Sulfur adsorption

Perdereau and Oudar s study20 of sulfur adsorption at Ni(110) surfaces showed a very similar behaviour to that of Cu(110). They identified a series of... 

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).

Azzaroni etal. [163] have used STM to study electrochemical reactivity of thiourea toward Au(lll). Sequential STM imaging has shown that thiourea adsorbs as striped arrays that evolve to the hexagonal close-packed structure when surface charge density is decreased. The transient hep structure undergoes electrooxidation to formamidine disulfide, which slowly yields adsorbed sulfur. Adsorption of thiourea on the pc-Au electrode from KCIO4 solutions has also been studied [164]. The film pressure and the Gibbs surface... 

Kruger, P. and Pollmann, J. lst-principles theory of sulfur adsorption on semi-infinite Ge(001). Physical Review Letters 64, 1808-1811 (1990). 

A. Thermodynamics of Sulfur Adsorption 1. Affinity of Sulfur for Metals... 

Therefore, it appears clearly from all the observations that sulfur adsorption from thiophene onto palladium is restricted to the surface area of metal particles, with part of this surface being sulfided through the decomposition of the sulfur compound. 

Such electronic transfer induced by sulfur adsorption was also pointed out by using cinnamic acid as a probe molecule (48). The UV-visible reflexion spectra of adsorbed cinnamic acid on nonpoisoned and partly poisoned platinum catalysts shows that adsorption on pure platinum induces a shift of the peaks toward the higher wavelengths and an appearance of fine structure. Sulfurization of platinum induces a further enhancement of higher wavelength peaks. Binding energy of cinnamic acid is thus increased by sulfur adsorption on Pt catalysts. 

In order to obtain deeper insight into such complex effects, the definition of the deactivation extent induced by sulfur adsorption can provide a lot of information about its action. The simplest technique to characterize a poison is to define its initial toxicity as the number of accessible metal atoms deactivated through adsorption of the first molecule of poison (74). 

Further, Maxted (36) studied the influence of several sulfur compounds on the activity of platinum black for the hydrogenation of crotonic acid in the liquid phase. He noticed that between 15 and 50°C the toxicity remains constant for a sulfur compound, pointing out the irreversibility of sulfur adsorption. Conversely, the toxicity of various compounds increases with the molecule size. For molecules containing two sulfur atoms, losing all freedom of rotation through this double adsorption, the toxicity is less than for molecules of the same length containing only one atom of sulfur. 

Boitiaux et al. (61) have examined the influence of palladium sulfuration on the hydrogenation and isomerization of 1-butene, 1,3-butadiene, and 1-butyne. The tested catalysts have been sulfided with thiophene to obtain an atomic ratio (sulfur per surface palladium) varying between 0 and 0.5. The thiophene in heptane solution is put in contact with the reduced palladium catalyst at 50°C, under 2 MPa hydrogen pressure. The butane evolution is followed during the sulfiding step (see above) and a control of total sulfur adsorption is performed by the analysis of the heptane after the sulfiding step and through X-ray fluorescence after the reaction step. 

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... 

Modification of selectivity induced by sulfur adsorption has been widely studied (41, 90, 75) and can be evaluated by comparison of the initial toxicides (see Table VI) for different parallel reactions. As an example, sulfur on platinum does not inhibit similarly benzene hydrogenation and the exchange with deuterium reactions. The H/D exchange is less deactivated than the hydrogenation. For instance, one sulfur atom (introduced as H2S) poisons five accessible platinum atoms for benzene hydrogenation, whereas it poisons only two platinum atoms for exchange. The mechanism of selectivity modification induced by sulfur adsorption is far from being explained. 

Competitive reactions and essentially competitive hydrogenations were often used to discuss the extent of the electronic transfers induced by poison adsorption. For instance, two model molecules with different electronic densities are chosen, e.g., benzene and toluene. In this case the electronic donor properties of the methyl group increase the electronic density on the insaturated bonds. During competitive hydrogenation of benzene and toluene, sulfur adsorption poisons the two reactions but is less toxic for toluene than for benzene hydrogenation (94-96). Sulfur, by its adsorption as an electron acceptor, is able to decrease the electronic density of the unpoisoned metallic surface area and could favor the adsorption of the reactant with the highest donor properties enhancing the hydro-... 

Nevertheless, two factors strongly influence the heat of sulfur chemisorption on metal surfaces relative coverage and crystallographic structure. Thus sulfur chemisorbs at high coordination sites and, as a result, a selective poisoning of structure-sensitive reactions, preferentially catalyzed by these sites, may occur. Such a simple geometrical model can be used to explain change in selectivities induced by sulfur adsorption. 

In hydrogenation conditions the effect of sulfur adsorption is the result of interactions between the metal, the hydrocarbon, and the sulfur-containing compound. As a consequence, for a given metal, the sulfur coverage, and its effect on the activity and selectivity of the unpoisoned metallic surface area, will be defined by the nature of the hydrocarbon. 

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