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Abstract

Iron-based catalysts are considered active for the hydrogenation of CO₂ toward high-order hydrocarbons. Here, we address the structural and chemical evolution of oxide-supported iron nanoparticles (NPs) during the activation stages and during the CO₂ hydrogenation reaction. Fe NPs were deposited onto planar SiO₂ and Al₂O₃ substrates by dip coating with a colloidal NP precursor and by physical vapor deposition of Fe. These model catalysts were studied in situ by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) in pure O₂, then in H₂, and finally in the CO₂ + H₂ (1:3) reaction mixture in the mbar pressure range and at elevated temperatures. The NAP-XPS results revealed the preferential formation of Fe(III)- and Fe(II)-containing surface oxides under reaction conditions, independently of the initial degree of iron reduction prior to the reaction, suggesting that CO₂ behaves as an oxidizing agent even in excess of hydrogen. The formation of the iron carbide phase, often reported for unsupported Fe catalysts in this reaction, was never observed in our systems, even on the samples exposed to industrially relevant pressure and temperature (e.g., 10 bar of CO₂ + H₂, 300 °C). Moreover, the same behavior is observed for Fe NPs deposited on nanocrystalline silica and alumina powder supports, which were monitored in situ by X-ray absorption spectroscopy (XAS). Our findings are assigned to the nanometer size of the Fe particles, which undergo strong interaction with the oxide support. The combined XPS and XAS results suggest that a core (metal-rich)–shell (oxide-rich) structure is formed within the Fe NPs during the CO₂ hydrogenation reaction. The results highlight the important role played by the oxide support in the final structure and composition of nanosized catalysts.