Structural changes in model oxide catalysts studied by operando XAFS

Bridging the pressure gap between surface science studies and industrial processes requires a combination of well-studied reactions, model systems as catalysts, and advanced techniques capable of detecting structural changes under realistic pressure conditions. Observations from such studies provide helpful insight into the present phases of the gas-surface boundary and transitions that happen during studied reactions.
This thesis utilizes operando X-ray absorption fine structure spectroscopy (XAFS) to study ultra-thin oxide films supported on single crystals under realistic reaction conditions. Grazing incidence geometry was used to probe only the topmost few layers of our surface. Ultra-thin CoOx and FeOx on Pt(111) were studied under ambient pressure exposure to CO and O2, and CO oxidation conditions, where XAFS analysis revealed structural changes of the oxide films, depending on the gas atmosphere. Furthermore, operando time resolved measurement of FeOx/Pt(111) indicate involvement of the trilayer phase in CO oxidation. The Sn/Pt surface alloys were studied during CO oxidation using GI-XANES, revealing the formation of Sn(II)
oxides on the Sn/Pt surfaces under CO oxidation conditions. The found oxide qualitatively agrees with calculated spectra of previously studied tin surface oxides. Refle-XAFS was used to study chemical surface changes of industrial alloys and model electrodes in harsh electrochemical environments, without being limited to thin samples. NiMo powder catalysts were studied in transmission mode Quick-XAFS, which allowed for almost simultaneous data acquisition on both edges during in situ sulfidation reaction.
We have shown successful measurements in ambient pressure gas phase reaction conditions and harsh electrochemical conditions in liquid. Combining GI-XAFS with traditional surface science techniques such as low energy electron diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and auger electron spectroscopy (AES) is shown to be a very promising way begin to understand the structure of the catalyst before, during and after the chemical reaction.