Suppressing Methane Formation in CO2 Hydrogenation to Olefins over Ag-Promoted Na–Fe Catalyst

Abstract Methane formation remains a major selectivity loss in Fe-based CO2 hydrogenation to olefins, particularly at moderate-to-high CO2 conversion. Here, Ag was introduced into Na-promoted Fe catalysts to regulate the modified Fischer–Tropsch pathway. The effect strongly depended on the mode of Ag incorporation. Fe–Ag coprecipitation followed by Na impregnation provided a better balance between CO2 conversion and CH4 suppression than Ag impregnation. At 320 °C, 2.0 MPa, H2/CO2 = 3, and 4320 mL gcat–1 h–1, Na-FeAg-CP-1 reached 43.1% CO2 conversion and 9.0% CO selectivity; the CO-free hydrocarbon distribution contained 7.7% CH4, while the C2–4 and C5+ fraction reached 34.2% and 58.1% with an olefin/paraffin ratio of 8.6. H2-TPR and in situ/ex situ XRD show that Ag promotes Fe reduction and changes the subsequent oxide-carbide evolution. H2-TPD and C3H6 pulse hydrogenation further indicate modified hydrogen adsorption and weaker secondary olefin hydrogenation after Ag addition. These results show that Ag can suppress methane formation without sacrificing CO2 conversion by regulating Fe phase evolution and hydrogenation behavior.