Date of Award

2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Abstract

Understanding the fundamentals of oxygen surface structures under a variety of conditions is pivotal to determining reactivity of heterogeneous catalysis. Exposure of catalytically active metal surfaces to high oxygen coverages results in a myriad of surface structures. A further complication is the formation of subsurface oxygen (Osub) or oxygen present in the near subsurface region of the metal. It is known to form in transition metals yet the absorption of oxygen and resultant formation of Osub is not equivalent across all catalytically relevant metals. As a result, it is difficult to predict the stability and efficacy of the formation of Osub in metals, as well as how the absorbed oxygen affects the reactivity of the metal. This dissertation investigates both Rh(111) and Ag(111) oxidized surfaces after exposure to gas-phase O atoms, utilizing a combination of surface science techniques including Auger electron spectroscopy (AES), low energy electron diffraction (LEED), temperature programmed desorption (TPD), and scanning tunneling microscopy (STM).Carbon monoxide (CO) oxidation over Rh(111) surfaces is a prototypical heterogeneous catalyzed reaction, especially when it comes to studying the oxygen species present during reaction. Investigation of the reactivity of adsorbed oxygen with CO shows insight into the mechanism of CO oxidation on surface oxygen. Through a combination of LEED, TPD, and STM experiments it was determined that CO oxidation on adsorbed oxygen shows a temperature dependence, with CO oxidation even occurring at and around room temperature. When Rh(111) is exposed to atomic oxygen at high temperatures, a myriad of structures form including oxides, adsorbed oxygen, and Osub. Studying the evolution of highly oxidized Rh(111) surfaces using STM and LEED lead to the discovery that upon Osub emergence from the bulk, the surface changes homogeneously. On Ag(111), Osub is temperature dependent, forming at temperatures < 500 K. Once 0.1 ML of Osub has formed on Ag(111), the surface uniformly reconstructs to a striped structure that persists at longer exposures. The maximum uptake of Osub in Ag(111) is at a temperature of 450 K. Using XPS, it was determined that originally the surface is covered in a single adsorbed O species yet as the oxygen coverage increases, a three-dimensional phase begins to form. These results indicate the importance of Osub in formation of oxygenaceous structures on Ag(111) and the conditions at which Osub forms. While planar surfaces allow for the study of oxygen uptake on catalytically relevant metals, using a curved crystal with well-defined step geometries and terrace widths allows for investigation of multiple surface structures simultaneously. An investigation of O-induced reconstructions of curved Ag(111) showed that A-type (100) steps were more conducive to O adsorption than B-type (110) steps. Furthermore, O uptake and reconstruction formation were more favorable on wide terraces since narrow B-type steps reconstruct less when compared to their A-type counterparts. The results illustrate the complexity of Oad reactivity with CO, the properties of Osub formation and its emergence, and the influence of step geometry on O adsorption on transition metal surfaces.

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