Guan, Xuze; (2024) Rational Design and Improvement of Transition Metal Catalysts for Selective Ammonia Oxidation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Various nitrogen-containing pollutants (NH3 and NOX) have raised concerns about public health and environmental protection, which has led to increasingly strict emission standards. As one of the most promising methods for removing ammonia, the selective catalytic oxidation of NH3 (NH3-SCO) to nitrogen has received increasing attention and will play a crucial role in the upcoming EU7 standard. However, achieving high activity and nitrogen selectivity simultaneously remains a challenge. Noble metals generally exhibit high activity, while excessive oxidation leads to low nitrogen selectivity above 300 °C in NH3-SCO. In comparison, transition metal catalysts are favourable for the formation of N2 but suffer from low activity. To replace expensive noble metals, a series of binary transition metal oxide catalytic systems were designed, and a wide range of characterization techniques was applied to study the structures and performances of the catalysts. Increasing the oxidation rates is an effective method for enhancing the NH3-SCO activity of transition metals, which can be achieved by activating the surface lattice oxygen of the catalysts. Cu-CeO2 single-atom catalysts (SACs) with different shapes are prepared and tested for the NH3-SCO. The interaction between Cu and CeO2 is crucial to regulating the interface structure and the content of oxygen vacancies. Among them, Cu-CeO2 SACs prepared by the one-step flame spray pyrolysis (FSP) method exhibited a high extent of ceria reduction at low temperatures and a drastically improved catalytic activity in the N-H bond dissociation. Altogether, the obtained results demonstrate that FSP is an appealing strategy for the synthesis of highly active SACs. Reducing the emission of NOX at high temperatures is an effective method for transition metals with high activities to achieve high N2 productivity. N2O is a major by-product of active Co-based catalysts in NH3-SCO. However, there remains an insufficient understanding of the fundamental methods for achieving satisfying low-temperature activity with less N2O emission. CuO-Co3O4 catalysts were designed to catalyze a cascade reaction that first forms N2O at an unprecedented rate from NH3 and then decomposes N2O back to N2, achieving high NH3-SCO performance. The ability to remove N2O at high temperatures can be further exploited to reduce N2O emissions from noble metal catalysts. Synthesizing innovative catalysts is essential for enhancing low-temperature catalytic activity and N2 selectivity. The mechanochemical method was employed to insert single Cu atoms into the subsurface of Fe2O3 support. The subsurface single-atom catalyst design strategy opens promising perspectives to initiate the lattice distortion and facilitate the activation of the inactive lattice oxygen. Controlling single atoms on the surface and subsurface resulted in very different adsorption properties of catalysts. The subsurface Cu single atoms in Fe2O3 remained isolated under either oxidative or reductive environments, while surface Cu single atoms on Fe2O3 sintered under reduction. The unique properties of these subsurface single-atom catalysts call for innovations and understandings in catalyst design.

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