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Article

Co and Ni Incorporated γ-Al2O3 (110) Surface: A Density Functional Theory Study

by
Huaxi Li
1,2,
Liu Shi
3,*,
Chengkai Jin
1,
Runping Ye
1 and
Rongbin Zhang
1,*
1
Key Laboratory of Jiangxi Province for Environment and Energy Catalysis, Institute of Applied Chemistry, College of Chemistry, Nanchang University, No. 999 Xuefu Road, Nanchang 330031, China
2
The Laboratory of the Emergency Management Technical Support Center, Jiangxi Academy of Emergency Management Science, No. 999 Wugong Mountain Road, Nanchang 330103, China
3
State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, No. 1037 Luoyu Road, Wuhan 430074, China
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(2), 111; https://doi.org/10.3390/catal12020111
Submission received: 29 December 2021 / Revised: 10 January 2022 / Accepted: 13 January 2022 / Published: 18 January 2022
(This article belongs to the Special Issue Heterogeneous Catalysis for Energy Conversion)
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Abstract

:
Investigation into the state and mechanisms of the active metal substitution into the γ-Al2O3 support is the basis for design of many catalysts. Periodic density functional theory (DFT) +U calculations were used to investigate the surface properties of transition metals Co3+ and Ni3+ cations substitute for the Al3+ cations of γ-Al2O3 (110) surface. It was found that the substitution energy of one Al3+ substituted by Co3+ and Ni3+ are −61 and −57 kJ/mol, respectively. The Co and Ni preferentially substitute the tetrahedral Al sites instead of the octahedral Al sites. Using thermodynamics, the Al atoms in the top layer of γ-Al2O3 (110) can be 100% substituted by Co and Ni. Ni is easier to substitute the Al atom than Co. There is no obvious structural distortion that occurs after Co and Ni substituted all the top layer Al atoms. While the band gaps of the substituted surface become narrower, resulting in the increase of surface Lewis acidity. In addition, the oxygen vacancy formation energies of the Co and Ni substituted surface are 268 and 53 kJ/mol, respectively. The results provide interface structure and physical chemistry properties of metal-doped catalysts.

Graphical Abstract

1. Introduction

γ-Al2O3 is commonly used as a catalyst support, due to its performance and excellent thermal stability, good adsorption and high specific surface area [1]. Transition metal substitution into γ-Al2O3 surface to form high-performance catalyst is widely used in the industrial field. Co3+ and Ni3+ cations are of interest to researchers because they are isovalent with Al3+ and can form stable oxides. More importantly, the Co doped γ-Al2O3 exhibits excellent catalytic activity in Fischer–Tropsch synthesis [2,3,4,5] vs. Ni doped γ-Al2O3 applied in hydrogenation [6,7], dehydrogenation reaction [8] and alcohol steam reforming reaction [9,10].
The properties of Co and Ni substituted γ-Al2O3 surfaces have been studied in previous experiments [11,12,13]. Some characterization techniques were applied to explore the Co and Ni substituted γ-Al2O3 surfaces properties, such as X-ray diffraction (XRD) [14,15], CO2 temperature programmed desorption (CO2-TPD) [14], and H2-temperature programmed reduction (H2-TPR) [16,17]. Jaroniec et al. [14] synthesized 10% of Co and Ni added into γ-Al2O3, they found the peaks of Co3O4, Ni2O3 and NiO in the XRD patterns. The Co3O4, Ni2O3 and NiO peaks disappeared when the temperature was higher than 823 K. Laine et al. [18] used the XRD and assumed that the Co substituted for the tetragonal site Al atoms. In addition, the phase diagrams showed that the structure of Ni/γ-Al2O3 and Co/γ-Al2O3 changed with the temperature and doped metal content [19,20]. The CO2-TPD results [14] detects that the doped Ni and Co enhanced acid-base properties of catalyst surface.
Using the first principal method to study the surface properties of doped catalysts is of interest to researchers [21,22,23,24,25,26,27]. Gu et al. [28] studied the surface properties of the Fe doped γ-Al2O3(111) surface. They found that the Me-O (Me indicates the transition metal atom) bond distances were increased after substitution reaction. Furthermore, the surface charge is redistributed, with part of Fe atoms only as electron acceptor, and the others Fe atoms as electron donor and acceptor. Benam et al. [29] studied the band gap of the IIIB transition metal substituted α-Al2O3 unit cell, and it was found that the bond gap became narrower with the ionic radius increase, as the order of 3d < 4d < 5d < 6d. Baltrusaitis et al. [30] found the Me-O bond distances of the Fe and Cr doped α-Al2O3(0001) surface had no obvious change. The empty Fe and Cr 3d orbitals increased after substitution, which changed the surface charge properties, resulting in different adsorption properties for CO. The surface properties of other metal-doped supports have also been studied. Pacchioni et al. [31] used DFT-GGA method to study the boron(B)-doped anatase TiO2, and they found two kinds of B structures of neutral and diamagnetic, which proved the two different XPS signals of B. Gu et al. [32] found the dopants of In and Sb could form stable geometries with the graphene. Yan [27] analyzed surface electronic structure of Ru/TiO2(101) surface, they found that the electrons were transferred from the Ru active metal to the TiO2 (101) support surface. Wang [33] reported that the clean and hydroxylation Pt4/γ-Al2O3 (110) surfaces exhibited different catalytic performances for CO2 hydrogenation, and the hydroxyl could weakness the interaction between Pt4 and γ-Al2O3 (110) surface. Zhang [34] studied the Ag atom doped into the hydroxylated γ-Al2O3 (110) and (100) surfaces, they found that the terminal hydroxyl groups bond and disperse the Ag active metal, which lead the Ag/nano-Al2O3 had high catalytic performance in HC-SCR of NO.
Our previous work [35] has used DFT+U method to investigate the Cu2+ substitution into the γ-Al2O3 (100) and (110) surfaces. The results show that the substitution energies of one Cu2+ and H+ substitution for γ-Al2O3 (100) and (110) surfaces were −120 and −261 kJ/mol, respectively. In addition, the substituted Cu atoms had better selectivity to the surface tetrahedral Al sites. The surface Al could be 100% substitution by Cu to form spinel-like structure, which was agreed well with experimental results. Moreover, the Fe, Co and Ni doped into the CuAl2O4 spinel surfaces were also studied [25]. The substitution energies of Me3+ cations substitution for tetrahedral Al3+ cations were in the order of Fe > Co > Ni; vs. Ni > Co > Fe for octahedral Al3+ cations.
In conclusion, the Co and Ni doped γ-Al2O3 exhibits excellent catalytic activity in industrial field. However, conventional experimental research usually focus on the catalytic performance, while the molecular level insights for the understanding of the interface structures and physical chemistry properties of Co and Ni substituted γ-Al2O3 are still not clear. It was reported that the active metals of Ni and Co were mainly doped into the most stable γ-Al2O3 surfaces (e.g., (110) [36], (220) [37] and (440) [38,39,40] surfaces in XRD pattern). Meanwhile, the theoretical calculations reported by Digne et al. showed that the γ-Al2O3 (110) surface dominates largely 74% of the total surface area of γ-Al2O3 surface [41,42]. This model agrees well with the experimental data i.e., bulk modulus, XRD, NMR, electronic density and the acid–base surface properties [43,44]. In addition, the γ-Al2O3 model has been widely used in related research of γ-Al2O3, such as Ag/γ-Al2O3 [34], Rh/γ-Al2O3 [45], Pd/γ-Al2O3 [33], Pt/γ-Al2O3 [46] and Fe doped γ-Al2O3 [28,47]. Investigation into the Ni and Co doped γ-Al2O3 (110) surface could provide interface structure and physical chemistry properties of Ni and Co doped γ-Al2O3 catalysts.
The present work was aimed to investigate the surfaces structures of the Co and Ni substitution into the γ-Al2O3 (110) surface. The surface charge properties, substitution mechanism by the projected density of states (PDOS), and surface stability via oxygen vacancy were analyzed.

2. Results and Discussion

2.1. Substitution of Al3+ by Co3+ and Ni3+

2.1.1. Substitution of Al3+ by Co3+

Figure 1 shows the structures of the Co3+ cations substituted γ-Al2O3 (110) surface. The results show that the most favorable substitution structure of one Co substituted γ-Al2O3 (110) surface is (110)-1Co-4, with the substitution reaction exothermic by −61 kJ/mol. The electronic structure of Co changes after substitution. The Bader charge of substituted Co(4)3c is +1.40 e, which is smaller than the Al(4)3c cation (+2.38 e) of the perfect γ-Al2O3 (110) surface. The substituted Co(4)3c atom bonds with the O(5)2c, O(6)2c and O(sublayer)4c with the Co-O bond distances of 183, 183 and 196 pm, respectively, which are longer than the Al(4)-O bond distances (171, 171 and 179 pm) of the perfect γ-Al2O3 (110) surface. It agrees well that the covalent radius of Co (126 pm) is larger than Al (121 pm) [48]. For the substitution of Al(1)4c and Al(3)4c by Co atom, the substitution energies are 26 and −29 kJ/mol, respectively. It is worth noting that the Al(4)3c site is tetrahedral structure, and the Al(1,2,4)4c sites are octahedral structure. It means that Co atoms are easier to substitution the γ-Al2O3 (110) surface tetrahedral Al atoms than the octahedral Al atoms. In addition, the substitution of sublayer Al(5)3c by Co is also calculated, with the substitution energy endothermic of 82 kJ/mol, which is thermodynamically unfavorable. This indicates that Co preferentially substitution surface Al sites rather than subsurface Al sites.
For two Co3+ cations substitution for Al3+ cations of γ-Al2O3 (110) surface. The substitution energies of (110)-2Co-12, (110)-2Co-23, (110)-2Co-24 and (110)-2Co-34 are 80, 24, −7 and −83 kJ/mol, respectively. It indicates that the substitution reaction is thermodynamically favorable at Al(2,4) and Al(3,4) sites, besides, the most thermodynamics favorable substitution structure is (110)-2Co-34. For the (110)-2Co-34 structure, the Bader charge of Co(3,4) are +1.42 and +1.41 e, and the average bond distances are 183 and 191 pm, respectively. For three Co3+ cations substitution for the Al3+ cations of γ-Al2O3 (110) surface. The substitution energies for (110)-3Co-123, (110)-3Co-124 and (110)-3Co-234 are 38, −30 and −67 kJ/mol, respectively. The most thermodynamics favorable substitution site is Al(2,3,4). For the (110)-3Co-234 structure, the Bader charge of Co(2,3,4) are +1.45, +1.41 and +1.41 e vs. 189, 181 and 191 pm for the average bond distances, respectively.
For Co3+ cations completely substitute the Al3+ cations top layer of γ-Al2O3 (110) surface, the substitution reaction of (110)-4Co-1234 structure is exothermic by −48 kJ/mol. It indicates that the Co3+ completely substituted all the top layer Al3+ cations are thermodynamically favorable. In the (110)-4Co-1234 structure, the Co(4)3c atom is in the tetrahedral position vs. the Co(1,2,3)4c atoms is in the octahedral position. In addition, after Co3+ substitution for the surface Al atoms, there is no large surface structure change, which is consistent with the Fe and Cr substituted for the α-Al2O3(0001) surface Al atoms [30].

2.1.2. Substitution of Al3+ by Ni3+

Figure 2 shows the structures of the Ni3+ cations substituted γ-Al2O3 (110) surface. The results present that the most favorable substitution structure of one Ni3+ cation substituted the γ-Al2O3 (110) surface is (110)-1Ni-4, with exothermic of −61 kJ/mol. The substitution energy of (110)-1Ni-4 is slightly smaller than the (110)-1Co-4 by 4 kJ/mol. The surface electronic structure is redistributed after the Ni3+ cations substitution. The Bader charge of Ni(4)3c is +1.22 e, which is slightly smaller to the doped Co of (110)-1Co-4 structure (+1.40 e). In addition, the structure of (110)-1Ni-4 is similar to the (110)-1Co-4. The substituted Ni atom bonds with two top layer O(5), O(6) atoms and one sublayer O atom, with the bond distances of 178, 178 and 195 pm, respectively. The Ni-O bond distances are slightly shorter than the Co-O, while it is longer than the Al-O bond distances. It is reported that the covalent radius of Ni is 124 pm [48]. The substitution energy for one Ni3+ cation substitutes Al(1)4c cation is 18 kJ/mol vs. −18 kJ/mol for the Al(3)4c site. In addition, the Ni3+ substitution of sublayer Al(5)3c atom is thermodynamically unfavorable.
It is interesting that the substitution properties of Ni3+ cations into γ-Al2O3 (110) surface are similar to the Co3+ cations. The Co and Ni atoms preferentially substitute for the tetrahedral Al sites rather than the octahedral Al sites, which consistent with previously reported experiments [18,49,50]. In additon, Co and Ni atoms preferentially substitute for the top layer Al sites rather than the sublayer Al sites.
For two Ni3+ cations substitution for Al3+ cations of γ-Al2O3 (110) surface, the substitution energies of (110)-2Ni-12, (110)-2Ni-23, (110)-2Ni-24 and (110)-2Ni-34 are 31, −1, −79 and −91 kJ/mol. It suggests that only the doping sites of Al(1,2) is thermodynamics unfavorable. For the most thermodynamics favorable substitution structure of (110)-2Ni-34, the Bader charge for Ni(3,4) is +1.25 and +1.15 e vs. 187 and 186 pm for the average bond distances, respectively. For three Ni3+ cations substitution for Al3+ cations of γ-Al2O3 (110) surface, the substitution energies of (110)-3Ni-123, (110)-3Ni-124 and (110)-3Ni-234 are −17, −79 and −96 kJ/mol. It can be found that the substitution reactions of three Ni3+ are thermodynamically favorable. For the most thermodynamics favorable substitution structure of (110)-3Ni-234, the Bader charge for Ni(2,3,4) is +1.22, +1.28 and +1.28 e vs. 185, 188 and 185 pm for the average bond distances, respectively.
For Ni3+ completely substituted all the Al3+ cations of γ-Al2O3 (110) surface, the substitution reaction of (110)-4Ni-1234 structure is exothermic by −95 kJ/mol. It means that the Ni3+ cations 100% substitution for the top layer Al3+ cations are thermodynamically favorable. In addition, the (110)-4Ni-1234 structure is similar to the 110-4Co-1234 structure. After all the top layer Al3+ cations have been substituted, there is no large structural distortion, and 25% Me (one Co3+/Ni3+) in tetrahedral position vs. 75% Me (three Co3+/Ni3+) in octahedral position.
Table 1 shows the average Me-O bond distances and average Co/Ni Bader charges of the Co and Ni doped γ-Al2O3 (110) surfaces. The substitution energies of Co substituted the (110) surface first increases and then decreases with the increase of Co atoms. It due to the Co-O bond distances are shortened and distorted by the (110) surface, especially the Co(2)-O(5) bond distance of 169 pm in the (110)-4Co-1234 structure. More details, it is worth noting that the Al(1) and Al(2), O(2) and O(3), O(5) and O(6) have the same chemical environment in the original γ-Al2O3 (110) surface. After the Co substitution, the Co(4)-O(5) and Co(4)-O(6), Co(1)-O(6) and Co(2)-O(5), Co(1)-O(3) and Co(2)-O(2) bonds distances are not equal in the (110)-4Co-1234 structure. For the Ni substituted γ-Al2O3 (110) surface, the Ni(1)-O and Ni(2)-O have the same bond distances in the (110)-4Ni-1234 structure. This results in a higher substitution energy of (110)-4Ni-1234 than (110)-4Co-1234.The average Co-O and Ni-O bond distances are in the range of 181~191 pm and 178~188 pm, respectively, which are slightly longer than the original Al-O bond distances (171~179 pm) of γ-Al2O3 (110) surface. It is due to the larger covalent radius of Co (126 pm) and Ni (124 pm) than Al (121 pm) [48]. In addition, the Bader charge of substituted Co and Ni are range in +1.39~+1.42 and +1.03~+1.22 e, which are similar to the Bader charges of Co(OH)3 (Co3+, +1.34 e) and Ni(OH)3 (Ni3+, +1.16 e), the Mulliken charge of the Fe doped γ-Al2O3 (111) surfaces (Fe3+, +1.47 e) [28].
Table 2 collects some Me substituted surfaces. It shows the substitution energy of Co substitution into γ-Al2O3 (110) surface is large than Co substitution into the CuAl2O4 (110) surface. While the substitution energy of the Ni doped γ-Al2O3 (110) surface is less than Ni doped into the CuAl2O4 (110) surface. Moreover, the substitution energies of Me doped γ-Al2O3 (110) surface are order of Cu > Co > Ni > Fe.

2.2. Electronic Structure Analysis

Using the DFT to study the electronic structure of doped surface could easily provide doping mechanism of active metals and support surfaces. The PDOS of perfect and Me doped γ-Al2O3 (110) surfaces top layer are shown in Figure 3. For the Co atoms substituted γ-Al2O3 (110) surface, it can be seen the occupied Co 3d orbital overlaps with O 2p orbital. It is worth noting that the band gap of the perfect γ-Al2O3 (110) surface is 3.3 eV [41], while the band gap for the γ-Al2O3 (110) surface after Co doping becomes narrower. For the four Co substituted, the band gap of (110)-4Co-1234 surface is 0.8 eV, which is 2.5 eV narrower than the perfect γ-Al2O3 (110) surface. For the Ni atoms substitute γ-Al2O3 (110) surface, the PDOS of Ni substituted (110) surface shows similar characters with Co substitute surface. The interaction mechanism for substituted Ni cations is the interaction of Ni 3d orbital with surface O 2p orbital. The band gap for (110)-4Ni-1234 is 0.4 eV. In addition, the band gaps change and become narrower as the increased of doping, which is consistent with the Fe and Cu substituted γ-Al2O3 surfaces [35,47], Fe and Cr substituted α-Al2O3(0001) surface [30].
Moreover, the experimental detection difficult to characterize electronic information of a given surface, the results of PDOS could provide theoretical data for comparison with experiments. The PDOS curves could facilitate characterize and compare the Lewis acidity of catalysts. In this study, the unoccupied 4s and 3d orbital of the substituted Co and Ni atoms are close to the Femi level, which could work as Lewis acid site. It can obviously change the catalytic performance. For example, the different sites of Fe and Cr doped α-Al2O3(0001) surface had different CO adsorption ability reported by Baltrusaitis et al. [30].

2.3. Thermodynamic Properties

2.3.1. Substitution Reaction under Given Conditions

The free energy change is used to investigate the thermodynamic properties of substitution reaction under the doped catalyst preparation conditions in forma (3). The corresponding function of substitution reaction free energy change (ΔG) of n and α(Al3+)/[α(Me3+)] under given temperature is expressed by Equation (1). Where the G[Al(4−n)(Me)nO6*] and G(Al4O6*) are the ΔG of the perfect and Me doped γ-Al2O3 (110) surface, respectively. The α(Al3+) and α(Me3+) are the activity of Al3+ and Me3+ in solution, respectively.
ΔG = G[Al(4−n)(Me)nO6*] + nG[Al(OH)3] − G(Al4O6*) − nG(Me(OH)3)] + RT × ln[α(Al3+)/α(Me3+)]n,
Figure 4 illustrates the function of substitution reaction free energy change (ΔG) of ln[α(Al3+)/α(Me3+)] under 300 K. It can be found that the substitution reaction of Co and Ni substitution into the γ-Al2O3 (110) surface is thermodynamically favored at 300 K. For the Co3+ cations substitution into the (110) surface, Co could form 100% (n = 4) substitution for γ-Al2O3 (110) surfaces top layer Al atoms when the value of ln[α(Al3+)/α(Co3+)] is lower than 114. When the value of ln[α(Al3+)/α(Co3+)] is higher than 114, the substitution reaction of Al3+ substitution by Co3+ cannot occur. For the Ni3+ cations substitution into (110) surface, all the surface Al3+ cations (n = 4) could be doped with the value of ln[α(Al3+)/α(Ni3+)] lower than 163 vs. ln[α(Al3+)/α(Ni3+)] higher than 163 the substitution reaction cannot occur.
It is worth noting that Ni3+ cations are more easily substituted into the (110) surface Al3+ cations than the Co3+ cations under the temperature of 300 K.

2.3.2. Surface O-Defects of the Co and Ni Substituted Surfaces

Surface oxygen vacancy is a typical state of oxide catalysts, which can affect the stability and performance of catalysts. In this part, oxygen vacancy formation energy was employed to explore the stability of the Co and Ni substituted γ-Al2O3 (110) surface. In the Equation (2), the E(O2), E[(Me)4O5*] and E[(Me)4O6*] are the total energies of the isolated gas-phase O2 molecule, O-defective surface, and four Ni and Co substituted γ-Al2O3 (110) surfaces, respectively.
Ef = E[(Me)4O5*] + 1/2E(O2) − E[(Me)4O6*],
Each oxygen atom was calculated to detect the most stable O-defective surface structure. The most stable one O-defective Ni and Co substituted γ-Al2O3 (110) surfaces have been shown in Figure 5. The computational results show that the O(5)3c is the preferred thermodynamically favorable that could be removed from the surface to form (110)-4Co-1234-dO5 structure, with the oxygen vacancy formation energy of 268 kJ/mol. After the removal of O(5)3c atom, the O(6)3c becomes tri-coordinated. The O(6)3c bonds with Co(1,2,4) with the bond distances are 191, 201 and 183 pm, respectively. For the (110)-4Co-1234 surface, the thermodynamically favorable O-defective site is O(5)3c, which has the same structure with (110)-4Co-1234-dO5. The oxygen vacancy formation energy of (110)-4Ni-1234-dO5 is 53 kJ/mol, which is lower than the (110)-4Co-1234-dO5 by 215 kJ/mol. Compared with other surfaces, the oxygen vacancy formation energy of the Co and Ni doped γ-Al2O3 (110) surfaces are lower than the Cu doped γ-Al2O3 (110) surface (367 kJ/mol). In addition, it reports that the oxygen vacancy formation energy of Co3O4 (110) [51], Co3O4 (110) [52], NiO (100) [53] are 59, 4 and 424 kJ/mol, respectively.

3. Computation Details

3.1. Computational Methods

The PBE functional was used for the present work, since we want to compare the results of the present work with our previous works on the NiCu-alumina [35,54], as well as other catalytic works on the alumina [55,56], Fe-alumina [28,47], Co-alumina [57], Ni-alumina [58], Cu-alumina [59,60], Ag-alumina [34], Pt-alumina [61,62] and Pd-alumina [63]. It had been proved that the PBE and PW91 functional work well for such systems for discussions on the reaction, adsorption and substitution energies.
All calculations were performed using the DFT with D3 dispersion correction [64] method with the GGA-PBE [65] functional in the Vienna ab initio Simulation Package [66,67]. Using the projector augmented wave (PAW) method report by Blöchl et al. [68,69] to express the interactions of electron-ion. All calculations are spin-polarized. Using DFT+U methods to correct the strong on-site Coulomb interaction of the Co and Ni 3d electrons with the U of 4.0 and 3.8. The Brillouin zone via the Monkhorst–Pack algorithm generated for the unit cell and surface slabs are 5 × 5 × 5 and 3 × 3 × 1 k-points meshes, respectively. All calculations were performed using the kinetic energy cutoff of 400 eV. The SCF energy, the total energies and the atomic forces are 1.0 × 10−5 eV, 1.0 × 10−4 eV and 0.03 eV/Å as the convergence criteria for structure optimizations. The 15 Å × 15 Å × 15 Å cell was used to calculate the gas phase molecules.
All reported most stable structures have only one true frequency. The structure vibration frequency is used to calculate the Gibbs free energy, including the oscillator, standard harmonic, ideal gas approximation and rigid rotor. The Gibbs free energy of solid phase surface slabs only consider the vibration contributions. The Gibbs free energy of gas phase substances considers vibration, rotation and translation contributions. In addition, the Bader charge [70] was used to determine the atoms charges.
The substitution reaction of Co and Ni substitution into the γ-Al2O3 (110) surface can be considered as Equation (3). Using Co(OH)3 and Ni(OH)3 instead of Co3+ and Ni3+ cations could more realistically represent Me substitution reaction in solution.
nMe(OH)3 + Al4O6*→Al(4−n)(Me)nO6* + nAl(OH)3,
The substitution energy of Me atoms substitute the (110) surface in the Equation (3) is defined as:
Esub = E[Al(4−n)(Me)nO6*] + nE[Al(OH)3] − nE[Me(OH)3] − E(Al4O6*),
where the E[Al(OH)3] and E[Me(OH)3] are the total energies of gas phase Al(OH)3 and Me(OH)3. The E[Al(4−n)(Me)nO6*] and E[Me(OH)3] are the energies of perfect and Me doped γ-Al2O3 (110) surfaces. The larger negative of the substitution energy, the more thermodynamics favorable for the substitution reaction in the Equation (3).

3.2. γ-Al2O3 Surface Model

The γ-Al2O3 (110) surface structure [41,42] was chosen as the calculation model. As shown in Figure 6, all calculations used an eight-layer slab structure. The top and bottom four layers were relaxed and fixed during structural optimization, respectively. In the γ-Al2O3 (110) surface, the top layer contains Al(1)4c, Al(2)4c, Al(3)4c and Al(4)3c, where the coordination numbers are tetra-coordination and tri-coordination. For surface O atoms, the O(1,2,3)3c and O(4,5,6)2c are in tri- and di-coordinated, respectively. Especially, part surface atoms have the same chemical environment, and for Al atoms are Al(1)4c and Al(2)4c, while for O atoms are O(5)2c and O(6)2c. In addition, the Al(5)3c is the sublayer Al atom.
All the surface Al3+ sites substituted by Co3+ and Ni3+ have been considered, as well as different magnetic moments of Co and Ni atoms. The most thermodynamically stable structure was shown and discussed. The Co and Ni substituted surface structures are identified as ‘(110)-nMe-p, q’. The name indicates that n Me (Co and Ni) doped for the Al(p) atoms of (110) surface, with the substitution energy of (110)-nMe-p structure is q kJ/mol. Such as the ‘(110)-2Co-12, 80′ means that two Co3+ cations substitute the Al(1) and Al(2) atoms. The substitution energy of (110)-2Co-12 structure is 80 kJ/mol. At the same time, the substituted Co3+ cations are named Co(1) and Co(2).

4. Conclusions

In this work, DFT+U method was performed to investigate the surface properties of the Co and Ni substituted γ-Al2O3 (110) surface.
In conclusion, the substitution of Co3+ and Ni3+ cations for Al3+ cations of γ-Al2O3 (110) surface are thermodynamically favorable. The substitution energies of one Al3+ substitution by Co3+ and Ni3+ are −61 and −57 kJ/mol, respectively. The order of substitution site is: Al3c > Al4c, as well as preferentially substituting the tetrahedral Al than the octahedral Al atom. Moreover, the substitution of sublayer Al3+ cation is thermodynamically unfavorable. After all the top layer Al3+ cations have been substituted, there is no large structural distortion, and 25% Me (one Co3+/Ni3+) in tetrahedral position vs. 75% Me (three Co3+/Ni3+) in octahedral position.
The interaction mechanism for the substituted Me3+ cations and γ-Al2O3 (110) surface is Me 3d orbital overlaps with surface O 2p orbital. The surface electronic structure is redistributed after the Co and Ni substitution. The band gap of doped surface is narrower than the perfect γ-Al2O3 surface, and become narrower while more Me3+ cations is substituted into the surface. The band gap of (110)-4Co-1234 and (110)-4Ni-1234 are 0.8 and 0.4 eV, respectively. The unoccupied 4s and 3d orbitals of the substituted Co3+ and Ni3+ cations are close to the Femi level, which could be used as Lewis acid sites.
In addition, the function of substitution reaction free energy change of ln[α(Al3+)/α(Me3+)] has been investigated using thermodynamics. It is found that both Co and Ni could form 100% substitution of top layer Al atoms in γ-Al2O3 (110) surfaces. Furthermore, the oxygen vacancy formation energies of (110)-4Co-12345 and (110)-4Ni-12345 are 268 and 53 kJ/mol, respectively.
Our work predicts the surface properties of transition metals Co and Ni substitution into the γ-Al2O3 (110) surface, providing the molecular level surface structure, doping mechanism and electronic structure characteristic. The calculation results would benefit to design of new high-performance metal doping catalysts.

Author Contributions

Investigation, writing—original draft, visualization, H.L.; writing—review and editing, validation, methodology, L.S.; data curation, formal analysis, C.J.; project administration, software, methodology, R.Y.; conceptualization, methodology, resources, supervision, funding acquisition, R.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work has been supported by the National Natural Science Foundation of China (Grants No. 21868016).

Data Availability Statement

Not applicable.

Acknowledgments

We would like to thank the teachers of the College of Chemistry, Nanchang University for their support in the equipment used for theoretical calculations.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Catalysts 12 00111 g001 550
Figure 1. Co3+ cations substituted for 1~4 Al3+ cations of γ-Al2O3 (110) surface. The red, gray and violet balls represent the surface O, Al and Co atoms, respectively. The coordination numbers and Bader charges of Co atoms are next to the violet balls. The substitution energies (kJ/mol), Bader charges (e) and bond distances (pm) have been shown in the figure.
Figure 1. Co3+ cations substituted for 1~4 Al3+ cations of γ-Al2O3 (110) surface. The red, gray and violet balls represent the surface O, Al and Co atoms, respectively. The coordination numbers and Bader charges of Co atoms are next to the violet balls. The substitution energies (kJ/mol), Bader charges (e) and bond distances (pm) have been shown in the figure.
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Catalysts 12 00111 g002 550
Figure 2. Ni3+ substituted for 1~4 Al3+ of γ-Al2O3 (110) surface. The red, gray and olive-green balls represent the surface O, Al and Ni atoms, respectively. The coordination numbers and Bader charges of Ni atoms are next to the olive-green balls. The substitution energies (kJ/mol), Bader charges (e) and bond distances (pm) have been shown in the figure.
Figure 2. Ni3+ substituted for 1~4 Al3+ of γ-Al2O3 (110) surface. The red, gray and olive-green balls represent the surface O, Al and Ni atoms, respectively. The coordination numbers and Bader charges of Ni atoms are next to the olive-green balls. The substitution energies (kJ/mol), Bader charges (e) and bond distances (pm) have been shown in the figure.
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Catalysts 12 00111 g003 550
Figure 3. PDOS curves for gas phase Co and Ni atom, perfect and metal substituted γ-Al2O3 (110) surfaces.
Figure 3. PDOS curves for gas phase Co and Ni atom, perfect and metal substituted γ-Al2O3 (110) surfaces.
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Catalysts 12 00111 g004 550
Figure 4. The function of substitution reaction free energy change (ΔG) of ln[α(Al3+)/α(Me3+)] under 300 K in the Equation (3).
Figure 4. The function of substitution reaction free energy change (ΔG) of ln[α(Al3+)/α(Me3+)] under 300 K in the Equation (3).
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Catalysts 12 00111 g005 550
Figure 5. The structures for (110)-4Co-12345 and (110)-4Ni-12345 with one O defect. The dotted circle represents the removed O atom.
Figure 5. The structures for (110)-4Co-12345 and (110)-4Ni-12345 with one O defect. The dotted circle represents the removed O atom.
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Catalysts 12 00111 g006 550
Figure 6. The side and top views of γ-Al2O3 (110) surface. The red and gray balls represent the surface O and Al atoms, respectively. The surface atoms are indexed by numbers. The surface O and Al atoms coordination numbers are shown by subscript numbers.
Figure 6. The side and top views of γ-Al2O3 (110) surface. The red and gray balls represent the surface O and Al atoms, respectively. The surface atoms are indexed by numbers. The surface O and Al atoms coordination numbers are shown by subscript numbers.
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Table
Table 1. The Esub, average Me−O bond distances and average Bader charges of substituted Me atoms for the most stable substituted structures.
Table 1. The Esub, average Me−O bond distances and average Bader charges of substituted Me atoms for the most stable substituted structures.
SubstitutedEsub
(kJ/mol)		Charge
(e)		Co-O
(pm)		SubstitutedEsub
(kJ/mol)		Charge
(e)		Co-O
(pm)
(110)-1Co-4−61+1.40187(110)-1Ni-4−57+1.22184
(110)-2Co-34−83+1.42183,191(110)-2Ni-34−91+1.10187,186
(110)-3Co-234−67+1.42181,191,189(110)-3Ni-234−96+1.03183,188,185
(110)-4Co-1234−48+1.39184,191,184,189(110)-4Ni-1234−95+1.08178,187,185,185
Table
Table 2. The Esub and calculation method of different metals substituted surfaces.
Table 2. The Esub and calculation method of different metals substituted surfaces.
SubstitutedEsub
(kJ/mol)		MethodRef.
Co and Ni doped γ-Al2O3 (110)−61/−57GGA-PBEThis work
Fe doped γ-Al2O3 (111)−22MN12-L[28]
Cu doped γ-Al2O3 (110)−261GGA-PBE[35]
Fe doped γ-Al2O3 (110)−53/−52GGA-PW91/PBE[47]
Co and Ni doped Al of CuAl2O4 (110)−30/−61GGA-PBE[25]
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Li, H.; Shi, L.; Jin, C.; Ye, R.; Zhang, R. Co and Ni Incorporated γ-Al2O3 (110) Surface: A Density Functional Theory Study. Catalysts 2022, 12, 111. https://doi.org/10.3390/catal12020111

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Li H, Shi L, Jin C, Ye R, Zhang R. Co and Ni Incorporated γ-Al2O3 (110) Surface: A Density Functional Theory Study. Catalysts. 2022; 12(2):111. https://doi.org/10.3390/catal12020111

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Li, Huaxi, Liu Shi, Chengkai Jin, Runping Ye, and Rongbin Zhang. 2022. "Co and Ni Incorporated γ-Al2O3 (110) Surface: A Density Functional Theory Study" Catalysts 12, no. 2: 111. https://doi.org/10.3390/catal12020111

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