# Strong d–p orbital hybridization in cobalt porphyrin cages promotes electrochemical nitrate reduction to ammonia

## TL;DR

A cobalt-based porphyrin catalyst efficiently converts nitrate to ammonia, offering a sustainable solution for pollution and industrial ammonia production.

## Contribution

The study introduces metalized porphyrin cages with strong d–p orbital hybridization for efficient nitrate reduction.

## Key findings

- PB-Co achieves 95.8% ammonia selectivity and a high yield rate of 995.5 µmol h⁻¹ mg⁻¹.
- Strong d–p hybridization in PB-Co enhances charge transfer and lowers activation energy for nitrate reduction.
- The d-band center and adsorption energies in PB-Co promote active hydrogen production and catalytic performance.

## 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 catalysts 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 possesses a high d-band center of −0.97 eV and high adsorption energies 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.

Three metalized porphyrin organic cages, PB-M (M = Co, Ni, Cu), were synthesized. Due to the more positive d-band center and stronger d–p hybridization, the highest faradaic efficiency and ammonia yield were achieved by PB-Co.