Structural and electronic effects in hydrocarbon reactions over metal oxides have been examined by comparing the reactions of methyl (-CH₃) and methylene (=CH₂) fragments on three different oxide single crystal surfaces: α-Cr₂O₃(101̅2), α-Cr₂O₃(0001), and α-Fe₂O₃(101̅2). The intermediates have been generated through the decomposition of halogenated hydrocarbons. The primary reactions of methyl and methylene over α-Cr₂O₃ are methyl dehydrogenation to methylene, and methylene coupling (C-C bond formation) to ethylene (CH₂=CH₂). The different surface geometric structures of α-Cr₂O₃(101̅2) and (0001) lead to an increase in the activation barrier for methylene surface migration, a critical step in the coupling reaction, of 5.9 kcal/mol over the (0001) surface. For methyl dehydrogenation, differences in the local site pair (cation/anion) geometry and the proximity of surface lattice oxygen to the methyl group do not result in a significant difference in the barrier for dehydrogenation, suggesting that the surface anions play a minor role in the dehydrogenation of methyl on these surfaces. Electronic differences in the Fe³⁺ (𝑑⁵) and Cr³⁺ (𝑑³) cations on structurally-similar α-Cr₂O₃(101̅2) and α-Fe₂O₃(101̅2) surfaces lead to major differences in reaction selectivity. α-Cr₂O₃(101̅2) is nonreducible under the reaction conditions of this study, but α-Fe₂O₃(101̅2) is highly reducible due to the difference in the d electron configuration. Hydrocarbons are formed over α-Cr₂O₃(101̅2), but nonselective oxidation products (CO₂, CO, H₂O) are formed over the stoichiometric α-Fe₂O₃(101̅2) surface along with surface reduction. Reduction of the α-Fe₂O₃(101̅2) leads to a shift in the product selectivity towards formaldehyde (CH₂O) and ethylene. For the limited number of systems examined in this study, examples of geometric structure sensitive (methylene coupling) and structure insensitive (methyl dehydrogenation) reactions have been found on α-Cr₂O₃, and electronic effects are observed for the reactions on α-Cr₂O₃(101̅2) and α-Fe₂O₃. For the structure sensitive reaction, the differences in surface geometry impact the reactions kinetics over Cr₂O₃ but not the types of products formed, while the electronic differences give rise to dramatic changes in the selectivity associated with the very different products formed over α-Cr₂O₃(101̅2) and α-Fe₂O₃(101̅2). / Ph. D.

Metal oxide reactions at metal oxide surfaces or at metal-metal oxide interfaces are of exceptional significance in areas such as catalysis, micro- and nanoelectronics, chemical sensors, and catalysis. Such reactions are frequently complicated by the presence of high electric fields and/or H2O-containing environments. The focus of this research was to understand (1) the iron oxide growth mechanism on Fe(111) at 300 K and 500 K together with the effect of high electric fields on these iron oxide films, and (2) the growth of alumina films on two faces of Ni3Al single crystal and the interaction of the resulting films with water vapor under non-UHV conditions. These studies were conducted with AES, LEED, and STM. XPS was also employed in the second study. Oxidation of Fe(111) at 300 K resulted in the formation of Fe2O3 and Fe3O4. The substrate is uniformly covered with an oxide film with relatively small oxide islands, i.e. 5-15 nm in width. At 500 K, Fe3O4 is the predominant oxide phase formed, and the growth of oxide is not uniform, but occurs as large islands (100 - 300 nm in width) interspersed with patches of uncovered substrate. Under the stress of STM induced high electric fields, dielectric breakdown of the iron oxide films formed at 300 K occurs at a critical bias voltage of 3.8 ± 0.5 V at varying field strengths. No reproducible result was obtained from the high field stress studies of the iron oxide formed at 500 K. Ni3Al(110) and Ni3Al(111) were oxidized at 900 K and 300 K, respectively. Annealing at 1100 K was required to order the alumina films in both cases. The results demonstrate that the structure of the 7 Å alumina films on Ni3Al(110) is k-like, which is in good agreement with the DFT calculations. Al2O3/Ni3Al(111) (γ'-phase) and Al2O3/Ni3Al(110) (κ-phase) films undergo drastic reorganization and reconstruction, and the eventual loss of all long-range order upon exposure to H2O pressure > 10-5 Torr. Al2O3/Ni3Al(110) film is significantly more sensitive to H2O vapor than the Al2O3/Ni3Al(111) film, and this may be due to the incommensurate nature of the oxide/Ni3Al(110) interface. STM measurements indicate that this effect is pressure- rather than exposure- dependent, and that the oxide instability is initiated at the oxide surface, rather than at the oxide/metal interface. The effect is not associated with formation of a surface hydroxide, yet is specific to H2O (similar O2 exposures have no effect).