ChemInform Abstract: The Reduction Behavior of Silica‐Supported and Alumina‐Supported Iron Catalysts: A Moessbauer and IR Spectroscopic Study.

Abstract

ChemInform Abstract Addition of an acidified Fe(NO3)3 solution at constant pH (pH 6) to a suspension of the support results in the deposition of highly dispersed α-FeOOH. The reduction of Fe/Al2O3 occurs in the sequence α-FeOOH → Fe3O4 → Fe1-xO → α-Fe (particles of 20-30 nm diameter after 15 h at 873 K). It is shown that the catalyst surface still contains traces of oxygen after reduction. The reduction of Fe/SiO2 proceeds via an Fe(II)-silicate which is partly reduced to α-Fe at higher temperatures. The iron crystallites appear to be partly encapsulated by an Fe(II)-silicate layer. The encapsulation of SiO2-supported Fe particles, the incomplete reduction of the Al2O3-supported iron, and the reoxidation occurring upon high-temp. evacuation presumably account for the small extent of room temp. hydrogen chemisorption usually observed for supported iron catalyst.

Infrared Spectroscopy Investigation of Fe-Promoted Rh Catalysts Supported on Titania and Ceria for CO Hydrogenation

“…The broad shoulder (*1970 cm -1 ) that develops as the iron concentration increases is attributed to Fe, likely in the form of a FeRh alloy on the catalyst surface. The adsorption of CO on reduced iron particles has been studied and it is widely accepted that peaks in the range of 2040-1990 cm -1 are a result of CO bonded linearly to Fe 0 sites whereas bands in the range of 1980-1880 cm -1 are a result of bridged CO species [28][29][30]. Our previous XRD results showed that no fraction of the iron added to the catalyst existed as Fe metal, only as alloy, and from this we can eliminate CO (l) -Fe 0 as the source of this new feature [1].…”

Synthesis of highly dispersed zirconia-supported iron-based catalysts for Fischer–Tropsch synthesis

“…With the common oxidic supports silica and alumina it is difficult to prevent reaction of the iron(II), which is initially produced during reduction, with the support to iron(II) silicate or iron(II) aluminate. As both iron(II) silicate and iron(II) aluminate are very difficult to reduce [6][7][8][9][10][11], silica and alumina are not very suitable as support for iron-based Fischer-Tropsch catalysts. Zirconia does not form bulk iron(II) zirconate, although the interaction with iron oxide is sufficient [12][13][14][15][16] to anchor supported metallic iron particles through an intermediate very thin iron oxide layer.…”

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

“…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.…”

“…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.…”

“…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. Also with TiO 2 and MgO supports mixed oxides are mentioned to account for the intimate contact between the precursor metal oxide and the support resulting in a lower reducibility of the iron oxide [13][14][15].…”