Metals: anomalously high

Schwerdtfeger, P. and Dolg, M. (1991) Anomalous high gold-metal bond... 

In size-fractionated samples, they found that concentrations of some metals generally increased with decreasing particle size. However, more upstream samples (nearer to the source) contained anomalously high concentrations in the coarsest fractions. Because Clark Fork River sediment is predominantly coarse-grained, coarse fractions significantly add to the bulk contaminant content of the system. 

TR-C, above 60 °K up to about 300 °K is a metal-like region. Following a sharp drop in anomalously high conductivity (when existing), the conductivity has a negative temperature coefficient. 

In the third series iridium trifluoride has a volume which is slightly larger than that for rhodium trifluoride, in line with the small increase in size of the metal atom. The anomalously high volume for gold trifluoride has already been discussed. 

It is believed that electron correlation plays an important role with the anomalously high resistivity exhibited in marginal metals. Unfortunately, although the Mott-Hubbard model adequately explains behavior on the insulating side of the M-NM transition, on the metallic side, it does so only if the system is far from the transition. Electron dynamics of systems in which U is only slightly less than W (i.e. metallic systems close to the M-NM transition), are not well described by a simple itinerant or localized picture. The study of systems with almost localized electrons is still an area under intense investigation within the condensed matter physics community. A dynamical mean field theory (DMFT) has been developed for the Hubbard model, which enables one to describe both the insulating state and the metallic state, at least for weak correlation. 

For fused metals anomalous results for the association factor x below 1 are found, whilst fused salts give peculiar results, sometimes indicating an association factor of 10. (In such cases, equation (1) must be used, since the critical temperatures are unknown and must be very high.) These are, however, extreme cases of application. The abnormalities in general seem to be connected with the value of the critical temperature, At present, it is fairly generally agreed that, whilst low values of k for liquids about room temperature point to association, the calculation of an association factor x by Ramsay and Shields s method is unjustified. ... 

The term electron deficiency was introduced by Dalla Betta and Bou-dart to account for the anomalously high hydrogenation activity of small Pt particles in zeolite Y (50). The electron deficiency was ascribed to an electron transfer from small Pt particles to the zeolite. X-Ray absorption has been applied to measure the Pt Lm white line area as an indication of the electron deficiency because the white line is related to the number of unoccupied electronic states in the 5d and the 6j bands (273). For reduced Pt/NaHY it appeared that the white line area, and hence the electron deficiency of Pt particles, are closely related to the proton concentration of the zeolites. For example, the relative white line areas for Pt/H4gY, Pt/ H19Y, and Pt foil are 1.6, 1.2, and 1, respectively. White line areas at the Liii X-ray absorption threshold to determine the if-band occupancy of supported metal catalysts were first reported by Lytle 274). The use of the white line area as an indication for electron deficiency has been questioned by Lewis, who argues that a decrease of the metal particle size will also lead to an increase of the white line area (275). 

High r factors are, however, not without some other complications since they imply porosity of materials. Porosity can lead to the following difficulties (a) impediment to disengagement of evolved gases or of diffusion of elec-trochemically consumable gases (as in fuel-cell electrodes 7i2) (b) expulsion of electrolyte from pores on gas evolution and (c) internal current distribution effects associated with pore resistance or interparticle resistance effects that can lead to anomalously high Tafel slopes (132, 477) and (d) difficulties in the use of impedance measurements for characterizing adsorption and the double-layer capacitance behavior of such materials. On the other hand, it is possible that finely porous materials, such as Raney nickels, can develop special catalytic properties associated with small atomic metal cluster structures, as known from the unusual catalytic activities of such synthetically produced polyatomic metal clusters (133). 

The adsorption of acids is assumed to be responsible for the catalyst deactivation observed at pH 2 which blocks further conversion of glyceric acid. This problem is alleviated simply by neutralising the acids. Thus, at pH 4, 5 and 6, conversion proceeds smoothly and there is an increase in the initial rate of reaction as the pH is increased. The anomalously high rate at pH 2 is assumed to be due to the initial rapid adsorption of glyceric acid on the platinum metal which increases the reaction rate at first since it brings the substrate into the immediate proximity of the bismuth. 

Rasmussen (2004b) and Rasmussen et al. (2008) showed that differences in metal concentrations between indoor dust and outdoor soil may become magnified when metal bioaccessibility is considered, depending on the sources of the element and its speciation. Particularly in residential environments where anomalously high metal concentrations are encountered, it is important to elucidate metal speciation in order to refine risk assessments of metal contaminants. Advanced... 

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