Scalable two-step annealing method for preparing ultra-high-density single-atom catalyst libraries

The stabilization of transition metals as isolated centres on suitably tailored carriers with high density is crucial to exploit the technical potential of single-atom heterogeneous catalysts, enabling their maximized productivity in industrial reactors. Wet-chemical methods are best suited for practical applications due to their amenability to scale up. However, achieving single-atom dispersions at metal contents above 2 wt.% remains challenging. We introduce a versatile approach combining impregnation and two-step annealing to synthesize ultra-high-density single-atom catalysts (UHD-SACs) with unprecedented metal contents up to 23 wt.% for 15 metals on chemically-distinct carriers. Translation to an automated protocol demonstrates its robustness and provides a path to explore virtually unlimited libraries of mono or multimetallic catalysts. At the molecular level, characterization of the synthesis mechanism through experiments and simulations shows that controlling the bonding of metal precursors with the carrier via stepwise ligand removal prevents their thermally-induced aggregation into nanoparticles, ensuring atomic dispersion in the resulting UHD‑SACs. The catalytic benefits of UHD-SACs are demonstrated for the electrochemical reduction of CO₂ to CO over NiN₄ motifs on carbon.