Modulating the Electronic Structure of Cobalt in Molecular Catalysts via Coordination Environment Regulation for Highly Efficient Heterogeneous Nitrate Reduction.

Ammonia (NH ₃ ) is pivotal in modern industry and represents a promising next-generation carbon-free energy carrier. Electrocatalytic nitrate reduction reaction (eNO ₃ RR) presents viable solutions for NH ₃ production and removal of ambient nitrate pollutants. However, the development of eNO ₃ RR is hindered by lacking the efficient electrocatalysts. To address this challenge, we synthesized a series of macrocyclic molecular catalysts for the heterogeneous eNO ₃ RR. These materials possess different coordination environments around metal centers by surrounding subunits. Consequently, electronic structures of the active centers can be altered, enabling tunable activity towards eNO ₃ RR. Our investigation reveals that metal center with an N ₂ (pyrrole)-N ₂ (pyridine) configuration demonstrates superior activity over the others and achieves a high NH ₃ Faradaic efficiency (FE) of over 90 % within the tested range, where the highest FE of approximately 94 % is obtained. Furthermore, it achieves a production rate of 11.28 mg mg _cat ^-1  h ^-1 , and a turnover frequency of up to 3.28 s ^-1 . Further tests disclose that these molecular catalysts with diverse coordination environments showed different magnetic moments. Theoretical calculation results indicate that variated coordination environments can result in a d-band center variation which eventually affects rate-determining step energy and calculated magnetic moments, thus establishing a correlation between electronic structure, experimental activity, and computational parameters.
(© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)