Matching Items (4)
Description
The conversion of water to hydrogen and of carbon dioxide to industrially relevant chemical precursors are examples of reactions that can be used to store renewable energy as fuels or chemical building blocks for creating sustainable chemical manufacturing cycles. Unfortunately, current industrial catalysts for these transformations are reliant on relatively expensive and/or rare materials, such as platinum in the case of hydrogen generation, or lack selectivity towards producing a desired chemical product. Such drawbacks prevent global-scale applications. Although replacing such catalysts with more efficient and earth-abundant catalysts could improve this situation, the fundamental science required for this is lacking. In the first part of this dissertation, the synthesis and characterization of a novel binuclear iron fused porphyrin designed to break traditional scaling relationships in electrocatalysis is presented. Key features of the fused porphyrin include: 1) bimetallic sites, 2) a π-extended ligand that delocalizes electrons across the multimetallic scaffold, and 3) the ability to store up to six reducing equivalents. In the second part of this thesis, the electrochemical characterization of benzimidazole-phenols as “proton wires” is described. These bioinspired assemblies model the tyrosine-histidine pair of photosystem II, which serves as a redox mediator between the light-harvesting reaction center P680 and the oxygen evolution complex that enables production of molecular oxygen from water in cyanobacteria, algae, and higher plants. Results show that as the length of the hydrogen-bond network increases across a series of benzimidazole-phenols, the midpoint potential of the phenoxyl/phenol redox couple becomes less oxidizing. However, benzimidazole-phenols containing electron-withdrawing trifluoromethyl substituents enable access to potentials that are thermodynamically sufficient for oxidative processes relevant to artificial photosynthesis, including the oxidation of water, while translocating protons over ~11 Å.
ContributorsReyes Cruz, Edgar Alejandro (Author) / Moore, Gary F (Thesis advisor) / Trovitch, Ryan J (Committee member) / Sayres, Scott G (Committee member) / Arizona State University (Publisher)
Created2023
Description
Chemical modification of (semi)conducting surfaces with soft-material coatings containing electrocatalysts provides a strategy for developing integrated constructs that capture, convert, and store solar energy as fuels. However, a lack of effective strategies for interfacing electrocatalysts with solid-state materials, and an incomplete understanding of performance limiting factors, inhibit further development. In this work, chemical modification of a nanostructured transparent conductive oxide, and the III-V semiconductor, gallium phosphide, is achieved by applying a thin-film polymer coating containing appropriate functional groups to direct, template, and assemble molecular cobalt catalysts for activating fuel-forming reactions. The heterogeneous-homogeneous conducting assemblies enable comparisons of the structural and electrochemical properties of these materials with their homogeneous electrocatalytic counterparts. For these hybrid constructs, rational design of the local soft-material environment yields a nearly one-volt span in the redox chemistry of the cobalt metal centers. Further, assessment of the interplay between light absorption, charge transfer, and catalytic activity in studies involving molecular-catalyst-modified semiconductors affords models to describe the rates of photoelectrosynthetic fuel production as a function of the steady-state concentration of catalysts present in their activated form. These models provide a conceptual framework for extracting kinetic and thermodynamic benchmarking parameters. Finally, investigation of molecular ‘proton wires’ inspired by the Tyrosine Z-Histidine 190 redox pair in Photosystem II, provides insight into fundamental principles governing proton-coupled electron transfer, a process essential to all fuel-forming reactions relevant to solar fuel generation.
ContributorsWadsworth, Brian Lawrence (Author) / Moore, Gary F (Thesis advisor) / Moore, Thomas A. (Committee member) / Trovitch, Ryan J (Committee member) / Arizona State University (Publisher)
Created2020
Description
Liquid-phase exfoliation (LPE) is a straightforward and scalable method of producing two-dimensional nanomaterials. The LPE process has typical been applied to layered van der Waals (vdW) solids, such as graphite and transition metal dichalcogenides, which have layers held together by weak van der Waals interactions. However, recent research has shown that solids with stronger bonds and non-layered structures can be converted to solution-stabilized nanosheets via LPE, some of which have shown to have interesting optical, magnetic, and photocatalytic properties. In this work, two classes of non-vdW solids – hexagonal metal diborides and boron carbide – are investigated for their morphological features, their chemical and crystallographic compositions, and their solvent preference for exfoliation. Spectroscopic and microscopic techniques are used to verify the composition and crystal structure of metal diboride nanosheets. Their application as mechanical fillers is demonstrated by incorporation into polymer nanocomposite films of polyvinyl alcohol and by successful integration into liquid photocurable 3D printing resins. Application of Hansen solubility theory to two metal diboride compositions enables extrapolation of their affinities for certain solvents and is also used to find solvent blends suitable for the nanosheets. Boron carbide nanosheets are examined for their size and thickness and their exfoliation planes are computationally analyzed and experimentally investigated using high-resolution transmission electron microscopy. The resulting analyses indicate that the exfoliation of boron carbide leads to multiple observed exfoliation planes upon LPE processing. Overall, these studies provide insight into the production and applications of LPE-produced nanosheets derived from non-vdW solids and suggest their potential application as mechanical fillers in polymer nanocomposites.
ContributorsGilliam, Matthew Scott (Author) / Green, Alexander A (Thesis advisor) / Wang, Qing Hua (Committee member) / Moore, Gary F (Committee member) / Arizona State University (Publisher)
Created2020
Description
Hydrogenase enzymes capable of catalyzing proton reduction to produce H2 have generated a considerable interest due to increasing motivation in finding sustainable carbon free energy sources. A considerable amount of research has been focused on producing synthetic structures mimicking the hydrogenase catalytic site, but the activity seen in hydrogenase enzymes in aqueous near neutral pH has yet to be replicated. It is now clear that the protein structure surrounding the H-cluster enables the high activity by fine tuning characteristics of the catalyst, but the structure and complexity of hydrogenase enzymes makes it difficult to predict exactly how the secondary coordination sphere affects catalysis. This work looks at incorporating both synthetic molecular catalysts and hydrogenase mimics into peptide scaffolds to improve the activity for photo-driven H2 production in aqueous solutions. The first chapter of this dissertation shows a de novo heme binding peptide improving the activity of cobalt protoporphyrin IX (CoPPIX) upon coordination inside a four-helix bundle. The peptide bound CoPPIX exhibited a 5.5-fold increase in anaerobic and an 8.3-fold increase in aerobic activity compared to free CoPPIX, while also showing dramatic increases to stability and solubility. In the second chapter, this work is expanded by using a randomly mutated cytochrome b562 library to identify beneficial attributes for downstream implementation of an ideal coordination site. A high-throughput assay was developed to measure H2 production using WO3/Pd deposited on a glass plate for a colorimetric first-pass screen. This assay successfully measured H2 production from CoPPIX bound cytochrome b562 in the periplasm of cells and identified a possible mutant showing 70% more H2 production compared to the wildtype. The third chapter incorporated a hydrogenase mimic into a four-helix bundle using a semi-synthetic strategy yielding a 3-fold increase in activity due to catalyst encapsulation. The method created will allow for easy modifications to the synthetic catalyst or peptide sequence in future work. The systems developed in this work were designed to facilitate the identification and implementation of beneficial characteristics for future development of an optimal secondary coordination sphere for a peptide bound molecular catalyst.
ContributorsHalloran, Nicholas Ryan (Author) / Ghirlanda, Giovanna (Thesis advisor) / Mills, Jeremy H (Committee member) / Moore, Gary F (Committee member) / Arizona State University (Publisher)
Created2021