Sulfur surface

Organosulfur Adsorbates on Metal and Semiconductor Surfaces. Sulfur compounds (qv) and selenium compounds (qv) have a strong affinity for transition metal surfaces (206—211). The number of reported surface-active organosulfur compounds that form monolayers on gold includes di- -alkyl sulfide (212,213), di- -alkyl disulfides (108), thiophenols (214,215), mercaptopyridines (216), mercaptoanilines (217), thiophenes (217), cysteines (218,219), xanthates (220), thiocarbaminates (220), thiocarbamates (221), thioureas (222), mercaptoimidazoles (223—225), and alkaneselenoles (226) (Fig. 11). However, the most studied, and probably most understood, SAM is that of alkanethiolates on Au(lll) surfaces. 

Frasch process for mining sulfur. Superheatad watar at 165°C is sent down through the outer pipe to form a pool of molten sulfur (mp = 119°C) at the base. Compressed air, pumped down the inner pipe, brings the sulfur to the surface. Sulfur deposits are often 100 m or more beneath the earth s surface, covered with quicksand and rock. 

Sulfur is widely distributed as sulfide ores, which include galena, PbS cinnabar, HgS iron pyrite, FeS, and sphalerite, ZnS (Fig. 15.11). Because these ores are so common, sulfur is a by-product of the extraction of a number of metals, especially copper. Sulfur is also found as deposits of the native element (called brimstone), which are formed by bacterial action on H,S. The low melting point of sulfur (115°C) is utilized in the Frasch process, in which superheated water is used to melt solid sulfur underground and compressed air pushes the resulting slurry to the surface. Sulfur is also commonly found in petroleum, and extracting it chemically has been made inexpensive and safe by the use of heterogeneous catalysts, particularly zeolites (see Section 13.14). One method used to remove sulfur in the form of H2S from petroleum and natural gas is the Claus process, in which some of the H2S is first oxidized to sulfur dioxide ... 

The mechanism proposed involves hydrogenation of the C2 C3 double bond, formation of 2-vinylthiophenol by an E2 elimination, and hydrocarbon elimination by homolysis of the S—Caryi bond. This pathway rationalizes the primary formation of (104) observed in some HDS reactions of (102) over Co/Mo/S catalysts, as well as the kinetic evidence that the rate-determining step on real catalysts is the removal of surface sulfur.158-160... 

Elemental sulfur is recovered from its ore deposits found throughout the world. It is obtained commercially by the Frasch process, recovery from wells sunk into salt domes. Heated water under pressure is forced into the underground deposits to melt sulfur. Liquid sulfur is then brought to the surface. Sulfur is recovered by distillation. Often the ore is concentrated by froth flotation. 

The XRD of the catalyst after 24 h on stream showed only the original W2C phase. No bulk tungsten sulfide phases were detected. The similarity in produce distribution to the Co-Mo-S/A1203 suggests that a surface sulfur layer might have formed on the carbide by reaction with the thiophene. This sulfur layer would be responsible for the reaction. 

The extent of sulfonation of 8% crosslinked styrene divinylbenzene co-polymer beads (Biorad Bio Bead SX 8) was investigated by comparing the surface composition after sulfonation of the beads using chlorosulfonic acid, sulfuric acid and fuming sulfuric (54). Each reagent was refluxed in methylene chloride for a similar period of time. A wide-scan spectrum indicates the presence of sulfur 2p and 2s electrons, indicative of sulfonation. The surface sulfur content was fairly similar in the surface region analyzed by XPS. The composition determined from beads sulfonated by the three methods is indicated in Table VI. 

AVERAGE GRAIN BOUNDARY FRACTURE SURFACE SULFUR CONTENT... 

Therefore, the suggested equation (2.19) reflects adequately a variation in the properties of luminescence quenchers and describes well the experimental data as a whole. In addition, the data suggest that for the colloids obtained with an excess of the sulfide ions, the quencher adsorption site is represented most likely by the negatively charged surface sulfur atoms, on which the cations adsorb readily while the anions adsorb poorly. Actually, in this case, the dominant surface defects are either the vacancies of the cadmium ions, or... 

The surface concentration of carbon in the used catalysts is consistent with the bulk carbon analysis, while surface carbon on the fresh and regenerated samples seems disproportionately high. Surface sulfur is consistently lower than the bulk values of Table I indicating the presence of metallic sites on the surface, since these levels on the used catalysts are incomplete for total sulfiding of Co and Mb. 

Bulatnikov et al. (340) studied the effects of promoters on sulfur resistance of iron by measuring the amount of radioactive H2S adsorbed on iron catalysts promoted with A1203 and/or K20. They reported irreversible deactivation of Fe promoted with A1203, A1203 + K20, or KzO after 0.8, 1.5, and 5 monolayers of sulfur had been adsorbed. In other words, the presence of K20 was responsible for increasing sulfur adsorption capacity, although it was not clear upon which portion of the surface sulfur had adsorbed. It was also reported that A1203 was necessary to prevent volatilization of K20. 

In the only study of regeneration using steam (237,270), it was found that up to 80% removal of surface sulfur from Ni steam reforming catalysts either unpromoted or promoted with Mg and Ca could be achieved at 973 K. The effluent gas analysis showed the presence of S02 and H2S, and the following reaction pattern was suggested ... 

Convincing evidence of recycling of volatiles to the mantle and their incorporation into diamonds has recently been reported from sulfur isotope studies of sulfide inclusions within diamond (Farquhar et al., 2002). SIMS analysis of sulfide inclusions has revealed anomalous mass-indepen-dently fractionated sulfur isotopic compositions from the Orapa kimberlite, which are explained as a consequence of the recycling of surface sulfur produced through photolytic chemistry in the Archean atmosphere. Such studies need to be extended to other diamond populations and combined with C-N isotope studies on the host diamonds. 

Thomas J. E., Jones C. F., Skinner W. M., and Smart R. St. C. (1998) The role of surface sulfur species in the inhibition of pyrrhotite dissolution in acid conditions. Geochim. Cosmo-chim. Acta 62, 1555—1565. 

Macias-Garcia et al. [208] did not cite any of these studies and thus came to very confusing conclusions, but they did analyze an intriguing set of chemically modified activated carbon samples (exposure to N at 900°C, exposure to H S at 900°C. exposure to SO2 and then H2S at 30°C followed by treatment in N2 at 200°C). Their results are summarized in Table 11. The characterization of the surface chemistry after various treatments is limited to statements about the formation of surface sulfur and S=0 groups [which] may belong to the species SO or SOj-. The authors then simply conclude that the most effective [method] to increase the adsorption capacity of AC [is] the heat treatment in N , which they attributed to the development of porosity in this sample (even though their own data—see Table 11—do not really support such an explanation). The authors do not comment on the fact that the uptake at low pH is electrostatically unfavorable and is expected to be even more so after the removal of acidic surface groups (unless these are reconstituted upon exposure of the sample treated in N at 900°C to room-temperature air see Refs. 82 and 84). Finally, a... 

We performed steam reforming runs with clean and sulfur poisoned RU/AI2O2 catalyst samples that had been treated separately to attain two levelE3 of surface sulfur coverage. The results of these experiraents are illustrated in Figures 1 and 2 ... 

Sulfur Sulfur can be found combined with mercury in cinnabar or with lead in galena. It also is found uncombined in underground deposits. When water heated to 160°C and compressed air are pumped into a deposit, the yellow solid melts and is forced to the surface. Sulfur has ten allotropes, more than carbon and oxygen combined. Figure 7-16 shows how a brittle allotrope that can be ground into a powder can be changed into one that is elastic. 

Pre-reduced catalysts. The previous experiments were repeated again but catalysts were reduced before poisoning. Reduction of the film at various temperatures before sulfur deposition decreased very dramatically the rate of reaction compared to fresh unreduced films. Total deactivation is attained at much lower levels of surface sulfur poisoning than in the case of the unreduced catalysts. For a S/Pd ratio of 0.18, conversion decrease from 60 (fresh unreduced) to 7% when the reduction temperature is as low as 200°C. For a reduction temperature of 300°C only a 1% conversion is measured, and no conversion is detected when the reduction temperature is 400°C. The AFM images of these catalysts show that the surface breaks up in islands of varying sizes. As the reduction temperature increases, the sizes of these islands decrease, but their heights increase. 

Evidence of the formation of multilayer or subsurface sulfides under conditions where bulk sulfides are not expected has been reported [24-28]. For both gold [26] and platinum [27] it was concluded that in aqueous acid medium, sulfur species formed from hydrogen sulfide, which are likely to be composed mainly of polysulphide and/or element sulfur layers on the metal sulfide. Buckley et al. [26] concluded that the multilayer sulfur deposit does not have the properties of bulk elemental sulfur, suggesting substantial interaction occurs between the multilayer sulfur and the underlying metal sulfide and substrate metal. Two models account for the characteristics of the deposit. The sulfur could grow as an extension of the initial metal sulfide lattice with the possible movement of some metal atoms from the substrate into the sulfur layer. Alternatively, the sulfur layer could develop as a conglomerate of chains, from the underlying metal sulfide by the formation of polysulphide bonds with surface sulfur atoms. 

The adsorption of anions on solid surfaces is of considerable interest, mainly because of its effect on the kinetics of electrochemical reactions. Several in-situ techniques have been applied toward this purpose. Infrared measurements were used to identify adsorbed species, estimate anion adsorption isotherms, and to gain information on anion interaction with electrode surfaces. " Sulfuric acid anions are possibly the conunonest anion adsorbates because of their specific adsorption on metal surfaces. Depending on the metal, its surface orientation, and the concentration of anion, either sulfate or bisulfate can be specifically adsorbed on the surface. Identifying the predominant adsorbate on platinum-group metals has engendered some controversy. While STM studies show that... 

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