Dynamic Evolution of Fe and Carbon Species over Different ZrO2 Supports during CO Prereduction and Their Effects on CO2 Hydrogenation to Light Olefins

The dynamic evolution of active Fe species and carbon species during CO prereduction was revealed for Fe–Zr catalysts with different (monoclinic and tetragonal) zirconia supports (m-ZrO2 and t-ZrO2) on CO2 hydrogenation to light olefins. As the Fe loading reached 15 wt %, the corresponding Fe–K/m-ZrO2 catalyst presented a remarkable CO2 conversion (38.8%) and a high selectivity to light olefins in the hydrocarbon (C–H) products (42.8%). Using in situ characterization techniques including in situ X-ray diffraction (XRD), Raman, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), the dynamic evolution (reduction and carburization) of iron oxides and the generation of diverse carbon species during CO prereduction on different ZrO2 supports were probed. The detected intermediate FeO species in the Fe–K/m-ZrO2 catalyst indicates a mild and sufficient reduction of Fe2O3 on m-ZrO2, promoting the carburization leading to smaller and more active iron species (Fe3O4 and χ-Fe5C2). More oxygen vacancies (Ov) on the surface of m-ZrO2 and the electron-donating ability of iron elements boosted the charge transfer between Fe and the ZrO2 support, forming a more active Fe species. For the Fe–K/t-ZrO2 catalyst, the low selectivity of CxHy implied its weak capacity to produce light olefins through CO2 hydrogenation. By comparison, the m-ZrO2 support provided relatively more strong basic sites, reducing the physically deposited carbon species and coke generation. The formation of more χ-Fe5C2 species contributed to the high yield of light olefins in products. Tuning the physicochemical properties and base microenvironment of supports could be an effective means to boost the iron catalyst activity for CO2 hydrogenation to olefins.