Degree

Doctor of Philosophy (PhD)

Department

Department of Chemical Engineering

Document Type

Dissertation

Abstract

The mechanisms of ambient-temperature reactions of heteroatomic compounds catalyzed by ceria (CeO2), an archetypical reducible oxide, for enzyme mimetics, environmental protection, and chemical synthesis are investigated in this dissertation using theoretical methods. CeO2 is modeled with thermodynamically stable low-index surfaces exposed by commonly studied ceria thin films and nano particles. To understand phosphatase-like dephosphorylation activity, stable adsorption states and surface reactions of model phosphates are examined. Binding of the central P-atom to surface lattice oxygen (Olatt) supplemented by phosphoryl O-Ce interaction is the only stable adsorption state for the un-dissociated molecule. Deprotonation of phosphate monoesters, which are multi-protic acids, is energetically favorable on the amphoteric CeO2 surfaces, and it mainly affects the P-O ester bond scission energies. Peptidase-like activity toward cleaving stable amide bonds is studied next on model amine compounds including acetamide. Nucleophilic attack by Olatt on the carbonyl C of the amide group forming a tetrahedral intermediate is key to C-N bond scission. When crystalline facet, dispersion interactions, and solvation effects are taken into consideration, the results suggest (110) to be the more active than the other two facets for catalyzing deamidation in aqueous phase. In addition to these hydrolysis reactions, a condensation reaction has also been investigated for a model oxygenate (acetaldehyde), which is relevant to upgrading of biomass feedstock. Conclusive evidence for C-C coupling to selectively form crotonaldehyde on CeO2(111) under flow conditions is obtained theoretically in collaboration with infrared experiment. Overall, the catalytic activity of CeO2 for these reactions at ambient temperature originates from neighboring acid(Ce)-base(Olatt) site pairs through characteristically Langmuir-Hinshelwood mechanisms, instead of Mars-van Krevelen mechanisms that are prevalent at higher temperatures.

Committee Chair

Xu, Ye

DOI

10.31390/gradschool_dissertations.5759

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