Iron Modified Bismuth Titanate Pyrochlore Photocatalysts for Environmental Remediation and Solar Fuel Production
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Authors
Ragsdale, William C.
Issue Date
2016
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Thesis
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AbstractPhotocatalysis represents and emerging field in material and chemical engineering that can provide solutions for specific problems in environmental engineering and sustainable energy production. What is unique about photocatalysis is that it represents a scope that combines traditional catalytic chemical engineering with electrochemical principles and techniques. Traditional catalysis involves the flow, transport, and adherence of reactants, typically in gas or liquid phase, to recyclable catalysts, while electrochemistry involves the extrusion of electrical charge carriers for direct electrical energy or redox reactions. Combining these two features allows us for a direct mechanism for converting sunlight into useable energy and lies directly under the purview of modern chemical engineers. Of particular interest in the field of photocatalysis are multi metal oxides due to their low cost of acquisition and band gap tunability. The work that follows represents three years of my time working in the SOLAR lab under the supervision of Dr. Ravi Subramanian developing novel mixed metal oxide photocatalysts to help alleviate environmental and energy related concerns. In the first chapter we examine the catalytic activity of a pyrochlore phase bismuth titanate (Bi2Ti2O7-BTO) in driving the photo-assisted decomposition of a model pollutant, methyl orange (MO). The photoactivity of the BTO was probed with the inclusion of Fe with BTO and with the addition of a co-catalyst- Pt external to the Fe-BTO. The addition of Fe was shown to enhance BTO photoactivity by ~38%, while the presences of Pt along with Fe demonstrated the most favorable increase at 74 % compared to the plain BTO. The MO degradation was analyzed following a pseudo first order kinetic rate law. Under 100% visible light illumination we note that all catalysts demonstrate photoactivity. Specifically, a 10%, 15%, and 21% degradation of MO with BTO, Fe-BTO, and Pt/Fe-BTO respectively, was observed. Stability analysis of the photocatalysts indicates that a mild oxidative treatment at 350°C is sufficient to recover ~ 80% of the photoactivity lost over 6 hours of exposure to photo-illumination in 2 h increments. Further, for the first time, complementary photo-electrochemical and optical measurement tools have been used to systematically probe the functioning of BTO in the presence of Fe and Pt. Electrochemical impedance, chronopotentiometry (intermittent illumination studies), and fluorescence measurements reveals Fe aids in visible light assisted charge separation, Pt is not as effective with visible light as it is with UV, and that a high concentration of hydroxyl radical in the Pt/Fe-BTO is the basis for improved photoactivity of the catalysts. Using bismuth titanate pyrochlore as a case study in this work, we demonstrate the approach to leverage optical and photoelectrochemical tools for systematic analysis of other multimetal oxides for future work.In the second chapter we again examine the same catalyst, a pyrochlore based bismuth titanate (BTO) photocatalyst with incorporated Fe (Fe_BTO), for the photocatalytic production of hydrogen. Detailed insights into the photocatalyst performance in a methanol-water mixture were obtained by examining the effects of catalyst loading, light intensity, methanol concentration, and catalyst stability under repeated use. Among the parameters examined, the hydrogen yield of 37 mL g-1 using 150 mg catalyst, 30,000 lux, and using methanol concentration of 20 M was determined the most effective for maximizing hydrogen generation. Additionally, all the time resolved hydrogen generation experiments indicated the presence of a methanol concentration dependent 2-zone region: zone 1- slow hydrogen generation and zone 2 - rapid hydrogen generation. The existence of the 2-zone region is attributed to the role of the intermediates formed during the methanol oxidation process. The accelerated hydrogen generation is attributed to the formation of the intermediate formic acid, which is thermodynamically favored for rapid oxidation over methanol. Repeated use of the photocatalyst leads to over 70% loss in the Fe-BTO photoactivity. The productivity loss is attributed to the formation of surface-functional groups. The groups may be removed by a simple oxidative surface treatment to recover the photocatalyst without impacting the surface or physical properties of Fe-BTO.
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