4d Metals

Fig. 3. (a) TEM Image of 4d-metal phase in high bum-up spent fuel (b) EDS spectrum of the particle. 

Indications from both microscopic analyses of metallic particles from corrosion tests and evidence from the Oklo natural reactors indicate that performance assessment calculations should not assume 99Tc is easily mobilized. It is entirely inappropriate to use "Tc release as a marker for fuel corrosion because Tc is not located in the fuel matrix. The TEM examinations of corroded e-particles have shown that Mo is preferentially leached from these phases, a behaviour that is similar to the one observed at Oklo. It is interesting to note that laboratory dissolution of billion-year old 4d-metallic particles for a chemical analysis required a corrosive mix of peroxide and acid (Hidaka Holliger 1998) similar to the experience at SNF reprocessing plants. It is doubtful that the oxidation potential at the surface of an aged fuel will be sufficient to move Tc(0) from the e-metal particles. 

The pentacoordinate trifluorophosphine complexes behave similarly. Thus the difference in energy between the 2E and 2E" states, Ae e"> increases markedly in going from Fe(PF3)s to Ru(PF3)s. This increase is caused by the change in energy of the 2E state, which, in turn, results from changing from a 3d to a 4d metal orbital. 

Noble metal Compounds 4d-metal Sd-metal Metal oxidation state Coordination type... 

It has turned out that the computation of metal nuclear shieldings and chemical shifts is much more difficult than the calculation of ligand shifts which were discussed previously. It appears that metal shieldings are more sensitive to the quality of the computed electronic structure and consequently larger influences due to the XC potential are observed. Regarding the 3d metals, it has turned out that hybrid functionals appear to be particularly well suited for NMR computations of the metal shielding constants. This cannot be easily extrapolated to all of the transition-metals, though, since counter examples are known for which nonhybrid functionals perform better. On the other hand, some 4d metals have been treated most successfully with hybrid functionals. 

For further data on DFT computations of 3d and 4d metal shieldings the reader is referred to Refs. [1,3,5]. 

More important than pushing the time resolution into the microsecond range is measuring X-ray absorption data of superior quality. Therefore, to evaluate data quality and time resolution of a particular experimental station, "real" catalysts under reaction conditions should be compared (e.g., 3d and 4d metal oxides at elevated temperature measured to k equal to 16 A, rather than metal foils). It must be kept in mind that kinetics data can also be obtained from complementary methods such as... 

X-edge absorption spectra of 3d and 4d metals (205,206) show similar XANES for metals with similar crystal structure. After recalibration of the energy scales, in order to remove the trivial kinetic energy dependence on lattice spacing, the observed peak positions are a signature of the environment of the absorber as determined by local geometry only. The amplitude of features due to specific bands are proportional to the hybridization strength or wave character (204). 

The known classes of open-framework materials, such as zeolites, phosphates, and oxides, are primarily dominated by oxo frameworks. In 1989, however, Bedard and Wilson at of UOP[31] extended this kind of material to include metal sulfide compounds R-M MS-n (R indicates organic SDA M is a 3d or 4d metal M is Ge, Sn, Sb, or In). Their frameworks are constmcted by the corner-sharing or bridging of connected MS, clusters... 

The constrained equilibrium description discussed up to now conveys the impression that a possible oxidation of the catalyst surface in the O-rich environments of oxidation catalysis would rather Meld bulklike thick oxide films on Ru but thin surface oxide structures on the more noble 4d metals. This reflects the decreasing heat of formation of the bulk oxides over the late TM series, and seems to suggest that it is primarily at Pd and Ag where oxide formation in the reactive environment could be self-limited to nanometer or subnanometer thin overlayers. Particularly for the case of Ru, Fig. 5.12 shows that the gas phase conditions t3q)ical for technological CO oxidation catalysis fall deep inside the stability regime of the bulk oxide, indicating that thermodynamically nothing should prevent a continued growth of the once formed oxide film. 

Fig. 8. Calculated surface energy for fcc(lll) surfaces of 3d and 4d metals (solid squares), compared with experiment (open circles) (Skriver and Rosengaard, 1992 tight-binding L.MTO-ASA, with Green function method). For the 3d metals, the dashed line connecting solid circles gives results from spin-polarized calculations. For the 4d metals, the dashed line connecting open triangles gives results from Methfessel et al. (1992 full potential LMTO, slab geometry).

For carbonylation reactions involving metal carbonyls as catalyst precursors, the best fit to requirements 1 and 2 apparently is given by 4d transition elements. It is known that when activation of CO is required, carbonyl complexes of 4d metals are better catalyst precursors than their 3d and 5d congeners. Pertinent to this point are the following experimental observations ... 

D(Ag+— L). This Au, Cu, Ag order parallels the decrease in D(M+ — L) from 3d to 4d metals from the same group as noted for other middle and late transition elements18. 

XANES studies at the Mg and A1 X-edges. Phys Chem Minerals 27 20-33 Mottana A, Robert J-L, Marcelli A, Giuli G, Della Ventura G, Paris E, Wu Z (1997) Octahedral versus tetrahedral coordination of A1 in synthetic micas determined by XANES. Am Mineral 82 497-502 Muller JE, Jepsen O, Wilkins JW (1982) X-ray absorption spectra X-edges of 3d transition metals, /-edges of 3d and 4d metals, andM-edges of palladium. Solid State Commun 42 365-368 Muller JE, Wilkins JW (1984) Band-structure approach to the X-ray spectra of metals. Phys Rev B29 4331-4348... 

A striking feature of fluorophosphine-transition metal complexes is their high thermal stability, and this often contrasts markedly with the corresponding carbonyl derivative, e.g., HCo(CO)4 decomposes to an appreciable extent below room temperature, whereas HCo(PF3)4 is stable up to 250°C. The only exceptions appear to be some 4d metal complexes,... 

Our group systematically studied single transition metal atoms M deposited on surfaces of ionic metal-oxide supports [252-256]. In some of these studies [252,254], relativistic contributions to the strength of M-0 bonds were thoroughly discussed and found to be rather significant for 4d-metal atoms. 

Our comparative scalar relativistic DKH DF BP study of Pds and Pt3 particles, free and deposited on the Al-terminated surface a-Al203(0001), provides another example of notably enhanced relativistic effects when going from 4d to 5d metal moieties [260]. This increase resulted in stronger and shorter bonds formed by 5d metals compared to 4d metals. 

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