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

Electrocatalytic Nitrate Reduction to Ammonia (NO₃RR) 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 Zn₅-NiS₄ 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 Zn₅ clusters adsorb and selectively reduce the NO₃ ^- to NO₂ ^-, whereas [NiS₄] acts as an analogue to the ferredoxins, subsequently boosts the reduction of NO₂ ^- to produce NH₃. To this end, the bimetallic Zn₅-NiS₄TP MOF was synthesized based on the redox-active ligand [Ni(C₂S₂(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 Zn₅-NiS₄TP MOF under neutral conditions. To validate the critical role of dual Zn₅-NiS₄ sites, Mn₅-NiS₄TP and Cd₂-NiS₄TP were synthesized as control samples, together with Zn-TTFTB, Zn-NiS₄Ph and other Zn₅-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.