Date of Award
Spring 2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemistry
First Advisor
Brudvig, Gary
Abstract
Our rapidly increasing population’s growing need for food, fertilizer, and clean energy necessitates an exploration into the oxidation of ammonia (NH3) for its application as a carbon-free fuel and a precursor for nitrite and nitrate production Traditional ammonia oxidation (AO) techniques, such as the Ostwald process, demand considerable energy and result in greenhouse gas emissions. In contrast, electrochemical AO emerges as an energy-efficient solution. This thesis investigates the development and characterization of copper and iridium-based molecular and heterogeneous catalysts targeting selective electrochemical AO in aqueous environments.Chapter 2 focuses on [Cu(bipyalk)]+, a mononuclear copper electrocatalyst exhibiting selective ammonia oxidation to nitrite (NO2−) and nitrate (NO3−) in water. Notably, this catalyst showcases a 65% faradaic efficiency (FE) for nitrate production and a remarkable 94% FE for total AO, while displaying high selectivity against water oxidation (WO)—an unprecedented trait in previous molecular AO electrocatalysts, as [Cu(bipyalk)]+ is not competent for WO. Chapter 3 explores Cu(pyalk)2, an originally developed electrochemical WO catalyst, for its effectiveness in chemoselective AO in water. While Cu(pyalk)2 exhibits WO efficacy, its WO activity is considerably diminished in the presence of aqueous ammonia. Conversely, it exhibits selective AO in water and primarily yields NO2− (FE 62.3%) rather than complete oxidation to NO3−. 15N isotope labeling, modified Griess colorimetric method, electrochemical kinetics studies, and H/D kinetic isotope effects are employed to provide initial mechanistic insights for the chemoselectivity. Chapter 4 shifts to developing heterogeneous AO catalysts. Conventional AO on heterogeneous noble metal electrodes often yields undesired N2 and leads to detrimental nitride surface poisoning. We now find that our previously reported “Blue Layer” (BL), an organometallic–inorganic hybrid anode based on [IrO2]x nanoclusters (x ~ 5), is active for selective AO in aqueous solution. Through adjustment of both pH and applied potential, we identified optimal operating conditions (pH 8.0, 1.00 V) for AO, where BL achieves optimum selectivity towards NO3− (FE 90.5%), effectively minimizing competitive WO and maintaining activity against ammonia-induced degradation of the electrode. Chapters 5 and 6 focus on water oxidation. Chapter 5 delves into the computational-aided molecular design and synthesis of a porphyrin–phenazine chromophore interconnected by an azo linkage. Numerous strategies were employed to synthesize this compound. Chapter 6 explores the design and synthesis of a potential N-heterocyclic carbene ligand and its proposed corresponding iridium complex for homogeneous WO. Extensive spectroscopic characterizations were employed to elucidate the true identity of the synthesized molecular catalyst.
Recommended Citation
Liu, Hanyu, "Transition Metal Complexes for Electrocatalytic Ammonia Oxidation" (2024). Yale Graduate School of Arts and Sciences Dissertations. 1343.
https://elischolar.library.yale.edu/gsas_dissertations/1343
