AVS 62nd International Symposium & Exhibition
    Surface Science Monday Sessions
       Session SS-MoA

Paper SS-MoA10
Enantioselective Adsorption on Platinum Surfaces

Monday, October 19, 2015, 5:20 pm, Room 113

Session: Organics and Ionic Liquids: Surfaces, Layers, Interfaces and Chirality
Presenter: Francisco Zaera, University of California
Authors: S. Karakalos, University of California
F. Zaera, University of California
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Enantioselectivity in the adsorption of chiral compounds on solid surfaces may lead to preferential crystallization, a possible route to chiral separation of racemic mixtures, and also to a way to design enantioselective catalytic chemical reactions. Tests on the uptake of simple molecules on metal single-crystal surfaces as a function of the enantiocomposition of the adsorbate has revealed several interesting kinetic and thermodynamics. Specifically, we have been exploring possible enantiospecific behavior during the uptake of propylene oxide (PO) on Pt(111) surfaces. Three related observations will be reported here. In the first set of experiments, the amplification of enantioselectivity during adsorption was demonstrated for the case of a surface seeded with a small amount of enantiopure PO and then dosed with propylene, a non-chiral molecule, as the amplifier. Chiral chemical titration and isothermal kinetic adsorption experiments using collimated effusive molecular beams indicated the possibility of reaching enantioselectivity excesses of over 60% this way. Monte Carlo simulations provided a kinetic explanation for this effect in terms of both an adsorbate-assisted adsorption process and a bias in the chiral configuration the propylene molecules are driven to upon adsorption on the surface. The second example refers to the uptake of PO itself, where lower saturation coverages are seen with the racemic mixtures versus enantiopure samples by approximately 20%. This behavior could be explained in terms of adsorbate-assisted adsorption with different probabilities for homo- versus hetero-enantiomeric pairs. Finally, it was determined that the enantiomeric composition of PO monolayers eventually reverses the direction of the enantiospecfic enrichment seen in the kinetic uptake.