Effective Electrochemical Nitrogen Reduction through π Back-donation Process in Mn³⁺ of Mn-doped g-C₃N₄

Ammonia (NH₃) synthesis via nitrogen reduction reaction under mild conditions is challenging due to the difficulty of activating nitrogen. Herein, Mn-doped g-C₃N₄ (x-Mn-CN, x = 5, 10, and 15) catalysts with efficiency NRR performance were synthesized. The resulting 5-Mn-CN exhibits higher NRR performance (NH₃ yield rate: 15.2 μg h⁻¹ mg_cat⁻¹, Faradaic efficiency: 7.1%) at -0.4 V (vs. RHE) than others. It is found that the active sites of Mn³⁺ are generated by electron transfer from Mn²⁺ to the N of g-C₃N_4, and active N₂ through the π back-donation process. This is evidenced by the experimental result that the NH₃ yield rate of 5-Mn-CN significantly decreases from 15.2 μg h⁻¹ mg_cat⁻¹ to 2.6 μg h⁻¹ mg_cat⁻¹ after lowering the concentration of Mn³⁺. The concentration of Mn³⁺ is reduced by treating the catalyst in the ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) solution. This study enhances the understanding of N₂ activation and provides insights into the transition metal-doped g-C₃N₄ as NRR catalysts.