Date of Award

8-2021

Document Type

Campus Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry/Physical/Analytical Chemistry

First Advisor

Michelle Foster

Second Advisor

Jason Green

Third Advisor

Neil Reilly

Abstract

CeO2 catalysts have proven to be effective in volatile organic compound (VOC) oxidation due to their oxygen storage potential and ability to chemisorb gas phase organic adsorbates. The surface of CeO2 is populated with numerous oxygen and hydroxyl species that facilitate chemisorption and oxygen donation during oxidation reactions. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is a popular tool for observing intermediate transformation mechanisms of VOC oxidation reactions over powder catalysts. DRIFTS traditionally uses a flow system where adsorbates are introduced to a catalytic surface via an inert carrier gas. These flow systems add a layer of uncertainty in DRIFTS analysis as the full removal of unreacted adsorbates from the surface is not accomplished. The presence of unreacted adsorbates allows the replenishing of transformed reactants and drastically hinders the observation of intermediate transformation trends. The removal of unreacted compounds before attempting to observe the transformation of intermediate species is necessary for a clear understanding. Here, a novel method of in situ DRIFTS analysis is developed that ensures the removal of unreacted adsorbates via chamber evacuation after initial adsorbate introduction. The evacuation of the sample chamber removes physisorbed VOC adsorbates while chemisorbed VOC intermediate species remain on the surface thus enabling clear observation of their transformation. This evacuated method is first utilized on the oxidation of methanol over CeO2 to add clarity to some open questions in the oxidation mechanism while validating the DRIFTS technique through comparison to traditionally obtained data. In this study the validity of the evacuated in situ DRIFTS method is confirmed. Additionally, new insight into the role surface hydroxyl species play in the chemisorption and oxidation of methanol on the surface is obtained. Finally, the intermediate pathway is clarified; namely, it is determined that the transformation of methanol requires a bridged (b) methoxy intermediate species as a precursor to formate production. After the validity of the evacuated in situ DRIFTS method was verified, the process is utilized to investigate the oxidation of toluene on the surface of CeO2. To date, it was unclear if benzyloxy and/or benzaldehyde intermediates species are present during the oxidation of toluene to benzoate on CeO2, clouding the oxidation pathway toluene undergoes on said substrate. In this study toluene is found to chemisorb via interaction with surface bridged hydroxyls and bare surface oxygen species resulting in chemisorbed benzyloxy species. The benzyloxy species are then further oxidized via interaction with surface bound hydroxyls to form benzoate. Next, the oxidation of toluene over 1% and 2% Ag/CeO2 is observed using DRIFTS for the first time in conjunction with the newly developed evacuated method allowing for the characterization of the mechanism for the oxidation of toluene to benzoate at room temperature over the Ag/CeO2 surface. During the reaction, toluene chemisorbs as benzyloxy and is oxidized to benzoate and then carbonate on the surface of Ag/CeO2 at 25 °C. The oxidation of benzoate to carbonate results in the benzene ring of benzoate being transferred to the catalytic surface and split apart. Any remaining benzoate is oxidized to carbonate as the temperature of the system is increased to 500 °C. Finally, the steam reforming of methanol reaction is observed over 1% Ag/CeO2 detailing the oxidation of methanol to carbonate at 25 °C. By comparing oxidized intermediate species present on CeO2 and Ag/CeO2 at 25 °C after introduction of methanol or toluene, low concentrations of Ag are confirmed to drastically increase the oxidative power of CeO2. The newly developed evacuated in situ DRIFTS method has been validated and utilized to add insight into the true intermediate transformation pathway for the oxidation of methanol and toluene on the surface of CeO2 and Ag/CeO2 in this dissertation.

Comments

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