Relationships between Gas-Phase Ionization Energies and Solution-Phase Oxidation Potentials: Applications to the Electrocatalytic Production of Hydrogen from Weak Acids

Persistent Link:
http://hdl.handle.net/10150/194534
Title:
Relationships between Gas-Phase Ionization Energies and Solution-Phase Oxidation Potentials: Applications to the Electrocatalytic Production of Hydrogen from Weak Acids
Author:
Sakamoto, Takahiro
Issue Date:
2010
Publisher:
The University of Arizona.
Rights:
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
Abstract:
The transfer of electrons to and from a molecule is one of the more fundamental and important chemical processes. One such important example is the reduction-oxidation (redox) cycles in catalysts and enzymes. In the hydrogenase enzymes, adding and removing electrons is one of the key processes for generating H₂ from water molecules. Finding a direct free energy relation between the vertical ionization energies (IE(V)) measured spectroscopically by gas-phase photoelectron spectroscopy and the oxidation potentials (E(1/2)) measured thermodynamically in solution by cyclic voltammetry (CV) for molecules is an important aspect for developing effective catalysts. In this study, a series of organometallic compounds such as metallocenes were used for investigating the free energy relationships and catalysts inspired by the active sites of [FeFe]-hydrogenases enzymes were evaluated for their ability to produce H₂ from electrocatalytic reduction of weak acids. The first part of the dissertation explores metallocenes of the form (η⁵-C₅H₅)₂M (M= Fe, Ru, Os, Co, Ni) as the model for developing the free energy relation between gas phase ionization energies (IE(V)) and solution oxidation potentials (E(1/2)). It was found that computing the electronic properties of Cp₂Fe, Cp₂Ru, and Cp₂Os using VWN-Stoll and OPBE density functional theory (DFT) functional was successful with root mean square deviation (RMSD) of 0.02 eV between the experimental and calculated ionization energies. However, calculated ionization energies of Cp₂Co and Cp₂Ni were less successful with RMSD of 0.3 eV between the experimental and calculated ionization energies. Introduction of the B3LYP or M06 hybrid DFT functionals yielded much improved results (0.1 eV) over the previous combinations of DFT functional for Cp2Co and Cp2Ni. The energy relation between the two experimental measurements was established and further computational studies revealed that the solvation energy was the largest energy contribution between IE(V) and E(1/2) in the five studied metallocenes. The RMSD of the calculated oxidation potentials, after adjusting for the error in gas-phase ionization energies, was 0.09 V. The second part of the dissertation explores a series of catalysts inspired by the active sites of [FeFe]-hydrogenase enzymes; μ-(2,3-pyrazinedithiolato)diironhexacarbonyl (PzDT-cat), Fe₂(μ-X₂C₅H₈O)(CO)₆ (where X = S, Se, Te), and Fe₂(μ-1,3-SC₃H₆X)(CO)₆ (where X = Se and Te) for their ability to produce H₂ from weak acids utilizing the computational techniques and knowledge gained from the metallocene study. Even though the overall electronic perturbation from μ-(1,2-benzenedithiolato)diironhexacarbonyl (BDT-cat) to μ-(2,3-pyridinedithiolato)diironhexacarbonyl (PyDT-cat) to PzDT-cat is found to be small, the reduction potential of PzDT-cat was found to be 0.15 V less negative than that of BDT-cat resulting in less energy required for initiating electrocatalytic H₂ production over the BDT-cat and PyDT-cat. Lower reorganization energy has been achieved by substitutions of larger chalcogens at the Fe₂S₂ core. However, the electrocatalytic production of H₂ from acetic acid in acetonitrile was found to be diminished upon going from analogous S to Se to Te species. This is ascribed to the increase in the Fe–Fe bond distance with a corresponding increase in the size of the chalcogen atoms from S to Se to Te, disfavoring the formation of a carbonyl-bridged structure in the anion which is thought to be critical to the mechanism of H₂ production.
Type:
text; Electronic Dissertation
Keywords:
Catalysis; Density Functional Theory; Electrochemistry; Hydrogenase; Metallocene; Photoelectron Spectroscopy
Degree Name:
Ph.D.
Degree Level:
doctoral
Degree Program:
Chemistry; Graduate College
Degree Grantor:
University of Arizona
Advisor:
Lichtenberger, Dennis L.
Committee Chair:
Lichtenberger, Dennis L.

Full metadata record

DC FieldValue Language
dc.language.isoenen_US
dc.titleRelationships between Gas-Phase Ionization Energies and Solution-Phase Oxidation Potentials: Applications to the Electrocatalytic Production of Hydrogen from Weak Acidsen_US
dc.creatorSakamoto, Takahiroen_US
dc.contributor.authorSakamoto, Takahiroen_US
dc.date.issued2010en_US
dc.publisherThe University of Arizona.en_US
dc.rightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.en_US
dc.description.abstractThe transfer of electrons to and from a molecule is one of the more fundamental and important chemical processes. One such important example is the reduction-oxidation (redox) cycles in catalysts and enzymes. In the hydrogenase enzymes, adding and removing electrons is one of the key processes for generating H₂ from water molecules. Finding a direct free energy relation between the vertical ionization energies (IE(V)) measured spectroscopically by gas-phase photoelectron spectroscopy and the oxidation potentials (E(1/2)) measured thermodynamically in solution by cyclic voltammetry (CV) for molecules is an important aspect for developing effective catalysts. In this study, a series of organometallic compounds such as metallocenes were used for investigating the free energy relationships and catalysts inspired by the active sites of [FeFe]-hydrogenases enzymes were evaluated for their ability to produce H₂ from electrocatalytic reduction of weak acids. The first part of the dissertation explores metallocenes of the form (η⁵-C₅H₅)₂M (M= Fe, Ru, Os, Co, Ni) as the model for developing the free energy relation between gas phase ionization energies (IE(V)) and solution oxidation potentials (E(1/2)). It was found that computing the electronic properties of Cp₂Fe, Cp₂Ru, and Cp₂Os using VWN-Stoll and OPBE density functional theory (DFT) functional was successful with root mean square deviation (RMSD) of 0.02 eV between the experimental and calculated ionization energies. However, calculated ionization energies of Cp₂Co and Cp₂Ni were less successful with RMSD of 0.3 eV between the experimental and calculated ionization energies. Introduction of the B3LYP or M06 hybrid DFT functionals yielded much improved results (0.1 eV) over the previous combinations of DFT functional for Cp2Co and Cp2Ni. The energy relation between the two experimental measurements was established and further computational studies revealed that the solvation energy was the largest energy contribution between IE(V) and E(1/2) in the five studied metallocenes. The RMSD of the calculated oxidation potentials, after adjusting for the error in gas-phase ionization energies, was 0.09 V. The second part of the dissertation explores a series of catalysts inspired by the active sites of [FeFe]-hydrogenase enzymes; μ-(2,3-pyrazinedithiolato)diironhexacarbonyl (PzDT-cat), Fe₂(μ-X₂C₅H₈O)(CO)₆ (where X = S, Se, Te), and Fe₂(μ-1,3-SC₃H₆X)(CO)₆ (where X = Se and Te) for their ability to produce H₂ from weak acids utilizing the computational techniques and knowledge gained from the metallocene study. Even though the overall electronic perturbation from μ-(1,2-benzenedithiolato)diironhexacarbonyl (BDT-cat) to μ-(2,3-pyridinedithiolato)diironhexacarbonyl (PyDT-cat) to PzDT-cat is found to be small, the reduction potential of PzDT-cat was found to be 0.15 V less negative than that of BDT-cat resulting in less energy required for initiating electrocatalytic H₂ production over the BDT-cat and PyDT-cat. Lower reorganization energy has been achieved by substitutions of larger chalcogens at the Fe₂S₂ core. However, the electrocatalytic production of H₂ from acetic acid in acetonitrile was found to be diminished upon going from analogous S to Se to Te species. This is ascribed to the increase in the Fe–Fe bond distance with a corresponding increase in the size of the chalcogen atoms from S to Se to Te, disfavoring the formation of a carbonyl-bridged structure in the anion which is thought to be critical to the mechanism of H₂ production.en_US
dc.typetexten_US
dc.typeElectronic Dissertationen_US
dc.subjectCatalysisen_US
dc.subjectDensity Functional Theoryen_US
dc.subjectElectrochemistryen_US
dc.subjectHydrogenaseen_US
dc.subjectMetalloceneen_US
dc.subjectPhotoelectron Spectroscopyen_US
thesis.degree.namePh.D.en_US
thesis.degree.leveldoctoralen_US
thesis.degree.disciplineChemistryen_US
thesis.degree.disciplineGraduate Collegeen_US
thesis.degree.grantorUniversity of Arizonaen_US
dc.contributor.advisorLichtenberger, Dennis L.en_US
dc.contributor.chairLichtenberger, Dennis L.en_US
dc.contributor.committeememberEnemark, John H.en_US
dc.contributor.committeememberGlass, Richard S.en_US
dc.contributor.committeememberEvans, Dennis H.en_US
dc.identifier.proquest11303en_US
dc.identifier.oclc752261150en_US
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