Nitrogen oxides have become a major source of air pollution from stationery and mobile sources, which can cause serious damage to the environment and human health. Selective catalytic reduction (SCR) of NO with NH3 has been the predominant technology for deNOx in the industry. Additionally, NH3 is emerging as a very promising energy vector due to its high hydrogen content, carbon-free combustion, and existing production and transportation infrastructure.

To address the issue of NH3 excess emissions in this process, the selective catalytic oxidation of ammonia (NH3-SCO) has been the main solution in the industry. Single atom catalyst (SAC) has been widely used in heterogeneous catalysis due to its high dispersion and maximized atom utilization efficiency. Herein, in this thesis, we prepared single atoms and nanoparticles and comprehensively compared and understanding their catalytic performance in NH3-SCR of NO reaction and NH3 oxidation reactions. Furthermore, the structure-performance relationships over single atoms and nanoparticles were elucidated by exploring catalytic performance and analyzing results from ex-situ and in-situ characterizations. 

In the first chapter, the state of the art in NH3-SCR of NO and NH3 oxidation reactions and reaction pathway were summarized. Additionally, the synthesis method, characterization technique and application of single atom catalysts were discussed in this chapter. Specifically, the challenges of Mn-based materials were discussed, and the objectives of this thesis were outlined accordingly. 

In the second chapter, we start the investigation by using Mn as an active phase. Mn single atoms and Mn nanoclusters were prepared and well characterized using advanced characterizations (HAADF-STEM, EXAFS, EPR, etc), and then their catalytic performance in NH3-SCR of NO reaction were explored. This chapter highlight the role of proximity in influencing the N2O selectivity. To further validate this concept, we extended our investigation to another oxidation reaction-NH3 oxidation and other elements (Co and Ni). Our results suggests that active-site distance plays a very important role in regulating the N2O formation and thus could guide the further design of superior catalysts to meet the industrial demand. 

In the third chapter,  to better understand the role of water in NH3-SCR of NO reaction, we comprehensively evaluated the catalytic performance of Mn single atoms and Mn nanoparticles in the presence of water in NH3-SCR of NO reaction. We investigate the differences in water effects between single atoms and nanoparticles samples using operando X-ray absorption spectroscopy (XAS) measurements.

In the fourth chapter, the valence effect on catalytic performance in the NH3-SCR of NO and NH3 oxidation reactions over Mn single-atom catalysts was investigated. This chapter highlighted the critical role of active site valence in single atom catalysts, as it can significantly impact the N2O formation in both reactions.