Unraveling Trans-metalation Mechanism for the Formation of Fe-N4 Sites during Pyrolysis

Abstract

It is now generally accepted that the Fe-N4 sites are responsible for the high oxygen reduction reaction (ORR) activity of Fe-N-C catalysts in acidic environments. It, however, remains elusive how the Fe-N4 sites are formed during pyrolysis. This knowledge gap hinders synthesis efforts to improve the ORR activity of Fe-N-C catalysts by increasing the Fe-N4 site density and/or utilization. Herein, we investigate the Fe-N4 site formation mechanism by studying the Fe-N-C catalyst synthesized via a two-step route: a Zn-N-C substrate was synthesized via pyrolysis of zeolitic imidazolate framework (ZIF-8) mixed with 1,10 phenanthroline, followed by the chemical vapor deposition (CVD) of FeCl3 onto the Zn-N-C to form a Fe-N-C catalyst with a high density of Fe-N4 sites. A combination of characterization techniques was utilized to trace the metal sites evolution at different stage of the synthesis route. We identified the single-atom Zn-N4 sites in the Zn-N-C substrate, and the formation of Fe-N4 sites in the Fe-N-C during pyrolysis, accomplished with the disappearance of Zn-N4 sites. In addition, we identified gas-phase ZnCl2 during the CVD by applying the temperature-programmed reaction (TPR). These results together lead us to propose the trans-metalation mechanism for the formation of Fe-N4 on ZIF-8 with pre-existing Zn-N4 sites derived Fe-N-C catalysts: Zn-N4 + FeCl3 (g) + X → Fe-N4 + ZnCl2 (g) + XCl, wherein X represents species that can bind Cl such as H, Cl, FeClx, etc. We believe this mechanism also governs the formation of Fe-N4 sites for traditional synthesis of ZIF-8 derived Fe-N-C catalysts by pyrolysis of the mixture of ZIF-8 and Fe precursors: Zn-N4 + Fe-O4 + 4Y → Fe-N4 + Zn (g) + 4YO, wherein Y represents species that can bind O such as H, C, Fe, etc. Mixing Fe precursors such as FeCl3 with ZIF-8 or derived N-C prior to pyrolysis will lead to the formation of Fe-O4 sites during pyrolysis prior to trans-metalation.1 The preservation of gas-phase FeCl3 enabled by CVD makes all the formed Fe-N4 sites gas-accessible, resulting in full site utilization. In addition, the lower boiling point of ZnCl2 (732 ℃) compared to that of Zn (907 ℃) allows for lower temperature formation of Fe-N4 sites by CVD. The Fe-N4 site formation mechanism for the Fe-N-C catalysts derived from materials without pre-existing Zn-N4 sites remains unclear.(1) Li, J.; Jiao, L.; Wegener, E.; Richard, L. L.; Liu, E.; Zitolo, A.; Sougrati, M. T.; Mukerjee, S.; Zhao, Z.; Huang, Y.; Yang, F.; Zhong, S.; Xu, H.; Kropf, A. J.; Jaouen, F.; Myers, D. J.; Jia, Q., J. Am. Chem. Soc. 2020, 142, 1417.