Research Projects

The electrocatalytic reduction of NO3 and NO2 to NH3 presents a sustainable approach for nitrogen recycling and holds particular relevance for environmental remediation applications, such as wastewater treatment. In biological systems, the selective 6e⁻/8H⁺ reduction of NO₂⁻ to NH₄⁺ is catalyzed by enzymes like cytochrome c nitrite reductase (CcNiR) and siroheme-dependent nitrite reductase (CsNiR), which have inspired the development of artificial molecular catalysts. A range of metal complexes—such as M(porphyrin), M(phthalocyanine), and other transition metal complexes with non-macrocyclic ligands—have been investigated for NO3 and NO2 reduction (NO3RR/NO2RR). However, most of these systems either operate stoichiometrically or yield a mixture of partially reduced products (e.g., NO, N2O, and NH2OH), resulting in poor selectivity and limited practical utility. Consequently, despite growing interest, this area remains relatively underexplored. The design principles that govern selective 6e/8H+ reduction—especially the role of the primary and secondary coordination spheres in preventing the release of intermediate products—are not yet fully understood. To address these knowledge gaps, this proposal aims to explore new ligand architectures featuring non-heme nitrogen/carbon donor sets at the primary coordination sphere and polar functional groups in the secondary coordination spheres. These tailored ligands are designed to modulate the electronic and proton-transfer properties of the metal centers and promote selective catalysis. We plan to synthesize a series of 3d transition metal complexes using these ligand scaffolds and systematically evaluate their electrocatalytic performance in NO3RR and NO2RR under buffered aqueous conditions. Product selectivity will be monitored using techniques such as ¹H NMR, GC, and UV–vis spectroscopy. In addition, kinetic analyses will be conducted to determine catalytic rates and mechanistic features. Importantly, we propose to investigate structure–activity relationships via linear free energy relationship (LFER) studies—an approach not yet investigated for NO3RR/NO2RR. By introducing electron-donating or electron-withdrawing substituents on the ligand backbone, we aim to tune the redox potential of the catalytic center and establish correlations with catalytic efficiency. Given the involvement of multi-proton/multi-electron (mH+/ne⁻) transfer steps, the inclusion of outer-sphere polar functional groups is expected to facilitate proton-coupled electron transfer (PCET) in the rate-limiting step, potentially enabling catalysis at lower effective overpotentials—thus breaking traditional scaling relationships. Moreover, limited mechanistic insights are currently available regarding key intermediates in the NO3RR/NO2RR pathways. To address this, we will employ in situ spectroelectrochemical techniques—including UV–vis, IR, and Raman spectroscopy—to detect and characterize metastable species. In addition, we wish to characterize isolable intermediate species through other spectroscopic techniques, including EPR, NMR, and X-ray absorption spectroscopy. Finally, to assess the practical applicability of these systems, selected catalysts will be immobilized on electrode surfaces through appropriate covalent linkers, and their performance in electrocatalytic NO3RR/NO2RR under operational conditions will be investigated. As a proof of concept, preliminary investigations using a Co(III) complex supported by a bis-pyridine-monooxime ligand framework demonstrated selective NO2- reduction to NH4+ with a Faradaic yield exceeding 99% and a turnover frequency of 9.5 x 104 s-¹. These findings validate our ligand design strategy and underscore the potential of this research to yield fundamental mechanistic insights and practical solutions for nitrogen-cycle electrocatalysis.