(Invited) In Situ Small-Angle X-Ray Scattering for the Analysis of Electrochemical Degradation of Metal Oxide Supported Pt Nanoparticles
We have performed electrochemical in situ anomalous SAXS experiments, for the first time, on metal oxide supported Pt nanoparticles. The support consisted of a mixed iridium oxide-titanium oxide (IrO2-TiO2). The anomalous changes of the elemental scattering cross sections around the Pt LIII and the Ir LIII absorption edges were used to separate the Pt scattering contribution from the support scattering. This approach of anomalous SAXS was previously used for in situ studies of conventional carbon supported Pt [2,3]. Our results not only show that it is possible in this way to obtain a precise net Pt scattering signal even in the presence of a strongly scattering support material, but they also demonstrate clearly the existence of a previously neglected scattering interference effect due to the spatial correlations between Pt nanoparticles and support particles. This effect can become very strong for support materials containing high-Z elements like the IrO2-TiO2 support investigated in this study. However, we could also show that these particle-support interferences are non-negligible even for weakly scattering carbon supports. We developed a novel analytical tool to take the particle-support interferences into account in the data fitting procedure with high accuracy . This approach highly improves the quantitative analytical power of SAXS studies on supported catalyst materials.
We applied the novel method to the analysis of in situ SAXS data from Pt/IrO2-TiO2 to study the degradation properties of this ORR catalyst during harsh electrochemical potential cycling between 0.5 V and 1.5 V vs. RHE in 0.1 M HClO4. This protocol simulates the oxidative conditions during PEFC start/stop cycles . The results not only reveal a Pt particle growth due to electrochemical Ostwald ripening and a concomitant Pt mass loss due to dissolution, but also indicate a possible influence of the X-ray beam on the catalyst degradation. Hence, on the one hand, the X-ray exposure time of the catalyst must be chosen long enough in order to achieve a good signal-to-noise ratio, but on the other hand, it must be short enough to prevent X-ray induced catalyst degradation. In summary, our findings pave the way to a detailed and quantitative in situ analysis of Pt nanoparticle degradation on various metal oxide support materials.
This work was supported by CCEM Switzerland and Umicore AG & Co KG within the project DuraCat. We acknowledge the Paul Scherrer Institut, Villigen, Switzerland, for provision of synchrotron radiation beamtime at the cSAXS beamline of the SLS.
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