Dual Zn-NiS Sites in a Redox-Active Metal-Organic Framework Enables Efficient Cascade Catalysis for Nitrate-to-Ammonia Conversion.

Electrocatalytic Nitrate Reduction to Ammonia (NORR) 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-NiS 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 clusters adsorb and selectively reduce the NO to NO , whereas [NiS] acts as an analogue to the ferredoxins, subsequently boosts the reduction of NO to produce NH. To this end, the bimetallic Zn-NiSTP MOF was synthesized based on the redox-active ligand [Ni(CS(TPCOOH))]. A maximum ammonia production rate of 23477.59 μg ⋅ h ⋅ mg and faradaic efficiency 92.87 % was achived by Zn-NiSTP MOF under neutral conditions. To validate the critical role of dual Zn-NiS sites, Mn-NiSTP and Cd-NiSTP were synthesized as control samples, together with Zn-TTFTB, Zn-NiSPh and other Zn-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.