Density Functional Theory Study of CO Adsorption on the CaFe₂O₄ (001) Surface

Abstract

The density functional theory (DFT) has been employed to investigate the relative stability of CaFe₂O₄ (001) surface at seven terminations and the adsorption of CO onto the six possible topmost sites of the CaFe₂O₄ (001) surface. The results showed the surface energies ranged from 8.16 to 23.71 eV·nm⁻² at the seven terminations and the surface with the O1 atom termination was the most stable structure. For the O1-terminated CaFe₂O₄ (001) surface, compared to the Ca, Fe1 and Fe2 atoms, CO molecule tended to adsorb to the topmost three O atoms. The adsorption energies were −3.19, −0.82 and −0.80 eV when CO molecule adsorbed on the O1, O2 and O3 atoms, respectively. More than 0.5 electrons were transferred to CO molecule from surface on the O sites, whereas CO molecule obtained few electrons with CO molecule adsorption on others atoms. Furthermore, two new C–O singe bonds, a new C–O double bonds, a new C–O and C–Fe single bond were generated with CO adsorption on O1, O2 and O3 sites, respectively, which implied the CO₂ species formation. However, the CO₂ dissociation energies were 1.80, −0.33 and 0.32 eV from the O1, O2 and O3 sites, respectively. Although CO preferentially adsorbed on O1 site to form a CaCO₃-like stable sturcture, the CO₂ dissociation was difficult from O1 site. Moreover, since the CO adsorption and CO₂ dissociation energies were negative values, CO molecule spontaneously adsorbed on O2 site and deprived oxygen bound to iron to form the CO₂ molecule.

1. Introduction

Calcium ferrite is the key bonding phase of the fluxed sinters, which plays an important role in solid–state reaction, liquid–phase formation and crystallization process in sintering process.¹⁾ The crystal structure and morphology of calcium ferrite directly affects the physical strength and metallurgical properties of sinters.²⁾ The reduction reaction of CO, H₂ and sinter occurs mainly in the lump ore zone of blast furnace, and the reduction behavior of calcium ferrite has significantly influence on the low temperature reduction degradation and degree of sinter in the blast furnace and subsequent the permeability of the blast furnace.³⁾ Hence, it is important to clarify the reduction behavior of calcium ferrite with CO and H₂.

In the past few decades, the reduction mechanism, equilibrium and kinetic of calcium ferrite with CO or H₂ have always been the research emphasis and frontier. Lv^4,5,6,7) and Taguchi⁸⁾ et al. studied the reduction mechanism of CaO–Fe₂O₃ binary calcium ferrite by CO and H₂. They found that the reduction path way of CaO·Fe₂O₃ was divided into four steps (CaO·Fe₂O₃→ CaO·FeO·Fe₂O₃→ CaO·3FeO·Fe₂O₃→2CaO·Fe₂O₃→Fe) at below 1000°C. CF reduced by H₂ had superior reducibility than CO. Moreover, Schürmann et al.⁹⁾ revealed that the reduction equilibria of the calcium ferrites in the Fe–Fe₂O₃–CaO system was a function of oxygen concentration and temperature. In addition, Lv and Ding et al.^4,5,7,10) investigated the reduction kinetic of the calcium ferrites by CO and proposed that the reduction of CaO·Fe₂O₃ was expressed initially by two-dimensional diffusion reaction and subsequently by a three-dimensional diffusion reaction. Furthermore, the unreacted-core shrinking model for six interfaces was proposed to take into account reduction reaction process of CaO·Fe₂O₃ of sinter in blast furnace by Usui et al.¹¹⁾ However, calcium ferrites in sinter are usually present as silico-ferrite of calcium and aluminum (SFCA) minerals and the reduction behaviors of binary and quaternary calcium ferrite are quite different from each other. Lv and Ding et al.^7,12,13) obtained that the reduction degree of the ternary calcium ferrite (CaO–Fe₂O₃–SiO₂) first decreased and then increased with the content of silica increased from 2 to 8 wt% and the reduction was highly accelerated and proceeded easily with 8% silica. Maeda et al.¹⁴⁾ found that the reduction behaviors of the ternary calcium ferrite (CaO–Fe₂O₃–Al₂O₃) with higher and lower Al₂O₃ contents were essentially the same as that quaternary and binary calcium ferrite, respectively. Due the formation of MgO·Fe₂O₃, the reduction degree of the ternary calcium ferrite (CaO–Fe₂O₃–MgO) decreased with the content of MgO increased from 2 to 8 wt%.^7,15) The reduction path and reducibility of columnar and acicular SFCA were compared by Cai et al.¹⁶⁾

Previous investigations mainly focused on the reduction behavior of micron-scale calcium ferrite, whereas there were a few literatures to reveal the reduction behavior of CaO·Fe₂O₃ in an atomic perspective. Although the adsorption, reaction and desorption behaviors of CO or H₂ on the surface of Fe₂O₃,^17,18,19) Fe₃O₄^20,21,22,23) and FeO^24,25) have been widely studied by the density functional theory (DFT). In present study, in order to clarify the adsorption behavior of CO on the surface of CaO·Fe₂O₃, the relative stability of the CaO·Fe₂O₃ (001) surface was firstly investigated with different terminations, and then the adsorption behaviors of CO molecule on the six possible on-top sites of the CaO·Fe₂O₃ (001) surface were calculated by DFT. Finally, aiming at, the CO₂ dissociations of the stable adsorption structure from the CaO·Fe₂O₃ (001) surface were analysed.

2. Computational Methods and Models

All DFT calculations were performed by using the Vienna Ab initio Simulation Package (VASP).^26,27,28) The pseudopotential was described with projector-augmented-wave (PAW) method.^29,30) The exchange-correlation energy was treated by adopting the Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional.³¹⁾ The cutoff energy for the plane wave expansion was set to be 400 eV. The force on each atom smaller than 0.03 eV/Å was set as convergence criterion. The Brillouin zone integration was performed within the Monkhorst-Pack scheme using the k-point set of 3 × 4 × 1 for surface optimization and adsorption studies. In addition, to correct for the strong electronic correlation of the Fe 3d-electrons in CaFe₂O₄, we employed generalized gradient approximation (GGA) by including the Hubbard parameter U (GGA+U) calculations.³²⁾ The U_eff value was set 4.7 in the study.

CaFe₂O₄ has an orthorhombic structure (Pnma, # 62) with lattice parameters a = 9.230 Å, b = 3.024 Å, and c = 10.705 Å.^33,34) The unit cell of CaFe₂O₄ was shown in Fig. 1. It consists of distorted FeO₆ octahedral sharing edges and corners with eight-coordinated calcium atoms. There are symmetrically two independent Fe atoms (Fe1 and Fe2), one Ca atom, and four independent O atoms (from O1 to O4) in the unit cell. Ca atom is surrounded by eight O atoms and Fe atoms are surrounded by six O atoms with a distorted octahedron shape.^35,36,37) In addition, CaFe₂O₄ has a magnetic property and the most stable structure is antiferromagnetic (AFM).^35,38,39) It is a consequence of the antiferromagnetic alignment of the magnetic moments on the two Fe atoms. Hence, the AFM of CaFe₂O₄ was also set in the present study.

Fig. 1.

Unit cell of CaFe₂O₄ (green, purple, khaki, gray, blue, red and azure balls represent the Ca, Fe1, Fe2, O1, O2, O3 and O4 atoms, respectively). (Online version in color.)

In this work, the low-index plane of CaFe₂O₄ (001) was studied. There are seven terminations on the CaFe₂O₄ (001) surface. To discuss these terminations, the supercell was constructed by a (1 × 2) Ca₁₆Fe₃₂O₆₄ slab and a 20 Å vacuum, in order to avoid the interaction between periodic structures. As shown in Fig. 2, there are two thickness layers for the CaFe₂O₄ (001) surfaces. In our calculations, the topmost 5 Å layers of all slabs are relaxed, while the corresponding bottom layers are fixed. The dipole correction was employed to correct potential spurious terms arising from the asymmetry of the slabs.

Fig. 2.

Seven terminations for CaFe₂O₄ (001) surface (green, purple, khaki, gray, blue, red and azure balls represent the Ca, Fe1, Fe2, O1, O2, O3 and O4 atoms, respectively). (Online version in color.)

For discussing and comparing the stability of surface terminations, we computed the surface energy γ_S, that describes the stability of a surface, is the energy required to cleave a surface from a bulk crystal. It is given by Eq. (1).⁴⁰⁾   

$${\gamma }_S=\frac{1}{2A}\left(E_S^{unrelax}-N{E}_b\right)+\frac{1}{A}\left(E_S^{relax}-E_S^{unrelax}\right)$$(1)

Where, γ_S is the surface energy, eV/nm²; A is the area of the surface considered, nm²; $E_S^{relax}$ and $E_S^{unrelax}$ the energies of the relaxed and unrelaxed surfaces, eV, respectively; N the number of atoms in the slab and E_b the bulk energy per atom, eV/atom.

The adsorption strength between CO molecule and the CaFe₂O₄ (001) surface was characterized by the adsorption energy (E_ad), which is defined as   

$$E_{\text{ad}}=E_{\text{CO}/\text{slab}}-E_{\text{slab}}-E_{\text{CO}}$$(2)

Where E_CO/slab, E_slab and E_CO are the energies of the surface with adsorbed CO molecule, the clean surface and isolated CO molecule, eV, respectively.

Furthermore, as CO bonded to the lattice oxygen and formed a CO₂ molecule, the formed CO₂ may desorb from the substrate. The corresponding desorption energy (E_de) is can be defined as   

$$E_{\text{de}}=\text{(}{E}_{\mathrm{s}\mathrm{l}\mathrm{a}\mathrm{b}+{\mathrm{V}}_{\mathrm{O}}}+E_{{\text{CO}}_{\text{2}}}\text{)}-E_{{\text{CO}}_{\text{2}}\text{/slab}}$$(3)

where $E_{{\text{CO}}_{\text{2}}\text{/slab}}$, $E_{\text{slab}+{\text{V}}_{\text{O}}}$ and $E_{{\text{CO}}_{\text{2}}}$ are the energies of the surface with a generated CO₂ molecule, the surface with an oxygen vacancy and isolated CO₂ molecule, eV, respectively.

3. Results and Discussion

3.1. CaFe₂O₄ (001) Surface Termination and Stability

Figure 3 shows the optimized structures of the CaFe₂O₄ (001) surface at different terminations. Figure 3(a) shows the structure of the CaFe₂O₄ (001) surface with O1 atom termination. It can be seen that Ca atom moved toward the top most layer. The Ca-O3 and Ca-O4 bond lengths expanded from 0.250 and 0.256 nm to 0.290 and 0.304 nm, respectively. At the same time, the angle of O1-Ca-O2 increased from 72.8 to 89.8 degree. Figures 3(b) and 3(c) shows the structures of the CaFe₂O₄ (001) surface with O2 and Fe2 atom termination. The Fe1-O4 bond broke and the O4 atom moved to the top most layer while the angle of O3-Fe2-O4 increased from 93.9 to 124.6 and 125.2 degree in Figs. 3(b) and 3(c), respectively. Meanwhile, the Fe1-O4 bond length increased from 0.202 nm to 0.326 and 0.345 nm while the Ca-O4 bond length decreased to about 0.22 nm for O2 and Fe2 atom termination, respectively. For the Fe1 termination (Fig. 3(d)), the Fe1 atom dipped in to the slab and the angle of O2-Fe1-O4 increased from 91.3 to 114.6 degree. The lengths of Fe1-O4 and Fe1-O2 bonds were decreased by 5.4% and 10.0%, respectively. Moreover, the Ca-O4 bond broken and the distance of them increased to 0.369 nm. Figures 3(e) and 3(f) shows the structure of the CaFe₂O₄ (001) surface with Ca and O3 atom termination, respectively. It can be seen that O3 atom closed to the Ca atom, the length of them decreased from 0.51 nm to 0.228 and 0.238 nm in Figs. 3(e) and 3(f), respectively. Furthermore, the angle of O2-Ca-O4 increased from 72.1 degree to 83.3 and 96.2 degree for Ca and O3 atom termination, respectively. In the case of O4 atom termination (Fig. 3(g)), it can be seen that the Ca-O4 bond lengths were 0.247 and 0.230 nm, short than the bulk values (0.258 and 0.257 nm).

Fig. 3.

Topmost layers of seven terminations for CaFe₂O₄ (001) surface (green, purple, khaki, gray, blue, red and azure balls represent the Ca, Fe1, Fe2, O1, O2, O3 and O4 atoms, respectively). (Online version in color.)

In addition, the surface energies at different terminations were calculated for the relative stability. The surface energies at different terminations were shown in Table 1. The surface energies ranged from 8.16 to 23.71 eV·nm⁻². The surface energy was the lowest at O1 atom termination, while the surface energies of other terminations were much higher than 10 eV·nm⁻². Hence, the structure with O1 atom termination was the most stable for CaFe₂O₄ (001) surface.

Table 1. Surface energy of different terminations for CaFe₂O₄ (001) surface.

Terminations	O1	O2	Fe2	Fe1	Ca	O3	O4
Ebulk/eV	−188.35	−188.35	−188.35	−188.35	−188.35	−188.35	−188.35
$E_S^{unrelax}$/eV	−740.62	−736.65	−723.02	−730.94	−726.72	−724.65	−734.35
$E_S^{relax}$/eV	−737.09	−733.24	−719.50	−723.83	−723.68	−721.65	−732.12
γS/(eV·nm−2)	8.16	11.76	23.71	13.55	20.86	22.72	14.83

3.2. CO Absorption on CaFe₂O₄ (001) Surface

Figure 4 shows the adsorption energies and the bonds length for CO molecule perpendicular adsorption on the different exposed atoms of the CaFe₂O₄ (001) surface with O1 termination. Figures 4(a), 4(b) and 4(c) shows a CO molecule perpendicular to the plane of surface with the C atom adsorbed to the O1, O2 and O3 atom, respectively. It can be seen that the C atom of CO molecule generated two new C–O single-bonds with two O atoms on the surface and formed a CaCO₃-like structure in Fig. 4(a). The lengths of new C–O single-bonds were 0.1377 and 0.1297 nm, respectively. The adsorption energy was −3.19 eV and the C–O triple-bond of CO changed to double-bond with the bond length of 0.1261 nm. Moreover, the generation of new C–O bond implied that there was a CO₂ species production. As we can see from Fig. 4(b), the C atom of CO molecule was also adsorbed to the O2 atom on CaFe₂O₄ (001) surface with new C–O double bond and formed the CO₂ species. The C–O triple bond of CO translated to a double bond and expanded from 0.1155 to 0.1165 nm, while the new C–O double bonds length was 0.1189 nm. They were similar to the C–O double bonds length of a free CO₂ molecule (0.1176 nm). The adsorption energy was −0.82 eV. For the configuration in Fig. 4(c), the CO molecule adsorbed on the surface with C–O and C–Fe singe bond, whereas this C–O and C–Fe bond length was 0.1264 and 0.2130 nm. Meanwhile, the C–O triple bond of CO molecule changed to a double bond and expanded from 0.1142 to 0.1207 nm. The formation of a new C–O double bond indicated the CO₂ species generation. The adsorption energy was −0.80 eV. Figures 4(d), 4(e) and 4(f) shows a CO molecule perpendicular adsorbed to the Ca and different Fe atoms of CaFe₂O₄ (001) surface, respectively. The C–O bond remained a triple bond and the bond length was 0.1140, 0.1146 and 0.1145 nm in Figs. 4(d), 4(e) and 4(f), respectively, which was shorter than that of pure CO (0.1142 nm). The adsorption energy of configurations in Figs. 4(d), 4(e) and 4(f) was −0.75, −0.66 and −0.19 eV, respectively. The distances between C and Ca, Fe1, Fe2 atom were 0.2804, 0.2370 and 0.2350 nm. In general, CO molecule preferentially adsorbed on the O1 atom, followed by the O2 and O3 atoms, and finally the Ca and Fe atoms.

Fig. 4.

Adsorption of CO on the CaFe₂O₄ (001) surface: (a) O1; (b) O2; (c) O3; (d) Ca; (e) Fe1; (f) Fe2. (black and magenta balls represent the C and O atoms of CO molecule, respectively; green, purple, khaki, gray, blue, red and azure balls represent the Ca, Fe1, Fe2, O1, O2, O3, O4 atoms of CaFe₂O₄ (001) surface, respectively). (Online version in color.)

3.3. Partial Density of States

The bonding mechanism of CO adsorbed on the CaFe₂O₄ (001) surface could be obtained by analysing the density of state (DOS) for the adsorbed CO molecule. The DOS of a pure CO and those of a single CO molecule adsorbed on the CaFe₂O₄ (001) surface is shown in Fig. 5. The pure CO molecule has the 4σ, 1π, 5σ and 2π* bands. In Fig. 5(a), there were two new peaks near −20 eV, which indicated the formation of two new C–O single bonds and the transformation of the C–O trip bonds of CO molecule to double bonds. Meanwhile, the 2π* band was broadened by the partial charge transfer. The 5σ band obviously broadened and shifted downward and overlapped with the 1π band. A new peak appeared near −25.4 eV in Fig. 5(b), which indicated the generation of the new C–O double bonds and the conversion of the C–O trip bonds in CO molecule. Compared to the DOS of pure CO, the bands of the adsorbed CO was shifted downward. The 2π* band was also broadened and shifted downward toward the Fermi level. As we can be seen from Fig. 5(c), there was a new peak near −24 eV due to the generation of a new C–O single bond. Moreover, since the new O–Fe single bond formed, the 5σ band overlapped with 1π band and split into two peaks near −7.6 eV. The bands of the adsorbed CO (Fig. 5(d)) was almost the same as those of free CO. At last, there was an apparent feature in the DOS of configurations Figs. 5(e) and 5(f), which 2π* orbital peaks split into two peaks and shifted downward toward the Fermi level. The d -2π* back-donation was the mainly reason of the shifting of the 2π* band. In general, there were the chemical adsorptions between CO molecule and O atoms, whereas the adsorptions between CO and Ca, Fe1, Fe2 atoms were physical.

Fig. 5.

DOS of the pure CO and the adsorbed CO on different atoms of CaFe₂O₄ (001) surface: (a) O1; (b) O2; (c) O3; (d) Ca; (e) Fe1; (f) Fe2. (Online version in color.)

3.4. Charge Transfer

To understand the mechanism of adsorption and reactions, the charge transfer between the adsorbed CO molecules and CaFe₂O₄ (001) surface was investigated based on the analysis of Bader charges. Table 2 shows the Bader charge analysis for CO adsorption on the different sites of CaFe₂O₄ (001) surface. As can be seen from the Table 2, CO molecule got more than 0.5 electron from the surface when CO adsorbed on the O atom sites of the CaFe₂O₄ (001) surface. Meanwhile, the charges of CO molecule were less than 0.04 electron at the cases of CO adsorption on the Ca, Fe1 and Fe2 atom sites of CaFe₂O₄ (001) surface. For CO molecule, the electrons were transferred from C atom to O atom. In addition, as the charge of CaFe₂O₄ (001) surface gathered toward the adsorption site atoms of CaFe₂O₄ (001) surface, the charges of O atoms decreased while those of Ca and Fe atoms increased. Based on the charge transfer of CO adsorption on different sites of CaFe₂O₄ (001) surface, it can be known that CO reacted with the surface in the case of CO adsorption on O atoms, and free CO molecule became adsorbed molecule.

Table 2. Bader charge analysis for CO adsorption on different sites of CaFe₂O₄ (001) surface.

Sites	O1	O2	O3	Ca	Fe1	Fe2
ΔqC/e−	2.03	2.11	1.53	1.02	1.01	1.02
ΔqO/e−	−1.17	−0.98	−0.95	−1.00	−1.05	−1.05
ΔqCO/e−	0.86	1.13	0.58	0.02	−0.04	−0.03
Δqsite/e−	−1.22	−1.16	−1.19	1.44	1.68	1.59

*Positive q indicates the electron donor, whereas, negative q indicates the electron acceptor.

3.5. CO₂ Dissociation

From Figs. 4(a), 4(b) and 4(c), it can be known that the strong adsorptions of CO on the O atoms of the CaFe₂O₄ (001) surface were chemisorptions. The stable structures were formed and implied the CO₂ species generation. Hence, the CO₂ molecules dissociated from the chemisorptions were investigated further. Figure 6 shows the bond lengths and the dissociation energy of CO₂. The dissociation energies of CO₂ were 1.80, −0.33 and 0.32 eV for the configuration in Figs. 4(a), 4(b) and 4(c). It indicated that CO₂ molecule was spontaneously dissociated from the O2 site, while it needed to overcome an energy barrier of 0.32 eV to form the CO₂ molecule from the O3 atom. In addition, since CO molecule was adsorbed on the O1 site to form a stable CaCO₃-like structure, the dissociation of CO₂ molecule needed to overcome the energy barrier of 1.80 eV. The dissociation of CO₂ molecule was difficult, which implied the adsorption of CO molecule on the O1 site was easily saturated. As the adsorption of CO molecule on the O1 atom reached saturation, CO molecule began to adsorb on O2 and O3 atoms. Due to the spontaneously dissociation of CO₂ molecule from O2 site, CO molecule continuously deprived O2 atom bound to iron. Meanwhile, it was necessary to provide the energy of 0.32 eV to dissociate CO₂ molecule so that CO molecule could be continuously adsorbed on the O3 site. The reaction rate is usually determined by the restrictive steps. CO molecule preferentially adsorbed on O1 atom but CO₂ molecule difficultly dissociated from O1 atom, the dissociation of CO₂ molecule was the restrictive step for O1 atom. Moreover, since CO molecule adsorption and CO₂ molecule dissociation energies were −0.82 and −0.33 eV, CO molecule spontaneously adsorbed on O2 site and deprived oxygen bound to iron to form the CO₂ molecule. Hence, the reduction reaction between CaFe₂O₄ (001) surface and CO molecule was easier reacted on the O2 site.

Fig. 6.

CO₂ dissociation from the chemisorption: (a) O1; (b) O2; (c) O3. (black and magenta balls represent the C and O atoms of CO molecule; green, purple, khaki, gray, blue, red and azure balls represent the Ca, Fe1, Fe2, O1, O2, O3, O4 atoms of CaFe₂O₄ (001) surface, respectively). (Online version in color.)

4. Conclusions

(1) There were seven terminations of the CaFe₂O₄ (001) surface, the surface energies ranged from 8.16 to 23.71 eV·nm⁻². The O1-terminated CaFe₂O₄ (001) surface had the lowest surface energy, which indicated that it was the most stable structure.

(2) The adsorption energies were −3.19, −0.82 and −0.80 eV with CO adsorbed on the O1, O2 and O3 atoms, respectively. Since the charges transferred to CO, the bands of adsorbed CO were all shifting downward and the new C–O and C–Fe bonds generated, which indicated those were chemiadsorptions.

(3) As CO adsorbed on the Ca, Fe1 and Fe2 atoms, the adsorption energies were −0.75, −0.66 and −0.19 eV, respectively. However, CO molecules obtained few electrons and DOS of adsorbed CO were similar to that of the pure CO, which implied the processes were physical adsorptions.

(4) The CO₂ dissociation energies were 1.80, −0.33 and 0.32 eV in the CO₂ dissociation processes from the O1, O2 and O3 sites of CaFe₂O₄ (001) surface, respectively. Considering the processes of CO adsorption and CO₂ dissociation, the reaction between CaFe₂O₄ (001) surface and CO molecule was most likely to react on O2 site, followed by O3 atom. It was difficult to react continuously on O1 site since the higher dissociation energy of CO₂ molecule.

Acknowledgements

We thank Professor Liang CHEN and Professor Ziqi TIAN of Ningbo Institute of Materials Technology and Engineering of Chinese Academy of Sciences for their helpful assistance during the calculation. This work was supported by the National Natural Science Foundation of China (No. 51904127), the Natural Science Foundation of Jiangxi Province (No. 20192BAB216018), the PhD Project (No. 2018-YYB-05) and Generalized System of Preferences-One Type Project (No. 2018-XTPH1-05) of Jiangxi Academy of Sciences.

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