Structure and Energetics of (111) Surface of γ-Al₂O₃: Insights from DFT Including Periodic Boundary Approach

The (111) surface of γ-alumina has been reexamined, and a new (111) surface model has been suggested. The local structure of this new surface of γ-alumina, (111)_n, has been optimized by the density functionals along with the full electron basis sets by using periodic boundary condition. This newly modeled (111)_n surface is characterized by the same stability as that of the (110) surface, and its surface energy amounts to 2.561 J/m², only about 0.002 J/m² larger than that of (110). Three different types of the tricoordinated Al centers have been identified on (111)_n. Molecular orbital (MO) analysis and the population analysis demonstrate that one type of Al, Al(I), is a nonpaired electron center. The singly occupied MO on Al(I) center is expected to play an important role in the catalytic activities of the γ-alumina. Moreover, the neighboring Al (Al(III)) on the (111)_n surface provides suitable acceptance position for the electron donating groups. The defected surfaces of (111)_n are found to be having a similar stability. The detachment of Al(I) from the (111)_n surface results in disappearance of the nonpaired electron centers. Meanwhile, the attachment of Al(I) on (111)_n surface will produce rich nonpaired electron centers on this new surface. Therefore, this newly defined surface is expected to attract the research interests in the catalytic activities of γ-alumina.