Tammann temperature-guided synthesis of efficient and stable core@shell ruthenium-free catalysts for ammonia decomposition

Abstract

Ammonia (NH3) is a promising liquid carrier for hydrogen (H2) storage, but its wide-scale adoption is hindered by the reliance on expensive ruthenium (Ru) catalysts for low-temperature catalysis. However, non-precious metal catalysts typically require high temperatures above 600 °C to achieve effective NH3 decomposition. To address these limitations, this study introduces a synergistic strategy for designing a heterostructure Ru-free core@shell catalyst. The innovative catalyst comprises well-dispersed non-precious cobalt (Co) nanoparticles encapsulated on a mixed-metal oxide shell. The thesis begins by introducing a simple and universal core@shell synthesis approach for the thermal-induced encapsulation of transition metal nanoparticles (MT NPs) through the achievement of unconventional strong metal-support interactions (SMSI). This approach leverages the Tammann temperature (TTam), which describes the increased mobility and reactivity of atoms in solid-state materials at temperatures higher than half of the melting point. In-situ X-ray diffraction (XRD), in-situ Raman spectroscopy, and in-situ scanning transmission electron microscopy (STEM) are employed to elucidate the encapsulation mechanism: Once a coating overlayer of low TTam compounds is formed on MT NPs, further overlayer functionalization occurs through a solid-state reaction, resulting in the formation of the highly thermally stable protective layer of the catalyst support encapsulating the metal core. By adhering to this universal guideline, we successfully obtain various core@shell nanocatalysts, including Cu@MgAl2O4-x, Ni@BaAl2O4-x, Co@BaAl2O4-x, and Co@BaTiO3-x. Following a series of optimizations, a highly efficient and cost-effective core@shell Co@BaAl2O4-x catalyst for NH3 decomposition at moderate temperatures of 500 °C is proposed. Building upon this catalyst design, the thesis delves into an investigation of the dynamic cyclic strain evolution mechanism over the catalyst, with a specific focus on the role of tensile strained surfaces. Comprehensive structural and lattice evolution analyses, conducted through ex-situ and in-situ experimental observations, along with electronic and occupied molecular orbitals analyses employing density functional theory (DFT) calculations, provide valuable insights into the interplay between strain modulation, NH3 adsorption, N-H dissociation, and strain restoration. Specifically, the tensile strain present in the Co lattice plays a crucial role in enhancing both NH3 adsorption and N-H dissociation processes. Furthermore, the release of strain upon NH3 adsorption enables efficient desorption of reaction products and guards against active site poisoning. These findings shed light on the strain-induced effects in catalysis, offering perspectives for strain engineering in the design of advanced catalysts for highly efficient NH3 decomposition and related reactions. In summary, this thesis provides valuable insights into the thermal-induced metal encapsulation principle guided by TTam, offering a straightforward synthesis approach to effectively design and optimize nanocatalysts with long-term robustness. The unique core@shell configuration with tensile strain presented by this strategy will not only pave the way for the practical application of Co-based catalysts in NH3 decomposition, but also significantly contribute to the progress of sustainable and efficient catalyst design in this field.