Fischer-Tropsch synthesis: Mössbauer investigation of iron containing catalysts for hydrogenation of carbon dioxide

CO₂ hydrogenation was investigated with a doubly promoted (Cu, K) silica containing iron catalyst. Hägg carbide (χ-Fe ₅C₂) is the dominant phase obtained under CO activation conditions at 543 K and 0.1 MPa and it was stable under FTS conditions typical for coal or biomass derived syngas using a H₂/CO ratio of 1:1 for at least ∼100 h TOS. An Fe-phase change occurs (χ-Fe₅C ₂ → Fe₃O₄) after switching from H ₂:CO:N₂ (1:1:2) to H₂:CO₂ (3:1). The distribution of hydrocarbon products changes significantly after switching to H₂:CO₂ (3:1), but then it slowly transformed to normal FTS products, albeit with over 3 times higher methane selectivity compared to FTS using H₂:CO:N₂ (1:1:2). A correlation was obtained between the rate of FTS and the % of Fe carbide indicating that iron carbide is the active phase for CO₂-based FT synthesis. Irrespective of conditions (i.e., either H₂:CO:N₂ = 1:1:2 or H₂:CO ₂ = 3:1) and the Fe phase, the methane selectivity appears to primarily depend on the H₂/CO ratio.