Date of Award

Fall 12-2023

Document Type

Campus Access Thesis

Degree Name

Master of Science (MS)

Department

Chemistry

First Advisor

Mariam N. Ismail

Second Advisor

Jonathan Rochford

Third Advisor

Michelle Foster

Abstract

The dry reforming of methane (DRM) reaction converts the two most abundant greenhouse gases, CO2 and CH4, into desirable fuels (i.e., syngas) under extreme heat in the presence of a catalyst. Although the high energy required to drive this reaction is traditionally provided by a furnace, resulting in the emission of the same harmful greenhouse gases. By coupling thermal energy with solar energy, known as photothermochemical dry reforming of methane (PTC-DRM), the sustainability and efficiency of the reaction has been shown to improve. Excellent catalytic activity and stability under PTC-DRM conditions were designed using scarce and high-cost noble metals, although their widespread use is unrealistic due to their limited accessibility. Earth abundant semiconductor cerium oxide (CeO2) is a promising candidate for PTC-DRM due to its enhanced redox properties, which promote oxygen vacancies in the semiconductors lattice structure. This leaves CeO2 with excellent oxygen storage capabilities since oxygen vacancies serve as adsorption sites for CO2. However, CeO2 is only activated by UV irradiation, and compared to other PTC-DRM supports it provides a small surface area which limits nanoparticle dispersion. Metallic nickel (Ni0) has been shown to narrow the bandgap of CeO2 and has promising catalytic activity by being an active site for CH4. While Ni catalysts show great catalytic potential, efforts lie in resisting deactivation due to nanoparticle aggregation and/or coking. Ensuring highly dispersed nanoparticles and reducing the acidity of the catalysts surface have been explored to address the downfalls of Ni-CeO2 catalysts. High concentrations of oxygen vacancies and strong metal support interactions have also been shown to enhance catalytic activity and stability under PTC-DRM conditions. In this work, a hollow rod-like MOF-derived CeO2 support was synthesized, providing high surface area, and enhancing nanoparticle dispersion proven by TEM imaging and EDS mapping. This feature should help nanoparticles resist aggregation and simultaneously increase the number of catalytic active sites. Amphoteric Ga2O3 was chosen to reduce acidity of the catalyst surface and aid in coke accumulation. It was confirmed through XRD and XPS that the catalyst support existed as cubic fluorite CeO2 and was decorated with Ga2O3 alongside two different nickel species. XPS confirmed that nickel existed as Ni in close coordination with CeO2 as well as Ni0. XPS and H2-TPR suggested this strong metal support interaction between Ni and CeO2 was attributed to the addition of Ga2O3 nanoparticles. UV-Vis spectroscopy revealed enhanced visible light absorption and bandgap narrowing of CeO2 due to Ni0. Oxygen vacancies were assessed through XPS and Raman spectroscopy, showing an improved number of vacancies in the CeO2 support with Ga2O3 and Ni0 nanoparticles. Full characterization of Ni-Ga2O3-CeO2 photocatalyst was achieved in this work, and the results showed promising characteristics for a PTC-DRM catalyst. Photocatalytic potential of this catalysts remains unknown, therefore catalytic experiments must be performed under varying intensities and wavelengths of light (PTC-DRM) and in the dark (DRM) to determine the effect of wavelength on catalytic activity. The explored research provides an option for utilizing sustainable energy while using bountiful materials to mitigate greenhouse gas concentrations in our atmosphere.

Comments

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