Reduction of supported iron oxide studied by temperature-programmed reduction combined with mössbauer spectroscopy and X-ray diffraction

Abstract

The reduction of iron oxide supported on alumina, anatase and rutile and the interactions between iron oxides and these supports have been studied using temperature-programmed reduction, Mossbauer spectroscopy and X-ray diffraction. The intermediates formed during reduction depend strongly on the nature of the support, the iron loading and the reduction conditions used. Several iron species such as bulk a-Fe,O,, superparamagnetic a-Fe203 and surface iron-aluminium oxides are found in the alumina-supported iron oxide samples. The ironaluminium species can be formed either by incorporation of A13+ into the structure of iron oxide in acidic solution during the preparation of the samples or by incorporation of Fe2+ into the structure of the alumina support under high-temperature reduction. For the as-prepared Fe/TiO, samples, the anatase and rutile have only weak interactions with the iron oxide as only bulk a-Fe,O, species have been detected. However, some intermediates such as (1x)FeTiO,-xFe203 solid solution, surface iron-titanium oxide and bulk FeTiO, phase can be formed during reduction because of the reducibility and mobility of titanium ions on the surface. Surface titanium ions can act as an electron-transfer medium for the reduction of Fe3+ to Fe2+ and can even be substituted for Fe3+ ions in the octahedral sites of magnetite formed during the reduction. In all cases, when the loading of iron oxide is low, Fe3+ and Fe2+ ions are stable during reduction due to the formation of iron oxide-support intermediates by strong interactions between iron oxides and the supports.

A Mössbauer study of In–Fe2O3/HZSM-5 catalysts for the selective catalytic reduction of NO by methane

“…It is pertinent to note that the ratio of the iron cations in octahedral to those in tetrahedral sites (parameter S), estimated by peak areas, was observed to decrease to a value of 1.5, as compared to a value of 2 for pure Fe 3 O 4 , indicating that there were more iron cations in tetrahedral sites in this sample than in pure Fe 3 O 4 . From the comparison of this behavior with the results of Topsøe et al, 26 Lund et al 21 and Gao et al 27 in their studies on magnetite-supported lead, silica-supported magnetite and titania-supported magnetite, it is suggested that the iron cations in octahedral sites in the first reduction stage of the In-Fe 2 O 3 /HZSM-5(I) catalyst were substituted by indium cations, resulting in the decrease in S. After the sample was treated at 630 C, the spectrum showed a doublet with parameters typical of high-spin Fe 2+ in tetrahedral vacancies, 16,28,29 a typical sextuplet of a-Fe and a singlet which possibly resulted from the small particles of metallic iron. 30,31 Since pure wustite is thermodynamically unstable at room temperature, 22,32 and the interaction between indium oxide and iron oxide was weak in the In-Fe 2 O 3 /HZSM-5(I) catalyst, the existence of stable ferric cations was apparently caused by the interaction between the iron oxide and the HZSM-5 support.…”

Reduction behaviour of Fe/ZrO2 and Fe/K/ZrO2 Fischer–Tropsch catalysts

“…For iron catalysts supported on Al 2 O 3 and SiO 2 the formation of ferrous aluminates and silicates has been reported [18,[20][21][22][23][24]. Wielers et al [21] observed the formation of a silicate layer during reduction of trivalent iron indicating that, as soon as divalent iron is formed, iron migrates into the support.…”

“…In addition, the Mössbauer measurements reveal the presence of high spin Fe 2+ species with quadrupole splittings at room temperature (2.7-1.7 mm/s) significantly higher than for bulk FeO (about 0.55 mm/s). These results point to the formation of ferrous species due to an intimate interaction with the support [9][10][11][13][14][15]18,[20][21][22][23][24]. One of the two iron(II) species is identified as a Fe 2+ which upon formation immediately migrates into the zirconia support to form a mixed oxide.…”

“…The reduction of bulk hematite (α-Fe 2 O 3 ) proceeds via magnetite (Fe 3 O 4 ) and woestite (FeO) to metallic iron [17,18]. However, the formation of FeO is not observed, because woestite is metastable below 843 K [17][18][19] at which temperature disproportionation into Fe 3 O 4 and Fe proceeds.…”

“…However, the formation of FeO is not observed, because woestite is metastable below 843 K [17][18][19] at which temperature disproportionation into Fe 3 O 4 and Fe proceeds.…”