Surface sulfate

Scale deposits create conditions for concentration-cell corrosion as they do not form uniformly over the metal surface. Sulfate-reducing bacteria thrive under these deposits, producing hydrogen sulfide and, consequently, increasing the rate of corrosion. Due to the following factors, the drilling fluid environment is ideal for scale deposition [189]. These factors are as follows ... 

To elucidate the higher BE peaks [S(2p) 168.9, 169.6 eV], we examined the Naflon film Itself because It has -SOj" cation exchange sites. Only C(ls) and F(ls) peaks were observed. No S(2p]f peak was observed even after Ar" bombardment. This result suggests that the surface concentration of cation exchange sites Involving -SO3 Is low. Sulfur located well below the surface would, of course, hot be detected by XPS, We assign the higher BE S(2p) peak to a surface sulfate species (11),... 

In contrast, with platinum, SO2 removal, while always greater than the unpromoted case, tends to decrease with increasing temperature. The presence of platinum increases the rate of oxidative adsorption of SO2 to the point that the capacity of alumina becomes the limiting factor rather than the rate. The capacity, limited by thermodynamics, decreases with increasing temperature because of the stability of surface sulfate species... 

The results from this "temperature programmed reduction" indicate that there are two regimes as indicated in Figure 16. In Regime 1, at temperatures above 1050°F there is virtually complete removal of SO2 under the conditions of our test. In Regime 2, characterized by the step jump downward at 1050°F in the amount of SO2 adsorbed in successive cycles, there is only partial removal of SO2. This observation suggests two kinds of surface sulfate species present on this material, one easily and another more difficultly removed as indicated by Andersson (47). 

Theoretical calculations for the simplified systems have shown that on the Ag(llO) surface, sulfate ions adsorb with the binding energy of 782 kj mol whereas for carbonate ions, this energy is equal to 949 kJ mol [104]. These... 

If one assigns, after Yao (38), the area occupied by a surface sulfate group as 30 A2, and if the sulfur content and the BET area are known, it is possible to estimate whether, indeed, sulfur retention is kept below one monolayer. Table VI shows a compilation of sulfur retention, recalculated as number of monolayers. The buildup of sulfur at a given temperature takes place during relatively short exposures to the exhaust, typically of the order of a few thousand miles of vehicle operation. When the temperature is changed, the extent of retention will change. Thus, a certain pelleted catalyst (42) accumulated in 3000 miles about 0.5% of sulfur,... 

Polystyrene latexes have been prepared using persulfate initiator for many years, but only recently have methods been developed to determine the number and loci of the sulfate surface groups. To determine these surface groups, the latex is cleaned to remove the adsorbed emulsifier and solute electrolyte, then the surface sulfate groups in the H+ form are titrated conductometrically with base. The latexes can be cleaned effectively by ion exchange (2-5) or serum replacement (6) dialysis is not effective in removing the adsorbed emulsifier and solute electrolyte (3,5,6). +... 

One possibility is hydroxyl endgroups, which may be formed by a side reaction of sulfate ion-radicals to form hydroxyl radicals (9) or hydrolysis of the surface sulfate groups. To determine if hydroxyl groups were present, the ion-exchanged latexes were oxidized by heating with persulfate and 10 silver ion at 90°, then ion exchanged and titrated conductometrically to determine the carboxyl groups. Table II (9) shows that some sul-... 

These results showed that monodisperse polystyrene particles prepared using persulfate initiator and bicarbonate buffer contain both sulfate and hydroxyl surface groups. Some sulfate groups are on the particle surface, while others are buried inside the particle. All of the hydroxyl groups are on the particle surface. These results also showed that careful control of the pH during polymerization can produce latex particles stabilized with only surface sulfate groups. 

Surface Sulfate Total Sulfate Sulfate + Hydroxyl... 

The foregoing latexes prepared using persulfate initiator and bicarbonate buffer contained surface sulfate groups and, in some cases, surface hydroxyl groups. None of these latexes contained carboxyl groups. These latexes had final pH values of 7-8. If the bicarbonate buffer was omitted, the final pH of the latex was 2-3 because of the bisulfate ion generated by the persulfate decomposition. 

Earlier work (3) has shown that cleaned monodisperse polystyrene latexes stabilized with surface sulfate (and perhaps a few hydroxyl) groups an be used as model colloids. For example, the distribution of H ions in the electric double layer as determined by conductometric titration has been correlated with the particle diameter determined by ultracentrifugation (3). The conductometric titration gives two measures of the concentration of H+ ions the initial conductance of the latex and the amount of base required for neutralization. The number of H+ ions determined by conductance is always smaller than the number determined by titration. This difference is attributed to the distribution of the H+ ions in the electric double layer those closest to the particle surface contribute least to the overall conductance. This distribution is expressed as the apparent degree of dissociation a, which is defined as the ratio H+ ions... 

The stability of latexes during and after polymerization may be assessed at least qualitatively by the theoretical relationships describing the stability of lyophobic colloids. The Verwey-Overbeek theory (2) combines the electrostatic forces of repulsion between colloidal particles with the London-van der Waals forces of attraction. The electrostatic forces of repulsion arise from the surface charge, e.g., from adsorbed emulsifier ions, surface sulfate endgroups introduced by persulfate initiator, or ionic groups introduced by using functional monomers. These electro-... 

Keating, D.H., Willits, M.G., Long, S.R. A Sinorhizobium meliloti lipopolysaccharide mutant Altered in cell surface sulfation. J Bacteriol 184 (2002) 6681-6689. 

Previous IR studies of metal oxides sulfation have evidenced two types of sulfate species, surface species characterized by one or more bands in the 1410-1370 cm l frequency range, and bulklike species leading to wide bands in the 1200-1000 cm l range (3, 9-12). In a recent paper (4), it has been shown that Pt does not affect the sulfate formation nor the nature of adsorbed species. Spectra reported in Fig.l show that only surface sulfate species are present on VxJZrOj (Fig.la) while both species are formed on Pt/Ce02 (Fig.lb) and Pt/Ce02-Zr02 (Fig.lc). 

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