Date of Award
Doctor of Philosophy
Patricia A. Thiel
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.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Chang, Sheng-Liang, "Kinetics of reactions of oxygen with the Pd(100) surface: Adsorption, desorption, reconstruction, and oxidation " (1988). Retrospective Theses and Dissertations. 9765.