Density Functional Theory Investigation into the B and Ga Doped Clean and Water Covered γ-Alumina Surfaces
The structures and energies of the B and Ga incorporated -alumina surface as well as the adsorption of water are investigated using dispersion corrected density functional theory. The results show that the substitution of surface Al atom by B atom is not so favored as Ga atom. The substitution reaction prefers to occur at the tricoordinated A(4) sites. However, the substitution reaction becomes less thermodynamically favored when more Al atoms are substituted by B and Ga atoms on the surface. Moreover, the substitution of bulk Al atoms is not so favored as the Al atoms by B and Ga on the surface. The -alumina surface is found to have stronger adsorption ability for water than the B and Ga incorporated surface. The total adsorption energy increases as water coverage increases, while the stepwise adsorption energy decreases. The studies show the coverage of water at 7.5 H2O/nm2 (five H2O molecules per unit cell) can fully cover the active sites and the further water molecule could only be physically adsorbed on the surface.
The γ-alumina is an important material in chemistry and materials science due to its widespread applications in chemical industry [1–3], ceramics, and semiconductors [4–7]. In order to improve the performance of the material, some heteroatoms were usually chosen to be incorporated into the γ-alumina bulk and surfaces [8, 9]. For example, the Fe atom was usually used to improve the catalytic performance of γ-alumina . Wan et al. investigated the Fe2O3/Al2O3 catalyst from coprecipitated and spray-dried method with Mösbauer spectroscopy  and found the reduction of Fe2O3 to FeO. The substitution of surface Al3+ by Fe3+ in alumina with mixed (Al1−xFex)2O3 surface formation is also confirmed by transmission Mösbauer spectroscopy. Integral low-energy electron Mösbauer spectroscopy and Fe K-edge X-ray absorption near-edge structure characterization observed the formation of iron nanoclusters from the transformation of γ-(Al1−xFex)2O3 to α-(Al1−xFex)2O3 and the iron distribution on the surface layers and in the cores of grains [12–14]. This field also attracts the interests of theoretical researches. Feng et al. calculated the structures and energies of the Fe promoted γ-alumina surface  and found that the incorporation of Fe atom into the γ-alumina surface is possible, while it is thermodynamically not so favored for the Fe substitution for the bulk Al atoms. The adsorption of water on the γ-alumina surface is stronger than that on the Fe2O3 covered surface. In addition, the electronic structures also change after the substitution of the Al atoms by the Fe atoms on the surface. Except for the incorporation of Fe into the alumina surface, the B and Ga atoms, which are often used as the trivalent substitution ions for the zeolites [15, 16], are also used in experiments for the preparation of high performance catalysis and semiconductors [8, 9, 17–19]. Kibar et al. prepared nanostructured boron doped alumina catalyst support  and found that the morphology of the supports can be modified from cracked surface to nanosphere formation by the introduction of boron. Jansons et al. introduced Ga into the alumina crystal and prepared a new complex luminescence band at about 5 eV . The Ga-related luminescence can be observed under the excitation of X-rays up to 600 K. In order to know the energies as well as the structures of the B and Ga incorporated γ-alumina surface at molecular level, the present work investigated the thermodynamic and structure properties of the B and Ga incorporated γ-alumina surface using dispersion corrected periodic density functional theory. Since γ-alumina is usually prepared and used in atmospheres containing water, the adsorption of water on the B and Ga incorporated γ-alumina is also investigated and compared with the water adsorption on pure γ-alumina surface.
2. Computational Details
The dispersion-corrected periodic density functional as implemented in the Vienna ab initio Simulation Package (VASP) was used for the calculations [21–23]. The DFT-D3 method of Grimme was used to take into account the dispersive interactions as previous work reported that the dispersion correction seriously influences the relative stability order and adsorption energies [24–28]. The exchange and correlation energies were calculated by the generalized gradient approximation (GGA) formulation with the PBE functional . The Kohn-Sham one-electron states were extended in accordance with plane-wave basis sets with a kinetic energy of 400 eV. The projector augmented wave (PAW) method was applied to describe the electron-ion interactions [29–31].
The Brillouin zone was sampled with and -points meshes generated by the Monkhorst-Pack algorithm, for the γ-alumina cell and γ-alumina (110) surface slab, respectively. The convergence criteria were set to be 10−4 eV for the SCF energy, 10−3 eV and 0.03 eV/Å for the total energy and the atomic forces, respectively.
The γ-alumina surface was described using the Digne’s model , and (110) surface was taken into account. As shown in Figure 1, γ-alumina surface was modeled using a supercell with an eight-layer slab, which contains sixteen Al2O3 units. A vacuum with thickness of 15 Å was employed to separate each slab from interactions. The top four layers and the adsorbates were fully relaxed, and the bottom four layers were fixed in their bulk position during the structure optimization. In order to facilitate the discussions, the surface layer Al and O atoms are indexed with number. The coordination number of each Al atom was expressed by subscript. As has been described in many previous works [33, 34], the Al atoms in bulk γ-alumina are in tetrahedral and octahedron sites. After cleavage, the tetrahedral and octahedron Al atoms expose as the tricoordinated and tetracoordinated Al, respectively, in the (110) surface. As shown in Figure 1, the was in tetrahedral site in the bulk, and , , and were in octahedral sites in the bulk. It could be observed from the top view that Al and Al atoms are in the same chemical environment.
For the substitution of surface Al3+ by ( = B and Ga) in reaction (1) the substitution energy is defined as , where and present the energies of the gas phase X(OH)3 and the isolated oxide surfaces ( presents the 2–8 layers alumina substrate), respectively. The positive substitution energy means substitution reaction is not thermodynamically favored.
The adsorption energy of H2O () on the oxide surface was defined as , where , , and are the total energies of the minima structures of () surface with adsorbed water, gas phase water molecule, and clean surface, respectively. Following this definition, a more negative value indicates stronger interaction between adsorbed species and the surface.
3. Results and Discussion
3.1. Substitution of Al by B and Ga on the Surface
Table 1 shows the substitution energies for the substitution of Al by B and Ga on the surface. As indicated by the calculated substitution energies, the substitution of surface Al by B atoms is not thermodynamically favored, since the calculated substitution energies are positive. The tricoordinated Al(4) is the most favored site for one B incorporation with the substitution energy of 1.31 eV. The tetracoordinated Al and Al(3) sites are slightly difficult with the substitution energies of 1.81 and 2.29 eV, respectively.
For two Al atoms substituted by B atoms on the surface, which corresponds to 50% surface, Al were replaced by B; the Al(1,2) are the most favored, with the substitution energy of 3.22 eV, versus Al(1,2,3) for 75% surface Al substitution by B, with the substitution energy of 5.32 eV. The substitution energies positively increase to 7.82 eV as all surface Al atoms were substituted by B atoms.
Figure 2 shows the structures for the B and Ga substituted γ-alumina surface. The corresponding bond distances of the surface layer atoms before and after the substitution are given in Table 1. It is found that the surface Al–O bond distances are in the range of 170–186 pm on the γ-alumina surface. After the substitution of Al by B atoms, the Al–O bond distance almost remains the same. It should be noted that the B–O bond distances (137–167 pm) are much shorter than those of the Al–O bonds, since the radius of B atom is much shorter than that of Al . In addition, the B atom prefers to be tricoordinated, for example, for B(1,2,3,4) in Figure 2, bond e is elongated to 255 pm and became broken.
Since the substitution energies for the substitution of Al by Ga are much less than those for B substitution, the Ga could be more easily incorporated into to the alumina surface than B. Particularly for the Ga(4) structure, the substitution energy is −0.06 eV, which indicates the substitution reaction at Al(4) site by Ga is thermodynamically favored. Similar to that of B substitution, the substitution energy increases as more Al atoms were replaced by Ga atoms, for example, the substitution energies are 0.11, 0.49, and 0.90 eV, respectively, for Ga(3,4), Ga(1,2,4), and Ga(1,2,3,4). Since the Ga atom radius is larger than that of Al , the Ga–O bond distance is much longer than that of Al–O bond distance. In order to map out whether it is possible for B and Ga substitution for the bulk Al atoms of γ-alumina, we also calculated the substitution energies for the substitution of the sublayer hexa- and tetracoordinated Al atom by B and Ga atoms. The calculated substitution energies for B replacing the sublayer hexa- and tetracoordinated Al atoms are 4.34 and 2.89 eV, respectively, versus 0.81 and 0.57 eV for Ga, which are larger than the substitution energies for Al substitution on the surface. It indicates that the substitution reaction should favor happening on the surface, rather than in the bulk. In addition, the substitution of tetrahedral Al sites is always more thermodynamically favored than the substitution of octahedral Al sites.
As reported in the previous work , the substitution of γ-alumina surface Al by Fe atoms is thermodynamically favored, as the substitution energy for the substitution of all surfaces Al by Fe atom is −0.87 eV. It indicates that the Fe should be more easily to be incorporated into the γ-alumina than B and Ga.
3.2. Adsorption of Water Molecules on the X2O3Surfaces (X = B, Al, Ga)
Figure 3 shows the structures and adsorption energies for water adsorption on the , , and surfaces. It should be noted that our calculated adsorption energies for the water adsorption on the surfaces are −2.62, −4.87, −6.84, −8.42, −9.63, and −10.50 eV, respectively, for 1–6 water molecules adsorbed on the γ-alumina surface. The stepwise adsorption energies for each water molecule are −2.62, −2.25, −1.97, −1.58, −1.21, and −0.86 eV. The previous works reported the adsorption of one water molecule on the γ-alumina (110) surface releases the energy of −2.49 eV [1, 32], which is slightly smaller than the results of present work (−2.62 eV). The reason is that the PBE-D3 methods were used in the present work and dispersion correction effects were taken into consideration. The optimized structures for the adsorption are the same as the previous works. The and show similar structures with one −OH group on the tricoordinated surface Al/Ga atom, and H atom bonds to the twofold coordinated surface O atoms. It is interesting to see that the surface B–O bond was broken after water adsorption, and the ‒OH group from the water bonds to the surface BO2 in coplanar. The H atom from the water adsorbs onto the surface of O atom forming an in-surface hydroxyl. It leads to a larger adsorption energy for the first water molecule adsorbed on the surface of than that of and .
For two water molecules adsorption on the , , and surfaces, the adsorption energies are −4.83, −4.87, and −4.03 eV, respectively. shows the largest adsorption energy. The second water molecule makes the , , and surfaces seriously distorted. As shown in Figure 3, the X(3) atom moves to surface X(4) atom for X = Al and Ga, and they share one −OH group from the water and both became tetracoordinated.
As the water coverage increases from one to six water molecules in one slab, the adsorption energy increases from −3.33, −4.83, −6.31, −6.43, and −8.07 to −8.80 eV, respectively. In comparison, the adsorption energy increases from −1.99, −4.03, −5.92, −6.79, and −8.40 to −9.15 eV for surface. Both are smaller than those for the surface. It indicates that the pure γ-alumina surface shows stronger water adsorption than and . It should be noted that the subsequent adsorption monotonously decreases for the water adsorption on surface. Since the water adsorption leads to the surface reconstruction, there are the turning points of the stepwise adsorption energy for 4-5 water molecules adsorption for the and slabs. In addition, the stepwise adsorption energy for the sixth water molecule adsorbent on the , , and surfaces is similar (−0.72, −0.86, and −0.75 eV, resp.). The main reason is that the former five water molecules have totally covered the active sites for water adsorption, and the sixth water molecule could only be physically adsorbed.
The dispersion corrected periodic density functional theory was used to investigate the structure and energies for the B and Ga incorporated γ-alumina surface. The results show that the substitution of Al by B is not thermodynamically favored on the surface. However, the substitution of Al by Ga is thermodynamically favored at low coverage on the surface. The substitution reaction prefers to occur at the tricoordinated A(4) sites. The substitution reaction becomes thermodynamically not favored as more and more B and Ga substitutions take place on the surface. The substitutions of Al by B and Ga are not so favored in the bulk as that for the surface.
The adsorption of water molecules on the B and Ga incorporated γ-alumina surface was also investigated and compared to that of the pure γ-alumina surface. It shows that the γ-alumina surface has the strongest adsorption ability for water adsorption. The total adsorption energy increases as water coverage increases from one to six water molecules in each slab, while the stepwise adsorption energy decreases. On the , , and surfaces, the sixth could only be physically adsorbed on the surface, since the former adsorbed five water molecules (at the coverage of 7.5 H2O/nm2) fully covered the active sites for water adsorption.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was supported by the Doctoral Scientific Research Foundation for Dr. Lihong Cheng of Jiangxi Science & Technology Normal University (2017.1-2020.12), Science Foundation of Jiangxi Department of Education (GJJ150827), and National Natural Science Foundation of Jiangxi Province (20151BAB204009).
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