Type

Text

Type

Dissertation

Advisor

Sears, Trevor | White, Michael G | Wong, Stanislaus | Black, Charles | Senanayake, Sanjaya.

Date

2015-12-01

Keywords

Chemistry | Ethanol, Fe-Rh alloy, syngas

Department

Department of Chemistry.

Language

en_US

Source

This work is sponsored by the Stony Brook University Graduate School in compliance with the requirements for completion of degree.

Identifier

http://hdl.handle.net/11401/77141

Publisher

The Graduate School, Stony Brook University: Stony Brook, NY.

Format

application/pdf

Abstract

The world’s dependence on the limited supply of fossil fuels has provided the motivation for research into alternative renewable fuel sources. Ethanol is a promising alternative due to its low toxicity, high energy density, and compatibility with the current fuel distribution system. An attractive route to ethanol production would be through the conversion of synthesis gas or ‘syngas’ (CO + H2), which is currently used to produce methanol, hydrogen, and synthetic fuels (diesel and gasoline). However, there are currently no commercially used catalysts for efficient ethanol production from syngas. In general, catalysts for syngas conversion to ethanol should promote C‒C coupling while suppressing methane formation. Previous studies have shown that oxide supported bi-metallic catalysts composed of noble and early transition metals can potentially meet these catalytic requirements, but identifying the structure and active phase of these systems under reaction conditions remains a significant challenge. This work focuses on in situ characterization of Fe-Rh bi-metallic catalysts supported on titania (TiO2) and ceria (CeO2). Synchrotron based techniques (X-ray diffraction, pair distribution function analysis, and X-ray absorption spectroscopy) are used to elucidate catalyst structure under reaction conditions, which is compared to the catalytic efficiency obtained via reactor studies from mass spectrometry and gas chromatography. The first catalytic system studied is bimetallic Fe‒Rh/TiO2, synthesized via two deposition methods: diblock co‒polymer reverse micelle template and incipient wetness impregnation. Both approaches are used to control the particle size and overall composition in order to determine the effect each have on the catalyst structure. In general, the addition of Fe results in alloying with Rh. Further increasing Fe loading serves to increase the alloy concentration, but alloy formation is limited as metallic Fe forms in catalysts that contain over 4 wt% Fe. Ethanol selectivity was found to be dependent upon the alloyed Fe concentration. Fe‒modification was also found to suppress the overall CO conversion when coupled with the methane suppression, the conclusion is that the alloying of Fe and Rh blocks active Rh sites responsible for methane formation and other side reactions. Fe from the alloy and metallic deposits was also found to be carburized into Fe3C during the reaction, but the presence of Fe3C had a negligible effect on the product distribution and CO conversion. The second major catalytic system studied consists of Fe‒promoted Rh/CeO2, synthesized via the incipient wetness impregnation method. The alloying of Fe and Rh is promoted on CeO¬2 compared to TiO2, where a larger concentration of Fe‒Rh alloy was observed at lower Fe loadings than on TiO2. Metallic Fe was not formed, but Fe‒Rh alloy formation began to plateau at ~5 wt% Fe, indicating that there is a limit to alloy formation. Fe from the alloy was carburized during the reaction, but as with TiO2, a negligible effect was observed on product distribution and CO conversion. Ethanol selectivity also peaked at a lower Fe loading than TiO2‒supported catalyst due to the promotion of Fe‒Rh alloy on CeO2. Additionally, the ethanol enhancement peaked at ~ 3 wt% alloyed Fe and a further increase of alloy concentration lead to the suppression of ethanol and a subsequent increase in methane selectivity. Lastly, CeO2 differs from TiO2 in that an interface with Fe promotes ethylene production. Overall, Fe‒Rh/CeO2 appears to have reactivity comparable to TiO2‒supported catalysts, where the maximum CO conversion and ethanol selectivity are arguably the same. | The world’s dependence on the limited supply of fossil fuels has provided the motivation for research into alternative renewable fuel sources. Ethanol is a promising alternative due to its low toxicity, high energy density, and compatibility with the current fuel distribution system. An attractive route to ethanol production would be through the conversion of synthesis gas or ‘syngas’ (CO + H2), which is currently used to produce methanol, hydrogen, and synthetic fuels (diesel and gasoline). However, there are currently no commercially used catalysts for efficient ethanol production from syngas. In general, catalysts for syngas conversion to ethanol should promote C‒C coupling while suppressing methane formation. Previous studies have shown that oxide supported bi-metallic catalysts composed of noble and early transition metals can potentially meet these catalytic requirements, but identifying the structure and active phase of these systems under reaction conditions remains a significant challenge. This work focuses on in situ characterization of Fe-Rh bi-metallic catalysts supported on titania (TiO2) and ceria (CeO2). Synchrotron based techniques (X-ray diffraction, pair distribution function analysis, and X-ray absorption spectroscopy) are used to elucidate catalyst structure under reaction conditions, which is compared to the catalytic efficiency obtained via reactor studies from mass spectrometry and gas chromatography. The first catalytic system studied is bimetallic Fe‒Rh/TiO2, synthesized via two deposition methods: diblock co‒polymer reverse micelle template and incipient wetness impregnation. Both approaches are used to control the particle size and overall composition in order to determine the effect each have on the catalyst structure. In general, the addition of Fe results in alloying with Rh. Further increasing Fe loading serves to increase the alloy concentration, but alloy formation is limited as metallic Fe forms in catalysts that contain over 4 wt% Fe. Ethanol selectivity was found to be dependent upon the alloyed Fe concentration. Fe‒modification was also found to suppress the overall CO conversion when coupled with the methane suppression, the conclusion is that the alloying of Fe and Rh blocks active Rh sites responsible for methane formation and other side reactions. Fe from the alloy and metallic deposits was also found to be carburized into Fe3C during the reaction, but the presence of Fe3C had a negligible effect on the product distribution and CO conversion. The second major catalytic system studied consists of Fe‒promoted Rh/CeO2, synthesized via the incipient wetness impregnation method. The alloying of Fe and Rh is promoted on CeO¬2 compared to TiO2, where a larger concentration of Fe‒Rh alloy was observed at lower Fe loadings than on TiO2. Metallic Fe was not formed, but Fe‒Rh alloy formation began to plateau at ~5 wt% Fe, indicating that there is a limit to alloy formation. Fe from the alloy was carburized during the reaction, but as with TiO2, a negligible effect was observed on product distribution and CO conversion. Ethanol selectivity also peaked at a lower Fe loading than TiO2‒supported catalyst due to the promotion of Fe‒Rh alloy on CeO2. Additionally, the ethanol enhancement peaked at ~ 3 wt% alloyed Fe and a further increase of alloy concentration lead to the suppression of ethanol and a subsequent increase in methane selectivity. Lastly, CeO2 differs from TiO2 in that an interface with Fe promotes ethylene production. Overall, Fe‒Rh/CeO2 appears to have reactivity comparable to TiO2‒supported catalysts, where the maximum CO conversion and ethanol selectivity are arguably the same. | 138 pages

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