Pyrolysis of Metal Organic Frameworks (MOF): Transformations Leading to Formation of Transition Metal-Nitrogen-Carbon Catalysts

Metal Organic Framework (MOF) materials link inorganic and organic units by strong bonds, forming porous structures. Pyrolysis consists of heat treatment in inert or reductive atmosphere. Pyrolysis of MOFs, which uses MOFs as a template, is a promising way to synthesize carbon-based oxygen reduction reaction (ORR) catalysts. The size, shape and composition of the pyrolysis product is controllable by choosing MOF-based precursors and tuning the pyrolysis parameters. Two MOFs that are frequently used in the ORR catalysis are selected in this study: ZIF-8 and ZIF-67, both of which can be synthesized under ambient temperature and pressure. Co-based ZIF-67 is considered as a good precursor to highly active Co-N-C ORR catalysts due to its high content of nitrogen and cobalt. Zn-based ZIF-8 has isostructural structure to ZIF-67, but contains Zn instead of Co. Currently, pyrolysis process is highly empirical, where process parameters are optimized via trial-and-error. In this study we aim to demystify pyrolysis process and provide understanding on transformation of ZIF-based precursors during pyrolysis. Our team has investigated the pyrolytic synthesis of both ZIF-8 and ZIF-67 via in situ STEM, EDS and XRD and used electrochemical techniques to assess pyrolyzed materials activity towards ORR. The structures of both MOFs shrunk in size after pyrolysis to 1050 °C under high-vacuum environment, indicating that material evaporation and decomposition happened. However, the structure of the ZIF-8 remained, while that of ZIF-67 did not. The product of pyrolyzing ZIF-8 contains graphitic planes along with amorphous carbon phases. On the other hand, metal carbide covered by graphitized shell was formed during the pyrolysis of ZIF-67. Ex situ XPS was conducted on the precursor, product of pyrolysis and the product after acid etching. The nitrogen contained in the product of pyrolyzing ZIF-8 will not be removed after acid etching. A low content of nitrogen was observed on the surface and few nanometers beneath the surface of the pyrolysis product of ZIF-67. Most of the cobalt that remained after pyrolyzing the Co-based ZIF-67 will be removed by acid-etching. The degree of graphitization is the main difference between the process of pyrolyzing ZIF-8 and ZIF-67. The metal that remained at high temperature (1050 °C) might have catalyzed the graphitization process and thus destroyed the structure. The electrochemical activity of the pyrolysis products of ZIF-8 and ZIF-67 before and after acid etching was assessed with a rotating ring disk electrode setup. The halfwave potential of ZIF-67 was very low (~0.6 V), whereas the halfwave potential of ZIF-67 was higher, at 0.7 V. ZIF-67 showed very low surface area, as the structure of the MOF was not preserved during synthesis, whereas ZIF-8 did preserve structure and featured high electrochemical surface area. Figure. XPS, STEM characterization and electrochemical activity assessment. (a) XPS survey results of ZIF-8, its pyrolysis product, and the product after acid etching. ArE: Ar Etching. (b) STEM images of ZIF-8 after pyrolysis. (c) XPS survey results of ZIF-67, its pyrolysis product, and the product after acid etching. ArE: Ar Etching. (d) STEM images of ZIF-67 when pyrolyzing to 1050 °C. (e) ring current density and (f) disk current density in O2-saturated electrolyte. Red solid: ZIF-8 pyrolyzed to 975 under H2 atmosphere. Red dash: ZIF-8 pyrolyzed to 975 under H2 atmosphere with acid etching. Black solid: ZIF-67 pyrolyzed to 975 °C under H2 atmosphere. Black dash: ZIF-67 pyrolyzed to 975 °C under H2 atmosphere with acid etching. Figure 1