Aberration corrected in-situ electron microscopy of nanoparticle catalysts

• Main Author: Walsh, Michael J.
• Other Authors: Ed, Boyes ; Pratibha, Gai
• Published: University of York 2012
• Subjects: 541.39
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.559115

Catalysts play a crucial role in much of the world’s energy, chemical processing and manufacturing technologies, whilst the development of more efficient, economical and durable catalysts is a prerequisite for the widespread introduction of future energy solutions such as fuel cells and biofuels. In this thesis, aberration corrected in-situ electron microscopy is used to provide atomic scale insights into the structure-property relationships of catalytic nanoparticles, as well as the deactivation mechanisms that affect them under reaction conditions. When reduced to just a few nanometres, gold nanoparticles have been reported to show remarkable catalytic activity for the low temperature oxidation of carbon monoxide. In this size range, one of the most energetically favourable morphologies is the decahedra, and through direct measurements of atomic column positions, we quantify the substantial inherent surface strain that results from the non-space filling structure. Density functional theory calculations based on the experimentally observed atomic displacements predict significantly enhanced activity for CO oxidation due to strain induced electronic band structure modifications. This is a new mechanism for the reactivity of gold nanoparticles and provides further explanation of the surprising activities reported. Exceptional catalytic properties for the water-gas shift reaction have been reported for cationic gold supported on ceria, although the nature of the active species and its interaction with the ceria support is uncertain. Atomic resolution Z contrast images reveal significant intensity increases for certain atomic columns’, suggesting the cationic Au is in the form of highly dispersed single atoms that substitute for Ce sites. The activation of Ni catalysts is observed in-situ, and a size dependent defect reduction mechanism is suggested. Upon reduction, the Ni particles are observed to sinter via an Ostwald ripening mechanism, and the effect of surface energetics is discussed to explain the variety of particle stabilities observed.