Sulfur-modified metal surfaces

KOESTNER ET AL. Clean and Sulfur-Modified Metal Surfaces... 

On metallic catalysts, sulfur is strongly adsorbed, and even if only minute amounts are found in the feedstock, accumulation can occur on a significant part of the metallic surface area. In the adsorbed state, the poison molecule will deactivate the surface on which it is adsorbed then the toxicity will depend on the number of geometrically blocked metal atoms. On the other hand, the chemisorption bond between the poison and the metal can modify the properties of the neighboring metallic atoms responsible for the adsorption of reactants. If the interaction between the poison and the metal is weak, the structure of the metal will remain unchanged, but it can induce a perturbation all around the adsorption site, which will be able to modify the catalytic properties of this surface. Yet if the interaction between the metal and the adsorbate is strong, it can go as far as to modify the metal-metal bond. The mobility of the surface atoms can be increased and a new superficial structure can appear. 

In a separate series of experiments, the influence of sulfur on the decomposition of a mixture consisting of CO/C2H4/H2 over iron was investigated. Previous work [17] had shown that while iron did not catalyze the decomposition of ethylene, even in the presence of hydrogen, when a small fraction of CO was added to the reactant, a dramatic increase in the rate of decomposition of the olefin was observed. This behavior was rationalized according to a model in which the presence of coadsorbed CO resulted in what is believed to be reconstruction of the iron to form a surface, which favors dissociative chemisorption of ethylene. In the current study, we have extended this study to include the case where sulfur is preadsorbed on the metal surface in an attempt to determine how such adatoms modify the coadsorption characteristics of CO and C2H4 on iron. 

The distribution of these impurities or minor alloy constituents near lattice discontinuities is known to affect the chemical and mechanical properties of the contaminated materials for example the presence of sulfur on a metal surface can promote ) or retard - o) corrosion, modify the surface energy ) or cause considerable increase in the surface self-diffusion coefficient ). Sulfur accumulation along grain boundaries may induce intergranular weakness and render otherwise ductile materials brittle ), either by formation of precipitates " ) or by enhancement of hydrogen adsorption >227)... 

Other elements can influence the formation of the coating by altering the rate of diffusion or by modifying the surface reactions. Sulfur and phosphorus have a poisoning effect on the chromizing media most other elements influence the rate of diffusion by virtue of their effect on the stability of the metallic phases present. 

SAMs on solid substrates discussed in this chapter fall into the latter category most of the SAM-modified substrates are prepared by exploiting either the strong interaction between sulfur-bearing organic molecules and metal surfaces [4] or the covalent siloxane bonds [5], The effect of metal-ion coordination in such chemisorbed layers, unlike that in the case of solution chemistry or physisorbed systems, is in influencing the local order in the SAM, while in most cases... 

In addition to adsorption, SO2, H2S, and CS2 also undergo catalytic conversions on the impregnated metal sites. The desorption experiments carried out by these workers showed" the role of chemical interactions and surface complexing in the adsorption of these sulfur compound on the cation impregnated carbons. It was found that only a small fraction (10 to 15%) of the adsorbed compound was desorbed from the cation modified carbon surface. The major fraction remained strongly bonded to the carbon surface or was catalytically converted, as evidenced by the presence... 

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