Platinum sulfur species adsorbed

Fig. 3. (A) Variation of the relative reflectivity of a platinum surface as a function of the coverage degree by adsorbed sulfur species. (B) Number of electrons exchanged per platinum site occupied by sulfur (Ncm) as a function of the degree of coverage.

The catalyst of Cheekatamarla and Lane lost 75% of its original surface area when etposed for more than 50 h to the sulfur containing feed, whereas only a 50% reduction in surface area was observed for the low sulfur feed [268]. In parallel, the dispersion of the platinum decreased from 51 to 35%. These workers explained these observations by two types of sulfur species being present on the catalyst surface firstly, physically adsorbed sulfur species and, secondly, their oxidised products, which were then irreversibly bound to the catalyst sites by chemical adsorption. The activity of the catalyst could be completely re-gained when it was reduced in 20 vol.% hydrogen (with a balance of helium) for 30 min at 800 °C. 

The chemisorption of sulfur from mixtures of H,S and H2 has been widely studied we have discussed some of the results. Nevertheless, introduction of irreversible and reversible adsorbed sulfur, which is in line with adsorption stoichiometries varying from more than 1 to 0.4 sulfur atom by accessible platinum atom, shows that different adsorbed species are involved in sulfur chemisorption. In fact, electrooxidation of adsorbed sulfur on platinum catalysts occurs at two different electrochemical potentials (42) in the same way, two different species of adsorbed sulfur were identified on gold by electrochemical techniques and XPS measurements (43,44). By use of 35S (45) it was pointed out that, according to the experimental conditions, reducible PtS2 or nonreducible PtS mono-layers can be created. 

The adsorption of anions on solid surfaces is of considerable interest, mainly because of its effect on the kinetics of electrochemical reactions. Several in-situ techniques have been applied toward this purpose. Infrared measurements were used to identify adsorbed species, estimate anion adsorption isotherms, and to gain information on anion interaction with electrode surfaces. " Sulfuric acid anions are possibly the conunonest anion adsorbates because of their specific adsorption on metal surfaces. Depending on the metal, its surface orientation, and the concentration of anion, either sulfate or bisulfate can be specifically adsorbed on the surface. Identifying the predominant adsorbate on platinum-group metals has engendered some controversy. While STM studies show that... 

Similar to platinum, controversial views were reported on the structure of adsorbed species formed from sulfuric acid on Au(lll) faces. According to Magnussen et al., a bisulfate layer is formed. In contrast to this, on the basis of thermodynamic analyses of charge-potential measurements, the existence of a sulfate adlayer was suggested by other authors. ... 

---