Understanding the Superior ORR Activity of the Fe-N2+2 to That of the Fe-N4 Site
We showed that the original macrocycle (denoted as Fe-N4), in which the N is pyrolylic type can be differentiated from the active sites formed upon high temperature pyrolysis (denoted as Fe–N2+2), in which the N is pyridinic type by using Δµ. Combined EXAFS and XANES showed that the central Fe in Fe–N2+2 is off the N4–plane and in high spin state, whereas the central Fe in Fe–N4 is precisely in the N4-plane and in intermediate spin state. Upon the adsorption of ORR intermediates at high potential region, the displacement, the oxidation state, and the spin state of the central Fe change in different ways between Fe–N4 and Fe–N2+2 sites. DFT calculations showed that this different Fe–N switching behavior accounts for the superior ORR activity of Fe–N2+2 to Fe–N4.
The PANI-derived catalysts were prepared and provided by Gang Wu and Piotr Zelenay (Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM 87545). Use of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL), was supported by the U.S. Department of Energy, Office of Basic Energy Sciences. This publication was made possible by the Center for Synchrotron Biosciences grant, P30-EB-009998, from the National Institute of Biomedical Imaging and Bioengineering (NBIB). Support from beamline personnel Dr. Erik Farquhar and Mark Chance (X3B) are gratefully acknowledged.
(1) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. Science 2011, 332, 443-447.
(2) Tylus, U.; Ramaswamy, N.; Jia, Q.; Mukerjee, S. Meeting Abstracts 2012, 225-225.
(3) Ramaswamy, N.; Mukerjee, S. Advances in Physical Chemistry 2012, 2012, 17.
(4) Roth, C.; Benker, N.; Buhrmester, T.; Mazurek, M.; Loster, M.; Fuess, H.; Koningsberger, D. C.; Ramaker, D. E. J. Am. Chem. Soc. 2005, 127, 14607-14615.
(5) Jia, Q.; Ramaker, D. E.; Ziegelbauer, J. M.; Ramaswamy, N.; Halder, A.; Mukerjee, S. The Journal of
Physical Chemistry C 2013, 117, 4585-4596.