Carbon-poisoned surface

The deleterious effect of water vapor was speculated to be due to its inhibition of carbon formation freeing the metal surface for interaction by H2S. Thus, sulfur poisoning of nickel at high temperature (above 673 K) may be more representative of a carbon-fouled surface, whereas at low temperatures it may be more characteristic of the clean metal surface. Again, this needs to be confirmed by direct measurements of carbon and sulfur adsorption. For Ni/Al203 and Ni/ZrOz the extent of sulfur deactivation was about fiftyfold at 673 K at 523 K the extent of deactivation was about 1000-fold. However, for Raney Ni the extent of sulfur deactivation was tenfold higher at 673 K than at 523 K this difference in behavior also needs confirmation and explanation. 

AES studies of in situ sulfur-poisoned Ni and Ru showed the complete absence of carbon on the surface or in the near-surface region for Co, on the other hand, a fraction of a monolayer of carbon was present but there was no bulk carburization. For Co which was carbon deactivated prior to sulfur poisoning, surface carbon and bulk carburization were both observed (205). 

The sequence of elementary steps shown in Fig. 13.2 suggests that one can formulate the problem of carbon poisoning in terms of the selectivity associated with the formation of C-0 vs. C-C bonds on Ni. In order to prevent carbon-induced deactivation, a catalyst should be able to selectively oxidize C atoms (and CH fragments) rather than form C-C bonds. This elementary step mechanism was the basis for the DFT calculations that focused on the identification of catalysts (mainly Ni-containing alloys), which preferentially oxidize C atoms rather than form C-C bonds [15, 16]. In these DFT calculations, the potential energy surfaces for the formation of C-C and C-0 bonds were calculated for different Ni alloys. The alloy model system used in these calculations contained mainly Ni, with some Ni atoms displaced by another atom in the surface layer. While we have examined a number of different alloys, we will focus our discussion on the alloy material (Sn/Ni). We note that this alloy material has also been studied by others previously [35, 38, 41, 49, 50]. 

Atomic C and O are yielded by the dissociation of CO in FTS, the C adsorption on Co surfaces is crucial for this reaction. As listed in Table 2, the adsorption of C on most Co models is much stronger than the adsorption of H and O. Weststrate et al. made a systematic investigation on the C adsorption on Co surfaces. The experimental results indicated that atomic carbon weakened the adsorption of CO and H2, whereas a saturated (reconstructed) atomic carbon-covered surface can still adsorb 60% of the CO and H compared to the clean surface. Thus, a high coverage of atomic carbon may not be a strong poison to FT cobalt catalysts. In other words, FTS on Co catalysts is feasible in spite of the strong adsorption of C atoms on Co surfaces. 

Occurrence. Carbon monoxide is a product of incomplete combustion and is not likely to result where a flame bums in an abundant air supply, yet may result when a flame touches a cooler surface than the ignition temperature of the gas. Gas or coal heaters in the home and gas space heaters in industry have been frequent sources of carbon monoxide poisoning when not provided with effective vents. Gas heaters, though properly adjusted when installed, may become hazardous sources of carbon monoxide if maintained improperly. Automobile exhaust gas is perhaps the most familiar source of carbon monoxide exposure. The manufacture and use of synthesis gas, calcium carbide manufacture, distillation of coal or wood, combustion operations, heat treatment of metals, fire fighting, mining, and cigarette smoking represent additional sources of carbon monoxide exposure (105—107). 

The mechanism of poisoning automobile exhaust catalysts has been identified (71). Upon combustion in the cylinder tetraethyllead (TEL) produces lead oxide which would accumulate in the combustion chamber except that ethylene dibromide [106-93-4] or other similar haUde compounds were added to the gasoline along with TEL to form volatile lead haUde compounds. Thus lead deposits in the cylinder and on the spark plugs are minimized. Volatile lead hahdes (bromides or chlorides) would then exit the combustion chamber, and such volatile compounds would diffuse to catalyst surfaces by the same mechanisms as do carbon monoxide compounds. When adsorbed on the precious metal catalyst site, lead haUde renders the catalytic site inactive. 

Nickel. As a methanation catalyst, nickel is presently preeminent. It is relatively cheap, it is very active, and it is the most selective to methane of all the metals. Its main drawback is that it is easily poisoned by sulfur, a fault common to all the known active methanation catalysts. The nickel content of commercial nickel catalysts is 25-77 wt %. Nickel is dispersed on a high-surface-area, refractory support such as alumina or kieselguhr. Some supports inhibit the formation of carbon by Reaction 4. Chromia-supported nickel has been studied by Czechoslovakian and Russian investigators. 

On the surface of metal electrodes, one also hnds almost always some kind or other of adsorbed oxygen or phase oxide layer produced by interaction with the surrounding air (air-oxidized electrodes). The adsorption of foreign matter on an electrode surface as a rule leads to a lower catalytic activity. In some cases this effect may be very pronounced. For instance, the adsorption of mercury ions, arsenic compounds, or carbon monoxide on platinum electrodes leads to a strong decrease (and sometimes total suppression) of their catalytic activity toward many reactions. These substances then are spoken of as catalyst poisons. The reasons for retardation of a reaction by such poisons most often reside in an adsorptive displacement of the reaction components from the electrode surface by adsorption of the foreign species. 

Gold is generally considered a poor electro-catalyst for oxidation of small alcohols, particularly in acid media. In alkaline media, however, the reactivity increases, which is related to that fact that no poisoning CO-hke species can be formed or adsorbed on the surface [Nishimura et al., 1989 Tremihosi-Filho et al., 1998]. Similar to Pt electrodes, the oxidation of ethanol starts at potentials corresponding to the onset of surface oxidation, emphasizing the key role of surface oxides and hydroxides in the oxidation process. The only product observed upon the electrooxidation of ethanol on Au in an alkaline electrolyte is acetate, the deprotonated form of acetic acid. The lack of carbon dioxide as a reaction product again suggests that adsorbed CO-like species are an essential intermediate in CO2 formation. 

The general advantage of using carbon tetrachloride or phosgene is that these compounds decompose at the reaction temperature to provide a uniform distribution of active carbon or carbon monoxide and chlorine at the reaction sites over the oxide surface. These reagents are, however, not as convenient to use as a carbon and chlorine mixture in large-scale operations. Besides, phosgene is poisonous. 

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