(51f) Understanding the Active Center of Na-Modified Iron-Based Catalysts for Light Olefin Production during CO2 Hydrogenation

Iron-based Fischer-Tropsch synthesis (FTS) is crucial for producing e-olefins from waste CO₂ and green H₂. During CO₂ hydrogenation, iron phase transformations and surface reconstruction occur, changing the stability of the catalyst and the selectivity for products. Alkaline dopants (e.g., Na, K) are widely used to modify iron-based catalysts to obtain higher selectivity for olefins, but the introduction of dopants complicates the identification of the active phase in iron catalysts during CO₂ hydrogenation to olefins. This work identifies the active sites for olefin production, providing new insights into the mechanisms by which alkali metals apparently enhance CO selectivity, by focusing on the interactions and dynamic phase transitions of the catalyst components.

Nanosized Fe₅C₂ was produced by carburizing nano-Fe₂O₃ (NANOCAT, 255 m²/g) using syngas, and nanosized Fe₃O₄. The Na-modified catalysts (PM-Na-FeC_x or PM-Na-FeO_y) were prepared by grinding Na₂CO₃ with Fe₃O₄ or Fe₅C₂ (n_Fe:n_Na = 1:0.02 molar). CO₂ hydrogenation was conducted in a continuous reactor with 1g of catalyst at 20 bar, 325°C. Catalysts were extensively characterized before and after reaction and regeneration (re-carburization).

The nanosized Fe₅C₂ nano-catalyst shows high FTS activity, while Fe₃O₄ only exhibits RWGS reactivity. Fe₅C₂ is easily oxidized to Fe₃O₄ during CO₂ hydrogenation, leading to decreased CO₂ conversion and increased CO selectivity. Re-carburization restores its activity and selectivity. A physical mixture of Fe₅C₂ and Na₂CO₃ enhances light olefin selectivity, while Fe₃O₄ and Na₂CO₃ alone are ineffective. Carburizing Fe₃O₄ to Fe₅C₂ and combining it with Na₂CO₃ improves olefin yield during CO₂ hydrogenation (see Figure 1), confirming that the active site is the interface between iron carbide and Na. However, Na₂CO₃ promotes phase oxidation and inhibits hydrogen dissociation, reducing methanation and FTS activity, so Na₂CO₃-modified Fe₅C₂ shows a higher selectivity for CO in light olefin production.