In the light of a recent study which identiÐed the beneÐcial inÑuence of poorly crystallised ferrihydrite on the activity of CO conversion catalysts comprising gold nanoparticles on oxidic iron, we Fe 5 HO 8 É 4H 2 O have investigated three series of ferrihydrite-rich samples prepared by coprecipitation. The samples were structurally and chemically characterised using X-ray di †raction and both 57Fe and 197Au Mo ssbauer spectroscopy, and tested for CO oxidation at room temperature using a microreactor with on-line GC. The highest activity, 100% conversion after 20 min on line, was observed in a dried sample that contained ferrihydrite and a non-crystalline and possibly hydrated gold oxyhydroxide phase, The AuOOH É xH 2 O. activity of the same materials after calcination, where the gold was transformed to 3È5 nm Au metal particles and the ferrihydrite to hematite, was less than ca. 7%. This is the Ðrst report of a synergistic interaction between and ferrihydrite resulting in an active catalyst for room temperature CO oxidation, AuOOH É xH 2 O and contrasts with previous work which has been interpreted in terms of the requirement for metallic Au nanoparticles.

Ferritin is a complex of a hollow, spherical protein and a hydrous, ferric oxide core of less than or equal to 4500 iron atoms inside the apoprotein coat; the apoprotein has multiple (ca. 12) binding sites for monoatomic metal ions, e.g., Fe(II), V(IV), Tb(III), that may be important in the initiation of iron core formation. In an earlier study we observed that the oxidation of Fe(II) vacated some, but not all, of the metal-binding sites, suggesting migration of some Fe during oxidation, possibly to form nucleation clusters; some Fe(III) remained bound to the protein. Preliminary extended X-ray absorbance fine structure (EXAFS) analysis of the same Fe(III)-apoferritin complex showed an environment distinct from ferritin cores, but the data did not allow a test of the Fe cluster hypothesis. In this paper, with improved EXAFS data and with Mössbauer data on the same complex formed with 57Fe, we clearly show that the Fe(III) in the distinctive environment is polynuclear (Fe atoms with Fe-Fe = 3.5 A and TB = 7 K). Moreover, the arrangement of atoms is such that Fe(III) atoms appear to have both carboxylate-like ligands, presumably from apoferritin, and oxo bridges to the other iron atoms. Thus the protein provides sites not only for initiation but also for nucleation of the iron core. Sites commodious enough and with sufficient conserved carboxylate ligands to accommodate such a nucleus occur inside the protein coat at the subunit dimer interfaces. Such Fe(III)-apoferritin nucleation complexes can be used to study the properties of the several members of the apoferritin family.

No abstract

X-ray absorption spectroscopy was used together with Mössbauer spectroscopy to study the chemical state and the microstructure of iron neutralized Nafion membranes. The cation oxidation state deduced from the x-ray absorption-near-edge structure (XANES) of the ionomers agrees with that deduced from the Mössbauer isomer shift. Extended x-ray absorption fine structure (EXAFS) results confirm the existence of Fe(H2O)62+ octahedra, with a Fe–O distance of 2.11±0.01 Å in hydrated ferrous ionomers. As water content decreases below 6 wt. %, both coordination number and distances determined from EXAFS are found to decrease. The hydrated ferric Nafion salt contains many isolated Fe(H2O)63+ complexes which convert to a ferric dimer in vacuum-dried ferric salts which has an isomer shift of 0.58±0.02 mm/s relative to iron and a quadrupole splitting of 1.73±0.04 mm/s at 80 K. The dimer is shown to be [(H2O)5Fe–O–Fe–(H2O)5]4+, having an Fe–H2O distance of 2.05±0.02 Å and a rather short Fe–O distance of 1.77±0.02 Å. The Fe–O–Fe bond angle is 155±20°.

No abstract