OXYGEN REDUCTION IN ACID-MEDIA CATALYZED BY HEAT-TREATED COBALT TETRAAZAANNULENE SUPPORTED ON AN ACTIVE CHARCOAL - CORRELATIONS BETWEEN THEPERFORMANCES AFTER LONGEVITY TESTS AND THE ACTIVE-SITE CONFIGURATION AS SEEN BY XPS AND TOF-SIMS

The influence of heat treatment (HT) on CoTAA supported on SX Ultra sa mples before and after electrochemical tests has been investigated usi ng the rotating disk electrode (RDE) technique and cyclic voltammetry, The heat treated sample configuration was also scrutinized by two dif ferent surface spectroscopic techniques: X-ray photoelectron spectrosc opy (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS ). After a 100 h polarization period at 500 mV (RHE) (diffusion activa tion domain), optimal performances were found to occur with the 600 de grees C heat treated samples (HT 600). However, the most salient featu re was the observation of a fair activity and stability for the 800 de grees C heat treated samples (HT 800). Similar behavior was exhibited both with H2SO4 and Nafion(R) electrolytes. Collected data could be in terpreted on the basis of RDE and cyclic voltammetry theories, suggest ing a fast dioxygen mass transfer within the catalytic material in com parison with interfacial process kinetics. After these electrochemical tests a complete demetallation was taking place, as seen by XPS withi n the HT 800 samples. A protonation process was also clearly identifie d on the nitrogen ions. An attempt has been made to propose a catalyti c cycle in which variations of the different nitrogen oxidation states are occuring. The major obstacle encountered lay in the lack of XPS e vidence of the high nitrogen oxidation states. From ToF-SIMS data the existence of bonding between C, N and O can be established with NO3- f ormation. Under the same treatment conditions this process occurs diff erently with H(2)TAA precursor. From the collected data it turned out that catalytic cycles involving nitrogen ions possess a lower turnover than those with cobalt ion participation.