UC Santa Barbara

Over the past two centuries, fossil fuels have been our main global energy currency of choice by exploiting the energy stored in C–H chemical bonds. Fossil fuels comprise roughly 86% of our primary global energy and their combustion contributes to irreversible climate change. The development of alternative energy storage platforms is considered necessary in the shift to decarbonize energy through use of clean alternative energy sources. However, due to the intermittent energy production of these, the development of new energy storage technologies is needed in order to meet an increasing global energy demand. The use of chemical energy vectors, such as H2, has been considered one of the most complementary platforms to reaching large-scale energy storage. Although H2 has many attractive qualities, there are many factors impeding us from utilizing it as our main energy currency. H2 has the highest gravimetric energy density of any clean fuel, but it has the lowest volumetric energy density, making the storage and transportation difficult. Therefore, H2 storage carriers – like NH3 – have been proposed to overcome this. Our focus in this work has been to utilize a homogenous approach with readily available transition metal frameworks, specifically a (salen)M≡N complex, to probe the requirements necessary to undergo NH3 oxidation to N2, to release the H2 equivalents stored within. A nitride homo–coupling reaction to N2 – a major step in catalytic NH3 oxidation – will be detailed with our results in isolating and characterizing a rare, mixed–valent MnIII–N≡MV species. Subsequent development of a genuine synthetic cycle for NH3 oxidation based on this chemistry will be described, in an effort toward generating a truly catalytic system.

Nuclear energy is considered one of the most important components in decarbonizing energy, but the inadvertent release of radioactive materials from storage repositories into the environment poses a potential threat to human health. Uranium (most commonly found as the uranyl ion; UO22+) is the key element in nuclear fuel and comprises > 96 % of spent nuclear waste, which can be reprocessed and reused for fuel, while reducing the long–term radiotoxicity and quantity of waste stored in geological repositories. Therefore, developing methods for the separation and recovery of UO22+ from lanthanides and trans-uranics is of upmost importance for the long–term viability and safety of the nuclear energy sector. Herein, we highlight our results in harnessing the redox-switchable chelating and donating properties of ortho-substituted closo- carboranes, leading to the controlled chemical or electrochemical sequestration and recovery of UO22+ in monophasic (organic) or biphasic (organic/aqueous) schemes. Building on these results, we also describe the development of a biphasic extraction system for selective UO22+ separation and recovery in aqueous mixtures of actinides(IV), lanthanides(III), and alkali(I) metals, commonly found in nuclear waste.