The field of catalysis is of paramount importance. Catalysts allow chemical reactions to be carried out with lower energy than otherwise possible. They play an important role in the reduction of harmful emissions from todays vehicles and are crucial for designing alternative energy sources for tomorrows vehicles. Many catalysts take the form of nano-sized particles which brings about challenges for their design and characterisation. In this thesis, aberration corrected electron microscopy is utilised for atomically resolved investigations of the structure of catalytically active nanoparticles as freshly produced and to provide insights of deactivation in treated and used catalysts.
Platinum and palladium nanoparticles are used for the reduction of harmful emissions from diesel car exhausts. Here, fresh insights are provided into the loss of activity in genuine road aged diesel oxidation catalysts. They include the segregation of alloys in the bimetallic variant in which the less active palladium moves to the surface where it can more easily form an oxide. Nano-beam diffraction was used in this study as well as for a model platinum system. The seldom used nano-beam diffraction technique was employed to provide additional structural information on very small nanoparticles, including those that contained defects. Using nano-beam diffraction, no oxides were found in the platinum model catalyst and loss of activity was due to sintering.
Ex-situ studies can only provide before and after information. Here, results from the latest developments in environmental scanning transmission electron microscopy are presented with model catalysts, namely atomically dispersed platinum and palladium. Single atoms of the metals were observed at temperatures as high as 500 degree-celsius and in O2 which represents the current state of the art in this field. The limitations of the Z contrast technique is also investigated for heavy atoms located on heavy supports, such as atomically dispersed gold on ceria.
Intricacies in the reduction of Co3O4 to CoO are provided using in-situ transmission electron microscopy in H2 at elevated temperatures. A new fuse wire like transformation is seen in large crystals in addition to dislocations to accomodate strain into the crystal structure
