Sulfur adsorbed, chemical state

Quantitative AES analysis was carried out using a standardization technique developed in our laboratory (55). Namely, a dry, thin layer of sulfate deposit was obtained on the Me(lll) template from a 0.3 M Na2S04 solution and subsequent water evaporation. This Na2S04 covered Me(lll) was used as a standard for work with monolayer (bi)sulfate adlattices. The procedure involved a comparison of the peak-to-peak (p/p) intensities of sulfur and oxygen at 131 eV and 516 eV, respectively, relative to the Me p/p (Au 69 eV, Rh 302 eV, Pt 64 eV) intensity of the clean Me(l 11) sample. As mentioned above, the chemical state of the adsorbate was interrogated by the Core Level Energy Loss Spectroscopy. The loss energies reported here were measured relative to the electron elastic peak of 500 eV electrons. 

The most stable chemical state of sulfur on surfaces is the adsorbed or chemisorbed state resulting from the segregation of sulfur from the bulk of the material or from dissociative adsorption of sulfur-containing molecules or ions present in the environment (in air or in aqueous solution). 

Once sulfur is present on the surface in the active chemical state (i.e., adsorbed), it has the same effects (which have already been described) irrespective of its origin. 

There is much evidence for the damaging effect of sulfur species in a wide range of corrosion-related service failures. The relation between the sulfur-induced corrosion mechanisms presented in this chapter and the implications in areas of practical importance can be rationalized on the basis of (a) the source of sulfur, (b) the transport process to the metal surface, and (c) the conditions of the reduction (or oxidation) of the sulfur species into the harmful chemical state of sulfur, i.e., the adsorbed (chemisorbed) sulfur (or the sulfide if the concentration of the sulfur species is high). Table 8.2 summarizes some of these... 

There are precautions that must be taken when attempting to separate heavy feedstocks or polar feedstocks into constituent fractions. The disadvantages in using ill-defined adsorbents are that adsorbent performance differs with the same feed and in certain instances may even cause chemical and physical modification of the feed constituents. The use of a chemical reactant such as sulfuric acid should only be advocated with caution since feeds react differently and may even cause irreversible chemical changes and/or emulsion formation. These advantages may be of little consequence when it is not, for various reasons, the intention to recover the various product fractions in toto or in the original state, but in terms of the compositional evaluation of different feedstocks, the disadvantages are very real. 

D. Spence and J. R. Scott t have concluded from their results that sulfur is present in rubber in three different states, free, adsorbed, and chemically combined. 

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