In situ Mossbauer spectrum

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)...

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]).

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.

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). 

In situ Mossbauer spectra were obtained at 300 K after various treatments. First, after evacuation and flushing in nitrogen flow, then the second spectmm was recorded after a treatment with 4 % toluene in purified nitrogen flow, then the third spectrum was recorded after a repeated evacuation/nitrogen flow treatment. All the mentioned treatments were performed at 720 K for 2 h. 

In situ Mossbauer Measurements. In situ Mossbauer spectra were taken during the room temperature oxidation of carbon monoxide on Sn-Pt/Si02 catalyst ((0-3) type (sample II-3 in ref. 139) both at 27 °C and -196 °C. These spectra are presented in Figure 21 A and B, respectively and the corresponding data are summarized in Table 12. ... 

Figures 21A and 21B show that the character of in situ Mossbauer spectra taken in the presence of CO -(- O2 at room temperature has been completely changed compared to the reduced form of the catalyst (compare spectra b and c). The oxidative atmosphere in the presence of CO resulted in (i) oxidation of both PtSn(a) and PtSn(b) alloy phases to Sn in a high proportion and (ii) the...

Fig. 5.93. In situ Mossbauer spectra of a tin electrode at various electrode potentials in a borate buffer solution (pH = 8.4) (based on data in [546])...

Fig. 5.94. In situ Mossbauer spectra of a film of Prussian blue deposited on a glassy carbon electrode in an aqueous solution of 1 M KCl (pH = 4) at E ce = 0.6 V (top) and E ce = —0.2 V (bottom) (based on data by [549])...

Quasi in situ Mossbauer spectra of Fe/Si02 (a) after 100% H2 reduction at 300 C for 2h,... 

FIGURE 10. In situ Mossbauer spectra for FePc dispersed on Vulcan XC-72 carbon subjected to the same heat treatment as specified in the caption to Figure 9, obtained at (A) 0.0 V and (B) -1.05 V versus Hg/HgO, OFI". Other conditions given in caption to Figure 9. 

FIGURE 12, In situ Mossbauer spectra of a 50% w/w Fe-enriched hydrated ferric oxide precipitated on Shawinigan black high-area carbon in the form of a Teflon-bonded electrode in 4 M KOH at (a) -0.3 V and (b) -1.1 V versus Hg/HgO, OH". Inset Cyclic voltammogram of the same, although nonenriched, material as in (a) in the form of a thin porous Teflon-bonded coating electrode deposited on an ordinary pyrolytic graphite electrode, in 4 M KOH. Scan rate 10 mV s" ... 

FIGURE 22. In situ Mossbauer spectra of a fuel cell type electrode containing a polymeric form of FePc operating as an oxygen cathode in 2.3 M H2SO4. Curves A and E were obtained before and after conducting the actual experiments. The other spectra were acquired between (B) 0 and 6 h, (C) 6 and 22 h, and (D) 22 and 71 h. 

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]).

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... 

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 ...

In-situ Mossbauer spectroscopic measurements were carried out on enriched catalyst samples rising an AME-50 Mossbauer spectrometer at room temperature. All spectra were computer-fitied to a Lorentzian line shape, and the isomer shifts are quoted relative to SnOz. 

Liquefaction. Montano et al. [ ] have investigated the transformation of pyrite to pyrrhotite in coal liquefaction environments. They conducted in situ Mossbauer spectroscopy measurements on coals maintained at 1.24 MPa nitrogen pressure and observed changes in the isomer shift at approximately 300°C that signalled the beginning of the transformation of pyrite to pyrrhotite. The transformation accelerated between 300 and 400°C, and from 20 to 80 percent of the pyrite in four different coals was transformed after one hour at 440°C. From examination of both the in situ spectra and the spectra of cooled residues, they concluded that the pyrrhotite underwent covalent bonding to the coal molecules, causing a catalytic effect on coal liquefaction. 

FIGURE 14. In situ Mossbauer transmission spectra for 11-nm Fe film in borate buffer (pH 8.4) at -0.4 V (metallic Fe) (curve A) and after passivation at + 1.3 V versus a-Pd/H (curve B). Curves C and D were obtained ex situ after drying the film and in situ (+1.3 V) after reintroducing the passive film in the same electrolyte. Spectrum E was recorded at + 1.3 V after two reduction-passivation cycles. See text for additional details. 

The example is typical for many applications of Mossbauer spectroscopy in catalysis a catalyst undergoes a certain treatment, then its Mossbauer spectrum is measured in situ at room temperature. Flowever, if the catalyst contains highly dispersed particles, the measurement of spectra at cryogenic temperatures becomes advantageous as the recoil-free fraction of surface atoms increases substantially at temperatures below 300 K. Secondly, spectra of small particles that behave superparamagne-... 

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. 

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