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 M N x structure presented in current synthesized M-N-C catalysts. In this study, we utilized metal phthalocyanine ( M Pc) as a model platform catalyst containing a uniform and well-defined M N 4 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 FeN 4 > CuN 4 > NiN 4 > MnN 4 > CoN 4 > ZnN 4 , and among which the FeN 4 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 CoN 4 and MnN 4 , the FeN 4 site not only has appropriate adsorption strength for NO x 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 NH 3 from both the experimental and computational perspectives.