In Situ X-ray Spectroscopy and Environmental TEM Study on Manganite Water Oxidation Catalysts

This thesis presents in situ studies on the active states of manganite water oxidation catalysts in water vapor. The study in chapter 2 probes the surface electronics of the strongly correlated Pr1-xCaxMnO3 (0<x<0.8) by in situ X-ray absorption and photoemission spectroscopy (XANES, XPS). Chapter 3 presents an environmental transmission electron microscopy (ETEM) study comparing the microscopic processes in Pr1-xCaxMnO3 (x = 0.1, 0.3) and the related layered Ruddlesden-Popper system Pr0.5Ca1.5MnO4. A discussion about gas phase electrochemistry and further information on the experimental techniques is provided in Chapter 1

In Situ XANES/XPS Investigation of Doped Manganese Perovskite Catalysts

Studying catalysts in situ is of high interest for understanding their surface structure and electronic states in operation. Herein, we present a study of epitaxial manganite perovskite thin films (Pr1−xCaxMnO3) active for the oxygen evolution reaction (OER) from electro-catalytic water splitting. X-ray absorption near-edge spectroscopy (XANES) at the Mn L- and O K-edges, as well as X-ray photoemission spectroscopy (XPS) of the O 1s and Ca 2p states have been performed in ultra-high vacuum and in water vapor under positive applied bias at room temperature. It is shown that under the oxidizing conditions of the OER a reduced Mn2+ species is generated at the catalyst surface. The Mn valence shift is accompanied by the formation of surface oxygen vacancies. Annealing of the catalysts in O2 atmosphere at 120 °C restores the virgin surfaces

An endstation for resonant inelastic X-ray scattering studies of solid and liquid samples.

A novel experimental setup is presented for resonant inelastic X-ray scattering investigations of solid and liquid samples in the soft X-ray region for studying the complex electronic configuration of (bio)chemical systems. The uniqueness of the apparatus is its high flexibility combined with optimal energy resolution and energy range ratio. The apparatus enables investigation of chemical analyses, which reflects the chemical imprints. The endstation is composed of a main sample chamber, a sample holder for either solid or liquid jet delivery system, and a soft X-ray grating spectrometer for 210–1250 eV with a resolving power of 1000. It combines for the first time liquid jet technology with a soft X-ray spectrometer based on the variable line spacing principle. This setup was commissioned at the soft X-ray beamline P04 at PETRA III of the Deutsches Elektronen-Synchrotron in Hamburg which is currently the most brilliant storage-ring-based X-ray radiation source in the world. The first results of liquid and solid samples show that this setup allows the detection of photons across an energy range of 300 eV. This covers simultaneously the emission lines of life- important elements like carbon, nitrogen and oxygen in a shot-based procedure

Environmental TEM Study of Electron Beam Induced Electrochemistry of Pr0.64Ca0.36MnO3 Catalysts for Oxygen Evolution

Environmental transmission electron microscopy (ETEVI) studies offer great potential for gathering atomic scale information on the electronic state of electrodes in contact with reactants. It also poses big challenges due to the impact of the high energy electron beam. In this article, we present an ETEM study of a Pr0.64Ca0.36MnO3 (PCMO) thin filth electrocatalySt for water splitting and Oxygen evolution in contact with water vapor: We show by means of off-axis electron holography and electrostatic modeling that the electron beam gives rise to a positive electric sample potential due to secondary electron emission. The value of the electric potential depends on the primary electron flux, the sample's electric Conductivity and grounding, and gas properties. We present evidence that two observed electrochemical reactions are driven by a beam induced electrostatic potential of the order of a volt. The first reaction is an anodic oxidation of oxygen depleted amorphous PCMO which results in recrystallization of the oxide. The Second reaction is oxygen evolution which can be detected by the oxidation of a silane additive and formation of SiO2-gamma at catalytically active surfaces. The quantification of beam induced potentials is an important step for future controlled electrochemical experiments in an ETEM

Environmental TEM Study of Electron Beam Induced Electrochemistry of Pr_0.64Ca_0.36MnO₃ Catalysts for Oxygen Evolution

Environmental transmission electron microscopy (ETEM) studies offer great potential for gathering atomic scale information on the electronic state of electrodes in contact with reactants. It also poses big challenges due to the impact of the high energy electron beam. In this article, we present an ETEM study of a Pr_0.64Ca_0.36MnO₃ (PCMO) thin film electrocatalyst for water splitting and oxygen evolution in contact with water vapor. We show by means of off-axis electron holography and electrostatic modeling that the electron beam gives rise to a positive electric sample potential due to secondary electron emission. The value of the electric potential depends on the primary electron flux, the sample’s electric conductivity and grounding, and gas properties. We present evidence that two observed electrochemical reactions are driven by a beam induced electrostatic potential of the order of a volt. The first reaction is an anodic oxidation of oxygen depleted amorphous PCMO which results in recrystallization of the oxide. The second reaction is oxygen evolution which can be detected by the oxidation of a silane additive and formation of SiO_2–<i>y</i> at catalytically active surfaces. The quantification of beam induced potentials is an important step for future controlled electrochemical experiments in an ETEM

Probing the Surface of La0.6Sr0.4MnO3 in Water Vapor by In Situ Photon In Photon Out Spectroscopy

Resonant inelastic X ray scattering RIXS is a promising method for elucidating detailed electronic structure of materials in a broad range of chemical and physical applications. Here, we use the fine fluorescence energy resolution of a RIXS spectrometer to obtain various proxies of the Mn L edge X ray absorption spectra XAS of the perovskite La0.6Sr0.4MnO3 LSMO as a model catalyst for the oxygen evolution reaction OER and evaluate the suitability for in situ surface studies of this electrocatalyst. We conclude that the inverse partial fluorescence yield IPFY of the O 2p 1s transition and the partial fluorescence yield of the 3s 2p transition 3s PFY are most suitable for determining changes at the surface of the perovskite, because distortions at grazing incidence measurements are low. In particular, the negligible angular dependence of the 3s PFY spectra can be perfectly simulated using a fluorescence model in the thin sample limit which is justified by low reabsorption of 3s photons. Remarkably, the 3s PFY reveals an influence of water vapor on the electronic reconstruction of the LSMO surface. Thus, our work paves the road for quantitative distortion free X ray spectroscopy of transition metal oxide surfaces under in situ conditions, which is needed to understand fundamental chemical processes such as corrosion and catalysi