The consecutive adsorption of formic acid and propargyl alcohol, as well as that of formic acid and ethanol on platinum in acid media were studied by on-line mass spectrometry (DEMS). Oxidation of the coadsorbed species remaining on the electrode surface after a flow-cell experiment yields only CO2. Using isotopically labeled H13COOH, the contribution of formic acid during the electro-oxidation of the coadsorbates can be distinguished from that of ethanol or propargyl alcohol residues. It is found that ethanol replaces formic acid residues, whereas the adsorption of propargyl alcohol is modified by the presence of formic acid on the surface. Formic acid cannot chemisorb on a platinum surface covered by propargyl alcohol residues, but reacts without replacement with platinum modified by ethanol residues.
organic adlayers; mass spectrometry; isotopic labeling; adsorbate replacement
