Promotional effect of Mg on Fe catalyst during CO₂ methanation

This study investigates CO₂ hydrogenation to CH₄ on a Mg-promoted Fe₂O₃ catalyst, which reveals significant methanation activity. The influence of Mg on the textural and structural properties of fresh, H₂-activated and spent catalysts was comprehensively examined using temperature-programmed reduction by H₂, temperature-programmed desorption of NH₃, CO₂ and H₂, X-ray photoelectron spectroscopy, N₂ physisorption, in-situ Raman spectroscopy as well as ex-situ and in-situ X-ray diffraction. Carbonaceous species formed during CO₂ hydrogenation were characterized by temperature-programmed hydrogenation. The mechanistic results show that Mg retards iron oxide reduction during H₂ activation leading to the formation of smaller Fe crystallites compared to the unpromoted catalyst. Moreover, Mg enhances CO₂ and H₂ adsorption and increases the relative abundance of surface and bulk iron carbides thereby promoting CH₄ formation. The Mg promoter also controls carburization stabilizing the catalytically active Fe₃C phase, while suppressing the formation of the carbon-rich and less active Fe₅C₂ phase. However, Mg simultaneously also facilitates the deposition of unreactive carbon species.