• Simonsen, Søren Bredmose

Abstract

This thesis presents a fundamental study of the sintering of supported nanoparticles in relation to<br/>diesel oxidation catalysts. The sintering of supported nanoparticles is an important challenge in<br/>relation to this catalyst, as well as many other catalyst systems, and a fundamental understanding of<br/>the sintering mechanisms of nanoparticles is important for making improvements to their long term<br/>catalytic activity.<br/>Diesel oxidation catalysts are usually composed of noble metal nanoparticles on a complex<br/>three-dimensional high surface area oxide. The complex support structure makes it difficult to<br/>directly observe dynamical processes such as particle sintering with the present state of the art<br/>microscope techniques, and consequently it is difficult to relate experimental observations and<br/>theoretical sintering models. To reduce the complexity, the present study uses planar model<br/>catalysts. These are composed of Pt, Pd and bimetallic Pt-Pd nanoparticles supported on a flat and<br/>homogeneous Al2O3 or SiO2 surface. By using in situ TEM on the planar model catalysts it was<br/>possible to directly monitor the detailed dynamical changes of the individual nanoparticles during<br/>exposure to oxidizing conditions at elevated temperatures. The time-resolved TEM images are<br/>presented and these offer direct insight into the fundamental dynamics of the sintering process at the<br/>nano-scale.<br/>For Pt, Pd and bimetallic Pt-Pd nanoparticles it is shown that the sintering process is governed by<br/>the Ostwald ripening mechanism in an oxidizing environment. The observations compare well with<br/>predictions from mean-field kinetic models for ripening, but deviations are revealed for the timeevolution<br/>for the individual nanoparticles. A better description of the individual nanoparticle<br/>ripening is obtained by kinetic models that include local correlations between neighbouring<br/>nanoparticles in the atom-exchange process. The sintering process was also presented statistically<br/>by particle size distributions extracted from the TEM images. The statistical data agreed only partly<br/>with the mean-field kinetic models for ripening, but the deviations could be accounted for by<br/>including more detailed information into the models, such as an observed size-dependence of the<br/>three-dimensional shape of the supported nanoparticles and the local correlations between the<br/>nanoparticles.<br/>

Topics

• nanoparticle
• impedance spectroscopy
• surface
• transmission electron microscopy
• sintering
• Ostwald ripening