Graduation Year

2023

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Chemical Engineering

Major Professor

Venkat Bhethanabotla, Ph.D.

Co-Major Professor

John Kuhn, Ph.D.

Committee Member

Anna Pyayt, Ph.D.

Committee Member

Xiaodong Shi, Ph.D.

Committee Member

Jing Wang, Ph.D.

Keywords

Carbon Dioxide Conversion, Density Functional Theory, Pelletized Catalyst, SiO2-supported Catalyst, Vibrational Frequency

Abstract

CO2 conversion to CO has the benefit of further conversion to hydrocarbon-based fuels and chemicals. Due to CO2 being an important greenhouse gas, its conversion to CO and an increase of the carbon cycling will help reduce the greenhouse effect. Reverse water-gas shift chemical looping (RWSG-CL) is a two-step process that can be employed for the conversion of CO2 to CO. The first step involves partial reduction of oxygen by hydrogen to form oxygen vacant oxide, and the second step is the conversion of CO2 to CO over these partially reduced materials to regain their original structures. The RWSG-CL process has the advantages of 100% selectivity of CO, lower energy requirement than other thermochemical processes, and higher reaction rate than photocatalysis processes. In previous research, perovskite oxides were shown to possess the ideal property of producing oxygen vacancy during reduction in the RWSG-CL process. Perovskite oxides are stable at high temperatures and can be made using earth-abundant elements, thus making them a viable choice for RWGS-CL. Currently, perovskite oxides, La0.75Sr0.25FeO3 (LSF) and La0.5Ba0.5FeO3 (LBF), have been proven to be viable materials for RWGS-CL. SiO2 was found to be an ideal support material. While these results were promising at a micro-scale reactor with powder form materials, additional research is needed to scale-up the materials to the required size for industrial use. This research is focused on the scale-up process of the powder perovskite oxides materials to pellet form to meet the purpose of industrial use and propose the surface mechanism of CO2 to CO conversion. The first approach consists of the synthesis of the powdered perovskite oxides and the formation of the pellet form the powdered materials. Supported-perovskite oxides powder (LBF/SiO2) is firstly formed by adding SiO2 to pure LBF during the sol-gel synthesis. Then, LBF/SiO2 is further made into pellets by extrusion or tableting method. Temperature-programmed reduction (TPR) and oxidation (TPO) experiments were performed and identified that the redox temperature is 550oC. The long-term chemical looping experiment combined with XRD and XPS were performed with the pellets revealed that these materials are highly stable during the CO2 to CO conversion. Crushing test were performed and showed a good mechanical property. The pellets showed CO yields of ~ 2.3 mmol/gLBF in 50 cycles semi-batch reactor experiments, indicating that LBF/SiO2 pellets are the candidate materials for further scale-up evaluation. To further illustrate the significant CO yield increasing after adding SiO2 supports (from ~ 0.2 to ~ 2.3 mmol/gLBF), during the second approach, the surface reaction was developed from the proposed reaction mechanism. XRD showed that the dominated surfaces are (100) for LBF, and (111) for LBF/SiO2. The density functional theory (DFT) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) methods were performed to find the vibrational frequencies of reaction intermediates. The results show that CO2 was adsorbed as carbonate then converted to CO. The reaction energies were calculated by DFT, and the results showed that the (111) plane requires lower energy than (100) plane. Combined with the XRD results, LBF/SiO2 then potentially converts more CO then LBF at the same temperature. This project is innovative in two ways; first, it targets the scale-up of materials for low-temperature CO2 to CO conversion from laboratory to industrial scale. Secondly, it proposes the reaction mechanism of the CO2 conversion. The results of this project establish a new industrial pathway for the mitigation of CO2 emission and provide an added benefit of producing a feedstock for renewable hydrocarbon fuel synthesis. The production of renewable hydrocarbon fuel will also help to slow the greenhouse effect.

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