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.