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Abstract:
The interaction of water with solid surfaces is important in many scientific fields such as corrosion, electrochemistry, geology, and heterogeneous catalysis. Chemistry at water/oxide interfaces plays a crucial role in surface's properties and reactivity. Employing model systems structurally and electronically characterized under well-defined conditions offers deep insights into the chemistry of the water/solid interfaces at an atomic level. This knowledge allows one to tune and optimize catalytic processes. Its strong relevance to the economical and environmental concerns (climate change) has driven growing worldwide attention to carbon dioxide (CO2). It is highly desirable to find a promising route to transform CO2 into valuable chemicals such as fuels for further applications. In this respect, iron oxides were considered as suitable catalysts for activation of CO2, not only because of their natural abundance, but also due to their important catalytic role in many industrial processes, such as the Fisher-Tropsch synthesis, the Haber process (NH3 synthesis), and the high-temperature water gas shift reaction.
The thesis aims to investigate the interaction of water and CO2 on magnetite (Fe3O4) surfaces and the role of water on CO2 activation. I also investigated the influence of surface orientation ((111) vs (001)) on the adsorption behavior. Well-defined magnetite surfaces were prepared as thin ordered films on metal substrates (Pt(111) and Pt(001)). For structural characterization and reactivity studies we used Temperature Programmed Desorption (TPD), low energy electron diffraction (LEED), Temperature-Programmed LEED (TP LEED), and Infrared Reflection Absorption Spectroscopy (IRAS).
First, I addressed the surface termination of the films using CO as a probe molecule. In the next step, individual adsorption of water and CO2 was studied. In the case of Fe3O4(111), water dissociates resulting in the formation of OsH and OwH hydroxyl groups, consisting of oxygen atoms from the magnetite surface (s) and water (w) molecule, respectively. These hydroxyl groups act as an anchor for the incoming water molecules to form a dimer complex, which ultimately forms (2x2) hexagonally ordered structure (seen by LEED) via hydrogen bond network. The latter is proved by DFT calculations to be thermodynamically favorable. In contrast to the (111) surface, the water molecularly adsorbs on Fe3O4(001)- (√2 × √2)R45°. Upon increasing coverage, water molecule starts to partially dissociate which reinforces the interaction between the formed dimer or trimer and the oxide surface. Water ordering directly observed by LEED suggests that the water ad-layer follows the symmetry of iron oxide underneath, thus ice-like layers are formed on (111) and (001) in hexagonal and square symmetry, respectively. This is the first time that water ordering has been experimentally observed in two different structures on the same oxide.
We believe that the experimental results provide a strong basis for theoretical calculations of water/oxide interfaces, and can even serve as benchmarks for the investigation of ice nucleation on solid surfaces. When compared to water, CO2 molecules are rather weakly interacting with both Fe3O4(111) and (001) surfaces. However, strongly bound CO2 species may be formed as minority species at a low coverage regime, which are, most likely, related to surface defects. Based on isotopic experiments, there is no evidence of CO2 dissociation. TPD spectra on (111) facet manifest a competition between CO2 and residual gases (water and CO) from the UHV background. In fact, even traces of water may considerably alter CO2 interaction with the oxide. Therefore, careful precautions have to be taken while studying CO2 interaction with the oxide surface. The results show that on the (111) surface, CO2 may adsorb more strongly in the presence of surface hydroxyls, resulting in CO2 desorption at 250 K (compared to ∼200 K on the clean surface). According to the TPD results, a water pre-covered magnetite (001) surface enhances CO2 interaction, probably via the formation of bicarbonate species which decompose at ∼350 K. We believe that the results presented in this Thesis shed more light on the complex interaction of "simple" molecules with oxide surfaces.
