Filamentous coke catalyst

As the metal particle size decreases the filament diameter should also decrease. It has been shown that the surface energy of thirmer filaments is larger and hence the filaments are less stable (11,17-18). Also the proportion of the Ni(l 11) planes, which readily cause carbon formation, is lower in smaller Ni particles (19). Therefore, even though the reasons are diverse, in practice the carbon filament formation ceases with catalysts containing smaller Ni particles. Consequently, well dispersed Ni catalysts prepared by deposition precipitation of Ni (average metal particle size below 2-3 nm) were stable for 50 hours on stream and exhibited no filamentous coke [16]. 

In the steam cracking of hydrocarbons, a small portion of the hydrocarbon feed gases decomposes to produce coke that accumulates on the interior walls of the coils in the radiant zone and on the inner surfaces of the transferline exchanger (TLX). Albright et identified three mechanisms for coke formation. Mechanism 1 involves metal-catalyzed reactions in which metal carbides are intermediate compounds and for which iron and nickel are catalysts. The resulting filamentous coke often contains iron or nickel positioned primarily at the tips of the filaments. This filamenteous coke acts as excellent collection sites for coke formed by mechanisms 2 and 3. Mechanism 2 results in the formation of tar droplets in the gas phase, often from aromatics. These aromatics are often produced by trimerization and other reactions involving acetylene. Some, but not all, of these droplets collect... 

The reactions shown in Eqs. 3a and 3c are endothermic in nature, whereas the water-gas shift (WGS) reaction shown in Eq. 3b is moderately exothermic. If these reactions are carried out externally (external reformaticm), the efficiency and operation of the cell are significantly affected. Hence, internal reforming is preferred however, Ni acts as an excellent coking catalyst. As a consequence, in the presence of carbonaceous fuels (and in the absence of sufficient water vapor), there is always a possibility of deposition of carbon filament on the surface of Ni. The mechanism involves carbon formation on the metal surface followed by dissolution of the carbon into the bulk of the metal and finally precipitation of graphitic carbon at some surface of the metal particles after it becomes supersaturated with carbon [6]. It not only reduces the active sites for reactions mentioned in Eqs. 2c-j and 3a-c but also destroys the whole anode over a period of time. The following three reactions are the most probable catalytic reactions that lead to carbon formation in high-temperature systems ... 

The activity and stability of catalysts for methane-carbon dioxide reforming depend subtly upon the support and the active metal. Methane decomposes to carbon and hydrogen, forming carbon on the oxide support and the metal. Carbon on the metal is reactive and can be oxidized to CO by oxygen from dissociatively adsorbed COj. For noble metals this reaction is fast, leading to low coke accumulation on the metal particles The rate of carbon formation on the support is proportional to the concentration of Lewis acid sites. This carbon is non reactive and may cover the Pt particles causing catalyst deactivation. Hence, the combination of Pt with a support low in acid sites, such as ZrO, is well suited for long term stable operation. For non-noble metals such as Ni, the rate of CH4 dissociation exceeds the rate of oxidation drastically and carbon forms rapidly on the metal in the form of filaments. The rate of carbon filament formation is proportional to the particle size of Ni Below a critical Ni particle size (d<2 nm), formation of carbon slowed down dramatically Well dispersed Ni supported on ZrO is thus a viable alternative to the noble metal based materials. 

In contrast to the Pt catalysts discussed above, Ni based catalysts (i.e., also when supported on ZrO usually form coke at such a rapid rate that most fixed bed reactors are completely blocked after a few minutes time on stream (see Fig. 8) [16], The coke formed with the Ni catalysts is filamentous. The Ni particle remaining at the tip of the filament hardly deactivates as the coke formed on its surface seems to be transported through the metal particle into the carbon fibre, but the drastic increase in volume causes reactor plugging and prevents use of the still active catalyst (see Fig. 8). The TEM photographs indicate that the carbon filaments have similar diameters to those of the Ni particles. 

R.T.K. Baker, D.J.C. Yates, and J.A. Dumesic, Filamentous Carbon Formation over Iron Surfaces, in Coke Formation on Metal Surfaces, in Coke Formation on Metal Surfaces, eds. L.G. Albright and R.T.K. Baker, American Chemical Society, Washington D.C., 1982, p. 1. D.J. Dwyer, Iron Fischer-Tropsch Catalysts Surface Synthesis at High Pressure, Prep. ACS Div. Pet. Chem. 29 (1984) 715. 

Coke on Nickel Catalysts. - Nickel catalysts are well known for their ability to form whisker (or filamentous) carbon. The mechanism of formation of... 

Figure 19.4 displays a picture obtained by scanning electron microscopy (SEM) for a nickel catalyst deactivated. A large amount of carbon filaments and agglomerates of coke can be easily identified on the catalyst surface. These filaments do not cause a direct deactivation since they consist of a-type carbon. They do, however, lead to reactor clogging. 

Three catalysts prepared in this study showed a relatively severe and fast deactivation than the commercial catalyst. To investigate the reasons of the catalyst deactivation, SEM and EPMA methods were used to characterize the surface property changes after the dechlorination reaction. SEM images of the catalyst before and after the dechlorination reaction at different temperatures are shown in Fig. S. Significant coke formation was observed only in the sample tested at 500°C. The coke was filamentous and its diameter was approximately 40nm. High reaction temperature of 500 C is considered to be the major reason of the coke formation. 

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