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

Abstract

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 O2–, 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.

Strong Enhancement of deSoot Activity of Transition Metal Oxides by Alkali Doping: Additive Effects of Potassium and Nitric Oxide

“…A proposed explanation for these observations is the different interaction with oxygen. Whereas magnetite is readily oxidized into the corresponding sesquioxide (c-Fe 2 O 3 /a-Fe 2 O 3 ) in the reaction conditions, cobalt spinel is oxidation resistant [50]. The interaction of the unpromoted Co 3 O 4 with oxygen leads to the formation of surface oxygen species, which participate in the soot oxidation.…”

Surface oxygen dynamics and H2 oxidation on cobalt spinel surface probed by 18O/16O isotopic exchange and accounted for by DFT molecular modeling: facile interfacial oxygen atoms flipping through transient peroxy intermediate

“…The catalyst precursor was filtered, dried overnight (T = 100°C), and calcined in air at 700°C for 3 h to obtain a good crystalline spinel structure. Following our recent work, such treatment leads to formation of the stoichiometric spinel both in the bulk [27] and the surface regions [28]. Purity of cobalt spinel phase was confirmed by X-ray diffraction, using CuKa radiation by means of a Rigaku Miniflex X-ray diffractometer equipped with a DeTEX detector.…”

Atomic-Level Dispersion of Bismuth over Co3O4 Nanocrystals—Outstanding Promotional Effect in Catalytic DeN2O

“…The maximal promotional effect was obtained for the optimal bismuth addition, when the tuning of electrodonor properties was accompanied by the increase in SSA. The precise adjustment of the dopant concentration was particularly important, since the active sites were related to the surface of an octahedral Co complex (reported in [11]); a higher concentration of bismuth led to the active sites blocking.…”

“…Many of oxide phases have been tested for the catalytic decomposition of nitrous oxide (deN 2 O) [9,10]. Based on the literature reports, among the transition metal oxide catalysts, the cobalt spinel (Co 3 O 4 ) has been recognized as the most active phase in the deN 2 O [11,12]. Extensive research on the cobalt spinel revealed many possible modifications leading to the improvement of its catalytic performance.…”