Oxygen reduction in acidic media catalyzed by pyrolyzed cobalt macrocycles dispersed on an active carbon: The importance of the content of oxygen surface groups on the evolution of the chelate structure during the heat treatment

In previous work the influence of heat treatment temperatures (from 500 to 900 degrees C) on two cobalt macrocycles (CoTMPP and CoTAA) support on active carbon with a high content of oxygen surface groups has been 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 electrochemical activity for different ranges of temperatures. The aim of this paper lies in the understanding of the role of the oxygenated groups content present at the carbon surface during the heat treatment process and its influence on the chelate thermal evolution. The thermal decomposition 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 macrocycle and the appearance of metallic particles. Moreover when large amounts of oxygenated groups were present on the carbon surface, cobalt oxides were detected. Electrochemical studies were also conducted to qualify the catalytic behavior of such structures (selectivity of the reduction, activity and ageing of the electrodes). Different results were observed compared to those reported in [1]: lower activities and higher deactivation process (except for the 800 degrees C heat treated catalysts) 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 destruction and sintering behavior and thereby lead to a slower ageing process. (C) 1997 Elsevier Science Ltd.