Publication date: 10th April 2024
Metal nanoparticles have attracted interest since decades due to their applications in catalysis, electrochemical reactions for energy conversion/storage and electronics. Redox exsolution is a cost- and time-efficient method to produce tunable metal particles. The exsolution involves four processes of diffusion, reduction, nucleation and growth. By exposure of perovskite oxides to H2 gas at high temperatures, the dopant cations diffuse from bulk to the surface and then get reduced and form metallic nanoparticles.
Electron microscopy techniques (e.g. TEM, SEM) are mainly used so far to study this process. However, they deliver only local information. To gain significant, macroscopic insights on the evolution of nanoparticles via exsolution, we designed and constructed an in-situ heating setup for X-ray scattering experiments. At the sample position, a maximum temperature of 1000 °C can be reached using a piece of customized ceramic heater. We performed exsolution experiments at different temperatures to study the kinetics (activation energy) of this process. The sample is connected to a gas-mixing unit to control the gas inlet and the outcoming gas is analyzed by a mass spectrometer. By switching the gases for reduction and oxidation procedures, we followed the exsolution and reincorporation/regeneration of nanoparticles.
This setup can be easily transferred to different synchrotron beamlines for various experiments. Using combined Small- and Wide-Angle X-Ray Scattering (SAXS/WAXS) techniques, we can follow the particle growth while the crystal structure of the perovskite matrix changes. Using X-ray absorptions spectroscopy method, we can obtain information about the valence state of the metallic phase during the reduction and re-oxidation processes. These X-ray techniques allow us to better understand the reaction mechanism and the structural modifications of the catalytic materials.
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