Highly Enhancing CO₂ Photoreduction by Metallization of an Imidazole-linked Robust Covalent Organic Framework

Converting CO₂ into value-added chemicals to solve the issues caused by carbon emission is promising but challenging. Herein, by embedding metal ions (Co²⁺ , Ni²⁺ , Cu²⁺ , and Zn²⁺ ) into an imidazole-linked robust photosensitive covalent organic framework (PyPor-COF), effective photocatalysts for CO₂ conversion are rationally designed and constructed. Characterizations display that all of the metallized PyPor-COFs (M-PyPor-COFs) display remarkably high enhancement in their photochemical properties. Photocatalysis reactions reveal that the Co-metallized PyPor-COF (Co-PyPor-COF) achieves a CO production rate as high as up to 9645 µmol g^-1 h^-1 with a selectivity of 96.7% under light irradiation, which is more than 45 times higher than that of the metal-free PyPor-COF, while Ni-metallized PyPor-COF (Ni-PyPor-COF) can further tandem catalyze the generated CO to CH₄ with a production rate of 463.2 µmol g^-1 h^-1 . Experimental analyses and theory calculations reveal that their remarkable performance enhancement on CO₂ photoreduction should be attributed to the incorporated metal sites in the COF skeleton, which promotes the adsorption and activation of CO₂ and the desorption of generated CO and even reduces the reaction energy barrier for the formation of different intermediates. This work demonstrates that by metallizing photoactive COFs, effective photocatalysts for CO₂ conversion can be achieved.