From Oxide–Spinel Matrix to Partially Reduced Ni–Fe Domains: Structure–Activity Relationships in Fe–Ni/Al₂O₃ Catalysts for Methane Dry Reforming

Methane dry reforming is a high-temperature catalytic process requiring simultaneous CH₄/CO₂ activation and resistance of catalyst to sintering, phase segregation, redox drift, and carbon deposition. In this work, Fe–Ni/Al₂O₃ catalysts are considered as structurally evolving interfacial systems rather than simple mixtures of supported metals and oxides. Their catalytic behavior is interpreted through the transformation of an initially stabilized oxide–spinel matrix into partially reduced Ni–Fe active domains. Calcination at 850°C in air forms a NiAl₂O₄-type/Ni–O–Al framework with highly dispersed FeOx fragments. This matrix anchors metal-containing species, suppresses uncontrolled particle growth, and provides structural stability under reducing and reaction conditions. Subsequent H₂ reduction, especially at 500 °C, partially converts the oxide–spinel system into catalytically active Ni–Fe domains while preserving residual Ni–O–Al/NiAl₂O₄-type and FeOx/Fe–O–Al stabilizing regions. This intermediate state is essential: excessive oxidation limits methane activation, whereas over-reduction promotes segregation, particle growth, and loss of Fe–Ni interfacial contact. SEM/EDS analysis reveals a heterogeneous, radially differentiated Fe/Ni/O distribution, with Ni–O-enriched regions mainly inside agglomerates and Fe-enriched zones closer to the periphery. This organization is catalytically meaningful: reduced Ni domains favor C–H bond activation, while Fe-containing regions may assist CO₂ activation, oxygen transfer, and suppression of carbonaceous intermediates. EPR/FMR spectra show broad asymmetric signals characteristic of collective exchange-coupled Fe–Ni domains, supporting the formation of structurally connected metal-containing regions. Catalytic tests demonstrate that calcination at 850°C followed by H₂ reduction at 500°C produces the most favorable working state. In the 801–821°C range, CH₄ and CO₂ conversions increase markedly, while the H₂/CO ratio approaches the stoichiometric value, reaching 0.96 at 821°C. Thus, Fe–Ni/Al₂O₃ operates as a hierarchically organized catalyst whose performance is governed by stable partially reduced Ni–Fe domains and favorable Fe/Ni/O spatial organization.