Carbon poisoning

Z)-Methyl styrene was easily obtained by hydrogenation of 1-phenyl-1-propyne using Lindlar s catalyst (5% palladium on calcium carbonate, poisoned with lead) in //-hexane under an atmospheric pressure of hydrogen. The mixture, containing 90% of (Z)-methyl styrene and 10% of the overreduced alkane, was used without further purification. 

One of the main problems associated with hydrocarbon steam reforming over Ni is the deactivation of the Ni catalyst as a result of the formation of carbon deposits on Ni. The C-induced deactivation of Ni has been studied extensively [10,18,28-35], For example, Rostrup-Nielsen reported that steam reforming of various hquid fuels on Ni leads to the formation of encapsulating, whisker-like, or pyrolytic carbon on the catalyst [18, 30], To illustrate the problem of carbon poisoning, in Fig. 13.1 we show a transmission electron micrograph (TEM) of a Ni particle taken after steam reforming of propane at steam to carbon ratio of 1.5. The micrograph shows that carbon deposits are formed on Ni [16],... 

In addition to carbon-tolerant catalysts mentioned above, which were mainly identified through empirical testing, there have also been a number of recent efforts to probe the underlying molecular mechanism associated with carbon poisoning... 

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]. 

The steam reforming is an endothermic reaction therefore, the reaction is used for hydrogen production and control of the temperature distribution due to the cooling ability by the endothermic reaction. The catalyst for internal reforming is generally a nickel-based material supported on an oxide such as MgO, calcium aluminate, or a-alumina. Molten carbonate poisons the reforming catalyst therefore, placement and improvement of the catalysts are important for the internal reforming system [2, 5]. 

Hydrogen and helium permeance (with the feed of 1 1 Hj and He mixture) through a 200 nm thick Pd-Ag membrane before and after being exposed to a carbon source at 600°C (Lin, 2001). Open symbols - before carbon poisoning closed symbols - after carbon poisoning. 

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