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Interaction of Acid/Base Probe Molecules with Specific Features on Well-Defined Metal Oxide Single-Crystal Surfaces
Abee, Mark Winfield
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Acid/Base characterizations of metal oxide surfaces are often used to explain their catalytic behavior. However, the vast majority of these studies have been performed on powders or supported oxides, and there is very little information available in the literature on the interaction of acid/base probe molecules with well-defined oxide surfaces of known coordination geometry and oxidation state. The well-defined, single crystal surfaces of Cu2O (111), SnO2 (110), and Cr2O3 (1012) were investigated for their acid/base properties by the interactions between the probe molecules and the well-defined surface features. The adsorption of NH3 at cation sites was used to characterize the Lewis acidity of SnO2 (110) and Cu2O (111) surfaces. The adsorption of CO2, a standard acidic probe molecule, was used to characterize the Lewis basicity of the oxygen anions on SnO2 (110), Cu2O (111) , and Cr2O3 (1012) surfaces. BF3, while not a standard probe molecule, has been tested as a probe of the Lewis basicity of the oxygen anions on SnO2 (110) and Cr2O3 (1012). By studying probe molecules on well-defined metal oxide surfaces with known coordination geometry and oxidation state, an overall evaluation of NH3, CO2, and BF3 as probe molecules can be made using the surfaces studied. NH3 probed differences in Lewis acidity of Sn cations on SnO2 (110), which had differences in coordination environments and oxidation states. But, NH3 adsorption failed to provide any direct information on differences in Lewis acidity of Cu cations in different local coordination geometries on Cu2O (111). CO2 is a poor probe of the Lewis basicity of oxygen anions on the metal oxide surfaces studied here. CO2 does not strongly adsorb to either SnO2 (110) or Cu2O (111). On Cr2O3 (1012), CO2 does interact with oxygen sites but in two different coordinations, which vary with surface condition, making a comparison of basicity difficult. In the cases studied here, CO2 either does not adsorb, or it does not provide a clear set of results that can be related simply to Lewis basicity. BF3 seems to be a much better probe of the Lewis basicity than CO2 for the well-defined metal oxide surfaces studied here. On SnO2 (110) and Cr2O3 (1012), the boron atom of BF3 directly interacts with oxygen sites by accepting their electrons. BF3 thermal desorption seems to provide a direct measure of the Lewis basicity of different surface oxygen species as long as they are thermally-stable in vacuum.
- Doctoral Dissertations