This thesis presents new research on the topics of oxygen evolution catalysts inspired by Photosystem II’s active site, the oxygen evolving complex (OEC). The approach uses a 2-pyrine methanol scaffolding ligand. A second project involves the synthesis of green burn rate modifiers for inducing a catalytic burn rate effect in solid propellants and replacing lead with a friendlier metal that is as effective. The shift toward renewable energy sources has led to increased attention to new methods of storing and harnessing energy. Solar and wind energy are becoming cost-effective and will continue to improve. A problem with such renewables is peak production times for electricity on off peak usage. Peak electricity use is in the evening, but peak renewable production is earlier in the day. This creates a mismatch in pairing availability of electricity in the grid and its usage. The solution to this is the storage of excess energy. Currently, excess energy produced from all inputs, such as coal and nuclear, are used to pump water vertically as a means of energy storage. This water stores energy as potential energy like a dam. When the energy is needed, the water is allowed to flow downward through a turbine. This process is very energy inefficient and wastes a lot of energy through friction and heat loss. The need for an efficient and scalable storage method is desired to replace the methods currently in place. Hydrogen gas can be used instead of this method, by burning or using hydrogen in an electrochemical cell. Water can be used as a feedstock for hydrogen generation. Electrolytically generated hydrogen gas is scalable for energy grid storage, but efficiency problems in electrolytic water splitting continue to hinder greater adaptation of the technology. An electrocatalyst is needed to bring hydrogen generation to the forefront of grid storage.
Nature has developed an enzyme that produces molecular oxygen and protons from water, known as Photosystem II (PSII). The interest in Photosystem II is to better understand the mechanism the oxygen evolving complex (OEC) undergoes when it transitions through its various oxidation states, and to explore catalysts that are functionally similar to OEC to both understand the OEC and design better catalysts for oxygen evolution. Taking inspiration from nature with the CaMnO5 cluster as the active site in PSII, an ideal catalyst could evolve oxygen at an anode while readying the protons for hydrogen evolution at a cathode. This is the motivation for our exploration of hemicubane motifs with manganese and calcium and exploring solubility and reactivity in water.
A functionally related but structurally and compositionally distinct enzyme in oxygen metabolism is cytochrome c oxidase. This enzyme uses a copper and Fe-heme active site to reduce oxygen to water. A binuclear copper site located away from this active site reduces the cytochrome c to then allow the electron to transfer through to the active site. The oxygen reduction seems to occur between the Cu and Fe-heme sites, and not the adjacent Fe-heme site. In multi-copper oxidases, three coppers bind oxygen and a fourth copper site transfers electrons to the other coppers.
A second project of this thesis is the exploration of alternative metals and the isolation of said metal ions with stabilizers in solid propellants. Metal ions create a phenomenon in rocket fuel known as burn rate modification, with the ability to increase the surface burn rate and pressure of the propellant upon burning. These characteristics combine for increased velocity and acceleration of the rocket. The most commonly used burn rate modifier (BRM) has been lead-based, whose use in the manufacturing process is toxic. Lead is being replaced by other metal BRMs, but all alternatives shorten shelf life of the propellant by increasing radical chain decomposition.
In chapter 3, we describe the preparation and isolation of a Mn3 trimer, that was synthesized in inert atmosphere and anhydrous solvents. The trimer has a distorted octahedral and two distorted trigonal bipyramidal coordination environments for the MnII ions. Two ligands coordinate to open sites on the two distorted trigonal bipyramidal metal ions, that could coordinate solvents.
In chapter 4 we describe the preparation and isolation of Mn hemicubanes that are water soluble and coordinate with water. Over time the cluster oxidizes from exposure to water, indicating some interaction with an oxidant that can cause the alcohol arm of the ligand scaffolding to oxidize to a carboxylate. This phenomenon does not occur with a different ligand scaffolding, it only occurs with a (4-(dimethylamino)pyridin-2-yl)methanol. The Mn4 clusters, when doped with calcium exhibit electrochemical stability and catalytic activity in water. Mn4 clusters are stabilized by the presence of calcium in solution, but do not prevent the oxidation of the cluster over longer time frames.
In chapter 5, we describe electrochemical experiments coupled with oxygen experiments to determine cluster reactivity in water. It is found that the (4-(dimethylamino)pyridin-2-yl)methanol scaffolding yields water soluble clusters capable of oxidation. Bulk electrolysis with coulometry showed very little oxygen or hydrogen peroxide formation. The cluster is a poor catalyst for oxidizing water. It has been shown that doping the solution with CaII stabilizes the catalyst depositing on the electrode. This new catalyst does not generate additional deposition with every sweep, or with a constant positive potential. The 2-pyridine methanol showed similar reactivity with water oxidation, and with CaII present. The 2-pyridine methanol cluster does not oxidize over time in solution though.
In chapter 6 we describe analogous attempts to synthesize cobalt, calcium, and copper clusters and their isolation. Calcium formed mononuclear complexes with the ligand hydroxide groups still protonated, whereas cobalt formed hemicubanes clusters with bridging alkoxide ligand arms. Cobalt also formed two different scaffolding motifs; one motif is similar to the chapter 4 Mn ion hemicubanes, and a new cluster motif, where the pyridyl nitrogens bonded to the same corner Co, and the alkoxide arms formed the cluster. This left coordination sites in the middle of the cluster, where solvent would ligate the complex.
In chapter 7 we describe the replacement of the pyridyl groups of 2-pyridine methanol with five-membered heterocyclic rings, and their reactions with manganese and cobalt precursors. These reactions yielded degraded ligand products, protonated ligands with an anion, or a polymer with an unknown origin for the ligand scaffolding. The five membered ring-based ligand scaffolds thus proved to be too unstable for the formation of clusters.
In chapter 8, we describe the electrochemical experiments of a CuII dimer that showed promise for catalytic oxygen reduction. We present preliminary evidence that a synthetic Cu-Cu complex is able to reduce oxygen when in a Cu(I) state exposed to oxygen. The dimer shows catalytic activity in acetonitrile with oxygen present and when oxygen is removed the catalytic wave is absent. The dimer is robust and does not decompose during electrochemical experiments, though the signatures the dimer shows in the absence of water indicate water is binding to the dimer and interacting with it.
In chapter 9 we present the combination of a stabilizer with feasible metal catalysts to produce a copper complex that is coordinated with stabilizers with two outer sphere oxidizers. The synthesis of a CuII complex surrounded by n-phenyl urea produced a CuI tetrahedral acetonitrile complex with a perchlorate counteranion. These two reactions are competitive to each other, where the formation of the ACN complex is much easier and hinders the synthesis of the CuII complex. The CuII complex is square planar, with open sites in the z-axis, but has perchlorate in proximity to the open coordination site. This complex could be used as a BRM that has a localized stabilizer, while the other Cu-ACN complex could be a potential primary explosive.
These topics all involve coordination chemistry and their application to build better catalysts for their respective fields. Oxygen evolution enables better access to molecular oxygen for hospitals and a source of hydrogen for molecular hydrogen. Oxygen reduction enables the ability to drive electrochemical cells to consume oxygen to generate energy from the reaction with hydrogen. Stabilizing burn rate modifiers allows for the expansion of potential metals as catalysts for propulsion. / 08/30/2029
| Identifer | oai:union.ndltd.org:TEMPLE/oai:scholarshare.temple.edu:20.500.12613/10613 |
| Date | 08 1900 |
| Creators | Absil, Christopher, 0009-0004-0401-9144 |
| Contributors | Dobereiner, Graham, Zdilla, Michael J., 1978-, Valentine, Ann M., Torchinsky, Darius H. |
| Publisher | Temple University. Libraries |
| Source Sets | Temple University |
| Language | English |
| Detected Language | English |
| Type | Thesis/Dissertation, Text |
| Format | 402 pages |
| Rights | IN COPYRIGHT- This Rights Statement can be used for an Item that is in copyright. Using this statement implies that the organization making this Item available has determined that the Item is in copyright and either is the rights-holder, has obtained permission from the rights-holder(s) to make their Work(s) available, or makes the Item available under an exception or limitation to copyright (including Fair Use) that entitles it to make the Item available., http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | http://dx.doi.org/10.34944/dspace/10575, Theses and Dissertations |
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