Effects of Different Reductive Agents on Zn-Promoted Iron Oxide Phases in the CO2–Fischer–Tropsch to Linear α-Olefins

Abstract

The pretreatment atmosphere has a significant impact on the performance of iron-based catalysts in carbon dioxide (CO2) hydrogenation. In this study, we investigated the effects of carbon monoxide (CO), syngas (H2/CO), and hydrogen (H2) on the performance of iron-based catalysts during the pretreatment process. To evaluate the structural changes in catalysts after activation and reaction, we analyzed their morphology and particle size, the surface and bulk phase composition, carbon deposition, the desorption of linear α-olefins and reaction intermediates using transmission electron microscope (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Mössbauer spectroscopy (MES), temperature-programmed desorption (TPD), and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). Raman and XPS showed that the H2 pretreatment catalyst caused the absence of iron carbides due to the lack of carbon source, and the CO and syngas pretreatment catalysts promoted the formation of carbon deposits and iron carbides. While the bulk phase of the CO and syngas pretreatment catalyst mainly consists of iron carbide (FeCx), XRD and MES revealed that the bulk phase of the H2 pretreatment catalyst primarily consisted of metallic iron (Fe) and iron oxide (FeOx). The composition of the phase is closely associated with its performance at the initial stage of the reaction. The formation of olefins and C5+ products is more encouraged by CO pretreatment catalysts than by H2 and syngas pretreatment catalysts, according to in situ DRIFTS evidence. Ethylene (C2H4)/propylene (C3H6)-TPD indicates that the CO pretreatment catalyst is more favorable for the desorption of olefins which improves the olefins selectivity. Based on the analysis of the TEM images, H2 pretreatment stimulated particle agglomeration and sintering. In conclusion, the results show that the CO-pretreatment catalyst has higher activity due to the inclusion of more FeOX and Fe3C. In particular, the presence of Fe3C was found to be more favorable for the formation of olefins and C5+ hydrocarbons. Furthermore, carbon deposition was relatively mild and more conducive to maintaining the balance of FeOx/FeCx on the catalyst surface.