Development of Low-Temperature Ammonia Cracking CatalystsTools Young, Benjamin (2025) Development of Low-Temperature Ammonia Cracking Catalysts. PhD thesis, University of Nottingham.
AbstractHydrogen shows promise as the energy vector of the future, but problems with storage and transport are significant. Storage of hydrogen as ammonia has the potential to solve these problems, but current catalysts for its cracking are not efficient enough to enable the large-scale application of ammonia. Carbon materials, such as carbon nanotubes (CNTs), have shown potential as supports for ammonia decomposition catalysts. This thesis investigates the use of graphitised nanofibers (GNFs), which offer high purity and graphitisation, as a support material for Ru catalysts. Ru/GNF was synthesised using magnetron sputtering and tested for catalytic activity in ammonia decomposition and the catalyst exhibited self-improvement over the course of the reaction. The evolution of the Ru nanoclusters on GNF was studied by Identical Location Scanning Transmission Electron Microscopy (IL-STEM). The analysis revealed that the Ru nanoclusters undergo significant morphological changes during the reaction - transforming from flat and amorphous structures to more three-dimensional crystalline nanoclusters. The step-edges on the GNF surface help to stabilise the Ru nanoclusters, preventing excessive growth and maintaining a high density of active sites. Spectroscopic analysis using in-operando EXAFS and ex-situ XPS provide further insights into the mechanism behind the self-improvement. EXAFS data suggest that the Ru nanoparticles undergo bulk nitridation during the reaction. This is supported by XPS analysis, which confirms the formation of a metal nitride species. It is proposed that the formation of bulk nitrided Ru nanoclusters leads to a change in the reaction mechanism, increasing the number of active sites and enhancing the catalyst’s activity. This thesis highlights the importance of studying the dynamic behaviour of catalysts and provides an understanding of the self-improvement mechanism in Ru/GNF. This knowledge can contribute to the design of more efficient and stable catalysts for low-temperature ammonia cracking, advancing sustainable hydrogen production technologies.
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