MOLECULAR CATALYSIS FOR ELECTROCHEMICAL NITRATE CONVERSION TO AMMONIA

Abstract

Electrochemical reduction of nitrate (NO3⁻) to ammonia (NH3) offers a pathway to decentralized nitrogen cycle remediation and sustainable NH3 synthesis. This thesis advances the application of molecular catalysts in electrochemical NO3⁻ reduction to NH3 by elucidating how active site coordination and the local atomic environment govern activity, selectivity, and stability. Across three manuscripts, (i) the impact of second shell coordination on metal-N4 macrocycles beyond first-shell electronics was established (ii) the active phase identity and degradation pathways of Fe- and Cu-based phthalocyanine/porphyrin catalysts under cathodic potentials were resolved, (iii) Design challenges of dual site molecular catalysts were identified using CuPc and FePc, and (IV) the impact of electronic properties and catalyst wettability on the performance of molecular catalysts in NO3- reduction using a series of functionalized FePc-R/CNTs was decoupled. Methodologically, in situ X-ray absorption spectroscopy was integrated with post-mortem microscopy/diffraction, density-functional theory, and coupled mass-transport/reaction modeling, and electrochemical evaluation was performed to identify performance descriptors in molecular catalysts. New discoveries include: (1) metal identity and second shell (porphyrin vs phthalocyanine) in molecular catalysts impacts the stability and activity (2) revealing peripheral substituents affect electronic properties and wettability and that electronic trends are frequently masked, or amplified, by local hydrophobicity, (3) a tandem Fe-Cu design paradigm, translated from molecular insights, that identify key challenges in dual site catalysts designs and key factors playing a role in obtaining synergy between the active sites. The major emphasis of the thesis is that coordination chemistry and local environment co-determine selectivity in an eight-electron nitrate reduction reaction, and that operando-validated molecular models can provide transferable rules for scalable architectures. The contributions to knowledge are actionable: design principles linking Hammett-type substituent metrics and wettability to NO3- reduction kinetics; operando criteria to validate active-phase identity; and a blueprint for dual site catalysis that bridges molecular precision with device-relevant performance.