The stability of adsorbed quinoline and cinchonine on poly- and monocrystalline platinum surfaces

The adsorption of quinoline and cinchonine on Pt (1 1 1), Pt (3 3 2) and polycrystalline Pt electrode has been studied by differential electrochemical mass spectrometry (DEMS). From the surface-coverage data and from the potential dependence of both the faradaic oxidation current and the rate of CO2 formation during the oxidation of quinoline and cinchonine on Pt (1 1 1), we conclude that both molecules are bound through the pi-system to the electrode surface. The only anodic desorption product found was CO2. Surface concentrations for both molecules were found to be around 0.1-0.2 nmol cm(-2). It was also found that quinoline completely desorbs from the Pt (1 1 1) electrode around 0 V, provided that the electrolyte in the thin-layer cell is exchanged for fresh electrolyte; in contrast, desorption from Pt (3 3 2) and polycrystalline Pt is incomplete. Cinchonine does desorb in part from polycrystalline Pt, but not notably from Pt (1 1 1) due to an additional binding interaction of the exocyclic vinyl group linked to the quinuclidine moiety. No decomposition products, e.g. alkanes, were detected during such cathodic potential sweeps. Further experiments revealed that coadsorption of CO on polycrystalline Pt notably reduces the amount of carbon dioxide formed during subsequent anodic potential sweeps for pre-adsorbed quinoline and cinchonine, pointing to a partial displacement of the modifiers. In contrast, ethene is coadsorbed without displacing the original adsorbate and can still undergo hydrogenation when a negative potential is applied. Benzene is also coadsorbed to some extent, but its hydrogenation, which usually occurs on an unmodified surface, is largely diminished. (c) 2006 Elsevier Ltd. All fights reserved.