M�ssbauer Spectra and Catalytic Behaviorof Perovskite-like SrFe0.7Al0.3O3-?

Abstract

O 3Àd (SFA) exhibits substantial catalytic activity towards partial oxidation of methane and can thus be considered as a component of monolithic ceramic reactors for synthesis gas generation, where the dense membrane and porous catalyst at the permeate-side surface are made of similar compositions. Surface modification of SFA powder and membranes with Pt has no essential effect on the performance of a model reactor, suggesting that the catalytic behavior of SFA is mainly determined by the surface states of iron and oxygen ions in the catalyst. The Mo¨ssbauer spectroscopy shows that reduction of SrFe 0:7 Al 0:3 O 3Àd , having cubic perovskite lattice in air, leads to the co-existence of perovskite-and brownmillerite-like domains, whilst the concentrations of metallic Fe and even Fe 2þ under typical operation conditions are lower than the detection limits. The amount of vacancy-ordered phase increases with decreasing oxygen content, the estimations of which were confirmed by coulometric titration data. The catalytic activity of ferrite-based materials may thus be associated with lattice instability characteristic of morphotropic phase transformations.

Oxygen transport in ferrite-based ceramic membranes: Effects of alumina sintering aid

“…For the latter group of mixed conductors, the phenomena related to formation of oxygen vacancy-ordered microdomains are even observed under oxidizing conditions at temperatures as high as 1100-1200 K (e.g. 9,22,25,30); the disordering processes induced by the Asite deficiency occur due to local distortion of the lattice near the cation vacancies, statistically distributed in the lattice. As the oxygen sublattice of La 0.5 Sr 0.5 FeO 3−δ is essentially disordered [8], the local structural distortions decrease ion mobility.…”

Stability, oxygen permeability and chemical expansion of Sr(Fe,Al)O3−δ- and Sr(Co,Fe)O3−δ-based membranes

“…The 4 K spectra (Fig. 10) are similar to each other and can be fitted to the model [18] using distributions of magnetic hyperfine fields. For SrFe 0.7 Al 0.3 O 3 − δ heated and rapidly cooled in argon, the sextet with lower magnetic hyperfine field (B hf ) and isomer shift (IS) typical for Fe 4+ was found nonnegligible, though very small; its relative area (I) is approximately 4% ( Table 2).…”

“…As for SrFe 0.7 Al 0.3 O 3 − δ reduced in H 2 -H 2 O-N 2 [18], the roomtemperature spectra of the prepared SrFe 0.7 Al 0.3 O 3 − δ samples display substantial peak broadening (Fig. 9A) due to the relaxation effects associated with the proximity of magnetic ordering temperature, which may be explained by the incorporation of non-magnetic Al 3+ .…”

“…The Mössbauer transmission spectra were collected at 4 K, with samples immersed in liquid He, and at 290 K on a conventional constant-acceleration spectrometer equipped with a 25 mCi 57Co (Rh) source, as described elsewhere [18][19][20]. The spectra of SrCo 0.8-Fe 0.2 O 3 − δ were fitted to Lorentzian lines using a non-linear leastsquares method; the relative areas and widths of peaks 1-6, 2-5 and 3-4 in each magnetic sextet were kept equal during refinement.…”

Integration of air separation and partial oxidation of methane in the La0.4Ba0.6Fe0.8Zn0.2O3− membrane reactor

“…However, this modification may reduce ionic conductivity and in turn oxygen permeability if the type of ion and/or its dose are not properly selected. For instance, SrAl x Fe 1−x O 3−ı [10] shows adequate stability but a lack of oxygen flux. Hence, an optimal substitution of Fe 3+ for Co 3+ ions in the B-site could better meet both demands.…”