Nitrogen oxide (NOx) in exhaust gas is one of the primary sources of urban air pollution. In this study, zeolitic imidazolate frameworks (ZIFs) were synthesized from Zn, Co co-precursors, and the optimized porous Co/Zn-doped carbon nanoparticles (50-Co/Zn-CNPs) with an extremely high NO adsorption capacity (irreversible and reversible NO adsorption capacities of 560 and 110 μmol NO/g adsorbent, respectively) under mild operating conditions (373 K, 10% O2, and 500 ppm NO), which were fabricated via thermal decomposition of Co/Zn hybrid ZIFs at 1073 K. The high NO adsorption capacity of 50-Co/Zn-CNPs was attributed to the highly dispersed Co species in their structure. The 50-Co/Zn-CNPs retained 85% of their NO adsorption capacity in the presence of 5% water vapor, and their NO adsorption capacity was significantly higher than that of the conventional Pd-*BEA zeolite-based passive NOx adsorbers (67%) with the same metal loading under the same operating conditions.

To the best of our knowledge, this is the first report on the rapid one-pot synthesis of a unique core−shell-structured zeolitic imidazolate framework (ZIF) using Co(III) and Zn(II) precursors. The key to obtaining this unique structure is the use of a Co(III) precursor as the starting material. Transmission electron microscopy (TEM) reveals that Co was present within a 30-nmthick shell layer of the ZIF material. Thermal decomposition of the ZIF material affords core−shell-structured carbon nanoparticles that have Co on the external surface of the carbon grain. We have previously demonstrated that this carbonaceous material obtained by thermal decomposition exhibited high performance as an adsorbent for nitric oxide, even in the presence of excess oxygen and water vapor, and therefore, it was a suitable material for NO x elimination at low temperatures. The growth mechanism of the synthesized ZIF particles and the differences between synthesized ZIF and conventional Co(II)-ZIF-67 are discussed. The reactivity of the Co(III) precursor is much lower than that of the Co(II) species, leading to slower precipitation of Co(III) than that of Zn(II), thus forming the core−shell structure.