Date of Award
Doctor of Philosophy
John Z. Larese
Robert J. Hinde, Michael J. Sepaniak, Paul Crilly
The thermodynamics of molecular hydrogen adsorbed on MgO (100) surfaces has been examined using high resolution volumetric adsorption isotherm techniques. These studies were undertaken using highly perfect, narrow size distributed MgO nanocubes with essentially single (100) facet exposure and extremely high chemical purity. A narrowly spaced (in temperature) series of isotherms were performed between the temperatures of ~7K and the triple point (13.8 K) using H2 and D2 gas.
A minimum of five, and in some cases seven, discrete adsorption steps are clearly observed in each isotherm trace. Analysis of these data indicate that the monolayer film forms a two dimensional phase that is much more compressed than what is observed in the most compact plane of the bulk hcp crystal. This is most likely due to a strong molecule – substrate interaction. As the film thickness increases and the distance between the substrate and adsorbing molecules increases, the molecule-molecule interactions begin to dictate the behavior of the layer growth. The system shows many characteristics of complete wetting behavior.
Using standard thermodynamic analysis, the isosteric heat of adsorption, twodimensional compressibility, heat of adsorption, and differential (relative to the bulk values) enthalpy and entropy of each layer is determined. Phase boundaries including potential locations of critical points (that may be associated with phase changes) were also identified and used to generate the most complete phase diagram for the H2-MgO system to date.
Additional interesting adsorption behavior is uncovered upon examination of the numerical derivative of these adsorption traces; a series of intralayer features appear. There is at least one substep feature that is found between each major adlayer vertical risers. These features are identified (in the text)as 2f, 3f, 4f, and 5f. These features are also located in the aforementioned phase diagram. Future work is needed to identify the microscopic process(es) responsible for their appearance.
Frazier, Lillian Ruth, "Molecular Hydrogen Adsorbed on MgO (100) Surfaces: A Thermodynamic Study. " PhD diss., University of Tennessee, 2008.