Enhancement of Nitrite Oxidation by Heat-Treated Cobalt Phthalocyanine Supported on High Area Carbon

Abstract

Our ongoing effort is to develop efficient electrocatalysts and sensors for nitrite detection. Nitrite is one of the major components of wastewater from nuclear power production and involved in the corrosion and bacterial process known as the nitrogen cycle. It also plays important physiological roles in the form of NO, for example, as an intraand intercellular messenger, a neurotransmitter, and an immune system mediator. The detection of nitrite, therefore, is important from an environmental and biological point of view. We have been utilizing transition-metal (particularly Fe and Co) phthalocyanines and porphyrins for this purpose as they often display catalytic activities toward many important electrochemical reactions such as oxygen reduction and CO oxidation. We found that iron phthalocyanine (FePc) is a very effective catalyst for nitrite reduction, undergoing structural changes on the surface as a function of the redox state. Since the discovery by Jahnke et al. that the heat treatment of metal-N4 systems under an inert atmosphere can ensure both catalytic activity and stability, much effort has been poured to elucidate the nature of catalysis. Now there seems to be a consensus that different catalyst structures result depending on the heat-treatment temperature, thus leading to different catalytic activities. For low and medium pyrolysis temperatures, the metal-N4 moiety or its fragment of the macrocycles is responsible for the activity. With treatment at higher temperature, metal-N bonds are no longer found but rather metallic clusters surrounded by a graphite envelope are observed. In this paper, we have extended our previous research on nitrite reduction with FePc and its μ-oxo dimer, (FePc)2O to cover nitrite oxidation with CoPc this time. The changes in catalytic activity induced by heat-treatment at high temperatures (500-1000 C) have been correlated with structural aspects monitored by XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure).