Adsorbed sulfur

Wierse DG, Lohrengel MM, Schultze JW (1978) Electrochemical properties of sulfur adsorbed on gold electrodes. J Electroanal Chem 92 121-131... 

Studies of Atomic Sulfur Adsorbed at Metal Surfaces... 

Figure 10.9 STM images showing structural changes induced by sulfur adsorption. (a) Clean Au(lll) surface showing very regular herringbone pattern, (b) Close-up of the disordered herringbone pattern at low coverage of sulfur (<0.1ML). (c) Atomically resolved images of the Au atoms underlying approximately 0.3 ML sulfur adsorbed Au(lll). (Reproduced from Ref. 34).

Another example of the flexibility of the Pt catalyst is the reconstruction of a stepped Pt(l 11) crystal with adsorbed sulfur upon exposure to CO [25]. Single-crystal Pt(l 1 1) cut at an angle of approximately 5° from the (1 1 1) direction consists of numerous terraces with a width of 20-60 A separated by steps with single-atom height. The adsorption of sulfur atoms restructures the clean stepped Pt(l 1 1) surface with single-atom steps into a sulfur-adsorbed surface with double-atom... 

A part of Figure 3 in Ref. 207, reproduced on the right, reports radial EXAFS data around the S Is absorption edge for sulfur adsorbed on the (100) plane of a g nickel single-crystal surface. The top trace corresponds to the deposition of atomic S sulfur by dehydrogenation of H2S, while g, the bottom data were obtained by adsorb- M ing thiophene on the clean surface at 100 K. Based on these data, what can be learned about the adsorption geometry of thiophene Propose a local structure for the sulfur atoms in reference to the neighboring nickel surface. 

In other papers by the same group, the effects of sulfur adsorbed or segregated on the Ni surface on corrosion or passivation were described, including the sulfur-induced enhancement of dissolution and the blocking of passivation. It was shown how the conditions of stability of adsorbed sulfur monolayers could be predicted on thermodynamical grounds and this was illustrated by a potential-pH diagram for adsorbed sulfur on nickel in water at 25 °C. (See Refs. 22, 25-29 and papers cited therein.)... 

However, the rate of H2-D2 equilibration reaction (at I25°C) on the Pt(lll) face, modified by sulfur adsorption, is increased at low sulfur coverages showing a maximum at 0S = 0.11 88, 89). In the same way, an activation effect of sulfur, adsorbed at low coverages on polycrystalline... 

Carbon-sulfur groups are extremely stable surface compounds which cannot be removed by thermal treatment up to 1470 K. Only by a reductive treatment with hydrogen it is possible to clean carbon from sulfur adsorbates. One source of sulfur is the fuel used for the generation of the carbon from which about 90% are covalently bonded to the carbon and 10% are segregated as adsorbate which can be removed by solvent extraction. The abundance of sulfur can amount to several wt%. Removal of the structural sulfur is possible by hydrogen reduction to H2S at about 1000 K. A collection of references on this subject is found in the literature [33]. In activated carbons sulfur can also be present in an oxidized form as sulfate or as C-S-O compounds. 

A number of recent surface studies (5, 6, 26-50) have provided useful information regarding structures of sulfur adsorbed on various faces of Ni (Table III). During the initial stages of sulfur adsorption on clean Ni single-... 

Dynamic LEED 31 Sulfur adsorbs in hollow sites 0.13 nm above the Ni surface interaction between S and Na during coadsorption —Na bonds to S rather than Ni, but electron transfer is small, < 1 electron/Na atom... 

Fig. 7. Coincidence (5 yJZ x 2) structure for sulfur adsorbed on Ni(l 11) at 0 = 0.40. Solid circles represent adsorbed sulfur atoms open circles, nickel atoms in the adsorbent surface layer (Ref. 47).

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. 

Because sulfur adsorbs very strongly on metals and prevents or modifies the further adsorption of reactant molecules, its presence on a catalyst surface usually effects substantial or complete loss of activity in many important reactions, particularly in hydrogenation reactions. Where the reaction network leads to two or more products, adsorbed sulfur can markedly affect the selectivity by reducing the rate of one of the reactions more than the other(s). In a few reaction systems these changes in selectivity are desirable however, in many others they are not. 

To enable quantitative determination of rates of sulfur deactivation, of extents of sulfur deactivation at very low gas-phase sulfur concentrations, of true dynamic equilibrium between gas-phase sulfur concentration and metal surface, and/or of the amount of sulfur adsorbed on the surface, the following requirements must be satisfied in the design of experimental apparatus ... 

An example of a system which most nearly meets these requirements is a quartz continuous-flow stirred-tank reactor (CFSTR) (99-101,140,1%, 197) with catalyst configurations in which all surface metal atoms are on the exterior surface of the support. It satisfies the relevant requirements listed above and allows investigation over a broad range of both product and reactant concentrations. Furthermore, true poisoning rates can be measured directly, without requiring assumption of a model for the poisoning. The amount of sulfur adsorbed can be directly determined as a function of time and gas-phase H2S concentration, and the catalytic activity of the metal can be measured as a function of sulfur on the surface. 

Comparison of the methanation activity in Fig. 28 with the H2S concentration transients in Fig. 29 indicates a prolonged adsorption of H2S during the 13-ppb deactivation with the H2S level reaching a steady state at about the same time as the methanation activity. The difference between the sulfur fed to the reactor and that in the effluent represents the sulfur that was adsorbed on the Ni surface, since the alumina support and the quartz reactor did not adsorb sulfur. Time integration of the amount of sulfur adsorbed, obtained from the effluent H2S concentration data, gives the total amount of sulfur adsorbed at equilibrium with the gas-phase H2S concentration. 

Comparison of the site densities from Table XIX with metal areas determined from H2 adsorption provides important insights into the nature of H2S adsorption on these catalysts. For example, the sulfur site density of 213 /tmol/g compared to the metal site density of 182 /rmol/g (from H2 adsorption) for 14% Ni/Al203 is equivalent to S/Nis = 0.6, in reasonable agreement with the earlier discussed studies (Section III,C) which show values of 0.5-0.8 and consistent with the value 0.6 determined for pure unsupported Ni. However, in the case of a typical molybdenum-containing catalyst, e.g., 10% Ni/20% Mo/A1203, the sulfur site density and H2 uptake are 693 and 72 /imol/g, respectively (S/Nis = 4.8), providing evidence that a considerable amount of sulfur adsorbs on molybdenum oxide sites which do not adsorb H2 a similar behavior is also observed for Raney Ni and nickel-boride catalysts. 

The total amount of sulfur adsorbed in mmol/g clearly shows that the SC-2 surface favors adsorption of sulfur containing species, especially hydrogen sulfide. Only on this sample, and only when HjS is involved, the sulfur retained on the surface reaches over 2 mmol/g. Since the amount adsorbed is few times greater than for the other samples the catalytic effect, probably on iron oxides, seems to govern the performance of this material. 

Table 1. HjS and SO2 breakthrough capacities [mg/g/ imnol/g], total amount of sulfur adsorbed...

A frontal analysis with a photo-ionization detector allowed the amoimt of sulfur adsorbed by the catalyst (Stot) to be measured. After the catalyst was saturated with sulfur, pure hydrogen was passed through the sample for 10 h, which led to the desorption of some sulftir (Srev). The difference between Stot and S v allows the determination of the amount of irreversible sulfur adsorbed at 500°C. 

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