Sulfur saturation coverage

The effect of chemisorbed sulfur on the HER has been studied using a sulfur radiotracer method in combination with polarization measurements [23]. In the case of single-crystal (110) platinum in acid, the chemisorbed sulfur causes a large decrease in the HER rate, apparently due to a blocking effect of sulfur on the sites of adsorption of the weakly bonded hydrogen. The HER, however, is not completely poisoned by sulfur, even at the sulfur-saturation coverage [6 =0.8). 

For sulfur adsorption on a Pt(l 10) surface, Bonzel and Ku (83a) reported four structures occurring with successively increasing coverage c(2 x 6), p(2 x 3), p(4 x 3), and c(2 x 4). Prior to the appearance of each of these four structures, they reported transition structures characterized by ill-defined spot positions in the LEED patterns. Similar observations were reported by Berthier et al. (84, 89) they noted that at saturation coverage c(2 x 4) and p(4 x 4) structures coexist. The structures observed during initial stages of sulfur adsorption were identical, however. During sulfur... 

Sulfur adsorption stoichiometries at saturation coverage for single-crystal surfaces of Pt (84, 85), Fe (72, 142), Mo (75,143, 144), Ag (53-56), and Cu (58,65), and for polycrystalline metal surfaces of Pt (145), Fe (101), Co (101), and Ru (101) have been reported. The general features observed for these metals are similar to those observed for Ni accordingly, only the more interesting observations will be discussed. 

Similar observations were reported by Heegemann et al. (85). They observed the value of 9 at saturation coverage to be 0.5 and 0.38 for the (100) and (111) planes, respectively, when S2 was adsorbed on these planes. Saturated layers of Pt(lll) and (100) adsorbed more sulfur at room temperature, giving values of 9 of 1.12 and 1.08, respectively. Evacuation at room temperature reduced the value of 9 to 0.92 for both surfaces, indicating some fraction of S2 to be in a physisorbed state. When both these surfaces were heated to temperatures above 575 K, values of 9 = 0.5 and 0.38 were obtained for the (100) and (111) planes, respectively further continued heating to 723 K resulted in no further reduction in the value of 9. 

Thus, based on the above observations, it is possible to generalize the stoichiometry of sulfur adsorption at saturation coverage for various metals. A value of 9 = 0.5 appears to have a general applicability in representing the saturation coverage of the metals of catalytic interest at high reaction... 

From the data in Fig. 13 for Ag, Cu, and Pt it appears that the heat of sulfur adsorption increases with increasing atomic roughness of the surface. In the case of Ag, the saturation coverage is reached at the same h2s/ h2 (3-2 x 10-3) on all three low-index planes [(100), (110), and (111)]. However,... 

Since preadsorbed sulfur generally blocks the adsorption of other molecules, it would only be logical to expect that it would also prevent the adsorption of H2S or S2. Previously discussed studies of sticking coefficients (73, 83, 92, 99, 101) and H2S adsorption on metals (57, 106, 112-115) provide evidence that the sticking coefficient and heat of adsorption for H2S or S2 decrease with increasing coverage. Thus, rates and strengths of sulfur adsorption on sulfur-saturated metal surfaces are clearly lower than those on a clean metal surface. 

Most of the previous studies mentioned to this point did not meet the experimental requirements outlined in Section V,A and involved H2S concentrations in the ppm range. In view of the previously discussed adsorption studies showing that reversible adsorption of H2S occurs only at ppb levels under typical methanation conditions, it is reasonable to expect that full saturation coverage of sulfur occurred on the catalysts used in these previous studies hence their steady-state activities should not vary significantly for a given metal such as Ni. To allow quantitative measurements of the rates of catalyst deactivation, of the steady-state activity in the presence of ppb concentrations of H2S, and to determine the catalytic activity as a function of surface coverage by sulfur, Katzer and co-workers (99-101,147, 205-208), used a reactor and catalyst configuration which satisfied all the... 

At 661 K and 13-ppb H2S in H2, sufficient sulfur was adsorbed to give a sulfur-to-surface-Ni atom ratio of 0.5, where the number of surface Ni atoms was determined by H2 chemisorption. This S/Nis ratio corresponds to saturation coverage of single-crystal Ni with sulfur (Section III). 

Figure 3 Correlation between the heat (enthalpy) of sulfur adsorption (A/f ) at half-saturation coverage and the corresponding heat (enthalpy) of sulfide formation (A//j). (From Ref 4.)...

All values refer to half-saturation coverage by sulfur. 

The conformational analysis of saturated multi oxygen and sulfur rings is very well developed. The most recent survey of this area (B-80MI22600) provides an excellent coverage of the topic, updating an important earlier review (B-69MI22600). Within the present chapter, theoretical approaches, ring shapes and structural determinations are covered in Sections 2.26.2.1, 2.26.2.2 and 2.26.2.3. The present section is thus concerned primarily with the conformational preferences of substituents and barriers to conformational inversion. 

When Berthier et al. (84) exposed the saturated surface of Pt(l 11) to 0.1 Torr H2S at 473 K, more sulfur was adsorbed than at higher temperatures giving 9sm = 0.59 however, heating the surface to 623 K resulted in desorption of a portion of the adsorbed sulfur, and a stable coverage of 6 = 0.42 was obtained. These workers considered 9 = 0.42 to represent a saturated layer (stable at higher temperatures), and assumed that the amount of sulfur adsorbed in excess of 9 — 0.42 was due to a weakly adsorbed sulfur. 

This plot of fractional H2 adsorption (H2 uptake at 300 K of catalysts presulfided in 5, 10, or 25 ppm H2S at 725 K divided by initial H2 uptake) versus mean sulfur coverage (in molecules H2S adsorbed per molecule of surface nickel), suggests a linear relationship between H2 uptake and sulfur coverage. Interestingly, the intercept at zero H2 coverage (saturation sulfur coverage) is H2S/Nis = 0.75, in excellent agreement with the adsorption stoichiometry reported by Oliphant et al. 112) for adsorption of H2S at 725 K. 

---