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Composition and structure of the alpha-AlF3(0001) surface.
A Wander1, B G Searle, C L Bailey
1CCLRC Daresbury Laboratory, Warrington, Cheshire, WA4 4AD, United Kingdom. a.wander@dl.ac.uk
The Journal of Physical Chemistry. B
|July 21, 2006
Summary
This study reveals the atomic structure of aluminum fluoride (AlF3) surfaces. The fluorine-rich surface reconstruction explains why AlF3 is inactive as a Lewis acid catalyst in industrial reactions.
Area of Science:
- Materials Science
- Surface Chemistry
- Catalysis
Background:
- Strong Lewis acid catalysts are crucial for industrial processes like Cl/F exchange reactions.
- Aluminum fluorides (AlF3) show promise as catalysts, but their surface structure is poorly understood.
- Surface properties significantly influence catalytic activity.
Purpose of the Study:
- To elucidate the atomic-scale structure of the basal plane surface of alpha-AlF3.
- To investigate the surface composition and termination under realistic conditions.
- To understand the implications of surface structure for Lewis acid catalytic activity.
Main Methods:
- Utilized advanced surface thermodynamics calculations.
- Employed hybrid-exchange density functional theory (DFT).
- Examined four potential terminations of the alpha-AlF3 (0001) surface.
Main Results:
- The alpha-AlF3 (0001) surface is consistently terminated by a layer with two fluorine atoms per unit cell across realistic fluorine pressures.
- Surface fluorine ions reconstruct to shield the aluminum (Al3+) ions from the surrounding environment.
- This reconstruction effectively masks the Lewis acid site.
Conclusions:
- The predicted surface structure of alpha-AlF3 is consistent with experimental observations of its inactivity as a Lewis acid catalyst.
- The fluorine-terminated and reconstructed surface explains the lack of catalytic activity.
- This fundamental understanding is vital for designing future catalysts.