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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 . Natural gas consists next to methane to a considerable part of light paraffins, especially
ethane and also propane, generally 3-30% . 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 . 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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