Degree Type

Dissertation

Date of Award

1988

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

Patricia A. Thiel

Abstract

The interactions between oxygen and the Pd(100) surface are studied by using video-LEED (low-energy electron diffraction), AES (Auger electron spectroscopy), and TDS (thermal desorption spectroscopy). The temperature-dependence of the initial sticking coefficient implies a mobile precursor to adsorption (probably molecular oxygen). A metastable c(2 x 2) structure can be formed when adsorption is carried out at low temperature and high pressure. Possible models to explain this result are discussed, including a model in which an ensemble of eight empty sites is required for adsorbing oxygen dissociatively. The diffusion barrier of atomic oxygen is estimated to be 12 ± 1 kcal/mol. Oxygen induces two reconstructions above 400 K. The activation energy between an unreconstructed surface and a reconstructed surface is estimated to be 44 ± 4 kcal/mol. TDS results show three distinct desorption states, [alpha], [beta], and [gamma], which can be correlated to the ordered structures revealed during adsorption. The disordered p(2 x 2) adlayer is related to the high temperature desorption state, [alpha], which shows second-order desorption characteristics. The [beta] state is formed by oxygen desorbing from the ordered c(2 x 2) at temperatures lower than the [alpha] state, which is due to the repulsive interactions between the next-nearest neighbor adatoms. The lowest temperature desorption state, [gamma], is sharp and narrow, and shows zero-order desorption characteristics. This state is ascribed to oxygen desorption taking place from two phases in equilibrium: a reconstructed and an unreconstructed phase;Oxidation occurs when adsorption takes place above 600 K, and is signaled by c(2 x 2) and hexagonal ring-like patterns. These structures display completely different properties than the above-mentioned ordered adlayers and are probably related to bulk PdO;Finally, a [theta]-T diagram of this system is constructed based on the experimental data. Experiments are performed to prove that it represents a thermodynamically-allowed phase diagram. The phase diagram is very complex due to the reconstructions induced by oxygen.

DOI

https://doi.org/10.31274/rtd-180813-13095

Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/

Copyright Owner

Sheng-Liang Chang

Language

en

Proquest ID

AAI8825907

File Format

application/pdf

File Size

251 pages

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