Sulfur coverage

Agronomic Properties and Nutrient Release Mechanisms. The mechanism of nutrient release from SCU is by water penetration through micropores and imperfections, ie, cracks or incomplete sulfur coverage, ia the coating. This is followed by a rapid release of the dissolved urea from the core of the particle. When wax sealants are used, a dual release mechanism is created. Microbes ia the soil environment must attack the sealant to reveal the imperfections ia the sulfur coating. Because microbial populations vary with temperature, the release properties of wax-sealed SCUs are also temperature dependent. 

The same trends regarding the effect of sulfur have been reported for NO adsorption on Pt(lOO)90 and Rh(100).6 In the case of Pt(100) dissociative adsorption is completely inhibited upon formation of a p(2x2) overlayer at a sulfur coverage equal to 0.25, while the binding strength of molecularly adsorbed NO is lowered by more than 50 kJ/mol, as calculated by analysis of NO TPD data. Due to this complete inhibition of dissociative adsorption, the CO+NO reaction is completely deactivated, although it proceeds easily on sulfur free Pt(100). In the case of Rh(100) a sulfur coverage of only 0.08 suffices to completely inhibit NO dissociation at 300 K. 

The influence of the presence of sulfur adatoms on the adsorption and decomposition of methanol and other alcohols on metal surfaces is in general twofold. It involves reduction of the adsorption rate and the adsorptive capacity of the surface as well as significant modification of the decomposition reaction path. For example, on Ni(100) methanol is adsorbed dissociatively at temperatures as low as -100K and decomposes to CO and hydrogen at temperatures higher than 300 K. As shown in Fig. 2.38 preadsorption of sulfur on Ni(100) inhibits the complete decomposition of adsorbed methanol and favors the production of HCHO in a narrow range of sulfur coverage (between 0.2 and 0.5). 

Effects of Sulfur Coverage. The kinetic results reported in the previous section are from reactions performed on surfaces that are initially clean. The surfaces after reaction were examined by AES and shown to be covered with carbon and sulfur at coverages close to a monolayer. It is interesting to note that although this is the case even after reaction times on the order of minutes the reaction rate is constant for a period of approximately one hour. Either the reaction is occurring on top of this oarbon/sulfur layer or these species are present as sulfur containing hydrocarbon fragments that are intermediates in the desulfurization process. 

Figure 6. An Arrhenius plot of the rate of methanatlon over a clean and sulfided Ru(OOOl) catalyst. Sulfur coverages (6g s) are expressed as fractional monolayers.

Figure 7. Relative methanatlon rate as a function of sulfur coverage on a Ru(OOOl) catalyst. Reaction temperature 550K.

Oudar and co-workers studied the dissociative chemisorption of hydrogen sulfide at Cu(110) surfaces between 1968 and 1971.3,14 As in the case of Ni(110) described below, a series of structures were identified, which in order of increasing sulfur coverage were described as c(2 x 2), p(5 x 2) and p(3 x 2). In contrast to nickel, the formation of the latter phase is kinetically very slow from the decomposition of H2S and could only be produced at high temperatures and pressures. The c(2 x 2) and p(5 x 2) structures were confirmed by LEED,15 17 but the p(3 x 2) phase has not been observed by H2S adsorption since Oudar and colleagues work. 

Ni(100) with sulfur coverages between zero and 0.50 monolayers. For 0g = 0.37 three binding states are evident (see text). 

Figure 10.2 Methanation rate as a function of phosphorus and sulfur coverage on a Ni(100) catalyst. Pressure = 120 torr, H2/CO = 4. Reaction temperature = 600 K. (From Goodman, D.W., Appl. Surf. Sci. 19, 1-13, 1984. Used with permission from Elsevier Scientific Publishers.)...

Fig. 18. Product distribution from the reaction of cyclopropane with hydrogen as a function of sulfur coverage over a Ni(l 11) catalyst. Temperature = 550K. Total pressure = 100 torn Hj/cyclopropane = 100. From Ref. 4.)...

FIG. 15. Zinc and sulfur coverages per cycle, after three EC ALE cycles, as a function of the potential used to deposit zinc. The sulfur-deposition potential was held constant at -1.00 V. 

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