Density Functional Theory Study On Adsorption Properties And NO Decomposition Mechanism Of Cu-Co Bimetallic Clusters

In this paper,geometry structures,adsorption properties and electronic properties of NO and O₂ adsorbed on Cu_mCo_n?2?m+n?7?clusters are discussed using Dmol³ program package based on the first-principles calculation method of density functional theory.Moreover,the geometry structures,adsorption properties and the effect of doping H₂O on the activity of adsorbed small molecules of co-adsorption systems after doping H₂O are also carried out.And the decomposition mechanism of NO on Cu_mCo_n?m+n=2?clusters were also studied.Main conclusions are listed as follows:?1?For obtained ground state structures of Cu_mCo_nNO clusters,it can be seen that NO molecules are adsorbed at top sites except Cu₃NO and Cu₅NO whose NO are at bridge sites.The analysis of second-order energy difference shows that values of Cu₂CoNO,Cu₃Co₃NO and CuCo₅NO clusters are larger,while the Cu₂CoNO,Cu₃Co₂NO and CuCo₄NO clusters are smaller.The HOMO-LUMO energy gap value of Cu₆CoNO is the largest,while Co₇NO cluster is the smallest.NO molecules obtain charges for those clusters where NO are at bridge sites,while lose charges for those clusters where NO are at top sites.When NO are adsorbed on Cu_mCo_n alloy clusters,N-O bond are elongated.The presence of charge transfer and N-O bond elongation indicate that chemical adsorption occur.The analysis of charge population and PDOS shows that the change of N-O bond is mainly due to a small amount of charge provided by Cu_mCo_n alloy clusters to 2?*anti-bonding orbital of NO molecules.?2?For Cu_mCo_nNO-H₂O clusters,NO and H₂O are at top sites except that NO of Cu₃NO-H₂O and Cu₅NO-H₂O are at bridge sites.The analysis of the second-order energy difference reveals that the thermodynamic stability of Co₄NO-H₂O is strong,and the stability of CuCo₃NO-H₂O clusters is the worst.The energy gap of the Cu₆CoNO-H₂O cluster is the largest,which indicates that its chemical stability is the strongest.And the energy gap of the Co₇NO-H₂O cluster is the smallest,that is,the chemical activity is the strongest.H₂O molecules provide charges to Cu-Co alloy clusters.The presence of H₂O has greater effect on Co₄NO-H₂O,Co₆NO-H₂O and Cu₃Co₄NO-H₂O clusters.The N-O bond length of Co₄NO-H₂O and Cu₃Co₄NO-H₂O clusters are obviously elongated,while the N-O bond length of Co₆NO-H₂O is obviously weakened.?3?For the Cu_mCo_nO₂ structures,O₂ molecules are adsorbed on Cu_mCo_n clusters at top sites.The second-order energy difference analysis shows that the thermodynamic stability of CuCo₃O₂ clusters is the worst.Cu₆CoO₂ has the largest energy gap value and the strongest chemical stability,while Co₇O₂ has the smallest energy gap value.Charges are transferred from Cu-Co alloy clusters to O₂ and O₂ molecules get charges.O-O bond length are elongated and O₂ activity increase after O₂ adsorption.And d orbit of Cu_mCo_n clusters transfer charges to the?*anti-bonding orbital of O₂,which is the main reason for the O-O bond length change.?4?In the Cu_mCo_nO₂-H₂O co-adsorption structures,O₂ and H₂O are both adsorbed at top sites.The thermodynamic stability of CuCo₅O₂-H₂O and Co₆O₂-H₂O clusters are strong,while CuCo₃O₂-H₂O and Co₅O₂-H₂O clusters are poor.The energy gap of the Cu₆O₂-H₂O cluster is the largest,indicating that it has the strongest chemical stability.And the Cu₂CoO₂-H₂O cluster has the smallest energy gap,which means that it has the strongest chemical activity.Adsorption energies of Co₄O₂-H₂O,CuCo₄O₂-H₂O and Co₅O₂-H₂O are relatively large,which indicates that the interaction between O₂,H₂O and Co₄,CuCo₄ and Co₅ are stronger.H₂O molecules provide charges to Cu-Co and H₂O molecules lose charges.The presence of H₂O has great influence on Cu₂Co₄O₂-H₂O,CuCo₅O₂-H₂O and Cu₄Co₃O₂-H₂O clusters.The addition of H₂O causes elongation of O-O in CuCo₅O₂-H₂O and Cu₄Co₃O₂-H₂O,while causes shortening of O-O in Cu₂Co₄O₂-H₂O clusters.?5?The reaction mechanism of NO on Cu_mCo_n?m+n=2?clusters to N2 was studied.It was found that NO formed an intermediate N₂O during the decomposition reaction on Cu₂cluster and N₂ was decomposed by N₂O.And NO undergoes a transition state containing N₂O units during the decomposition reaction on CuCo cluster and decomposes to N₂.In the decomposition reaction of NO on Co₂ cluster,N2 is directly decomposed by the two NOs that have been adsorbed,and no intermediate N₂O is formed.