In situ Mossbauer

Figure 1. In Situ MOssbauer Spectra. A) Spectrum collected while the sample (sample 1) was at room temperature In flowing helium gas. The sample had been previously treated for 3 hours In water-gas shift synthesis gas at 613 K B) Spectrum collected while the sample was at 613 K In flowing water-gas shift synthesis gas C) Spectrum collected while the sample was at room temperature In flowing helium gas. The sample had been previously treated for 25 hours In water-gas shift synthesis gas at 613 K D) Spectrum collected at room temperature In flowing helium gas. The sample had been previously treated for 65 hours In flowing water-gas shift synthesis gas at 613 K.

Figure 10, In situ Mossbauer spectra for a Vulcan XC-72 carbon electrode containing FePc obtained at 0.0 V vs. Hg/Hg0,0H, (Figure k) and at -1.05 V vs. Hg/HgO,OH, (Figure B)(19).

Figure 1. In situ Mossbauer spectra of the Na+ and NH4+ forms of the Si/Fe = 27 sample (the top two pairs of spectra were recorded at 300 K, the bottom pair at 490 K)...

In Situ Mossbauer Spectroscopy under Reaction Conditions... 

Figure 5.12 In situ Mossbauer spectra of a reduced Felr/Si02 catalyst at a reaction temperature of 525 K and during CO hydrogenation when the catalyst is in its steady state (methanol-producing state). The bottom spectrum represents the difference between the two upper spectra it is characteristic of an iron carbide in superparamagnetic state (courtesy of Hyung Woo, adapted from [8]).

Figure 9.19 In situ Mossbauer emission spectra of 57Co in (left) a series of sulfided Co-Mo/A1203 catalysts and (right) MoS2 particles doped with different amounts of cobalt, corresponding to Co/Mo ratios of a) about 3 parts per million, b) 0.05 and c) 0.25. The Co-Mo-S phase, active in the HDS reaction, has a spectrum unlike that of any bulk cobalt sulfide and is most clearly observed in the spectra of Co-Mo/Al203 catalysts of low Co content, and in the MoS2 particles doped with ppms of cobalt (from Wivel et al. [70] and Topspe et al. [71]).

Applications of In Situ Mossbauer Spectroscopy to the Study of Transition Metal Oxides... 

This work summarizes some applications of in situ Mossbauer spectroscopy to the study of certain aspects of the electrochemistry of iron and iron containing transition metal oxides. A number of illustrations of the use of this technique to the investigation of a wide variety of interfacial phenomena may be found in two recent monographs. (2 ... 

One of the first in situ Mossbauer investigations of the behavior of iron oxides in electrochemical enviroments was the result of a fortuitous incident in which a specimen containing iron phthalo-... 

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

In situ Mossbauer parameters for iron oxides and oxyhydroxides dispersed on high area Schawinigan Black carbon electrode... 

Fig. 5. In situ Mossbauer spectra of the same electrode as in a) Fig. 4 at -0.3 V, after a potential step (Curve a), and after a potential sweep (Curve b), from -1.1 V. Curve c was obtained at a potential of -1.2 V.

The presence of iron in nickel oxyhydroxide electrodes has been found to reduce considerably the overpotential for oxygen evolution in alkaline media associated with the otherwise iron free material.(10) An in situ Mossbauer study of a composite Ni/Fe oxyhydroxide was undertaken in order to gain insight into the nature of the species responsible for the electrocatalytic activity.(IT) This specific system appeared particularly interesting as it offered a unique opportunity for determining whether redox reactions involving the host lattice sites can alter the structural and/or electronic characteristics of other species present in the material. 

The in situ Mossbauer experiments were conducted with 90% - Fe enriched 9 1 Ni/Fe oxyhydroxide films which were deposited in the fashion described above onto a gold on Melinex support(12) in a conventional electrochemical cell. Prior to their transfer into the in situ Mossbauer cell, the electrodes were cycled twice between 0 and 0.6 V vs. Hg/HgO,OH" in 1 M KOH. Two such films were used in the actual Mossbauer measurements in order to reduce the counting time. The in situ Mossbauer cell involved in these experiments was previously described. 

In situ Mossbauer parameters for an iron-nickel mixed oxyhydroxide... 

Typically, many researchers assume that the active site in heat-treated macrocycles is similar in nature to the original macrocycle, thus the mechanism is thought to be the same. Bouwkamp-Wijnoltz recently presented evidence to back up this assumption.49 With in situ Mossbauer analysis, the researchers showed similar changes occur in the oxidation state of Fe while applying a voltage to a traditional macrocycle cathode and a heat-treated (up to 700 °C) macrocycle cathode. However, by the authors own admission, more evidence is needed to link this redox cycle to the active site. Additionally, in samples treated at higher temperatures the redox cycle was no longer observed. 

The structural information has been provided by means of in situ Mossbauer emission spectroscopy (MES) and in situ EXAFS (for both the Mo and the Co K-edges). 

Figure 1. Examples of in situ Mossbauer emission spectra of unsupported Co-Mo catalysts, a) Co/Mo = 0.0625 b) Co/Mo = 0.50.

In parallel with the emission measurements, in situ Mossbauer absorption measurements on hematite suspensions treated in a similar manner as in the emission measurements were performed to check the effects of aqueous phase pH on the substrate. The absorption spectra obtained in the pH region 5-12 consisted of the same well-defined sextet as dry hematite powder, indicating that no appreciable change occurred in the state of dispersion and particle size of hematite in the studied pH range. 

In Situ Mossbauer Measurement on Hematite/Divalent Co-57. The adsorption behavior of cobaltous ions on hematite surfaces was essentially the same as that on silica reported by James and Healy (12). Appreciable adsorption begins at about pH 4 followed by an abrupt increase in adsorption between pH 6 and 8. Beyond pH 9, adsorption is practically complete. Emission Mossbauer spectra of Fe-57 arising from the divalent Co-57 ions at the interface between hematite particles and the 0.1 mol/dm3 NaCl solutions of different pH at room temperature are shown in Figure 3 The emission spectra show a marked dependence on the pH of the aqueous phase. No emission lines ascribable to paramagnetic iron species are recognized in... 

In Situ Mossbauer Measurement on Hematite/Pentavalent Sb-119. The Mossbauer measurement on Sb-119 was continued for 1 - 3 days after shaking the suspension for 30 min and waiting for settling of the... 

It should be noted here that in addition to collecting in situ Mossbauer spectra (as described above), it may be advantageous to perform dynamic experiments in the Mossbauer spectroscopy cell, i.e., the simultaneous collection of the Mossbauer spectrum and the measurement of the catalytic reaction rate over the sample. This point has recently been discussed by Dumesic et at. 102a), and simple cells for this purpose have been described elsewhere 102a, 102b). 

Fig. 9.19 In-situ Mossbauer emission spectra of 57Co in a series of sulfided C0-M0/AI2O3 catalysts (left) and M0S2 particles doped with different amounts of cobalt (right), corresponding to Co/Mo ratios of (a) about 3 ...

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