Surface Chemistry of Oxygen-functionalized Liquid Organic Hydrogen Carriers and Dehydrogenation Catalysts Studied by In Situ XPS

Liquid Organic Hydrogen Carriers (LOHCs) provide an attractive option for chemical hydrogen storage that is compatible to a large extent with the existing infrastructure for liquid fossil fuels. In this approach, hydrogen is covalently bound to a diesel-like organic carrier and can be released again on demand, whereby loading and unloading occurs in reversible heterogeneously-catalyzed reactions. A recent field of interest are oxygen-functionalized LOHCs, owing to their often high storage capacities and potential for energy-efficient low-temperature thermal or electrochemical cycling strategies. In this thesis, synchrotron radiation-based X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) were used to study the surface reactions of different alcohol/carbonyl functionalized LOHC couples under well-defined model conditions in ultra-high vacuum (UHV). Surface science studies provide fundamental molecular-level insights into the catalytic processes and in this way support the targeted optimization of these carrier systems and catalysts on a knowledge-driven basis. The surface chemistry of the LOHC system acetophenone/1-cyclohexylethanol (H0-AP/H8-AP) was investigated on Pt(111). To this end, dedicated in situ TPXPS experiments of both compounds and of two possible intermediates, 1-phenylethanol (H2-AP) and 1-cyclohexylethanone (H6-AP), were carried out. H8-AP was found to dehydrogenate first at the alcohol group, yielding the hydrogen-rich ketone H6-AP at 210 K. Further dehydrogenation at the cyclohexyl group of the ketone occurred gradually above 250 K. This second dehydrogenation step was found to be accompanied by an abstraction of one hydrogen atom from the methyl group of the molecule, resulting in the formation of an acetophenone-like phenyl-C(O)-CH2 species at the Pt surface at about 340 K. The same species was also observed in the surface reactions of H0-AP, H2-AP, and H6-AP. Heating above 350 K resulted in the decomposition of the intermediate species into CO and CH-fragments. Secondly, the surface chemistry of the LOHC system benzaldehyde/cyclohexylmethanol (H0-AP/H8-BA) was studied on Pt(111) by high-resolution TPXPS, assisted by TPD measurements and DFT simulations. In contrast to the aforementioned H0-AP/H8-AP couple, in which hydrogen is stored within a cyclohexyl group and a secondary alcohol, the H0-BA/H8-BA couple stores hydrogen in a cyclohexyl group and a primary alcohol function. The experimental results revealed again a two-step dehydrogenation sequence: The alcohol group of H8-BA is dehydrogenated first at 220 K, followed by dehydrogenation of the cyclohexyl group between 250 and 350 K. However, contrasting the results for H8-AP, a decomposition reaction was found to occur parallel to the cyclohexyl dehydrogenation step, yielding CO and a cyclohexyl- resp. phenyl fragment (C6H11-x/C6H5) depending on the progress of dehydrogenation. Conversion to the desired product H0-BA was not observed. This suggests a lower thermal stability of the primary alcohol, specifically, of the aldehyde intermediate H6-BA as compared to the ketone H6-AP formed in the surface reaction of the secondary alcohol. DFT simulations supported this theory, indicating that the decomposition of H6-BA is thermodynamically favored over that of H6-AP. Another important field of research in the context of the LOHC technology is the choice of catalyst material. In the scope of this thesis, a composition spread PtRu/Pt(111) near-surface alloy was prepared under UHV conditions and investigated using synchrotron radiation-based HR-XPS. Bimetallic Platinum-Ruthenium catalysts represent the current state-of-the-art in terms of anode materials for the direct electrochemical conversion of alcohol-based LOHCs in fuel cells but are also considered promising for efficient thermochemical hydrogen release. The alloy was prepared by physical vapor deposition (PVD) of a wedge-shaped Ru gradient on the Pt(111) surface and subsequent flash-annealing to 850 K to promote the intermixing of both metals. This procedure was found to result in a predominantly Pt-terminated surface alloy, with a wedge-like gradient of Ru in the near-surface region. Significant changes in the electronic structure of the surface Pt atoms, most likely due to vertical electronic ligand and/or compressive strain effects, were evidenced by changes in the surface component of the Pt 4f XP signal. Exposure of the as-prepared alloy to CO until saturation revealed a decreasing CO population of Pt bridge sites with increasing local Ru content in the sample; the population of Pt on-top sites was found to be constant over the investigated compositional range. In temperature-programmed experiments, CO was found to desorb at lower temperature from Pt sites of the near-surface alloy than from Pt(111), indicating weaker binding in the former case. Offering O2 resulted in the formation of Ru oxide, most likely via Ru surface segregation driven by the strong binding between Ru and O. Notably, the relative amount of Ru oxide, that is, the RuOx:Ru ratio, was found to increase with increasing local Ru content in the sample. This trend may be interpreted as a direct correlation between the Ru content and the oxidizability of the surface alloy.