Quantitative Electron Microscopy Studies of Metal Nanoparticle Catalysts: Nanostructure, Support Interaction and Ageing Effects

Doctoral thesis, 2017

Heterogeneous catalysis plays a major role in modern society, for example in chemical production, sustainable energy production and emission control technologies. Metal nanoparticles (NPs) supported on oxide materials are frequent catalytic systems in this field. Although used and investigated for decades, open questions about the structure of supported catalysts and correlation with their catalytic properties remain. Some of these questions involve the three-dimensional structures of the catalysts, which become increasingly accessible by modern characterisation techniques, as well as the nanoscale structures down to the atomic level. In this work, we focused on both of these aspects. We developed a specimen preparation method to reveal the three-dimensional structures of supported NP catalysts using transmission electron microscopy (TEM). We also refined the imaging of the catalysts’ structures in the size range of a few nanometres down to individual atoms by using high-resolution dark-field scanning TEM (STEM) imaging, reaching a precision of 2 pm. Structural aspects that were investigated included sintering (e.g. coalescence) of NPs in realistic catalysts at different temperatures and in different gas atmospheres, as well as sintering of NPs on model systems to investigate the effect of support surface corrugation. We used the developed specimen preparation method to study the three-dimensional distribution of NPs on the oxide support in a realistic catalyst as a function of ageing temperature. The structural properties were correlated to the catalytic activity, which was evaluated using a continuous flow reactor and simulations. The interaction at the interface between NPs and different support materials was studied by STEM imaging. The high spatial precision of 2 pm enabled the measurement of strain distributions within supported NPs and at external interfaces. This work has given new insights into the detailed three-dimensional nanoscale structure of some of the most commonly used supported catalysts and improved the understanding of the relation between their structural properties and catalytic activity. The observation of interfacial strain indicates the possibility to tailor the catalytic activity by tuning the NP-support interaction.