Atomic Structure and Stability of Magnetite (001)

Magnetite is a material used for catalysts in some of today’s most important industrial processes,the Haber-Bosch and Fischer-Tropsch processes. It has raised attention due to its catalyticbehaviour in the water-gas shift reaction, its potential use in spintronic devices and hierarchicalmaterials. There are however still a lot of open questions concerning these potential applications,because the mechanisms at the surface that play an important role in these are notfully understood. The structure of the √2x√2R45° reconstruction that forms on the cleanmagnetite (001) upon preparation in ultra-high vacuum, which was formerly explained by abulk-truncated model, had to be revised due to new observations that were unexplainable withthe former model and led to the proposition of a new model. This was also invalidating formerinterpretations of lifting processes of this reconstruction due to significant differences betweenthe models. To solve some of these open questions, surface X-ray diffraction measurementshave been performed on the (001) surface of magnetite exposed to several different conditions.Following the surface science approach, the first system investigated was the structure of theclean, reconstructed surface to revalidate the newly proposed structural model as a base for allof the following experiments. Due to an excellent agreement with the surface X-ray diffractiondata measured on this surface, the new, non-stoichiometric structural model was confirmed.A further structural refinement was performed, but only led to some minor changes in themodel. In addition, the impact of certain features of the model on the data was investigated bysimulations to get a better understanding on the sensitivity of the measurements for these.Based on this, the structural changes in the surface upon dissociative adsorption of formic acidwas investigated. The formate that forms during this process and adsorbs on the surface isa proposed intermediate in the water-gas shift reaction, and acts as a probe molecule for theinteraction of other carboxylic acids with the surface. Dosing formic acid was known to lift the √2x√2R45° reconstruction of the surface, and, following a structural analysis of the obtained diffraction data of this surface, it could be shown for the first time that the surface changes backto bulk-stoichiometry, involving the reorganisation of iron cations at the surface as well as irondiffusion from lower layers towards the surface. A full structural refinement was performed,and the resulting model agreed well with the adsorption geometry proposed in literature.In addition, water vapor and atomic hydrogen, both also known to lift the surface reconstructionat room temperature, were also dosed on the clean reconstructed surface and its structureinvestigated. The pressure region in which water vapor lifts the reconstruction was found tobe between 1x10-4 mbar and 1x10-3 mbar, which agrees nicely with the reported onset ofhydroxylation of the surface reported in literature. Both water vapor and atomic hydrogen werefound to not only completely lift the reconstruction, but also lead to an atomic rougheningof the surface within the first two layers. For atomic hydrogen, this was also confirmed byScanning-Tunneling Microscopy measurements. This indicates significant iron diffusion and,subsequently, a surprisingly high mobility of iron cations at the surface under these conditions.No different phase was formed at the surface, and the analysis indicates ordered hydroxyl to bepresent at the surface during water dosing as well as after pumping back down.To complement the X-ray diffraction measurements, X-ray photoelectron spectroscopy measurementswere performed before and after dosing of formic acid, water vapor and atomichydrogen. A rather large amount of carbon was found to be present on the surface after watervapor dosing. The same observation was made upon exposure to atomic hydrogen, but incontrast to the water vapor exposed surface, the amount of carbon increased even further overtime. This indicates that the hydroxylated surface is attracting carbon from the residual gaseven under ultra-high vacuum conditions.Finally the reversible lifting of the surface reconstruction upon heating was investigated. Theresulting data from the lifting process agreed well to what was described in literature. The causeof this was found to be a decrease in the long-range ordering at the surface due to diffusion ofiron at the surface, exhibiting a fundamentally different lifting mechanism than what was foundunder the previous conditions