First Advisor

Theresa McCormick

Date of Publication

Spring 4-11-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.) in Chemistry

Department

Chemistry

Language

English

Subjects

Catalysis, Density functionals, Chemistry

DOI

10.15760/etd.6268

Physical Description

1 online resource (xx, 210 pages)

Abstract

The widespread use of solar energy has been limited in part by the issue of effective storage. Water splitting is a means of converting solar energy into chemical fuel, in the form of hydrogen gas. It can be performed through both photochemical and electrochemical processes, via the two half reactions of water oxidation and proton reduction. Electrochemically, the catalyst receives electrons from an external circuit, which can be coupled to a renewable source such as a solar cell. Photochemically, a light-absorbing molecule provides an excited electron to a catalyst, which either generates oxygen or hydrogen gas. The work described herein studies both photochemical and electrochemical proton reduction using two different systems made of inexpensive transition metal and organic components. Nickel pyridine 2-thiolate (Ni(PyS)₃-) (PyS=pyridinthiolate) has been demonstrated to have good stability and activity as a proton reduction catalyst. It is applied here as an electrocatalyst, due to the wealth of mechanistic information that can be obtained through electrochemical experiments. Six derivatives of Ni(PyS)₃- were synthesized through uniform ligand modification to all three PyS- ligands using a series of electron rich or poor substituents. The physical properties of interest were investigated experimentally through electrochemical methods and UV-vis absorbance spectroscopy. Specifically, the desired properties for a hydrogen production catalyst are high proton affinity, quantified through the pKa, and low overpotential, quantified through E⁰. Each compound was also studied in depth using computational modeling of the various possible catalytic pathways. By combining the results of computational study with experimental results, mechanistic insight could be gained. The same joint theoretical and experimental methodology has been used to study the effect of non-uniform ligand modification. Four heteroleptic compounds were selected for study, two containing electron poor ligands and two containing electron rich ligands in varied ratios. What is found is that not only do the electronics of each ligand influence physical properties, but the placement of each ligand matters as well. Finally, photochemical hydrogen production was performed using polyvinylpyrrolidone (PVP)-coated carbon quantum dots (CQDs) as a photosensitizer, and nickel nanoparticles (NiNPs) as a catalyst. Total hydrogen production by CQD/NiNP composites as a function of the amount of PVP coating was investigated as well as various mechanistic and photophysical properties.

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Persistent Identifier

https://archives.pdx.edu/ds/psu/25711

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