Dual Zn₅−NiS₄ Sites in a Redox‐Active Metal–Organic Framework Enables Efficient Cascade Catalysis for Nitrate‐to‐Ammonia Conversion

Abstract

Abstract Electrocatalytic Nitrate Reduction to Ammonia (NO 3 RR) offers a promising solution to both environmental pollution and the sustainable energy conversion. Here we propose an efficient cascade catalytic mechanism based on a dual Zn 5 −NiS 4 sites, orderly assembled in a redox‐active metal–organic framework structure, which separately promotes the reaction kinetics of nitrate‐to‐nitrite and nitrite‐to‐ammonia conversions. Specifically, the Zn 5 clusters adsorb and selectively reduce the NO 3 − to NO 2 − , whereas [NiS 4 ] acts as an analogue to the ferredoxins, subsequently boosts the reduction of NO 2 − to produce NH 3 . To this end, the bimetallic Zn 5 −NiS 4 TP MOF was synthesized based on the redox‐active ligand [Ni(C 2 S 2 (TPCOOH) 2 ) 2 ]. A maximum ammonia production rate of 23477.59 μg ⋅ h −1 ⋅ mg −1 cat. and faradaic efficiency 92.87 % was achived by Zn 5 −NiS 4 TP MOF under neutral conditions. To validate the critical role of dual Zn 5 −NiS 4 sites, Mn 5 −NiS 4 TP and Cd 2 −NiS 4 TP were synthesized as control samples, together with Zn‐TTFTB, Zn−NiS 4 Ph and other Zn 5 ‐cluster‐based MOFs applied for the investigation of electrocatalytic nitrate reduction. Our results indicated that substitution by ‐thienyl instead of ‐phenyl group increases the S‐heteroatom content, improves the conductivity and facilitates electron transfer. Furthermore, Density Functional Theory (DFT) calculations of the energy changes for the reduction of each species could rationalize experimental results.