Defect sites at the (001) surface of mordenite: An ab initio study
- Author(s)
- Tomas Bucko, Lubomir Benco, Juergen Hafner
- Abstract
The mechanisms and energetics of the formation of various defects upon dehydration of the surface of pure-siliceous and Al-monosubstituted mordenite are investigated using a periodic ab initiodensity functional theory technique. An energetically favorable defect at the pure-siliceous surface is a strained two-membered Si–O ring (2MR) formed via elimination of a water molecule from a pair of neighboring terminal silanol groups. Assuming the formation of two-membered rings, the dehydration-energy of the (001) surface of pure-silica mordenite is 133 kJ/mol. A relatively high reaction barrier of 179 kJ/mol coincides with the experimental observation that these defects are formed at high temperatures >700 K. Despite a short Si–Si distance of 2.35 Å across the 2MR which is comparable to the bond length between Si atoms in silicon in diamond structure, the electron-localization function reveals no bonding interaction between Si atoms on the 2MR. In the Al-substituted surfaces, the dehydration proceeds via proton transfer from the Brønsted-acid site (BA) to a neighboring terminal hydroxyl group. The low values of two subsequent energetic barriers of dehydration of 13 and 10 kJ/mol suggest that the surface BA sites are likely to be destroyed at even modest temperatures. The most stable defects formed in this mechanism are ones containing a threefold-coordinated Al atom and a defect with both an Al atom and a bridging OH group located on a two-membered ring. The heat of reaction of only 9 kJ/mol and the activation energy of the transformation between these two configurations of 26 kJ/mol suggest that both defects occur with similar probability. © 2003 American Institute of Physics
- Organisation(s)
- Computational Materials Physics
- Journal
- Journal of Chemical Physics
- Volume
- 118
- Pages
- 8437-8445
- No. of pages
- 9
- ISSN
- 0021-9606
- DOI
- https://doi.org/10.1063/1.1565321
- Publication date
- 2003
- Peer reviewed
- Yes
- Austrian Fields of Science 2012
- 103009 Solid state physics, 103015 Condensed matter, 103025 Quantum mechanics, 103036 Theoretical physics
- Portal url
- https://ucris.univie.ac.at/portal/en/publications/defect-sites-at-the-001-surface-of-mordenite-an-ab-initio-study(e36f0273-602a-4b0b-82cc-85f80802cb54).html