Date on Master's Thesis/Doctoral Dissertation

5-2023

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Degree Program

Chemistry, PhD

Committee Chair

Thompson, Lee

Committee Co-Chair (if applicable)

Kozlowski, Pawel

Committee Member

Kozlowski, Pawel

Committee Member

Sumanasekera, Gamini

Committee Member

Buchanan, Robert

Author's Keywords

Physical chemistry; physics; inorganic chemistry; computational chemistry; microkinetic modeling

Abstract

Single-site transition-metal doped photocatalysts are a class of materials in which small transition-metal oxide clusters are embedded onto the surface of zeolites, graphene, and bulk semiconductors. Experimental findings demonstrate that VO4, CrO4, NbO4, and WO4 clusters embedded on an MCM- 41 mesoporous amorphous SiO2 are effective photocatalysts for oxidative reactions. The selective direct partial oxidation of methane (POM) to form methanol is essential for efficiently utilizing natural gas. Photocatalytic methane oxidation has advantages over traditional thermal catalysis as it can be deployed at small-scale facilities to reduce methane release into the atmosphere due to flaring potentially. Our work has aimed to examine the nature of the V, Cr, Nb, and W transition-metal oxide doped MCM-41 amorphous SiO2 (MOx/MCM-41) materials for their viability in the POM reaction towards the formation of methanol. To examine if the excited state electronic structures are localized to MOx species and if the low-lying excited states possess electronic structures that facilitate the POM methanol-producing reaction. Photoexcited state manifolds are examined to determine the nature of electronic excitation in these MOx/MCM-41 materials. We then elucidate the POM reaction mechanisms using computational studies involving density functional theory (DFT). We examine the role of terminal versus bridging oxygens at the photocatalytic center in enhancing photocatalytic efficiency. We calculate the energetics of the POM reaction on the various MOx/MCM-41 materials across reaction maps showing multiple possible photocatalytic reaction pathways. The calculated energetics are then analyzed through microkinetic modeling (MKM) to locate rate-determining intermediates and the reaction time scale of the POM cycle to find which photocatalytic MOx/MCM-41 material would most efficiently facilitate the selective formation of methanol.

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