Integrating Single Atoms with Different Microenvironments into One Porous Organic Polymer for Efficient Photocatalytic CO2 Reduction.

The precise identification of single-atom catalysts (SACs) activity and boosting their efficiency toward CO2 conversion is imperative yet quite challenging. Herein, for the first time a series of porous organic polymers is designed and prepared simultaneously, containing well-defined M-N4 and M-N2 O2 single-atom sites. Such a strategy not only offers multiactive sites to promote the catalytic efficiency but also provides a more direct chance to identify the metal center activity. The CO2 photoreduction results indicate that the introduction of salphen unit with Ni-N2 O2 catalytic centers into pristine phthalocyanine-based Ni-N4 framework achieves remarkable CO generation ability (7.77 mmol g-1 ) with a high selectivity of 96% over H2 . In combination with control experiments, as well as theoretical studies, the Ni-N2 O2 moiety is evidenced as a more active site for CO2 RR compared with the traditional Ni-N4 moiety, which can be ascribed to the M-N2 O2 active sites effectively reducing the energy barrier, facilitating the adsorption of reaction radicals *COOH, and improving the charge transportation. This work might shed some light on designing more efficient SACs toward CO2 reduction through modification of their coordination environments.