Ambient Pressure XPS Study of Mixed Conducting Perovskite-type SOFC Cathode and Anode Materials under Well-Defined Electrochemical Polarization

The oxygen exchange activity of mixed conducting oxide surfaces has been widely investigated, but a detailed understanding of the corresponding reaction mechanisms and the rate-limiting steps is largely still missing. Combined in situ investigation of electrochemically polarized model electrode surfaces under realistic temperature and pressure conditions by near-ambient pressure (NAP) XPS and impedance spectroscopy enables very surface-sensitive chemical analysis and may detect species that are involved in the rate-limiting step. In the present study, acceptor-doped perovskite-type La0.6Sr0.4CoO3-δ (LSC), La0.6Sr0.4FeO3-δ (LSF), and SrTi0.7Fe0.3O3-δ (STF) thin film model electrodes were investigated under well-defined electrochemical polarization as cathodes in oxidizing (O2) and as anodes in reducing (H2/H2O) atmospheres. In oxidizing atmosphere all materials exhibit additional surface species of strontium and oxygen. The polaron-type electronic conduction mechanism of LSF and STF and the metal-like mechanism of LSC are reflected by distinct differences in the valence band spectra. Switching between oxidizing and reducing atmosphere as well as electrochemical polarization cause reversible shifts in the measured binding energy. This can be correlated to a Fermi level shift due to variations in the chemical potential of oxygen. Changes of oxidation states were detected on Fe, which appears as FeIII in oxidizing atmosphere and as mixed FeII/III in H2/H2O. Cathodic polarization in reducing atmosphere leads to the reversible formation of a catalytically active Fe0 phase

Production of a dual-species Bose-Einstein condensate of Rb and Cs atoms

We report the simultaneous production of Bose-Einstein condensates (BECs) of $^{87}$Rb and $^{133}$Cs atoms in separate optical traps. The two samples are mixed during laser cooling and loading but are separated by $400 \mu$m for the final stage of evaporative cooling. This is done to avoid considerable interspecies three-body recombination, which causes heating and evaporative loss. We characterize the BEC production process, discuss limitations, and outline the use of the dual-species BEC in future experiments to produce rovibronic ground state molecules, including a scheme facilitated by the superfluid-to-Mott-insulator (SF-MI) phase transition

Comparison of novel Ni doped exsolution perovskites as methane dry reforming catalysts

Three perovskite-type materials with a different amount of B-site Ni doping have been tested for their catalytic performance during me-thane dry reforming (MDR) followed by characterization with X-ray dif-fraction (XRD) and scanning electron microscopy (SEM). They could be activated via a reductive treatment (either during a pre-reduction step or di-rectly in reducing reaction atmosphere), the main activating mechanism be-ing the formation of Ni nanoparticles on the surface by exsolution. The catalytic activity increased with the particle size and density. The particle distribution properties could be improved by increasing the amount of Ni doping from 3 % to 10 %, by using an A-site sub-stoichiometric perovskite and by choosing a higher annealing temperature during material prepara-tion. A deactivation over time was observed, due to segregation of CaCO3 on the surface, but no coking or particle sintering occurre

Combined UHV/high-pressure catalysis setup for depth-resolved near-surface spectroscopic characterization and catalytic testing of model catalysts

An ultra-high vacuum (UHV) setup for “real” and “inverse” model catalyst preparation, depth-resolved near-surface spectroscopic characterization, and quantification of catalytic activity and selectivity under technologically relevant conditions is described. Due to the all-quartz reactor attached directly to the UHV-chamber, transfer of the catalyst for in situ testing without intermediate contact to the ambient is possible. The design of the UHV-compatible re-circulating batch reactor setup allows the study of reaction kinetics under close to technically relevant catalytic conditions up to 1273 K without contact to metallic surfaces except those of the catalyst itself. With the attached differentially pumped exchangeable evaporators and the quartz-microbalance thickness monitoring equipment, a reproducible, versatile, and standardised sample preparation is possible. For three-dimensional near-surface sample characterization, the system is equipped with a hemispherical analyser for X-ray photoelectron spectroscopy (XPS), electron-beam or X-ray-excited Auger-electron spectroscopy, and low-energy ion scattering measurements. Due the dedicated geometry of the X-ray gun (54.7°, “magic angle”) and the rotatable sample holder, depth analysis by angle-resolved XPS measurements can be performed. Thus, by the combination of characterisation methods with different information depths, a detailed three-dimensional picture of the electronic and geometric structure of the model catalyst can be obtained. To demonstrate the capability of the described system, comparative results for depth-resolved sample characterization and catalytic testing in methanol steam reforming on PdGa and PdZn near-surface intermetallic phases are shown

Ambient Pressure XPS Study of Mixed Conducting Perovskite Type SOFC Cathode and Anode Materials under Well Defined Electrochemical Polarization

The oxygen exchange activity of mixed conducting oxide surfaces has been widely investigated, but a detailed understanding of the corresponding reaction mechanisms and the rate-limiting steps is largely still missing. Combined in situ investigation of electrochemically polarized model electrode surfaces under realistic temperature and pressure conditions by near-ambient pressure (NAP) XPS and impedance spectroscopy enables very surface-sensitive chemical analysis and may detect species that are involved in the rate-limiting step. In the present study, acceptor-doped perovskite-type La_0.6Sr_0.4CoO_3‑δ (LSC), La_0.6Sr_0.4FeO_3‑δ (LSF), and SrTi_0.7Fe_0.3O_3‑δ (STF) thin film model electrodes were investigated under well-defined electrochemical polarization as cathodes in oxidizing (O₂) and as anodes in reducing (H₂/H₂O) atmospheres. In oxidizing atmosphere all materials exhibit additional surface species of strontium and oxygen. The polaron-type electronic conduction mechanism of LSF and STF and the metal-like mechanism of LSC are reflected by distinct differences in the valence band spectra. Switching between oxidizing and reducing atmosphere as well as electrochemical polarization cause reversible shifts in the measured binding energy. This can be correlated to a Fermi level shift due to variations in the chemical potential of oxygen. Changes of oxidation states were detected on Fe, which appears as Fe^III in oxidizing atmosphere and as mixed Fe^II/III in H₂/H₂O. Cathodic polarization in reducing atmosphere leads to the reversible formation of a catalytically active Fe⁰ phase

Enhancing Electrochemical Water Splitting Kinetics by Polarization Driven Formation of Near Surface Iron 0 An In Situ XPS Study on Perovskite Type Electrodes

In the search for optimized cathode materials for high-temperature electrolysis, mixed conducting oxides are highly promising candidates. This study deals with fundamentally novel insights into the relation between surface chemistry and electrocatalytic activity of lanthanum ferrite based electrolysis cathodes. For this means, near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and impedance spectroscopy experiments were performed simultaneously on electrochemically polarized La0.6Sr0.4FeO3−δ (LSF) thin film electrodes. Under cathodic polarization the formation of Fe0 on the LSF surface could be observed, which was accompanied by a strong improvement of the electrochemical water splitting activity of the electrodes. This correlation suggests a fundamentally different water splitting mechanism in presence of the metallic iron species and may open novel paths in the search for electrodes with increased water splitting activity

Water splitting on model-composite La_0.6Sr_0.4FeO_3-δ (LSF) electrodes in H₂/H₂O atmosphere

Mixed conducting cathodes for solid oxide electrolysis cells (SOECs) offer a promising alternative to the nowadays used Ni/YSZ cermet. Here, the water splitting kinetics of mixed conducting perovskite-type La0.6Sr0.4FeO3-δ (LSF) thin film electrodes was investigated in humid reducing atmospheres at 600 – 650 °C. Under equilibrium conditions an area specific surface resistance of ca. 15 Ωcm2 was obtained on freshly prepared electrodes. Upon cathodic polarization of more than 20 mV a strong decrease of the surface resistance was observed. This acceleration of the water splitting kinetics was accompanied by the formation of metallic iron particles on the LSF surface, which was observed by means of near-ambient pressure XPS experiments

In situ NAP XPS spectroscopy during methane dry reforming on ZrO2 Pt 111 inverse model catalyst

Due to the need of sustainable energy sources, methane dry reforming is a useful reaction for conversion of the greenhouse gases CH4 and CO2 to synthesis gas (CO + H2). Syngas is the basis for a wide range of commodity chemicals and can be utilized for fuel production via Fischer-Tropsch synthesis. The current study focuses on spectroscopic investigations of the surface and reaction properties of a ZrO2/Pt inverse model catalyst, i.e. ZrO2 particles (islands) grown on a Pt(1 1 1) single crystal, with emphasis on in situ near ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) during MDR reaction. In comparison to technological systems, model catalysts facilitate characterization of the surface (oxidation) state, surface adsorbates, and the role of the metal-support interface. Using XPS and infrared reflection absorption spectroscopy we demonstrated that under reducing conditions (UHV or CH4) the ZrO2 particles transformed to an ultrathin ZrO2 film that started to cover (wet) the Pt surface in an SMSI-like fashion, paralleled by a decrease in surface/interface oxygen. In contrast, (more oxidizing) dry reforming conditions with a 1:1 ratio of CH4 and CO2 were stabilizing the ZrO2 particles on the model catalyst surface (or were even reversing the strong metal support interaction (SMSI) effect), as revealed by in situ XPS. Carbon deposits resulting from CH4 dissociation were easily removed by CO2 or by switching to dry reforming conditions (673-873 K). Thus, at these temperatures the active Pt surface remained free of carbon deposits, also preserving the ZrO2/Pt interface

Water Splitting on Model Composite La0.6Sr0.4FeO3 d LSF Electrodes in H2 H2O Atmosphere

Mixed conducting cathodes for solid oxide electrolysis cells (SOECs) offer a promising alternative to the nowadays used Ni/YSZ cermet. Here, the water splitting kinetics of mixed conducting perovskite-type La_0.6Sr_0.4FeO_3-δ (LSF) thin film electrodes was investigated in humid reducing atmospheres at 600 – 650 °C. Under equilibrium conditions an area specific surface resistance of ca. 15 Ωcm² was obtained on freshly prepared electrodes. Upon cathodic polarization of more than 20 mV a strong decrease of the surface resistance was observed. This acceleration of the water splitting kinetics was accompanied by the formation of metallic iron particles on the LSF surface, which was observed by means of near-ambient pressure XPS experiments