Effect of Partial Fe Substitution in La0.9Sr0.1NiO3 Perovskite-Derived Catalysts on the Reaction Mechanism of Methane Dry Reforming

The partial substitution of Ni in LaNiO3 perovskite-derived catalysts by Fe has been shown to considerably improve the catalyst stability and coke resistance in dry reforming of methane (DRM). In this study, we have investigated the reaction mechanism of DRM on La0.9Sr0.1Ni0.5Fe0.5O3-derived catalysts and shown how it differs from that on La0.9Sr0.1NiO3. By using isotopically labeled reactants and in situ DRIFTS analysis, we observe that lanthanum oxycarbonates are active intermediates for CO2 activation and oxidation of carbonaceous intermediates from methane on the La0.9Sr0.1NiO3 catalyst, whereas the oxidation of carbonaceous intermediates occurs solely through a MvK-type redox mechanism by support lattice oxygen in La0.9Sr0.1Ni0.5Fe0.5O3. The catalysts were thoroughly characterized by XRD, H2-TPR, XAS, XPS, O2- and CO2-TPD-MS, H2 chemisorption, TGA–DTA, and TEM, and the catalyst properties were correlated with the proposed reaction mechanism. It was observed that in contrast to La0.9Sr0.1NiO3, which almost completely decomposed to form Ni/La2O3-xSrO after H2 reduction pretreatment, La0.9Sr0.1Ni0.5Fe0.5O3 could partially retain a perovskite structure after reduction and in the DRM atmosphere. Detailed characterization of the La0.9Sr0.1Ni0.5Fe0.5O3 catalyst after reduction and exposure to reactant gases reveals that this perovskite phase is reversibly decomposed into La2O3 and metal (NiFe) upon exposure to CH4 and is reconstructed upon exposure to CO2 or CH4/CO2 mixture. The cyclic decomposition and formation of the perovskite phase is accompanied by the release and capture of oxygen from and into the perovskite lattice and the redox cycling of Fe between Fe0 (in NiFe alloy) and Fe3+/Fe4+ (in perovskite) states. The La0.9Sr0.1NiO3, on the other hand, forms the lanthanum oxycarbonate phase in CO2 atmosphere, with no observable reconstruction of the perovskite phase. We attribute the reversibility of the formation of the stable La1–xSrxFeO3 perovskite phase under DRM conditions and its high oxygen storage/release capacity as the primary reason for the dominance of the redox mechanism and high coke resistance of La0.9Sr0.1Ni0.5Fe0.5O3.