ZnO_x overlayer confined on ZnCr₂O₄ spinel for direct syngas conversion to light olefins

ZnCrO_x oxides coupled with zeolites (OXZEO) allow direct conversion of syngas into light olefins, while active sites in the composite oxides remain elusive. Herein, we find that ZnO particles physically mixed with ZnCr₂O₄ spinel particles can be well dispersed onto the spinel surfaces by treatment in syngas and through a reduction-evaporation-anchoring mechanism, forming monodispersed ZnO_x species with uniform thickness or dimension on ZnCr₂O₄ up to a dispersion threshold ZnO loading of 16.0 wt% (ZnCr₂O₄@ZnO_x). A linear correlation between CO conversion and surface ZnO loading clearly confirms that the ZnO_x overlayer on ZnCr₂O₄ acts as the active structure for the syngas conversion, which can efficiently activate both H₂ and CO. The obtained ZnCr₂O₄@ZnO_x catalyst combined with SAPO-34 zeolite achieves excellent catalytic performance with 64% CO conversion and 75% light olefins selectivity among all hydrocarbons. Moreover, the ZnO_x overlayer is effectively anchored on the ZnCr₂O₄ spinel, which inhibits Zn loss during the reaction and demonstrates high stability over 100 hours. Thus, a significant interface confinement effect is present between the spinel surface and the ZnO_x overlayer, which helps to stabilize ZnO_x active structure and enhance the catalytic performance.