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

摘要

Manganese, iron, and cobalt model spinel catalysts were systematically investigated for understanding the roots of their divergent performance in N2O 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 Fe3O4 and Mn3O4 spinels exhibit redox-labile Mn2+/Mn3+ and Fe2+/Fe(3+)constituents, and under the conditions of the deN(2)O reaction these catalysts have a pronounced tendency for stoichiometric overoxidation. The redox properties of Co3O4 are highly anisotropic, with Co2- being reluctant to undergo oxidation but Co3+ being prone to easy reduction. The stability of the Co3O4 catalyst is then controlled by partial reduction of octahedral Co3+ cations, due to the surface oxygen release at elevated temperatures in lean oxygen environments. The N2O decomposition was studied by temperature programmed surface reaction (TPSR) and pulse experiments using O-18 labeling of the catalysts. It was shown that Co3O4 provides a sustainable redox Co3+/Co4+ couple for catalytic decomposition of N2O, 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 deN(2)O mechanism evolves from suprafacial to intrafacial recombination of the oxygen intermediates. Fe3O4 decomposes nitrous oxide in a stoichiometric way via irreversible two-electron reduction of oxygen intermediates into O2-, giving rise to lattice expansion and formation of a gamma-Fe2O3 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 Mn3O4, yet a rather weak thermodynamic driving force makes this catalyst kinetically more stable. At higher reaction temperatures, the incipient gamma-Mn2O3 layer may be decomposed back to the parent Mn spine], when oxygen pressure is low. To quantify gradual oxidation of the investigated spinels during the N2O 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 Co3O4 in comparison to Fe3O4 and Mn3O4. The catalytic behavior of the latter two spinels is actually governed by a sesquioxide shell, produced spontaneously in the course of the deN(2)O reaction.