THE ELECTROCHEMICAL STABILITY OF MODEL INHIBITORS - A DEMS STUDY ON ADSORBED BENZENE, ANILINE AND PYRIDINE ON MONOCRYSTALLINE AND POLYCRYSTALLINE PT, RH AND PD ELECTRODES

The different metals show remarkable differences concerning the stabil ity of the adsorbed layers. On Pt, benzene is Completely oxidized to C O2 at potentials where oxygen adsorbes, whereas on Rh and Pd a part of the adsorbed benzene starts being desorbed without oxidation at poten tials where the oxide layer is formed. A negative going sweep after ad sorption leads to formation of benzene and/or cyclohexane. From Pt and Pd, cathodic desorption is nearly complete, whereas from Rh only 15% of the adsorbate is cathodically desorbable. The anodic desorption pro ducts of preadsorbed aniline are CO2 (on Pd) or CO2, HCN and NO (on Pt and Rh). A quantitative evaluation shows that the largest part of the total adsorbed nitrogen yields NO3-, being the final product of a rea ction sequence with HCN and NO as intermediates. Oxidation of pyridine on Pt leads to formation of CO2 and, at higher potentials, HCN and NO . Oxidation therefore starts at the carbon atoms. Surface concentratio ns indicate eta6-adsorption for aniline and benzene on Pt and Pd and p yridine on Pt. On Rh, the maximum coverages are less than one monolaye r, probably due to strong competitive adsorption of sulfate.