Maximizing Active Site Density in Tin Single-Site Catalysts for Selective Carbon Dioxide to Formate Electroreduction.

The electrocatalytic conversion of CO2 into value-added chemicals is governed by catalyst structure-reactivity relationships that remain incompletely understood, particularly regarding how coordination environment and active-site density influence product selectivity. Herein, we report a mechanochemically assisted synthesis of high-density tin single-site catalysts, in which atomically dispersed Sn centers are stabilized at loadings of approximately 7.5 wt.%, exceeding the typical ∼2 wt.% limit reported for Sn single-site systems without detectable aggregation. By tailoring the nitrogen-rich precursor chemistry, catalysts with distinct coordination environments and controlled site densities were obtained, providing a platform to elucidate how these two factors govern the CO2 reduction pathway. Operando spectroscopy combined with density functional theory reveals that the local coordination environment governs intrinsic activity and selectivity by stabilizing key intermediates along the *OCHO pathway, whereas the mechanochemical process primarily enhances the density of electrochemically accessible Sn sites. The optimized catalyst achieves formate Faradaic efficiencies above 90% over a wide current-density range of 50-500 mA cm-2 in a gas-fed flow cell, reaching 92.5% at 100 mA cm-2, and maintains stable operation for over 100 h at 100 mA cm-2. This work provides mechanistic insights and design principles for selective and high-rate CO2 electroreduction based on single-site catalysts.