In situ Mossbauer parameters

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... 

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

TABLE III. In Situ Mossbauer Parameters for Small Particles of a Hydrated Ferric Oxyhydroxide, FeOOH (hydrated), Dispersed on High-Area Vulcan XC-72 Carbon"... 

TABLE V. In situ Mossbauer Parameters for Iron Oxides and Oxyhydroxides Dispersed on High-Area Shawinigan Black Carbon Electrode... 

TABLE VIII, In Situ Mossbauer Parameters for an Iron-Nickel Mixed Oxyhydroxide... 

TABLE X. In situ Mossbauer Parameters for a Gas-Fed Electrode Containing a Polymeric Form of Iron Phthalocyanine Dispersed on High-Area Carbon s ... 

TABLE XL In situ Mossbauer Parameters for Tin in Borate Buffer (pH 8.4)... 

Supported non-framework elements, as well as substituted or doped framework atoms, have been important for zeolite catalyst regeneration. By incorporating metal atoms into a microporous crystalline framework, a local transition state selectivity can be built into the active site of a catalytic process that is not readily attainable in homogeneous catalysis. The use of zeolites for carrying out catalysis with supported transition metal atoms as active sites is just beginning. The local environment of transition metal elements as a function of reaction parameters is being defined by in situ Mossbauer spectroscopy, electron spin echo measurements, EXAFS, and other novel spectroscopic techniques. This research is described in the second part of this text. 

The aim of this chapter is to provide a brief background to Mossbauer spectroscopy within the context of phase transformations. The relevant parameters are summarised and the effect of temperature and pressure are discussed, particularly with reference to identifying phase transformations and characterising the electronic and structural environment of the Mossbauer nuclei. Instrumentation is summarised, particularly as it relates to in situ measurements of phase transformations, and a brief survey of applications is given. The appendix includes a worked example that illustrates the methodology of investigating a phase transformation using in situ Mossbauer spectroscopy. Numerous textbooks and review chapters have been written on Mossbauer spectroscopy, and a selection of the most relevant ones as well as some useful resources are listed in Table 1. 

The electrochemical cell employed in the experiments is illustrated in Figure 6b. A typical in situ Mossbauer spectrum obtained for an Fe film (ca. 11 nm thickness) polarized at -0.4 V versus a-Pd/H in borate buffer, pH 8.4, is shown as curve A of Figure 14. The two peaks correspond to the inner components of the sextet characteristic of bulk metallic iron. Upon stepping the potential to 1.3 V, an additional doublet with parameters 8 = 0.41 mm s A = 1.09mms and T = 0.83 mm s" attributed to the passive film was obtained after about 24 h of data acquisition (curve B, Figure 14). The electrode polarization was then interrupted, the electrolyte drained, and the film washed with distilled water and stored in a desiccator. 

The advantage of the technique is that the particle size may be determined with the sample in a controlled atmosphere and at a temperature different from 300 K, i.e., in situ particle size measurement, and measurement of changes in particle size may be possible. The problem, however, is that the quantitative relation between the Mossbauer parameters and particle size is rather complex and in some cases not theoretically available. Therefore, the application of the Mossbauer effect to particle size measurement is often facilitated through an experimental calibration of the Mossbauer parameters to particle size for the particular catalyst system of interest, i.e., the measurement of the parameters for a set of samples of known particle size as determined by other experimental methods. This point will become clearer below, as the effects of particle size on the Mossbauer parameters are discussed. 

Ex situ Mdssbauer spectra were recorded at 293, 77, and 4.2K with a constant acceleration spectrometer using a Co in Rh source. The source was kept at room temperature. The Mossbauer parameters were resolved by fitting the collected spectra with Lorentzian shaped profiles. Isomer shifts are given relative to sodium nitroprusside at room temperature. Hyperfine fields (Heff) were calibrated against the 515 kOe field of a-Fc203 at 300K. The accuracies of the parameters are Isomer shift (IS) 0.02 mm s", quadrupole splitting (QS) 0.04 mm s, and the spectral contribution 5%. 

The spin and valence states of iron in the lower-mantle minerals have been investigated bya number of synchrotron X-ray and optical laser spectroscopic techniques in a high-pressure diamond anvil cell (DAC). Mossbauer spectroscopy has been instrumental in our understanding of this topic because this method is specific to Fe-containing minerals and provides hyperfine QS and CS parameters [5,19-22]. Specifically, synchrotron Mossbauer spectroscopy (SMS) with a highly intense and focused beam coupled with the X-ray transparent DAC permits in situ observations of the Mossbauer spectra with reasonable data collection times at extreme P-T conditions [20-22]. 

Mdssbauer Spectra of Ir—Fe/Si02 and Fe/Si02 Treated with H2 or O2 Fig. 29.3 displays the quasi in situ Fe Mossbauer spectra of Ir-Fe/Si02 catalyst before and after H2 reduction, as well as reoxidation with O2. For comparison, the spectra of Fe/Si02 sample pretreated under similar conditions are presented in Fig. 29.4. The corresponding Mossbauer parameters are listed in Tables 29.4 and 29.5. 

The Mossbauer parameters for the passive film obtained in quasi in situ conversion electron measurements conducted in the same borate buffer medium (Figure 15) were found to yield good agreement with those obtained in the transmission mode (5 = 0.38, A = 1.03). The sharper doublet in Figure 15 corresponds to the inner lines of metallic iron. 

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