Strong d-p Orbital Hybridization in Cobalt Porphyrin Cages Promotes Electrochemical Nitrate Reduction to Ammonia

Abstract

The electrocatalytic reduction of nitrate (NO3RR) to ammonia presents a viable solution for addressing nitrate pollution and offers an environmentally-friendly, energy-efficient alternative for industrial ammonia synthesis. However, the absence of efficient electrocatalysts impedes its industrial application. In this study, we constructed a porphyrin organic cage (PB-2) through the covalent-bonded self-assembly. Subsequently, metalized porphyrin organic cages PB-M (M = Co, Ni, Cu) were synthesized via post-modification of PB-2. These PB-M were utilized to elucidate the reaction pathway and intrinsic structure-performance relationship of the NO3RR. Experimental results indicate that PB-Co exhibits the highest activity and ammonia selectivity (FENH3 = 95.8 ± 1.06%, NH3 yield rate = 995.5 ± 28.4 µmol h−1 mgcat−1). Theoretical calculations reveal that the d-p orbital hybridization between the Co 3d orbital in PB-Co and the NO3– 2p orbital is the strongest one. PB-Co with a high d-band center of –0.97 eV and high adsorption energy for NO3– and H2O, promoting charge transfer and the production of active hydrogen, thereby reducing the activation energy barrier of NO3–. This research illuminates the intrinsic structure-activity relationship of metalized PB-M for the NO3RR, potentially providing valuable insights for the design of efficient electrocatalysts.