Structure of a Nanophase Iron Oxide Catalyst

Zhao, Jun; Huggins, Frank E.; Feng, Zhen; Lu, Fulong; Shah, Nimesh; Huffman, G.P.

doi:10.1006/jcat.1993.1293

Cited by 94 publications

(67 citation statements)

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“…The structural interpretation by Zhao et al (1993Zhao et al ( , 1994 is also doubtful for steric reasons. Based on Fe-O bond distance calculation (<d(VtFem-O)> = 2.01 and <d(rvFem-O)> = 1.86 /k), reduction of Fe-Fe distance from 3.14 to 3.12/~ upon drying is far too small to coincide with a diminished coordination number of surface Fe atoms, whose proportion has been estimated as 35% (Zhao et al 1993).…”

Section: Prepeak Intensity Enhancement In Ferrihydritementioning

confidence: 97%

“…This large intensity variation was interpreted as a reduction from 6-to 4-fold coordination of surface Fe atoms in going from the wet to dry state, that is, as the result of dehydroxylation. Although the presence of a few percent tetrahedral Fe (or even 5-fold coordinated Fe) cannot be completely discarded based on results currently available, the finding that up to 20-30% rvFe(III) exists (Zhao et al 1993;Zhao et al 1994) is questionable because the prepeak intensity is not uniquely related to the coordination number of cations in solids, but also to the geometry of the Fe sites (compare background section). In addition, it will be shown later on that this coordination change conflicts with both mainedge and EXAFS results.…”

Section: Prepeak Intensity Enhancement In Ferrihydritementioning

confidence: 99%

“…In finely dispersed ferrihydrite grains, up to 35% Fe may be located at the surface (Zhao et al 1993) and Fe atoms can be coordinated to O, OH and H20 ligands (Russell 1979;Stanjek and Weidler 1992;Drits et al 1993). Based on simple valence charge balance considerations, one will find that oxygens are linked to several Fe atoms, whereas H20 can only be bonded to a single Fe atom.…”

Section: Prepeak Intensity Enhancement In Ferrihydritementioning

confidence: 99%

“…The 1st is to bring attention to instrumental and operator sources of error that are root causes of diverging interpretations of the coordination of Fe in ferrihydrite, and describe experimentally how this relates to the crystal chemical information obtained. The 2nd goal is to propose a simple, geometric model for the structure of surface Fe octahedra in ferrihydrite that builds on the earlier model of Drits et al (1993), and to show how this model can explain several of the inconsistencies in the quantitative interpretation of XANES and EXAFS spectra by Zhao et al (1993Zhao et al ( , 1994. Energy (eV)…”

Section: Introductionmentioning

confidence: 99%

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Surface Structural Model for Ferrihydrite

Manceau

Gates

1997

Clays and clay miner.

135

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Abstract--A structural model for the geometry of Fe(III) octahe-dra near the surface of finely divided ferrihydrite was elaborated based on the bond-valence theory and by considering the interaction of water molecules in the 2 nearest hydration spheres. In contrast to bulk Fe atoms, which are bonded to bridging oxo (O) and hydroxo (OH) ligands, surface Fe atoms are also octahedrally coordinated to HzO ligands forming the 1st hydration shell ((H20)0. In the wet state, external water molecules of the 2nd hydration shell ((H20)n) are singly H-bonded to (H20) 1, while they are doubly coordinated in the dry state. Accordingly, wet ferrihydrite contains twice as many sorbed water molecules as dry ferrihydrite, and the structural difference due to the 2nd hydration shell accounts quantatively for the 15% increase of ferrihydrite weight experimentally measured in moist atmosphere. The interaction of surface Fe atoms with their 2 nearest hydration spheres modifies the geometry of surface Fe octahedra as compared to bulk octahedra, and idealized Fe-OH and Fe-H20 bond lengths in the wet and dry state were evaluated by the bond-valence theory. Our structural model provides a sound crystal-chemical basis to describe many apparent incongruities of Fe X-ray absorption near edge structure (K-XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopic data that have led to differing interpretations of the coordination environment of Fe in ferrihydrite by various investigators.

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Section: Prepeak Intensity Enhancement In Ferrihydritementioning

confidence: 97%

Section: Prepeak Intensity Enhancement In Ferrihydritementioning

confidence: 99%

Section: Prepeak Intensity Enhancement In Ferrihydritementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Surface Structural Model for Ferrihydrite

Manceau

Gates

1997

Clays and clay miner.

135

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“…To our knowledge, transformations among the forms of ferrihydrite with varying numbers of peaks have not been demonstrated. Despite an early suggestion that 36% of the iron in 6LFh is in tetrahedral coordination (Eggleton and Fitzpatrick, 1988), synchrotron techniques have shown that essentially all of the iron in the interiors of 6LFh crystals is octahedrally coordinated (e.g., Combes et al, 1989Combes et al, , 1990Manceau and Drits, 1993;Zhao et al, 1993;Shinoda et al, 1994;Waychunas et al, 1996). Intermediate-range structures involving possible corner-, edge-, and face-sharing between octahedra remain subjects of active investigation (Combes et al, 1989(Combes et al, , 1990Waychunas et al, 1993Waychunas et al, , 1996Manceau and Drits, 1993;Shinoda et al, 1994) as do coordination environments of iron atoms at crystal surfaces (Zhao et al, 1994;Manceau and Gates, 1997).…”

confidence: 99%

Transmission Electron Microscopy of Synthetic 2- and 6-Line Ferrihydrite

Janney

Cowley

Buseck

2000

Clays and clay miner.

204

161

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Abstraet~High-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), annular dark-field scanning transmission electron microscope (STEM) images, and electron nanodiffraction were used to examine structures of synthetic 2-and 6-line ferrihydrite specimens. 086, 0.093, 0.107, 0.112, 0.119, 0.125, and 0.135 nm are visible in strongly exposed SAED patterns. Small quantities of hematite, magnetite or maghemite, and an acicular material tentatively identified as goethite were observed in the 6-line ferrihydrite, but these quantities do not contribute significantly to the overall diffracted intensity from the sample.

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Biopolymer Networks for the Solid‐State Production of Porous Magnetic Beads and Wires

Bakandritsos

Bourlinos

Tzitzios

et al. 2007

Adv Funct Materials

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Aqueous solutions of sodium carboxymethyl cellulose are used for the morphosynthesis of spherical and wire‐shaped biopolymer networks, in which Fe3+ cations serve as a crosslinking and hardening agent. Their morphology remains intact upon drying, resulting in monolithic beads (1 mm) and wires (ca. 80 μm), which are exploited as reaction vessels to pre‐encapsulate poly(ethylene glycol) 400 (PEG 400) and cobalt cations. A solid‐state reaction in an inert atmosphere at 600 °C affords porous carbonaceous xerogels, macroscopically shaped as beads or wires and decorated with nanocrystalline magnetic iron oxide, metallic iron, or iron–cobalt alloy particles, thus imparting magnetic properties to the products. As such the reduction of Fe3+ species to α‐Fe nanoparticles can be achieved without H2 treatment, since poly(ethylene glycol) serves as a reducing agent and the encapsulated Co2+ aids in the subsequent growth of the metallic iron particles. Particularly interesting are the magnetic properties of the carbon–α‐Fe composite, in which the size of the magnetic particles, estimated near the boundaries of the single magnetic domain, gives rise to increased coercivity compared with that of bulk iron.

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Structure of a Nanophase Iron Oxide Catalyst

Cited by 94 publications

References 0 publications

Surface Structural Model for Ferrihydrite

Surface Structural Model for Ferrihydrite

Transmission Electron Microscopy of Synthetic 2- and 6-Line Ferrihydrite

Biopolymer Networks for the Solid‐State Production of Porous Magnetic Beads and Wires

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