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

Abstract

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 (10<u>1</u>2) 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 (10<u>1</u>2) 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 (10<u>1</u>2). 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 (10<u>1</u>2), 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 (10<u>1</u>2), 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.