Thermodynamics, bulk sulfides

Whether such bulk gettering effects are transient or steady state is unclear. The mechanisms of such effects and whether they are due to bulk or surface phenomena is also unknown. Thus it is clear that bulk thermodynamic information used in a cursory way does not help in rationalizing observed sulfur-poisoning behavior. Furthermore, since most metals of catalytic interest do not form stable bulk sulfides under typical reaction conditions, the observed severe poisoning by sulfur suggests that surface rather than bulk thermodynamics may be required. 

Only a limited amount of thermodynamic data are available for 2-D sulfides of metals other than nickel. Data reported for Ag (57), Co (252), Cu (255), Fe (252, 154), Mo (255, 255), Ru (256), and Pt (84, 85) indicate that the heats of sulfur adsorption are generally 20-40% larger than the heats of formation of the most stable bulk sulfides. Indeed, Benard et al. (255) have shown a linear correlation between the heats of adsorption of sulfur and the... 

In our pressurized tests [6,7] we found that when the HjS content of the gas was as high as 500 ppm the conversion of ammonia started to increase compared to lower sulfur levels. We also carried out additional experiments to see the effects of increasing HjS to a level where bulk sulfide formation is expected to form based on thermodynamics. Fig. 1 shows the effect of the high H S content of the synthetic gasification gas on the conversion of toluene, methane and ammonia at 900°C, 20 bar SV=15000 1/h. The experiments were performed by keeping the catalyst at each of the HjS levels for 4 - 6 hours to stabilize the catalyst performance. At high sulfur levels (> 5(X) ppmv) the stabilization of the catalyst... 

U. Balachandran of Argonne National Laboratories has operated Pd-alloy cermets successfully to temperatures as high as 900°C [10, 22], At such extreme temperatures, most thin films of supported Pd and its alloys would be hard pressed not to ball up or form pinholes. Operation at elevated temperatures is desired, for example, in the extraction of hydrogen from gas mixtures containing compounds of sulfur. At such elevated temperatures, and in the presence of sufficient hydrogen, bulk sulfides of Pd are not thermodynamically stable, and the Pd-based cermets acquire some degree of tolerance to sulfur [22, 50]. In addition, permeability of Pd increases at elevated temperatures and becomes comparable to that of Ta and V (see Fig. 8.1). 

The thermodynamic properties of bulk metal sulfides are fairly well documented 13-20). Multiple phases similar to those of Ni are observed for Fe, Co, and other metal-sulfur systems of catalytic interest. Figure 2 shows... 

From the values of and A it can be concluded that the activity decay caused by coking was similar for all the catalysts. On the other hand, no specific trend concerning thioresistance could be established from the "k " values listed in Table 3. Thus, it seems that the different behavior of the catalysts regarding sulfur poisoning was mainly related to Kj values (parameter C in Table 3). The Pt-Re/Al202 catalysts which were the most deactivated by sulfur exhibited the highest Kj values The formation of a bulk Re sulfide is thermodynamically... 

At low H2S levels just two poisoning reaction types occur physical and chani-cal absorptions on nickel surface by reactions (5.8) and (5.9) replacement of carbonate ions with sulfide and sulfate ions by reactions (5.1)-(5.4). hi fact, formations of bulk nickel sulfides by reaction (5.7) are thermodynamically forbidden. 

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