Another new approach to control coke formation in the

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Unformatted text preview: f coke formation but decreases the rate of steam reforming too. Another new approach to control coke formation in the STR of methane is based on the idea of preventing coke formation by the use of metal dopants, which react with Ni inhibiting carbide formation [18, 72]. Ag, Sn, Pb, As, Sb, Bi and Zn were found to reduce coke formation significantly, but they also reduced the rate of steam reforming over the catalyst. The reasons for the reduction of coke formation are not clear yet, but all the metals that had a positive effect on coking also form clusters on the surface with Ni. This may indicate ensemble size control like in the passivation with sulphur [18, 78]. Also the addition of small amounts of Mo a Ni/Al catalyst were found to reduce the rate of coking in the STR of n-butane. Also here the exact reason is not yet clear [79]. Natural gas consists next to methane to a considerable part of light paraffins, especially ethane and also propane, generally 3-30% [11]. Since higher paraffins could lead to coke accumulation in the methane steam reformer, pre-reformers are used in the STR of natural gas [11, 17]. As the light paraffins are more reactive than methane the prereformer works at lower temperatures than the actual steam reformer. In the prereformer also Ni catalysts are used. It is noteworthy that the Ni catalysts in the prereformer are not prepared by impregnation as conventional normal steam reforming catalysts but by coprecipitation or deposition precipitation because such catalysts are more active [17]. The formulation of catalysts used for the steam reforming of light paraffins (C2H6 - C4H10) and higher hydrocarbons especially naphtha is more complicated than for methane because of their greater tendency to form coke deposits. Excessive coke formation can be suppressed by basic additives to the catalyst as already mentioned. 1. Introduction 26 ytrtrghrhrhhrhrhhffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff Unlike for the STR of methane, which is generally performed over α-Al2O3 supported Ni catalysts, for the STR of light paraffins only basic supports are suitable such as MgAl2O4, CaAl2O4, BaAl2O4, SrAl2O4, Z...
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