Core–Shell Zn–Co Zeolitic Imidazolate Framework-Derived Catalysts for the Reverse Water–Gas Shift Reaction

The reverse water–gas shift (RWGS) reaction is a promising route for converting CO2 into CO, an important feedstock for synthetic fuels and chemicals. In this study, Zn–Co zeolitic imidazolate frameworks (ZIFs), including ZIF-8, ZIF-67, ZIF-67-8, and the core–shell structures ZIF-8@67 and ZIF-67@8, were synthesized as catalyst precursors and thermally activated prior to catalytic testing. Transmission electron microscopy and elemental mapping confirmed the formation of well-defined core–shell architectures, while synchrotron X-ray diffraction verified the characteristic sodalite-type framework. Thermogravimetric analysis revealed substantial framework decomposition during activation at 700 °C. In situ time-resolved X-ray absorption spectroscopy (TR-XAS) showed that Zn remained predominantly in the Zn2+ state throughout heating, whereas Co2+ underwent progressive reduction to metallic Co0 at temperatures approaching 600 °C. Ex situ X-ray absorption spectroscopy confirmed the presence of Zn2+ species and metallic cobalt after activation. Catalytic testing of the activated ZIF-derived materials showed that Co-containing catalysts exhibited significantly higher RWGS activity than Zn-only ZIF-8-derived catalyst. Among the investigated samples, ZIF-67@8_C-500 achieved the highest performance, producing 2.50 μmol CO (equivalent to 50 μmol g−1 catalyst) at 600 °C with a H2/CO2 ratio of 2:1. The strong dependence of activity on ZIF-67 core loading indicates that metallic cobalt generated from the Co-rich core plays a dominant role in CO2 conversion. Thermal activation transformed the highly porous ZIF precursors into metallic Co-containing carbonaceous catalysts. The resulting structural evolution, rather than retention of the original porous MOF framework, governed the catalytic performance in the RWGS reaction.