Date of Award
2023-05-01
Degree Name
Doctor of Philosophy
Department
Environmental Science and Engineering
Advisor(s)
Dr. Geoffrey Saupe
Abstract
Modern advances have enabled rapid industrialization and urbanization. However, the consequences of global modernization have included significant pollution to the air and water resources. Consequently, many changes to energy use must come to address and mitigate damages to our future. Solar energy derived hydrogen gas is a clean, renewable fuel that does not produce carbon dioxide or other pollutants. My research investigates the use of heterogeneous photocatalysis, by focusing on solar photochemical energy conversion, which is the making of hydrogen from water using sunlight. The production of clean, renewable hydrogen fuel will enable the utilization of low-cost, pollution-free energy resources to help provide clean, pollution-free air. This work expands the potential of using metal oxide photocatalysts to directly photolyze water into low-cost hydrogen gas for use as a renewable fuel. The systems studied use tailored metal oxides to absorb light to create excited state electrons in the catalyst, and the electrons then reduce hydrogen ions in water into molecules of hydrogen gas. The electrons flow out of the catalyst and are depleted, and hence they must be replaced for the process to continue. The replacement process is accomplished by using electron donor molecules in the solution. Good electron donors are organic species, which become oxidized as the reaction proceeds. These donors are a key part of the overall process for making hydrogen gas via water photolysis. However, little is known about the effect of electron donor characteristics on the efficiency of water photolysis systems. Typically, small highly mobile molecules like methanol or ethanol are used to optimize the hydrogen generation process. However, these are relatively expensive donors, and practical low-cost options must be identified for solar hydrogen to become a realistic energy alternative. The goal of the research reported in Chapter 2 is to study the effect of using bulkier, heavier more complex electron donors in water photolysis reactions using heterogeneous metal oxide photocatalysts. Using nano TiO2 (Degussa P25) as a standard photocatalyst, several water-soluble electron donors were selected, used, and compared for their hydrogen generation activity under ultraviolet light. The donors studied were ethanol, isobutyl alcohol, xylose, d-glucose, and sucrose. The rate of diffusion of donor molecules towards the catalyst surfaces is a prime factor in determining their reactivity, so we hypothesized that the larger, heavier (i.e., slower) donors would perform worse than lighter ones. With the exception of ethanol, the trend observed was in fact the opposite. The donors were ranked as best to worst, as follows: ethanol (46.07 g/mol) & gt; sucrose (342.3 g/mol) & gt; d-glucose (180.16 g/mol) & gt; xylose (150.13 g/mol) & gt; iso-butyl alcohol (74.12 g/mol). Possible reasons for this trend are discussed. Chapter 3 reports on experiments using layered metal oxide materials to create novel porous photocatalysts for water photolysis. The parent layered materials used in the syntheses were KTiNbO5 and KNb3O8. The goal was to synthesize unique hybrid oxide materials by combining these two parent materials. To do this these two parent materials were chemically exfoliated into aqueous nanosheet colloids. By mixing colloidal nanosheet solutions of two different metal oxides, and by restacking the nanosheet mixtures using the sensitizer cation to cause the restacking, new hybrid porous metal oxide agglomerate photocatalysts were synthesized. The colloidal nano sheets were restacked (reassembled) into porous agglomerates using the cationic visible light sensitizer dye, ruthenium tris-bipyridine, Ru(bpy)32+. The sensitizer enabled these materials to absorb visible light, and thus render the catalysts active under sunlight irradiation. The porous agglomerates also possessed high surface areas, which is ideal for enhancing photocatalytic activity. The photophysical properties of the hybrid materials were expected to be intermediate between the two parent materials. Hydrogen evolution experiments were performed on various samples of hybrid catalysts using ethanol as the electron donor. However, the hybrid photocatalyst data proved to be inconclusive. Some of the issues encountered with these visible light hybrid catalysts are being addressed in future work. The use of porous nanosheet agglomerates continues to be a promising field for discovering new photocatalyst with high surface areas, high activity, and properties that can be tailored for catalyst optimization.
Language
en
Provenance
Recieved from ProQuest
Copyright Date
2023-05-01
File Size
p.
File Format
application/pdf
Rights Holder
Dr. Niveditha Nanda
Recommended Citation
Nanda, Dr. Niveditha, "Catalytic Water Photolysis For Generating Renewable Hydrogen Fuel" (2023). Open Access Theses & Dissertations. 3833.
https://scholarworks.utep.edu/open_etd/3833