The article presents a novel approach to fabricating ultra-high-density single-atom catalysts (SACs) by controlling metal diffusion during pyrolysis at reduced pressure. This method significantly reduces the aggregation of single atoms, leading to almost three times higher single-atom loadings compared to ambient pressure conditions. Molecular dynamics and computational fluid dynamics simulations reveal that reduced pressure enhances metal diffusion due to increased mean free paths, minimizing agglomeration. The prepared SACs exhibit superior performance in electrocatalytic oxygen reduction reaction (ORR) and Ullmann-type carbon-oxygen coupling reactions. The study demonstrates the robustness and scalability of the pressure-controlled metal diffusion approach, paving the way for more economically feasible catalyst systems.The article presents a novel approach to fabricating ultra-high-density single-atom catalysts (SACs) by controlling metal diffusion during pyrolysis at reduced pressure. This method significantly reduces the aggregation of single atoms, leading to almost three times higher single-atom loadings compared to ambient pressure conditions. Molecular dynamics and computational fluid dynamics simulations reveal that reduced pressure enhances metal diffusion due to increased mean free paths, minimizing agglomeration. The prepared SACs exhibit superior performance in electrocatalytic oxygen reduction reaction (ORR) and Ullmann-type carbon-oxygen coupling reactions. The study demonstrates the robustness and scalability of the pressure-controlled metal diffusion approach, paving the way for more economically feasible catalyst systems.