Boosting Selectivity toward Long-Chain α-Olefins in CO 2 Hydrogenation through Strengthening ZnO–Fe Interactions with Favorable Iron Phase Regulation for Fischer–Tropsch Synthesis

Developing excellent catalysts for CO2 hydrogenation to long-chain linear α-olefins (LAOs) offers a promising route toward value-added chemicals. However, it remains a considerable challenge to achieve high CO2 conversion along with superior selectivity to LAOs. In this study, we prepared a series of Zn-modified iron-based catalysts using urea precipitation for the CO2 hydrogenation reaction, and they exhibited a much higher selectivity to C4+ LAOs of 61.7%, with a CO2 conversion of 38.5%, over Fe2Zn1-U catalyst, compared to the selectivity of 50.5% over Fe2Zn1-A catalyst with same elemental composition prepared using ammonia precipitation. A lower selectivity to methane was obtained, from 13.7% to 8.8%. Moreover, this process resulted in a record-breaking space-time yield (STY) of 0.544 gC4+LAOs·gcat–1·h–1. HRTEM characterization over spent catalysts showed that Fe2Zn1-U exhibited smaller-size mixed iron-phase particles (Fe3O4, χ-Fe5C2, and θ-Fe3C) where the ZnO phase was more homogeneously dispersed and had stronger interactions with Fe species compared to the observations for Fe2Zn1-A. In situ XPS analysis suggested that this structure promoted Zn-to-Fe electron donation along with intimate interaction over Fe2Zn1-U. XANES spectra demonstrated a more difficult reduction of Fe species with weaker Fe–C coordination for carburization, thereby resulting in a higher Fe3O4 content in spent Fe2Zn1-U. In situ IR spectroscopy under reaction conditions confirmed that significantly more surface species of CO*, HCOO–, *HCO, *CHx, and *C═CH were present over the Fe2Zn1-U catalyst, derived from strengthened synergistic catalysis between ZnO, Fe3O4, and iron carbides. Also, Mössbauer spectroscopy analysis revealed that more θ-Fe3C phase was observed over spent Fe2Zn1-U, which was beneficial for carbon–carbon coupling, owing to its wavy protrusion structure, demonstrated by DFT calculations to be like *C5H10 and *C6H12 coupling with *CH2 species, showing a much lower energy barrier to form longer α-olefins. In addition, it was found that the introduction of Cu could enhance the CO2 conversion, selectivity to LAOs, and catalyst stability. As a result, the Fe3Zn1-U-12Cu catalyst gave stable production of LAOs with a high selectivity from CO2 hydrogenation over 400 h reaction time on stream.