In-Plane Palladium and Interplanar Copper Dual Single-Atom Catalyst in Bulk-Like Carbon Nitride for Cascade CO 2 Photoreduction.

Dual single-atom catalysts (DSACs) are promising for breaking the scaling relationships and ensuring synergistic effects compared with conventional single-atom catalysts (SACs). Nevertheless, precise synthesis and optimization of DSACs with specific locations and functions remain challenging. Herein, dual single-atoms are specifically incorporated into the layer-stacked bulk-like carbon nitride, featuring in-plane three-coordinated Pd and interplanar four-coordinated Cu (Pd 1 -Cu 1 /b-CN) atomic sites, from both experimental results and DFT simulations. Using femtosecond time-resolved transient absorption (fs-TA) spectroscopy, it is found that the in-plane Pd features a charge decay lifetime of 95.6 ps which is much longer than that of the interplanar Cu (3.07 ps). This finding indicates that the in-plane Pd can provide electrons for the reaction as the catalytically active site in both structurally and dynamically favorable manners. Such a well-defined bi-functional cascade system ensures a 3.47-fold increase in CO yield compared to that of bulk-like CN (b-CN), while also exceeding the effects of single Pd 1 /b-CN and Cu 1 /b-CN sites. Furthermore, DFT calculations reveal that the inherent transformation from s-p coupling to d-p hybridization between the Pd site and CO 2 molecule occurs during the initial CO 2 adsorption and hydrogenation processes and stimulates the preferred CO 2 -to-CO reaction pathway.