CBE PhD Thesis Defense: “Modeling Diverse Processes at Oxide Interfaces”

In this thesis, ab initio methods including density functional theory are used in concert with molecular dynamics, enhanced sampling techniques, and microkinetic modeling to study oxide materials as applied to electrochemical ammonia synthesis, carbon mineralization, and the oxygen evolution reaction. Special attention is directed towards discussion of model selection and its relationship to the experimental system.

Perovskite oxide BaZrO3-based ceramic electrolytes are shown to favor migration of protons from the bulk to the surface followed by hydrogen evolution without the need for a recombination catalyst. Using this same BaZrO3 electrolyte model surface, microkinetic modeling of ammonia synthesis leads to the proposal of a new experimental methodology for improving selectivity to ammonia at elevated temperatures. The dissolution rates of Mg- and Ca-containing mineral oxides are shown to be surface dependent. Methods for calculating the lowest energy facets and terminations at different water chemical potentials are presented and discussed. Similarly, transition metal oxides under acidic oxygen evolution reaction conditions expose different facets and have different adsorbate coverages depending on material and reaction conditions. Activity and stability are intimately related to these condition-dependent changes. Multi-surface Pourbaix diagrams are presented that allow for targeting model systems that are most likely to compare well with experiment. To help reduce the inherent complexity of these materials, a model for predicting the hybridization energy due to interactions between adsorbates and metal oxide surfaces is presented pointing out the key electronic structure features dictating the strength of this adsorption interaction.

Agreement between theory and experiment for metal oxides depends strongly on model selection subject to the reaction environment constraints. Further insight can be gleaned from physical models such as the generalized concerted coupling model, which offers insights into how the oxide electronic structure can be tuned for an application.