Density Functional Theory Study Of The Structures And Properties Of Transition Elements Doped(MgO)_n(2≤n≤12) Clusters

As a new pattern multifunctional inorganic material, nano-sizedmagnesium oxide （MgO） possesses some particular physical chemistryproperties different from bulk materials, except for thermostability andcorrosion resistance. Therefore, magnesium oxide （MgO） has been widelyapplied to catalyst carrier, antibiosis, ceram, refractory material, painting andso on. In this thesis, the geometry structures, stabilities, electronic propertiesand adsorption performances of transition elements doped （MgO）_n（2≤n≤12）clusters were detailed and systematic discussed by using the density functionaltheory methods and GGA-PBE functions. The main conclusions aresummaried as follows:1. We have obtain a series of possible Mn-doped （MgO）_n（n=2–10）configurations. The optimized geometries show that the impurity Mn atomprefers to replace Mg atom which has low coordination number in all the lowest-energy MnMg_n-1O_n（n=2–10） structures. The stability analysis clearlyrepresents that the average binding energies of the doped clusters are largerthan that of the corresponding pure （MgO）_nclusters. Maximum peaks of thesecond order energy differences are observed for MnMg_n-1O_nclusters at n=6,9, implying that these clusters exhibit higher stability than their neighboringclusters. By investigating the HOMO-LUMO gaps of the lowest-energyMn-doped magnesia clusters, it is found that MnMg₂O₃, MnMg₅O₆, andMnMg₈O₉exhibit high chemical inactive. In addition, all the Mn-dopedmagnesia clusters exhibit high total magnetic moments with the exception ofMnMgO₂which has3.00μ_B. Their magnetic behavior is attributed to theimpurity Mn atom, the charge transfer modes, and the cluster size ofMnMg_n-1O_nclusters.2. CO adsorption on TM-doped magnesia nanotubes （TM=Ni, Pd and Pt）have been studied. Firstly, we calculated the structural properties andstabilities of Ni, Pd，Pt doped （MgO）₁₂nanotubes. The results show that theinitial nanotube slightly deforms under the inducing of these impurity atoms,but their stabilities almost have not changed. The calculation results of COadsorption on TM-doped magnesia nanotubes demonstrate that CO favoursadsoption on TM-doped magnesia nanotubes in the form of C atom bondingwith TM atom. Fukui indices analysis clearly exhibits that doping of impurityTM atom allows for a noticeably enhancement of nucleophilic reactivityability of magnesia nanotube. The adsorption energies demonstrate that CO molecule is more strongly bound on the3-fold TM atoms than the4-fold TMatoms. This finding is well confirmed by TM-C bond length, charge transferand C–O vibrational frequency. The high adsorption energy of2.55eV isfound when CO adsorbs on3-fold Pt in Pt-doped magnesia nanotubes,implying the kind of the doping TM atom has a significant influence on thechemical reactivity.3. The adsorption of SO₂on pure and Cu, Au atoms doped （MgO）₁₂nanotubes were systematically studied. Firstly, the structural properties andstabilities of Cu, Au atoms doped （MgO）₁₂ nanotubes have been calculated.The geometry optimization shows that the kind of impurity atom and thedifferent of substitute site have a great effect on the doped configuration. Theaverage binding energy results demonstrate that Cu, Au atoms prefer toreplace the3-fold Mg atoms in （MgO）₁₂ nanotube. Next, we considered all thepossible adsorption sites and patterns of SO₂molecule on pure （MgO）₁₂nanotube. Structural optimization indicates that when SO₂molecule adsorbedon the （MgO）₁₂ nanotube, the η2model is the most stable. η2model refers tothe O-S-O bond in SO₂is parallel with the Mg-O-Mg bond in the4-memberedring of （MgO）₁₂ and the S atom in SO₂molecule bond with the O atom in thetop6-membered ring. The bond length analysis point out that （MgO）₁₂nanotube has activated the S-O bonds in SO₂molecule, and the adsorption ofSO₂molecules on the （MgO）₁₂ has stretched or even broken the Mg-O bondswhich paralleled with the S-O bonds in SO₂molecule. Finally, we researched the adsorption of SO₂molecule on the Cu, Au atoms doped （MgO）₁₂nanotubes. The geometry optimization shows that there is severe deformationoccurs after the adsorption of SO₂on the original structures. The results ofS-O bond lengths, S-O bond orders, charge transfers and adsorption energiesindicate that Au1-SO₂model is the most stable. Au1-SO₂model refers to the Satom in SO₂bond with the O atom （connected to the Au atoms） which in thetop6-membered ring of Au （3-fold） doped nanotube. Part of the TM-O andMg-O bonds in all the adsorption models are stretched or even fracture due tochemisorption of SO₂molecules. In addition, regardless of the SO₂moleculeadsorption on pure or Cu, Au doped （MgO）₁₂nanotubes, SO₂molecule endedin the form of sulfite, and the S-O bond vibrational frequencies move towardthe lower frequency region compared with a separate SO₂molecule.