In situ imaging of gold nanocrystals during the CO oxidation reaction studied by Bragg Coherent Diffraction Imaging

Author(s)

Ana Flávia Suzana

• Document type: Doctoral Thesis
• Press: Araraquara. 2019-04-22.
• Institution: Universidade Estadual Paulista (Unesp). Instituto de Química. Araraquara
• Defense date: 2019-03-26
• Advisor: Sandra Helena Pulcinelli; Florian Meneau

Abstract

The fundamental aim of heterogeneous catalysis research is to understand mechanisms at the nanoparticle level, and then to design and synthesise catalysts with desired active sites. In this regard, the in situ/operando characterisation of defects is crucial as they are preferential catalytic sites for the reaction occurrence. The main part of this thesis was the investigation of the morphology and structure evolution of gold nano-catalysts supported on titanium dioxide. We worked with different methods of preparation (seed-growth, ionic liquid stabilizer) to synthesise gold catalysts with controlled sizes. Those catalytic materials were evaluated for the model CO oxidation reaction, chosen for its environmental relevance and “simplicity” to be reproducible within our X-ray imaging study. We used the Bragg Coherent Diffraction Imaging technique to follow in situ the 3D morphology changes under catalytic reaction conditions. A dedicated in situ heating reactor has been built. We correlated the 3D displacement field and strain distribution of the gold nanoparticles to the catalytic properties of the material. In particular, for a 120 nm gold nanoparticle, we quantified under working conditions the adsorbate-induced surface stress on the gold nanocrystal, which leads to restructuration and defects identified as a nanotwin network. Finally, by using scanning X-ray nanodiffraction to characterise few tenths of nanometers single gold nanorods, we were able to obtain a strain map over the Bragg well oriented gold crystals. This characterisation revealed the presence of a strain pattern encoded in the nanorods structure consistent with the so-called Rayleigh Instability. This kind of study proved to be very important as the presence of instability does affect the properties of nanorods. (AU)

• FAPESP's process: 14/27127-3 - Genesis of gold nanoparticles: nucleation, growth and stabilization
• Grantee: Ana Flávia Suzana
• Support Opportunities: Scholarships in Brazil - Doctorate