Surface saturation state, sulfur

The upper plateau of the isotherm of sulfur chemisorption allows definition of a surface saturation state of sulfur for each metal. Such a state can be reached in a large range of temperatures and partial pressures of hydrogen sulfide. Using 35S, Oudar (21) listed the values obtained on different metals for a maximum concentration of sulfur before the appearance of solid sulfide. On the (100) faces of nickel and platinum, this saturation state corresponds to one sulfur atom for two accessible metallic atoms. On the (111) faces, it is slightly lower than one sulfur atom for two metal atoms. On the (110) faces, it is, respectively, equal to 0.71 on nickel... 

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... 

Saturation of a carbohydrate double bond is almost always carried out by catalytic hydrogenation over a noble metal. The reaction takes place at the surface of the metal catalyst that absorbs both hydrogen and the organic molecule. The metal is often deposited onto a support, typically charcoal. Palladium is by far the most commonly used metal for catalytic hydrogenation of olefins. In special cases, more active (and more expensive) platinum and rhodium catalysts can also be used [154]. All these noble metal catalysts are deactivated by sulfur, except when sulfur is in the highest oxidation state (sulfuric and sulfonic acids/esters). The lower oxidation state sulfur compounds are almost always catalytic poisons for the metal catalyst and even minute traces may inhibit the hydrogenation very strongly [154]. Sometimes Raney nickel can... 

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. 

Intermediates and causes them to abstract hydride Ions more rapidly from Isobutane or any other potential donor. Increased hydride transfer converts more of the carbonlum Ions at the add Interface to saturates faster, yielding product while minimizing polymerization and side reactions. It Is also likely that the surfactants physically block alkyl Ions from one another in the surface film and thus Impede Ion + olefin polymerization. In such a film the carbonlum Ion concentration must also be lower than In the absence of surfactant and mass law effects will therefore also lead to less polymerization and cracking. The fact that steady state hydride transfer rates In H2SO are subject to control through the use of acid modifiers which act In the bulk acid and at the acid-hydrocarbon Interface Is the key to the control of sulfuric acid alkylation. 

Nesbitt et al. (1995) conducted a detailed study of the oxidation of arsenopyrite in oxygenated solutions. Arsenic and sulfur were observed to exist in multiple oxidation states near the pristine surface. After reaction with air-saturated distilled water, Fe(III) oxyhydroxides formed the dominant iron surface species, and As(V), As(III), and As(I) were as abundant as As(—I) surface species. An appreciable amount of sulfate was observed on the mineral surface. Arsenic was more readily oxidized than sulfur, and similar rates of the oxidation of As(—I) and Fe(II)" surface species were observed. Nesbitt et al. (1995) concluded that continued dilfusion of arsenic to the surface under these conditions can produce large amounts of As " " and As, promoting rapid selective leaching of arsenites and arsenates. 

Steady-state current-distance approach curves, derived from the long-time portion of transients, are shown in Fignre 13.18 for dissolution experiments involving saturated copper sulfate solutions with sulfuric acid at concentrations of 10.2, 7.3, 6.4, 3.6, and 2.8mol/dm. As the concentration of sulfuric acid decreased, the dissolution characteristics moved away from a diffusion-controlled situation (the dashed line in Figure 13.18). This was due to the increase in the activity and diffusion coefficient of Cu + in solution, which increased mass transfer in the UME/crystal domain and pushed the dissolution reaction toward surface control. An excellent lit to the experimental data was found for a process that was first order in interfacial undersaturation (Figure 13.18). 

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