Operando Spectroscopic Study of Dynamic Structure of Iron Oxide Catalysts during CO2 Hydrogenation

Understanding of dynamic structure of active sites is of paramount importance for rational design of industrial catalysts. This work revealed the structure evolution of iron active phases for CO 2 hydrogenation over iron-based catalysts. With a combination of operando Raman spectroscopy and X-ray Diffraction coupled with online gas chromatography, the panoramic structure evolution of iron oxides (α-Fe 2 O 3 and γ-Fe 2 O 3 ) during activation and CO 2 hydrogenation were elaborated, that is, α-Fe 2 O 3 (γ-Fe 2 O 3 )→α-Fe 3 O 4 (γ-Fe 3 O 4 )→α-Fe (γ-Fe)→χ-Fe 5 C 2 (θ-Fe 3 C). Both iron carbides showed high catalytic activities while χ-Fe 5 C 2 exhibited higher selectivity to lower olefins but weaker chain growth probability than θ-Fe 3 C. Graphical Using operando techniques including Raman spectroscopy (ORS)-gas chromatography (GC) and operando X-ray diffraction (OXRD)-GC, we demonstrated that χ-Fe 5 C 2 and θ-Fe 3 C driven from α-Fe 2 O 3 and γ-Fe 2 O 3 precursors, respectively, are the active phases that are responsible for the different distribution of products.