Unveiling the activity tendency of well-defined metal-N4 sites for electrocatalytic nitrate reduction

ABSTRACT Metal-nitrogen-carbon single-atom catalysts (M-N-C SACs) have emerged as a highly promising material for ammonia synthesis from electrocatalytic nitrate reduction due to their isolated metal site and capacity to prevent the N–N coupling. However, understanding the structure-activity relationship at molecular level remains challenging because of the inhomogeneous MNx structure presented in current synthesized M-N-C catalysts. In this study, we utilized metal phthalocyanine (MPc) as a model platform catalyst containing a uniform and well-defined MN4 center to unravel their intrinsic activity tendency toward ammonia synthesis from nitrate reduction, both experimentally and theoretically. Our experimental results exhibit a significant activity difference for ammonia production in the order of FeN4 > CuN4> NiN4> MnN4 > CoN4 > ZnN4, and among which the FeN4 site delivers much higher faradic efficiency and the highest turnover frequency of 83.3% and 4395.2 h–1 at –1.0 V vs. RHE, respectively. Density-functional theory calculations indicates that, compared to CoN4 and MnN4, the FeN4 site not only has appropriate adsorption strength for NOx intermediate species, but also has certain inhibitory effects on hydrogen evolution reaction process. These findings provide systematic and reliable guidance for catalyst synthesis toward nitrate reduction to NH3 from both the experimental and computational perspectives.