Co and Ni Incorporated γ -Al 2 O 3 (110) Surface: A Density Functional Theory Study

: Investigation into the state and mechanisms of the active metal substitution into the γ -Al 2 O 3 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 Co 3+ and Ni 3+ cations substitute for the Al 3+ cations of γ -Al 2 O 3 (110) surface. It was found that the substitution energy of one Al 3+ substituted by Co 3+ and Ni 3+ 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 -Al 2 O 3 (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. 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 -Al 2 O 3 (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.


Introduction
γ-Al 2 O 3 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-Al 2 O 3 surface to form high-performance catalyst is widely used in the industrial field. Co 3+ and Ni 3+ cations are of interest to researchers because they are isovalent with Al 3+ and can form stable oxides. More importantly, the Co doped γ-Al 2 O 3 exhibits excellent catalytic activity in Fischer-Tropsch synthesis [2][3][4][5] vs. Ni doped γ-Al 2 O 3 applied in hydrogenation [6,7], dehydrogenation reaction [8] and alcohol steam reforming reaction [9,10].
The properties of Co and Ni substituted γ-Al 2 O 3 surfaces have been studied in previous experiments [11][12][13]. Some characterization techniques were applied to explore the Co and Ni substituted γ-Al 2 O 3 surfaces properties, such as X-ray diffraction (XRD) [14,15], CO 2 temperature programmed desorption (CO 2 -TPD) [14], and H 2 -temperature programmed reduction (H 2 -TPR) [16,17]. Jaroniec et al. [14] synthesized 10% of Co and Ni added into γ-Al 2 O 3 , they found the peaks of Co 3 O 4 , Ni 2 O 3 and NiO in the XRD patterns. The Co 3 O 4 , Ni 2 O 3 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-Al 2 O 3 and Co/γ-Al 2 O 3 changed with the temperature and doped metal content [19,20]. The CO 2 -TPD results [14] detects that the doped Ni and Co enhanced acid-base properties of catalyst surface. Figure 1 shows the structures of the Co 3+ cations substituted γ-Al 2 O 3 (110) surface. The results show that the most favorable substitution structure of one Co substituted γ-Al 2 O 3 (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-Al 2 O 3 (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-Al 2 O 3 (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-Al 2 O 3 (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.  Figure 1 shows the structures of the Co 3+ 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 Co 3+ cations substitution for Al 3+ 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  For two Co 3+ cations substitution for Al 3+ cations of γ-Al 2 O 3 (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)  For Co 3+ cations completely substitute the Al 3+ cations top layer of γ-Al 2 O 3 (110) surface, the substitution reaction of (110)-4Co-1234 structure is exothermic by −48 kJ/mol. It indicates that the Co 3+ completely substituted all the top layer Al 3+ 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 Co 3+ 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-Al 2 O 3 (0001) surface Al atoms [30]. Figure 2 shows the structures of the Ni 3+ cations substituted γ-Al 2 O 3 (110) surface. The results present that the most favorable substitution structure of one Ni 3+ cation substituted the γ-Al 2 O 3 (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 Ni 3+ 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 Ni 3+ cation substitutes Al(1) 4c cation is 18 kJ/mol vs. −18 kJ/mol for the Al(3) 4c site. In addition, the Ni 3+ substitution of sublayer Al(5) 3c atom is thermodynamically unfavorable.

Substitution of Al 3+ by Ni 3+
It is interesting that the substitution properties of Ni 3+ cations into γ-Al 2 O 3 (110) surface are similar to the Co 3+ 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 Ni 3+ cations substitution for Al 3+   It is interesting that the substitution properties of Ni 3+ cations into γ-Al2O3 (110) surface are similar to the Co 3+ 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.

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-Al 2 O 3 (110) surfaces top layer are shown in Figure 3. For the Co atoms substituted γ-Al 2 O 3 (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-Al 2 O 3 (110) surface is 3.3 eV [41], while the band gap for the γ-Al 2 O 3 (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-Al 2 O 3 (110) surface. For the Ni atoms substitute γ-Al 2 O 3 (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-Al 2 O 3 surfaces [35,47], Fe and Cr substituted α-Al 2 O 3 (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-Al 2 O 3 (0001) surface had different CO adsorption ability reported by Baltrusaitis et al. [30].
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].
It is worth noting that Ni 3+ cations are more easily substituted into the (110) surface Al 3+ cations than the Co 3+ cations under the temperature of 300 K. than 114. When the value of ln[α(Al 3+ )/α(Co 3+ )] is higher than 114, the substitution reaction of Al 3+ substitution by Co 3+ cannot occur. For the Ni 3+ cations substitution into (110) surface, all the surface Al 3+ cations (n = 4) could be doped with the value of ln[α(Al 3+ )/α(Ni 3+ )] lower than 163 vs. ln[α(Al 3+ )/α(Ni 3+ )] higher than 163 the substitution reaction cannot occur. It is worth noting that Ni 3+ cations are more easily substituted into the (110) surface Al 3+ cations than the Co 3+ cations under the temperature of 300 K.

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) 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.

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-Al 2 O 3 (110) surface. In the Equation (2) 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-Al 2 O 3 (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-Al 2 O 3 (110) surfaces are lower than the Cu doped γ-Al 2 O 3 (110) surface (367 kJ/mol). In addition, it reports that the oxygen vacancy formation energy of Co 3 O 4 (110) [51], Co 3 O 4 (110) [52], NiO (100) [53] are 59, 4 and 424 kJ/mol, respectively.

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], Nialumina [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 Co 3+ and Ni 3+ cations could more realistically represent Me substitution reaction in solution.
The substitution energy of Me atoms substitute the (110) surface in the Equation (3) is defined as:

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], Nialumina [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-Al 2 O 3 (110) surface can be considered as Equation (3). Using Co(OH) 3 and Ni(OH) 3 instead of Co 3+ and Ni 3+ cations could more realistically represent Me substitution reaction in solution.

γ-Al 2 O 3 Surface Model
The γ-Al 2 O 3 (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-Al 2 O 3 (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 Al 3+ sites substituted by Co 3+ and Ni 3+ 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 Co 3+ 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 Co 3+ cations are named Co(1) and Co(2).

Conclusions
In this work, DFT+U method was performed to investigate the surface properties of the Co and Ni substituted γ-Al 2 O 3 (110) surface.
In conclusion, the substitution of Co 3+ and Ni 3+ cations for Al 3+ cations of γ-Al 2 O 3 (110) surface are thermodynamically favorable. The substitution energies of one Al 3+ substitution by Co 3+ and Ni 3+ are −61 and −57 kJ/mol, respectively. The order of substitution site is: Al 3c > Al 4c , as well as preferentially substituting the tetrahedral Al than the octahedral Al atom. Moreover, the substitution of sublayer Al 3+ cation is thermodynamically unfavorable. After all the top layer Al 3+ cations have been substituted, there is no large structural distortion, and 25% Me (one Co 3+ /Ni 3+ ) in tetrahedral position vs. 75% Me (three Co 3+ /Ni 3+ ) in octahedral position.
The interaction mechanism for the substituted Me 3+ cations and γ-Al 2 O 3 (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-Al 2 O 3 surface, and become narrower while more Me 3+ 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 Co 3+ and Ni 3+ cations are close to the Femi level, which could be used as Lewis acid sites.
Our work predicts the surface properties of transition metals Co and Ni substitution into the γ-Al 2 O 3 (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.