Sulfur site blocking

In this way, the conjunct polymers serve as a reservoir of hydride ions. Under some conditions, the polymers are a source of hydride ions, but they accept these ions under other conditions. Substantial amounts of the saturated products are supposedly formed via this route with sulfuric acid. In zeolites, species similar to conjunct polymers also form. The heavy hydrocarbon molecules, which deactivate the catalyst by pore blocking or by site blocking, are generally termed soft coke or low-temperature coke , because of the absence of aromatic species. 

The site responsible for CH30 formation can be identified with the use of CO as a probe molecule. As increasing amounts of sulfur are added to Ni(100), the desorption state characteristic of CO on the clean surface disappears, and two new states appear at 315 and 380 K, respectively. These states persist from about 0g - 0.15 to 0gO. 46 (see Figure 5). In this coverage range one sulfur atom blocks adsorption of one CO molecule (15). 

Studies of the effect of sulfur on the activation of methane on a Ni(lOO) surface showed results in line with these observations. It was concluded that the poisoning effect was a simple site-blocking process which could be described by eq(l) with a=3. 

As demonstrated in recent HREELS and TPD studies of CO adsorbed on Ni (100), the majority of this local interaction may be explained by geometric effects or simple site blocking (5,6). A schematic diagram of a clean Ni(100) surface and a Ni(100) surface covered with a p(2x2) sulfur overlayer are shown in Figure 3. On clean Ni, CO adsorbs linearly on a single Ni atom, or atop site up to a coverage of 0 = 0.5. monolayer (6,7). Above this coverage so-... 

Site Blocking by Sulfur Let us consider the interaction of coadsorbed sulfur with thiophene which occurs during the hydrodesulfurization of thiophene on... 

Sulfur adsorbed (S ds) on metal surfaces blocks or delays the formation of the passive film by blocking the adsorption sites for OH, which are the precursors of passive oxide film formation. This site blocking effect is represented in Fig. 3-22. 

Fig. 27. The site-blocking effect of sulfur on deuterium adsorption on Mo(lOO) as determined by deuterium thermal desorption. (0) = sulfur layer disordered, (x) = sulfur layer ordered. The broken line is a theoretical prediction of the site-blocking effect assuming that one sulfur atom blocks one deuterium atom adsorption site and that deuterium molecules chemisorb dissociatively in adjacent, unoccupied sites...

Figure 5.21. Primary ensemble effect (a) multimolecular atom adsorption to a metal surface (b) decrease in heat of adsorption by site blocking. S indicates an adsorbed sulfur atom, (schematic denotes a site consisting of one or more surface metal atoms).

A large molecule, e.g., a hydrocarbon, may attach to a metal surface with several bonds. This is schematically illustrated in Figure 5.21. In this figure dissociative adsorption of pentane occurs by attachment of two of its carbon atoms to the metal surface. This mode of adsorption becomes suppressed when sulfur or carbon atoms are coadsorbed. The S or C atoms decrease the probability for a surface to have neighboring-reactive sites. As a result, the probability for pentane to form multiple bonds with the metal surface becomes suppressed. It will only adsorb with one of its carbon atoms attached to the surface. As a result of this site blocking effect, the heat of adsorption of pentane decreases and the probability for dissociative adsorption decreases as well. 

The Eastman Chemicals from Coal faciUty is a series of nine complex interrelated plants. These plants include air separation, slurry preparation, gasification, acid gas removal, sulfur recovery, CO /H2 separation, methanol, methyl acetate, and acetic anhydride. A block flow diagram of the process is shown in Eigure 3. The faciUty covers an area of 2.2 x 10 (55 acres) at Eastman s main plant site in Kingsport, Teimessee. The air separation plant is... 

A selective poison is one that binds to the catalyst surface in such a way that it blocks the catalytic sites for one kind of reaction but not those for another. Selective poisons are used to control the selectivity of a catalyst. For example, nickel catalysts supported on alumina are used for selective removal of acetjiene impurities in olefin streams (58). The catalyst is treated with a continuous feed stream containing sulfur to poison it to an exacdy controlled degree that does not affect the activity for conversion of acetylene to ethylene but does poison the activity for ethylene hydrogenation to ethane. Thus the acetylene is removed and the valuable olefin is not converted. 

It is well established that sulfur compounds even in low parts per million concentrations in fuel gas are detrimental to MCFCs. The principal sulfur compound that has an adverse effect on cell performance is H2S. A nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Chemisorption on Ni surfaces occurs, which can block active electrochemical sites. The tolerance of MCFCs to sulfur compounds is strongly dependent on temperature, pressure, gas composition, cell components, and system operation (i.e., recycle, venting, and gas cleanup). Nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Moreover, oxidation of H2S in a combustion reaction, when recycling system is used, causes subsequent reaction with carbonate ions in the electrolyte [1]. Some researchers have tried to overcome this problem with additional device such as sulfur removal reactor. If the anode itself has a high tolerance to sulfur, the additional device is not required, hence, cutting the capital cost for MCFC plant. To enhance the anode performance on sulfur tolerance, ceria coating on anode is proposed. The main reason is that ceria can react with H2S [2,3] to protect Ni anode. 

---