Covalent stabilization of copper porphyrin into carbon nitride for the catalytic reduction of CO₂

The development of active, selective, and durable (photo)electrocatalytic hybrid systems by combining molecular catalysts and semiconductor substrates is crucial for efficiently converting solar light into high-value products. Herein, a one-step synthesis method to obtain carbon nitride (CN) nanosheets, which allows the direct covalent polymerization with molecular catalysts, was developed. Copper-porphyrin (CuPor) units were embedded in the CN structure as a case-study. The single and hybrid materials were fully characterized by combining microscopic and spectroscopic techniques and tested as (photo)electrocatalysts for the CO₂ reduction reaction (CO₂RR) in aqueous solution. Experimental evidence confirmed an effective boost of the CN photoelectrocatalytic activity by introducing the CuPor units. Formate was identified as the only CO₂RR product on both CuPor and CuPor-CN, simultaneously with hydrogen from the competitive hydrogen evolution reaction. However, the formate/hydrogen ratio was higher when the hybrid material was used as catalyst, suggesting a synergetic effect between CuPor and CN, that favors the CO₂RR and hinders the HER. In addition, the stability of the CuPor units in the CN matrix under catalytic conditions was studied by in situ X-ray absorption. No changes of the Cu porphyrin structure or formation of copper clusters/nanoparticles was observed as a function of the applied potential nor after an accelerated ageing treatment, suggesting that the CN matrix is able to stabilize the Cu sites avoiding their agglomeration.