Iron oxides interatomic distances

EXAFS confirmed that unpyrolyzed ClFeTMPP adsorbed intact on Vulcan and was still intact up to 325°C. After pyrolysis at 700°C (the optimum temperature for catalytic activity) the first shell of atoms around the Fe ion was still Fe-N4, but the interatomic distance contracted and the Fe ion was now in the N4 plane (Fe ion was above that plane for the intact molecule). The Mossbauer spectrum of samples heat treated at 800°C revealed the formation of magnetic iron oxides that could be chemically leached out. The leached material yielded a spectrum similar to that recorded at 700°C and attributed to the Fe-N4 moiety. Van Veen and collaborators proposed a reaction scheme of what they believed happened to the ClFeTMPP chelates upon pyrolysis. This scheme is illustrated in Figure 3.5. However, they warned the reader that, in Figure 3.5, while the overall structure of the iron catalytic site remained close to the original Fe-N4 moiety, there must be substantial local variation in how this unit is attached to the subjacent carbon support. This study is in agreement with an earlier study by McBreen and collaborators", who analyzed 9 wt% CoTMPP and FeTMPP chelates (about 0.7 wt% metal) adsorbed on Vulcan XC-72 and heat treated in inert atmosphere between 600 and 1,000°C. They found that the heat treatment of the macrocycles... 

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