Oxygen Activity Tuning via FeO6 Octahedral Tilting in Perovskite Ferrites for Chemical Looping Dry Reforming of Methane
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Abstract
Rational design of perovskite-type redox catalysts is still challenging due to the limited understanding of the correlation between structural distortion and lattice oxygen activity. Herein, a series of model catalysts with a composition of LaFe0.8M0.2O3 (M = Al, Ga, Fe, and Sc) were designed to shed light on the structure–activity relationship. Combined experimental results and DFT calculations verify that the tilting degree of the FeO6 octahedron rather than the Fe–O bond length plays a dominant role in modulating the oxygen activity. Reducing the octahedral tilting via doping redox-inert cations with a smaller radius can greatly enhance the Fe–O bond covalency and reduce the oxygen vacancy formation energy. This not only contributes to the activation of reactants but also accelerates the oxygen mobility. Consequently, LaFe0.8Al0.2O3 with the smallest FeO6 octahedral tilting (6.6° tilting) manifests superior syngas productivity that is 70% higher than that of LaFe0.8Sc0.2O3 (9.5° tilting) with simultaneously better CO2 conversion for chemical looping dry reforming of methane. This discovery highlights the importance of geometric effect in adjusting the oxygen activity, paving an avenue for developing prospective redox catalysts for chemical looping processes and reactions proceeding via a Mars–van Krevelen mechanism.
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