ELUCIDATING INTERMEDIATE-SURFACE INTERACTIONS ON LA-BASED PEROVSKITES UNDER ELECTROCHEMICAL REDUCING  CONDITIONS FROM FIRST PRINCIPLES

Ariel  Whitten

Electrochemical systems operate far from equilibrium and often at high temperatures inducing harsh destabilizing conditions for the catalysts used to drive reactions. Although pure metal catalysts have been studied in the past to be used under these conditions, many recent studies utilize oxides to increase the stability of the catalysts and reduce cost of materials as noble metals, which are highly active are quite costly. Even though the stability increases with oxides, they do suffer from reduced activity as compared to noble or pure metal catalysts, thus this needs to be studied further. This dissertation details work performed on three different oxide systems under electric fields: CO2 reduction reaction on La-based perovskite surfaces, water-splitting on a Co3In2S2 surface and water formation on oxidized Rh tips. La-based perovskites have been widely studied to replace Ni/YSZ as cathode materials to conduct CO2 reduction to CO. Ni/YSZ is highly active for CO2 reduction but quickly degrades under electrochemical conditions. While La-based perovskites tend to be highly stable under the same conditions, they are not as active thus this must be improved. One of the major bottlenecks to this improvement is the lack of understanding of the underlying mechanism for CO2 reduction. Therefore, this dissertation studies three La-based perovskite surfaces (LaNiO3, LaCoO3 and LaFeO3) to establish an understanding of the surface adspecies through modelling XPS spectra and understanding the effects of coverage on the adsorption of CO2. Through this research, it is found that at differing coverages CO2 effects the electronic state of the surface differently augmenting the activity and changing the favorable adsorption configuration. In the second project covered in this dissertation, the formation of a hydroxide layer over Co3In2S2 under electrochemical water-splitting conditions is studied using both experimental and theoretical methods. The experimental observations showed that a hydroxide indium layer was formed over the bulk Co3In2S2 surface under the redox conditions. This layer both protected the surface from degradation and improved catalytic activity. Via theoretical modelling, we confirmed that Co3In2S2 had Weyl points in its band structure which confirm that this material is in fact a topological Weyl semimetal. Further, the calculations showed the formation of a hydroxide layer likely formed due to the presence of *OOH adsorbates on the surface thus it occurs under redox conditions. In the final project, we investigated the influence of applied voltages on the oxidation of Rh tips under H2 oxidation conditions. H2 oxidation is a complex reaction that exhibits oscillatory behavior as such is an area of active research since this occurs on nanoparticles as well. This work utilized field ion microscopy and field electron microscopy that operate using high positive or negative applied voltages on a metallic tip that mimics a nanoparticle surface. The field polarity was found to drastically change the reaction mechanism. The data detailed in this dissertation will be further used to improve a previous model of the surface to understand the reaction mechanism on the surface in more detail.

ELUCIDATING INTERMEDIATE-SURFACE INTERACTIONS ON LA-BASED PEROVSKITES UNDER ELECTROCHEMICAL REDUCING CONDITIONS FROM FIRST PRINCIPLES

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