THE INFLUENCE OF ORGANIC ADSORBATES ON THE UPD PROCESS - OXIDATION OFFORMIC-ACID AT UPD LEAD-MODIFIED PLATINUM-ELECTRODES

The oxidation of formic acid at Pt electrodes in the presence of under potentially deposited (UPD) Pb has been studied using an electrochemic al quartz crystal microbalance (EQCM). Although the current associated with the UPD process is largely obscured by current from the oxidatio n of formic acid, the mass response is dominated by the changes in UPD coverage. Thus examination of mass responses accompanying cyclic volt ammetric and constant-potential experiments reveals both variations in UPD coverage and the manner in which the underpotential deposits are affected by adsorbates derived from formic acid. At low concentrations of formic acid there is some suppression of the underpotential deposi t and data suggest that strongly adsorbing intermediates form most rap idly in the hydrogen adsorption region of potential. Mass responses al so indicate slight increases in UPD coverage upon removal of strongly adsorbed species by oxidation. Oxidation of high concentrations (0.1 M ) of formic acid induces a significant positive shift in the potential for removal of the UPD deposit on the positive scan, and on the subse quent negative scan the rapid reaction between the oxidized Pt surface and formic acid removes the oxide at a higher potential than normal a nd consequently allows the UPD process to begin at a more positive pot ential. Adsorption of Pb2+ at oxidized Pt surfaces is also inhibited b y the presence of formic acid.