Interaction of Acid/Base Probe Molecules with Specific Features on Well-Defined Metal Oxide Single-Crystal Surfaces

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Date
2001-09-05Author
Abee, Mark Winfield
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Show full item recordAbstract
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.
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- Doctoral Dissertations [13032]