Co and Ni single sites on the (111)n surface of γ-Al2O3 – a periodic boundary DFT study†

Abstract

The influences of increasing the number of d-electrons in the single metal (Fe-like) substituted (111)_n surface of γ-Al₂O₃ 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-Al₂O₃ 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-Al₂O₃. 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 d⁸ electron configuration in all three spin states, and similarly, Co_I has a unique d⁷ 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-Al₂O₃ is likely to provide a more stable electron configuration in the metal catalytic center.

Keywords: Co substituted surface of γ-Al₂O₃; Ni substituted surface of γ-Al₂O₃; (111)_n surface; Periodic boundary DFT approach; Metal catalytic center.