(2la) Structure-Performance Relationship of Fe-Based Catalysts for CO2 Directly Hydrogenation to Linear ?-Olefins

CO₂ by direct hydrogenation to value-added products, provides a possible solution to the carbon neutrality. Due to the advantage of iron-based catalysts for CO₂ hydrogenation to linear α-olefins, which has gained increasingly attention in recent years^[1]. By leveraging advanced operando spectroscopic techniques and ex-situ characterizations, the surface/bulk evolution of precipitated iron-zinc catalysts in activation, reaction process of CO₂ hydrogenation was elucidated in this presented work.

The Fe-Zn-Na catalyst achieved excellent performance with a CO₂ conversion of 43.5%, a C_4-20 olefin selectivity of 60.7%, and an α-olefin selectivity of 89.3% under conditions of 1.5 MPa, 330 ℃, and 15000 h^-1. The results from In-situ Raman, XRD and other characterizations revealed a dynamic evolution of surface-subsurface-bulk structure during the catalyst activation. After the Fe-Zn mixed oxide precursor was pre-activated with 10% CO, Fe³⁺ was completely converted to χ-Fe₅C₂^[2]. For the first time, it was revealed that ZnO migrated to the surface during the activation process, and Zn-Na exhibited a co-aggregation trend, which regulated the surface geometry and electronic structure of the active phase^[3].

The performance evaluation showed that the addition of Zn increased the yield of linear α-olefins. Through temperature program desorption and in-situ FT-IR characterization, it was found that the addition of Zn provided a new CO₂ activation path, which improved the CO₂ conversion and chain growth ability. The results from in-situ XPS indicated a mixed valence form of Zn⁰ and Zn²⁺ on the catalyst surface, wherein Zn⁰ had an electron donating effect to Fe₅C₂, which promoted the desorption of olefins and inhibited the secondary hydrogenation of olefins. After activation, the activated catalysts were exposed to the CO₂ reaction atmosphere, triggering the reoxidation on the surface. Fe₅C₂ on the surface was oxidized to FeO_x, and the ratio of Zn⁰/Zn²⁺ decreased. Moreover, the introduction of Zn promoter significantly reduced the degree of reoxidation of the catalyst. ZnO-Zn/Fe₅C₂-FeO_x formed by the interaction between Zn and Fe, was the active phase of CO₂ hydrogenation to produce linear α-olefins^[4].