Sophisticated IrO2(110)-based model electrodes are prepared by deposition of a 10 nm thick single-crystalline IrO2(110) layer supported on a structure directing RuO2(110)/Ru(0001) template, exposing a regular array of mesoscopic roof-like structures. With this model electrode together with the dedicated in-situ synchrotron based techniques (SXRD, XRR) and ex-situ characterization techniques (SEM, ToF-SIMS, XPS) the corrosion process of IrO2(110) in acidic environment is studied on different length scales. Potential-induced pitting corrosion starts at 1.48 V vs. SHE and is initiated at so-called surface grain boundaries, where three rotational domains of IrO2(110) meet. The most surprising results is, however, that even when increasing the electrode potential to 1.94 V vs. SHE still 60-70 % of the IrO2 film stays intact down to the mesoscale and atomic scale and no uniform thinning of the IrO2(110) layer is encountered. Neither flat IrO2(110) terraces nor single steps or grain boundaries, where only two rotational domains meet, are attacked. Ultrathin single-crystalline IrO2(110) layers seem to be much more stable in the anodic corrosion than hitherto expected.
Passivity determines corrosion resistance and stability of highly-alloyed stainless steels, and passivity breakdown is commonly believed to occur at a fixed potential due to formation and dissolution of Cr(VI) species. In this work, the study of a 25Cr-7Ni super duplex stainless steel in 1 M NaCl solution revealed that the passivity breakdown is a continuous degradation progress of the passive film over a potential range, associated with enhanced Fe dissolution before rapid Cr dissolution and removal of the oxide. The breakdown involves structural and compositional changes of the passive film and the underlying alloy surface layer, as well as selective metal dissolution depending on the anodic potential. The onset of passivity breakdown occurred at 1000 mV/ Ag/AgCl , and Fe dissolved more on the ferrite than the austenite phase. With increasing potential, the passive film became thicker but less dense, while the underlying alloy surface layer became denser indicating Ni and Mo enrichment. Rapid Cr dissolution occurred at ≥1300 mV/ Ag/AgCl .
Down
to a cathodic potentials of −1.20 V versus the reversible
hydrogen electrode, the structure of IrO2(110) electrodes
supported by TiO2(110) is found to be stable by in situ
synchrotron-based X-ray diffraction. Such high cathodic potentials
should lead to reduction to metallic Ir (Pourbaix diagram). From the
IrO2 lattice parameters, determined during cathodic polarization
in a H2SO4 electrolyte solution (pH 0.4), it
is estimated that the unit cell volume increases by 1% due likely
to proton incorporation, which is supported by the lack of significant
swelling of the IrO2(110) film derived from X-ray reflectivity
experiments. Ex situ X-ray photoelectron spectroscopy suggests that
protons are incorporated into the IrO2(110) lattice below
−1.0 V, although Ir remains exclusively in the IV+ oxidation
state down to −1.20 V. Obviously, further hydrogenation of
the lattice oxygen of IrO2(110) toward water is suppressed
for kinetic reasons and hints at a rate-determining chemical step
that cannot be controlled by the electrode potential.
We have developed an electrochemical cell for in situ 2-Dimensional Surface Optical Reflectance (2D-SOR) studies during anodization and cyclic voltammetry. The 2D-SOR signal was recorded from electrodes made of polycrystalline Al, Au(111), and Pt(100) single crystals. The changes can be followed at a video rate acquisition frequency of 200 Hz and demonstrate a strong contrast between oxidizing and reducing conditions. Good correlation between the 2D-SOR signal and the anodization conditions or the cyclic voltammetry current is also observed. The power of this approach is discussed, with a focus on applications in various fields of electrochemistry. The combination of 2D-SOR with other techniques, as well as its spatial resolution and sensitivity, has also been discussed.
Visualizing and measuring the gas distribution in close proximity to a working catalyst is crucial for understanding how the catalytic activity depends on the structure of the catalyst. However, existing methods are not able to fully determine the gas distribution during a catalytic process. Here we report on how the distribution of a gas during a catalytic reaction can be imaged in situ with high spatial (400 μm) and temporal (15 μs) resolution using infrared planar laser-induced fluorescence. The technique is demonstrated by monitoring, in real-time, the distribution of carbon dioxide during catalytic oxidation of carbon monoxide above powder catalysts. Furthermore, we demonstrate the versatility and potential of the technique in catalysis research by providing a proof-of-principle demonstration of how the activity of several catalysts can be measured simultaneously, either in the same reactor chamber, or in parallel, in different reactor tubes.
The anodic oxidation of metals such as aluminum and titanium can lead to the development of selfordering pores. These pores make excellent templates for a range of nanoscale objects with many applications in nanoscience. Theoretical studies on pore formation have proposed several models for the establishment, growth, and ordering of these pores; however, experimental verification has mostly been limited to ex situ measurements. Here we show that the lateral and vertical pore structure can be probed in situ with high precision, using grazing transmission X-ray scattering. By making use of the high flux available at modern synchrotrons and fitting only the difference between scattering patterns we show the nearly real-time evolution of the pore's arrangement. We observe no dependence on the substrate crystallographic orientation for domain size or pore separation. We do however observe an anisotropy in the oxide growth rate for the different substrate surfaces. This experimental approach can be applied to the study of a large variety of electrochemically produced materials such as magnetic nanowires, novel solar cell designs, and catalysts.
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