Unlocking the full potential of CO2 as a mild oxidant in propane dehydrogenation (CO2–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)Ox catalyst, synthesized as layered double hydroxide. The ensuing stabilization of oxidized iron within Mg(Fe,Al)Ox yields intricate redox dynamics during CO2–PDH. Fe-normalized time yields between 2.0 and 5.6 mmolC3H6·molFe–1·s–1 place Mg(Fe,Al)Ox among the most efficient Fe-based systems reported to date. The catalyst withstands CO2–PDH – O2 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 CO2–PDH pathways. First, iron reversibly cycles between Fe3+ in a surface MgFe2O4 phase and dispersed Fe2+ species, via Fe3+⇌Fe3+/2+⇌Fe2+ transitions. This Mars-van Krevelen pathway enables carbon removal, but can also overoxidize hydrocarbons. In a parallel Langmuir–Hinshelwood pathway, these Fe2+ 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 Fe3+⇌Fe3+/2+⇌Fe2+ transitions lead to MgFe2O4 depletion and aggregation of FeOx, the latter promoting carbon formation. These structure–activity relationships break the long-standing code of active site identity and dynamics in Fe-based CO2–PDH catalysts, highlighting both Fe dispersion and the Fe3+/Fe2+ speciation as critical levers for optimizing performance.
D’ooghe et al. (Fri,) studied this question.