Effects of hydrothermal aging on co and no oxidation activity over monometallic and bimetallic pt‐pd catalysts

By combining scanning transmission electron microscopy, CO chemisorption, and energy dispersive X-ray spectroscopy with CO and NO oxidation light-off measurements we investigated deactivation phenomena of Pt/Al$_{2}$O$_{3}$, Pd/Al$_{2}$O$_{3}$, and Pt-Pd/Al$_{2}$O$_{3}$ model diesel oxidation catalysts during stepwise hydrothermal aging. Aging induces significant particle sintering that results in a decline of the catalytic activity for all catalyst formulations. While the initial aging step caused the most pronounced deactivation and sintering due to Ostwald ripening, the deactivation rates decline during further aging and the catalyst stabilizes at a low level of activity. Most importantly, we observed pronounced morphological changes for the bimetallic catalyst sample: hydrothermal aging at 750 °C causes a stepwise transformation of the Pt-Pd alloy via core-shell structures into inhomogeneous agglomerates of palladium and platinum. Our study shines a light on the aging behavior of noble metal catalysts under industrially relevant conditions and particularly underscores the highly complex transformation of bimetallic Pt-Pd diesel oxidation catalysts during hydrothermal treatmen

Contrast of Backscattered Electron SEM Images of Nanoparticles on Substrates with Complex Structure

This study is concerned with backscattered electron scanning electron microscopy (BSE SEM) contrast of complex nanoscaled samples which consist of SiO2 nanoparticles (NPs) deposited on indium-tin-oxide covered bulk SiO2 and glassy carbon substrates. BSE SEM contrast of NPs is studied as function of the primary electron energy and working distance. Contrast inversions are observed which prevent intuitive interpretation of NP contrast in terms of material contrast. Experimental data is quantitatively compared with Monte-Carlo- (MC-) simulations. Quantitative agreement between experimental data and MC-simulations is obtained if the transmission characteristics of the annular semiconductor detector are taken into account. MC-simulations facilitate the understanding of NP contrast inversions and are helpful to derive conditions for optimum material and topography contrast

Nature and Functionality of La0.58_{0.58}Sr0.4_{0.4}Co0.2_{0.2}Fe0.8_{0.8}O3−δ_{3-δ} / Gd0.2_{0.2}Ce0.8_{0.8}O2−δ_{2-δ} / Y0.16_{0.16}Zr0.84_{0.84}O2−δ_{2-δ} Interfaces in SOFCs

Interdiffusion phenomena and secondary phase formation at the interface La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF) / Gd0.2Ce0.8O2-δ (GDC) / Y0.16Zr0.84O2-δ (YSZ) are correlated to linear and non-linear losses in symmetrical and full SOFC cells. FIB/SEM (focussed ion beam / scanning electron microscopy) tomography is applied for determining the local distribution of the primary phases LSCF, GDC, and YSZ and elemental analysis via STEM/EDXS (scanning transmission electron microscopy / energy dispersive X-ray spectroscopy) provides information on the secondary phase SrZrO3 (SZO) and the interdiffusion between GDC and YSZ (ID). This reveals the effect of GDC co-sintering temperature (varied from 1100°C to 1400°C), alongside the sintering of LSCF at 1080°C, on these multi-layered microstructures. Electrochemical impedance spectra on symmetrical cells show that the polarization resistance (ASRcat) of the cathode/electrolyte interface is pronouncedly affected by three orders of magnitude, changing the overall power density of anode supported SOFC single cells at 0.8V by as much as a factor of 20. In conclusion, the chemical composition of the ID has a tremendous impact on the electrochemical efficiency of the investigated LSCF/GDC/YSZ interface, and processing parameters of anode supported cells with LSCF cathode have to be carefully chosen for individual SOFC cell concepts

Low friction of metallic multilayers by formation of a shear-induced alloy

During sliding of metallic surfaces, the near surfaces undergo significant changes in terms of topography, composition and microstructure. Since friction and wear behavior of the materials are strongly influenced by sub-surface deformations, it is fundamental to investigate these effects. Therefore, the present study aims towards a better understanding of the behavior of friction depending on well-defined initial microstructures. By performing sliding experiments on Au-Ni multilayer samples under ultrahigh vacuum (UHV) conditions, we observe that the individual layer thickness of multilayer systems has a strong influence on friction behavior due to the transition in the dominant deformation mechanism near the surface. The experiments reported here provide a new route for lowering the friction force of metallic material systems in dry contact by providing more stable microstructures and alloy formation. Through ultrafine grains present in the alloy formed by mechanical mixing the number of grain boundaries strongly increases and hence, grain boundary-mediated deformation results in the low friction coefficient

Multi-scale characterization of ceramic inert-substrate-supported and co-sintered solid oxide fuel cells

Understanding cell performance is essential for selecting cell components and the processing parameters for solid oxide fuel cells. The scale of relevant microstructural features in electrodes, electrolyte and supporting substrate covers several orders of magnitude. This contribution will demonstrate how advanced correlative multi-scale tomography can be used to identify those parameters: ranging from millimeter to nanometer scale. We employ optical microscopy, X-ray computed tomography (μ-CT), focused ion beam-scanning electron microscopy tomography and energy-dispersive X-ray spectroscopy– scanning transmission electron microscopy. Additional investigations by selected area electron diffraction allow a determination of the underlying crystal structures. An SOFC design based on the co-sintering of an inert substrate with various functional layers on top is used as a blueprint, allowing further methodological development. The effect of interdiffusion between phases and development of secondary phases on microstructure and chemical composition will be shown. Furthermore, porosity and tortuosity extracted individually from all porous layers will allow modeling of gas diffusion loss contributions within the co-fired cell structure. This exemplifies how correlative tomography helps to understand specific contributions to overall cell performance

Grain-size dependence of the deterioration of oxygen transport for pure and 3 mol% Zr-doped Ba0.5Sr0.5Co0.8Fe0.2O3-δ induced by thermal annealing

In this study, the influence of long-term annealing at intermediate temperatures on oxygen transport of Ba0.5Sr0.5Co0.8Fe0.2O3 d (BSCF) and 3 mol% Zr-doped BSCF (BSCF-Z3) ceramics with different grain sizes was studied by means of in situ electrical conductivity relaxation (ECR) measurements. Ceramics with different grain sizes in the range of 3–80 mm were obtained by varying the temperature and dwell time during sintering. For both compositions, the apparent values of the chemical diffusion coefficient Dchem and surface exchange coefficient kchem extracted from the data of ECR measurements are found to decrease with the time of annealing. The strongly correlated decreases in Dchem and kchem, being greater in magnitude at a lower grain size and temperature, are observed significantly more pronounced for BSCF than for BSCF-Z3. The results from microstructural analysis provide a clear rationale for the observations from ECR. High-resolution transmission electron microscopy (TEM) images recorded before and after annealing under pure oxygen at 700 C for 13 d show excessive formation of hexagonal and plate-like lamellar precipitates at the grain boundaries and in the interior of grains of BSCF ceramics during the annealing process, whilst secondary phase formation is restricted solely to the grain boundary regions in BSCF-Z3. The possible importance of grain boundaries in determining the oxygen surface exchange kinetics is emphasized

Impact of the Support on the Catalytic Performance, Inhibition Effects and SO2SO_{2} Poisoning Resistance of Pt-Based Formaldehyde Oxidation Catalysts

Formaldehyde emissions in addition to the methane-slip have recently been addressed for natural gas fueled combustion engines and thus require an aftertreatment catalyst system. In this work, two Pt-based catalysts supported on Al2O3 and TiO2–SiO2 were investigated by considering their activity, inhibition and stability towards SO2-poisoning. The catalysts were analyzed by X-ray diffraction, electron microscopy, BET surface area, ex situ and operando X-ray absorption spectroscopy, which revealed differences in the oxidation state of Pt depending on the support material and that slightly reduced Pt sites seem to be the active species for this reaction. Systematic activity tests showed that NOx presence leads to a significant inhibition, possibly due to oxidation of the active species, whereas CO addition promotes formaldehyde oxidation above the onset temperature of CO oxidation. These results were supported by spatially resolved concentration profiles along the channel of a catalyst coated monolith. Addition of small amounts of SO2 to the gas mixture resulted in a complete loss of the low-temperature activity, slightly more pronounced for the TiO2–SiO2 supported catalyst

Nature and functionality of La0.58Sr0.4Co0.2Fe0.8O3-delta/Gd0.2Ce0.8O2-delta/Y0.16Zr0.84O2-delta interfaces in SOFCs

Interdiffusion phenomena and secondary phase formation at the interface La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF) / Gd0.2Ce0.8O2-δ (GDC) / Y0.16Zr0.84O2-δ (YSZ) are correlated to linear and non-linear losses in symmetrical and full SOFC cells. FIB/SEM (focussed ion beam / scanning electron microscopy) tomography is applied for determining the local distribution of the primary phases LSCF, GDC, and YSZ and elemental analysis via STEM/EDXS (scanning transmission electron microscopy / energy dispersive X-ray spectroscopy) provides information on the secondary phase SrZrO3 (SZO) and the interdiffusion between GDC and YSZ (ID). This reveals the effect of GDC co-sintering temperature (varied from 1100°C to 1400°C), alongside the sintering of LSCF at 1080°C, on these multi-layered microstructures. Electrochemical impedance spectra on symmetrical cells show that the polarization resistance (ASRcat) of the cathode/electrolyte interface is pronouncedly affected by three orders of magnitude, changing the overall power density of anode supported SOFC single cells at 0.8V by as much as a factor of 20. In conclusion, the chemical composition of the ID has a tremendous impact on the electrochemical efficiency of the investigated LSCF/GDC/YSZ interface, and processing parameters of anode supported cells with LSCF cathode have to be carefully chosen for individual SOFC cell concepts