Date of Award
Doctor of Philosophy (PhD)
With the rapid development of the economy, increasing combustion of fossils fuels has caused an increase in the atmospheric carbon dioxide (CO2) level, and has led to global climate change. As a mitigation approach, CO2 capture and conversion (CCC) can not only capture CO2, but also convert it to useable products, such as hydrocarbon fuels. Photocatalytic reduction is an attractive CCC option that directly harnesses inexpensive and abundant solar energy. Titanium dioxide (TiO2) is a widely used semiconductor for photocatalysis, and graphene nanosheets are a promising material for use in fabricating graphene-TiO2 hybridized photocatalysts. To realize the application of these materials for CO2 photoreduction, synthesis methods and pertinent material properties need to be investigated. In addition, a simple but practical kinetic model that can predict CO2 photoreduction performance is also needed, not only to save cost, but also to guide synthesis procedures.
Research presented in this dissertation bridges scientific and engineering gaps in CO2 photoreduction technologies that incorporate TiO2 with or without graphene materials. The work was conducted in three broad steps: material synthesis, photocatalyst design and testing, and kinetic model development.
(1) Synthesis of crumpled graphene-based materials and study of their chemical and physical properties
A furnace aerosol reactor (FuAR) was used to synthesize crumpled graphene oxide (CGO) nanoparticles, and their mobility and charging characteristics were systematically investigated. The projected area equivalent diameter was found to be approximately equal to the electrical mobility diameter for the CGO particles, regardless of their morphology. A differential mobility analyzer (DMA) is a convenient and efficient tool to classify CGO particles according to their physical diameters. In the charging characteristics study, Fuch’s theory fits the experimental data better than Boltzmann’s theory for most of the measured charging fractions, indicating that the conductivity of CGO particles plays an important role in affecting their charging characteristics. In addition to studying FuAR synthesized material, we also characterized the carbon material formed as a side product in a NASA plasma reactor for methane post-processing. It was crumpled graphene with a low oxygen content, and was only a few layers in thickness. This crumpled graphene material was used to synthesize the anodes of lithium-ion batteries, which showed high electrochemical performance.
(2) Design and performance testing of graphene-TiO2 based CO2 photocatalysts
We used a single-step aerosol method to synthesize aminated, reduced graphene-based nanocomposites consisting of crumpled r-GO with encapsulated TiO2 nanoparticles, thus creating open core-shell nanostructures (referred to as CGOATI). Furthermore, we synthesized TiO2/nitrogen doped reduced graphene oxide (NrGO) composites by a one-step urea-assisted hydrothermal method. Both two types of N-modified graphene-TiO2 materials were used for simultaneous gas-phase CO2 adsorption and photoreduction. The mechanisms of graphene and N modification in enhancing the CO2 photoreduction performance were investigated systematically.
Aerosol chemical vapor deposition (ACVD) was used to synthesize TiO2 thin films with columnar morphologies. To increase the CO yield of this columnar TiO2, oxygen vacancies and Ti-related defects (OTDs) were generated by applying electric potential to TiO2 under UV irradiation. We gained a mechanistic understanding of the formation and healing of these OTDs, and of enhanced CO2 photoreduction by defective TiO2 (TiO2-x) with electric potential.
(3) Kinetic model development and CO2 photoreduction performance prediction
We developed a kinetic model of gas-phase CO2 photoreduction over TiO2 thin films in a continuous flow reactor, considering three aspects: 1) light activation, 2) surface adsorption of reactants and products, and 3) reaction kinetics. The reaction rate constants were estimated by fitting the model with the experimental data, and used to further predict the CO yield for TiO2 thin films with different morphology parameters.
This work identifies and addresses several key issues regarding the synthesis of (crumpled) graphene-TiO2 based materials for CO2 photoreduction. The findings and conclusion from this work, will impact such fields as the aerosol processing of materials, and the morphology design of photocatalysts.
Pratim John D. Biswas Fortner
Rajan Chakrabarty, Elijah Thimsen, Wei-Ning Wang,