Co and Ni single sites on the (111) n surface of γ-Al 2 O 3 – a periodic boundary DFT study

Abstract

Coordination of Co and Ni single-atom on the most active tri-coordinated Al/Metal single-site (111) n surface of γ-Al 2 O 3 leads to stable electron configuration of d 7 for Co and d 8 for Ni on the surface of γ-Al 2 O 3 . The influences of increasing the number of d-electrons in the single metal (Fe-like) substituted (111) n surface of γ-Al 2 O 3 on its possible catalytic effects were explored. The energetic properties, local structures, and in-site electron configurations of the most active tri-coordinated Co and Ni single-site (111) n surface of γ-Al 2 O 3 have been studied using the density functional theory (DFT) approach under periodic boundary conditions. The replacement of Al by a Co or Ni atom on the I position of the (111) n surface leads to significant elongations of metal–O distances. The energy released from the substitution process on the Al I site of the (111) n surface follows the sequence Ni I (164.85 kcal mol −1 ) > Co I (113.17 kcal mol −1 ) > Fe I (44.30 kcal mol −1 ). The triplet and quintet (ground state) of the Co I substituted complex are energy degenerate. Also, the doublet and quartet (ground state) of the Ni I substituted complex have the same stable energy. This energy degeneracy comes from the α–β electron flipping on the p-orbital of the neighboring O that is next to the substituted Co I or Ni I site on the (111) n surface of γ-Al 2 O 3 . Different from the Fe I substituted single-site (111) n surface, in which the electron configuration of Fe I varies according to its spin-multiplicity state, substituted Ni I has a unique d 8 electron configuration in all three spin states, and similarly, Co I has a unique d 7 electron configuration in all three open shell spin states. An increase of the population of d-electrons in the single metal substituted (111) n surface of γ-Al 2 O 3 is likely to provide a more stable electron configuration in the metal catalytic center.