Single-atom catalysts (SACs) represent a transformative class of materials in catalysis, uniquely positioned at the interface between homogeneous and heterogeneous systems. By maximizing metal atom utilization while enabling tunable coordination environments, SACs offer exceptional reactivity, selectivity, and sustainability. However, challenges remain in the scalable and controlled synthesis of SACs with defined active sites and structural stability under operational conditions. This thesis begins with a comprehensive introduction to the field of SACs (Chapter 1), outlining their fundamental principles, synthesis strategies, coordination chemistry, and emerging use cases. Key challenges, including metal aggregation, low loading limits, and limited substrate scopes, are discussed, setting the stage for the experimental contributions that follow. Chapter 2 explores the catalytic capabilities of a ZIF-11-derived Zn-based SAC (Znâ â NC) for the selective hydrogenation and transfer hydrogenation of polycyclic aromatic hydrocarbons (PAHs). The catalyst features isolated Znâ Nâ sites within a nitrogen-doped carbon matrix and shows high activity and selectivity for the production of 9,10-dihydroanthracene. Control experiments confirm the essential role of Zn single atoms and reveal that solvent choice dramatically impacts catalyst stability. Specifically, isopropanol effectively suppresses coking and maintains performance over multiple cycles. The catalyst also demonstrates a degree of functional group tolerance, extending the utility of Zn SACs in arene hydrogenation. In Chapter 3, a general, bottom-up polycondensation strategy is developed to create high-density SACs (HD-SACs) incorporating up to 27.5 wt% of atomically dispersed metal species. This method employs polycondensation of metalâ phenanthroline complexes with benzenetetramine to form amorphous polymeric networks with tailored coordination environments. The process applies to a broad range of transition metals, enables bimetallic catalyst design, and is shown to be scalable and automatable. The resulting HD-SACs demonstrate outstanding stability and performance in electrocatalytic applications, including COâ reduction and hydrogen evolution, highlighting their robustness and potential for integration into high-throughput, data-driven materials discovery platforms. In Chapter 4, a Co-based SAC is developed based on the work in the previous chapter for the selective hydrogenolysis of 5-hydroxymethylfurfural (5-HMF) to 5-methylfurfural (5-MF), a high-value compound in biomass upgrading. The catalyst, featuring Coâ Nâ coordination sites, achieves 93.7% selectivity to 5-MF at full 5-HMF conversion under transfer hydrogenation using formic acid. Extensive characterization links the unprecedented selectivity to the Coâ Nâ environment, and additional experiments identify (5-formylfuran-2-yl)methyl formate (FMF) as a reaction intermediate. The catalyst remains stable over multiple cycles and enables 5-MF production from more complex substrates such as fructose and cellulose in ionicliquid-assisted tandem reactions. Together, these studies establish new synthetic methodologies, mechanistic insights, and applications for SACs. This thesis advances the field of single-atom catalysis by addressing key bottlenecks in SAC design and demonstrating their potential for selective, sustainable chemical transformations across energy and biomass domains.