Graduation Year

2016

Document Type

Thesis

Degree

M.S.C.H.

Degree Name

MS in Chemical Engineering (M.S.C.H.)

Degree Granting Department

Chemical Engineering

Major Professor

Venkat Bhethanabotla, Ph.D.

Co-Major Professor

John Kuhn, Ph.D.

Committee Member

Scott Campbell, Ph.D.

Keywords

Photocatalysis, Renewable Energy, Reactor Design, Upconversion

Abstract

With an ever-increasing human population, the importance in having sustainable energy resources is becoming increasingly evident, as the current energy habits have brought about massive atmospheric pollution in the form of CO2 emissions, resulting in a rise in the average global temperature. To battle the effects of climate change, many alternative energy resources have been investigated. Among these, photocatalytic conversion of CO2 to renewable hydrocarbon fuels such as methane and methanol is one of the most desirable, as it provides the opportunity to utilize the sun’s energy to convert CO2 to renewable fuels. The work in this study is primarily focused on developing a batch photoreactor system to improve the integrity of photocatalytic experiments and using that system to test the performance of Er-doped solid solutions of ZnO/GaN (ZG) towards photocatalytic reduction of CO2.

To upgrade the abilities from previous photoreactor systems, a novel photoreactor was deigned in SolidWorks and fabricated in-house. The photoreactor was designed to increase surface area at the gas-solid interface, improve utilization of the light source, and promote larger mass transfer rates of reactants to the catalyst surface. These goals were accomplished by immobilizing the catalyst on a transparent porous support, incorporating a threaded mount on top of the photoreactor for mounting an interchangeable LED to illuminate the catalyst bed, and recirculating the gas mixture through a closed loop system with a compressor, respectively.

Pure and Er-doped ZG photocatalyst samples were synthesized through the nitridation of Zn/Ga/CO3 layered double hydroxide (LDH) precursors. Erbium was chosen as a dopant to potentially enhance the photocatalyst by utilizing its upconversion photoluminescence properties. The LDH precursors were synthesized using a coprecipitation method. Levels of erbium doping were varied by [Er]/[Zn] = 0, 0.025, 0.05, and 0.10. ZnO/GaN solid solutions were chosen for their low bandgap energy so that visible light, roughly 40% of the solar spectrum [1], can be used to activate the catalyst. Diffuse reflectance spectroscopic data of the pure and Er-doped ZG samples were measured and used to calculate the bandgap energy. Bandgap values of EG = 2.53, 2.52, 2.56, and 2.56 eV were obtained for the [Er]/[Zn] = 0, 0.025, 0.05, and 0.10 samples, respectively. XRD data of the LDH samples indicated the formation of Zn/Ga/CO3 LDH and the Zn(OH)2, β-Ga2O3, α-GaOOH, and ZnGa2O4 impurity phases. Moreover, the broadening of the diffraction peaks in the Er-doped LDH samples suggested Er3+ ions substituted the Ga3+ ions in the LDH structure. XRD data of the pure and Er-doped ZG samples revealed strong peaks at 2θ = 31.86, 34.37, and 36.31°, indicating the formation of a solid solution of ZnO and GaN. Additionally, peaks at 2θ = 29.27, 48.79, and 57.86° indicate the formation of the secondary phase of Er2O3 in the Er-doped samples. Consequently, it was concluded that the Er3+ ions did not go into the crystal structure of the oxynitride solid solution. These findings were supported by the SEM images revealing hexagonal nanoplates and nanoprisms that coincide with the solid solution along with additional nanostructures corresponding to the Er2O3 phase.

During photocatalytic experiments with the pure and Er-doped ZG samples, CO2, and UV light (405 nm nominal wavelength), hydrocarbon production was observed to increase with increasing [Er/Zn]. However, results from control experiments with no catalyst while varying the nominal LED wavelength and the o-ring material suggested that hydrocarbon formation was partially or entirely the result of the o-ring photochemically degrading in the presence of UV light. An o-ring comprised of a silicone material yielded zero hydrocarbon formation in the presence of UV light, while this was not the case for o-ring materials of Viton® and Kalrez®. These findings can be applied to other research groups that plan to perform photocatalytic experiments in a photoreactor with o-rings while using a UV light source.

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