Environmental-(S)TEM of dynamic Catalyst Nanostructures

The production of intelligently designed catalysts requires an understanding of, not only the specific reaction pathways, but also the effects of reactive species on catalyst materials. Gas composition, temperature and pressure can alter the physical characteristics of a catalyst material. The size, shape, crystallographic topography, chemical state and material composition of catalytic nanostructures can be highly dynamic under reaction conditions. E(S)TEM is uniquely suited to observe the structure of nanomaterials in real time under simulated reaction conditions. The oxidation of Ni nanoparticles was investigated using dynamic in-situ imaging in ETEM. These observations show a change in reaction mechanism as the Ni-NiO structure evolves over the course of the reaction. Initially, pyramidal NiO crystals grow on the Ni surface. The overlap and outward growth of these crystallites leads to an oxide shell around a metallic core. At a critical oxide thickness, the mechanism switches and further structural transformations are observed due to differential diffusion. Fast diffusion of cation vacancies lead to the formation of Kirkendall voids and ultimately hollow NiO structures. Nanoparticle based catalysts for industrial methanation reactions were studied using ESTEM. The study focuses on the origin of enhanced catalytic activity via the addition of a ceria promoter. Oxygen vacancies at the ceria surface were found to be active sites for the activation of carbon oxides. A correlation was found between decreasing promoter crystallite size and the enhancement of catalytic activity. The best promotional effect was observed when the ceria takes the form of a highly dispersed atomic-scale species. E(S)TEM was used to observe the evolution of Pd nanoparticles under redox conditions. Pd model systems utilising carbon, silica and alumina supports were tested. The amount of sintering was found to be determined by both: gas environment and the support material.