Thermodynamic Stability, Redox Properties, and Reactivity of Mn3O4, Fe3O4, and Co3O4 Model Catalysts for N2O Decomposition: Resolving the Origins of Steady Turnover

Manganese, iron, and cobalt model spinel catalysts were systematically investigated for understanding the roots of their divergent performance in N 2 O decomposition. The catalysts were characterized by XRD, RS, N 2 -BET, SEM, and STEM/EELS techniques before and after the reaction. Their redox properties and the thermodynamic stability range were thoroughly examined by survey and narrow scan TPR/TPO cycles. The results were accounted for by the constructed size-dependent Ellingham diagrams. It was shown that Fe 3 O 4 and Mn 3 O 4 spinels exhibit redox-labile Mn 2+ /Mn 3+ and Fe 2+ /Fe 3+ constituents, and under the conditions of the de N 2 O reaction these catalysts have a pronounced tendency for stoichiometric overoxidation. The redox properties of Co 3 O 4 are highly anisotropic, with Co 2+ being reluctant to undergo oxidation but Co 3+ being prone to easy reduction. The stability of the Co 3 O 4 catalyst is then controlled by partial reduction of octahedral Co 3+ cations, due to the surface oxygen release at elevated temperatures in lean oxygen environments. The N 2 O decomposition was studied by temperature-programmed surface reaction (TPSR) and pulse experiments using 18 O labeling of the catalysts. It was shown that Co 3 O 4 provides a sustainable redox Co 3+ /Co 4+ couple for catalytic decomposition of N 2 O, which operates along a reversible one-electron process, leading to formation of O – surf intermediates that recombine next into dioxygen. As the reaction temperature increases, the de N 2 O mechanism evolves from suprafacial to intrafacial recombination of the oxygen intermediates. Fe 3 O 4 decomposes nitrous oxide in a stoichiometric way via irreversible two-electron reduction of oxygen intermediates into O 2–, giving rise to lattice expansion and formation of a γ-Fe 2 O 3 shell, as discerned by Raman spectroscopy. Postreaction STEM/EELS imaging confirmed a magnetite-core and a maghemite-shell morphology of the catalyst grains. A similar tendency for autogenous oxidation was observed for Mn 3 O 4, yet a rather weak thermodynamic driving force makes this catalyst kinetically more stable. At higher reaction temperatures, the incipient γ-Mn 2 O 3 layer may be decomposed back to the parent Mn spinel, when oxygen pressure is low. To quantify gradual oxidation of the investigated spinels during the N 2 O decomposition, size-dependent thermodynamic 3D diagrams were developed and used for rationalization of the experimental observations. The obtained results reveal the dynamic nature of the investigated spinels under varying redox conditions and explain the remarkable performance of Co 3 O 4 in comparison to Fe 3 O 4 and Mn 3 O 4 . The catalytic behavior of the latter two spinels is actually governed by a sesquioxide shell, produced spontaneously in the course of the de N 2 O reaction.