INVESTIGATION OF THE OSCILLATORY ELECTROOXIDATION OF FORMALDEHYDE ON PT AND RH ELECTRODES BY CYCLIC VOLTAMMETRY, IMPEDANCE SPECTROSCOPY ANDTHE ELECTROCHEMICAL QUARTZ-CRYSTAL MICROBALANCE

The oscillatory oxidation of formaldehyde is studied on platinum and r hodium polycrystalline electrodes in alkaline and acidic solutions. Bo th potentiostatic current and galvanostatic potential oscillations are found under most circumstances, except in alkaline solution on platin um where only current oscillations are found and no potential oscillat ions. The role of a sufficiently large series resistance (either inter nal or external to the cell) is shown to be essential in the generatio n of the potentiostatic current oscillations. The main destabilizing p rocess in both systems is shown to be a (hidden) negative faradaic imp edance, which is identified by impedance spectroscopy. Surface mass me asurements with an electrochemical quartz crystal microbalance demonst rate the important role of oxide layer formation in the oscillations. The results are discussed in relation with earlier mechanistic studies of the formaldehyde electro-oxidation.