The Structure, Stability And Adsorption Of Transition Metal Atoms Decorated Alumina And Aluminium Nitride Clusters: DFT Study

Alumina nanostructures play an important role in many fields, such asadsorbents, coatings, soft abrasives, and catalysts or catalytic supports.Transition metal atoms decorated alumina and aluminium nitride cluster havebeen widely applied. Transition metal atoms decorated alumina clusters haveuses in semiconductor devices, gas-sensitive sensors, catalyst for reforming ofmethane in petrochemical industry and hydrodesulfurization catalysts forpetroleum fractions. In addition, aluminium nitride cluster can be used asgas-sensitive sensors, such as the hydrogen storage material. Doping oftransition metal elements has been found to increase the hydrogen storagecapabilities of aluminium nitride cluster. In this thesis, the structures andadsorption performances of transition metal atoms decorated alumina andaluminium nitride clusters are investigated by the density functional theory（DFT）. In summary, the main and important conclusions are shown as follows:1The geometrical structures and stability of pure （Al₂O₃）_n（n=9,10,12,15） clusters and Ni-doped （Al₂O₃）_9-10clusters are investigated by densityfunctional theory. Especially, the structural and electronic properties of fourdifferent Ni-doped （Al₂O₃）₉clusters and one Ni-doped （Al₂O₃）₁₀cluster arefocused on. In comparison with the HOMO-LUMO gaps of pure clusters, theNi-doped （Al₂O₃）_9-10clusters have narrower gaps. The calculation resultsindicate that the modified Ni atom is mainly responsible for the reduction ofthe HOMO-LUMO gaps. The characteristic vibration peak at about1030cm^-1is found in the vibrational frequencies of the Ni-doped （Al₂O₃）_9-10clusters,which is caused by the asymmetric Al-O-Al stretching vibration. Another peakis at around826cm^-1involving the characteristic vibration of Ni-O bond.2We have investigated the geometry optimization, bonding characters,electronic structures and vibrational frequencies of thiophene adsorption onMAl₂₀O₃₀（M=Co, Ni and Mo） clusters by density functional theory.Transition metal atoms in MAl₂₀O₃₀（M=Co, Ni and Mo） clusters promote theactivity of catalyst. It is confirmed by an elongation of the S-C bond amongall thiophene adsorption modes. The analysis of charge transfer and molecularorbital pictures of different adsorption modes explains that π-backdonationexists in η²and η⁵bonding modes. The chemical hardness and theHOMO-LUMO gaps are large as adsorption energies increase. Moreover, incomparison with the vibrational frequencies of free thiophene, the ν（C=C）_sympeak of η⁵bonding mode is shifted to lower wavenumbers.3We have studied the feasibility of bare and Ni decorated Al₁₂N₁₂ clusters as hydrogen storage materials by using density functional theory.Each Al atom in bare Al₁₂N₁₂cluster can be adsorbed one H2molecule withthe average adsorption energy of-0.17eV. Moreover, it is shown thathydrogen remains inside the Al₁₂N₁₂cluster in molecular form. In Nidecorated Al₁₂N₁₂clusters, most stable site of Ni atom is the bridge site overthe Al-N bond shared by the two six-membered rings. Up to three hydrogenmolecules is found to adsorb on Ni atom of the NiAl₁₂N₁₂cluster. Thecalculation result indicates that up to20hydrogen molecules can be storedinside and on the exterior surface of the NiAl₁₂N₁₂cluster. As the capacity ofhydrogen storage is increasing to6.5wt%（weight percentage）, the value ofthe Gibbs free energy is positive at25K. It is demonstrated that NiAl₁₂N₁₂clusters cannot be as high weight percentage of hydrogen storage materials.