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Transition and Rare-Earth metal oxides coatings: Surface characterization, Thermo-mechanical properties, and chemical reactivity

Miran, Hussein (2018) Transition and Rare-Earth metal oxides coatings: Surface characterization, Thermo-mechanical properties, and chemical reactivity. PhD thesis, Murdoch University.

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This thesis describes a number of scientific studies investigating different interesting properties of rare earth metal oxides desired in different applications via combined experimental measurements and accurate density functional theory (DFT) calculations. The electronic, structural, mechanical and thermodynamic properties of cubic lanthanide sesquioxides are first reported, with a particular focus on the most common dioxide in the lanthanide family, ceria (CeO2). This is followed an investigation into the effect of Hf and Zr dopants on the reduction energies of pure ceria. The reduction enthalpies of Ce1-xHfxO2 and Ce1-xZrxO2 and Ce1-2xHfxZrxO2 solid solutions are computed as a function of the reduction extent (x). Alloying with Hf and Zr is found to systematically reduce the energies required to remove oxygen atoms from bulk of ceria. The computed coefficients in the Born-Huang criterion infer a mechanical stability of all cubic lanthanide sesquioxides. Acquired electronic parameters encompass Bader’s atomic charges and Partial Density of States (PDOS).

An important part of the thesis focuses on the catalytic capacity of CeO2 in acting as a stand-alone environmental catalyst toward the decomposition of a series of chlorinated volatile organic compounds, namely chloroethene, chloroethane and chlorobenzene. Guided by recent experimental measurements, the pyrolytic and oxidative decomposition of selected chlorinated compounds have been modelled on the most stable ceria surface, CeO2 (111). Dissociative addition (surface-assisted fission of the C-Cl bond) and direct elimination pathways (departure of a stable hydrocarbon entity with the co-adsorption of H and Cl atoms on the surface) assume comparable importance. Fission of the C-Cl bond over oxygen vacancies systematically necessitates lower energy barriers in reference to perfect surfaces. We have illustrated that observed catalytic deactivation in the experiment is attributed to the profound stability of adsorbed hydrocarbon adduct. Decomposition of an adsorbed phenyl moiety proceeds via addition of oxygen molecules to partially reduced surfaces. A simplified kinetic model plots the temperature-conversion profiles for the three compounds against corresponding experimental profiles, where a reasonable agreement has been attained.

Surfaces of terbium dioxide (TbO2) possess an important catalytic feature in that they are capable of producing hydrogen by splitting water molecules. We have computed a large array of thermo-mechanical properties including heat capacities, bulk modules and thermal expansions of bulk TbO2 as a function of temperatures and pressures based on the quasi-harmonic approximation (QHA) approach. Our calculated lattice constant and band gap were in good agreement with analogous experimental findings. A surface truncated along the (111), terminated with O atoms and with oxygen vacant site (111): O+1Vo incurs a higher thermodynamic stability across all values of oxygen chemical potential. Nonetheless, in the vicinity of the lean-limit of chemical potential the surface terminated with Tb atoms (111): Tb becomes more stable. The implications of these geometries on OH-H fission reactions have been discussed.

Magnetron sputtered CeO2 films as optically transparent materials, deposited onto crystalline silicon substrates at various oxygen-argon mixture gas, have been intensively studied and characterized by correlating their structural and chemical bonding states. All the thin films exhibit a polycrystalline character with cubic fluorite – structure for cerium dioxide along (111), (200) and (222) orientations. The XPS survey scans of the CeOx coatings revealed that that Ce, O, C elements are present in all of the obtained spectra of the studied films. XPS analysis demonstrated that the atomic percentages of Ce and O atoms increase as oxygen-argon mixture increases. Two oxidation states of CeO2 and Ce2O3 are present in the films prepared at lower oxygen/argon flow ratios; whereas the films are completely oxidized into CeO2 as the oxygen/argon flow ratio increase. Reflectance data obtained from UV-Vis examinations were utilized to calculate the optical constants such as absorption coefficient (α), the real and imaginary parts of the dielectric function (ε1, ε2), the refractive index (n) and the extinction coefficient (k). Our analysis indicates that the CeO2 films display indirect optical band gaps residing in the range of 2.25 - 3.1 eV. We utilized DFT calculations to estimate optical constants of a CeO2 cluster at ground state. The computed electronic density of states (DOSs) of the optimized unit cell of CeO2 yields a band gap that agrees well with the corresponding experimental value. The measured and DFT-computed absorption coefficient (α) exhibit a similar trend with similar values in the wavelength range from 100 to 2500 nm. Overall, a satisfactory correlation between the theoretical and experimental findings is demonstrated.

Spinal oxides of CuxCo3-xO4 thin films as one of metal mixed oxide systems, synthesized by sol-gel method and annealed at various temperatures ranging from º200 to º500 with interval 100, are deeply studied and characterized by various structural and optical characterization techniques. XRD data indicates that as annealing progresses, all the coatings possess a crystalline phase of Cu0.56Co2.44O4 (ICSD 78-2175) with preferential orientation along (400) reflection plane. Optical analysis reveals that the solar selectivity of the studied films improves as the annealing progresses. Bader’s charge analysis calculated by DFT implemented in VASP code points out that the Cu and Co atoms in all the stoichiometries hold positive charges whereas the O atoms are linked with negative charges. Our model reveals a covalent character for Cu-Co bond in all the system and ionic characteristics for Cu-O and Co-O bonds.

Finally, the influence of the variation in the Hubbard parameter U on the activation and reaction energies on CeO2-catalyzed reactions is studied. This has been achieved by surveying the change in activation and reaction energies for reactions underpinning the partial and full hydrogenation of acetylene over the CeO2 (111) surface. A positive correlation between the U values and reaction and activation energies reported. It is suggested that kinetic modeling against experimental profiles of products could be used as an approach to optimize the U value.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Engineering and Information Technology
Supervisor(s): Altarawneh, Mohammednoor, Jiang, Zhong-Tao, Veder, Jean-Pierre and Rahman, Mahbubur
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