Surface sulfide ions

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. 

The rate of reaction is controlled by the diffusion process, as the sulfide ion must first diffuse to the surface of the zinc oxide to react. High temperature (>250°F) increases the diffusion rate and is normally used to promote the reaction rate. 

Fig. 4.9 Schematic illustration of the electrosynthesis procedure using a sulfur-modified gold electrode and alternating exposure to indium- and sulfide ion containing aqueous baths, (a) A polycrystalline gold surface is first modified with a sulfur layer, (b) Indium is plated onto this layer forming indium sulfide, (c) Indium continues to deposit atop the indium sulfide layer, (d) Transfer back to a sulfide ion containing bath results in the suMdization of the residual indium sites. (Reprinted from [95], Copyright 2009, with permission from Elsevier)...

It was concluded from this and related works that suppression of the photodissolution of n-CdX anodes in aqueous systems by ions results primarily from specific adsorption of X at the electrode surface and concomitant shielding of the lattice ions from the solvent molecules, rather than from rapid annihilation of photogenerated holes. The prominent role of adsorbed species could be illustrated, by invoking thermodynamics, in the dramatic shift in CdX dissolution potentials for electrolytes containing sulfide ions. The standard potentials of the relevant reactions for CdS and CdSe, as well as of the sulfide oxidation, are compared as follows (vs. SCE) [68] ... 

Microsensors have been used to develop profiles in mixed species biofilms. Figure 10 shows concentration profiles of sulfide, oxygen, and pH in a biofilm accumulated on the surface of a mild steel corrosion coupon. The concentration of sulfide is highest near the metal surface, where iron sulfide forms quickly and covers the steel surface if both ferrous and sulfide ions are available. At low ferrous ion concentrations, adherent and temporarily protective films of iron sulfides are formed on the steel surface, with a consequent reduction in corrosion rate. High rates of SRB-induced corrosion of mild steel are maintained only in high concentrations of ferrous ion. 

The corrosion of metal surfaces and the precipitation of a metal sulfide by an aqueous acid solution can be prevented by an aldol-amine adduct. Aldol (from acetaldehyde) CH3CH(OH)CH2CHO has been utilized as a H2S scavenger that prevents the precipitation of metal sulfides from aqueous acid solutions. However, when the aldol or an aqueous solution of the aldol is stored, the solution separates quickly into two layers, with all of the aldol concentrated in the bottom layer. The bottom layer is not redispersible in the top layer or in water or acid. In addition, the aldol in the bottom layer has very little activity as a sulfide scavenger. Thus the use of aldol as a H2S scavenger in aqueous acid solutions can result in unsatisfactory results [245,247]. However, the aldol can be reacted with an amine, such as monoethanoleamine (=aminoethanol), to form an aldol-amine adduct to overcome these difficulties. The amine utilized to prepare the aldol-amine adduct must be a primary amine. The aldol-amine adduct preferentially reacts with sulfide ions when they are dissolved in the... 

The products of hydrolysis and dissociation depend on the pH. In an acid medium, hydrogen sulfide, which has no depressing action, evolves. It is, therefore, necessary to use alkaline circuits in which HS, predominates. These sulfide ions are adsorbed on the copper sulfide mineral surface and react with the surface previously coated with cuprous xanthate. The reaction causes desorption of the collector, and as a result of this desorption the copper sulfide minerals generally become hydrophilic. There is, however, no action of the sulfide ions on molybdenite, and so molybdenite retains its hydrophobic character. 

Sol-gel techniques can be used to produce thin layers, as described in Section 19.1. For example, cadmium sulfide layers for photocells are easily made by spraying an ammoniacal solution of cadmium chloride and thiourea (which hydrolyzes to give sulfide ion) onto a substrate surface and baking the resulting CdS film at up to 500 °C ... 

One interpretation presumes that the photocurrent onset in the absence of sulfide is determined by electron-hole recombination. The sulfide ions on the surface are then supposed to be bound to these surface recombination levels rendering them unavilable for recombination reactions. The charge transfer reactions could then proceed at lower voltages. In this case the corrosion suppression role of the sulfide ions would be to reduce the oxidized corrosion site before a cadmium ion could go into solution. A variation on this theme is to consider the corrosion site to be the recombination state, i.e., the site on the surface that normally leads to corrosion when oxidized by a photoexcited hole can be... 

Figure 3. Surface density N of adsorbed sulfide ions calculated from the change in flatband potential V/6 reported in Ref. 26 (27J.

It is not the objective here to choose between these alternatives. Additional experimental work is needed to do this. The objective of this work is to show how both the corrosion sites and adsorbed sulfide ions can be treated as surface states and to further suggest that quantitative treatment of these surface states can be useful in considering experiments to elucidate the role of the sulfide ions. 

Another quantitative consideration involves the rate of replacement of the sulfide ions on the surface during illumination assuming that they are the states with which the hol s react. The rate at which sulfide ions reach the CdS surface for J x 10j M concentration can be estimated to be greater than lCr1 cm sec 2 for curve 2 of Figure k the first... 

The objective of the preceding discussion of CdS electrodes has been to point out the possible role of two types of identifiable surface states one a state intrinsic to the CdS surface, presumably associated with Cd, which is eventually ionized by reaction with a hole and dissolved into solution the other is a sulfide ion deposited on the CdS surface from solution. By considering these as surface states capable of inelastic... 

Amount of Sulfide Needed for Optimum Sensitization, sturmer and Blackburn (128) used radioactive sulfur to measure the concentration of sulfide ions formed on a silver bromide grain surface by thiosulfate. The number at optimum sensitization depended on the exposure irradiance to be used. The number was about 10,000 ions/pm for a high-irradiance exposure and about 20,000 for a medium-irradiance exposure. 

A kinetic study on the photobleaching relaxation at the addition of PWi2 and MV to colloidal CdS prepared in an excess of the sulfide ions, when the colloidal particles have the negatively charged surface, shows the opposite results as compared to those obtained... 

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... 

Since the surface properties of the colloid have a strong influence on the photoreduction kinetics, it seemed interesting to elucidate the effect of the added surface-active substances on the kinetic regularities of the reactions photosensitized by semiconductor colloid. The surfactant molecules are known to concentrate near the surface of the colloidal particle, so they may affect strongly the kinetics of photocatalytic reactions proceeding at the particles surface [52,53]. Fig. 2.30 presents the temperature dependencies of the initial rate of the MV photoreduction over colloidal CdS prepared at the excess of the sulfide ions. These dependencies were obtained at the addition of different amounts of PAA. One may see that both the initial rate and the observed activation energy of the methylviologen photoreduction do not depend, within the experimental error, on the concentration of... 

Figure 35 shows the photolumincsccnce spectrum of CdS supported on PVG with a relatively high loading 196). Peaks are observed near 520, 560, and 680 nm. The 680-nm peak is associated with the sulfur vacancy since the presence of excess sulfide ions quenches the photoluminescence however, the presence of excess cadmium has no effect on the emission. The 520- and 560-nm photoluminescence are associated with the major bulk emission 197-199). The 520-nm emission is attributed to the band-to-band transition, and the 560-nm emission is attributed to a typical radiative clcctron-hole recombination at the particle surface. As shown in Fig. 35 (b), the addition of H2O to the catalyst has a significant effect on the spectrum. The 560-nm photoluminescence is completely quenched, as expected if the radiative recombination of electrons and holes occurs at the surfaces where H2O molecules easily interact with these electrons and holes, thereby reducing the energy and intensity of the photoluminescence. On the other hand, the 520-nm emission from the bulk emitting sites is not affected by the addition of H2O. The photoluminescence... 

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