Mossbauer hydrated

Three series of Au nanoparticles on oxidic iron catalysts were prepared by coprecipitation, characterized by Au Mossbauer spectroscopy, and tested for their catalytic activity in the room-temperature oxidation of CO. Evidence was found that the most active catalyst comprises a combination of a noncrys-taUine and possibly hydrated gold oxyhydroxide, AUOOH XH2O, and poorly crystalhzed ferrihydrate, FeH0g-4H20 [421]. This work represents the first study to positively identify gold oxyhydroxide as an active phase for CO oxidation. Later, it was confirmed that the activity in CO2 production is related with the presence of-OH species on the support [422]. 

The verification of the presence of hydrogen in the film has proved more controversial, primarily because many of the structural investigations have been carried out after the film has been dried in vacuo. An example of the problems here is the fact that electron diffraction, which has to be carried out in vacuo, reveals a relatively well-crystallised spinel lattice whose origin may be the comparatively high sample heating encountered in the electron beam. Moreover, the use of in situ techniques, such as Mossbauer and X-ray absorption spectroscopy, clearly reveals marked differences between the spectra of the films in situ and the spectra of the same films ex situ as well as the spectra of y-Fe203 and y-FeOOH standards. These differences are most naturally ascribed to hydration of the spinel forms. 

In situ Mossbauer parameters for small particles of a hydrated ferric oxyhydroxide, FeOOH(hydrated), dispersed on high area Vulcan XC-72 carbon ... 

The ex situ Mossbauer spectrum for the partially dried electrode yielded a doublet with 6 — 0.34 and A 0.70 mm-s l. A decrease in the value of A was found in the in situ spectra of the same electrode immersed in 4 M KOH at -0.3 V vs Hg/HgO,OH ( see Table III, and Curve a, Fig. 4 ), in direct analogy with the behavior observed for the heat treated FePc. It is thus conceivable that this material is the same as that found after the thermal decomposition of FePc dispersed on carbon and that reported by other workers, and that the variations in the value of A are simply due to differences in the degree of hydration of the lattice. 

V.I. Gol danskii, Yu. F. Krupyanskii, and V. N. Fleurov, Rayleigh scattering of Mossbauer radiation (RSMR) data, hydration effects and glass-like dynamical model of biopolymers, Phys. Scr. 33, 527-540 (1986). 

Surface site interactions, as measured with the Mossbauer isotope present in the chemisorbed species, are illustrated in the work of Karasev et al. (229, 230). In Fig. 34, the Mossbauer spectra of Sn(CH3)4 before and after chemisorption on y-Al203 are shown at room temperature. Upon hydration of the alumina at 770 K and subsequent chemisorption, the peak at 2.5 mm sec-1 nearly disappears, leaving only the peak centered at the zero of velocity. Thus, the spectrum (Fig. 34) corresponding to chemisorbed Sn(CH3)4 is not a spectral doublet, but is instead composed of two spectral singlets. This is suggestive of two different adsorption sites. The peak centered at zero velocity is suggestive of a Sn02-like species, which could be formed... 

Cytochromes from bacterial, yeast, and mammalian sources have been investigated by Mossbauer spectroscopy (114—117). Horseheart cytochrome c and the c-type cytochrome from T. utilis show spectra characteristic of low-spin Fe(III) in the oxidized form of the protein and low-spin Fe(II) for the reduced form of the protein. Lang et al. (115) have analyzed the Mossbauer data in terms of a low-spin Hamiltonian in some detail. Cooke and Debrunner (116) present quadrupole data on dehydrated forms of oxidized and reduced cytochrome c the quadrupole splittings for hydrated and dehydrated forms of the reduced protein are quite similar in contrast to a difference of the oxidized form. No spin-state change is reported for either form of cytochrome c. 

Fig. 12.65. A polymeric film of hydrate iron oxide consistent with Mossbauer spectra of passive film on iron.

Several investigators have noted that the Al/Fe ratio in the product phases tends to be higher than that of the starting material, and have concluded that an iron(III) oxide or hydroxide is also formed. Teoreanu et al. (T33), using Mossbauer spectroscopy and XRD, concluded that the hydration products formed from C2F or C4AF at ordinary temperatures included FH3. With C2F above 75°C, this was replaced by hematite. Fukuhara et al. (F29)... 

Usha and Wittebort (1989) studied the NMR of crystalline cram-bin. At 140 K the protein hydrate is stationary, with t = 1 msec. Above 200 K changes in the signal with temperature are consistent with a glass transition or melting of the hydration water. This broad transition parallels closely the changes with temperature found for the heat capacity, Mossbauer spectroscopic, and other properties of hydrated protein crystals. At room temperature no more than 12 water molecules are orien-tationally ordered. The average rotational correlation time of the hydration water is about 40 times longer than that for bulk water. 

Likhtenshtein and colleagues (Belonogova et al., 1978, 1979 Likhten-shtein, 1976) carried out a series of measurements on the hydration dependence of the mobility of spin labels covalently bound to several proteins. The results were correlated with Mossbauer spectroscopic data obtained in parallel experiments. Spin-labeled human serum albumin and a-chymotrypsin showed a critical hydration level for onset of motion at relative humidity 0.8, equivalent to 0.2 h. The temperature dependence of the spin label spectrum showed a critical temperature of 230 K, below which motion was frozen. Serum albumin labeled at surface sites... 

Mossbauer labels covalently attached to proteins, presumably at the protein surface, exhibit temperature dependences similar to that described above for the heme iron and hydration dependences showing motion developing above a slightly higher hydration level than that found for the heme iron (Belonogova et ai, 1979 Likhtenshtein, 1976). 

Mossbauer spectroscopic measurements suggest that the hydration water of myoglobin and the internal motions of the protein are coupled. [ Fe]Ferricyanide diffused into the solvent of myoglobin crystals exhibits (x ) values equal to those for the heme iron for temperatures below 250 K, and greater than those for the heme iron at higher temperatures (50% greater at 300 K) (Parak, 1986). The [ Fe]ferricyanide in the crystal monitors motions of the hydration water [ Fe]ferricyanide in bulk water shows no Mossbauer spectrum. 

---