The Structural And Electronic Properties Of Spinel MnCo₂O₄ Bulk And Low Index Surfaces:From First Principles Studies

Recently,the MnCo₂O₄ compound,which belongs to an important class of AB₂O₄ spinel structure,is widely applied as a catalyst.In order to analyze the relationship between its surface characteristics and the catalytic properties,we use first-principles projector augmented plane wave?PAW?method to study the structural and electronic properties of the bulk and low index?001?and?110?surfaces of the spinel MnCo₂O₄.The results show that the MnCo₂O₄ compound with inverse spinel structure is energetically more stable.The MnCo₂O₄ inverse spinel is an insulating material with a small band gap.We also found that the Mn prefers to exhibit as high-spin Mn³⁺ion occupying the octahedral sites in the bulk structure.On the other hand,there are two different types of the cobalt ions:namely the tetrahedral sites Co²⁺and octahedral sites Co³⁺.We obtained stable slab models through calculating the surface energies of the low index surfaces?001?and?110?,and the surface energies of the two surfaces are 0.036 eV/?²and 0.101eV/?².Further analysis of the projected density of states of the manganese atoms and cobalt atoms,we get more detailed information about the surface electronic structures:the surface of Mn and the tetrahedral site Co atoms are both very stable,and their electronic structures resemble their bulk characters.However,due to the lowered Co-O coordination,the electronic structure of the octahedral Co atoms changes substantially from the bulk state t³_2g???t³_2g???into the surface state t³_2g???e_g¹???t²_2g???at the MnCo₂O₄?001?surface?5-coordinated Co?.At the?110?surface,the surface Co atom is 4-coordinated?there are 4 Co-O bonds for each Co atom?and therefore the surface cobalt is even more active,and thus have higher catalytic activity.The Co atoms at the?110?surface have electron configuration of t³_2g???e_g²???t¹_2g???e_g¹???.These results illustrate that the active catalytic sites in MnCo₂O₄ spinel is the Co³⁺ions at the octahedral sites.