Foreign metals

Fremd-gas, n, foreign gas. -kdrper, m. foreign substance foreign body, -metall, n. foreign metal. 

When Europe exploded into war in 1914, scientists largely abandoned their studies to go to the front. Marie Curie, with her daughter Irene, then 17 years old. organized medical units equipped with X-ray machinery. These were used to locate foreign metallic objects in wounded soldiers. Many of the wounds were to the head French soldiers came out of the trenches without head protection because their government had decided that helmets looked too German. In November of 1918, the Curies celebrated the end of World War I France was victorious, and Marie s beloved Poland was free again. 

Figure 12. Schematic drawing of the theoretical positions of the foreign metal ions in (a) 7 A phyl-lomanganates and (b) 10 A phyllomanganates. The foreign ions can be located above a manganese vacancy (between the Mn - O layer and the sheet of water molecules) and within the layer, respectively (adapted from [47]).

Lead can be used, because the corrosion itself forms a rather dense passivating layer of lead dioxide that protects the underlying material against fast corrosion [28]. If foreign metals like copper are used they have to be covered thoroughly by a dense layer of lead. 

Silver monolayer deposited on foreign metal single crystal. 

The use of adatoms of foreign metals obtained by imderpotential deposition on the platinum surface is another convenient method for investigating the effect of a promoter on the electrocatalytic properties of platinum. However, the effect of adatoms in this case has been shown to be not as effective for electrooxidation of methanol as for the oxidation of other organic molecules such as formic acid adatoms of tin, however, showed a positive effect on the rate of methanol oxidation. ... 

EFFECT OF FOREIGN METALS ALLOYED TO Pt COMPARISON BETWEEN ETHANOL AND METHANOL... 

Adzic RR. 1984. Electrocatalytic properties of surfaces modified by foreign metal adatoms. In Gerisher H, Tobias CW, eds. Advances in Electrochemistry and Electrochemical Engineering. Volume 13. New York Wiley-Interscience. pp. 159-260. 

Spasojevic MD, Adzic RR, Despic AR. 1980. Electrocatalysis on surfaces modified by foreign metal adatoms Oxidation of formaldehyde on platinum. J Electroanal Chem 109 261-269. 

Adzic RR, Tripkovic AV, Markovic NM. 1983. Structural effects in electrocatalysis oxidation of formic acid and oxygen reduction on single-crystal electrodes and the effects of foreign metal adatoms. J Electroanal Chem 150 79-88. 

When a metal is in contact with its metal ion in solution, an equilibrium potential is established commonly referred to as Nernst potential (Er). Metal deposition occurs at potentials negative of Er, and dissolution for E > Er. However, when a metal is deposited onto a foreign metal substrate, which will always be the case for the initial stages of deposition, it is frequently observed that the first monolayer on the metal is deposited at potentials which are positive of the respective Nernst potential [37, 38]. This apparent violation for Nernst s law simply arises from the fact that the interaction between deposit metal and substrate is stronger than that between the atoms of the deposit. This effect has been termed underpotential deposition (upd), to contrast deposition processes at overpotentials. (One should keep in mind, however, that despite the symmetrical technical terms the physical origins of both effects are quite different. While the reason for an overpotential is solely due to kinetic hindrance of the deposition process, is that for underpotential deposition found in the energetics of the adatom-substrate interaction.)... 

Both effects make the adsorption of an adsorbate on a foreign metal with a higher work function more favorable than bulk deposition. 

The underpotential deposition (UPD) of metals on foreign metal substrates is of importance in understanding the first phase of metal electrodeposition and also as a means for preparing electrode surfaces with interesting electronic and morphological properties for electrocatalytic studies. The UPD of metals on polycrystalline substrates exhibit quite complex behavior with multiple peaks in the linear sweep voltammetry curves. This behavior is at least partially due to the presence of various low and high index planes on the polycrystalline surface. The formation of various ordered overlayers on particular single crystal surface planes may also contribute to the complex peak structure in the voltammetry curves. 

It is likely that the decreases observed can be rationalized in terms of two contributions. Changes in surface optical properties resulting from modification by the foreign metal have been shown to decrease the electromagnetic enhancement contribution to SERS. However, for the case of Pb UPD on Ag, this effect has been shown to account for only ca. 40% of the decrease in going from zero coverage to one monolayer.(14) Moreover, this model does not account for the relatively rapid decrease in intensity observed with the deposition of small (i.e., less than 20% of a monolayer) amounts of Pb on the Ag surface. 

Underpotential deposition is described as less than monolayer metal deposition on a foreign metal substrate, which occurs at more positive potentials than the equilibrium potential of a metal ion deposed on its own metal, expressed by the Nemst equation. Kolb reviewed state-of-the-art Underpotential deposition up to 1978. As Underpotential deposition is a process indicative of less than a monolayer metal on a substrate, it is expected to be quite sensitive to the surface stmcture of the substrate crystal a well-defined single-crystal electrode preparation is a prerequisite to the study of Underpotential deposition. In the case of Au and Ag single-crystal electrodes, Hamelin and co-workers extensively studied the necessary crystal surface structure, as reviewed in Ref. 2. 

In industrial applications of metal deposition a metal M is deposited either on the native metal substrate M or on a foreign metal substrate S. As an example of the former, Cu is electrodeposited on a Cu substrate formed by electroless Cu deposition on an activated nonconductor in the fabrication of printed circuit boards. As an example of the latter, Ni is electrodeposited on Cu in the fabrication of contact pads in the electronics industry. 

The procedure is depicted schematically in Figure 7.22. For example, uniform-size Cu clusters were produced and distributed evenly over an atomically flat Au(l 11) electrode surface (76). This example is of the type of deposition of a metal on a foreign metallic substrate and involves UPD (defined in Section 7.15) (80). The procedure was made fully automated through the use of microprocessors in controlling the x-, y-, and z-movements of the tip. Arrays of up to 400 individual clusters were made at a rate of 50 clusters per second (76). The average height of the Cu clusters was 0.8 nm (75,80). 

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