Mechanistic Studies of Copper and Iron Catalyzed Ammonia Oxidation

Abstract

Global energy demand is increasing because of industrialization and population growth. Fossil fuels (oil, natural gas, and coal) provide about 88% of the energy needed to meet current demand. The two major problems associated with fossil fuels are well established. First, fossil fuel resources are limited on our planet and will last approximately 120 (coal), 60 (natural gas), and 40 (oil) years, respectively. Second, burning fossil fuels releases greenhouse gases into the atmosphere which causes global warming. We are under tremendous pressure to find sustainable sources of energy. Hydrogen-enriched fuels such as ammonia (NH3), hydrogen (H2), ethanol (CH3CH2OH), and methanol (CH3OH) have recently gained attention as a hydrogen carrier fuels for storage of hydrogen. In contrast to other chemical fuels, hydrogen and ammonia are the only carbon-free renewable chemical fuels and can be produced from green sources. Green hydrogen is formed via water splitting powered by solar energy and ammonia is generated by the reaction of green hydrogen with nitrogen from the air. Among all environmentally friendly energy sources, hydrogen has attracted perhaps the greatest interest, as water is the only byproduct. However, the large scale applications of flammable H2 technology has been strongly hindered by concerns about safety, effectiveness, economical and regulated storage of hydrogen especially for its application as a fuel in road transport. Construction and operation costs for ammonia pipelines are more economical than the hydrogen pipelines. Due to its widespread use as a feedstock for nitrogen fertilizers, there is extensive infrastructure for ammonia storage and transport including rail, truck, oceangoing tankers, and pipeline. The transportation network has a key role in ammonia economy, more precisely, maritime trading is essential part of global distribution networks by having a comprehensive network of ports that delivers ammonia at large scale that makes ammonia promising energy vector. Analogous to C−C and C−H bonds in hydrocarbons, the N−H bonds in ammonia store chemical energy that can be converted to other forms of energy such as electricity. To increase efficiency of ammonia oxidation as a green fuel, cost-effective approaches for electrocatalytic oxidation of ammonia into electrical power are required. Homogeneous electrocatalysts are attractive methods to outline the fundamental mechanisms, spectroscopic, kinetic, and thermodynamic through which electrocatalytic ammonia oxidation may occur which is necessary to design catalysts with higher activity and selectivity. Herein we describe mechanistic studies that uncover mechanism of ammonia oxidation by copper and iron catalysts. Research from our laboratory on some β-diketiminato copper(II) amides [CuII]-NHR suggests that they are unstable towards reductive coupling to [CuI] and hydrazines RNH-NHR which are easily oxidized to diazenes RN=NR. These findings demonstrate that related [CuII]-NH2 intermediates could undergo N-N coupling to give [CuI] and H2N-NH2, which may finally disproportionate to NH3 and N2. Electrochemical studies reveal that copper(I) β-diketiminate complexes may be used as electrocatalysts for ammonia oxidation. Detailed mechanistic studies suggest that cationic copper(II) ammine complexes {[CuII]-NH3}+ are unstable towards deprotonation in the presence of a base to form reactive [CuII]-NH2 intermediates. The unique electronic structure of the β-diketaminatato copper(II) amides leads to a low energy pathway for N-N bond formation to generate N2H4. We also describe an electrocatalytic system for the oxidation of ammonia based on ferrocene (Cp2Fe), an inexpensive, robust catalyst utilizing Earth-abundant iron. Ferrocenium (Cp2Fe+), the 1-electron oxidized form of ferrocene, cleanly oxidizes ammonia to generate nitrogen gas (N2) and protons captured by excess ammonia as NH4+ with electrons reducing ferrocenium to ferrocene. This process occurs under electrocatalytic conditions to generate N2 with sustained current. Simple modification of ferrocene through sulfonation allows for solubility in liquid ammonia to enable electrocatalysis in highly concentrated, energy dense solutions of ammonia. Kinetic and computational analysis provides mechanistic insight into the oxidation of ammonia by ferrocenium.