Characterization of Fe Substitution into La-Hexaaluminate Systems and the Effect on N₂O Catalytic Decomposition

Abstract

Compared to Ba-hexaaluminates with β-Al 2 O 3 structure, La-hexaaluminates with magnetoplumbite (MP) structure have more substituted Al sites in the preferentially exposed mirror plane, which should favor N 2 O decomposition. In this regard, LaFe x Al 12−x O 19 catalysts with x = 1 and 5 were herein prepared using the coprecipitation method. 57 Fe Mossbauer spectroscopy and X-ray diffraction structure refinements revealed that Fe 3+ ions originating in Fe 2 O 3 species mainly entered tetrahedral Al(2) and trigonal bipyramidal Al(5) sites. Meanwhile, Fe 3+ ions in octahedral sites of perovskite-type LaFeO 3 intermediates preferentially accommodated in octahedral Al(3) sites in the mirror plane of La-hexaaluminates. Correlation of normalized rates (moles of N 2 O per hour per square meter) of the catalysts at 500 °C with the occupancy of Fe ions in different Al crystallographic sites of the MP phase indicated that Fe 3+ ions in the Al(3) and Al(5) sites were highly active for N 2 O decomposition. In contrast, Fe 2+ derived from a Fe 3+ → Fe 2+ reduction under high-temperature calcination played a negative role.

Thermal Evolution Crystal Structure and Fe Crystallographic Sites in LaFexAl12–xO19 Hexaaluminates

“…Accordingly, the formation of Fe-substituted La-hexaaluminate at high Fe concentration ( x = 6) can be concluded in two routes, namely, solid-state reactions between γ-Al 2 O 3 and dispersed La species, or LaFeO 3 . These results are consistent with the conclusions in our previous contributions . However, higher threshold temperature (1200 °C) was required to obtain the final monophasic MP phase in x = 6 than that in x = 1 and 3 samples (1100 °C), which could be attributed to the formation of LaFeO 3 intermediates that essentially suppressed the mobility of La species.…”

“…These results are consistent with the conclusions in our previous contributions. 27 However, higher threshold temperature (1200 °C) was required to obtain the final monophasic MP phase in x = 6 than that in x = 1 and 3 samples (1100 °C), which could be attributed to the formation of LaFeO 3 intermediates that essentially suppressed the mobility of La species. Similar thermal evolutions were observed in x = 9 and 11 samples, whereas modulations associated with residual α-Fe 2 O 3 in x = 9 and α-Fe 2 O 3 and LaFeO 3 in x = 11 were still verified, even under calcination at 1300 °C.…”

Ceria Nanoparticles’ Morphological Effects on the N2O Decomposition Performance of Co3O4/CeO2 Mixed Oxides

“…The abatement of N 2 O emissions is of paramount importance and the direct catalytic decomposition of nitrous oxide to molecular nitrogen and oxygen (deN 2 O process) is considered to be a highly efficient remediation method. Thus far, several catalytic systems, such as supported noble metals [5][6][7], perovskites [8][9][10], hexaaluminates [11][12][13][14], spinels [15][16][17][18], zeolites [19][20][21][22] and mixed oxides [23][24][25][26][27], have been used for N 2 O decomposition. Although noble metals exhibit satisfactory activity for the deN 2 O process, their high cost and the deterioration of their catalytic efficiency from gases present in the exhaust gas stream (e.g., O 2 ) act as inhibiting factors for practical applications [1,28].…”

Single-Step Production of Bio-Based Methyl Methacrylate from Biomass-Derived Organic Acids Using Solid Catalyst Material for Cascade Decarboxylation–Esterification Reactions

“…Hexaaluminate is considered as a promising material for high-temperature catalytic applications, such as CO 2 reforming, catalytic combustion, oxidative coupling of methane, and ammonia decomposition, because of its remarkable sintering resistance and thermal stability. , Such remarkable properties of hexaaluminate are ascribed to the surface defects and unique peculiar layered structure of alternately stacked spinel blocks separated by mirror planes in which large cations (Ba, La, Sr, or Ca) are located. In addition, hexaaluminate materials possess excellent acid–base properties due to the presence of basic cations within the proximity of aluminate framework …”