Platinum sulfur block

AH commercial processes for the manufacture of caprolactam ate based on either toluene or benzene, each of which occurs in refinery BTX-extract streams (see BTX processing). Alkylation of benzene with propylene yields cumene (qv), which is a source of phenol and acetone ca 10% of U.S. phenol is converted to caprolactam. Purified benzene can be hydrogenated over platinum catalyst to cyclohexane nearly aH of the latter is used in the manufacture of nylon-6 and nylon-6,6 chemical intermediates. A block diagram of the five main process routes to caprolactam from basic taw materials, eg, hydrogen (which is usuaHy prepared from natural gas) and sulfur, is given in Eigute 2. 

According to Bagotsky et alM, this is the rate determining step. Therefore, rt would be limited by this step and be independent of what reaction would follow. In sulfuric add, it is well-known that bisulfate anions adsorb on platinum and possibly block the reaction sites. Therefore, rt would be higher in dilute adds than in concentrated adds. Indeed, in dilute adds, the initial total current is larger. 

In the presence of Pb(II) ions in sulfuric acid, potential oscillations have been observed for galvanostatic oxidation of hydrogen on platinum electrode [129]. This behavior has been attributed to ad-sorption/oxidation/desorption processes of lead on the platinum surface. Lead at high values of coverage is oxidized to insoluble PbS04, which blocks the Pt surface. 

Comparisons of reactions of cis- versus fran -DDP with sulfur donors and other cellular components may afford some insight into their relative biological effects, however. /rans-DDP is much more reactive than the cis isomer with GSH (6, 27). Depletion of GSH with use of the inhibitor BSO had no effect on the cisplatin sensitivity of two human ovarian carcinoma cell lines, but made them 2.7 times more sensitive to trans-DY) (4). In a related finding, it was necessary to add 14 times more trans- than ds-DDP to cells in culture to achieve the same percent inhibition of in vivo SV40 DNA replication (23). These results suggest that part of the differential activity of these isomers in biological systems may arise from the differential reactivity of fran -DDP toward cellular components other than DNA. Thus trani-DDP could be more effectively inactivated prior to encountering DNA, or be better blocked as monofunctional platinum-DNA cross-links. 

Proton donor electrodes consisting of platinized platinum in contact with a thin layer of sulfuric acid or glycerol were used by Magnan and Kahane 112) for d.c. measurements of pure ice at temperatures from —80°C. to —120°G. and potential differences from 10 to 50 k.v. The current through the samples was non-ohmic (space-charge limited proton current). The effectiveness of these electrodes in preventing blocking effects was not discussed. 

The effect of chemisorbed sulfur on the HER has been studied using a sulfur radiotracer method in combination with polarization measurements [23]. In the case of single-crystal (110) platinum in acid, the chemisorbed sulfur causes a large decrease in the HER rate, apparently due to a blocking effect of sulfur on the sites of adsorption of the weakly bonded hydrogen. The HER, however, is not completely poisoned by sulfur, even at the sulfur-saturation coverage [6 =0.8). 

Chalcogenides are commonly synthesized by this method. The metal sulphide syntheses have been carried out in ethanol, water, " and ethylenediamine, " whereas the sources of metal ions have been acetates or the chlorides. The precursor for sulfur is usually thioacetamide or thiourea. Nanoparticle synthesis of d-block elements have also been carried out, for example, platinum, gold, cobalt, iron, palladium, gold, nickel, and bimetallic alloys such as Co/Cu, 52i Pt/Ru, i Au/Pd, i Fe/Co. ... 

The adsorbed sulfur deactivates Pt by blocking the access to a given number of Pt atoms and modifies the electronic properties of the neighboring atoms. For example, S not only decreases the amount of CO adsorbed but also decreases the binding energy between CO and Pt (138) and also with other adsorbates (87). It has been established that certain amount of sulfur remains adsorbed on the metal, even after 30 h at 500° C in H2 flow. This is called irreversible sulfur, and this amount does not depend on the sulfiding conditions (139). This amount is approximately 0.4 atoms of sulfur for each atom of accessible platinum, in Pt, Pt-Re, and Pt-Ir catalysts. 

Sulfur. Sulfur commonly exists as mercaptans, disulfides, or thiophenes in reformer feeds (Fig. 4). Mercaptans and disulfides are significantly more reactive than thiophene during hydrotreating (4). Sulfur contamination results in poisoning of the platinum metal. Studies have shown that the sulfur reacts with the platinum, changing the reactivity of the platinum in a manner more extensive than a simple blocking of metal sites (5). Typical maximum allowable concentrations of sulfur are 0.5 ppm or less. With proper hydrotreating, levels of 0.2-0.3 wt. ppm are reached and these lower levels are beneficial to reformer performance. 

The primary contaminants of a PEFC are carbon monoxide (CO) and sulfur (S). Carbon dioxide (CO2) and unreacted hydrocarbon fuel act as diluents. Reformed hydrocarbon fuels typically contain at least 1 percent CO. Even small amounts of CO in the gas stream, however, will preferentially adsorb on the platinum catalyst and block hydrogen from the catalyst sites. Tests indicate that as little as 10 ppm of CO in the gas stream impacts cell performance (35, 36). Fuel processing can reduce CO content to several ppm, but there are system costs associated with increased fuel purification. Platinum/ruthenium catalysts with intrinsic tolerance to CO have been developed. These electrodes have been shown to tolerate CO up to 200 ppm (37). 

Sperm heads are treated with dilute mineral acid (hydrochloric or sulfuric) (Kossel, 1929). By adding a solution of anion such as picrate to the acid extract, protamine is precipitated as an insoluble salt, which is further converted into sulfate or hydrochloride. In the original method of preparation of a protamine from testes of Rhine salmon, Miescher (1897) used 1—2 % hydrochloric acid for the extraction and platinum chloride for the precipitation of protamine. Kossel (1929) used 1 % sulfuric acid while Stedman and Stedman (1951) used 0.1—0.5 N sulfuric acid in their complete extraction procedure . Block et al. (1949) isolated the sulfate salt of clupeine and salmine on a larger scale, using 0.2 N hydrochloric acid and metaphosphate, respectively, as the extracting and precipitating reagents. 

---