THE ADSORPTION OF UNSATURATED ORGANIC-SPECIES AT SINGLE-CRYSTAL ELECTRODES STUDIED BY DIFFERENTIAL ELECTROCHEMICAL MASS-SPECTROMETRY

Using a thin layer cell for differential electrochemical mass spectrometry, we studied the influence of the crystallographic surface orientation of Pt single crystals on the adsorption of benzene, ethene and perchloroethylene. Whereas on Pt(110), adsorbed benzene is completely desorbed at negative potentials under hydrogenation to cyclohexane, it is desorbed as such from the (111) face. - Several adsorbate states are found when ethene is adsorbed on these surfaces. Only part of the adsorbate can be desorbed under hydrogenation to ethane and also dimerization to butane; the extent of this hydrogenative desorption being much higher in the case of the (110) surface. The remaining part of the adsorbate can be oxidized to CO2 at potentials where oxygen is coadsorbed. In addition, at the (110) face and, at higher adsorption potentials also at the (111) face, some CO-like adsorbate is formed during adsorption. - In the case of perchlorethylene, the adsorption is reductive, i.e. the C-Cl-bonds dissociate upon adsorption. Cathodic desorption also yields methane besides ethane pointing also to a C-C bond dissociation.