Atomically Precise Integration of Multiple Functional Motifs in Catalytic Metal–Organic Frameworks for Highly Efficient Nitrate Electroreduction

Abstract

Ammonia production plays a central role in modern industry and agriculture with a continuous surge in its demand, yet the current industrial Haber–Bosch process suffers from low energy efficiency and accounts for high carbon emissions. Direct electrochemical conversion of nitrate to ammonia therefore emerges as an appealing approach with satisfactory sustainability while reducing the environmental impact from nitrate pollution. To this end, electrocatalysts for efficient conversion of eight-electron nitrate to ammonia require collective contributions at least from high-density reactive sites, selective reaction pathways, efficient multielectron transfer, and multiproton transport processes. Here, we report a catalytic metal–organic framework (two-dimensional (2D) In-MOF In8) catalyst integrated with multiple functional motifs with atomic precision, including uniformly dispersed, high-density, single-atom catalytic sites, high proton conductivity (efficient proton transport channel), high electron conductivity (promoted by the redox-active ligands), and confined microporous environments. These eventually lead to a direct and efficient electrochemical reduction of nitrate to ammonia and record high yield rate, FE, and selectivity for NH3 production. A novel “dynamic ligand dissociation” mechanism provides an unprecedented working principle that allows for the use of a high-quality MOF crystalline structure to function as highly ordered, high-density, single-atom catalyst (SAC)-like catalytic systems and ensures the maximum utilization of the metal centers within the MOF structure. Further, the atomically precise assembly of multiple functional motifs within a MOF catalyst offers an effective and facile strategy for the future development of framework-based enzyme-mimic systems.

Highly Efficient Electrochemical Nitrate Reduction to Ammonia in Strong Acid Conditions with Fe2M‐Trinuclear‐Cluster Metal–Organic Frameworks

“…In addition, through Raman spectroscopy, it can be concluded that the nitrogen atoms of N 2 ‐ligand were protonated to form ‐NH after immersion in an acidic environment (a new peak appeared at ≈1330 cm −1 in Fe 2 Co‐MOF at pH=1, see Figure 3d), [47] which presumably results in more rapid proton conduction. Furthermore, the significantly enhanced electron negativity and delocalized conjugation in N 2 ligand (over N 1 ligand) presumably facilitate their hydrogen bonding interactions with water molecules or hydronium ions, providing a possible proton conductive pathway at the MOF‐electrolyte interface, which is key to the efficient proton supply during multi‐proton NO 3 RR process [26, 48, 49] . Overall, the di‐nitrogen atoms in the H 4 TPBD ligand collectively enable more efficient electron transfer and proton transport in the framework, leading to more effective hydrogenation and reduction of nitrate.…”

“…To this end, single-crystalline metal-organic frameworks (MOFs) offers enormous potentials in the construction of porous electrocatalytic systems, with their highdensity mono-dispersed catalytic sites, tunable electron and proton transport characteristics from ligand design, multistage pore channels that facilitates mass diffusions, [23][24][25] and structure-function correlations down to atomic precision for the mechanistic clarifications. [26,27] These unique advantages make MOF an ideal framework to host and stabilize the transition metal ions. A major obstacle that has prevented most MOFs from widespread use in electrocatalysis is that the metal centers are typically saturated with the coordination of linkers, leading to the underlined catalytic efficiencies, [28] despite the efforts on defect engineering [29,30] or dynamic ligand dissociation.…”

“…While the inhibition of HER on noble metal surface is fundamentally challenging, increasing the stability of high‐performance transition‐metal‐catalysts such as Fe/Co by means of immobilization appears to be a more viable solution. To this end, single‐crystalline metal–organic frameworks (MOFs) offers enormous potentials in the construction of porous electrocatalytic systems, with their high‐density mono‐dispersed catalytic sites, tunable electron and proton transport characteristics from ligand design, multi‐stage pore channels that facilitates mass diffusions, [23–25] and structure‐function correlations down to atomic precision for the mechanistic clarifications [26, 27] . These unique advantages make MOF an ideal framework to host and stabilize the transition metal ions.…”

“…These unique advantages make MOF an ideal framework to host and stabilize the transition metal ions. A major obstacle that has prevented most MOFs from widespread use in electrocatalysis is that the metal centers are typically saturated with the coordination of linkers, leading to the underlined catalytic efficiencies, [28] despite the efforts on defect engineering [29, 30] or dynamic ligand dissociation [26] . Here, we developed a series of MOF structure with Fe‐based trinuclear clusters [31, 32] as catalytic nodes (Fe 2 M‐MOF, M=Fe, Co, Ni, Zn) coupled with dinitrogen ligands.…”

“…A major obstacle that has prevented most MOFs from widespread use in electrocatalysis is that the metal centers are typically saturated with the coordination of linkers, leading to the underlined catalytic efficiencies, [28] despite the efforts on defect engineering [29,30] or dynamic ligand dissociation. [26] Here, we developed a series of MOF structure with Fe-based trinuclear clusters [31,32] as catalytic nodes (Fe 2 M-MOF, M = Fe, Co, Ni, Zn) coupled with dinitrogen ligands. Each trinuclear clusters enables unsaturated metal sites to ensure the high turnover adsorption and reduction of nitrate (with their intrinsically limited HER competition), whereas the flexible, redoxactive and protonated (in acidic environment) dinitrogen ligand simultaneously facilitates the transports of electron, proton and nitrate reactants, to further boost the catalytic efficiency.…”