Surface sulfiding

Copper ore minerals maybe classified as primary, secondary, oxidized, and native copper. Primaryrninerals were concentrated in ore bodies by hydrothermal processes secondary minerals formed when copper sulfide deposits exposed at the surface were leached by weathering and groundwater, and the copper reprecipitated near the water table (see Metallurgy, extractive). The important copper minerals are Hsted in Table 1. Of the sulfide ores, bornite, chalcopyrite, and tetrahedrite—teimantite are primary minerals and coveUite, chalcocite, and digenite are more commonly secondary minerals. The oxide minerals, such as chrysocoUa, malachite, and azurite, were formed by oxidation of surface sulfides. Native copper is usually found in the oxidized zone. However, the principal native copper deposits in Michigan are considered primary (5). 

Very little is known of the constitution of the surface sulfides of carbon. Sykes and White (130) assumed that the same type of surface sulfide results from the action of sulfur or carbon disulfide. A speculative structure was postulated ... 

Surface sulfide formation was attempted by Wibaut and van der Kam (122). The results were negative. However, it seems doubtful whether a sufficiently finely divided diamond powder was used. Otherwise, the analytic methods used by the authors would have been too crude for the detection of the extremely small sulfur concentrations. 

Another interpretation would be to suppose that the adsorbed sulfide ion forms a surface state that can be directly oxidized by a hole in the valence band. In this case the shift in current onset to lower voltages would be due to an increase in the charge transfer rate rather than the decrease in the recombination rate discussed in the preceeding paragraph. The corrosion suppression associated with the sulfide could then be partially attributed to the rapid kinetics of hole capture by these surface sulfide ions and partially due to reduction of oxidized corrosion sites by sulfide ions in solution. 

We can attempt to apply the same type of model to the H2S data, however there are two additional unknown factors involved. First, we do not have a measurement of the sea surface concentrations of H2S. Second, the piston velocity of H2S is enhanced by a chemical enrichment factor which, in laboratory studies, increases the transfer rate over that expected for the unionized species alone. Balls and Liss (5Q) demonstrated that at seawater pH the HS- present in solution contributes significantly to the total transport of H S across the interface. Since the degree of enrichment is not known under field conditions, we have assumed (as an upper limit) that the transfer occurs as if all of the labile sulfide (including HS ana weakly complexed sulfide) was present as H2S. In this case, the piston velocity of H2S would be the same as that of Radon for a given wind velocity, with a small correction (a factor of 1.14) for the estimated diffusivity difference. If we then specify the piston velocity and OH concentration we could calculate the concentration of H2S in the surface waters. Using the input conditions from model run B from Figure 4a (OH = 5 x 106 molecules/cm3, Vd = 3.1 m/day) yields a sea surface sulfide concentration of approximately 0.1 nM. Figure S illustrates the diurnal profile of atmospheric H2S which results from these calculations. 

Sulfur adsorption studies of single-crystal faces of other metals and in particular of Ag (52-57), Cu (58-65), Fe (66-73), Mo (74-76), Ru (77-79), Pd (80), Pt (81-90), and Pb (91) indicate surface metal sulfides as having structures qualitatively similar to those of Ni. In all the metals studied, there are two steps involved in sulfur adsorption (1) adsorption on high-coordination metal sites, and (2) formation of a 2-D surface sulfide. 

The interaction of H2S, organic sulfides, and other sulfur compounds may involve a number of consecutive steps including reversible molecular adsorption of the sulfur compound, its dissociation, reorientation or reconstruction of the metal surface, formation of a 2-D surface sulfide, and at still higher H2S/H2 ratios, formation of a three-dimensional (3-D) (bulk) metal sulfide. Kinetic information about these processes may generally be helpful in elucidating the adsorption mechanism. Unfortunately, such quantitative kinetic information is not adequately available, with one exception, formation of bulk sulfides (9, 96). 

In practice, thermodynamic data such as heats and free energies of formation of surface sulfides are very difficult to obtain, mainly because quantitative measurements must be made of adsorbed and gas-phase sulfur at extremely low concentrations. Only during the last decade has the develop-... 

Free Energies of Formation of Surface Sulfides Compared to Free Energies of Formation of Bulk Metal Sulfides... 

Table VII compares the free energies of formation of surface sulfides on various metal surfaces with those of corresponding bulk sulfides. With the exception of Ag and Cu [which are less sensitive to sulfur adsorption due to their lower (—AG)S], it is clear that the free energies of formation of surface sulfides are at least 40 kj/mol more stable than the corresponding bulk metal sulfides. Hence it is generally true for all metals that the surface sulfides are considerably more stable than the bulk sulfides.

Although more information is needed to determine details concerning factors that favor inactive coke formation, relatively high levels of surface sulfides probably promote formation of such coke. On the other hand, metal oxides on the surface likely favor production of active coke. Sulfiding the reactor tube immediately upon completion of the decoking step would form metal sulfides. An aluminized surface, such as provided by the alonized Incoloy 800 reactor, also has been found to be an effective way to prevent the production of active coke. Quite possibly, the initial type of coke formed on the just-cleaned tube would have an important effect on the length of time a reactor tube could be used in a commercial plant before decoking would be required. 

Migdisov A. A., Williams-Jones A. E., Lakshtanov L. Z., and Alekhin Y. V. (2002) Estimates of the second dissociation constant of H2S from the surface sulfidation of crystalline sulfur. Geochim. Cosmochim. Acta 66, 1713-1725. 

The necessity of a larger ensemble for the dissolution of carbon into nickel than for activating methane corresponds to observations in surface physics. Adsorbed carbon atoms result in a distortion of the metal atom geometry whereas the bonding of adsorbed methane may require only 3-4 free nickel atoms. In simple terms, the two-dimensional surface sulfide prevents a distortion of the surface being necessary for the diffusion of surface carbon atoms into the bulk nickel phase. 

The formation of this surface sulfide prevented carbon deposition from the Boudouard reaction (2CO -> C + CO2). It was believed that sulfur blocked the nickel surface and as a consequence prevented the diffusion of carbon through the metal particles (ref. 11). Moreover under certain reaction conditions sulfur can dissolve in the bulk metal and undergo chemical reaction to form a three dimensional sulfide (ref. 

The industrial catalyst is initially a mixed oxide of Mo and Co supported on -y-AljOj. In the presence of H2 and the H2S and organosulfur compounds that form H2S, the oxide is converted into a surface sulfide. The catalyst consists of small crystallites of MoSj dispersed on the AI2O3 support. Some of the Co is present at the crystallite edges in the M0S2 structure and forms the catalytic sites. ... 

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