A Covalent Organic Framework Bearing Single Ni Sites as a Synergistic Photocatalyst for Selective Photoreduction of CO₂ to CO

Photocatalytic reduction of CO₂ into energy-rich carbon compounds has attracted increasing attention. However, it is still a challenge to selectively and effectively convert CO₂ to a desirable reaction product. Herein, we report a design of a synergistic photocatalyst for selective reduction of CO₂ to CO by using a covalent organic framework bearing single Ni sites (Ni-TpBpy), in which electrons transfer from photosensitizer to Ni sites for CO production by the activated CO₂ reduction under visible-light irradiation. Ni-TpBpy exhibits an excellent activity, giving a 4057 μmol g^-1 of CO in a 5 h reaction with a 96% selectivity over H₂ evolution. More importantly, when the CO₂ partial pressure was reduced to 0.1 atm, 76% selectivity for CO production is still obtained. Theoretical calculations and experimental results suggest that the promising catalytic activity and selectivity are ascribed to synergistic effects of single Ni catalytic sites and TpBpy, in which the TpBpy not only serves as a host for CO₂ molecules and Ni catalytic sites but also facilitates the activation of CO₂ and inhibits the competitive H₂ evolution.