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

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• 作者：Jan Kaczmarczyk ; Filip Zasada ; Janusz Janas ; Paulina Indyka ; Witold Piskorz ; Andrzej Kotarba ; Zbigniew Sojka
• 刊名：ACS Catalysis
• 出版年：2016
• 出版时间：February 5, 2016
• 年：2016
• 卷：6
• 期：2
• 页码：1235-1246
• 全文大小：743K
• ISSN：2155-5435

文摘

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, N₂-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⁺/Fe³⁺constituents, and under the conditions of the deN₂O 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 ¹⁸O 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 deN₂O 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 O^2–, giving rise to lattice expansion and formation of a γ-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 γ-Mn₂O₃ 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 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 deN₂O reaction.