Pyrolysis iron acetate

Jaouen, F. et al.. Oxygen reduction catalysts for polymer electrolyte fuel cells from the pyrolysis of iron acetate adsorbed on various carbon supports, J. Phys. Chem. B, 107, 1376, 2003. 

Figure 3.20. Comparison between changes with the pyrolysis temperature, of the catalyst activity (top panel) and the relative intensity of FeN2Cy (bottom panel), measured in cataysts obtained by adsorbing iron acetate (0.2 wt% Fe) on prepyrolyzed PTCDA and heat treating that material in inert atmosphere at various temperatures, ranging from 400 to 1,000 C. Prepyrolyzed PTCDA is obtained by heat treating PTCDA at 90O°C in H2 At NH3 (1 1 2) (according to Figure 8 in ref. [100] reproduced with permission of the American Chemical Society).

Table 3.1. Relative Abundance in % of FeN Cy" " Ions as a Function of the Pyrolysis Temperature for Catalysts Using Iron Acetate (0.2 wt% Fe) as Fe precursor (Reproduced from ref. [105] with Permission of the American Chemical Society)...

Before ending this section, it is worth mentioning two particular cases for which the carbon support was made from a carbon precursor during the pyrolysis step that also generated the catalyst. The first case was that of PTCDA, which was introduced in Section 2.2.3. Active catalysts were produced by adsorbing a metal precursor on that carbon precursor, then heat treating the resulting material, usually at 900°C in NH3 atmosphere. Our best results in fuel cells were obtained with catalysts made by this procedure, either with iron acetate (0.2 wt% Fe), or with ClFeTMPP (2 wt% Fe) as an Fe precursor. ... 

The apparent number of electrons transferred during ORR, , has been measured for several Fe-based catalysts obtained by the pyrolysis of various Fe precursors, including Fe-N4 chelates, iron phenanthroline or salts such as iron acetate. Values of n and the associated yields of hydrogen peroxide, %H202, are reported in Table 3.2 for Fe precursors and in Table 3.3 for Co precursors. %H202 is obtained from the following equation " %H202 = 100 (4 - n)tl. 

In the previous section, we came to the conclusion that two catalytic sites were always obtained simultaneously, but not in the same proportions, when an Fe precursor and an N precursor were present at the same time in the pyrolysis reactor. This demonstration was performed with the help of ToF SIMS analysis of either heat-treated iron acetate or CIFeTMPP adsorbed on N-enriched prepyrolyzed PTCDA . The two catalytic sites were labeled Fe-N4/C and Fe-N2/C, according to the relative abundance of their typical ions detected by ToF SIMS. While Fe-N4/C corresponds to the catalytic site proposed by van Veen and illustrated in Figure 3.5, the full coordination of Fe-N2/C, illustrated in Figure 3.19, is not completely known. Possible Fe-N2+2/C catalytic structures have been proposed by various authors > but have not yet been confirmed. In the following discussion we will continue to use the Fe-N4/C and Fe-N2/C labels to identify these catalytic sites. 

Fig. 10.7 Catalytic activity in O2 satiffated H2SO4 solution at pH 1 vs. pyrolysis tune for N234 pristine carlxtn black that was first etched in NH3 at 950 °C and then impregnated with 0.2 wt% Fe (as iron acetate) half-shaded star). A second pyrolysis of this material performed at 950 °C in pure NH3 black squares) ot in Ar black circles) resulted in a sharp increase in the catalytic activity (according to Fig. 5 in ref. [31] reproduced with permission of the American Chemical S(x iety)...

Following the discovery of the two important factors (the N and micropore contents) that govern the activity of catalysts made by the impregnation of a carbon black with 0.2 wt% Fe as iron acetate and its pyrolysis in NH3 at high temperature, in 2008, we proposed a structure that would replace FeN2/C, the incomplete catalytic site structure depicted in Fig. 10.4 that was previously introduced as a possible part of the most active type of catalytic site. 

In 1989, Gupta and coauthors showed that highly active catalysts can be prepared from less complex molecules [63]. In their work, they impregnated a carbon black with polyacrylonitril (PAN) and an iron or cobalt acetate. The precursors were heat-treated at different temperatures and the ORR activity was measured. It was found that one can generalize the preparation of Me-N-C catalysts Whenever a metal precursor is heat-treated with nitrogen and carbon sources at temperatures of >600°C (Co) or >700°C (Fe), an active catalyst can be obtained. A scheme of their preparation route and the achieved ORR activities (as a function of pyrolysis temperature) are given in Fig. 16.18. 

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