Metastable intermetallic compound Zn₃Co alloying from porous coordination polymer pyrolysis

MOF- or ZIF-derived carbon (MDC/ZDC) has attracted significant attention especially in catalysis applications, because well-dispersed metal or oxide nanoparticles over the MDC/ZDC matrix with retained MOF/ZIF porosity can be obtained with direct pyrolysis. Although it seems to be a facile approach, pyrolysis parameters could significantly alter the MDC/ZDC properties. However, an in-depth study on the pyrolytic factors has not been discussed. Herein, we manipulated ZIF structure, pyrolytic atmosphere, temperature, incubation time, and ramping/cooling rates for ZDC structural analyses. Compared to pure ZIF-67 derived carbon with only Co metal nanoparticles, it was observed that the carbonization process in core-shell ZIF-8@ZIF-67 (Z8@Z67) initiated the interdiffusion of reduced metal ions at the core-shell interface, which triggered the unique Zn-Co alloying behavior forming Zn₃Co when they were in close proximity. The resultant intermetallic Zn₃Co compound is unstable and has never been experimentally observed. By employing the in situ powder X-ray diffraction (PXRD) technique, we observed the fast-heating plus cooling process arrested or circumvented the phase segregation, and kept the metastable intermetallic Zn₃Co. For the first time, we report a method to maintain it at ambient conditions, protected by the porous MOF derivative from ZIF pyrolysis.