OSCILLATING ELECTROCATALYTIC SYSTEMS .2. INSITU UV-VISIBLE REFLECTANCE SPECTROSCOPIC INVESTIGATION OF FORMALDEHYDE OXIDATION ON RHODIUM IN ALKALINE-MEDIUM

In the first part of this series it was shown that the electrochemical oxidation of formaldehyde on rhodium in alkaline medium could lead to strong and well reproducible current oscillations under potentiostatic control. In this part, an extensive "in situ" investigation of the interface under working conditions is carried out by means of UV-visible reflectance spectroscopy, in order to determine whether rhodium oxides are, or are not, involved in the oscillating electrochemical process. Firstly, to serve as reference, various types of rhodium oxides were grown under given experimental conditions of pH and potential, and characterized by their reflectance spectra. Then, observing the rhodium surface during the electrocatalytic oxidation of formaldehyde, it was possible to show that the current oscillations were accompanied by in-phase significant periodic reflectivity changes. Finally, the dependence of the magnitude of the reflectivity oscillation on wavelength allowed us to suggest that periodic buildup-removal of superficial film oxides are involved in the mechanism.