Heterostructure-Derived Heterovalent Fe(OH)2/Fe Pair Sites: Tuning Adsorption of Intermediates and Enhancing Utilization of Atomic *H for Efficient Nitrate Reduction to Ammonia

Electrocatalytic nitrate reduction (NO3RR) to valued ammonia is an ideal supplementary route to the Haber-Bosch method and a strategy for the removal and utilization of nitrate pollutants. However, due to the fact that NO3RR goes through a complicated multi-electron/proton transfer, catalysts with monovalent metal sites are difficult to tackle multitasking that it involves, leading to unsatisfactory nitrate conversion efficiency and ammonia selectivity. Herein, heterovalent Fe(OH)(2)/Fe pair sites supported onto carbon nanotubes (Fe(OH)(2)/Fe@CNTs) are presented via electrochemical reconstruction of CNTs-supporting FeS/Fe2C heterostructure. Fe(OH)(2)/Fe@CNTs exhibits a high NH3 yield rate of 0.67 mmol h(-1) cm(-2) with a FE of 95.1% at -0.4 V versus RHE, which is mainly attributed to the regulated electronic structure and cooperation of heterovalent iron pair sites. Meanwhile, the adsorption of nitrogen-containing species is adjusted and the utilization of *H is enhanced. Moreover, a balanced content of Fe(OH)(2) and Fe creates "buffering effect" to maintain its activity and stability. Theoretical calculations combined with in situ FTIR and in situ Raman spectra reveal a novel multiple reaction pathway on heterovalent Fe(OH)(2)/Fe pair sites, entirely different from a single pathway on monovalent Fe or Fe(OH)(2). Clearly, this study offers a creative strategy for the design of advanced catalysts with multivalent metal sites.