Sulfur accumulation

As is obvious, hydrogen sulfide active under current conditions is one of the reaction products. Its aggressiveness, the ability to interact with S02 even under natural conditions, promotes sulfur accumulation in the system, affects synthesis of condensation products and can alter their catalytic activity. Therefore, despite high effectiveness, oxidative catalytic dehydrogenation of ethylbenzene by sulfur dioxide may be found unacceptable for production management in relation to ecology and protection of the environment. 

Costonis et al. (33) were unable to demonstrate a positive correlation between ambient sulfur dioxide levels and inorganic sulfur ion accumulation in the needles of pines which are injured by relatively low levels of sulfur dioxide in the air. White pines growing in air polluted with more than 0.25 ppm of sulfur dioxide are often stunted, and a direct correlation can be obtained between plant growth and ambient S02 levels (34). However, tissue analysis does not reveal a measurable rise in sulfur level as inorganic sulfate ions. It is posible that the excess sulfur is incorporated into cell protein, but the data conflict (28, 35). For these reasons, it is not possible to follow sulfur accumulation in plants chronically exposed to low levels of ambient S02 without resorting to labeled S02. 

For propane oxidation, sulfation with SO2 induces an inhibiting effect on monometallic pfatinum catalysts which increases with the amount of sulfur accumulated on the catalyst (Fig 3b). On the other hand, sulfur storage enhances the activity of coimpregnated platinum-rhodium catalysts oxidized before hydrocarbon oxidation. However, it seems that an optimum sulfur storage exists since catalyst activity decreases as the amount of sulfur stored on the sample increases (Fig 3b). We examined also the effect of sulfation on catalyst activity for... 

The distribution of these impurities or minor alloy constituents near lattice discontinuities is known to affect the chemical and mechanical properties of the contaminated materials for example the presence of sulfur on a metal surface can promote ) or retard - o) corrosion, modify the surface energy ) or cause considerable increase in the surface self-diffusion coefficient ). Sulfur accumulation along grain boundaries may induce intergranular weakness and render otherwise ductile materials brittle ), either by formation of precipitates " ) or by enhancement of hydrogen adsorption >227)... 

Various workers have estimated the rate of pyrite formation. Berner (1972) summed the sulfur accumulation rates of various sediment types in proportion to their areal coverage and found a flux of about 10% of the river flux. Li (1981) carried out a similar calculation and finds 30% of the river flux, probably indicative of the uncertainty of the approach. Toth and Lerman (1977) established that the decrease of sulfate with depth in sediment pore waters is a function of sedimentation rate. This information was used to estimate the diffusive flux of sulfur into sediments driven by pyrite formation, again a value about 10% of the river flux. Apparently, pyrite... 

Sulfur from SO2 can poison noble metal catalysts by its strong bonding with the metal, forming the metal sulfide and even penetrating into the bulk metal l 3. When alumina is used as the catalyst support, irreversible deactivation can result from the formation of Al2(S04)3 with concurrent substantial reduction in surface area and pore volume " . Similar activity loss with decreased surface area and pore volume accompanying sulfur accumulation in the catalyst can result from the formation and deposition of sulfates of ammonia, particularly at lower operating temperatures, but these effects can usually be reversed by heating 2,47... 

Matsuoka et al. studied the deactivation of a V205/Ti02 catalyst due to formation of sulfates of ammonia on the catalyst, and subsequent regeneration, at 200-300 °C. The normal composition of their reaction gas was 200 ppm NO, 240 ppm NH3,50 ppm SO2, 3% O2,10% H2O, balance N2. The water soluble components of those catalysts which had been deactivated at 200 °C were found to contain NH4HSO4, NH4V(S04)2, and 2(804)3. No sulfates of Ti were detected. Both the surface area and pore volume of deactivated catalysts were found to be inversely proportional to the sulfur content of the catalyst. The percent reduction in the rate constant also correlated well with the extent of sulfur accumulation in the catalyst. The rate and extent of deactivation at 200 °C were observed to be independent of space velocity, but at 300 °C they were inversely proportional to the space velocity. To explain this, the authors proposed a different mechanism of deactivation at the higher temperature. 

Matisoff, G., Holdren Jr., G.R., 1995, A model for sulfur accumulation in soft water lake sediments. Water Resources Res. 31, 1751-1760. 

Initial exposure indicated that static areas existed in the chambers as indexed by varied symptom expression as a function of chamber location. Teflon baffles were installed in the chambers to improve turbulent mixing. Flow rates were adjusted such that the chamber volumes were replaced once every two minutes. This flow rate did not cause perceptible movement of foliage in the chamber. Subsequent studies showed that symptom development on the indicator species was uniform throughout the chamber after this modification. Foliar sulfur accumulation was also independent of chamber location after these revisions. 

There exists a correlation between support affinity for sulfur and HDS catalytic activity and it is suggested that, in contrast to conventional alumina supports, the narrow slit-shaped micropores in AC, while inaccessible to bulky reactant molecules, lower the vapor pressure of sulfur to such an extent as to create a driving force for sulfur transfer from the active compound to the micropores (sink effect), thus generating active vacancies. It is also suggested that the pore shape is a necessary condition for the sink effect, probably because the slitshaped pore allows the sulfur accumulated between the pore w ls to react with hydrogen from any side of the pore, to desorb as H2S. This is not possible in cylindrical micropoies of comparable dimensions in fact, the sink effect was not observed for microporous silica. 

Experimental work with bryophytes (Gilbert, 1968a) indicates that the difference between sensitive and resistant species does not lie in the resistant ones possessing a less efficient mechanism for sulfur accumulation. From this point the possibilities for speculation are endless. Sulfur could be removed by chelation in resistant species and quickly oxidized to sulfate alkaline cell sap could render it harmless, and it has been suggested that low wetability of the thallus might be protective. 

In the case of gaseous catalyst poisons, a distinction can be made between permanent poisons causing an irreversible loss of catalytic activity and temporary poisons which lower the activity only while present in the synthesis gas. This distinction is fully discussed in the book by Nielsen. Permanent poisons such as sulfur accumulate upon the catalyst surface and may be detected by chemical and spectroscopic analysis, while temporary poisons do not interact nearly as strongly with the catalyst. It is very difficult to detect temporary poisons by means of post-analytical methods. The principal temporary poisons are oxygen, carbon oxides, and water. Since the catalyst also contains percent amounts of oxygen... 

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