Pyridine adsorption on polycrystalline platinum studied by the radioactive-labeling method

The adsorption of pyridine on a polycrystalline platinum electrode was measured by using a radioactive-labeling technique. The potential dependence and concentration dependence of adsorption were measured in 0.1 M HClO4, and two exchange experiments were performed. The potential dependence revealed behavior typical for a strongly adsorbed organic species; a broad plateau of maximum adsorption occurs in the double-layer region with decreased adsorption toward potential extremes. No exchange of surface species with added unlabeled solution pyridine occurred at 0.2 V (vs Ag/AgCl, 1 M Cl-), but slow exchange was observed at -0.2 V over about 2 h; slow hydrogenation is the most likely cause of the loss of adsorbate t this potential. Two different concentration dependence profiles were observed depending on the treatment of the platinum electrode surface; a clean surface gave consistently higher packing densities as a function of pyridine concentration than one that had been pretreated at 10-6 M. The observed packing densities correspond to horizontally oriented pyridine, formed at low concentrations, and vertically oriented pyridine, formed at high concentrations