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The photocatalytic conversion of CO2 into widely adjustable syngas (CO/H2 mixture) has been considered as an extraordinarily promising but challenging strategy to realize high value-added utilization of CO2 to alleviate worldwide environmental problems and energy crisis. To achieve this goal, we report a newly designed crystalline covalent organic framework (COF-TVBT-Bpy) that embedded a series of metal active sites to efficiently convert CO2 and H2O into syngas under visible-light illumination. COF-TVBT-Bpy not only provides a coordination environment for the metal center, but also enhances electron delocalization by the extended conjugated moiety, thus facilitating electron transfer. The optimized Co@COF-TVBT-Bpy achieves the highest photocatalytic CO2-to-syngas conversion efficiency in pure CO2 (up to 2291.1 μmol g−1 h−1, CO/H2 ≈ 1 : 1). The intrinsic reason for a CO/H2 ratio of 1 : 1, electron transfer pathway, and syngas generation mechanism over Co@COF-TVBT-Bpy are explored by the combination of in situ measurements and theoretical calculations. The experimental results confirm that the ratio of CO/H2 can be widely modulated by adjusting the metal active sites and photoreaction conditions. This work proves the immense potential of utilizing COFs as platforms to anchor metal active sites for syngas formation and provides a facile method to regulate the ratio of syngas in a wide range. [ABSTRACT FROM AUTHOR]

A β-ketoenamine-based COF is used as a photocatalyst to convert CO2 and H2O into CO and O2 under visible-light irradiation without using additional photosensitizers and sacrificial agents in a gas–solid system. When CO2 concentration was controlled at 30.0% at 80 °C, TpBb-COF exhibited a CO production rate of 89.9 μmol g−1 h−1, which was higher than that (52.8 μmol g−1 h−1) in pure CO2, indicating that low concentrations of CO2 are conducive to the photocatalytic reduction of CO2. DFT calculations indicate that the adsorption of H2O on TpBb-COF was more preferential, which promoted the adsorption and reduction of CO2. In light of this finding, the mechanism or reaction pathway was explored. The rate equation of CO2 photocatalytic reduction was derived to correlate the association of the CO2 concentration and CO production rate, which is consistent with the experimental results and theoretical calculations. [ABSTRACT FROM AUTHOR]

We prepare a novel COF for CO2 photoreduction with 99.9% CO selectivity in aqueous solution without a cocatalyst. DFT shows that the preferential adsorption of H+ on the COF results in increased CO2 adsorption energy providing an anchoring site of CO2, and with the cooperation of an ethylene group, CO2 reduction is triggered. [ABSTRACT FROM AUTHOR]

We prepare a novel COF for CO2 photoreduction with 99.9% CO selectivity in aqueous solution without a cocatalyst. DFT shows that the preferential adsorption of H+ on the COF results in increased CO2 adsorption energy providing an anchoring site of CO2, and with the cooperation of an ethylene group, CO2 reduction is triggered.