OXYGEN REDUCTION IN ACIDIC MEDIA CATALYZED BY PYROLYZED COBALT MACROCYCLES DISPERSED ON AN ACTIVE-CARBON - THE IMPORTANCE OF THE CONTENT OFOXYGEN-SURFACE GROUPS ON THE EVOLUTION OF THE CHELATE STRUCTURE DURING THE HEAT-TREATMENT

In previous work the influence of heat treatment temperatures (from 50 0 to 900 degrees C) on two cobalt macrocycles (CoTMPP and CoTAA) suppo rt on active carbon with a high content of oxygen surface groups has b een investigated [1, 2]. The structural configuration of the chelates has been defined by applying two surface analysis spectroscopies (XPS and ToF SIMS) and setting on correlations with the corresponding elect rochemical activity for different ranges of temperatures. The aim of t his paper lies in the understanding of the role of the oxygenated grou ps content present at the carbon surface during the heat treatment pro cess and its influence on the chelate thermal evolution. The thermal d ecomposition of CoTAA, which has been deposited and heat treated onto the previously deoxygenated active carbon was investigated through XPS analysis. The XPS data exhibit a faster decomposition of the macrocyc le and the appearance of metallic particles. Moreover when large amoun ts of oxygenated groups were present on the carbon surface, cobalt oxi des were detected. Electrochemical studies were also conducted to qual ify the catalytic behavior of such structures (selectivity of the redu ction, activity and ageing of the electrodes). Different results were observed compared to those reported in [1]: lower activities and highe r deactivation process (except for the 800 degrees C heat treated cata lysts) were found. The main conclusion in the present work resides in the fact that large amounts of oxygenated groups on the surface of the active carbon can protect the chelate structure from rapid destructio n and sintering behavior and thereby lead to a slower ageing process. (C) 1997 Elsevier Science Ltd.