Breaking the Code of Active Sites in CO ₂ -Assisted Propane Dehydrogenation over Mg(Fe,Al)O _<i>x</i>

Unlocking the full potential of CO 2 as a mild oxidant in propane dehydrogenation (CO 2 –PDH) hinges on mastering the redox chemistry of metal oxide catalysts, that are suited to activate C–H bonds, while mitigating deactivation. This work explores a Mg(5 wt % Fe,Al)O x catalyst, synthesized as layered double hydroxide. The ensuing stabilization of oxidized iron within Mg(Fe,Al)O x yields intricate redox dynamics during CO 2 –PDH. Fe-normalized time yields between 2.0 and 5.6 mmol C3H6 ·mol Fe –1 ·s –1 place Mg(Fe,Al)O x among the most efficient Fe-based systems reported to date. The catalyst withstands CO 2 –PDH – O 2 regeneration cycles without loss of initial activity. While gradual deactivation occurs across 20 h time-on-stream, the propylene selectivity stabilizes at 78.9%, underscoring the potential for prolonged operation. The intricate redox dynamics were investigated using time-resolved XAS and XRD with modulation-excitation. This approach enabled the decoding of two distinct CO 2 –PDH pathways. First, iron reversibly cycles between Fe 3+ in a surface MgFe 2 O 4 phase and dispersed Fe 2+ species, via Fe 3+ ⇌Fe 3+/2+ ⇌Fe 2+ transitions. This Mars-van Krevelen pathway enables carbon removal, but can also overoxidize hydrocarbons. In a parallel Langmuir–Hinshelwood pathway, these Fe 2+ species, stabilized by an MgO-like environment in a likely distorted coordination, serve as highly selective sites for C–H bond activation. Although only ∼ 1% of Fe participates, it governs the catalytic performance. In contrast, irreversible Fe 3+ ⇌Fe 3+/2+ ⇌Fe 2+ transitions lead to MgFe 2 O 4 depletion and aggregation of FeO x, the latter promoting carbon formation. These structure–activity relationships break the long-standing code of active site identity and dynamics in Fe-based CO 2 –PDH catalysts, highlighting both Fe dispersion and the Fe 3+ /Fe 2+ speciation as critical levers for optimizing performance.