Transition state for alkyl group hydrogenation on Pt(111)
Pingping Ye1, Andrew J Gellman
1Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA.
Journal of the American Chemical Society
|June 6, 2008
Summary
Substituent effects on platinum surfaces influence hydrogenation barriers. Increasing alkyl group polarizability and fluorination degree raises hydrogenation energy barriers, indicating a cationic transition state.
Area of Science:
- Surface chemistry
- Catalysis
- Physical chemistry
Background:
- Understanding substituent effects is crucial for designing catalytic processes.
- The Pt(111) surface is a model system for studying surface reactions.
Purpose of the Study:
- To investigate substituent effects on the hydrogenation transition state of alkyl groups on Pt(111).
- To correlate substituent properties with hydrogenation barriers using linear free energy relationships.
Main Methods:
- Dissociative adsorption of alkyl and fluoroalkyl iodides on Pt(111).
- Coadsorption with hydrogen followed by thermal desorption.
- Temperature-programmed reaction spectroscopy to measure hydrogenation barriers (ΔE(H)‡).
Main Results:
- Hydrogenation barriers increase with both alkyl group polarizability and degree of fluorination.
- A linear free energy relationship was established between substituent constants and ΔE(H)‡.
- Field reaction constant (ρF) = 27 ± 4 kJ/mol and polarizability reaction constant (ρα) = 19 ± 3 kJ/mol were determined.
Conclusions:
- The transition state for alkyl group hydrogenation on Pt(111) exhibits a slightly cationic character.
- Substituent effects significantly modulate surface reaction energetics.
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