Sulfur sensitivity

Sulfur is a potential problem even at low levels for synthesis gas systems using certain types of catalysts. The production of methanol from synthesis gas, for example, uses catalysts that are poisoned by sulfur. Some tar cracking catalysts are also sulfur sensitive. In those systems, thorough removal of sulfur will be required. Fuel cell systems are also sulfur sensitive. 

Current SR catalysts based on Ni are extremely sulfur-sensitive and deactivate considerably in the presence of traces of sulfur. 

If the product is a low- or medium-Btu gas, produced for direct combustion, the stream will probably be desulfurized for environmental reasons. In this instance, approximately 95% sulfur removal is typical of a reasonable level of purification - better than direct combustion of the coal by a factor of two, yet not requiring an excessive energy penalty. If the gaseous product is a synthesis gas, as in the production of methane, methanol, or Fishcher-Tropsch liquids, extreme desulfurization is required to protect the sulfur-sensitive downstream catalysts. 

Sensitization of Model Systems. Silver bromide sheet crystals can be sulfur-sensitized easily, and have been studied extensively (56-58). Evaporated silver halide layers and sprayed emulsions likewise can be sulfur-sensitized. 

Pearlman and associates (63) found that sprayed emulsions prepared from silver bromide could be sulfur-sensitized by conventional means, but the increase in sensitivity was only twofold. The increase for grains prepared with 99% AgBr plus 1% Agl, on the other hand, was tenfold. They did not detect a difference in structure between the pure bromide and the iodo-bromide grains. The response of equimolar chlorobromide grains to S-sensitization also was greater than that of the pure bromide. 

Model systems have been used to demonstrate that S-sensitization is important in determining the location of the latent image centers. Moisar s experiments with core/shell emulsions showed that, when the surface of the core was sulfur-sensitized, the latent image formed by exposure of the core/shell grains was situated predominantly at or very close to the core/shell interface (124). 

Cash (129) obtained a lower number, about 4000, but he used a gelatin that induced optimum sensitization upon heating even without added sulfur sensitizer. His gelatin may have contained a sensitizer that contributed to the optimum sensitization. 

Since there is an enormous density of such equivalent sites on the surface of a silver halide microcrystal, it would be surprising if a single speck of Ag2S were formed on each grain even under optimum conditions of sulfur sensitization. The sensitizer forms an adsorption silver complex which reacts to form an adsorbed molecule of Ag2S. 

Birch, Farnell, and Flint (141) studied the effect of sulfur sensitization on the location of latent image centers on grains that had both cubic and octahedral faces. In the emulsions without deliberate chemical sensitization, the latent image centers formed preferentially on the cubic faces, but progressive sulfur sensitization caused a gradual shift in location from the cubic to the octahedral faces. Based on the assumption that the latent image centers formed at the sulfide center sites, they concluded that silver sulfide formed predominantly on the octahedral faces. 

They reasoned that in the vicinity of a silver sulfide speck the conduction band is bent downward and photoelectrons could reach the surface of the crystal more easily, and hence more readily form surface image. However, the thickness of the silver sulfide layer in their experiments was about 4 pm, which far exceeds that of any sensitivity center on emulsion grains. The relevance of their observation to S-sensitization of emulsions is doubtful. Starbov (147) found no evidence of change in the surface potential when either the (111) or the (200) surface of an evaporated silver bromide layer was sulfur-sensitized. 

In summary, there is evidence that the multitalented sulfur sensitization product can trap electrons, can trap holes to reduce recombination, can stabilize photolytic silver atoms, and can accelerate reduction sensitization. Conceivably, each of these properties could be of importance for latent image formation under at least some conditions. The silver sulfide centers are not of uniform size, they probably are not uniformly related energetically to the silver halide matrix, and they may differ in chemical consititution. 

Sheppard s early patent on sulfur sensitization (155) included sensitization by selenium compounds, such as seleno-ureas. He assumed that the two types of sensitization were... 

Koslowsky in 1936 observed that certain gold salts, such as aurous thiocyanate and aurous thiosulfate complexes, are sensitizers, but this discovery remained a trade secret for several years (157-159). Gold salts used alone are relatively poor sensitizers, but in combination with sulfur sensitizers they can increase the sensitivity beyond that obtainable by S-sensitiza-tion alone (160). 

The last three involve the capture of a charged carrier at an oppositely charged center. In all of these events except the free-hole trapped-electron recombination, the free carrier is the electron and the trapping center has a charge of +e/2. The key assumption is that the cross section for electron capture is determined by the coulombic attraction. On this basis, Hamilton derived an equation that includes one term to cover low-intensity reciprocity failure and another which is a first-order approximation of high-intensity reciprocity failure. Its predictions were in good accord with experimental data on the effects of sulfur sensitization. 

Sulfur sensitization increases both the depth of shallow electron traps and the stabilization energy of the silver-atom center. These changes are sufficient to account for the overall increase in sensitivity" and its subsequent decrease at concentrations above the optimum. The population of free electrons is so reduced that recombination is now largely between free holes and trapped electrons. 

Sulfur sensitization either introduces electron traps or deepens existing traps. If, however, a sulfide nucleus provides a trap deep enough to hold an electron until an interstitial... 

Sulfur sensitization does not change the number of latent image centers formed per grain for low irradiance of the mono-disperse fine-grain emulsions. Sulfur sensitivity centers cannot be deep enough to affect the chance establishment of a single stable latent subimage center. In coarse, polydisperse... 

Burge does not exclude the possibility that electrons trapped at centers such as those supplied by sulfur sensitization may form silver atoms by the Gurney-Mott mechanism, but he regards these atoms as mobile with a higher probability of aggregation in the region where they are formed than in other regions. 

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