Insights into the Structural Evolution Process of Na/ZnFe₂O₄ Spinel Catalyst in CO₂ Hydrogenation

The CO2 hydrogenation to olefins process over Fe-based catalysts offer a promising route for the production of high-value chemicals from non-fossil route. Herein, we present a Na promoted ZnFe2O4 spinel catalyst and their structural evolution from fresh state to reduction and to reaction period was elucidated. The clear phase segregation between Fe and Zn occurred after reduction/reaction because of crystalline phase transformation in the order of ZnFe2O4 -> ZnO+Fe7C3+chi-Fe5C2 -> FeO+ZnO+Fe7C3+chi-Fe5C2. The formation of iron carbide phases (Fe7C3 and chi-Fe5C2) was enhanced dramatically by Na promoter. The adjacent Na promoter increase the electron density in Fe-Zn interfaces and enhance its ability to dissociate CO* to form adsorbed CHx* species, thus promoting the formation of olefins. This Na promoted rich electronic Fe-Zn interfaces alters the balance between iron oxides and iron carbides on the catalyst surface, which accelerate the chain growth reaction and facilitate the coupling efficiency of tandem reaction. The various catalysts were characterized by N-2 physisorption, SEM, HR-TEM, HAADF-STEM, ex-situ /in situ XRD, M & ouml;ssbauer spectra, ICP, in situ XPS, and operando DRIFTs.