Tuning Composition and Morphology of Noble Metal Catalysts for Enhanced Ammonia Synthesis at Mild Conditions

Abstract

This PhD research focuses on the design and development of Ru-based catalysts for mainly ammonia (NH₃) synthesis under mild conditions, aiming to address the limitations of the traditional energy-intensive Haber-Bosch process. Ammonia decomposition catalysts are also investigated. The study investigates the effects of Ru loading on the active site dispersion and catalytic efficiency, while also exploring the incorporation of secondary metals such as Co and Mo to enhance the N₂ activation step via electronic and structural modifications. Two main categories of support materials are explored: LDH-derived (MgFeOx) and MOF-derived (CeO₂) supports. The goal is to optimize Ru metal growth on these oxide supports, tailoring metal-support interactions and assessing the influence of support materials on the catalytic performance. A comprehensive suite of characterization techniques, including ex-situ and insitu methodologies, such as X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), Mössbauer spectroscopy, and X-ray photoelectron spectroscopy (XPS), is employed to investigate the structural and electronic properties of the catalysts. Additionally, density functional theory (DFT) calculations is used to complement the experimental findings, providing deeper insights into the process mechanisms, including N₂ adsorption/activation and reaction pathways on various active sites. This research offers valuable insights into the rational design of the next-generation catalysts for sustainable NH₃ production. In Contribution 1, the study investigates Ru single-atom alloys (SAA) grown on MgFeOx support for stable NH₃ synthesis. The chapter focuses on optimizing Ru loading, ranging from 0.2 wt.% to 5 wt.%, using chemical reduction methods. Key characterization techniques, such as XAS and DFT calculations, are employed to understand the atomic-level interaction between Ru and Fe, enhancing catalytic activity and stability. The results demonstrate that Ru can be atom-efficiently deployed on MgFeOx supports to achieve exceptionally high NH₃ formation rates under mild conditions (Chapter 4), with the optimal 0.1 wt.% Ru SAA catalyst showing both outstanding activity and >150 h stability. The Contribution 2 builds on the findings from Chapter 1, and explores the impact of bimetallicity (CoRu and MoRu) on MgFeOx support towards sustainable NH3 catalysts development. In particular, Co and Mo are used as co-metals, replacing 50% of the total Ru content in the catalyst formulation. Bimetallic Ru/Co and Ru/Mo catalysts are characterized using advanced techniques such as Mössbauer spectroscopy, XAS, in-situ XRD, and XPS to examine structural and electronic modifications that inherently improve NH₃ synthesis and cracking performance. Incorporation of secondary metals (Co/Mo) along with Ru on MgFeOx (Chapter 5) reveals that Co–Ru and Mo-Ru combinations strike an optimal balance for NH₃ synthesis with high stability for 150h, while Mo based monometallic catalyst excel in NH₃ decomposition to H₂, highlighting dual-functionality achievable at low noble-metal content. The Contribution 3 explores CeO₂-based catalysts derived from a sacrificial template strategy using Ceria-MOF. The effect of varying Ru loadings (0.2, 0.5, 1, and 2 wt.%) on metal dispersion and particle size is studied. The role of the support oxygen vacancies and their impact on the catalytic performance for both NH₃ synthesis and cracking is investigated using various in-situ and ex-situ techniques, providing insights into the importance of structural defects for catalytic activity enhancement. Ru supported on oxygen-vacancy–rich CeO₂ derived from MOFs (Chapter 6) exhibits enhanced N₂ activation and H₂ evolution due to strong spillover effect and defect-engineered surfaces, with the 0.5Ru/CeO₂ catalyst delivering superior activity and stability.

Overall, by integrating experimental and computational approaches, this thesis contributes to the rational development and understanding of more efficient and sustainable catalysts for NH₃ synthesis, paving the way towards more energy-efficient chemical processes with limited CO2 emissions.