Catalysts by pyrolysis: Transforming metal-organic frameworks (MOFs) precursors into metal-nitrogen-carbon (M-N-C) materials

Abstract

Metal-organic framework (MOF) materials are promising precursors to transition into metal-nitrogen-carbon (M-N-C) catalysts with high catalytic performance. Pyrolysis is one of the most common ways to convert MOFs into electrically conductive MOF-derived M-N-C catalysts. General interests in using MOFs as precursors derive from changing the MOFs’ porous structure into the carbonaceous frame. However, this favorable structure is not always maintained, with the reason unidentified since pyrolysis is currently still more of an empirical process. Understanding the pyrolysis processes leading to the transformation of MOFs into M-N-C catalysts is essential to broaden the development of MOF-derived catalysts. We report here complete chemical and morphological evolution and transformation of two commercial MOFs with the same sodalite topology, the ZIF-8 and ZIF-67, via series of in situ and ex situ techniques. The results reveal that the structure of the ZIF-67 collapses after the pyrolysis due to the graphitization of the carbon. Graphitization is catalyzed by the metallic cobalt particles formed when the temperature rose above 460 °C. The ZIF-8 was found to be a promising precursor material since most of the metal evaporates at the early stages and does not aggregate to form metal particles. However, the insufficient metal-N active sites and intrinsic low electrocatalysis availability of Zn limit the performance of the final catalyst. To maintain the porous structure of MOFs it is important to control the metal species and prevent them from agglomeration. Direct utilization of high-concentration MOFs as pyrolysis precursors is not recommended without rational design to inhibit agglomeration and graphitization that are undesirable for catalyst activity.