Adsorption and stability of (R)-(+)- and (S)-(-)-1-(1-naphthyl) ethylamine on a series of platinum single crystal surfaces: Implications for heterogeneous chiral hydrogenation

The adsorption, desorption, and stability of the powerful chiral modifier precursors R- and S- naphthylethylamine (NEA) on the {111}, {211}, {331}, {643}(R), and "polycrystalline" platinum surfaces have been examined by cyclic voltammetry. NEA is effectively irreversibly adsorbed at room temperature to a saturation coverage that corresponds to occupancy of similar to80% of the available metal area and there is no detectable transport of adsorbate between the various adsorption sites present. Most importantly, the molecule adsorbs indiscrimminately and with essentially equal rates at terrace and step sites, and in the case of the chiral {643}(R) surface, the adsorption behavior of the enantiomers R- and S-NEA is the same. Specifically, the intrinsically chiral kink, sites neither fa or nor disfavor the adsorption of either enantiomer. In every case, desorption of NEA occurs under electro-reducing conditions with {100} facets showing an enhanced desorption rate on die {643}(R), {211}, and polycrystalline surfaces. The implications of these findings for our understanding of heterogeneous enantioselective hydrogenation at platinum surfaces are discussed.