Study on the enhancement of stability in ambient pressure CO2-FTS for the production of α-olefins by Pt-modified FeZn catalysts

The CO₂-FTS under ambient pressure for α-olefins synthesis offers advantages such as safety, lower operating cost, and saving equipment investment. However, the reaction has encountered challenges including high selectivity for CO and CH₄, as well as poor catalyst stability. To address these issues, Cu, Co, Ru, and Pt-modified FeZn catalysts were designed and synthesized for the ambient pressure CO₂-FTS reaction. The results show that the introduction of Pt significantly improves the stability of CO₂ conversion, reducing the selectivity for by-products CO and CH₄, and promoting the formation of valuable C₂₊ hydrocarbon products. When optimized content of 6% Pt was incorporated into the FeZn catalyst, the CO₂ conversion rate increased from 22.5% to 27.3%, and no obvious deactivation was observed over 20 h. Correspondingly, the average selectivity for CO and CH₄ decreased obviously from 93.1% and 52.9% to 80.0% and 20.5%, respectively. Especially, when the H₂/CO₂ ratio was increased from 3 to 7, CO₂ conversion enhanced up to 48.6%, CO selectivity sharply decreased to 24.0%, and at the same time, the selectivity for C₂₊ hydrocarbon products increased noticeably. More importantly, the undesired selectivity to CH₄ was further lower toward about 13.1%, high α-olefins content was achieved like 90% ${\text{C}}_2^{=}-{\text{C}}_4^{=}$ in the C₂−C₄ hydrocarbon fraction, and the selectivity to CO and CH₄ became stable. Characterization results from XRD, H₂-TPR, CO-TPD, CO₂-TPD, H₂-D₂ exchange, TEM, Raman, XPS and MES indicate that the introduction of Pt significantly enhances the adsorption and dissociation of CO, as well as the dissociation of H₂, thus promoting the hydrogenation and coupling of surface CO* species to form C₂₊ hydrocarbons and reducing the carbon deposits. It should be mentioned that the addition of Pt into FeZn catalyst has eliminated the formation of graphite carbon and reduced the thickness of amorphous carbon layer around the iron carbide nanoparticles. Moreover, Pt improves the carbonization ability of Fe species in the catalyst, increases the content of the active phase χ-Fe₅C₂ for FTS, and decreases the crystaline size of χ-Fe₅C₂ particles.