Mossbauer elements employed

Gold ( Au) is another Mossbauer element that can possibly be employed in archaeological applications. However, Au studies vdll be fairly limited in practice, since samples must be cooled dovm to very low temperatures. 

Mossbauer spectroscopy is a selective tool for the quantitative analysis and spe-ciation of a very limited number of elements. It has been mainly used to study iron compounds—e.g., ceramics, as it gives valuable information about iron-bearing oxide and silicate minerals. This technique has been applied to the identification of the provenance of clay and used raw materials—the manufacturing method employed in pottery and, to a lesser extent, to the characterization of pigments and weathering crusts formed on stone monuments [23]. 

In the preparation of iridium-containing catalysts, the temperature of calcination in air is critical. Mossbauer spectra at 25°C are shown in Figure 4.34 for platinum, iridium, and bimetallic platinum-iridium catalysts (samples B-600, C-600, and D-600, respectively) which were prepared by calcination in air at 600°C prior to the final reduction in hydrogen at 500°C. The catalysts have the same elemental compositions as the catalysts for which spectra are shown in Figure 4.32, but they exhibit structural differences due to the higher calcination temperature (600 vs. 260<>C) employed in their preparation. 

These materials are characterized by common techniques such as H NMR, IR, elemental analysis, DSC, and TGA.54 58 59 60 64 68 Other techniques such as electron spin resonance (ESR),54 magnetic susceptibilities54 and Mossbauer spectroscopy (when they apply),56,58 60 64 71 uV-visible spectroscopy,58,59 60 61 64-67 and solid-state electric conductivity measurements5456 59-63 66 67 71 were also employed. These materials were carefully compared to model bis(mono-isocyanide) adducts for better understanding of the physical properties. Important solubility problems are often observed when no alkyl side chain is used. So, these more soluble substituents are incorporated either on the macrocycles or the bridging ligands for better characterization. 

It is useful at this juncture to point out that some forms of Fe, as well as other transition elements, can exist in diamagnetic forms as well as in forms containing an even number of electrons. For the first there can be no EPR since there is no magnetic moment. For the latter situation, exemplified by high-spin Fe(II) and Mn(III), EPR may be observable using special methods, or may not be observable at the conventional frequencies. (For details, see [22].) In the case of Fe, however, one has the option of employing Mossbauer spectroscopy. It is useful in all oxidation states and spin-coupling schemes. 

The influence of the zeolite environment on the XPS BE of dispersed ions (vide supra) means that reference compoimds for this type of investigation are not easily available. This is not so much a problem for the starting material for which the highest oxidation state of the element is often plausible, but the identification of intermediate states, and sometimes of the final state of reduction, is not straightforward. As a first approximation, BE shifts known from bulk components (e.g., coordination compounds) are often used in the analysis of zeolite systems. Combination with bulk techniques sensitive to electronic structure can provide additional information notwithstanding possible differences between the conditions in the bulk crystallite and the surface layer. Thus, IR of adsorbed CO has been used to differentiate between Pt andPt(O) atoms in H-ZSM-5 [131], EXAFS was able to detect very small intra-zeolite Cu(0) clusters formed from Cu+ with almost identical XPS/XAES signature [108], Mossbauer spectroscopy suggested the presence of Fe in zeolites with doubtftil Fe 2p satellites [116], and ESR was employed to support the occurrence of Pd+ in the reduction of intra-zeolite Pd(II) [126,127]. 

The source most commonly employed with Fe Mossbauer spectroscopy is elemental Co, which is incorporated into rhodium or copper metal. During the radioactive decay of the cobalt isotope into Fe, the needed gamma radiation is emitted. For measurements with tin ( Sn), sources of CaSnOs or BaSnOs enriched with Sn are used, which again release the proper radiation during their radioactive decay. The source is moved at constant positive and subsequently negative accelerations (i.e. linearly varying speed) to probe the resonant absorption. 

Second, the ability of source experiments to use Mossbauer parent atoms permits many more elements to be used than in absorption experiments by employing the excited Mossbauer ievei. The nature of an experiment will determine how Mossbauer 7-emitters are introduced into a sample. Using an energetic Rl beam, it is possible to implant Mossbauer probes to depths of several hundred micrometers and with straggling widths as wide as several tenths of a micrometer. The measurement duration can be controlled by using a probe nuclide with an appropriate half-life for example, for Mn, the Mossbauer effect can be observed for a few minutes after implantation, whereas an in-beam experiment using Coulomb excitation and recoil implantation has a duration of only several hundred nanoseconds after implantation. 

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