Atomic-level Electron Microscopy of Metal and Alloy Electrocatalysts

Research output: Book/ReportPh.D. thesis – Annual report year: 2015Research

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This thesis presents the application of transmission electron microscopy techniques towards the characterisation of novel metal nanoparticle catalysts. Two main subjects have been covered: first, the sintering-resistance behaviour of monomodal mass-selected Pt cluster catalysts have been studied by means of ex situ Scanning Transmission Electron Microscopy (STEM) in combination with in situ indirect nanoplasmonic sensing. Secondly, electron microscopy imaging and spectroscopy have been used for the characterisation of novel metal alloy nanoparticle electrocatalysts for the Oxygen Reduction Reaction (ORR) and for the electrochemical production of hydrogen peroxide. This has been done in the context of an extensive investigation including electrochemical measurements, theoretical calculations, and surface science studies.
Pt cluster catalysts of selected mass have been deposited on different flat surfaces and exposed to different sintering conditions. Ex situ STEM imaging has been used to monitor the variation of the particle dimensions through the analysis of particle area distributions. Clusters with a monomodal size distribution exhibited intrinsic sintering resistance on different supports and under different reaction conditions, in contrast to cluster with broader and bimodal size distributions. The reason for the stability has been assigned to the suppression of the Ostwald ripening mechanism. In Ostwald ripening, mass transport from small particles to bigger ones is driven by the difference in their chemical potential. By mass-selecting the clusters, sharp and narrow particle distributions are achieved and the driving force for Ostwald ripening mechanism is eliminated.
PtxY and PtxGd nanoparticles exhibit electrochemical activity among the most active catalysts for the ORR, more than 5 times higher activity compared to the best commercial Pt nanoparticles. Furthermore, they both retain more than 60% of the activity after accelerated ageing tests and they still perform more than 3 times better than Pt. By employing STEM X-ray Energy Dispersive Spectroscopy (EDS) spectrum imaging, the elemental distribution of the PtxY, before and after the electrochemical tests, has been determined. A core-shell structure is formed after the ORR chemical treatment, with an alloyed core embedded by a ~1 nm Pt-rich shell, due to the segregation of the Y from the first few atomic layers of the particle and its dissolution into the electrolyte. The formed Pt-rich shell prevents further dissolution of the rare earth metal protecting the alloyed core.
Pt−Hg and Pd−Hg have been identified by Density Functional Theory (DFT) calculations as promising candidates for the electrochemical production of hydrogen peroxide H2O2. The active surface is predicted to be formed by reactive Pt or Pd atoms surrounded by more inert Hg atoms. Electrochemical measurements on the two catalysts have shown performance exceeding the current state-of-the-art in both forms of extended surface and nanoparticles. Electron microscopy has been used to elucidate the structure and composition of the nanoparticle alloys to confirm the theoretical predictions. In Pt−Hg nanoparticles, EDS spectroscopy has indicated the presence of both elements on single particles, although high-resolution imaging has shown particles where pure Pt was the only matching structure. In the case of Pd−Hg, a core-shell structure has been found, with a pure Pd core and a Pd-Hg shell. Through atomic resolution STEM, the structure of the alloy in the shell of different particles has been revealed, showing the formation of an ordered alloy structure.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark (DTU)
Number of pages184
Publication statusPublished - 2014
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