Surface carbon, reactivity nickel catalyst

J.G. McCarty, P.Y. Hou, D. Sheridan, and H. Wise, Reactivity of Surface Carbon on Nickel Catalysts Temperature-Programmed Surface Reaction with Hydrogen and Water, in Coke Formation on Metal Surfaces, eds. L.G. Albright and R.T.K. Baker, American Chemical Society, Washington D.C., 1982, p. 253. 

Reactivity of Surface Carbon on Nickel Catalysts Temperature-Programmed Surface Reaction with Hydrogen and Water... 

The nature of the carbon deposits formed on an alumina-supported nickel catalyst have been characterized by their reactivity with H2 and H 0 during temperature-programmed surface reaction (TPSR). 

Carbon can exist on the metal surfaces of nickel catalysts in a variety of forms. Hydrocarbon exposure to nickel crystallites at elevated temperature (> 700 K) can rapidly produce a mass of long-growing carbon filaments (1, 2) as identified in numerous experiments analyzed by transmission electron microscopy (TEM). Yet very reactive forms of surface carbon can exist, since carbon atoms chemisorbed on nickel surfaces apparently play a central role in the mechanism of several nickel-catalyzed reactions, such as hydrocarbon synthesis, (3, U, 5) hydrocarbon steam reforming, (6, 7) and hydrogenolysis (8). 

The morphology and properties of the carbon deposition of the nickel-based catalysts for carbon dioxide reforming of methane are investigated. Silica supported nickel catalysts were more facile to carbon deposition than alumina supported catalysts. The decomposition of methane resulted in the formation of at least three kinds of surface carbon species on supported nickel catalysts. Carbidic C , carbonaceous Cp and carbidic clusters Cy surface carbon species formed by decomposition of methane showed different thermal stability and reactivity. The carbidic carbon was a very active and important intermediate in the carbon dioxide reforming of methane and the carbidic clusters Cy species might be the precursor of the surface carbon deposition. The partially dehydrogenated Cp species can react with H2 or CO2 to form CH4 or CO. 

JG McCarty, PV Hou, D Sheridan, H Wise. Reactivity of surface carbon on nickel catalysts temperature-programmed surface reaction with hydrogen and water. In LF Albright, RT Baker, eds. Coke formation on metal surfaces. Washington, DC American Chemical Society, 1982. 

As for heterogeneous catalysts, the addition of hydrogen is catalyzed by a large variety of materials, but synthetically useful procedures generally employ nickel or the platinum metals. In the latter case, the best results are obtained if the metal is finely divided over the surface of an inert support. Many materials can be used as catalyst supports, however, carbon or alumina are suitable for the majority of reactions. Calcium and barium carbonate or sulfate are also frequently used if less reactive catalysts are desired. The influence of the support is generally small compared to the effect of the metal3. The choice of metal is especially important when stereoselectivity is desired because different metals can catalyze the formation of different diastereomers upon hydrogenation. 

When fuel contains heavier hydrocarbons than methane, or it is biofuel, or contains alcohols, the feedstock often contains additional compounds such as sulphur and phosphorus, that is, fertiliser impurities. In the petrochemical industry, gas-borne reactive spedes (i.e., sulphur, arsenic, chlorine, mercury, zinc, etc.) or unsaturated hydrocarbons (i.e., acetylene, ethylene, propylene and butylene) may act as contaminating agents (Deshmukh et al, 2007). These impurities cause catalyst deactivation by poisoning. The effect of a poison on an active surface is seen as site blockage or atomic surface structure transformation (Babita et a/., 2011). Therefore, it is important to choose poisoning-resistant catalyst materials. For example, nickel is not the most effective MSR catalyst although it is widely used in industry due to its low market price compared to ruthenium and rhodium. Both Ru and Rh are more effective in MSR and less carbon is formed in these systems, than in the case of Ni. However, due to the cost and availability of precious metals, these are not widely used in industrial applications. 

---