Untersuchung von Grenzflächen zwischen zwei Flüssigkeiten mittels Röntgenstreumethoden

Die strukturelle Untersuchung von Phasengrenzen zwischen zwei Flüssigkeiten ist aufgrund der Tatsache, dass sie nicht direkt zugänglich sind (sog. vergrabene Grenzflächen), der hohen thermischen Beweglichkeit der Teilchen, sowie der Kapillarwellen experimentell sehr anspruchsvoll und durch den Mangel an geeigneten Methoden bisher kaum durchgeführt worden. Röntgenstreuung, insbesondere Reflektivität und Streuung unter streifendem Einfall, stellt eine ideale Untersuchungsmethode für derartige Phasengrenzen dar, da sie eine Untersuchung auf atomarer Skala ermöglicht und eine hohe zeitliche Auflösung bietet, so dass durch äußere Einflüsse hervorgerufene Änderungen der Grenzflächenstruktur in situ beobachtet werden können. In dieser Arbeit wurde unter Verwendung von Röntgenstreumethoden die Grenzflächenstruktur zwischen einer flüssigen Quecksilberelektrode und verschiedenen wässrigen Elektrolyten in Abhängigkeit des Potentials sowie von Lipidmonoschichten an der hydrophoben Grenzfläche zwischen Wasser und Perfluorohexan untersucht. An der Grenzfläche zwischen Quecksilber und Natriumfluoridlösung wurde mittels Röntgenreflektivität eine atomare Schichtung des Quecksilbers ähnlich derjenigen an der Quecksilber-Gas-Grenzfläche gefunden. Die in situ durchgeführten Untersuchungen zur Bestimmung des Potentialeinflusses auf die Grenzflächenstruktur zeigten, dass die atomare Schichtung potentialunabhängig ist. Es handelt sich bei der Schichtung daher wahrscheinlich um eine intrinsische Eigenschaft des Quecksilbers. Für die Grenzflächenrauhigkeit wurde dagegen eine Potentialabhängigkeit gefunden, die neben dem nach der Kapillarwellentheorie erwarteten Anteil einen zusätzlichen Beitrag in der Größenordung der durch Polarisation der Leitungselektronen zu erwartenden Änderung des Grenzflächenprofils aufwies. Für eine Elektrolytlösung, die zusätzlich zum Natriumfluorid auch Natriumbromid und Blei(II)-bromid, d. h. spezifisch adsorbierende Spezies enthielt, wurde ein potentialinduziertes, reversibles Wachstum einer 0.76 nm dicken PbFBr-Adsorbatschicht beobachtet. Ihre kristalline Struktur konnte anhand der Reflektivitätsmessungen auf atomarer Skala aufgelöst werden. Für außerdem entstandene PbFBr-Kristallite wurde mittels Streuung unter streifendem Einfall eine Vorzugsorientierung nachgewiesen. In Übereinstimmung mit weiteren Untersuchungen zur Wachstumsdynamik kann von einer Templatwirkung der kristallinen Schicht für das Wachstum der Kristallite ausgegangen werden. Weiterhin konnten Reflektivitätsmessungen an der kontrastarmen, hydrophoben Grenzfläche zwischen Wasser und Perfluorohexan durchgeführt und deren Grenzflächenrauhigkeit bestimmt werden. Für die Grenzfläche zwischen Wasser und Lösungen der Lipide POPC und DPPC in Perfluorohexan wurde die Ausbildung einer Lipidmonoschicht geringer Packungsdichte beobachtet. Weiterhin wurde die für die Anlagerung der Lipide an der Grenzfläche erwartete Erhöhung der Grenzflächenrauhigkeit gefunden.The structural investigation of liquid-liquid interfaces is experimentally challenging because of the fact, that they are buried interfaces, the high thermal mobility of the particles, and the capillary waves. As a result, only a few suitable methods are available and structural investigations of these interfaces are rare to date. To investigate the liquid-liquid interface, X-ray scattering techniques, particularly reflectivity and grazing incidence diffraction, are ideal, since they provide both high spatial and temporal resolution. Thus, structural changes at the liquid-liquid interface induced by external influences can be investigated in situ with these methods. In this thesis, the structure of the interface between a liquid mercury electrode and an aqueous electrolyte solution in dependence of the electrode potential and that of lipid monolayers self-assembled at the hydrophobic interface between water and perfluorohexane have been studied with X-ray scattering techniques. Using X-ray reflectivity, atomic layering of mercury was observed at the interface between mercury and a sodium fluoride solution, similar to that found at the mercury-vapour interface. In situ investigations of the influence of the electrode potential showed that the atomic layering is potential-independent. Thus, it is presumably an intrinsic property of the mercury. In contrast the interfacial roughness exhibited a potential dependence. It was found that in addition to the expected contribution of the capillary waves, a potential-dependent roughness is required to describe the interfacial profile, which can be explained by a polarisation of the conduction electrons at the interface. For another electrolyte solution, additionally containing sodium bromide and lead bromide, i.e. specifically adsorbing species, a potential induced, reversible growth of a 0.76 nm PbFBr adsorbate layer was observed. With X-ray reflectivity it was possible to resolve the crystalline structure of this layer on the atomic scale. In addition, growth of PbFBr crystallites, which exhibited a preferred orientation, was determined by grazing incidence X-ray diffraction. In agreement with further measurements of the growth dynamics it can be shown that the crystalline PbFBr layer acts as a template for the crystallite growth. Furthermore, reflectivity measurements were carried out at the low contrast, hydrophobic interface between water and perfluorohexane and its interfacial roughness was determined. At the interface between water and solutions of the lipids POPC and DPPC in perfluorohexane the formation of a lipid monolayer was observed. In addition, an increase in interfacial roughness was found, as is expected for the assembly of lipids at the interface

Temperature- and potential-dependent structure of the mercury-electrolyte interface

The atomic-scale structure of the mercury-electrolyte (0.01 M NaF) interface was studied as a function oftemperature and potential by x-ray reflectivity and x-ray diffuse scattering measurements. The capillary wavecontribution is determined and removed from the data, giving access to the intrinsic surface-normal electrondensity profile at the interface, especially to the surface layering in the Hg phase. A temperature dependentroughness anomaly known from the Hg-air interface is found to persist also at the Hg-electrolyte interface.Additionally, a temperature dependence of the layering period was discovered. The increase in the layer spacingwith increasing temperature is approximately four times lager than the increase expected from thermal expansion.Finally, the interface is found to broaden towards the electrolyte side as the potential becomes more negative, inagreement with the Schmickler-Henderson theory. Our results favor a model for the interface structure, which isdifferent to the model formerly used in comparable studies

Intracluster atomic and electronic structural heterogeneities in supported nanoscale metal catalysts

This work reveals and quantifies the inherent intracluster heterogeneity in the atomic structure and charge distribution present in supported metal catalysts. The results demonstrate that these distributions are pronounced and strongly coupled to both structural and dynamic perturbations. They also serve to clarify the nature of the dynamic bonding of nanoscale catalytic metal clusters with their supports, and the mediation of these properties due to the presence of adsorbates. These findings are supported by theoretical modeling and experimental data measured for an exemplary supported metal catalyst, Pt supported on silica, using in situ high energy resolution X-ray absorption and emission spectroscopies; in situ diffuse reflectance infrared Fourier transform spectroscopy; and ex situ scanning transmission electron microscopy

Comparative in Operando Studies in Heterogeneous Catalysis: Atomic and Electronic Structural Features in the Hydrogenation of Ethylene over Supported Pd and Pt Catalysts

There exists an emerging opportunity, engendered by advances made in experimental methods of research, to address long-standing questions about the nature of the molecular mechanisms that are operative in important heterogeneous catalytic processes, as well as the nature of the complex atomic and electronic structural features that mediate them. Of particular interest in this regard is the understanding of the dynamical attributes of catalytic processesan understanding that might allow design principles to be applied to optimize the atomic and electronic structure of heterogeneous catalysts to sustain their performance in essentially any operating process condition. The current work explores these ideashighlighting capabilities of in operando methods of spectroscopic characterization as applied to an exemplary heterogeneous catalytic process, olefin hydrogenation. No heterogeneous catalytic process has been studied more intensively than olefin hydrogenation. The extensive literature available establishes important features by which metal catalysts activate and efficiently transform the bonding of the hydrogen and alkene reactants to generate a product alkane. Even so, many important mechanistic questions remain poorly understood due to the inherent multiscale complexity associated with heterogeneous catalytic transformations, as well as the paucity of methods suitable for their characterization in operando. The recent literature documents the development of new capabilities for characterization afforded by in situ and in operando methods. Of these, X-ray absorption spectroscopy (XAS) has become a particularly important technique for studying the mechanisms of catalytic reactions due to its capabilities for elucidating the nature of the atomic and electronic structural features of operating catalysts. Many important questions can now be addressed, in particular those that follow from the unique dynamical impacts and patterns of reactivity that occur in higher pressure (non-UHV) environments. In this Perspective, we examine important structure–property correlations for an exemplary model reactionethylene hydrogenationas elucidated in operando for two efficient catalyst materialsnanoscale Pd and Pt clusters supported on SiO₂. The examined features include the following: the structural dynamics of the metal clusters and their sensitivity to the composition of the reactant feed; the role of hydrogen, and metal- and/or support-bonded forms of adsorbates more generally, in forming intermediates and products; the influences of adsorbate bonding states (e.g., hydrogen) on reactivity; the role played by carbonaceous deposits (and the mechanisms of their formation); the quantitative nature of the atomistic features that exist within the structure–sensitivity correlations of this catalytic reaction; and mechanisms that mediate the sintering of catalysts operating in high-pressure ambient environment. Here we present a comparative overview of the hydrogenation of ethylene over ≈1 nm-sized Pd and Pt catalysts supported on SiO₂. The reaction was characterized in various mixed hydrogen and ethylene atmospheres at ambient conditions by in operando XAS and complemented with scanning transmission electron microscopy (STEM). Pronounced changes in the atomic and electronic structures of both catalysts (e.g., defined transitions between hydrogen- and hydrocarbon-covered surfaces, carbide-phase formation, hydrogen (de)­intercalation, and particle coarsening) are found to occur during the reaction. The evolution of the catalysts features, however, has only minimal impact on the largely reversible patterns of reactivity. These findings demonstrate remarkable dynamic structural complexity within the mechanisms of alkane formation over both types of supported catalysts