PROFILES OF ADSORPTION DURING THE OXIDATION OF SMALL ORGANIC-MOLECULES - OXIDATION OF FORMIC-ACID AT POLYCRYSTALLINE PT IN ACID-SOLUTIONS

The electrochemical quartz crystal microbalance has been used to monit or changes in adsorption at Pt electrodes during the oxidation of form ic acid in 0.1 mol dm-3 HClO4 solutions. This has been achieved throug h cyclic voltammetric, injection and open-circuit potential decay expe riments where mass profiles are recorded alongside the electrochemical response. Adsorption in the H underpotential deposition (UPD) region causes the mass to increase relative to the background electrolyte whe reas in the double-layer region of potential, increased coverage of st rongly adsorbed intermediates has the reverse effect. Removal of these latter species can be followed from the mass response because it lead s to a characteristic mass step. Subsequent to this process there is a region of potential where oxidation of formic acid occurs through con sumption of adsorbed OH or PtOH and mass responses reveal that, as con centration increases, there is increased adsorption of organic residue s here. The consumption of surface oxy species (OH(ads), PtOH or PtO) by formic acid also results in irreversible oxidation of the electrode surface being shifted to higher potentials with increasing formic aci d concentration, since it is only at the higher potentials that the ra te of the place exchange process can compete effectively with the reac tion with formic acid. The mass decrease associated with removal of th e surface oxide is also accelerated at higher formic acid concentratio ns and occurs at higher potentials.