Atomic Oxygen Adsorption and Absorption on RH(111) and AG(111)

A central question in the field of heterogeneous catalysis is how surface structure and subsurface species influence catalytic behavior. One key to answering that question is determining which surface structures and subsurface species are present under catalytically relevant conditions. This dissertation presents results of Auger electron spectroscopy, low energy electron diffraction, temperature programmed desorption, and scanning tunneling microscopy experiments on oxidized Rh(111) and Ag(111) crystals. Exposing Rh(111) to O2 produced a predominately (2 × 1) adlayer, but even after extended dosing, (2 × 2) domains were also present. Exposing Rh(111) to atomic oxygen yielded O coverages greater than 0.5 ML and (1 × 1) domains were observed to form along terrace step edges. However, (2 × 1) and (2 × 2) domains were still present. Atomic oxygen was used to oxidize Ag(111) in order to study the effect of sample temperature as well as oxygen flux and energy. When atomic oxygen was generated using a lower temperature thermal cracker, a variety of previously reported surface structures were observed. When O was generated using a higher filament temperature, the surface became highly corrugated, layers of Ag2O appeared to form, and little subsurface oxygen was observed. To investigate the role of sample temperature, the Ag(111) sample was held at various temperatures while being exposed to atomic oxygen. For short doses, sample temperature had minimal effect on surface reconstruction. For longer doses, changes in sample temperature in the range of 490 K to 525 K had a substantial impact on surface reconstruction and subsurface oxygen absorption. Higher temperature dosing yielded the same surface structures which were observed after short doses. Lower temperature dosing with atomic oxygen resulted in subsurface oxygen formation and new structures which covered the surface. The results indicate the rich complexity of oxygen/transition metal interactions and illustrate how reactive species can be used to produce high coverage surface structures under UHV conditions

Thermally Selective Formation of Subsurface Oxygen in Ag(111) and Consequent Surface Structure

A long-standing challenge in the study of heterogeneously catalyzed reactions on silver surfaces has been the determination of what oxygen species are of greatest chemical importance. This is due to the coexistence of several different surface phases on oxidized silver surfaces. A further complication is subsurface oxygen (Osub). Osub are O atoms absorbed into the near surface of a metal, and are expected to alter the surface in terms of chemistry and structure, but these effects have yet to be well characterized. We studied oxidized Ag(111) surfaces after exposure to gas-phase O atoms to determine how Osub is formed and how its presence alters the resultant surface structure. Using a combination of surface science techniques to quantify Osub formation and the resultant surface structure, we observed that once 0.1 ML of Osub has formed, the surface dramatically, and uniformly, reconstructed to a striped phase at the expense of all other surface phases. Furthermore, Osub formation was hindered at temperatures above 500 K. The thermal dependence for Osub formation suggests that at industrial catalytic conditions of 475 – 500 K for the epoxidation of ethylene-to-ethylene oxide, Osub would be present and is a factor in the subsequent reactivity of the catalysts. These findings point to the need for the incorporation of Osub into catalytic models as well as further theoretical investigation of the resultant structure observed in the presence of Osub

Double-Stranded Water on Stepped Platinum Surfaces

The interaction of platinum with water plays a key role in (electro)catalysis. Herein, we describe a combined theoretical and experimental study that resolves the preferred adsorption structure of water wetting the Pt(111)-step type with adjacent (111) terraces. Double stranded lines wet the step edge forming water tetragons with dissimilar hydrogen bonds within and between the lines. Our results qualitatively explain experimental observations of water desorption and impact our thinking of solvation at the Pt electrochemical interface

Monitoring the long-term degradation behavior of biomimetic bioadhesive using wireless magnetoelastic sensor

The degradation behavior of a tissue adhesive is critical to its ability to repair a wound while minimizing prolonged inflammatory response. Traditional degradation tests can be ex-pensive to perform, as they require large numbers of samples. The potential for using magnetoelastic resonant sensors to track bioadhesive degradation behavior was investigated. Specifically, biomimetic poly (ethylene glycol)- (PEG-) based adhesive was coated onto magnetoelastic (ME) sensor strips. Adhesive-coated samples were submerged in solutions buffered at multiple pH levels (5.7, 7.4 and 10.0) at body temperature (37°C) and the degradation behavior of the adhesive was tracked wirelessly by monitoring the changes in the resonant amplitude of the sensors for over 80 days. Adhesive incubated at pH 7.4 degraded over 75 days, which matched previously published data for bulk degradation behavior of the adhesive while utilizing significantly less material (∼103 times lower). Adhesive incubated at pH 10.0 degraded within 25 days while samples incubated at pH 5.7 did not completely degrade even after 80 days of incubation. As expected, the rate of degradation increased with increasing pH as the rate of ester bond hydrolysis is higher under basic conditions. As a result of requiring a significantly lower amount of samples compared to traditional methods, the ME sensing technology is highly attractive for fully characterizing the degradation behavior of tissue adhesives in a wide range of physiological conditions