two adjacent surface Cusuf1 [d(Cusuf1–O1) = 1.937 Å] and the S atoms in the first atomic layer [d(S–O1) = 1.548 Å] (Figure ).

Figure 7

CO2 molecule dissociation to the CO molecule on the (110)_II surface in configuration C in a (1 × 2) supercell. The left panel illustrates the side view, while the top view is illustrated in the right panel. Red, blue, and yellow spheres represent Cu, Fe, and S surface atoms, respectively, while O and C atoms of the CO2 and CO molecules are represented by green and black spheres, respectively.

CuFeS2(110)_III Surface

As described earlier, the relaxed CuFeS2(110)_III surface is a stable surface similar to the CuFeS2(110)_I surface with the same energy surface of 0.51 J/m2. In this surface, Fe and Cu atoms in the first atomic layer significantly displace inward toward the bulk, while the surface S atoms relocate slightly outward toward the surface (Figure k,l). We found similar adsorption sites on this surface, while the CO2 molecule binds with different adsorption energy values.

The CO2 molecule binds weakly in exothermic fashion (−11.7 kJ/mol) on this surface on placing it parallel to the surface in the y-direction in the sublayer in the middle of the space of the molecular ring, consisting of the Fesuf1, Cusuf1, Cusub1, and three S atoms (configuration A) (Figure c). In this configuration, the molecule binds at a surface Fesuf1 site through only one of its oxygen atoms, O1, [d(Fesuf1–O1) = 2.257 Å], as shown in Figure a. We note that the Fesuf1 atom moves slightly upward while the bonding with the S atom in the second atomic layer decreases from 2.289 to 2.262 Å. As a result, the CO2 molecule holding nearly its linear structure with α(OCO) changing to 177.4° and skewed relative to the surface normal, whereas the C–O bonds are slightly stretched to 1.188 and 1.166 Å for the two oxygen atoms O1 and O2, respectively.

Figure 8

CO2 molecule adsorbed on the (110)_III surface in (a) configuration A and (b) configuration B in a (1 × 2) supercell. Left panels illustrate the side views, while the top views are illustrated in the right panels. Red, blue, and yellow spheres represent Cu, Fe, and S surface atoms, respectively, while O and C atoms of the CO2 molecule are represented by green and black spheres, respectively. Bond length values are in angstroms.

We found another exothermic adsorption configuration by placing the molecule perpendicular to the surface in the z-direction on top of the S atom in the second atomic layer, which has a bond with the Fesuf1 atom in the first atomic layer (configuration B) (Figure a). We found that, similar to configuration A, the Fesuf1 atom moves up by increasing the bonding with the S atom in the second atomic layer from 2.289 to 2.338 Å, whereas only one of the oxygen atoms, O1, of the molecule binds at a surface Fesuf1 site [d(Fesuf1–O1) = 2.319 Å], as shown in Figure b. We note that the CO2 molecule slightly loses the linearity and bends to 177.4°, and the C–O bonds are slightly stretched to 1.181 and 1.169 Å for the two oxygen atoms O1 and O2, respectively. The CO2 adsorbs exothermally to the surface with an adsorption energy of −3.6 kJ/mol.

The adsorption energies and representative parameters of these two configuration are summarized in Table . Similar to the SuFeS2(110)_I surface, it shows that the most stable adsorption site on the SuFeS2(110)_III surface is the Fe atom in the first atomic layer, but the CO2 molecule adsorbs weaker on this surface than the SuFeS2(110)_I surface with the adsorption energy of −11.7 and −3.6 kJ/mol.

Table 4

CO2 Adsorption Energies and Representative Parameters for the CuFeS2(110)_I Surface

adsorption site	bonding type	Eads (kJ/mol)	d(C–O1) (Å)	d(C–O2) (Å)	α(OCO) (deg)	d(Fe–O1) (Å)	figures
Fesuf1	config. A	–11.7	1.188	1.166	177.4	2.257	7a
Fesuf1	config. B	–3.6	1.181	1.169	177.4	2.319	7b

All other possible sites of the surface were explored to study the CO2 adsorption on the surface, and the other configuration leading to endothermic adsorption energy is provided in the Supporting Information as Figure S3a–j.

Similar to the two surfaces described above, the CO2 molecule dissociation to the CO and O species was also found on the CuFeS2(110)_III surface. The molecule dissociated when we placed it perpendicular to the surface in the z-direction in the sublayer in the middle of the space of the molecular ring, which contains the Cusuf4, Cusub1, Fesub2, and three S atoms (configuration C) (Figure c). We also note that the molecule dissociates similar to the configuration C by placing it near the Cusuf4 in the same molecular ring. In both configuration, the bonds between the Fesub2 and the adjacent lower S atom break and the dissociated O species, O1, binds at the Fesub2 [d(Fesub2–O1) = 1.940 Å] and S atoms [d(S–O1) = 1.573 Å] (Figure ). The CO moiety with a bond length of 1.178 Å binds through the C atom at the S atom in the first atomic layer [d(S–C) = 1.568 Å], which its bonds with two adjacent Cu atoms are broken.

Figure 9

CO2 molecule dissociation to the CO molecule on the (110)_III surface in configuration C in a (1 × 2) supercell. The left panel illustrates the side view, while the top view is illustrated in the right panel. Red, blue, and yellow spheres represent Cu, Fe, and S surface atoms, respectively, while O and C atoms of the CO2 and CO molecules are represented by green and black spheres, respectively.

Electronic Charge Transfer Analysis

Because the CO2 molecule may receive electrons into its lowest unoccupied molecular orbital to form a negatively charged bent species (CO2–δ), we have quantified the electron transfer from the surface to the CO2 molecule by performing Bader charge analysis on the most exothermic CuFeS2–CO2 adsorption configurations on all three surfaces (Figures a, 6a, and 8a). A clear charge transfer from the surfaces to the adsorbed CO2 molecule has been observed calculated through the difference of the charges on the CO2 molecule [Δq(CO2)] in the adsorption configurations to the isolated CO2 molecule. We note that the charge transfer is more prominent in the most exothermic adsorption configuration on the CuFeS2(110)_II (Figure a), where a net charge of 1.10 e– is transferred to the adsorbed CO2 molecule. In the most exothermic adsorption configurations on the CuFeS2(110)_I (Figure a) and CuFeS2(110)_III (Figure a); however, the CO2 molecule gained a net charge of 0.08 and 0.10 e–, respectively, from the surface species. We plotted the electronic charge density difference in order to originate further insight into local charge rearrangement of the surfaces because of the CO2 adsorption. The electronic charge density difference can be obtained by subtracting from the charge density of the total adsorbate–substrate system, the sum of the charge densities of the CO2 molecule, and the clean CuFeS2(110) surfaces, calculated using the same geometry as the adsorbate–substrate system. Shown in Figure are the electron density difference isosurface plots, illustrating the electron redistribution within the CO2–CuFeS2 systems. We observe that the adsorbed CO2 molecule is activated with a net negative charge and is localized on the oxygen atoms binded to the surfaces. As noted, we observed a significant structural change in the CO2 molecule as a result of its activation, changing from a neutral linear molecule to a negatively charged (CO2–δ) slightly or considerably bent species, with elongated C–O bond distances (Tables , 2, and 3).

Figure 10

Electronic density difference plot of CO2 adsorption structures on (a) CuFeS2(110)_I, (b) CuFeS2(110)_II, and (c) CuFeS2(110)_III surfaces in the most stable exothermic configurations, showing charge transfer in the regions between the CO2 and the atoms in the first atomic layer. The left panel illustrates the side view, while the top view is illustrated in the right panel. Orange contours indicate the electron density increases by 0.02 electrons/Å3, and green contours indicate the electron density decreases by 0.02 electrons/Å3.

Conclusions

We have used the DFT + U methodology to describe CuFeS2 surface structures and presented the analyses of the CO2 adsorption and activation on the CuFeS2(110) surfaces. Plane-wave density functional calculations have been applied to study the reconstruction of the (110) CuFeS2 surfaces. Three possible stable surface terminations for the (110) surface have been proposed, and reconstruction has been explained in detail.

Among three different surface configurations, the two terminations are found to be more stable, and for these surface terminations, we found that the most stable site for adsorption is the iron atom binded to one of the oxygen atoms with an adsorption energy equal to ∼−22.8 and −11.7 kJ/mol, whereas the CO2 molecule holding nearly its linear structure and skewed relative to the surface normal, and the C–O bonds are elongated for the two oxygen atoms. However, the other (110) surface exhibits stronger adsorption (∼−29.8 kJ/mol), and the most stable site for adsorption is the iron atom binded to the oxygen atoms and the sulfur atom binded to the carbon atom, whereas the molecule loses linearity, and the two C–O bonds are also elongated. We also notice spontaneous dissociation of the CO2 molecule into CO and O species on all the three surfaces, where one of the oxygen atoms binds with two surface atoms in the first and second atomic layer.

We note that the CO2 molecule accepts electrons into its lowest unoccupied molecular orbital to form a negatively charged species on the surfaces as confirmed by Bader charge analysis, and the structure of the surface plays a significant role in the activation of CO2. Elongation of the C–O bonds is observed in the adsorbed molecule on these surfaces compared to the gas-phase molecule, showing activation of the CO2 molecule over these surfaces. Future work will include investigations of reaction pathways for the CO2 conversion on the different surfaces of CuFeS2. Catalytic processes are extremely complex in nature because of occurring in a multicomponent environment, and various environmental parameters such as temperature, pressure, and electrode potential could affect the process. However, DFT calculations at 0 K, as presented in this paper, would provide mechanistic insight into CO2 activation on the (110) CuFeS2 surface, and this fundamental understanding would still be relevant for applications at higher temperatures.