Magnesium surface

To minimize the effects of this difficulty, an initiator is frequentiy employed. Among the numerous suggestions in the Hterature, the most satisfactory industrial procedure is to retain a portion of the Grignard from the preceding batch and to add this portion to the initial ether charge. The purpose of this procedure is to eliminate residual water and to clean the magnesium surface. Once this initiator has been added, the hahde is added at a rate deterrnined by the temperature and the pressure in the reaction vessel. 

The species R—X and Mg " are radical ions." The subscript s is meant to indicate that the species so marked are bound to the surface of the magnesium. Its is known that this is a surface reaction." It has been suggested that some of the R-radicals diffuse from the magnesium surface into the solution and then return to the surface to react with the XMg. There is evidence both for" and against" this suggestion. Another proposal is that the fourth step is not the one shown here, but... 

That carbon monoxide could be oxidised in a facile reaction at cryogenic temperature (100 K) was first established in 1987 by XPS at an aluminium surface.21 The participation of reactive oxygen transients O 1 (s) was central to the mechanism proposed, whereas the chemisorbed oxide O2 state present at 295 K was unreactive. This provided a further impetus for the transient concept that was suggested for the mechanism of the oxidation of ammonia at a magnesium surface (see Chapter 2). Of particular relevance, and of crucial significance, was Ertl s observation by STM in 1992 that oxygen chemisorption at Al(lll) resulted in kinetically hot adatoms (Figures 4.1 and 4.7). 

A passive film is formed on the Mg electrode surface in alkaline solutions, which creates problems when the Mg-air cell is to be discharged. However Mg and especially some Mg alloys are suitable to be used in magnesium-air cells with NaCl-electrolyte [4], where the formation of the passive film on the magnesium surface is not observed. 

D. Reactions of Magnesium Surfaces and Films with Organic Substrates. 172... 

It must be emphasized that when high purity magnesium is converted into turnings, chips or powder, special precautions have to be taken in order to avoid contamination [2]. It should also be noted that magnesium surfaces, exposed to air, rapidly become covered with an oxide film. Methods for activating magnesium (see below) usually depend on the removal or penetration of this surface film. 

Following the prophetic speculations of Kharasch and Reinmuth [A] and the crucial CIDNP experiments of the Amsterdam group [22], the main features of the mechanism are now clear. Electron transfer from the magnesium surface to adsorbed organic halide gives rise to alkyl radicals and halide ions. Further recombination steps lead to the alkylmagnesium halide. Two matters especially remain controversial whether the electron transfer step involves a discrete radical anion intermediate, and to what extent radicals leave the metal surface to diffuse into the solution. Recent arguments have been summarized [23, 24], commented on [25], and pursued [26]. The implications for practical work may be summarized briefly as follows ... 

Surface Finishes. Of almost equal importance are surface finishes and surface coatings. For instance, E-coating (or electrophoretic painting) is an effective contaminant-containment for many machined metal surfaces such as aluminum and magnesium surfaces, and for magnets. Other surface treatments applied in past and current IBM disk drives are listed in Table A1. 

The results for the removal of PAHs from the 5 surface substrates are summarized in Table 16. In general, the 23 PAHs listed in the table averaged removal rates around 90% from the smooth surfaces and over 80% for the porous cast magnesium surface. In contrast, supercritical fluid extraction studies using CO2 for the removal of PAHs from soils for environmental applications have shown relatively poor removal efficiencies for many of the compounds listed in the table often requiring the addition of secondary solvents to the C02. However, it appears that from the results on the removal of the PAHs shown in Table 16, surface contamination is... 

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